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Efficient ITO-free semitransparent perovskite solar cells with metal transparent electrodes
Solar Energy Materials and Solar Cells 196 (2019) 1–8
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Efficient ITO-free semitransparent perovskite solar cells with metal transparent electrodes
T
Se-Phin Chob, Seok-in Nab, Seok-Soon Kima,∗ a
Department of Nano and Chemical Engineering, Kunsan National University, 558 Daehangno, Kunsan, Jeollabuk-do, 573-701, Republic of Korea Professional Graduate School of Flexible and Printable Electronics, Polymer Materials Fusion Research Center, Chonbuk National University, 664-14, Deokjin-dong, Deokjin-gu, Jeonju-si, Jeollabuk-do, 561-756, Republic of Korea
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Perovskite solar cells ITO-Free Semitransparent Metal thin film
In parallel with soaring efficiency, interest in multi-functional semitransparent perovskite solar cells (PeSCs) has emerged for special applications such as windows and tandem cells. To demonstrate air-stable as well as efficient ITO-free semitransparent PeSCs, ultra thin Cu and Ni, which are well-known non-precious metals having sufficient electrical conductivity, are investigated as bottom transparent electrode and optimization of devices is systematically carried out. Semitransparent Ni/NiOX based PeSC exhibits the average visible transmittance (AVT) of 22% and power conversion efficiency (PCE) of 8.20%. Considering required AVT for the application of power-generating window of ∼25% and recent reports, this result is one of the best performances reported to date.
1. Introduction
semitransparent applications due to its high optical transparency and low scattering [12,13]. Generally, because there is always trade-off between transparency and power generating performance in semitransparent solar cells, increasing the transmittance of full device without loss of PCE is important issue to demonstrate excellent semitransparent solar cells. To achieve semitransparent PeSC, the thickness and surface morphology of perovskite layer has been adjusted and their relationship with final performance has been reported by several groups [14–16]. Discontinuous perovskite layer composed of ‘perovskite islands’ absorbing enough sunlight and ‘de-wet regions’ providing visible transparency resulted from the spontaneous de-wetting of perovskite precursor has been demonstrated by G. E. Eperon et al. [17–19]. With the optical transparency of absorbing perovskite layer, highly transparent and conductive top and bottom electrodes are necessary to complete semitransparent PeSCs. As a simple way to realize semitransparent PeSCs, replacement of top opaque metal electrode in typical transparent conducting oxide (TCO) based devices with various transparent electrodes such as Ag nanowire, carbon based nanomaterials, and thin metal or metal/oxide/metal multilayers has been studied [20–24]. Meanwhile, ITO, a commonly used transparent bottom TCO electrode, has many critical problems such as limited indium source, high temperature and high cost process, poor transparency in the violet region, and mechanical brittleness [25–27]. Hence, conducting polymer, carbon-based nanomaterials, metal nanowire, and thin metal film have
Organic-inorganic metal halides have attracted much attention as promising light harvesting materials for solar cells owing to their unique properties such as long carrier diffusion length, high charge carrier mobility, intense broadband absorption, ambipolar semiconducting characteristics, low-cost, and simple solution processability [1–4]. Since the first report on the hybrid solar cells based on meso-superstructured organometal halide perovskites in 2009, the PCE has dramatically increased and reached over 22% as a result of the considerable efforts devoted to optimizing perovskite absorbing layer and charge transporting in device architecture [5–7]. In parallel with soaring efficiency, interest in multi-functional and aesthetic semitransparent PeSCs has emerged due to their potential in specific applications such as building-integrated photovoltaics and transportation by the integration to windows, skylight, and sunroof. Furthermore, semitransparent PeSCs can be used to combine with typical inorganic solar cells to demonstrate highly efficient tandem cells [8–10]. Various device configurations such as n-i-p and p-i-n layouts, mesoporous metal oxide scaffold based architecture, and planar heterojunction are possible due to ambipolar semiconducting property of perovskite [11]. Although mesoporous TiO2 based PeSCs have been extensively studied and shown higher PCE, a planar heterojunction architecture consisting with planar photoactive perovskite layer and selective charge transport layers is considered as a better candidate for ∗
Corresponding author. E-mail address: [email protected] (S.-S. Kim).
https://doi.org/10.1016/j.solmat.2019.03.002 Received 1 November 2018; Received in revised form 11 February 2019; Accepted 2 March 2019 0927-0248/ © 2019 Published by Elsevier B.V.
