- Email: [email protected]
Flexible Perovskite Solar Cells
Review
Flexible Perovskite Solar Cells Hyun Suk Jung,1,* Gill Sang Han,1 Nam-Gyu Park,2,* and Min Jae Ko3,*
Since the first report on solid-state perovskite solar cells (PSCs) with 9.7% efficiency and 500-h long-term stability in 2012, PSCs have achieved an amazing power-conversion efficiency (PCE) of 24.2%, exceeding the PCEs of multi-crystalline Si (22.3%), thin-film crystalline Si (21.2%), copper indium gallium selenide (22.6%), and CdTe-based thin-film SCs (22.1%), and are suitable for transforming into flexible solar cells based on plastic substrates. The light weight and flexibility of flexible-PSCs (F-PSCs) allows their use in niche applications such as portable electric chargers, electronic textiles, large-scale industrial roofing, and power sources for unmanned aerial vehicles (UAVs). However, the F-PSCs always exhibit inferior efficiency compared to rigid PSCs, i.e., champion-cell efficiency of F-PSCs is 19.11%, which is apparently lower than that of rigid cells. Also, the world-best module efficiency for rigid perovskite module is 17.18% (30 cm2) higher than that for flexible perovskite module efficiency, 15.22% (30 cm2). Moreover, the F-PSCs have not shown better long-term stability in comparison with rigid PSCs. In this review paper, we investigate fundamental challenges of F-PSCs regarding relatively low efficiency and stability and demonstrate the recent efforts to overcome big hurdles. Also, current attempts for the commercialization of F-PSCs are introduced.
Introduction In photovoltaic history, there has been no solar cell like the perovskite solar cell (PSC), which has achieved an amazing power conversion efficiency (PCE) of 24.2% in such a short time of 7 years. This efficiency exceeds the PCEs of multi-crystalline Si (22.3%), thin-film crystalline Si (21.2%), copper indium gallium selenide (22.6%), and CdTe-based thin-film SCs (22.1%), owing to the excellent optical, electrical, and chemical properties of perovskite materials.1,2 In 2012, the study on adopting the perovskite materials in a solid-state sensitized cell reported the achievement of 9.7% PCE,3 which triggered the efficiency race in PSCs. In 2019, more than 12,000 research papers cumulatively and many commercial companies, including Oxford PV, Dyesol, and Frontier Energy Solutions, are developing their own technologies for the commercialization of perovskite solar modules. The cost-effective material,4 low formation energy,5 mechanical durability,6 and potential for a solution-based roll-to-roll (R2R) process make the PSC suitable for realizing a flexible thin-film solar cell that employs a plastic substrate such as polyethylene terephthalate (PET). Furthermore, the flexible PSC (F-PSC) using plastic substrate would produce the most competitive power per weight among the solar cells (Figure 1A).7 Therefore, F-PSCs can be used in niche applications such as portable electric chargers, electronic textiles, large-scale industrial roofing, and power sources for unmanned aerial vehicles (UAVs). In contrast with rigid PSC based on glass substrate, the charge transport layer, especially the electron transport layer (ETL), like TiO2, ZnO, and SnO2, should be formed at a low temperature of below 150 C in plastic-based F-PSCs, which is a non-trivial
1850 Joule 3, 1850–1880, August 21, 2019 ª 2019 Elsevier Inc.
Context & Scale In a short time of 7 years, perovskite solar cells (PSCs) have achieved an amazing power conversion efficiency (PCE) of 24.2%, which exceeds the PCEs of multi-crystalline Si (22.3%), thinfilm crystalline Si (21.2%), copper indium gallium selenide (22.6%), and CdTe-based thin-film SCs (22.1%). Owing to low process temperature, mechanical durability, and the potential for the solution-based roll-to-roll (R2R) process, the PSC has a strong potential of being utilized in the form of flexible solar cell based on a plastic substrate. This flexible-PSC (F-PSC) is expected to be used in niche applications such as portable electric chargers, electronic textiles, large-scale industrial roofing, and power sources for unmanned aerial vehicles (UAVs). However, the champion-cell efficiency of the F-PSC is 19.11%, which is apparently lower than that of the rigid cell (24.2%). Also, the world-best perovskite module efficiency on a rigid substrate is 17.1%, outstripping the efficiency of flexible perovskite module (11.7%). Moreover, the F-PSCs have not shown superior longterm stability to rigid cells. To commercialize the F-PSC, the efficiency needs to be comparable to the glass-based rigid PSC as well as the long-term stability.
challenge to achieving high efficiency. Since the F-PSC is potentially used in bendable or foldable electronic devices, robust bending durability is another important issue. In this regard, there has been remarkable progress in developing F-PSCs with high efficiency as well as with bending durability. In 2014, the F-PSC with a meaningful efficiency of 10.2% was exploited by using ZnO nanocolloids ETL (5 nm).8 The high-density and uniform ZnO layer was successfully prepared without a sintering process. In 2015, Kim et al.6 realized a high PCE of 12.2% flexible cell by fabricating a TiO2 ETL using plasma-enhanced atomic layer deposition (PEALD). They performed the bending durability test of F-PSC for the first time and characterized the excellent bending durability of perovskite materials. Park et al.9 also proved the super-flexibility of F-PSCs by demonstrating the complete shape recovery of the device under repeated crumpling condition without significant mechanical damage. The PCE was maintained up to 60% of initial value even after 50 cycles of complete crumpling (Figures 1B and 1C). Another breakthrough was achieved by exploiting the inverted structured F-PSC, which adopted an organic charge transport layer.10 Owing to the use of PEDOT:PSS as an organic hole-transport layer (HTL), the maximum processing temperature for manufacturing flexible PSS was 90 C. Stretchable and ultra-lightweight F-PSC with the inverted structure was successfully fabricated, implying that the F-PSC could be applied to power aviation models.7 The photovoltaic performance of ultrathin F-PSC was displayed as a function of compression condition (Figure 1D). Recently, there have been many review papers introducing recent progress in F-PSCs.11–16 In contrast with the previously published review papers, this review paper pays attention to the key scientific challenges underlying the inherently low PCEs in F-PSCs compared to PSCs employing rigid substrates. So far, championcell efficiency of the flexible cell is 19.11%,17 which is apparently lower than that of the rigid cell, 24.2%. Also, the world-best efficiency for rigid perovskite module is 17.1%, which is fairly higher compared to flexible perovskite module,18 11.7% and 15.2% for the area of 703 and 30 cm2, respectively, as displayed in Figure 1E.19–21 Moreover, the F-PSCs have not shown good long-term stability in comparison with rigid cells. The fundamental reason for the inferior performance of F-PSCs is related to the difference in chemical and physical properties of substrate materials that are glass and plastic. In particular, the restrictive process temperature for plastic substrate hinders the fabrication of highly conductive transparent conducting oxide (TCO) and recombination-free charge transport layer, which makes it difficult to achieve high efficiency. Also, the optical transmittance of plastic substrate is lower than that of the glass substrate, which is another reason for inferior photovoltaic performance for F-PSC. The PET is permeable to oxygen and water molecules, and is not able to perfectly block the attack from oxygen and water, unlike glass materials.22,23 In this review paper, we investigate the fundamental challenges of F-PSCs regarding relatively low efficiency and stability compared to glass substrate-based PSCs and demonstrate the recent attempts to overcome big hurdles. Moreover, this paper deals with the efforts that have been made to facilitate the commercialization of F-PSC.
In this review paper, we investigate the fundamental challenges of F-PSCs such as the optical transmittance of flexible substrates and electrical conductivity of flexible transparent conducting oxides, uniform coating technology with a large area on flexible substrates, the high moisture permeability of plastic flexible substrates, and super flexibility. We also introduce recent efforts for overcoming the aforementioned issues as well as for facilitating the commercialization of F-PSCs. As a perspective, we suggest the future direction of research and development of F-PSCs such as the module technology involving assembling multiple subcells and the flexible tandem devices including flexible PSC/CIGS or flexible PSC/organic photovoltaics (OPVs).
1School
of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea
2School
of Chemical Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea
3Department
Fundamental Challenges in Flexible Perovskite Solar Cells Challenge in Substrate Materials for the Flexible Solar Cells First, a general review of the substrate materials for flexible solar cells is provided.24 Flexible substrates are among the most important components required to realize
of Chemical Engineering, Hanyang University, Seoul 04763, Republic of Korea *Correspondence: [email protected] (H.S.J.), [email protected] (N.-G.P.), [email protected] (M.J.K.) https://doi.org/10.1016/j.joule.2019.07.023
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Figure 1. The Superiority of F-PSCs and Progress of PCE in F-PSCs/Module (A) Comparison of power-per-weight for the various solar cells. Reprinted with permission from Kaltenbrunner et al. 7 Copyright 2015 Springer Nature. (B) Shape recoverable F-PSCs after random crumpling followed by thermal treatment. (C) Photovoltaic performance before and after crumpling test. Reprinted with permission from Park et al. 9 Copyright 2015 John Wiley and Sons. (D) Stretchable F-PSCs and their current-voltage characteristics as a function of compression. Reprinted with permission from Kaltenbrunner et al. 7 Copyright 2015 Springer Nature. (E) Progress of PCEs in rigid-type cells and F-PSCs for unit cell and module.2,17–19,21
high-performance flexible electronics. Many performances strongly depend on substrate characteristics. Here, several requirements for the flexible substrates of photovoltaic devices are listed, as follows. (1) Optical properties: because solar cells are light-absorbing devices, the substrate should optically transmit as much light as possible, especially more than 90% transmission in the visible-light region. (2) Barrier properties: most electronic devices are vulnerable to oxygen or moisture, which leads to significant degradation of performance. The substrate should act as a barrier layer to avoid the penetration of oxygen and water vapor for long-term stable performance. (3) Higher conductivity: charge-collecting layers, such as TCOs, are usually deposited on the substrate to function as transparent conductive electrodes (TCEs) consisting of the TCO and substrate. The sheet resistance of TCEs is closely related to the photovoltaic performance, especially the fill factor and photocurrent density of solar cells. (4) Chemical properties: the substrates are exposed to a wide range of process chemicals such as gas and solvents during fabrication, and hence must be stable to these chemicals. PET and polyethylene naphthalate (PEN) films, which are widely used polymeric substrates, show good resistance to various solvents. (5) Mechanical properties: flexible substrates should comply with deformable transformation under severe stress and strain and effectively release stress without
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losing their original functions. In addition, dimensional stability, that is, the ability of materials to maintain its original dimensions while being used or processed, should be ensured. (6) Thermal and thermomechanical properties: two parameters, i.e., the thermal transition temperature and the coefficient of thermal expansion (CTE), have to be considered. The softening temperature of substrate materials, especially the so-called glass transition temperature (Tg) for the polymer, must be compatible with the maximum processing temperature (Tmax) that films can be subjected to. The substrates would be expanded or contracted during temperature change. Significant mismatch of thermal properties between the neighboring layers would exert additional stress on the lattice, resulting in the creation of strain and cracking during heating and cooling processes. The limit of tolerable thermal mismatch is expressed by jDCTE,DTj %0:1% 0:3%, where DCTE and DT are the difference in CTE between the base substrate and the neighboring layers that are subsequently deposited, and the temperature excursion during processing, respectively. (7) Surface roughness: rough surface of the substrate could not assure the conformal coating of layers and avoidance of electrical short circuiting in the device. When the roughness is very large, additional planarization processes are necessary like polishing and coating for the steel-based substrates and polymer, respectively, which would increase the processing cost. Three types of materials, i.e., ultrathin flexible glass, metal foils, and polymers, are available for the substrate of flexible photovoltaics. Glass substrates with a thickness of several hundred mm become flexible. The flexible glass can meet most of the standards for the requirements of the flexible substrate, described above. The strong temperature tolerance of up to over 600 C, good dimensional stability, low CTE of 106/ C, and perfect gas barrier properties are very attractive. However, fragility, heavy weight, and high cost would be big concerns, as compared to other candidate materials. By contrast, thin metal foils such as stainless steel and Ti retain very good mechanical properties and resistance to process chemicals; however, the impermeability of light would limit its application to the substrate of flexible solar cells. Polymer substrate would be ideal in terms of flexibility, light weight, low cost, and R2R processability. However, there are many drawbacks such as low heat resistance, high CTE, and poor gas barrier properties for oxygen and moisture. In particular, because of the low Tg and high CTE of the polymer substrate, shrinkage or expansion of polymer substrate would occur with temperature change. Therefore, the whole fabrication processes should be completely redesigned and compared to the processes capable of high-temperature heat treatments. In addition, according to the tolerance thermal mismatch rule explained above, even a small mismatch would cause cracked or strained TCO or device. Therefore, the process windows for a polymer-based solar device become very narrow. In addition, regarding the barrier properties of polymer substrates, most of the polymer substrates do not satisfactorily meet the demanding requirements for oxygen and moisture protection. As a result, the extra encapsulations are highly necessary for the long-term stable operation of F-PSCs. Additionally, it is desirable that the functions that are antireflection (AR) and ultraviolet (UV) proof (AR+UV-proof) are added to the encapsulating layer for further light absorption and long-term stability, respectively. Optical Transmittance of Flexible Substrates and Electrical Conductivity of Flexible Transparent Conducting Oxides In this regard, this discussion focuses on polymer-substrate-based F-PSCs, because polymer substrates would be ideal platforms for the realization of all flexible electronics. A number of polymers could be applied to materials for flexible substrates. Biaxially oriented PET and PEN films, using heat-setting for the crystallization and
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Table 1. Property Parameters for Polymer Substrate Substrate
PEN
PET
PI
PC
Tg ( C)
120–155
70–110
155–270
145
Tm ( C)
269
115–258
250–452
115–160
Density (g/cm3)
1.36
1.39
1.35–1.43
3
2–4.1 3 10
3
2.5 3 10
3
1.20–1.22 2.0–2.6 3 103
Modulus (MPa)
0.1–0.5 3 10
Work temp. ( C)
–
50–150
<400
40–130
CTE (ppm/ C)
20
15–33
8–20
75
Water absorption (%)
0.3–0.4
0.4–0.6
1.3–3.0
0.16–0.35
Solvent resistance
good
good
good
poor
Dimensional stability
good
good
fair
fair
Tm: melting temperature
stabilization of the films, are widely used as conventional and commercially available polymer substrates, despite several drawbacks, such as low heat resistance and dimensional instability during processing. Polycarbonate (PC) and polyethersulfone (PES) are optically more transparent and have a relatively high Tg compared with PET and PEN, but they lack process chemical resistance and have poor gas barrier properties. Polyimide (PI) exhibits excellent thermal properties but is not optically clear, which is critical for a substrate material of light-absorbing devices, although colorless PI has been recently developed. Table 1 compares the properties of flexible polymer substrates.25 For use as TCEs, TCOs are subsequently deposited on the flexible substrates. As base materials of TCOs, indium oxide (In2O3), tin oxide (SnO2), and zinc oxide (ZnO) are typically used. By doping specific metal ions to those, the transparency and conductivity noticeably increase because of the extrinsic doping effects. Among the TCOs, indium tin oxide or tin-doped indium oxide (ITO) and fluorine-doped tin oxide (FTO) are commercially and widely used in optoelectronic devices. The deposition process of TCO on polymer substrates such as PET and PEN is limited because of thermal sensitivities of polymers. The flexible TCEs with higher optical transmittance and conductivity are basic essentials for the high performance of F-PSC. However, typically, the overall performance of plastic-based F-PSC would be inferior to that of rigid glass-based PSCs due to the lower optical transmittance and lower electrical conductivity of polymer-based TCE. The optical transmittance of substrates is very critical for PCE because photoactive materials should absorb as much light as possible. Polymeric materials inherently absorb ultraviolet and even visible light to some extent, due to the electronic transition of the chromophore in polymers, leading to the light-harvesting loss for the photoactive materials. Zhao et al.26 demonstrate that the flexible ITO/PET substrates show relatively lower transmittance in the visible range, compared to ITO/ glass, due to the lower transparency of the polymer (Figure 2A). The optical properties of F-PSCs are also affected by those of TCOs that are generally transparent in visible and infrared regions due to the large bandgap (Eg) of over 3.0 eV. The transmission window defined by the two imposed boundaries (lg and lp) is also observed. The lg at the short wavelength of UV regions is related to photon absorption with energy larger than Eg. The second transmission edge (lp) is referred to as the plasma wavelength, where the frequency of the light is the same as the
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Figure 2. The Optical and Electrical Properties of TCOs (A) Comparison of the optical transmittance of ITO (100 nm), PET (125 um), and ITO (130 nm) and glass. Reprinted with permission from Zhao et al. 26 Copyright 2014 Royal Society of Chemistry. (B) The wavelength dependence of optical transmittance of TCO. 27 (C) The temperature dependence of sheet resistance of TCE. Reprinted with permission from Zardetto et al. 29 Copyright 2011 John Wiley and Sons.
frequency of the collective oscillation of electrons in materials. The lp is generally detected at the longer wavelength around the near-infrared region. For wavelengths larger than lp, the transmittance steeply decreases as the reflectance caused by the plasma resonance of the electrons becomes dominant.27 The two absorption edges (lg and lp) are shifted to a shorter wavelength as the carrier concentration of TCOs increases, as shown in Figure 2B. Since the carrier concentration affects both the electrical and the optical properties of TCOs, optimization of carrier concentration is necessary. Ideally, TCO should have large transmission windows with higher electric conductivity. The ITO is the most widely used TCO for polymer substrates, due to the better transmittance and lower sheet resistance than others. It is generally known that the electrical properties of ITO strongly depend on the film composition and deposition parameters such as sputtering power, oxygen pressure, film thickness, and substrate deposition temperature.28 Zardetto et al.29 reported that the sheet resistance of PET/ITO increases at over 180 C and the sheet resistance of PEN/ITO increases at 235 C. However, this would not be an issue for the polymer substrates in which a low-temperature process is essential. On the other hand, the conductivity of plastic-based TCE employing ITO is typically lower than that of a glass-based one, due to the relatively lower carrier concentration of ITO films on polymers. The microstructure of ITO without high-temperature thermal treatments is generally amorphous or partially crystalline, leading to poor electrical properties. For the fabrication of ITO-based glass, ITO is dc-magnetron sputtered at glass substrates while the substrate is maintained at approximately 300 C–400 C, or sputtered on the cold substrate, followed by annealing at 200 C. Such a heat treatment would result in a high degree of the crystalline or partially crystalline microstructure of ITO. Meanwhile, the conduction electrons are created because oxygen deficiencies could donate a maximum of two unbound electrons by removing oxygen from the cluster, which is beneficial for better electrical conductivity. Besides, dopant S4+ would replace In3+ by extrinsic doping during sputtering, further producing one more electron in the conduction band of In2O3x. Both the oxygen vacancies and tin dopants in substitutional lattice positions would simultaneously contribute to electrical conductivity. On the other hand, such contributions as doping and crystal effects would not be expected in the ITO deposited at room-temperature processing limited by the polymer substrate. Bellingham et al.30 reported that the higher
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resistivity of the amorphous or less crystalline ITO films would be a consequence of the inability to activate the impurity dopant, i.e., limited diffusion of Sn4+ from interstitial locations and grain boundaries into an In3+ site, caused by the low deposition temperature. In this case, the conductivity solely depends on oxygen vacancies. Furthermore, the electron transports in the amorphous or less crystalline ITO films would not be as effective as those in the ordered crystalline structure, due to the relatively large amount of defects and charge traps. Figure 2C exhibits the sheet resistance of PET/ITO and glass/ITO. The sheet resistances of PEN/ITO and PET/ITO significantly increase after thermal treatment at 200 C and 240 C, respectively.29 As described previously, the morphological, electrical, and optical properties of ITO films change depending on the deposition condition and types of substrates. However, except for the ITO crystalline quality and preferred orientation, no significant difference in chemical composition or chemistry according to the process temperature was reported.31–36 Lee investigated the relationship between microstructure of ITO and electron scattering mechanisms at the various process conditions.33 For the less crystalline ITO, the grain boundary scattering was dominant while ITO with a high crystallinity was governed by the ionized-impurity scattering mechanism due to the higher carrier concentration. Deposition of thicker TCO on plastic substrates is another strategy to compensate for the low conductivity. However, because the absorption of TCO films rises strongly with increasing thickness, there are some limitations to meet both the desired properties together. Recently, F-PSCs have been fabricated using various transparent electrodes, such as metal-based materials (nanowires, grids, and networks), conducting polymers, carbon-based materials (carbon nanotube and graphene), and oxide–metal–oxide multilayers.37,38 Table 2 summarizes the transmittance and sheet resistance of various flexible PCEs for the F-PSCs reported. Challenges in Uniform Coating Technology with a Large Area on Flexible Substrates With the dramatic boosting of the certified efficiency of up to 24% for rigid glassbased PSCs,2 there has been much interest and effort for the enlargement of the device area and upscaling. A few reviews of research on the processing and upscaling of PSC modules have recently been reported.48–52 High-quality perovskite films with a small area (25 cm2) can be fabricated using a simple solution method, such as spin coating. However, despite a few reports on the fabrication of large-area PSCs using spin coating, it is generally accepted that there are limitations to the extension of the process to large-area modules (25 cm2 or larger), because it is difficult to control the morphology and kinetics of perovskite crystals over a large area without pinholes. However, in terms of cost and productivity, flexible solar devices are more promising than rigid glass ones. For the commercialization of F-PSCs, it is important to develop a process to make mass production with an R2R process possible. The printing and coating methods can be divided into contact and noncontact methods depending on whether the coating materials are in direct contact with the coating plate (Figure 3). Screen printing, flexography, gravure printing, and soft lithography belong to the contact methods that have been widely used recently in printed electronics. Recently, Cruz et al.25 reported the relationship between process parameters and printing quality for the fabrication of printed electronics on polymer substrates.