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been investigated as alternatives to ITO [28–31]. Among these materials, smooth thin metal film, which can be easily deposited by thermal evaporation, is suitable for roll-to-roll based mass production and its electrical conductivity and flexibility is excellent [32–34]. For that reason, thin metal films have emerged as ITO-alternatives in semitransparent solar cells as well as typical opaque devices. Feng et al. have reported ITO-free semitransparent PeSCs exhibiting considerable performance and superior flexibility to ITO based PeSCs by using thin Au bottom electrode [35]. W.eC. Lai et al. demonstrated ITO-free opaque PeSC with oxidized Ni/Au bottom electrode prepared by evaporation of Ni/Au bi-layer followed by thermal oxidation [36]. By varying the Ni/Au composition and annealing process, appropriate optical transparency and electrical conductivity can be obtained and consequently comparable PCEs to ITO based devices has been successfully demonstrated. In the above two reports, they used Au with high workfunction of 5.1 eV to collect holes. However, use of precious metal such as Au limits the commercialization of PeSCs as cost efficient solar cells. In this study, we investigated ITO-free semitransparent PeSCs based on ultra thin Cu and Ni film. It is well-known that Cu and Ni are nonprecious metal and their electrical conductivity is sufficient to serve as electrodes. Their high workfunctions are also suitable to collect holes. For versatile applications of semitransparent PeSCs, AVT of full device of 20- 25%, suitable performance, and long-term stability is required. To demonstrate air-stable as well as efficient semitransparent PeSCs, thin Cu and Ni were studied as bottom electrode and their effect on overall performance and air-stability of biplanar PeSCs consisting with thin metal films/hole transporting layer (HTL)/perovskite/[6,6]phenyl-C61 butyric acid methyl ester (PCBM)/Cu was investigated. Very recently, J. Zhao et al. has found that Cu is more stable to the chemical reaction with perovskite layers than Ag or Al, even Au [37]. Based on above report and our research results, we used Cu thin layer as top electrode to maximize air-stability. Furthermore, because very thin metal layer acts as bottom and top electrode to obtain suitable transparency (AVT of ∼20%), electrical properties of electrodes are poor comparing to conventional thick ITO based opaque device and thus which are unfavorable for charge extraction & collection to each electrode. Hence, control of interfaces of transparent Cu or Ni bottom electrode/perovskite and PCBM/Cu top electrode is also systematically carried out to minimize the contact barrier and improve long-term stability.
Fig. 1. (a) Schematic layout of ITO-free semitransparent PeSCs and (b) energy level diagram of each layer.
PEDOT:PSS (VPAI 4083) or NiOX were deposited as HTL via spincoating process. Typical PEDOT:PSS was spin-coated by two-step (at 500 rpm for 5 s and 5000 rpm for 40 s) and annealed at 120 °C for 10 min in atmospheric condition. NiOX was spin-coated at 3000 rpm for 40 s using a solution of 0.1 M nickel acetate (Sigma-Aldrich) in ethanol with 6 vol % ethanolamine followed by annealing at 350 °C for 30 min. Perovskite active layers were fabricated by spin-coating of a 25 wt% CH3NH3PbI3 solution in 2-methoxyethanol prepared by dissolving methyl ammonium iodide and PbI2 (1:1 M ratio) with N-cyclohexyl-2pyrrolidone (CHP) additive as described previous report [38]. 20 μl CH3NH3PbI3 solution was spin-coated at 7000 rpm for 90 s and dried on a hot plate at 140 °C for 20 s in N2-filled glove box to obtain optimal thickness and morphology. PCBM dissolved in chlorobenzene (20 mg/ ml) was spin-coated at 1000 rpm for 60 s on the perovskite layers. Afterward, 0.5 wt% polyethylenimine (PEIE) in methanol was subsequently spin-coated at 5000 rpm for 40 s. Finally, thin Cu (8 nm) was thermally evaporated as transparent top metal electrode in a vacuum of 10 −7 Torr. After calibration with reference Si solar cell (SRC-1000-TC-KG5-N, VLSI standards, Inc.), current density-voltage (J-V) curves were measured in an atmospheric condition (23 °C, humidity of ∼45–55%) using a Keithley 2400 equipment under standard solar irradiation (AM 1.5 G 1sun condition). The film morphologies of various HTLs were observed using a field emission scanning electron microscope (FE-SEM, Hitachi S4800) and optical transparency and AVT was evaluated using a UV–vis spectrophotometer (VarianAU/DMS-100S). The electrical properties of transparent electrodes were studied using a 4-point-probe measurement (FPP-RS9, Dasol Eng.). The work function values of electrodes and HTLs were measured using ultraviolet photoelectron spectroscopy (UPS, ESCALAB 201) with a He I (21.2 eV) source. Change in efficiency was recoded as a function of exposed time in air without any encapsulation.
2. Experimental Prior to deposition of transparent Cu and Ni bottom electrodes, glass substrates (area 1.5 X 1.5 cm2, AMG Tech.) were cleaned by ultra-sonication in deionized water, acetone, and isopropyl alcohol followed by drying in an oven at 100 °C overnight. Cu and Ni with a thickness of 15 nm and 6 nm, respectively, were evaporated at a pressure of 10 −7 Torr and then substrates were treated with UV-O3 for 20 min. Because there is a trade-off between optical transparency for more efficient use of incident light and electrical conductivity for sufficient charge collection, optimization of metal thickness was carried out and optimal thickness of Cu and Ni was decided to 15 nm and 6 nm, respectively. For careful study, we check “tooling factor” expressing the ratio between monitored thickness and actual thickness deposited on substrate measured by FE-SEM cross-sectional images. The tooling factor is calculated as follows:
Fm = Fi
Tm Ti
where, Fi is the initial tooling factor, Ti is the film thickness indicated by the instrument, and Tm is the actual, independently measured thickness of the deposited film. We calibrated the real thickness (Tm) of very thin metal layers (Cu of 15 nm and Ni of 6 nm) by using calculated Fm.
3. Result and discussion As mentioned, planar heterojunction architecture is considered as a 2
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Fig. 2. (a) Transmittance and (b) RSheet of 15 nm Cu and 6 nm Ni.
metal oxide have been tried to replace PEDOT:PSS [44–47]. Recently, solution processed NiOX has shown their potential as a HTL for achieving high performance PeSCs due to high optical transparency, ptype conductivity resulted from two positively charged holes accompanying Ni2+ vacancy, and deep-lying VB of 5.2–5.4 eV [48–51]. Hence, we applied sol-gel derived NiOX as HTLs on thin metal TE and compared with conventional PEDOT:PSS systems. Also, PCBM/Cu interface was modified with solution processed polyethylenimine ethoxylate (PEIE) to minimize the contact barrier between PCBM and Cu top electrode [52–54]. Fig. 1 (b) illustrates the energy band diagram for each layer derived from UPS measurement and literature survey [52,55]. VB of sol-gel processed NiOX was evaluated as 5.3 eV. This value is well-aligned with VB of CH3NH3PbI3, and thus efficient hole transport from perovskite can be expected. The workfunction of Cu is decreased to ∼4 eV by the modification with PEIE due to the formation of surface dipole. It indicates that the contact barrier between the top electrode and the PCBM is improved and efficient electron extraction to the top electrode is allowed. From these results, higher device performance can be expected by the use of NiOX and PEIE. Thin metal bottom TE in solar cells requires both excellent optical transparency for more efficient use of incident light and electrical