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Table 2. Transmittance and Sheet Resistance of Various Flexible Substrates for the F-PSCs Substrates
Electrodes
Transmittance (%)
Sheet Resistance (U/cm2)
Structure of PSCs
PCE (%)
Ref.
PET
AZO/Ag/AZO
81
7.5
PET/AZO/Ag/AZO/PEDOT:PSS/PolyTPD/MAPbI3/PCBM/Au
7
10
Ti Foil
Ag(8 nm)
–
–
Ti foil/TiO2 BL/MP-TiO2/ MAPbI3/Spiro/Ag
0.77
39
Ti Foil
Ag(12 nm)
–
–
Ti foil/TiO2 BL/MP-TiO2/ MAPbI3/Spiro/Ag
6.15
39
Ti Foil
Ag(16 nm)
–
–
Ti foil/TiO2 BL/MP-TiO2/ MAPbI3/Spiro/Ag
3.85
39
Ti Foil
Ag(20 nm)
–
–
Ti foil/TiO2 BL/MP-TiO2/ MAPbI3/Spiro/Ag
2.99
39
Ti Foil
CNTs
60–80
–
Ti foil/TiO2 NTs+MAPbI3/CNTs+Spiro
8.31
40
Ti
Ag(1 nm) +ITO
>80
107.14
Ti/TiO2 BL/TiO2+MAPbI3/ Spiro/Ag+ITO
11.01
41
PET
PEDOT:PSS+EG
>60
28
PET/PEDOT:PSS(PH1000)+EG/PEDOT:PSS(VPAI 4083)/ MAPbI3/PCBM/TiOx/Al
4.9
42
Willow glass
ITO
81.5
31.3
MgF2/Willow glass/ITO/SnO2/ FAMACs/Spiro/MoOx/Al
17.3
43
Willow glass
AZO
85.8
17.8
MgF2/Willow glass/ITO/SnO2/ FAMACs/Spiro/MoOx/Al
12.2
43
Willow glass
IZO
76.4
11.6
MgF2/Willow glass/ITO/SnO2/ FAMACs/Spiro/MoOx/Al
18.1
43
PET
AuCl3-GR
>80
75
Al/PCBM/MAPbI3/PEDOT:PSS/AuCl3-GR/APTES/PET
17.9
37
PES
AZO/AgNW/ AZO
88.6 at 550 nm
11.86
Au/Spiro/MAPbI3/ZnO/AZO/AgNW/AZO
11.23
44
PEN
graphene
~97
552.0
PEN/Graphene/MoO3/PEDOT:PSS/MAPbI3/C60/BCP/LiF/Al
16.8
45
PET
ITO
70
15
PET/ITO/PEDOT:PSS(VPAI 4083)/MAPbI3/PCBM/TiOx/Al
4.3
42
Glass
ITO
80
8
Glass/ITO/PEDOT:PSS(VPAI 4083)/MAPbI3/PCBM/TiOx/Al
9
42
Glass
ITO
–
–
Glass/ITO/PEDOT:PSS/MAPbI3/PCBM/Al
11.5
46
PET
ITO
–
–
PET/ITO/PEDOT:PSS/MAPbI3/PCBM/Al
9.2
46
PET
PEDOT:PSS
–
–
PET/PEDOT:PSS/ CH3NH3PbI3-xClx/PTCDI/ Cr2O3/Cr/Au
12
47
Cellulose paper
MoOx/Au/MoOx
62.5 (top electrode)
9
paper/Au/SnO2/meso-TiO 2/CH3NH3PbI3/Spiro-OMeTAD/ MoOx/Au/MoOx
2.7
38
They proposed that the coating quality on the substrates mainly depends on the surface wettability, spreadability, and adhesion. In particular, when polymer substrates are used, the low surface energy of polymer substrates gives rise to poor coating or printing quality, requiring additional surface treatment before processing. Such additional processes generally increase the cycle time and production costs. The coating quality generally depends on the surface tension of the bottom layer. Therefore, a deep understanding of the relationship between process parameters and printing quality is required for the fabrication of high-performance F-PSCs with large areas. Conformal coating or printing without pinholes on floppy substrates is crucial to the performance of F-PSCs. In particular, the control of the crystallization kinetics of the perovskite layer should be uniformly controlled over a large area during solvent evaporation. Moreover, the profiles of plastic substrates as supporting layers generally remain intact, even after multiple coatings of upper layers. Therefore, the planarization for uniform coating is important. In addition, accumulated internal stress or strain is generated when the mechanical and thermal shocks are applied to the films, leading to delamination or debonding. This is more severe for large-area devices. High Moisture Permeability of Plastic Flexible Substrates Although the light absorption ability of organic-inorganic hybrid perovskite is excellent, the ionic nature of perovskite crystals makes them inherently vulnerable to moisture as well as oxygen, resulting in the degradation and phase transition of the perovskite. To avoid such deterioration, the substrate requires complete
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Figure 3. Schematic Illustration of Contact (Upper Three Figures) and Noncontact (Lower Three Figures) Methods for Perovskite Layer Coating on a Flexible Substrate
protection from the intrusion of moisture. By contrast, glass substrates can block the penetration of moisture through the substrates. However, F-PSCs with polymer substrates have severe stability problems because of the much higher water vapor transmission rate (WVTR) or oxygen transmission rate (OTR) of polymeric substrates. The WVTR and OTR of polymer materials are generally approximately 1–10 g/(m 2day) and about 1–10 mL/(m2day), requiring additional lamination or encapsulation to achieve those values in the range of 106. The reasons for the poor barrier stability and higher gas permeability are related to the morphology of polymers in the films. The microstructure of polymers can be divided into a crystalline part and an amorphous structure, depending on whether the arrangements of the polymers are regular or irregular. PET and PEN, which are widely used as substrates, are semi-crystalline polymers in which crystalline and amorphous are mixed, thus easing penetration of gas or moisture molecules. However, inorganic materials, such as glass SiO2, are regularly arranged in an atomic arrangement. As a result, such a dense structure of inorganic materials effectively suppresses the penetration of the molecules, such as moisture or oxygen. The factors affecting the moisture permeability are (1) film thickness, (2) film density, and (3) crystallinity.53 Gas molecules diffuse through the nanoscale or micron-scale pores, called ‘‘free volume,’’ in the polymers. There are three stages of penetration of gas molecules: (1) polymer adsorption of penetrating molecules, (2) diffusion due to the difference in concentration of the molecules inside and outside the polymer, and (3) desorption from the polymer film. These three series of steps are continuous, especially water molecules penetrating the interior. The thermal motion of polymer segments facilitates this penetration. Considering these sequential steps, the thicker the polymer substrate, the longer the diffusion distance of the molecules becomes, resulting in lower moisture permeability. However, because the gas transmittance decreases as the thickness increases, it is important to select an appropriate thickness of polymer substrate because of the trade-off relationship with optical transmittance. Additionally, as the density of the film increases, the size of the free volume pore becomes smaller, so diffusion becomes difficult, and the moisture permeability becomes lower. Finally, the free volume occupies the amorphous region rather than the crystalline portion, so the moisture permeability decreases as the crystalline part
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increases. Naturally, as the amorphous area increases, the free volume and moisture permeability increase. To relieve concerns about the stability issue, flexible film packaging employing a gas-barrier encapsulating function should be developed for F-PSCs. The encapsulating package includes flexible front/back sheets, flexible encapsulant, edge seal, and electrical joints. A typical configuration of flexible packaging for F-PSCs is a transparent front sheet/encapsulant/F-PSC/encapsulant/back sheet.54 In addition to the gas-barrier property, the materials embedded in barrier films should exhibit a relatively higher dielectric constant, optical transparency, thermal and UV stabilities, good adhesion, chemical inertness, processability, and mechanical strength with scratch resistance. Organic encapsulating materials are a good candidate to meet most of the requirements above, except gas barrier properties. Recently, organic-inorganic multilayer films have been commonly used to obtain better barrier properties.55 For further performance development of barrier film, the encapsulation and packaging technology developed in organic light-emitting diodes (OLEDs) that requires a WVTR of 106 g/(m2day) would provide deep insight, because OLEDs require a higher WVTR of 106 g/(m2day) than solar cells. Challenges in Super Flexibility The excellent mechanical properties of organic-inorganic hybrid perovskite materials make the possibility of flexible solar cells promising. Ko et al.9 reported the mechanical properties of all layers of organic-inorganic hybrid F-PSCs using nanoindentation and theoretical analyses. The Young’s modulus (E) of the hybrid perovskite layer was noticeably lower (approximately one-tenth) than that of metal-oxide-based perovskite, thus leading to the higher flexibility of the device. The finite-element analysis demonstrated that significantly small plastic strain was generated over a small region within the F-PSCs under bending. In terms of microstructure, such exceptional flexibility is thought to originate from the weak van der Waals bonding caused by extended alkyl chains (R) in R-NH3+ between perovskite layers, as well as a hydrogen/ionic bonding of NH3+ within the perovskite sheets. On the other hand, Jing Feng reported the elastic and anisotropic properties of CH3NH3BX3 (B = Sn, PB; X = Br, I),56 suggesting that the elastic properties are not determined by the interaction among organic and inorganic ions, but by the type and strength of the metal–halogen (B–X) bond. The ratio of the bulk modulus (B) to the shear modulus (G), or B/G would be a good index for the evaluation of the ductile ability of materials. The hybrid perovskite materials have a relatively lower shear modulus, implying a larger B/G with high ductility, which would be desirable as the absorbing layer of flexible solar cells. In addition, the Poisson ratio (n) is another parameter to determine the softness of materials. Taking into the consideration that n values for hybrid organic-inorganic perovskites and ductile solids are greater than 0.3 and 0.26, respectively, it is conjectured that the hybrid perovskite would be adequately soft as a photon absorber on the polymer substrates. Further studies on the molecular origin of super-flexibility of perovskite materials are necessary. For the evaluation of flexibility, bending tests are widely used for flexible electronics. During bending, the outer and inner surfaces experience tensile and compression stress, respectively. The plane inside the device, where no uniaxial stress is applied by balancing the tensile and compression stress, is defined as a neutral plane. If the neutral plane is located at the most fragile layer, the mechanical durability is sustained. On the other hand, the relationship between the film strain and the radius of bending curvature is expressed by e = d/2r, where e, d, and r are the strain, device
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thickness, and radius of curvature (bending radius), respectively. According to this relation, the thinner devices could ensure the bending stability, because a relatively smaller strain would be applied to the devices. In this regard, the hybrid perovskites would be ideal photo-absorbing materials because of the high absorption coefficient and intrinsic soft character, enabling thinner devices. However, ITOs coated on polymer substrates do not have high bending durability compared with perovskite because of the ITO’s small Poisson ratio and large modulus. Thus, during repeated bending tests, cracks occur in the ITO layer, inducing degradations in the perovskite layer. Therefore, for the realization of super F-PSCs, strategies to utilize new TCO materials with a ductile nature should be introduced. Recent Efforts on Improving Performance of Flexible Perovskite Solar Cells Because of the restriction on process temperature for fabricating transparent conducting oxide, ETL and HTL, and perovskite absorber, the PCE of flexible cells is not that much higher than those of glass-based rigid cells because of high series resistance and facilitated recombination of charges that are inherent issues for F-PSCs. Also, the employment of brittle TCOs like ITO and aluminum-doped ZnO (AZO) may not enable enough flexibility to PSCs. In this section, recent efforts on improving flexibility as well as enhancing charge collection for high efficiency and flexibility are discussed. The Exploitation of New Flexible TCO Substrates ITO or FTO has been abundantly utilized in both rigid and F-PSCs. However, from the viewpoint of cost-effectiveness, photovoltaic performance, and flexibility, these TCO materials may not be the best way to achieve a goal. Efforts have been made to replace these TCO materials with new compositional TCO materials. Kim et al.57 exploited new TCO materials that are composed of W-doped ITO (IWO) for F-PSC. These IWO materials, prepared by plasma arc ion plating method, were analyzed to possess high mobility (61.7 cm2/Vs) and high transmittance of 96% at 550 nm. Also, work function for IWO was 4.85 eV, much higher than that for ITO (4.65 eV). This high work function may enhance Voc. To overcome the brittleness of TCO, AZO/Ag/AZO, that is, double-sandwich structure, was provided. This new type of TCO showed fairly low sheet resistance of 7.5 U/sq and the resultant flexible cell maintained 98.5% of initial PCE after 50 cycles of bending test at 2.75 cm radius of curvature.10 So far, comparative studies of photovoltaic performance of PSCs, depending on various TCO materials, have not been reported, which is of great importance to decide the right TCO materials for F-PSCs. Dou et al.43 performed a comparative study regarding the influence of TCO materials on the photovoltaic performance of PSCs (Figure 4). They demonstrated that the F-PSCs employing the In-doped ZnO (IZO) achieved the highest efficiency of 18.1% due to the high Voc and FF. The other TCO materials, such as ITO and AZO, showed relatively inferior PSC. Especially, the AZO materials showed the lowest PCE of 12.2% due to the chemical reaction between AZO and perovskite layer, verified by analyzing surface stoichiometry and change in photoluminescence spectra of perovskite materials.43 Although ITO films have the same composition, the photovoltaic performance of F-PSCs may be different depending on the fabrication method of ITO. Kim et al.58 compared the cell performance containing ITO films prepared by (1) ion-plated process and (2) sputtering process. The ion-planted ITO film showed better electrical and optical properties such as sheet resistance and optical transmittance in comparison with the ITO films prepared by a sputtering process. The resultant PCE of PSC based on ion-planted ITO was measured to be 15.4%, which implies that the
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Figure 4. The Influence of TCO Materials on the Photovoltaic Performance of F-PSCs (A) Schematic representation of device architecture for the F-PSCs, where different transparent conducting oxide (TCO), AZO (aluminum-doped zinc oxide), ITO (tin-doped indium oxide), and amorphous IZO (indium-zinc oxide) were investigated. X-ray diffraction (XRD) patterns and transmittance of AZO, ITO, and IZO are shown on the left. Current density-voltage (J–V) characteristics (reverse scans) under 100 mW/cm 2 AM1.5G for champion Cs0.04 MA 0.16 FA 0.80 Pb 1.04 I 2.6 Br 0.48 perovskite devices with (B) willow glass-AZO, (C) willow glass-ITO, and (D) willow glass-IZO. Reprinted with permission from Dou et al. 43 Copyright 2017 American Chemical Society.
photovoltaic performance of F-PSC may change according to the electrical and optical quality of TCO, which may be determined by preparation method.58 Metal foils have been one of the promising candidates as a flexible electrode owing to high flexibility, high electrical conductivity, and a wide process temperature window. In particular, Ti metal foils have been abundantly utilized as a flexible substrate due to easy formation of TiO2 ETL. An attempt to replace Ti with cheap Cu foil has been made.59 Cu foil is capable of simply forming CuI HTL. Although many efforts have been made, it is challenging to achieve a high efficiency due to the inherent optical reflectivity of metal, however. Many studies have focused on the exploitation of top electrodes that possess a high conductivity and high optical transmittance. Ag metal film has been well utilized as a translucent top electrode. Since the translucent Ag metal itself does not show high electrical conductivity as well as high optical transmittance, the modification of Ag film has been proposed.39,41,60 For example, Ag nanowire electrodes showed the improved PCE of 7.45% compared to the PCE of bare Ag electrode cells (6.15%). The F-PSCs employing 13 nm thick Ag layer coated with amorphous ITO (a-ITO) exhibited 11.01%. Another Ni mesh metal grid was utilized to replace Ag materials.61 In this study, a transparent conductive adhesive (TCA) was adopted to reduce interfacial resistance between the metal grid and
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Figure 5. Metal-Polymer Hybrid Transparent Flexible Electrode (A) Image of the large-area flexible PET substrate with embedded Ag-mesh (FEAM) substrate. (B) Diagram for the hybrid electrode (PET–Ag-mesh–PH1000). (C) The structure of the FEAMs substrate with detail parameters. (D) J–V curves in reverse and forward scan measured under 100 mW/cm2 AM 1.5G illumination and dark for the champion flexible PET–Ag-mesh–PH1000–PEDOT:PSS–MAPbI 3 –PCBM–Al solar cell: inset shows photograph of corresponding ultra-thin F-PSCs. Reprinted with permission from Li et al. 67 Copyright 2016 Springer Nature.
hole-conducting layer, which yielded 10.3% of PCE. Carbon-based nanomaterials such as carbon nanotube (CNT) and graphene are other good candidates for transparent top electrode materials in Ti foil-based F-PSCs.40,62 Especially, the anodized Ti-metal-based PSC employing a graphene top electrode exhibited a PCE of 15%.62 Instead of metal-foil substrate, flexible CNT or graphene materials that are directly deposited on plastic substrates may be one of the alternatives to brittle TCO materials.63 Liu et al.64 adopted a graphene layer instead of TCO and achieved a super-light flexible solar cell with a power density of 5.07 W/g. Efficiency of graphenebased F-PSCs has been significantly improved by combining with PEDOT:PSS layer and incorporating dopants such as AuCl3 and MoO3 into graphene.37,45 The F-PSC containing AuCl3-doped graphene layer with PEDOT:PSS showed a high PCE of 17.4%.37 CNTs have been also utilized as flexible transparent electrode materials. Although the photovoltaic performance of CNT-based cells is inferior to that of graphene-based cells, the superior mechanical stability and large-scale fabrication possibility make it appropriate for commercialization of F-PSC.65 Silver nanowire (Ag NW) or mesh has been well investigated as transparent electrode materials in F-PSCs owing to high figure of merit, relatively good chemical stability, and bending durability. These materials are usually combined together with PEDOT:PSS to enhance the electrical conductivity of the transparent electrode. The Ag NWs/PEDOT:PSS electrode-based cell exhibited 11% PCE and the efficiency was retained after 10,000 cycles of bending test under 5 mm of a radius of curvature.66 Moreover, to overcome the difference in work function between Ag and PEDOT:PSS, polymer or graphene oxide (GO) interlayers were inserted.67,68 The PCE of Ag-mesh/PH10000/PEDOT:PSS based cell was 14% achieved by Li et al.67 (Figure 5). This cell retained 95.4% of initial PCE after 5,000 bending cycles under radius curvature of 5 mm.
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Advanced Electron Transport and Hole Transport Layer for Flexible Cells In n-i-p structure flexible cell, low-temperature formation of an ETL such as ZnO, TiO2, and SnO2 layer on plastic-based TCO substrate is challenging. To overcome inferior electron-transport properties of low-temperature-treated ETL, interface modification method with C60 or graphene and new composition oxide ETL has been exploited. Also, various carbon-based materials have been used as ETL layers. ZnO compared with TiO2 is relatively suitable for the low-temperature process.69 Also, ZnO possesses better electrical conductivity and carrier lifetime in comparison with TiO2, which led to a high PCE of 15.6% in F-PSCs.69 TiO2 ETLs have been well studied in F-PSCs. There have been mostly two ways to form TiO2 ETL in a flexible plastic substrate. First, low-temperature deposition using vacuum-based thin-film deposition systems such as the evaporator, sputter, and atomic layer deposition (ALD); second, deposition of well-crystallized TiO2 colloidal particles and subsequent treatment such as UV treatment or low-temperature annealing treatment.70–72 The RF-sputtering method for deposition of TiO2 layer has been well known. Usually, amorphous TiO2 layer can be obtained using the low-temperature sputtering method. Yang et al.73 compared carrier transport, recombination, and lifetime characteristics of both amorphous and anatase TiO2 layer. Fermi level of amorphous TiO2 was 4.15 eV, slightly lower than that of anatase TiO2 (4.01 eV). This leads to superior electron-transport property of an amorphous layer, achieving a PCE of 15.07% for F-PSC.73 The PEALD method is favorable for precise control of thickness and reducing oxygen vacancies in TiO2 ETL. Kim et al.6 realized F-PSC with a PCE of 12.2% by fabricating TiO2 ETL with the aid of low-temperature PEALD process, as shown in Figure 6. The charge transport properties of TiO2 ETL prepared by PEALD are enhanced by interface modification with PCBM, which yields 17.7% PCE.74 Moreover, Giacomo et al.72 were able to fabricate F-PSC modules by combining TiO2 ETL prepared by PEALD and mesoscopic TiO2 layer. TiO2 layer can be deposited by electron-beam evaporation and the resultant cell efficiency was reported to be 13.5%.75 Highly crystalline anatase nanoparticles may be directly deposited as ETL. However, the ligand or residual organics attached on the surface of TiO2 would be detrimental to electron transport in the ETL. Therefore, further low-temperature treatment or UV treatment is needed to achieve a high PCE. So far, UV treatment is more efficient in removing organics and attaching each TiO2 particles than low-temperature treatment (150 C).70,71 The flexible cell employing the low-temperature UV-treated Nb:TiO2 ETL exhibited 16.01% PCE due to the enhanced electron transport. Since the stability and hysteresis issues of TiO2 materials, SnO2 has been conceived as another alternative to ETL materials due to its enhanced charge transport property.76 Usually, SnO2 layer has been prepared by a solution coating method that contains precursor solution coating and subsequent heat treatment. However, this heat-treatment process is not suitable for obtaining SnO2 with good crystal quality in plastic-based flexible cells due to limited heat-treatment temperature. Plasma treatment may be a good method to overcome the temperature issue. N2 plasma treatment broke alkoxy and hydroxy group to form Sn-O bonding at a low temperature, as shown in Figure 7. The resultant PCE of F-PSC was 18.1%.77 The employment of highly crystalline SnO2 colloids is another way to achieve high efficiency. Park et al.78 synthesized highly crystalline and dispersed SnO2 colloids. This SnO2
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Figure 6. Amorphous TiO2 ETL for F-PSCs Using PEALD at the Low-Temperature Process (A) The structure and best performing device of F-PSCs on ITO–PEN, with a TiO 2 ETL by using PEALD. Cross-sectional SEM image of the inorganic-organic halide perovskite planar heterojunction flexible solar cell and schematic of the flexible device structure. J–V curve measured under the simulated solar light for the best performing PEN–ITO–TiO x –CH3 NH3 PbI 3-x )Clx –spiroMeOTAD–Ag flexible device. Reprinted with permission from Kim et al. 6 Copyright 2015 Royal Society of Chemistry. (B) F-PSCs with hybrid double ETL (PCBM and TiO2 prepared by PEALD). A cross-sectional TEM image of the as-prepared a-TiO2 –PCBM ETL.-based PSCs. A schematic representation and J–V curve of an a-TiO 2 –PCBM ETL-based PSC on the ITO-PEN substrate. Reprinted with permission from Kim et al. 74 Copyright 2018 John Wiley and Sons.
colloid ETL exhibited excellent charge-transport property, which enabled a PCE of 17.7%. PEALD-synthesized ETL shows also the excellent photovoltaic performance with a PCE of 18.36%.21 In addition, the low-temperature-processed EDTA (ethylene diamine tetraacetic acid)-complexed SnO2-based F-PSC achieved a PCE over 18.4%.79 To enhance the charge-transport property of SnO2 ETL, doping metal ions or mixing graphene oxide have been attempted.80,81 New composition ETLs have been exploited to enhance charge transport as well as to reduce the hysteresis phenomenon in F-PSC. Zn2SnO4 (ZSO) is a good example, possessing better optical transmittance than SnO2.82 The PCE of ZSO-based F-PSCs has been improved by tailoring particle size and stacking structure, which leads to yielding 16.5% PCE.83 Also, the combination of PCBM with ZSO was reported to facilitate electron transport in F-PSC.84 Nb-doped WOx (W(Nb)Ox) was used as ETL. By controlling the thickness of W(Nb)Ox, PCE and hysteresis properties were improved.85 Recently, fairly interesting ETL material that is nanocrystalline titanium metal-organic frameworks (nTi-MOF) was proposed. The addition of PCBM increased the electrical conductivity and enhanced the charge injection properties of nTi-MOF. The flexible cell employing nTi-MOF/PCBM demonstrated a PCE of 17.43%.86 Large bandgap materials such as Nb2O5 and Al2O3 were utilized as ETLs. The electron transport from the perovskite layer to TCO was enabled by quantum tunneling effect.87,88 However, the inherent high series resistance is still challenging to achieve high efficiency. As an alternative, electrically conductive organic materials, including the modified C60 and PCBM, have been utilized as ETLs. Stearyl dimethyl benzyl ammonium chloride (SDBAC)-doped [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) was used as ETL wherein SDBAC enhanced the conductivity and therefore
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Figure 7. New Composition ETLs (A) Schematic representation of the SnO 2 thin-film fabrication methodology using a modified sol–gel technique by employing low-power RF N 2 plasma exposure. Cross sectional SEM image of PET–ITO–SnO 2 –m-Al2 O 3 – ((FA 0.83 MA 0.17 ) 0 . 95 Cs 0 . 05 PbI 2 . 5 Br 0 . 5 ) –spiro-OMeTAD–Au flexible device and architecture schematic of F-PSC. Reprinted with permission from Subbiah et al. 77 Copyright 2018 American Chemical Society. (B) A uniform SnO 2 QD thin film by using ligand-capped ultrafine SnO 2 quantum dots. Reprinted with permission from Park et al. 78 Copyright 2018 American Chemical Society. (C) Schematic of the synthetic protocol for nTi-MOF. The successive linked TiO2 clusters with BDC molecules create small nanocrystals. Device structure of a F-PSC incorporating nTi-MOF/PCBM. The best performing J-V curve of the flexible nTi-MOF–PCBM PSCs. Reprinted with permission from Ryu et al. 86 Copyright 2018 American Chemical Society.
enhanced the PCE to 11.8%. The polyallylamine-coupled C60 showed an excellent charge extraction and the increased Voc owing to work function shift, thereby realizing a highly efficient F-PSC with a PCE of 15.2%.89 Yang et al.90 used solid-state ionic liquids (ss-IL) for ETL. The large bandgap, anti-reflection, high electron mobility, and proper work function for ss-IL ETL enabled a high efficiency of 16.09%. Moreover, the ss-IL ETL reduced the electron trap-state density of the perovskite layer, leading to effectively suppressed hysteresis. In p-i-n structure, HTL has been actively studied because HTL requires high optical transmittance, bending durability, hole injection, and hole-transport properties in p-i-n F-PSC. In this regard, PEDOT:PSS have been proposed as suitable HTL materials.47 However, the relatively poor charge transport in the out of plane direction due to its lamellar structure, the potential energy loss at the interface between PEDOT:PSS and perovskite layer due to low work function, and corrosion capability to the adjacent layers due to its acidity are critical issues.91 Therefore, new organic HTL materials such as 1,4-bis(4-sulfonatobutoxy)benzene and thiophene moieties (PhNa-1T) and (N-(4-(9H-carbazol-9-yl)phenyl)-7-(4-(bis(4-methoxyphenyl)amino) phenyl)-N-(7-(4-(bis(4-methoxyphenyl)amino)phenyl)-9,9-dioctyl-9H-fluoren-2-yl)9,9-dioctyl-9H-fluoren-2-amine (CzPAF-TPA)) have been exploited.91,92 For example, the F-PSC using PhNa-1T as HTL showed significantly higher 14.7% PCE in comparison
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with the PEDOT:PSS-based one (8.4%), which was ascribed to the decreased potential energy loss at the interface between perovskite and HTL as well as to efficient hole injection.91 Since the organic HTLs exhibit relatively low stability compared with inorganic HTLs, research on developing new HTLs has been shifted to exploit new inorganic HTLs. In particular, NiOx materials have received a great deal of attention due to the well-aligned band structure with perovskite layer, good optical transparency, and electron blocking capability.91 The NiOx HTLs have been deposited by vacuum thin-film fabrication tools such as ALD, RF-sputtering, and evaporation.93–95 However, in F-PSC for potential roll to roll process, low-temperature solution coating has been under active research.96–98 Najafi et al.98 achieved 16.6% PCE in F-PSC, which exhibited fairly good stability. Over 85% of the maximum stabilized output efficiency was retained after 1,000 h aging employing a thin MAPbI3 perovskite.98 Also, NiOx derivatives have been studied to improve photovoltaic performance.99–101 Cu-doped NiOx (Cu:NiOx) HTL-based F-PSC demonstrated 17.16% because of the improved charge injection and the improved surface morphology.100 Noticeably, the hybrid HTL composed of PEDOT:PSS–NiOx made it possible to achieve 18.0% PCE in F-PSC.101 Cu-based HTL materials such as CuyCrzO2 and CuSCN have been introduced.102,103 However, these materials are fairly challenging due to the problem of formation of CuI coming from inter-diffusion of Cu and In ions.102,103 In p-i-n structure, n-type ETL needs efficient electron extraction, full coverage on the perovskite layer, no damage on perovskite layer during the coating process, and no chemical reaction between ETL and perovskite under operating conditions.104 In this regard, fullerene-based ETL such as PCBM has been the most popular in p-i-n structured PSC. However, the wettability of PCBM on the perovskite layer is not better than polymer-based ETL, which may induce the deterioration of long-term stability related to insufficient coverage of PCBM. Therefore, polymeric ETLs, including the naphthalene diimide (NDI)-based polymer with strong electron withdrawing dicyanothiophene (P(NDI2DT-TTCN)) and triphenylamine (TPA)-(3-cyano-4,5,5-trimethyl-2(5H)-furanylidene)malononitrile (3CN), have been developed in F-PSC. Representatively, the flexible cell employing the P(NDI2DT-TTCN) as ETL achieved 17.0% PCE with good long-term stability owing to the improved charge extraction ability. Also, the mechanical durability of this material was better than PCBM.104 High-Quality Perovskite Film for Flexible Cells The formation of high-quality perovskite film with large grain size, low trap density, high coverage, and high crystallinity, etc., is one of the major challenges for high-efficiency F-PSCs on the plastic substrate. In general, perovskite film exhibits poor film quality on the flexible substrate, because of the low surface energy, poor wettability, and wrinkle of the plastic substrate. Therefore, it is necessary to develop a suitable perovskite precursor for the same perovskite film quality on a flexible substrate, compared with high-quality perovskite film on a rigid substrate. Bi et al.105 investigated the various ratios of FAI:MABr concentration in perovskite precursors, the optimized composition of which exhibits the long recombination lifetime, small trap state density, and pin-hole-free film, resulting in 18.1% of PCE in F-PSCs. One of the strategies for high-quality perovskite film is to add the additives in the perovskite precursor. H2O is critical to controlling the crystallization rate and grain size, which achieved the 13.11% PCE (Figure 8A).106 Moreover, the dual additives of DIO and H2O in perovskite precursors can induce gradient evaporation during
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Figure 8. Additive Induced High-Quality Perovskite Film (A) Schematic illustration of the different morphology development for CH 3 NH 3 PbI 3x Cl x films with and without H 2 O additive. Reprinted with permission from Du et al. 106 Copyright 2017 American Chemical Society. (B) Illustration of the F-PSC structure and J–V curves of F-PSCs under both reverse and forward scan directions using MAPbI 3 –DS (dimethyl sulfide) as the absorber layer. Reprinted with permission from Feng et al. 107 Copyright 2018 John Wiley and Sons. (C) Cross-sectional SEM image and J-V curves of a F-PSC with the Rb1 K 4 CsFAMA absorber. Reprinted with permission from Cao et al. 17 Copyright 2019 Royal Society of Chemistry.
the drying and heat process. This leads to high-quality perovskite film with large grain and uniformity, achieving a PCE of 18.0% for F-PSC.101 A dimethyl sulfide (DS) additive has been successfully developed as it chelates with Pb2+ to form an intermediate complex and slows down the crystallization rate of perovskite film. The resultant PCE of F-PSC was 18.4% with good mechanical tolerance (Figure 8B).107 Recently, a quintuple-cation perovskite film containing Rb+, K+, Cs+, FA+, and MA+ was proposed to reduce the trap density in the perovskite film, which a champion PCE for F-PSC of 19.11% was achieved with less hysteresis (Figure 8C).17 Ultra-Flexible Perovskite Solar Cells F-PSCs can be potentially used as a flexible power source for mobile devices as well as a portable outdoor electricity generator. For example, the recent advent of foldable smartphones is expected to open a new era of flexible electronics. In foldable devices, there are two types of folding modes: in-folding and out-folding. In-folding mode requires an extremely small bending radius of curvature like 2 mm, which is fairly challenging in foldable devices composed of inorganic electrical components. Also, many F-PSCs contain inorganic materials including TCO, ETLs, and HTL that are not favorable for reliable flexibility. The F-PSC adopting PEN/Sn-doped indium oxide (ITO) showed 1,000 times bending durability at the radius of curvature of 10 mm. However, at the radius of curvature of 4 mm, the cell performance significantly deteriorated because of the fracture of ITO layer (Figure 9).6 Efforts have been made to improve the bending reliability of F-PSCs by replacing or modifying inorganic TCO and substrate materials. Nanostructured ITO demonstrated the improved flexibility of PSCs. The inverted nanocone ITO showed
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Figure 9. Highly Bending and Durable F-PSCs (A) Normalized PCE measured after bending the substrate within a specified radius of 400 mm to 1 mm and normalized PCE as a function of bending cycles with different radii of 400 nm, 10 nm, and 4 mm. The real images attached on the human neck, wrist, and finger corresponding to 400 nm, 10 nm and 4 nm bending radii, respectively (inset within upper figure). Real images taken during the bending tests (inset within lower figure). Reprinted with permission from Kim et al. 6 Copyright 2015 Royal Society of Chemistry. (B) Device structure of Gr-Mo/PEN based F-PSC and Normalized PCEs as a Function of Bending Cycles at a Fixed Bending Radius of 4 mm for the Gr-Mo–PEN and ITO–PEN devices. The inset photograph shows the Gr-Mo–PEN device bent at a bending radius. Reprinted with permission from Yoon et al. 45 Copyright 2017 Royal Society of Chemistry.
improved bending durability compared with the planar ITO based cell. The F-PSC employing inverted cone retained 95% of the initial PCE after 200 cycles of bending test under a 6 mm radius of curvature, which was ascribed to strain relaxing effect of nanocone structure.108 Metal nanowires possess excellent flexibility, which enables to replace the relatively brittle TCO. However, the roughness and adhesion issues remain. These issues have been resolved by combining an oxide layer or hybrid layer.109,110 The composite of crystalline ITO metal nanowire-GFRHybrimers is one of the representative examples. The flexible cell containing this nanocomposite showed excellent bending durability. Under 2.5 mm radius of curvature, this cell showed 20% degradation to initial PCE after 500 cycles of bending test.110 The PEDOT:PSS system has been frequently utilized for realizing ultra-F-PSCs owing to its flexible nature of the polymer. The F-PSCs employing PEDOT:PSS materials do not possess any cracks from the fracture of PEDOT:PSS in contrast with ITO materials under a severe bending condition such as a 4 mm radius of curvature.111 The F-PSCs
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based on highly conductive nitric acid treated PEDOT:PSS with 50 nm thickness exhibited 90% of the initial efficiency after 1,000 cycles of bending test under 5 mm of the radius of curvature.112 Combination of Ag mesh and PEDOT:PSS enhances the electrical conductivity of the transparent electrode, which results in a PCE of 14%.67 Moreover, this flexible cell exhibits excellent bending durability, retaining over 95.4% of its initial PCE value even after 5,000 fully bending cycles under 5 mm radius of curvature. Carbon nanomaterials such as CNT and graphene have shown excellent flexibility with proper electrical conductivity compared with TCO. All-carbon-electrode-based F-PSC employing graphene as the transparent bottom electrode and CNT as the top electrode retains 84% of the initial PCE after 2,000 cycles of the bending test under a 4 mm radius of curvature.63 MoO3-decorated graphene-based F-PSC demonstrates super flexibility, i.e., only 10% degradation in PCE is observed after 1,000 cycles of the bending test at a radius of curvature = 2 mm. Moreover, this cell achieved 16.8% PCE, which exhibits the great potential for the highly efficient foldable photovoltaic cell.45 The flexibility of PSC has been improved by adopting a new type of flexible substrates with conducting layers such as Ti plate–TiO2 substrate.113 The highly bendable PSC containing TiO2 nanolayer-coated Ti plates does not show any degradation in PCE after 1,000 cycles bending test at 4 mm radius of curvature, which is explained in terms of high crystallinity with low oxygen vacancy that improves bending durability.113 Toward Commercialization Owing to abundant cheap precursor materials and low-cost processes, the perovskite solar module has been believed to be economically viable, which will enable it to be a leader in the future photovoltaic market. The manufacturing costs for the perovskite solar module has been expected to be one-third of those based on bulk silicon PV technologies.12 Moreover, the levelized cost of the electricity (LCOE) for perovskite solar modules could reach 6 US cents/KWh, comparable to the LCOE for the fossil fuels once the module efficiency and lifetime exceeded 12% and 15 years, respectively. High efficiency and bending durability for F-PSCs provide prolific application opportunities. These flexible solar cells have the capability of being installed on the curved surface, which is suitable for building integrated photovoltaics (BIPV). Moreover, lightweight with high efficiency enables to achieve a high-power density of 2 W/g,67 which makes them possible to be utilized as a power source in high-altitude unmanned aerial vehicles (HAUAVs). To commercialize this F-PSC, however, several issues remain: (1) developing uniformly large-area coating technology (Table 3), (2) ensuring long-term stability by encapsulation technology, and (3) exploiting emerging unique flexible cells for special applications need to be solved. Uniform and Large Area Coating Technologies for Flexible Modules Spin coating method has been adopted in the fabrication of small-sized cells, which is not suitable for obtaining uniform ETL, HTL, and perovskite light-absorbing layers due to deflection of large size flexible substrate. Rather, the flexible substrate makes it easy to realize mass production based on R2R process. In R2R, slot-die coating method has been utilized to fabricate large size F-PSC owing to facile mass production of thin films with uniform thickness. Research on slot-die R2R coating has focused on optimizing some process parameters to (1) improve the uniformity of each ETL, HTL, and Perovskite layer; (2) control crystallization kinetics of perovskite;
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Table 3. The Uniform and Large-Area Coating Methods for Perovskite Modules, and Comparison of Device Structure, Substrate Type, Active Area, and Performance of Perovskite Modules Scalable Coating Layer
Coating Method
Substrate Type
Device Structure
Active Area (cm2)
Jsc (mA/cm2)
Voc (V)
Fill Factor (%)
PCE (%)
Ref.