better candidate for semitransparent applications owing to higher optical transparency and lower scattering than the mesoporous oxide based PeSCs. Among various planar heterojunction configurations and composition, p-i-n structured PeSCs consisting with transparent electrode (TE)/HTL/CH3NH3PbI3/PCBM/top electrode was investigated in this study. As shown in Fig. 1 (a), thin Cu and Ni were used as transparent bottom electrodes to replace typical ITO and Cu, which is known to high resistance to the chemical reaction with perovskite, was used as top electrode, respectively. Because the main purpose of this study is to demonstrate air-stable as well as efficient ITO-free semitransparent PeSCs, Cu top electrode was used instead of typical Ag or Al electrode. Interfaces of TE/photoactive perovskite layer and PCBM/top electrode also have a significant effect on the device efficiency and stability. Although PEDOT:PSS, which is commonly used HTL in polymer solar cell, was successfully applied to PeSCs, the acidic and hygroscopic nature of PEDOT:PSS causing device instability is still one of the big issues regarding to this type of solar cells [39–41]. Additionally, large mismatch between valence band (VB) of PEDOT:PSS (∼5 eV) and CH3NH3PbI3 (∼5.4 eV) induces insufficient hole transport and collection [42,43]. Therefore, various promising alternatives such as 2-D carbonaceous materials, polymer composite, CuSCN, and transition 3
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Fig. 3. (a) Representative J-V characteristics and (b) variation in photovoltaic parameters of PeSCs based on Cu and Ni with PEDOT:PSS and NiOx HTLs. Table 1 Important parameters for PeSCs based on ITO and metal thin films. Device
VOC (V)
JSC (mA/cm2)
FF (%)
PCE (%)
ITO-based PeSCs
Opaque (Cu 40 nm) Semitransparent (Cu 8 nm)
0.98 ± 0.01 0.96 ± 0.02
16.46 ± 0.44 13.45 ± 0.50
72.45 ± 2.39 65.02 ± 1.43
11.64 ± 0.27 (12.27) 8.85 ± 0.19 (9.19)
ITO-free semitransparent PeSCs
Cu 15 nm/PEDOT:PSS Cu 15 nm/NiO Ni 6nm/PEDOT:PSS Ni 6nm/NiO
0.81 1.04 0.91 1.00
4.71 ± 0.32 7.13 ± 0.20 8.47 ± 0.44 12.20 ± 0.15
67.05 49.87 67.64 69.66
2.18 3.20 5.19 8.47
± ± ± ±
0.02 0.03 0.02 0.08
± ± ± ±
2.05 3.35 5.95 0.19
± ± ± ±
0.18 0.26 0.43 0.14
(2.15) (3.68) (5.92) (8.65)
*Best PCE is presented in parenthesis.
poor transmittance in the visible region between 400 and 800 nm compared to a typical ITO bottom electrode. A Ni film has a higher sheet resistance of ∼63 Ω/cm2 than that of Cu of ∼5 Ω/cm2 due to relatively lower thickness. To evaluate the potential of Cu and Ni as TE, ITO-free semitransparent PeSCs were fabricated and their performance was compared with typical ITO based devices. Fig. 3 (a) shows representative current density-voltage (J-V) curves of PeSCs with PEDOT:PSS and NiOX HTL. All devices showed negligible hysteresis behavior at all scan directions. The ITO based reference opaque PeSC with NiOX HTL and
conductivity for sufficient charge collection, but there is a trade-off between them. Hence, optimization of metal thickness is important to obtain desirable device performance. Based on the relationship between metal thickness and performance, we decided the optimal thickness of Cu and Ni to 15 nm and 6 nm, respectively, and their optical and electrical properties were characterized. Although thinner Cu has enough electrical conductivity, 15 nm is required to successful operation of devices. Fig. 2 (a) and (b) show transmittance spectra and sheet resistance of metal films deposited on glass substrates. As can be seen from photographs and transmittance spectra, both Cu and Ni exhibited
4
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Fig. 4. (a) Transmittance spectra of Cu and Ni with NiOx and (b) FE-SEM surface images of Cu and Ni, Cu and Ni with NiOx before and after thermal annealing, and perovskite layers on each layer.