Perovskite
doctor-blade
rigid
glass/FTO/TiO2/perovskite/spiro
10.1
4.29
4.11
58.14
10.26
119
doctor-blade
rigid
glass/FTO/TiO2/ perovskite/P3HT
100
0.83
9.61
53.79
4.30
119
doctor-blade
rigid
glass/ITO/PTAA/perovskite/ fullerene (C60)/bathocuproine (BCP)
33
1.19
18.1
71.3
15.3
120
doctor-blade
rigid
glass/ITO/PTAA/perovskite/ fullerene (C60)/bathocuproine (BCP)
57.2
1.26
17.04
67.5
14.6
120
bar
rigid
glass/FTO/TiO2/ perovskite (double-layered)/P3HT
24.97
2.72
8.66
72.6
17.1
18
drop casting
rigid
glass/FTO/TiO2/ZrO2- perovskite/ carbon
31
4.9
3.72
57.5
10.46
121
drop casting
rigid
glass/FTO/TiO2/ZrO2- perovskite/ carbon
70
1.772
9.63
62.9
10.74
121
drop casting
rigid
glass/FTO/TiO2/ZrO2- perovskite/ carbon
49
2
9.3
56
10.4
122
soft-cover deposition
rigid
glass/FTO/TiO2/ perovskite/spiro
36.1
1.97
10.5
75.7
15.7
123
spray
rigid
glass/FTO/TiO2/ perovskite/PTAA
40
2.10
10.5
70.16
15.5
124
ETL or HTL
slot-die
rigid
glass/ITO/TiO2/perovskite/spiro
168.75
0.692
21.2
67.9
10
125
spin
rigid
glass/ITO/TiO2/perovskite/spiro
4
19.9
0.91
75
13.6
126
spin
rigid
glass/ITO/PEDOT/perovskite/PCBM
60
1.9
8.1
57
8.7
127
spin
rigid
glass/ITO/PEDOT/perovskite/PCBM
40
2.0
10.1
63.7
12.9
128
spin
rigid
glass/ITO/PTAA/PFN/perovskite/ PCBM/PFN/Ag
9.06
16.86
3.27
73
14.1
129
ALD + screen printing
flexible
PET/ITO/ALD-TiO2/TiO2 scaffold/ perovskite/spiro-OMeTAD/Au
7.92
5.2
3.39
71
3.1
72
RF-sputtering + evaporation
flexible
AZO/LiF/C60/Perovskite/spiroOMeTAD/Au
10.2
2.1
7.3
67.5
10.5
130
slot-die
flexible
PET/ITO/SnO2/perovskite/spiroOMeTAD/Au
16.09
3.28
6.727
69
15.22
20
spin
flexible
PEN/ITO/MFGO/perovskite/ PC61BM/BCP/Ag
10
4.34
3.76
49.1
8.1
131
spin
flexible
PET/ITO/SnO2/meso-TiO2/ perovskite/spiro-OMeTAD/Au
12
3.12
5.014
55.9
8.8
132
spin
flexible
PET/ITO/SnO2/perovskite/spiroOMeTAD/Au
10
6.479
3.075
62
12.31
133
spin
rigid
glass/FTO/SnO2/perovskite/spiroOMeTAD/Au
25
1.98
11.2
69.0
15.3
134
spin
rigid
glass/FTO/SnO2/perovskite/spiroOMeTAD/Au
100
1.02
20.2
0.68
14.03
134
and (3) enhance the PCE of the flexible device (Figure 10).114 However, the research on manufacturing F-PSC using the slot-die method is at an early stage. This research is still ongoing for achieving the exploitation of commercially viable F-PSCs. Since slot-die coating of organic materials such as PEDOT:PSS and spiro-OMETAD is relatively favorable compared with inorganic ETL and perovskite materials, efforts have been made to fabricate uniform ETLs or perovskite layers.20,114–116 Bu et al.20 developed a strategy to form high quality SnO2 ETLs using the slot-die coating method. By modifying the SnO2 surface with KOH, the surface defects at the interface between SnO2 and perovskite layer were significantly reduced owing
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Figure 10. Photographic Images of the R2R-Coated Perovskite Layer and an Example of the Manufactured Flexible Devices Reprinted with permission from Galagan et al. 114 Copyright 2018 John Wiley and Sons.
to the formation of KBr, which resulted in improving efficiency and reducing hysteresis on F-PSCs. The efficiency of flexible perovskite module with a size of 5 3 6 cm2 was 15.22%. PEDOT:PSS as HTL in the p-i-n structure was coated using a slot-die R2R process. To improve film coverage and morphology of the perovskite layer, 3-aminopropanoic acid was adopted as a self-assembled monolayer containing a carboxyl group. This carboxyl group was found to act as nuclei for crystallization of perovskite, which resulted in a much smoother perovskite surface morphology together with a PCE enhancement.116 The feasibility of upscaling F-PSC to a width of 30 cm based on slot-die R2R coating was demonstrated by Galagan et al.114 They performed R2R deposition of ETL and perovskite layers at the web speed of 3–5 m/min with nontoxic solvents, which showed a potential for future R2R manufacturing and commercialization of F-PSCs. Beside the slot-die R2R coating method, solution-shearing and spray-coating methods have been introduced. Crystal nucleation and growth in large-area MAPbI3 thin films was confined by using solution shearing, which yielded near single-crystalline perovskite microarrays with a high degree of controlled macroscopic alignment and crystal orientation and consequently exhibited significant improvements in optical and optoelectronic properties.117 Also, TiO2 ETL and perovskite layers with high uniformity, crystallinity, and surface coverage were obtained by using an ultrasonic spray-coating (USC) method.118 The TiO2 ETL deposited by USC was cured by high-density pulsed beam of infrared light, realizing F-PSCs with a PCE of 8.1% that are robust under mechanical stress. Recent Encapsulation Technologies for Long-Term Stability Although remarkable advances in the efficiency of F-PSC have been achieved, the toxicity of the water-soluble lead compound and the poor stability are big challenges for commercialization. Encapsulation technology is able to block the emission of lead as well as extend lifetime of PSC by prohibiting exposure to ambient air containing moisture and oxygen.
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Weerasinghe et al.135 demonstrated that encapsulation of PSC devices using high-quality flexible barrier materials and simple encapsulation architectures can significantly improve long-term stability under ambient storage conditions. Also, the Ca film tests revealed that the ingress of moisture and oxygen through the adhesive layers and around electrical wire contacts is a significant lifetime-limiting factor, which highlights the importance of developing new encapsulation architectures. The combination of polyurethane resin encapsulation and Cr2O3–Cr interlayer demonstrated the improved air stability of ultra-thin F-PSCs. Especially, the employment of a chromium oxide–chromium interlayer was found to effectively protect the metal top contacts from reactions with the perovskite, which was beneficial for extending the long-term stability.7 Functional encapsulating materials possessing their own unique properties have been introduced. Nanocone PDMS, which possesses antireflection and water-repellent functions, was attached to the front side of the flexible substrate. These nanocone PDMS encapsulation materials contributed to improving the optical transmittance as well as to achieving the water-repelling effect (Figure 11A).136 The multilayer encapsulating film consisting of ultra hydrophobic and relatively hydrophilic layers significantly enhanced the stability of PSCs under very humid conditions.137,138 The ultra-hydrophobic film containing poly(methyl methacrylate) (PMMA), polyurethane (PU), and SiO2 nanoparticles in contact with environments served to repel water in humid environments and prevent water permeation into the vulnerable perovskite layer, as shown in Figure 11B.138 The moderately hydrophilic PMMA layer acted as a desiccant to extract residual water from the perovskite layer itself during the solar cell operation. The dual function of the coating film retained the PCE of PSCs at 17.3% for 180 min when exposed to over 95% humidity. Emerging Flexible Photovoltaics for Pioneering a New Market The exploitation of new concept F-PSCs can extend applicability to various devices or systems, which facilitate opening and cornering an emerging photovoltaic market. Since the manufacturing process of PSCs is relatively simple and cost effective compared with other photovoltaic cells, the realization of new concept solar cells using perovskite absorber is viable. The F-PSC is capable of achieving high power-per-weight if the ultra-thin and light plastic substrate is employed. Kaltenbrunner et al.7 used 1.4 mm PET film as substrate and achieved a power-per-weight of 23 W/g, which is a fairly outstanding result because the power-per-weight of silicon solar cells is 1 W/g (Figure 12A). Potential future applications are power sources for UAV for environmental and industrial monitoring, rescue and emergency response, and tactical security applications.7 On the other hand, Lucarelli et al.139 reported the F-PSC that exhibits a PCE of 10.8% and 12.1% at low luminance of 200 lx and 400 lx, respectively. In typical office and building environments, indoor lighting could supply enough energy to drive low-power sensors equipped with F-PSC, implying another potential application of F-PSC. Once the F-PSC uses biocompatible textiles (Figure 12B),140 wire-shaped substrates,141 or cellulose papers38 instead of conventional plastic substrates, this cell can be used as reliable power sources for wearable electronics. The PSC was simply stacked onto a textile by using an elastomer with a good adhesive property. The PCE of textile-based F-PSC was 15% with improved ambient stability and washable
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Figure 11. Strategies for Resisting Water in Humid Environments (A) Schematic structure of the PSC device with nanocone PDMS film attached on the top and SEM image of PDMS nanocone with 1 mm pitch and 1 mm depth. Inset image is a drop of water on the surface of the nanocone PDMS layer, with AR 1.0 showing a large contact angle of 155 . Reprinted with permission from Tavakoli et al. 136 Copyright 2015 American Chemical Society. (B) SEM image for the cross-section of the PMMA-PU-SiO 2 –PMMA-PU double-layer coated PSCs. A change in PCE of the PSCs coated with the PMMA-PU-SiO 2 –PMMA-PU double layer in the ambient conditions of 100% humidity for 180 min. Schematic illustration of the passivation effect of the PMMA-PU-SiO 2 –PMMA-PU double layer. Reprinted with permission from Yoo et al. 138 Copyright 2017 Springer Nature.
capability owing to the encapsulation effect of the elastomer and demonstrating the promising potential for the wearable device applications.140 Recently, Castro-Hermosa et al.38 reported the F-PSC fabricated on a paper substrate that exhibits a maximum PCE of 2.7% by adopting a top semitransparent electrode. Semitransparent F-PSC possesses a wide variety of potential applications such as solar glass for buildings and solar curtains for upholstery, to wearable electronics and fashion (Figure 12C).112 The semitransparent F-PSC without TCO was realized by utilizing highly conductive n-PEDOT:PSS as both the bottom and top transparent electrodes. The n-PEDOT:PSS was stacked onto Spiro-OMeTAD by using a stamp-transfer process with PDMS. The resultant PCE was measured as 10.3% and maintained more than 90% of the initial value after 1,000 cycles of bending test at a radius of curvature of 5 mm.112 Recently, self-healing F-PSC based on self-repairing of electrical pathways in mechanically damaged metals has been exploited (Figure 12D).142 The composite of core-shell-structured liquid metal microcapsules (LMCs) synthesized via in situ
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Figure 12. Niche Applications of F-PSCs (A) Power sources for UAV. Reprinted with permission from Kaltenbrunner et al. 7 Copyright 2015 Springer Nature. (B) The biocompatible textile-based F-PSCs. Reprinted with permission from Lam et al. 140 Copyright 2017 Royal Society of Chemistry. (C) Semi-transparent PSCs by using stamp-transfer process with PDMS. Reprinted with permission from Zhang et al. 112 Copyright 2018 John Wiley and Sons. (D) A solar-powered smart watch embedded with self-healing conductors. Reprinted with permission from Chu et al. 142 Copyright 2018 John Wiley and Sons.
polymerizations of urea-formaldehyde onto liquid metal colloids passivated the metal top electrode. Once the LMC passivation film was ruptured by cutting or pressing, the LMC released and transported liquid metal to damaged sites, which led to recovering the broken electrical pathway. The average recovery ratio of the PCEs was revealed to be 99% after recovering the damaged sites, which demonstrates the great potential of self-repairing devices beneficial to mechanical issues related to the repeated deformation.142 Perspectives For the successful realization of flexible, foldable, and stretchable electronics for future portable devices, it is essential to develop a power supplying system that integrates all of them including the storage system together. Flexible solar cells are expected to offer those functions. Among them, F-PSCs have the advantages of both the high power of inorganic solar cells as well as the flexibility and light weight of organic solar cells, making them ideal power-generating systems. In this paper, we have summarized the fundamental challenges and recent efforts for the development of F-PSCs. As discussed, the reported PCEs of F-PSCs do not exceed those of the conventional rigid PSCs, mainly due to the limitations in processing and material selection. Nevertheless, the advantages over rigid PSCs are very clear, such as the
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Figure 13. Schematic of F-PSC Module with Monolithic Interconnection P1, P2, and P3 stand for first, second, and third laser scribing process. PET and c-TiO 2 represent poly(ethylene terephthalate) and compact-TiO 2 . ITO, c-TiO2 , and spiro-MeOTAD can be replaced by different TCO, oxide, and hole-transporting materials. Au can be also replaced by cheaper and more efficient metal electrode materials.
light weight and versatility of applications due to the customized and integrated devices. In this part, we emphasize the module technology for the future developments of F-PSCs. Although several papers have been published on the fabrication of largesized F-PSCs using printing technology, their direct application to actual operating devices has not been achieved. For the practical application of F-PSCs as power supplying devices for soft and conformal electronics such as those employed for the internet of things (IoT) devices and sensors, the module technology, which involves assembling multiple subcells, should be developed for boosting the power to levels that would meet the minimum input powers of related electronics and power storage systems. For this purpose, many subcells are generally interconnected in a monolithic form or a grid-type structure; the former is generally thought to be more proper for F-PSCs due to the overall coating process and compatibility of the layer materials.51 Through series interconnection, the parasitic resistive loss originating from the series and shunt resistances could be reduced when the solar cells are enlarged. Recently, there have been only a few reports on the F-PSC module.20,51,132,143 Bu et al.20 introduced a facile interface passivation strategy with potassium treatment for the SnO2 ETL and demonstrated the fabrication of an F-PSC module with a PCE of 15.22% and large size (5 3 6 cm2) using the slot-die printing method. The potassium treatments induced hysteresis-free, stable, and high PCE by facilitating the growth of perovskite grains. Li et al.132 demonstrated a sequential deposition method for the formation of pinhole-free CH3NH3PbI3 perovskite thin films for the fabrication of F-PSC modules (active area of 16 cm2) with PCE over 8%. Three scribing processes, referred to as P1, P2, and P3 scribing steps, are essential for the fabrication of the monolithic interconnected module.51 As depicted in Figure 13, the P1, P2, and P3 processes represent the isolation of the TCO between cells, selective removal of the ETL/perovskite layer/HTL for the connection from the top contact of the unit cells to the bottom TCO, and isolation of the top metal electrode between neighboring cells, respectively. The patterned area for P1, P2, and P3 belong to the dead area where power cannot be produced in the solar-cell module. Conventionally, nanosecond lasers or mechanical scribing tools are widely used for the P1, P2, and P3 processes. Rakocevic et al.144 compared mechanical and laser patterning technologies for the fabrication of glass-based PSC module. They found that the mechanical scribing is a reliable and relatively cost-effective patterning technique, whereas the laser ablation can provide greater variability with respect to substrates and layer materials. Further study is necessary to compare the scribing technologies for the plastic-based F-PSC. Recently, Dagar et al.132
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reported an F-PSC module with a 12 cm2 active area patterned with a laser that exhibits a PCE of 8.8%. This F-PSC module was designed using three scribing steps (P1, P2, and P3 scribing steps) based on an efficient, fully laser scribing procedure. For the F-PSCs, especially on ITO-polymers, patterned lines are not as thin, sharp, and clean as those for the glass-based PSCs, implying that the high performance of unit cells in F-PSCs might not be reproducible in modules with a large area. Therefore, the scribing adaptability, as well as the high performances and large area processability, should be considered together. Recently, intensive and extensive studies have been conducted on perovskite-based tandem solar cells, and a PCE of 28.0% was achieved for the PSC/silicon tandem cells.2 Owing to the considerable interest in PSC-based tandem devices, we predict that flexible tandem devices such as flexible PSC/CIGS or flexible PSC/organic photovoltaics (OPV) could be realized as future generation devices if new architectures, materials, and processes are developed.
ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Ministry of Science and ICT: NRF-2012M3A6A7054861, NRF2014M3A6A7060583, NRF-2012M3A6A7054856, and NRF-2017R1A2B3010927 (Global Frontier R&D Program on Center for Multiscale Energy System); NRF-2016M3D1A1027663 and NRF-2016M3D1A1027664 (Future Materials Discovery Program); NRF-2018R1A2B2006708 (Research Program); and NRF2015M1A2A2053004 and NRF-2015M1A2A2056827 (Climate Change Management Program).
AUTHOR CONTRIBUTIONS H.S.J., M.J.K., and N.-G.P. perceived scientific challenges and recent efforts as the flexible perovskite solar cell topics in this review article and wrote the manuscript. G.S.H. revised the manuscript, collected permissions, and corrected typos.
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117. Li, J., Liu, Y., Ren, X., Yang, Z., Li, R., Su, H., Yang, X., Xu, J., Xu, H., Hu, J.-Y., et al. (2017). Solution coating of superior large-area flexible perovskite thin films with controlled crystal packing. Adv. Opt. Mater. 5, 1700102. 118. Das, S., Yang, B., Gu, G., Joshi, P.C., Ivanov, I.N., Rouleau, C.M., Aytug, T., Geohegan, D.B., and Xiao, K. (2015). High-performance flexible perovskite solar cells by using a combination of ultrasonic spray-coating and low thermal budget photonic curing. ACS Photonics 2, 680–686. 119. Razza, S., Di Giacomo, F., Matteocci, F., Cina, L., Palma, A.L., Casaluci, S., Cameron, P., D’Epifanio, A., Licoccia, S., Reale, A., et al. (2015). Perovskite solar cells and large area modules (100 cm(2)) based on an air flowassisted PbI2 blade coating deposition process. J. Power Sources 277, 286–291. 120. Deng, Y., Zheng, X., Bai, Y., Wang, Q., Zhao, J., and Huang, J. (2018). Surfactant-controlled ink drying enables high-speed deposition of perovskite films for efficient photovoltaic modules. Nat. Energy 3, 560–566. 121. Priyadarshi, A., Haur, L.J., Murray, P., Fu, D.C., Kulkarni, S., Xing, G.C., Sum, T.C., Mathews, N., and Mhaisalkar, S.G. (2016). A large area (70 cm(2)) monolithic perovskite solar module with a high efficiency and stability. Energy Environ. Sci. 9, 3687–3692. 122. Hu, Y., Si, S., Mei, A.Y., Rong, Y.G., Liu, H.W., Li, X., and Han, H.W. (2017). Stable large-area (10 x 10 cm(2)) printable mesoscopic perovskite module exceeding 10% efficiency. Sol. RRL 1, 1600019. 123. Chen, H., Ye, F., Tang, W., He, J., Yin, M., Wang, Y., Xie, F., Bi, E., Yang, X., Gra¨tzel, M., et al. (2017). A solvent- and vacuum-free route to large-area perovskite films for efficient solar modules. Nature 550, 92–95. 124. Heo, J.H., Lee, M.H., Jang, M.H., and Im, S.H. (2016). Highly efficient CH3NH3PbI3xClx mixed halide perovskite solar cells prepared by re-dissolution and crystal grain growth via spray coating. J. Mater. Chem. A 4, 17636– 17642. 125. Di Giacomo, F., Shanmugam, S., Fledderus, H., Bruijnaers, B.J., Verhees, W.J.H., Dorenkamper, M.S., Veenstra, S.C., Qiu, W., Gehlhaar, R., Merckx, T., et al. (2018). Upscalable sheet-to-sheet production of high efficiency perovskite module and solar cells on 6-in. substrate using slot die coating. Sol. Energy Mater. Sol. Cells 181, 53–59. 126. Qiu, W., Merckx, T., Jaysankar, M., Masse de la Huerta, C.M., Rakocevic, L., Zhang, W., Paetzold, U.W., Gehlhaar, R., Froyen, L., Poortmans, J., et al. (2016). Pinhole-free perovskite films for efficient solar modules. Energy Environ. Sci. 9, 484–489. 127. Seo, J., Park, S., Kim, Y.C., Jeon, N.J., Noh, J.H., Yoon, S.C., and Sang, S.I. (2014). Benefits of very thin PCBM and LiF layers for solutionprocessed p-i-n perovskite solar cells. Energy Environ. Sci. 7, 2642–2646. 128. Heo, J.H., Han, H.J., Kim, D., Ahn, T.K., and Im, S.H. (2015). Hysteresis-less inverted CH3NH3PbI3 planar perovskite hybrid solar cells with 18.1% power conversion efficiency. Energy Environ. Sci. 8, 1602–1608.