obtained due to higher RS in thinner Cu electrode system, but still high FF of 66.77% indicates that 8 nm Cu is enough to collect electrons from PCBM. Thick Cu in the opaque PeSC serves as a rear reflector inducing increase in path length of incident light in the perovskite layer. It indicates more harvesting of photons. In contrast, the transmittance of thin Cu permits passing through of nonaborbed photons, resulting in loss of JSC. ITO-free semitransparent PeSCs showed lower PCE than ITO based control devices. Both Cu and Ni based devices exhibited improved performance by replacing typical PEDOT:PSS to NiOX HTL, but Cu based device still showed poor performance of less than 4%. In case of Ni based device, performance was prominently improved by using NiOX
40 nm Cu top electrode showed open-circuit voltage (VOC) of 0.94 V, short-circuit current density (JSC) of 16.73 mA/cm2, fill factor (FF) of 77.82%, and PCE of 12.27%. As described above, adjustment of thickness and surface morphology of perovskite is essential to demonstrate proper performance as well as transparency and ∼120 nm perovskite layer with smooth surface is selected as optimal condition for our semitransparent PeSCs. Here, PCE of ∼12% is reasonable values considering relatively thin perovskite layer of ∼120 nm. Comparing to opaque device, semitransparent ITO based PeSC with 8 nm Cu top electrode exhibited lower PCE of 9.19% with lower FF and JSC. Series resistance (RS) values of opaque and semitransparent PeSCs were calculated to 3.12 Ω cm2 and 13.06 Ω cm2, respectively. Lower FF was 5
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PEEOT:PSS. However, because Cu and Ni electrodes with NiOX HTL have still poor transmittance comparing to ITO/NiOX (AVT of ∼81%), all ITO-free devices showed lower photocurrent than that of ITO based devices. The higher VB of NiOX of 5.30 eV than that of PEDOT:PSS (4.95–5.00 eV) makes better energy matching with VB of CH3NH3PbI3 (5.4 eV) inducing higher VOC than PEDOT:PSS based devices. Furthermore, since NiOX is known to have small electron affinity, effective blocking of electrons can minimize recombination with the holes at the front electrode [56,57]. Consequently, Ni based device showed significantly improved PCE of 8.20% by replacing PEDOT:PSS to NiOX. On the other hand, Cu/NiOX based devices still showed poor performance of less than 4%, despite increased transmittance and better energy matching with CH3NH3PbI3. To find the reason of poor performance in Cu based systems, surface morphologies of metal thin layer with and without NiOX, which strongly affect the quality of perovskite layer and overall performance of PeSCs, were characterized. As shown in Fig. 4 (b), surfaces of Ni without and with NiOX were smooth and dense and perovskite film grown on Ni/NiOX exhibited homogeneous pinhole free surface morphology. On the other hand, smooth and dense Cu surface changed to porous layer after NiOX formation due to cracking of Cu films during thermal annealing of spin-coated NiOX at ∼350 °C. Cracking of Cu leads to degradation of electrical conductivity of Cu front electrode. Moreover, rough surface results in non-uniform surface morphology and pinholes on perovskite film. For both of these reasons, Cu/NiOX based devices still showed poor performance of less than 4%. As mentioned, the purpose of this study is to demonstrate semitransparent PeSCs with high stability as well as high efficiency. To evaluate the stability of the ITO-free semitransparent PeSCs, devices were kept in atmospheric environment (at ∼25 °C with a humidity of 45–60%) without any encapsulation and change in PCE was recorded. As shown in Fig. 5, Cu based devices showed poor stability compared to Ni based system. In particular, the PCE of Cu/PEDOT:PSS device was rapidly decreased and plummeted to ∼0% only after 6 h. On the contrary, PCE of the Ni/NiOX based device remains 60% of the initial efficiency after 144 h exposure in air.
Fig. 5. Stability of PeSCs based on Cu and Ni with PEDOT:PSS and NiOx HTLs.
as follows: VOC = 0.98 V, JSC = 11.90 mA/cm2, FF = 70.02%, and PCE = 8.20%. As shown in photograph in Fig. 3 (a), ITO-free semitransparent solar cell was successfully demonstrated and the average visible transmittance (AVT) of completed Ni based solar cell, which is determined by taking an average of the transmittance in the visible region, was evaluated to ∼22%. In order to realize a versatile applications of semitransparent PeSCs, it is necessary to satisfy a required AVT of 20- 25% of the full device with proper photovoltaic performance. The performance and optical transparency reported to date is vary depending on the device structure and materials (see summarized recent reports on semitransparent PeSCs in table S1. in supporting information). PeSCs achieving AVT of ∼20% exhibited PCE of ∼10% or lower. Furthermore, ITO-free semitransparent PeSCs showed poor PCE comparing to ITO based devices. Therefore, we conclude that our ITOfree semitransparent PeSCs are comparable to the best performance reported to date. Even better, Ni/NiOX based devices exhibited extremely small fluctuations in performance as shown in Fig. 3 (b) and Table 1. Here, we recorded deviations of important parameters within 8 devices fabricated with each configuration to verify the reliability of device performance. For further understanding on the effect of HTLs on the performance, optical, electrical, and structural changes after preparation of HTL on Cu and Ni thin layers were investigated. Cu and Ni with PEDOT:PSS showed still poor transmittance in the whole wavelength region and thus low photocurrent was obtained (see Fig. S1 in supporting information). In particular, because thicker film of 15 nm was required for Cu based device operation, Cu/PEDOT:PSS showed lower transmittance than Ni/PEDOT:PSS and leaded to lower JSC of ∼4 mA/cm2. As mentioned, by replacing typical PEDOT:PSS HTL to NiOx, both Cu and Ni based devices exhibited improved performance. Improvement in VOC and JSC was observed for both cases. Fig. 4 (a) shows the transmittance of Ni and Cu with NiOX HTL. As shown in transmittance spectra and photographs in Fig. 4 (a), Ni and Cu with NiOX showed higher transmittance of ∼70% and ∼60%, respectively, than that of Ni and Cu without and with typical PEDOT:PSS HTL. Increase in transmittance is attributed to the partial oxidation of thin metal electrode during thermal annealing of sol-gel processed NiOX and therefore higher photocurrent can be obtained by using NiOX HTL instead of
4. Conclusion In summary, ITO-free semitransparent PeSCs based on ultra thin Cu and Ni films, which are well-known non-precious metal having sufficient electrical conductivity and high work-function, were studied. To demonstrate air-stable as well as efficient semitransparent PeSCs, thin Cu and Ni were characterized as bottom electrode and their effect on overall performance of biplanar PeSCs consisting with thin metal film/ HTL/perovskite/PCBM/PEIE/Cu was investigated. Both Cu and Ni based devices exhibited improved performance by replacing typical PEDOT:PSS HTL to NiOX. In particular, Ni/NiOX based PeSC exhibiting the average visible AVT of 22% showed prominently improved performance as follows: VOC = 0.98 V, JSC = 11.90 mA/cm2, FF = 70.02%, and PCE = 8.20%. Considering required AVT for the application of power-generating window of ∼25% and recent reports, this result is one of the best performances reported to date. However, Cu based device still showed poor performance of less than 4% due to lower transmittance than that of Ni system and cracking of Cu films during thermal annealing of spin-coated NiOX. Cracking of Cu caused decrease in the electrical conductivity of Cu electrode and uneven surface morphology and pinholes in the upper perovskite layer. Furthermore, Ni/NiOX based device showed superior stability to Cu based systems. Acknowledgements This research was financially supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2018R1A2B6001206) & the Kunsan National University's Long6
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term Overseas Research Program for Faculty Member in the year 2016.
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Efficient ITO-free semitransparent perovskite solar cells with metal transparent electrodes
T
Se-Phin Chob, Seok-in Nab, Seok-Soon Kima,∗ a
Department of Nano and Chemical Engineering, Kunsan National University, 558 Daehangno, Kunsan, Jeollabuk-do, 573-701, Republic of Korea Professional Graduate School of Flexible and Printable Electronics, Polymer Materials Fusion Research Center, Chonbuk National University, 664-14, Deokjin-dong, Deokjin-gu, Jeonju-si, Jeollabuk-do, 561-756, Republic of Korea
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Perovskite solar cells ITO-Free Semitransparent Metal thin film
In parallel with soaring efficiency, interest in multi-functional semitransparent perovskite solar cells (PeSCs) has emerged for special applications such as windows and tandem cells. To demonstrate air-stable as well as efficient ITO-free semitransparent PeSCs, ultra thin Cu and Ni, which are well-known non-precious metals having sufficient electrical conductivity, are investigated as bottom transparent electrode and optimization of devices is systematically carried out. Semitransparent Ni/NiOX based PeSC exhibits the average visible transmittance (AVT) of 22% and power conversion efficiency (PCE) of 8.20%. Considering required AVT for the application of power-generating window of ∼25% and recent reports, this result is one of the best performances reported to date.