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129. Hong, S., Lee, J., Kang, H., Kim, G., Kee, S., Lee, J.H., Jung, S., Park, B., Kim, S., Back, H., et al. (2018). High-efficiency large-area perovskite photovoltaic modules achieved via electrochemically assembled metalfilamentary nanoelectrodes. Sci. Adv. 4, eaat3604. 130. Pisoni, S., Fu, F., Widmer, R., Carron, R., Moser, T., Groening, O., Tiwari, A.N., and Buecheler, S. (2018). Impact of interlayer application on band bending for improved electron extraction for efficient flexible perovskite mini-modules. Nano Energy 49, 300–307. 131. Yeo, J.S., Lee, C.H., Jang, D., Lee, S., Jo, S.M., Joh, H.I., and Kim, D.Y. (2016). Reduced graphene oxide-assisted crystallization of perovskite via solution process for efficient and stable planar solar cells with module-scales. Nano Energy 30, 667–676. 132. Dagar, J., Castro-Hermosa, S., Gasbarri, M., Palma, A.L., Cina, L., Matteocci, F., Calabro`, E., Di Carlo, A., and Brown, T.M. (2018). Efficient fully laser-patterned flexible perovskite modules and solar cells based on low-temperature solution-processed SnO2/ mesoporous-TiO2 electron transport layers. Nano Res. 11, 2669–2681. 133. Bu, T., Shi, S., Li, J., Liu, Y., Shi, J., Chen, L., Liu, X., Qiu, J., Ku, Z., Peng, Y., et al. (2018). Lowtemperature presynthesized crystalline tin
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oxide for efficient flexible perovskite solar cells and modules. ACS Appl. Mater. Interfaces 10, 14922–14929. 134. Han, G.S., Kim, J.S., Han, B., Kim, S., Gong, Y.J., Lee, O.Y., Ko, P.M.J., and Jung, H.S. (2019). Spin coating process for 10cm 3 10cm perovskite solar modules enabled by selfassembly of SnO2 nanocolloids. ACS Energy Lett. 4, 1845–1851. 135. Weerasinghe, H.C., Dkhissi, Y., Scully, A.D., Caruso, R.A., and Cheng, Y.-B. (2015). Encapsulation for improving the lifetime of flexible perovskite solar cells. Nano Energy 18, 118–125. 136. Tavakoli, M.M., Tsui, K.H., Zhang, Q.P., He, J., Yao, Y., Li, D.D., and Fan, Z.Y. (2015). Highly efficient flexible perovskite solar cells with antireflection and self-cleaning nanostructures. ACS Nano 9, 10287–10295. 137. Han, G.S., Yoo, J.S., Yu, F.D., Duff, M.L., Kang, B.K., and Lee, J.K. (2017). Highly stable perovskite solar cells in humid and hot environment. J. Mater. Chem. A 5, 14733– 14740. 138. Yoo, J.S., Han, G.S., Lee, S., Kim, M.C., Choi, M., Jung, H.S., and Lee, J.K. (2017). Dual function of a high-contrast hydrophobichydrophilic coating for enhanced stability of perovskite solar cells in extremely humid environments. Nano Res. 10, 3885–3895.
139. Lucarelli, G., Di Giacomo, F., Zardetto, V., Creatore, M., and Brown, T.M. (2017). Efficient light harvesting from flexible perovskite solar cells under indoor white light-emitting diode illumination. Nano Res. 10, 2130–2145. 140. Lam, J.-Y., Chen, J.-Y., Tsai, P.-C., Hsieh, Y.-T., Chueh, C.-C., Tung, S.-H., and Chen, W.-C. (2017). A stable, efficient textile-based flexible perovskite solar cell with improved washable and deployable capabilities for wearable device applications. RSC Adv. 7, 54361–54368. 141. Adams, G.R., and Okoli, O.I. (2018). A review of perovskite solar cells with a focus on wireshaped devices. Renew. Energy Focus 25, 17–23. 142. Chu, K., Song, B.G., Yang, H.-I., Kim, D.-M., Lee, C.S., Park, M., and Chung, C.-M. (2018). Smart passivation materials with a liquid metal microcapsule as self-healing conductors for sustainable and flexible perovskite solar cells. Adv. Funct. Mater. 28, 1800110. 143. Li, K.P., Xiao, J.Y., Yu, X.X., Li, T.H., Xiao, D., He, J., Zhou, P., Zhang, Y.W., Li, W.N., Ku, Z.L., et al. (2018). An efficient, flexible perovskite solar module exceeding 8% prepared with an ultrafast PbI2 deposition rate. Sci. Rep. 8, 442. 144. Rakocevic, L., Gehlhaar, R., Merckx, T., Qiu, W.M., Paetzold, U.W., Fledderus, H., and Poortmans, J. (2017). Interconnection optimization for highly efficient perovskite modules. IEEE J. Photovoltaics 7, 404–408.
Flexible Perovskite Solar Cells Hyun Suk Jung,1,* Gill Sang Han,1 Nam-Gyu Park,2,* and Min Jae Ko3,*
Since the first report on solid-state perovskite solar cells (PSCs) with 9.7% efficiency and 500-h long-term stability in 2012, PSCs have achieved an amazing power-conversion efficiency (PCE) of 24.2%, exceeding the PCEs of multi-crystalline Si (22.3%), thin-film crystalline Si (21.2%), copper indium gallium selenide (22.6%), and CdTe-based thin-film SCs (22.1%), and are suitable for transforming into flexible solar cells based on plastic substrates. The light weight and flexibility of flexible-PSCs (F-PSCs) allows their use in niche applications such as portable electric chargers, electronic textiles, large-scale industrial roofing, and power sources for unmanned aerial vehicles (UAVs). However, the F-PSCs always exhibit inferior efficiency compared to rigid PSCs, i.e., champion-cell efficiency of F-PSCs is 19.11%, which is apparently lower than that of rigid cells. Also, the world-best module efficiency for rigid perovskite module is 17.18% (30 cm2) higher than that for flexible perovskite module efficiency, 15.22% (30 cm2). Moreover, the F-PSCs have not shown better long-term stability in comparison with rigid PSCs. In this review paper, we investigate fundamental challenges of F-PSCs regarding relatively low efficiency and stability and demonstrate the recent efforts to overcome big hurdles. Also, current attempts for the commercialization of F-PSCs are introduced.
Introduction In photovoltaic history, there has been no solar cell like the perovskite solar cell (PSC), which has achieved an amazing power conversion efficiency (PCE) of 24.2% in such a short time of 7 years. This efficiency exceeds the PCEs of multi-crystalline Si (22.3%), thin-film crystalline Si (21.2%), copper indium gallium selenide (22.6%), and CdTe-based thin-film SCs (22.1%), owing to the excellent optical, electrical, and chemical properties of perovskite materials.1,2 In 2012, the study on adopting the perovskite materials in a solid-state sensitized cell reported the achievement of 9.7% PCE,3 which triggered the efficiency race in PSCs. In 2019, more than 12,000 research papers cumulatively and many commercial companies, including Oxford PV, Dyesol, and Frontier Energy Solutions, are developing their own technologies for the commercialization of perovskite solar modules. The cost-effective material,4 low formation energy,5 mechanical durability,6 and potential for a solution-based roll-to-roll (R2R) process make the PSC suitable for realizing a flexible thin-film solar cell that employs a plastic substrate such as polyethylene terephthalate (PET). Furthermore, the flexible PSC (F-PSC) using plastic substrate would produce the most competitive power per weight among the solar cells (Figure 1A).7 Therefore, F-PSCs can be used in niche applications such as portable electric chargers, electronic textiles, large-scale industrial roofing, and power sources for unmanned aerial vehicles (UAVs). In contrast with rigid PSC based on glass substrate, the charge transport layer, especially the electron transport layer (ETL), like TiO2, ZnO, and SnO2, should be formed at a low temperature of below 150 C in plastic-based F-PSCs, which is a non-trivial
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Context & Scale In a short time of 7 years, perovskite solar cells (PSCs) have achieved an amazing power conversion efficiency (PCE) of 24.2%, which exceeds the PCEs of multi-crystalline Si (22.3%), thinfilm crystalline Si (21.2%), copper indium gallium selenide (22.6%), and CdTe-based thin-film SCs (22.1%). Owing to low process temperature, mechanical durability, and the potential for the solution-based roll-to-roll (R2R) process, the PSC has a strong potential of being utilized in the form of flexible solar cell based on a plastic substrate. This flexible-PSC (F-PSC) is expected to be used in niche applications such as portable electric chargers, electronic textiles, large-scale industrial roofing, and power sources for unmanned aerial vehicles (UAVs). However, the champion-cell efficiency of the F-PSC is 19.11%, which is apparently lower than that of the rigid cell (24.2%). Also, the world-best perovskite module efficiency on a rigid substrate is 17.1%, outstripping the efficiency of flexible perovskite module (11.7%). Moreover, the F-PSCs have not shown superior longterm stability to rigid cells. To commercialize the F-PSC, the efficiency needs to be comparable to the glass-based rigid PSC as well as the long-term stability.
challenge to achieving high efficiency. Since the F-PSC is potentially used in bendable or foldable electronic devices, robust bending durability is another important issue. In this regard, there has been remarkable progress in developing F-PSCs with high efficiency as well as with bending durability. In 2014, the F-PSC with a meaningful efficiency of 10.2% was exploited by using ZnO nanocolloids ETL (5 nm).8 The high-density and uniform ZnO layer was successfully prepared without a sintering process. In 2015, Kim et al.6 realized a high PCE of 12.2% flexible cell by fabricating a TiO2 ETL using plasma-enhanced atomic layer deposition (PEALD). They performed the bending durability test of F-PSC for the first time and characterized the excellent bending durability of perovskite materials. Park et al.9 also proved the super-flexibility of F-PSCs by demonstrating the complete shape recovery of the device under repeated crumpling condition without significant mechanical damage. The PCE was maintained up to 60% of initial value even after 50 cycles of complete crumpling (Figures 1B and 1C). Another breakthrough was achieved by exploiting the inverted structured F-PSC, which adopted an organic charge transport layer.10 Owing to the use of PEDOT:PSS as an organic hole-transport layer (HTL), the maximum processing temperature for manufacturing flexible PSS was 90 C. Stretchable and ultra-lightweight F-PSC with the inverted structure was successfully fabricated, implying that the F-PSC could be applied to power aviation models.7 The photovoltaic performance of ultrathin F-PSC was displayed as a function of compression condition (Figure 1D). Recently, there have been many review papers introducing recent progress in F-PSCs.11–16 In contrast with the previously published review papers, this review paper pays attention to the key scientific challenges underlying the inherently low PCEs in F-PSCs compared to PSCs employing rigid substrates. So far, championcell efficiency of the flexible cell is 19.11%,17 which is apparently lower than that of the rigid cell, 24.2%. Also, the world-best efficiency for rigid perovskite module is 17.1%, which is fairly higher compared to flexible perovskite module,18 11.7% and 15.2% for the area of 703 and 30 cm2, respectively, as displayed in Figure 1E.19–21 Moreover, the F-PSCs have not shown good long-term stability in comparison with rigid cells. The fundamental reason for the inferior performance of F-PSCs is related to the difference in chemical and physical properties of substrate materials that are glass and plastic. In particular, the restrictive process temperature for plastic substrate hinders the fabrication of highly conductive transparent conducting oxide (TCO) and recombination-free charge transport layer, which makes it difficult to achieve high efficiency. Also, the optical transmittance of plastic substrate is lower than that of the glass substrate, which is another reason for inferior photovoltaic performance for F-PSC. The PET is permeable to oxygen and water molecules, and is not able to perfectly block the attack from oxygen and water, unlike glass materials.22,23 In this review paper, we investigate the fundamental challenges of F-PSCs regarding relatively low efficiency and stability compared to glass substrate-based PSCs and demonstrate the recent attempts to overcome big hurdles. Moreover, this paper deals with the efforts that have been made to facilitate the commercialization of F-PSC.
In this review paper, we investigate the fundamental challenges of F-PSCs such as the optical transmittance of flexible substrates and electrical conductivity of flexible transparent conducting oxides, uniform coating technology with a large area on flexible substrates, the high moisture permeability of plastic flexible substrates, and super flexibility. We also introduce recent efforts for overcoming the aforementioned issues as well as for facilitating the commercialization of F-PSCs. As a perspective, we suggest the future direction of research and development of F-PSCs such as the module technology involving assembling multiple subcells and the flexible tandem devices including flexible PSC/CIGS or flexible PSC/organic photovoltaics (OPVs).
1School
of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea
2School
of Chemical Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea
3Department
Fundamental Challenges in Flexible Perovskite Solar Cells Challenge in Substrate Materials for the Flexible Solar Cells First, a general review of the substrate materials for flexible solar cells is provided.24 Flexible substrates are among the most important components required to realize
of Chemical Engineering, Hanyang University, Seoul 04763, Republic of Korea *Correspondence: [email protected] (H.S.J.), [email protected] (N.-G.P.), [email protected] (M.J.K.) https://doi.org/10.1016/j.joule.2019.07.023
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Figure 1. The Superiority of F-PSCs and Progress of PCE in F-PSCs/Module (A) Comparison of power-per-weight for the various solar cells. Reprinted with permission from Kaltenbrunner et al. 7 Copyright 2015 Springer Nature. (B) Shape recoverable F-PSCs after random crumpling followed by thermal treatment. (C) Photovoltaic performance before and after crumpling test. Reprinted with permission from Park et al. 9 Copyright 2015 John Wiley and Sons. (D) Stretchable F-PSCs and their current-voltage characteristics as a function of compression. Reprinted with permission from Kaltenbrunner et al. 7 Copyright 2015 Springer Nature. (E) Progress of PCEs in rigid-type cells and F-PSCs for unit cell and module.2,17–19,21
high-performance flexible electronics. Many performances strongly depend on substrate characteristics. Here, several requirements for the flexible substrates of photovoltaic devices are listed, as follows. (1) Optical properties: because solar cells are light-absorbing devices, the substrate should optically transmit as much light as possible, especially more than 90% transmission in the visible-light region. (2) Barrier properties: most electronic devices are vulnerable to oxygen or moisture, which leads to significant degradation of performance. The substrate should act as a barrier layer to avoid the penetration of oxygen and water vapor for long-term stable performance. (3) Higher conductivity: charge-collecting layers, such as TCOs, are usually deposited on the substrate to function as transparent conductive electrodes (TCEs) consisting of the TCO and substrate. The sheet resistance of TCEs is closely related to the photovoltaic performance, especially the fill factor and photocurrent density of solar cells. (4) Chemical properties: the substrates are exposed to a wide range of process chemicals such as gas and solvents during fabrication, and hence must be stable to these chemicals. PET and polyethylene naphthalate (PEN) films, which are widely used polymeric substrates, show good resistance to various solvents. (5) Mechanical properties: flexible substrates should comply with deformable transformation under severe stress and strain and effectively release stress without
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losing their original functions. In addition, dimensional stability, that is, the ability of materials to maintain its original dimensions while being used or processed, should be ensured. (6) Thermal and thermomechanical properties: two parameters, i.e., the thermal transition temperature and the coefficient of thermal expansion (CTE), have to be considered. The softening temperature of substrate materials, especially the so-called glass transition temperature (Tg) for the polymer, must be compatible with the maximum processing temperature (Tmax) that films can be subjected to. The substrates would be expanded or contracted during temperature change. Significant mismatch of thermal properties between the neighboring layers would exert additional stress on the lattice, resulting in the creation of strain and cracking during heating and cooling processes. The limit of tolerable thermal mismatch is expressed by jDCTE,DTj %0:1% 0:3%, where DCTE and DT are the difference in CTE between the base substrate and the neighboring layers that are subsequently deposited, and the temperature excursion during processing, respectively. (7) Surface roughness: rough surface of the substrate could not assure the conformal coating of layers and avoidance of electrical short circuiting in the device. When the roughness is very large, additional planarization processes are necessary like polishing and coating for the steel-based substrates and polymer, respectively, which would increase the processing cost. Three types of materials, i.e., ultrathin flexible glass, metal foils, and polymers, are available for the substrate of flexible photovoltaics. Glass substrates with a thickness of several hundred mm become flexible. The flexible glass can meet most of the standards for the requirements of the flexible substrate, described above. The strong temperature tolerance of up to over 600 C, good dimensional stability, low CTE of 106/ C, and perfect gas barrier properties are very attractive. However, fragility, heavy weight, and high cost would be big concerns, as compared to other candidate materials. By contrast, thin metal foils such as stainless steel and Ti retain very good mechanical properties and resistance to process chemicals; however, the impermeability of light would limit its application to the substrate of flexible solar cells. Polymer substrate would be ideal in terms of flexibility, light weight, low cost, and R2R processability. However, there are many drawbacks such as low heat resistance, high CTE, and poor gas barrier properties for oxygen and moisture. In particular, because of the low Tg and high CTE of the polymer substrate, shrinkage or expansion of polymer substrate would occur with temperature change. Therefore, the whole fabrication processes should be completely redesigned and compared to the processes capable of high-temperature heat treatments. In addition, according to the tolerance thermal mismatch rule explained above, even a small mismatch would cause cracked or strained TCO or device. Therefore, the process windows for a polymer-based solar device become very narrow. In addition, regarding the barrier properties of polymer substrates, most of the polymer substrates do not satisfactorily meet the demanding requirements for oxygen and moisture protection. As a result, the extra encapsulations are highly necessary for the long-term stable operation of F-PSCs. Additionally, it is desirable that the functions that are antireflection (AR) and ultraviolet (UV) proof (AR+UV-proof) are added to the encapsulating layer for further light absorption and long-term stability, respectively. Optical Transmittance of Flexible Substrates and Electrical Conductivity of Flexible Transparent Conducting Oxides In this regard, this discussion focuses on polymer-substrate-based F-PSCs, because polymer substrates would be ideal platforms for the realization of all flexible electronics. A number of polymers could be applied to materials for flexible substrates. Biaxially oriented PET and PEN films, using heat-setting for the crystallization and
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Table 1. Property Parameters for Polymer Substrate Substrate
PEN
PET
PI
PC
Tg ( C)
120–155
70–110
155–270
145
Tm ( C)
269
115–258
250–452
115–160
Density (g/cm3)
1.36
1.39
1.35–1.43
3
2–4.1 3 10
3
2.5 3 10
3
1.20–1.22 2.0–2.6 3 103
Modulus (MPa)
0.1–0.5 3 10
Work temp. ( C)
–
50–150
<400
40–130
CTE (ppm/ C)
20
15–33
8–20
75
Water absorption (%)
0.3–0.4
0.4–0.6
1.3–3.0
0.16–0.35
Solvent resistance
good
good
good
poor
Dimensional stability
good
good
fair
fair
Tm: melting temperature
stabilization of the films, are widely used as conventional and commercially available polymer substrates, despite several drawbacks, such as low heat resistance and dimensional instability during processing. Polycarbonate (PC) and polyethersulfone (PES) are optically more transparent and have a relatively high Tg compared with PET and PEN, but they lack process chemical resistance and have poor gas barrier properties. Polyimide (PI) exhibits excellent thermal properties but is not optically clear, which is critical for a substrate material of light-absorbing devices, although colorless PI has been recently developed. Table 1 compares the properties of flexible polymer substrates.25 For use as TCEs, TCOs are subsequently deposited on the flexible substrates. As base materials of TCOs, indium oxide (In2O3), tin oxide (SnO2), and zinc oxide (ZnO) are typically used. By doping specific metal ions to those, the transparency and conductivity noticeably increase because of the extrinsic doping effects. Among the TCOs, indium tin oxide or tin-doped indium oxide (ITO) and fluorine-doped tin oxide (FTO) are commercially and widely used in optoelectronic devices. The deposition process of TCO on polymer substrates such as PET and PEN is limited because of thermal sensitivities of polymers. The flexible TCEs with higher optical transmittance and conductivity are basic essentials for the high performance of F-PSC. However, typically, the overall performance of plastic-based F-PSC would be inferior to that of rigid glass-based PSCs due to the lower optical transmittance and lower electrical conductivity of polymer-based TCE. The optical transmittance of substrates is very critical for PCE because photoactive materials should absorb as much light as possible. Polymeric materials inherently absorb ultraviolet and even visible light to some extent, due to the electronic transition of the chromophore in polymers, leading to the light-harvesting loss for the photoactive materials. Zhao et al.26 demonstrate that the flexible ITO/PET substrates show relatively lower transmittance in the visible range, compared to ITO/ glass, due to the lower transparency of the polymer (Figure 2A). The optical properties of F-PSCs are also affected by those of TCOs that are generally transparent in visible and infrared regions due to the large bandgap (Eg) of over 3.0 eV. The transmission window defined by the two imposed boundaries (lg and lp) is also observed. The lg at the short wavelength of UV regions is related to photon absorption with energy larger than Eg. The second transmission edge (lp) is referred to as the plasma wavelength, where the frequency of the light is the same as the
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Figure 2. The Optical and Electrical Properties of TCOs (A) Comparison of the optical transmittance of ITO (100 nm), PET (125 um), and ITO (130 nm) and glass. Reprinted with permission from Zhao et al. 26 Copyright 2014 Royal Society of Chemistry. (B) The wavelength dependence of optical transmittance of TCO. 27 (C) The temperature dependence of sheet resistance of TCE. Reprinted with permission from Zardetto et al. 29 Copyright 2011 John Wiley and Sons.
frequency of the collective oscillation of electrons in materials. The lp is generally detected at the longer wavelength around the near-infrared region. For wavelengths larger than lp, the transmittance steeply decreases as the reflectance caused by the plasma resonance of the electrons becomes dominant.27 The two absorption edges (lg and lp) are shifted to a shorter wavelength as the carrier concentration of TCOs increases, as shown in Figure 2B. Since the carrier concentration affects both the electrical and the optical properties of TCOs, optimization of carrier concentration is necessary. Ideally, TCO should have large transmission windows with higher electric conductivity. The ITO is the most widely used TCO for polymer substrates, due to the better transmittance and lower sheet resistance than others. It is generally known that the electrical properties of ITO strongly depend on the film composition and deposition parameters such as sputtering power, oxygen pressure, film thickness, and substrate deposition temperature.28 Zardetto et al.29 reported that the sheet resistance of PET/ITO increases at over 180 C and the sheet resistance of PEN/ITO increases at 235 C. However, this would not be an issue for the polymer substrates in which a low-temperature process is essential. On the other hand, the conductivity of plastic-based TCE employing ITO is typically lower than that of a glass-based one, due to the relatively lower carrier concentration of ITO films on polymers. The microstructure of ITO without high-temperature thermal treatments is generally amorphous or partially crystalline, leading to poor electrical properties. For the fabrication of ITO-based glass, ITO is dc-magnetron sputtered at glass substrates while the substrate is maintained at approximately 300 C–400 C, or sputtered on the cold substrate, followed by annealing at 200 C. Such a heat treatment would result in a high degree of the crystalline or partially crystalline microstructure of ITO. Meanwhile, the conduction electrons are created because oxygen deficiencies could donate a maximum of two unbound electrons by removing oxygen from the cluster, which is beneficial for better electrical conductivity. Besides, dopant S4+ would replace In3+ by extrinsic doping during sputtering, further producing one more electron in the conduction band of In2O3x. Both the oxygen vacancies and tin dopants in substitutional lattice positions would simultaneously contribute to electrical conductivity. On the other hand, such contributions as doping and crystal effects would not be expected in the ITO deposited at room-temperature processing limited by the polymer substrate. Bellingham et al.30 reported that the higher
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resistivity of the amorphous or less crystalline ITO films would be a consequence of the inability to activate the impurity dopant, i.e., limited diffusion of Sn4+ from interstitial locations and grain boundaries into an In3+ site, caused by the low deposition temperature. In this case, the conductivity solely depends on oxygen vacancies. Furthermore, the electron transports in the amorphous or less crystalline ITO films would not be as effective as those in the ordered crystalline structure, due to the relatively large amount of defects and charge traps. Figure 2C exhibits the sheet resistance of PET/ITO and glass/ITO. The sheet resistances of PEN/ITO and PET/ITO significantly increase after thermal treatment at 200 C and 240 C, respectively.29 As described previously, the morphological, electrical, and optical properties of ITO films change depending on the deposition condition and types of substrates. However, except for the ITO crystalline quality and preferred orientation, no significant difference in chemical composition or chemistry according to the process temperature was reported.31–36 Lee investigated the relationship between microstructure of ITO and electron scattering mechanisms at the various process conditions.33 For the less crystalline ITO, the grain boundary scattering was dominant while ITO with a high crystallinity was governed by the ionized-impurity scattering mechanism due to the higher carrier concentration. Deposition of thicker TCO on plastic substrates is another strategy to compensate for the low conductivity. However, because the absorption of TCO films rises strongly with increasing thickness, there are some limitations to meet both the desired properties together. Recently, F-PSCs have been fabricated using various transparent electrodes, such as metal-based materials (nanowires, grids, and networks), conducting polymers, carbon-based materials (carbon nanotube and graphene), and oxide–metal–oxide multilayers.37,38 Table 2 summarizes the transmittance and sheet resistance of various flexible PCEs for the F-PSCs reported. Challenges in Uniform Coating Technology with a Large Area on Flexible Substrates With the dramatic boosting of the certified efficiency of up to 24% for rigid glassbased PSCs,2 there has been much interest and effort for the enlargement of the device area and upscaling. A few reviews of research on the processing and upscaling of PSC modules have recently been reported.48–52 High-quality perovskite films with a small area (25 cm2) can be fabricated using a simple solution method, such as spin coating. However, despite a few reports on the fabrication of large-area PSCs using spin coating, it is generally accepted that there are limitations to the extension of the process to large-area modules (25 cm2 or larger), because it is difficult to control the morphology and kinetics of perovskite crystals over a large area without pinholes. However, in terms of cost and productivity, flexible solar devices are more promising than rigid glass ones. For the commercialization of F-PSCs, it is important to develop a process to make mass production with an R2R process possible. The printing and coating methods can be divided into contact and noncontact methods depending on whether the coating materials are in direct contact with the coating plate (Figure 3). Screen printing, flexography, gravure printing, and soft lithography belong to the contact methods that have been widely used recently in printed electronics. Recently, Cruz et al.25 reported the relationship between process parameters and printing quality for the fabrication of printed electronics on polymer substrates.
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Table 2. Transmittance and Sheet Resistance of Various Flexible Substrates for the F-PSCs Substrates
Electrodes
Transmittance (%)
Sheet Resistance (U/cm2)
Structure of PSCs
PCE (%)
Ref.
PET
AZO/Ag/AZO
81
7.5
PET/AZO/Ag/AZO/PEDOT:PSS/PolyTPD/MAPbI3/PCBM/Au
7
10
Ti Foil
Ag(8 nm)
–
–
Ti foil/TiO2 BL/MP-TiO2/ MAPbI3/Spiro/Ag
0.77
39
Ti Foil
Ag(12 nm)
–
–
Ti foil/TiO2 BL/MP-TiO2/ MAPbI3/Spiro/Ag
6.15
39
Ti Foil
Ag(16 nm)
–
–
Ti foil/TiO2 BL/MP-TiO2/ MAPbI3/Spiro/Ag
3.85
39
Ti Foil
Ag(20 nm)
–
–
Ti foil/TiO2 BL/MP-TiO2/ MAPbI3/Spiro/Ag
2.99
39
Ti Foil
CNTs
60–80
–
Ti foil/TiO2 NTs+MAPbI3/CNTs+Spiro
8.31
40
Ti
Ag(1 nm) +ITO
>80
107.14
Ti/TiO2 BL/TiO2+MAPbI3/ Spiro/Ag+ITO
11.01
41
PET
PEDOT:PSS+EG
>60
28
PET/PEDOT:PSS(PH1000)+EG/PEDOT:PSS(VPAI 4083)/ MAPbI3/PCBM/TiOx/Al
4.9
42
Willow glass
ITO
81.5
31.3
MgF2/Willow glass/ITO/SnO2/ FAMACs/Spiro/MoOx/Al
17.3
43
Willow glass
AZO
85.8
17.8
MgF2/Willow glass/ITO/SnO2/ FAMACs/Spiro/MoOx/Al
12.2
43
Willow glass
IZO
76.4
11.6
MgF2/Willow glass/ITO/SnO2/ FAMACs/Spiro/MoOx/Al
18.1
43
PET
AuCl3-GR
>80
75
Al/PCBM/MAPbI3/PEDOT:PSS/AuCl3-GR/APTES/PET
17.9
37
PES
AZO/AgNW/ AZO
88.6 at 550 nm
11.86
Au/Spiro/MAPbI3/ZnO/AZO/AgNW/AZO
11.23
44
PEN
graphene
~97
552.0
PEN/Graphene/MoO3/PEDOT:PSS/MAPbI3/C60/BCP/LiF/Al
16.8
45
PET
ITO
70
15
PET/ITO/PEDOT:PSS(VPAI 4083)/MAPbI3/PCBM/TiOx/Al
4.3
42
Glass
ITO
80
8
Glass/ITO/PEDOT:PSS(VPAI 4083)/MAPbI3/PCBM/TiOx/Al
9
42
Glass
ITO
–
–
Glass/ITO/PEDOT:PSS/MAPbI3/PCBM/Al
11.5
46
PET
ITO
–
–
PET/ITO/PEDOT:PSS/MAPbI3/PCBM/Al
9.2
46
PET
PEDOT:PSS
–
–
PET/PEDOT:PSS/ CH3NH3PbI3-xClx/PTCDI/ Cr2O3/Cr/Au
12
47
Cellulose paper
MoOx/Au/MoOx
62.5 (top electrode)
9
paper/Au/SnO2/meso-TiO 2/CH3NH3PbI3/Spiro-OMeTAD/ MoOx/Au/MoOx
2.7
38
They proposed that the coating quality on the substrates mainly depends on the surface wettability, spreadability, and adhesion. In particular, when polymer substrates are used, the low surface energy of polymer substrates gives rise to poor coating or printing quality, requiring additional surface treatment before processing. Such additional processes generally increase the cycle time and production costs. The coating quality generally depends on the surface tension of the bottom layer. Therefore, a deep understanding of the relationship between process parameters and printing quality is required for the fabrication of high-performance F-PSCs with large areas. Conformal coating or printing without pinholes on floppy substrates is crucial to the performance of F-PSCs. In particular, the control of the crystallization kinetics of the perovskite layer should be uniformly controlled over a large area during solvent evaporation. Moreover, the profiles of plastic substrates as supporting layers generally remain intact, even after multiple coatings of upper layers. Therefore, the planarization for uniform coating is important. In addition, accumulated internal stress or strain is generated when the mechanical and thermal shocks are applied to the films, leading to delamination or debonding. This is more severe for large-area devices. High Moisture Permeability of Plastic Flexible Substrates Although the light absorption ability of organic-inorganic hybrid perovskite is excellent, the ionic nature of perovskite crystals makes them inherently vulnerable to moisture as well as oxygen, resulting in the degradation and phase transition of the perovskite. To avoid such deterioration, the substrate requires complete
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Figure 3. Schematic Illustration of Contact (Upper Three Figures) and Noncontact (Lower Three Figures) Methods for Perovskite Layer Coating on a Flexible Substrate
protection from the intrusion of moisture. By contrast, glass substrates can block the penetration of moisture through the substrates. However, F-PSCs with polymer substrates have severe stability problems because of the much higher water vapor transmission rate (WVTR) or oxygen transmission rate (OTR) of polymeric substrates. The WVTR and OTR of polymer materials are generally approximately 1–10 g/(m 2day) and about 1–10 mL/(m2day), requiring additional lamination or encapsulation to achieve those values in the range of 106. The reasons for the poor barrier stability and higher gas permeability are related to the morphology of polymers in the films. The microstructure of polymers can be divided into a crystalline part and an amorphous structure, depending on whether the arrangements of the polymers are regular or irregular. PET and PEN, which are widely used as substrates, are semi-crystalline polymers in which crystalline and amorphous are mixed, thus easing penetration of gas or moisture molecules. However, inorganic materials, such as glass SiO2, are regularly arranged in an atomic arrangement. As a result, such a dense structure of inorganic materials effectively suppresses the penetration of the molecules, such as moisture or oxygen. The factors affecting the moisture permeability are (1) film thickness, (2) film density, and (3) crystallinity.53 Gas molecules diffuse through the nanoscale or micron-scale pores, called ‘‘free volume,’’ in the polymers. There are three stages of penetration of gas molecules: (1) polymer adsorption of penetrating molecules, (2) diffusion due to the difference in concentration of the molecules inside and outside the polymer, and (3) desorption from the polymer film. These three series of steps are continuous, especially water molecules penetrating the interior. The thermal motion of polymer segments facilitates this penetration. Considering these sequential steps, the thicker the polymer substrate, the longer the diffusion distance of the molecules becomes, resulting in lower moisture permeability. However, because the gas transmittance decreases as the thickness increases, it is important to select an appropriate thickness of polymer substrate because of the trade-off relationship with optical transmittance. Additionally, as the density of the film increases, the size of the free volume pore becomes smaller, so diffusion becomes difficult, and the moisture permeability becomes lower. Finally, the free volume occupies the amorphous region rather than the crystalline portion, so the moisture permeability decreases as the crystalline part
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increases. Naturally, as the amorphous area increases, the free volume and moisture permeability increase. To relieve concerns about the stability issue, flexible film packaging employing a gas-barrier encapsulating function should be developed for F-PSCs. The encapsulating package includes flexible front/back sheets, flexible encapsulant, edge seal, and electrical joints. A typical configuration of flexible packaging for F-PSCs is a transparent front sheet/encapsulant/F-PSC/encapsulant/back sheet.54 In addition to the gas-barrier property, the materials embedded in barrier films should exhibit a relatively higher dielectric constant, optical transparency, thermal and UV stabilities, good adhesion, chemical inertness, processability, and mechanical strength with scratch resistance. Organic encapsulating materials are a good candidate to meet most of the requirements above, except gas barrier properties. Recently, organic-inorganic multilayer films have been commonly used to obtain better barrier properties.55 For further performance development of barrier film, the encapsulation and packaging technology developed in organic light-emitting diodes (OLEDs) that requires a WVTR of 106 g/(m2day) would provide deep insight, because OLEDs require a higher WVTR of 106 g/(m2day) than solar cells. Challenges in Super Flexibility The excellent mechanical properties of organic-inorganic hybrid perovskite materials make the possibility of flexible solar cells promising. Ko et al.9 reported the mechanical properties of all layers of organic-inorganic hybrid F-PSCs using nanoindentation and theoretical analyses. The Young’s modulus (E) of the hybrid perovskite layer was noticeably lower (approximately one-tenth) than that of metal-oxide-based perovskite, thus leading to the higher flexibility of the device. The finite-element analysis demonstrated that significantly small plastic strain was generated over a small region within the F-PSCs under bending. In terms of microstructure, such exceptional flexibility is thought to originate from the weak van der Waals bonding caused by extended alkyl chains (R) in R-NH3+ between perovskite layers, as well as a hydrogen/ionic bonding of NH3+ within the perovskite sheets. On the other hand, Jing Feng reported the elastic and anisotropic properties of CH3NH3BX3 (B = Sn, PB; X = Br, I),56 suggesting that the elastic properties are not determined by the interaction among organic and inorganic ions, but by the type and strength of the metal–halogen (B–X) bond. The ratio of the bulk modulus (B) to the shear modulus (G), or B/G would be a good index for the evaluation of the ductile ability of materials. The hybrid perovskite materials have a relatively lower shear modulus, implying a larger B/G with high ductility, which would be desirable as the absorbing layer of flexible solar cells. In addition, the Poisson ratio (n) is another parameter to determine the softness of materials. Taking into the consideration that n values for hybrid organic-inorganic perovskites and ductile solids are greater than 0.3 and 0.26, respectively, it is conjectured that the hybrid perovskite would be adequately soft as a photon absorber on the polymer substrates. Further studies on the molecular origin of super-flexibility of perovskite materials are necessary. For the evaluation of flexibility, bending tests are widely used for flexible electronics. During bending, the outer and inner surfaces experience tensile and compression stress, respectively. The plane inside the device, where no uniaxial stress is applied by balancing the tensile and compression stress, is defined as a neutral plane. If the neutral plane is located at the most fragile layer, the mechanical durability is sustained. On the other hand, the relationship between the film strain and the radius of bending curvature is expressed by e = d/2r, where e, d, and r are the strain, device
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thickness, and radius of curvature (bending radius), respectively. According to this relation, the thinner devices could ensure the bending stability, because a relatively smaller strain would be applied to the devices. In this regard, the hybrid perovskites would be ideal photo-absorbing materials because of the high absorption coefficient and intrinsic soft character, enabling thinner devices. However, ITOs coated on polymer substrates do not have high bending durability compared with perovskite because of the ITO’s small Poisson ratio and large modulus. Thus, during repeated bending tests, cracks occur in the ITO layer, inducing degradations in the perovskite layer. Therefore, for the realization of super F-PSCs, strategies to utilize new TCO materials with a ductile nature should be introduced. Recent Efforts on Improving Performance of Flexible Perovskite Solar Cells Because of the restriction on process temperature for fabricating transparent conducting oxide, ETL and HTL, and perovskite absorber, the PCE of flexible cells is not that much higher than those of glass-based rigid cells because of high series resistance and facilitated recombination of charges that are inherent issues for F-PSCs. Also, the employment of brittle TCOs like ITO and aluminum-doped ZnO (AZO) may not enable enough flexibility to PSCs. In this section, recent efforts on improving flexibility as well as enhancing charge collection for high efficiency and flexibility are discussed. The Exploitation of New Flexible TCO Substrates ITO or FTO has been abundantly utilized in both rigid and F-PSCs. However, from the viewpoint of cost-effectiveness, photovoltaic performance, and flexibility, these TCO materials may not be the best way to achieve a goal. Efforts have been made to replace these TCO materials with new compositional TCO materials. Kim et al.57 exploited new TCO materials that are composed of W-doped ITO (IWO) for F-PSC. These IWO materials, prepared by plasma arc ion plating method, were analyzed to possess high mobility (61.7 cm2/Vs) and high transmittance of 96% at 550 nm. Also, work function for IWO was 4.85 eV, much higher than that for ITO (4.65 eV). This high work function may enhance Voc. To overcome the brittleness of TCO, AZO/Ag/AZO, that is, double-sandwich structure, was provided. This new type of TCO showed fairly low sheet resistance of 7.5 U/sq and the resultant flexible cell maintained 98.5% of initial PCE after 50 cycles of bending test at 2.75 cm radius of curvature.10 So far, comparative studies of photovoltaic performance of PSCs, depending on various TCO materials, have not been reported, which is of great importance to decide the right TCO materials for F-PSCs. Dou et al.43 performed a comparative study regarding the influence of TCO materials on the photovoltaic performance of PSCs (Figure 4). They demonstrated that the F-PSCs employing the In-doped ZnO (IZO) achieved the highest efficiency of 18.1% due to the high Voc and FF. The other TCO materials, such as ITO and AZO, showed relatively inferior PSC. Especially, the AZO materials showed the lowest PCE of 12.2% due to the chemical reaction between AZO and perovskite layer, verified by analyzing surface stoichiometry and change in photoluminescence spectra of perovskite materials.43 Although ITO films have the same composition, the photovoltaic performance of F-PSCs may be different depending on the fabrication method of ITO. Kim et al.58 compared the cell performance containing ITO films prepared by (1) ion-plated process and (2) sputtering process. The ion-planted ITO film showed better electrical and optical properties such as sheet resistance and optical transmittance in comparison with the ITO films prepared by a sputtering process. The resultant PCE of PSC based on ion-planted ITO was measured to be 15.4%, which implies that the
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Figure 4. The Influence of TCO Materials on the Photovoltaic Performance of F-PSCs (A) Schematic representation of device architecture for the F-PSCs, where different transparent conducting oxide (TCO), AZO (aluminum-doped zinc oxide), ITO (tin-doped indium oxide), and amorphous IZO (indium-zinc oxide) were investigated. X-ray diffraction (XRD) patterns and transmittance of AZO, ITO, and IZO are shown on the left. Current density-voltage (J–V) characteristics (reverse scans) under 100 mW/cm 2 AM1.5G for champion Cs0.04 MA 0.16 FA 0.80 Pb 1.04 I 2.6 Br 0.48 perovskite devices with (B) willow glass-AZO, (C) willow glass-ITO, and (D) willow glass-IZO. Reprinted with permission from Dou et al. 43 Copyright 2017 American Chemical Society.
photovoltaic performance of F-PSC may change according to the electrical and optical quality of TCO, which may be determined by preparation method.58 Metal foils have been one of the promising candidates as a flexible electrode owing to high flexibility, high electrical conductivity, and a wide process temperature window. In particular, Ti metal foils have been abundantly utilized as a flexible substrate due to easy formation of TiO2 ETL. An attempt to replace Ti with cheap Cu foil has been made.59 Cu foil is capable of simply forming CuI HTL. Although many efforts have been made, it is challenging to achieve a high efficiency due to the inherent optical reflectivity of metal, however. Many studies have focused on the exploitation of top electrodes that possess a high conductivity and high optical transmittance. Ag metal film has been well utilized as a translucent top electrode. Since the translucent Ag metal itself does not show high electrical conductivity as well as high optical transmittance, the modification of Ag film has been proposed.39,41,60 For example, Ag nanowire electrodes showed the improved PCE of 7.45% compared to the PCE of bare Ag electrode cells (6.15%). The F-PSCs employing 13 nm thick Ag layer coated with amorphous ITO (a-ITO) exhibited 11.01%. Another Ni mesh metal grid was utilized to replace Ag materials.61 In this study, a transparent conductive adhesive (TCA) was adopted to reduce interfacial resistance between the metal grid and
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Figure 5. Metal-Polymer Hybrid Transparent Flexible Electrode (A) Image of the large-area flexible PET substrate with embedded Ag-mesh (FEAM) substrate. (B) Diagram for the hybrid electrode (PET–Ag-mesh–PH1000). (C) The structure of the FEAMs substrate with detail parameters. (D) J–V curves in reverse and forward scan measured under 100 mW/cm2 AM 1.5G illumination and dark for the champion flexible PET–Ag-mesh–PH1000–PEDOT:PSS–MAPbI 3 –PCBM–Al solar cell: inset shows photograph of corresponding ultra-thin F-PSCs. Reprinted with permission from Li et al. 67 Copyright 2016 Springer Nature.
hole-conducting layer, which yielded 10.3% of PCE. Carbon-based nanomaterials such as carbon nanotube (CNT) and graphene are other good candidates for transparent top electrode materials in Ti foil-based F-PSCs.40,62 Especially, the anodized Ti-metal-based PSC employing a graphene top electrode exhibited a PCE of 15%.62 Instead of metal-foil substrate, flexible CNT or graphene materials that are directly deposited on plastic substrates may be one of the alternatives to brittle TCO materials.63 Liu et al.64 adopted a graphene layer instead of TCO and achieved a super-light flexible solar cell with a power density of 5.07 W/g. Efficiency of graphenebased F-PSCs has been significantly improved by combining with PEDOT:PSS layer and incorporating dopants such as AuCl3 and MoO3 into graphene.37,45 The F-PSC containing AuCl3-doped graphene layer with PEDOT:PSS showed a high PCE of 17.4%.37 CNTs have been also utilized as flexible transparent electrode materials. Although the photovoltaic performance of CNT-based cells is inferior to that of graphene-based cells, the superior mechanical stability and large-scale fabrication possibility make it appropriate for commercialization of F-PSC.65 Silver nanowire (Ag NW) or mesh has been well investigated as transparent electrode materials in F-PSCs owing to high figure of merit, relatively good chemical stability, and bending durability. These materials are usually combined together with PEDOT:PSS to enhance the electrical conductivity of the transparent electrode. The Ag NWs/PEDOT:PSS electrode-based cell exhibited 11% PCE and the efficiency was retained after 10,000 cycles of bending test under 5 mm of a radius of curvature.66 Moreover, to overcome the difference in work function between Ag and PEDOT:PSS, polymer or graphene oxide (GO) interlayers were inserted.67,68 The PCE of Ag-mesh/PH10000/PEDOT:PSS based cell was 14% achieved by Li et al.67 (Figure 5). This cell retained 95.4% of initial PCE after 5,000 bending cycles under radius curvature of 5 mm.
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Advanced Electron Transport and Hole Transport Layer for Flexible Cells In n-i-p structure flexible cell, low-temperature formation of an ETL such as ZnO, TiO2, and SnO2 layer on plastic-based TCO substrate is challenging. To overcome inferior electron-transport properties of low-temperature-treated ETL, interface modification method with C60 or graphene and new composition oxide ETL has been exploited. Also, various carbon-based materials have been used as ETL layers. ZnO compared with TiO2 is relatively suitable for the low-temperature process.69 Also, ZnO possesses better electrical conductivity and carrier lifetime in comparison with TiO2, which led to a high PCE of 15.6% in F-PSCs.69 TiO2 ETLs have been well studied in F-PSCs. There have been mostly two ways to form TiO2 ETL in a flexible plastic substrate. First, low-temperature deposition using vacuum-based thin-film deposition systems such as the evaporator, sputter, and atomic layer deposition (ALD); second, deposition of well-crystallized TiO2 colloidal particles and subsequent treatment such as UV treatment or low-temperature annealing treatment.70–72 The RF-sputtering method for deposition of TiO2 layer has been well known. Usually, amorphous TiO2 layer can be obtained using the low-temperature sputtering method. Yang et al.73 compared carrier transport, recombination, and lifetime characteristics of both amorphous and anatase TiO2 layer. Fermi level of amorphous TiO2 was 4.15 eV, slightly lower than that of anatase TiO2 (4.01 eV). This leads to superior electron-transport property of an amorphous layer, achieving a PCE of 15.07% for F-PSC.73 The PEALD method is favorable for precise control of thickness and reducing oxygen vacancies in TiO2 ETL. Kim et al.6 realized F-PSC with a PCE of 12.2% by fabricating TiO2 ETL with the aid of low-temperature PEALD process, as shown in Figure 6. The charge transport properties of TiO2 ETL prepared by PEALD are enhanced by interface modification with PCBM, which yields 17.7% PCE.74 Moreover, Giacomo et al.72 were able to fabricate F-PSC modules by combining TiO2 ETL prepared by PEALD and mesoscopic TiO2 layer. TiO2 layer can be deposited by electron-beam evaporation and the resultant cell efficiency was reported to be 13.5%.75 Highly crystalline anatase nanoparticles may be directly deposited as ETL. However, the ligand or residual organics attached on the surface of TiO2 would be detrimental to electron transport in the ETL. Therefore, further low-temperature treatment or UV treatment is needed to achieve a high PCE. So far, UV treatment is more efficient in removing organics and attaching each TiO2 particles than low-temperature treatment (150 C).70,71 The flexible cell employing the low-temperature UV-treated Nb:TiO2 ETL exhibited 16.01% PCE due to the enhanced electron transport. Since the stability and hysteresis issues of TiO2 materials, SnO2 has been conceived as another alternative to ETL materials due to its enhanced charge transport property.76 Usually, SnO2 layer has been prepared by a solution coating method that contains precursor solution coating and subsequent heat treatment. However, this heat-treatment process is not suitable for obtaining SnO2 with good crystal quality in plastic-based flexible cells due to limited heat-treatment temperature. Plasma treatment may be a good method to overcome the temperature issue. N2 plasma treatment broke alkoxy and hydroxy group to form Sn-O bonding at a low temperature, as shown in Figure 7. The resultant PCE of F-PSC was 18.1%.77 The employment of highly crystalline SnO2 colloids is another way to achieve high efficiency. Park et al.78 synthesized highly crystalline and dispersed SnO2 colloids. This SnO2
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Figure 6. Amorphous TiO2 ETL for F-PSCs Using PEALD at the Low-Temperature Process (A) The structure and best performing device of F-PSCs on ITO–PEN, with a TiO 2 ETL by using PEALD. Cross-sectional SEM image of the inorganic-organic halide perovskite planar heterojunction flexible solar cell and schematic of the flexible device structure. J–V curve measured under the simulated solar light for the best performing PEN–ITO–TiO x –CH3 NH3 PbI 3-x )Clx –spiroMeOTAD–Ag flexible device. Reprinted with permission from Kim et al. 6 Copyright 2015 Royal Society of Chemistry. (B) F-PSCs with hybrid double ETL (PCBM and TiO2 prepared by PEALD). A cross-sectional TEM image of the as-prepared a-TiO2 –PCBM ETL.-based PSCs. A schematic representation and J–V curve of an a-TiO 2 –PCBM ETL-based PSC on the ITO-PEN substrate. Reprinted with permission from Kim et al. 74 Copyright 2018 John Wiley and Sons.
colloid ETL exhibited excellent charge-transport property, which enabled a PCE of 17.7%. PEALD-synthesized ETL shows also the excellent photovoltaic performance with a PCE of 18.36%.21 In addition, the low-temperature-processed EDTA (ethylene diamine tetraacetic acid)-complexed SnO2-based F-PSC achieved a PCE over 18.4%.79 To enhance the charge-transport property of SnO2 ETL, doping metal ions or mixing graphene oxide have been attempted.80,81 New composition ETLs have been exploited to enhance charge transport as well as to reduce the hysteresis phenomenon in F-PSC. Zn2SnO4 (ZSO) is a good example, possessing better optical transmittance than SnO2.82 The PCE of ZSO-based F-PSCs has been improved by tailoring particle size and stacking structure, which leads to yielding 16.5% PCE.83 Also, the combination of PCBM with ZSO was reported to facilitate electron transport in F-PSC.84 Nb-doped WOx (W(Nb)Ox) was used as ETL. By controlling the thickness of W(Nb)Ox, PCE and hysteresis properties were improved.85 Recently, fairly interesting ETL material that is nanocrystalline titanium metal-organic frameworks (nTi-MOF) was proposed. The addition of PCBM increased the electrical conductivity and enhanced the charge injection properties of nTi-MOF. The flexible cell employing nTi-MOF/PCBM demonstrated a PCE of 17.43%.86 Large bandgap materials such as Nb2O5 and Al2O3 were utilized as ETLs. The electron transport from the perovskite layer to TCO was enabled by quantum tunneling effect.87,88 However, the inherent high series resistance is still challenging to achieve high efficiency. As an alternative, electrically conductive organic materials, including the modified C60 and PCBM, have been utilized as ETLs. Stearyl dimethyl benzyl ammonium chloride (SDBAC)-doped [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) was used as ETL wherein SDBAC enhanced the conductivity and therefore
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Figure 7. New Composition ETLs (A) Schematic representation of the SnO 2 thin-film fabrication methodology using a modified sol–gel technique by employing low-power RF N 2 plasma exposure. Cross sectional SEM image of PET–ITO–SnO 2 –m-Al2 O 3 – ((FA 0.83 MA 0.17 ) 0 . 95 Cs 0 . 05 PbI 2 . 5 Br 0 . 5 ) –spiro-OMeTAD–Au flexible device and architecture schematic of F-PSC. Reprinted with permission from Subbiah et al. 77 Copyright 2018 American Chemical Society. (B) A uniform SnO 2 QD thin film by using ligand-capped ultrafine SnO 2 quantum dots. Reprinted with permission from Park et al. 78 Copyright 2018 American Chemical Society. (C) Schematic of the synthetic protocol for nTi-MOF. The successive linked TiO2 clusters with BDC molecules create small nanocrystals. Device structure of a F-PSC incorporating nTi-MOF/PCBM. The best performing J-V curve of the flexible nTi-MOF–PCBM PSCs. Reprinted with permission from Ryu et al. 86 Copyright 2018 American Chemical Society.
enhanced the PCE to 11.8%. The polyallylamine-coupled C60 showed an excellent charge extraction and the increased Voc owing to work function shift, thereby realizing a highly efficient F-PSC with a PCE of 15.2%.89 Yang et al.90 used solid-state ionic liquids (ss-IL) for ETL. The large bandgap, anti-reflection, high electron mobility, and proper work function for ss-IL ETL enabled a high efficiency of 16.09%. Moreover, the ss-IL ETL reduced the electron trap-state density of the perovskite layer, leading to effectively suppressed hysteresis. In p-i-n structure, HTL has been actively studied because HTL requires high optical transmittance, bending durability, hole injection, and hole-transport properties in p-i-n F-PSC. In this regard, PEDOT:PSS have been proposed as suitable HTL materials.47 However, the relatively poor charge transport in the out of plane direction due to its lamellar structure, the potential energy loss at the interface between PEDOT:PSS and perovskite layer due to low work function, and corrosion capability to the adjacent layers due to its acidity are critical issues.91 Therefore, new organic HTL materials such as 1,4-bis(4-sulfonatobutoxy)benzene and thiophene moieties (PhNa-1T) and (N-(4-(9H-carbazol-9-yl)phenyl)-7-(4-(bis(4-methoxyphenyl)amino) phenyl)-N-(7-(4-(bis(4-methoxyphenyl)amino)phenyl)-9,9-dioctyl-9H-fluoren-2-yl)9,9-dioctyl-9H-fluoren-2-amine (CzPAF-TPA)) have been exploited.91,92 For example, the F-PSC using PhNa-1T as HTL showed significantly higher 14.7% PCE in comparison
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with the PEDOT:PSS-based one (8.4%), which was ascribed to the decreased potential energy loss at the interface between perovskite and HTL as well as to efficient hole injection.91 Since the organic HTLs exhibit relatively low stability compared with inorganic HTLs, research on developing new HTLs has been shifted to exploit new inorganic HTLs. In particular, NiOx materials have received a great deal of attention due to the well-aligned band structure with perovskite layer, good optical transparency, and electron blocking capability.91 The NiOx HTLs have been deposited by vacuum thin-film fabrication tools such as ALD, RF-sputtering, and evaporation.93–95 However, in F-PSC for potential roll to roll process, low-temperature solution coating has been under active research.96–98 Najafi et al.98 achieved 16.6% PCE in F-PSC, which exhibited fairly good stability. Over 85% of the maximum stabilized output efficiency was retained after 1,000 h aging employing a thin MAPbI3 perovskite.98 Also, NiOx derivatives have been studied to improve photovoltaic performance.99–101 Cu-doped NiOx (Cu:NiOx) HTL-based F-PSC demonstrated 17.16% because of the improved charge injection and the improved surface morphology.100 Noticeably, the hybrid HTL composed of PEDOT:PSS–NiOx made it possible to achieve 18.0% PCE in F-PSC.101 Cu-based HTL materials such as CuyCrzO2 and CuSCN have been introduced.102,103 However, these materials are fairly challenging due to the problem of formation of CuI coming from inter-diffusion of Cu and In ions.102,103 In p-i-n structure, n-type ETL needs efficient electron extraction, full coverage on the perovskite layer, no damage on perovskite layer during the coating process, and no chemical reaction between ETL and perovskite under operating conditions.104 In this regard, fullerene-based ETL such as PCBM has been the most popular in p-i-n structured PSC. However, the wettability of PCBM on the perovskite layer is not better than polymer-based ETL, which may induce the deterioration of long-term stability related to insufficient coverage of PCBM. Therefore, polymeric ETLs, including the naphthalene diimide (NDI)-based polymer with strong electron withdrawing dicyanothiophene (P(NDI2DT-TTCN)) and triphenylamine (TPA)-(3-cyano-4,5,5-trimethyl-2(5H)-furanylidene)malononitrile (3CN), have been developed in F-PSC. Representatively, the flexible cell employing the P(NDI2DT-TTCN) as ETL achieved 17.0% PCE with good long-term stability owing to the improved charge extraction ability. Also, the mechanical durability of this material was better than PCBM.104 High-Quality Perovskite Film for Flexible Cells The formation of high-quality perovskite film with large grain size, low trap density, high coverage, and high crystallinity, etc., is one of the major challenges for high-efficiency F-PSCs on the plastic substrate. In general, perovskite film exhibits poor film quality on the flexible substrate, because of the low surface energy, poor wettability, and wrinkle of the plastic substrate. Therefore, it is necessary to develop a suitable perovskite precursor for the same perovskite film quality on a flexible substrate, compared with high-quality perovskite film on a rigid substrate. Bi et al.105 investigated the various ratios of FAI:MABr concentration in perovskite precursors, the optimized composition of which exhibits the long recombination lifetime, small trap state density, and pin-hole-free film, resulting in 18.1% of PCE in F-PSCs. One of the strategies for high-quality perovskite film is to add the additives in the perovskite precursor. H2O is critical to controlling the crystallization rate and grain size, which achieved the 13.11% PCE (Figure 8A).106 Moreover, the dual additives of DIO and H2O in perovskite precursors can induce gradient evaporation during
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Figure 8. Additive Induced High-Quality Perovskite Film (A) Schematic illustration of the different morphology development for CH 3 NH 3 PbI 3x Cl x films with and without H 2 O additive. Reprinted with permission from Du et al. 106 Copyright 2017 American Chemical Society. (B) Illustration of the F-PSC structure and J–V curves of F-PSCs under both reverse and forward scan directions using MAPbI 3 –DS (dimethyl sulfide) as the absorber layer. Reprinted with permission from Feng et al. 107 Copyright 2018 John Wiley and Sons. (C) Cross-sectional SEM image and J-V curves of a F-PSC with the Rb1 K 4 CsFAMA absorber. Reprinted with permission from Cao et al. 17 Copyright 2019 Royal Society of Chemistry.
the drying and heat process. This leads to high-quality perovskite film with large grain and uniformity, achieving a PCE of 18.0% for F-PSC.101 A dimethyl sulfide (DS) additive has been successfully developed as it chelates with Pb2+ to form an intermediate complex and slows down the crystallization rate of perovskite film. The resultant PCE of F-PSC was 18.4% with good mechanical tolerance (Figure 8B).107 Recently, a quintuple-cation perovskite film containing Rb+, K+, Cs+, FA+, and MA+ was proposed to reduce the trap density in the perovskite film, which a champion PCE for F-PSC of 19.11% was achieved with less hysteresis (Figure 8C).17 Ultra-Flexible Perovskite Solar Cells F-PSCs can be potentially used as a flexible power source for mobile devices as well as a portable outdoor electricity generator. For example, the recent advent of foldable smartphones is expected to open a new era of flexible electronics. In foldable devices, there are two types of folding modes: in-folding and out-folding. In-folding mode requires an extremely small bending radius of curvature like 2 mm, which is fairly challenging in foldable devices composed of inorganic electrical components. Also, many F-PSCs contain inorganic materials including TCO, ETLs, and HTL that are not favorable for reliable flexibility. The F-PSC adopting PEN/Sn-doped indium oxide (ITO) showed 1,000 times bending durability at the radius of curvature of 10 mm. However, at the radius of curvature of 4 mm, the cell performance significantly deteriorated because of the fracture of ITO layer (Figure 9).6 Efforts have been made to improve the bending reliability of F-PSCs by replacing or modifying inorganic TCO and substrate materials. Nanostructured ITO demonstrated the improved flexibility of PSCs. The inverted nanocone ITO showed
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Figure 9. Highly Bending and Durable F-PSCs (A) Normalized PCE measured after bending the substrate within a specified radius of 400 mm to 1 mm and normalized PCE as a function of bending cycles with different radii of 400 nm, 10 nm, and 4 mm. The real images attached on the human neck, wrist, and finger corresponding to 400 nm, 10 nm and 4 nm bending radii, respectively (inset within upper figure). Real images taken during the bending tests (inset within lower figure). Reprinted with permission from Kim et al. 6 Copyright 2015 Royal Society of Chemistry. (B) Device structure of Gr-Mo/PEN based F-PSC and Normalized PCEs as a Function of Bending Cycles at a Fixed Bending Radius of 4 mm for the Gr-Mo–PEN and ITO–PEN devices. The inset photograph shows the Gr-Mo–PEN device bent at a bending radius. Reprinted with permission from Yoon et al. 45 Copyright 2017 Royal Society of Chemistry.
improved bending durability compared with the planar ITO based cell. The F-PSC employing inverted cone retained 95% of the initial PCE after 200 cycles of bending test under a 6 mm radius of curvature, which was ascribed to strain relaxing effect of nanocone structure.108 Metal nanowires possess excellent flexibility, which enables to replace the relatively brittle TCO. However, the roughness and adhesion issues remain. These issues have been resolved by combining an oxide layer or hybrid layer.109,110 The composite of crystalline ITO metal nanowire-GFRHybrimers is one of the representative examples. The flexible cell containing this nanocomposite showed excellent bending durability. Under 2.5 mm radius of curvature, this cell showed 20% degradation to initial PCE after 500 cycles of bending test.110 The PEDOT:PSS system has been frequently utilized for realizing ultra-F-PSCs owing to its flexible nature of the polymer. The F-PSCs employing PEDOT:PSS materials do not possess any cracks from the fracture of PEDOT:PSS in contrast with ITO materials under a severe bending condition such as a 4 mm radius of curvature.111 The F-PSCs
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based on highly conductive nitric acid treated PEDOT:PSS with 50 nm thickness exhibited 90% of the initial efficiency after 1,000 cycles of bending test under 5 mm of the radius of curvature.112 Combination of Ag mesh and PEDOT:PSS enhances the electrical conductivity of the transparent electrode, which results in a PCE of 14%.67 Moreover, this flexible cell exhibits excellent bending durability, retaining over 95.4% of its initial PCE value even after 5,000 fully bending cycles under 5 mm radius of curvature. Carbon nanomaterials such as CNT and graphene have shown excellent flexibility with proper electrical conductivity compared with TCO. All-carbon-electrode-based F-PSC employing graphene as the transparent bottom electrode and CNT as the top electrode retains 84% of the initial PCE after 2,000 cycles of the bending test under a 4 mm radius of curvature.63 MoO3-decorated graphene-based F-PSC demonstrates super flexibility, i.e., only 10% degradation in PCE is observed after 1,000 cycles of the bending test at a radius of curvature = 2 mm. Moreover, this cell achieved 16.8% PCE, which exhibits the great potential for the highly efficient foldable photovoltaic cell.45 The flexibility of PSC has been improved by adopting a new type of flexible substrates with conducting layers such as Ti plate–TiO2 substrate.113 The highly bendable PSC containing TiO2 nanolayer-coated Ti plates does not show any degradation in PCE after 1,000 cycles bending test at 4 mm radius of curvature, which is explained in terms of high crystallinity with low oxygen vacancy that improves bending durability.113 Toward Commercialization Owing to abundant cheap precursor materials and low-cost processes, the perovskite solar module has been believed to be economically viable, which will enable it to be a leader in the future photovoltaic market. The manufacturing costs for the perovskite solar module has been expected to be one-third of those based on bulk silicon PV technologies.12 Moreover, the levelized cost of the electricity (LCOE) for perovskite solar modules could reach 6 US cents/KWh, comparable to the LCOE for the fossil fuels once the module efficiency and lifetime exceeded 12% and 15 years, respectively. High efficiency and bending durability for F-PSCs provide prolific application opportunities. These flexible solar cells have the capability of being installed on the curved surface, which is suitable for building integrated photovoltaics (BIPV). Moreover, lightweight with high efficiency enables to achieve a high-power density of 2 W/g,67 which makes them possible to be utilized as a power source in high-altitude unmanned aerial vehicles (HAUAVs). To commercialize this F-PSC, however, several issues remain: (1) developing uniformly large-area coating technology (Table 3), (2) ensuring long-term stability by encapsulation technology, and (3) exploiting emerging unique flexible cells for special applications need to be solved. Uniform and Large Area Coating Technologies for Flexible Modules Spin coating method has been adopted in the fabrication of small-sized cells, which is not suitable for obtaining uniform ETL, HTL, and perovskite light-absorbing layers due to deflection of large size flexible substrate. Rather, the flexible substrate makes it easy to realize mass production based on R2R process. In R2R, slot-die coating method has been utilized to fabricate large size F-PSC owing to facile mass production of thin films with uniform thickness. Research on slot-die R2R coating has focused on optimizing some process parameters to (1) improve the uniformity of each ETL, HTL, and Perovskite layer; (2) control crystallization kinetics of perovskite;
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Table 3. The Uniform and Large-Area Coating Methods for Perovskite Modules, and Comparison of Device Structure, Substrate Type, Active Area, and Performance of Perovskite Modules Scalable Coating Layer
Coating Method
Substrate Type
Device Structure
Active Area (cm2)
Jsc (mA/cm2)
Voc (V)
Fill Factor (%)
PCE (%)
Ref.
Perovskite
doctor-blade
rigid
glass/FTO/TiO2/perovskite/spiro
10.1
4.29
4.11
58.14
10.26
119
doctor-blade
rigid
glass/FTO/TiO2/ perovskite/P3HT
100
0.83
9.61
53.79
4.30
119
doctor-blade
rigid
glass/ITO/PTAA/perovskite/ fullerene (C60)/bathocuproine (BCP)
33
1.19
18.1
71.3
15.3
120
doctor-blade
rigid
glass/ITO/PTAA/perovskite/ fullerene (C60)/bathocuproine (BCP)
57.2
1.26
17.04
67.5
14.6
120
bar
rigid
glass/FTO/TiO2/ perovskite (double-layered)/P3HT
24.97
2.72
8.66
72.6
17.1
18
drop casting
rigid
glass/FTO/TiO2/ZrO2- perovskite/ carbon
31
4.9
3.72
57.5
10.46
121
drop casting
rigid
glass/FTO/TiO2/ZrO2- perovskite/ carbon
70
1.772
9.63
62.9
10.74
121
drop casting
rigid
glass/FTO/TiO2/ZrO2- perovskite/ carbon
49
2
9.3
56
10.4
122
soft-cover deposition
rigid
glass/FTO/TiO2/ perovskite/spiro
36.1
1.97
10.5
75.7
15.7
123
spray
rigid
glass/FTO/TiO2/ perovskite/PTAA
40
2.10
10.5
70.16
15.5
124
ETL or HTL
slot-die
rigid
glass/ITO/TiO2/perovskite/spiro
168.75
0.692
21.2
67.9
10
125
spin
rigid
glass/ITO/TiO2/perovskite/spiro
4
19.9
0.91
75
13.6
126
spin
rigid
glass/ITO/PEDOT/perovskite/PCBM
60
1.9
8.1
57
8.7
127
spin
rigid
glass/ITO/PEDOT/perovskite/PCBM
40
2.0
10.1
63.7
12.9
128
spin
rigid
glass/ITO/PTAA/PFN/perovskite/ PCBM/PFN/Ag
9.06
16.86
3.27
73
14.1
129
ALD + screen printing
flexible
PET/ITO/ALD-TiO2/TiO2 scaffold/ perovskite/spiro-OMeTAD/Au
7.92
5.2
3.39
71
3.1
72
RF-sputtering + evaporation
flexible
AZO/LiF/C60/Perovskite/spiroOMeTAD/Au
10.2
2.1
7.3
67.5
10.5
130
slot-die
flexible
PET/ITO/SnO2/perovskite/spiroOMeTAD/Au
16.09
3.28
6.727
69
15.22
20
spin
flexible
PEN/ITO/MFGO/perovskite/ PC61BM/BCP/Ag
10
4.34
3.76
49.1
8.1
131
spin
flexible
PET/ITO/SnO2/meso-TiO2/ perovskite/spiro-OMeTAD/Au
12
3.12
5.014
55.9
8.8
132
spin
flexible
PET/ITO/SnO2/perovskite/spiroOMeTAD/Au
10
6.479
3.075
62
12.31
133
spin
rigid
glass/FTO/SnO2/perovskite/spiroOMeTAD/Au
25
1.98
11.2
69.0
15.3
134
spin
rigid
glass/FTO/SnO2/perovskite/spiroOMeTAD/Au
100
1.02
20.2
0.68
14.03
134
and (3) enhance the PCE of the flexible device (Figure 10).114 However, the research on manufacturing F-PSC using the slot-die method is at an early stage. This research is still ongoing for achieving the exploitation of commercially viable F-PSCs. Since slot-die coating of organic materials such as PEDOT:PSS and spiro-OMETAD is relatively favorable compared with inorganic ETL and perovskite materials, efforts have been made to fabricate uniform ETLs or perovskite layers.20,114–116 Bu et al.20 developed a strategy to form high quality SnO2 ETLs using the slot-die coating method. By modifying the SnO2 surface with KOH, the surface defects at the interface between SnO2 and perovskite layer were significantly reduced owing
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Figure 10. Photographic Images of the R2R-Coated Perovskite Layer and an Example of the Manufactured Flexible Devices Reprinted with permission from Galagan et al. 114 Copyright 2018 John Wiley and Sons.
to the formation of KBr, which resulted in improving efficiency and reducing hysteresis on F-PSCs. The efficiency of flexible perovskite module with a size of 5 3 6 cm2 was 15.22%. PEDOT:PSS as HTL in the p-i-n structure was coated using a slot-die R2R process. To improve film coverage and morphology of the perovskite layer, 3-aminopropanoic acid was adopted as a self-assembled monolayer containing a carboxyl group. This carboxyl group was found to act as nuclei for crystallization of perovskite, which resulted in a much smoother perovskite surface morphology together with a PCE enhancement.116 The feasibility of upscaling F-PSC to a width of 30 cm based on slot-die R2R coating was demonstrated by Galagan et al.114 They performed R2R deposition of ETL and perovskite layers at the web speed of 3–5 m/min with nontoxic solvents, which showed a potential for future R2R manufacturing and commercialization of F-PSCs. Beside the slot-die R2R coating method, solution-shearing and spray-coating methods have been introduced. Crystal nucleation and growth in large-area MAPbI3 thin films was confined by using solution shearing, which yielded near single-crystalline perovskite microarrays with a high degree of controlled macroscopic alignment and crystal orientation and consequently exhibited significant improvements in optical and optoelectronic properties.117 Also, TiO2 ETL and perovskite layers with high uniformity, crystallinity, and surface coverage were obtained by using an ultrasonic spray-coating (USC) method.118 The TiO2 ETL deposited by USC was cured by high-density pulsed beam of infrared light, realizing F-PSCs with a PCE of 8.1% that are robust under mechanical stress. Recent Encapsulation Technologies for Long-Term Stability Although remarkable advances in the efficiency of F-PSC have been achieved, the toxicity of the water-soluble lead compound and the poor stability are big challenges for commercialization. Encapsulation technology is able to block the emission of lead as well as extend lifetime of PSC by prohibiting exposure to ambient air containing moisture and oxygen.
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Weerasinghe et al.135 demonstrated that encapsulation of PSC devices using high-quality flexible barrier materials and simple encapsulation architectures can significantly improve long-term stability under ambient storage conditions. Also, the Ca film tests revealed that the ingress of moisture and oxygen through the adhesive layers and around electrical wire contacts is a significant lifetime-limiting factor, which highlights the importance of developing new encapsulation architectures. The combination of polyurethane resin encapsulation and Cr2O3–Cr interlayer demonstrated the improved air stability of ultra-thin F-PSCs. Especially, the employment of a chromium oxide–chromium interlayer was found to effectively protect the metal top contacts from reactions with the perovskite, which was beneficial for extending the long-term stability.7 Functional encapsulating materials possessing their own unique properties have been introduced. Nanocone PDMS, which possesses antireflection and water-repellent functions, was attached to the front side of the flexible substrate. These nanocone PDMS encapsulation materials contributed to improving the optical transmittance as well as to achieving the water-repelling effect (Figure 11A).136 The multilayer encapsulating film consisting of ultra hydrophobic and relatively hydrophilic layers significantly enhanced the stability of PSCs under very humid conditions.137,138 The ultra-hydrophobic film containing poly(methyl methacrylate) (PMMA), polyurethane (PU), and SiO2 nanoparticles in contact with environments served to repel water in humid environments and prevent water permeation into the vulnerable perovskite layer, as shown in Figure 11B.138 The moderately hydrophilic PMMA layer acted as a desiccant to extract residual water from the perovskite layer itself during the solar cell operation. The dual function of the coating film retained the PCE of PSCs at 17.3% for 180 min when exposed to over 95% humidity. Emerging Flexible Photovoltaics for Pioneering a New Market The exploitation of new concept F-PSCs can extend applicability to various devices or systems, which facilitate opening and cornering an emerging photovoltaic market. Since the manufacturing process of PSCs is relatively simple and cost effective compared with other photovoltaic cells, the realization of new concept solar cells using perovskite absorber is viable. The F-PSC is capable of achieving high power-per-weight if the ultra-thin and light plastic substrate is employed. Kaltenbrunner et al.7 used 1.4 mm PET film as substrate and achieved a power-per-weight of 23 W/g, which is a fairly outstanding result because the power-per-weight of silicon solar cells is 1 W/g (Figure 12A). Potential future applications are power sources for UAV for environmental and industrial monitoring, rescue and emergency response, and tactical security applications.7 On the other hand, Lucarelli et al.139 reported the F-PSC that exhibits a PCE of 10.8% and 12.1% at low luminance of 200 lx and 400 lx, respectively. In typical office and building environments, indoor lighting could supply enough energy to drive low-power sensors equipped with F-PSC, implying another potential application of F-PSC. Once the F-PSC uses biocompatible textiles (Figure 12B),140 wire-shaped substrates,141 or cellulose papers38 instead of conventional plastic substrates, this cell can be used as reliable power sources for wearable electronics. The PSC was simply stacked onto a textile by using an elastomer with a good adhesive property. The PCE of textile-based F-PSC was 15% with improved ambient stability and washable
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Figure 11. Strategies for Resisting Water in Humid Environments (A) Schematic structure of the PSC device with nanocone PDMS film attached on the top and SEM image of PDMS nanocone with 1 mm pitch and 1 mm depth. Inset image is a drop of water on the surface of the nanocone PDMS layer, with AR 1.0 showing a large contact angle of 155 . Reprinted with permission from Tavakoli et al. 136 Copyright 2015 American Chemical Society. (B) SEM image for the cross-section of the PMMA-PU-SiO 2 –PMMA-PU double-layer coated PSCs. A change in PCE of the PSCs coated with the PMMA-PU-SiO 2 –PMMA-PU double layer in the ambient conditions of 100% humidity for 180 min. Schematic illustration of the passivation effect of the PMMA-PU-SiO 2 –PMMA-PU double layer. Reprinted with permission from Yoo et al. 138 Copyright 2017 Springer Nature.
capability owing to the encapsulation effect of the elastomer and demonstrating the promising potential for the wearable device applications.140 Recently, Castro-Hermosa et al.38 reported the F-PSC fabricated on a paper substrate that exhibits a maximum PCE of 2.7% by adopting a top semitransparent electrode. Semitransparent F-PSC possesses a wide variety of potential applications such as solar glass for buildings and solar curtains for upholstery, to wearable electronics and fashion (Figure 12C).112 The semitransparent F-PSC without TCO was realized by utilizing highly conductive n-PEDOT:PSS as both the bottom and top transparent electrodes. The n-PEDOT:PSS was stacked onto Spiro-OMeTAD by using a stamp-transfer process with PDMS. The resultant PCE was measured as 10.3% and maintained more than 90% of the initial value after 1,000 cycles of bending test at a radius of curvature of 5 mm.112 Recently, self-healing F-PSC based on self-repairing of electrical pathways in mechanically damaged metals has been exploited (Figure 12D).142 The composite of core-shell-structured liquid metal microcapsules (LMCs) synthesized via in situ
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Figure 12. Niche Applications of F-PSCs (A) Power sources for UAV. Reprinted with permission from Kaltenbrunner et al. 7 Copyright 2015 Springer Nature. (B) The biocompatible textile-based F-PSCs. Reprinted with permission from Lam et al. 140 Copyright 2017 Royal Society of Chemistry. (C) Semi-transparent PSCs by using stamp-transfer process with PDMS. Reprinted with permission from Zhang et al. 112 Copyright 2018 John Wiley and Sons. (D) A solar-powered smart watch embedded with self-healing conductors. Reprinted with permission from Chu et al. 142 Copyright 2018 John Wiley and Sons.
polymerizations of urea-formaldehyde onto liquid metal colloids passivated the metal top electrode. Once the LMC passivation film was ruptured by cutting or pressing, the LMC released and transported liquid metal to damaged sites, which led to recovering the broken electrical pathway. The average recovery ratio of the PCEs was revealed to be 99% after recovering the damaged sites, which demonstrates the great potential of self-repairing devices beneficial to mechanical issues related to the repeated deformation.142 Perspectives For the successful realization of flexible, foldable, and stretchable electronics for future portable devices, it is essential to develop a power supplying system that integrates all of them including the storage system together. Flexible solar cells are expected to offer those functions. Among them, F-PSCs have the advantages of both the high power of inorganic solar cells as well as the flexibility and light weight of organic solar cells, making them ideal power-generating systems. In this paper, we have summarized the fundamental challenges and recent efforts for the development of F-PSCs. As discussed, the reported PCEs of F-PSCs do not exceed those of the conventional rigid PSCs, mainly due to the limitations in processing and material selection. Nevertheless, the advantages over rigid PSCs are very clear, such as the
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Figure 13. Schematic of F-PSC Module with Monolithic Interconnection P1, P2, and P3 stand for first, second, and third laser scribing process. PET and c-TiO 2 represent poly(ethylene terephthalate) and compact-TiO 2 . ITO, c-TiO2 , and spiro-MeOTAD can be replaced by different TCO, oxide, and hole-transporting materials. Au can be also replaced by cheaper and more efficient metal electrode materials.
light weight and versatility of applications due to the customized and integrated devices. In this part, we emphasize the module technology for the future developments of F-PSCs. Although several papers have been published on the fabrication of largesized F-PSCs using printing technology, their direct application to actual operating devices has not been achieved. For the practical application of F-PSCs as power supplying devices for soft and conformal electronics such as those employed for the internet of things (IoT) devices and sensors, the module technology, which involves assembling multiple subcells, should be developed for boosting the power to levels that would meet the minimum input powers of related electronics and power storage systems. For this purpose, many subcells are generally interconnected in a monolithic form or a grid-type structure; the former is generally thought to be more proper for F-PSCs due to the overall coating process and compatibility of the layer materials.51 Through series interconnection, the parasitic resistive loss originating from the series and shunt resistances could be reduced when the solar cells are enlarged. Recently, there have been only a few reports on the F-PSC module.20,51,132,143 Bu et al.20 introduced a facile interface passivation strategy with potassium treatment for the SnO2 ETL and demonstrated the fabrication of an F-PSC module with a PCE of 15.22% and large size (5 3 6 cm2) using the slot-die printing method. The potassium treatments induced hysteresis-free, stable, and high PCE by facilitating the growth of perovskite grains. Li et al.132 demonstrated a sequential deposition method for the formation of pinhole-free CH3NH3PbI3 perovskite thin films for the fabrication of F-PSC modules (active area of 16 cm2) with PCE over 8%. Three scribing processes, referred to as P1, P2, and P3 scribing steps, are essential for the fabrication of the monolithic interconnected module.51 As depicted in Figure 13, the P1, P2, and P3 processes represent the isolation of the TCO between cells, selective removal of the ETL/perovskite layer/HTL for the connection from the top contact of the unit cells to the bottom TCO, and isolation of the top metal electrode between neighboring cells, respectively. The patterned area for P1, P2, and P3 belong to the dead area where power cannot be produced in the solar-cell module. Conventionally, nanosecond lasers or mechanical scribing tools are widely used for the P1, P2, and P3 processes. Rakocevic et al.144 compared mechanical and laser patterning technologies for the fabrication of glass-based PSC module. They found that the mechanical scribing is a reliable and relatively cost-effective patterning technique, whereas the laser ablation can provide greater variability with respect to substrates and layer materials. Further study is necessary to compare the scribing technologies for the plastic-based F-PSC. Recently, Dagar et al.132
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reported an F-PSC module with a 12 cm2 active area patterned with a laser that exhibits a PCE of 8.8%. This F-PSC module was designed using three scribing steps (P1, P2, and P3 scribing steps) based on an efficient, fully laser scribing procedure. For the F-PSCs, especially on ITO-polymers, patterned lines are not as thin, sharp, and clean as those for the glass-based PSCs, implying that the high performance of unit cells in F-PSCs might not be reproducible in modules with a large area. Therefore, the scribing adaptability, as well as the high performances and large area processability, should be considered together. Recently, intensive and extensive studies have been conducted on perovskite-based tandem solar cells, and a PCE of 28.0% was achieved for the PSC/silicon tandem cells.2 Owing to the considerable interest in PSC-based tandem devices, we predict that flexible tandem devices such as flexible PSC/CIGS or flexible PSC/organic photovoltaics (OPV) could be realized as future generation devices if new architectures, materials, and processes are developed.
ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Ministry of Science and ICT: NRF-2012M3A6A7054861, NRF2014M3A6A7060583, NRF-2012M3A6A7054856, and NRF-2017R1A2B3010927 (Global Frontier R&D Program on Center for Multiscale Energy System); NRF-2016M3D1A1027663 and NRF-2016M3D1A1027664 (Future Materials Discovery Program); NRF-2018R1A2B2006708 (Research Program); and NRF2015M1A2A2053004 and NRF-2015M1A2A2056827 (Climate Change Management Program).
AUTHOR CONTRIBUTIONS H.S.J., M.J.K., and N.-G.P. perceived scientific challenges and recent efforts as the flexible perovskite solar cell topics in this review article and wrote the manuscript. G.S.H. revised the manuscript, collected permissions, and corrected typos.
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