1. Introduction
semitransparent applications due to its high optical transparency and low scattering [12,13]. Generally, because there is always trade-off between transparency and power generating performance in semitransparent solar cells, increasing the transmittance of full device without loss of PCE is important issue to demonstrate excellent semitransparent solar cells. To achieve semitransparent PeSC, the thickness and surface morphology of perovskite layer has been adjusted and their relationship with final performance has been reported by several groups [14–16]. Discontinuous perovskite layer composed of ‘perovskite islands’ absorbing enough sunlight and ‘de-wet regions’ providing visible transparency resulted from the spontaneous de-wetting of perovskite precursor has been demonstrated by G. E. Eperon et al. [17–19]. With the optical transparency of absorbing perovskite layer, highly transparent and conductive top and bottom electrodes are necessary to complete semitransparent PeSCs. As a simple way to realize semitransparent PeSCs, replacement of top opaque metal electrode in typical transparent conducting oxide (TCO) based devices with various transparent electrodes such as Ag nanowire, carbon based nanomaterials, and thin metal or metal/oxide/metal multilayers has been studied [20–24]. Meanwhile, ITO, a commonly used transparent bottom TCO electrode, has many critical problems such as limited indium source, high temperature and high cost process, poor transparency in the violet region, and mechanical brittleness [25–27]. Hence, conducting polymer, carbon-based nanomaterials, metal nanowire, and thin metal film have
Organic-inorganic metal halides have attracted much attention as promising light harvesting materials for solar cells owing to their unique properties such as long carrier diffusion length, high charge carrier mobility, intense broadband absorption, ambipolar semiconducting characteristics, low-cost, and simple solution processability [1–4]. Since the first report on the hybrid solar cells based on meso-superstructured organometal halide perovskites in 2009, the PCE has dramatically increased and reached over 22% as a result of the considerable efforts devoted to optimizing perovskite absorbing layer and charge transporting in device architecture [5–7]. In parallel with soaring efficiency, interest in multi-functional and aesthetic semitransparent PeSCs has emerged due to their potential in specific applications such as building-integrated photovoltaics and transportation by the integration to windows, skylight, and sunroof. Furthermore, semitransparent PeSCs can be used to combine with typical inorganic solar cells to demonstrate highly efficient tandem cells [8–10]. Various device configurations such as n-i-p and p-i-n layouts, mesoporous metal oxide scaffold based architecture, and planar heterojunction are possible due to ambipolar semiconducting property of perovskite [11]. Although mesoporous TiO2 based PeSCs have been extensively studied and shown higher PCE, a planar heterojunction architecture consisting with planar photoactive perovskite layer and selective charge transport layers is considered as a better candidate for ∗
Corresponding author. E-mail address: [email protected] (S.-S. Kim).
https://doi.org/10.1016/j.solmat.2019.03.002 Received 1 November 2018; Received in revised form 11 February 2019; Accepted 2 March 2019 0927-0248/ © 2019 Published by Elsevier B.V.
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been investigated as alternatives to ITO [28–31]. Among these materials, smooth thin metal film, which can be easily deposited by thermal evaporation, is suitable for roll-to-roll based mass production and its electrical conductivity and flexibility is excellent [32–34]. For that reason, thin metal films have emerged as ITO-alternatives in semitransparent solar cells as well as typical opaque devices. Feng et al. have reported ITO-free semitransparent PeSCs exhibiting considerable performance and superior flexibility to ITO based PeSCs by using thin Au bottom electrode [35]. W.eC. Lai et al. demonstrated ITO-free opaque PeSC with oxidized Ni/Au bottom electrode prepared by evaporation of Ni/Au bi-layer followed by thermal oxidation [36]. By varying the Ni/Au composition and annealing process, appropriate optical transparency and electrical conductivity can be obtained and consequently comparable PCEs to ITO based devices has been successfully demonstrated. In the above two reports, they used Au with high workfunction of 5.1 eV to collect holes. However, use of precious metal such as Au limits the commercialization of PeSCs as cost efficient solar cells. In this study, we investigated ITO-free semitransparent PeSCs based on ultra thin Cu and Ni film. It is well-known that Cu and Ni are nonprecious metal and their electrical conductivity is sufficient to serve as electrodes. Their high workfunctions are also suitable to collect holes. For versatile applications of semitransparent PeSCs, AVT of full device of 20- 25%, suitable performance, and long-term stability is required. To demonstrate air-stable as well as efficient semitransparent PeSCs, thin Cu and Ni were studied as bottom electrode and their effect on overall performance and air-stability of biplanar PeSCs consisting with thin metal films/hole transporting layer (HTL)/perovskite/[6,6]phenyl-C61 butyric acid methyl ester (PCBM)/Cu was investigated. Very recently, J. Zhao et al. has found that Cu is more stable to the chemical reaction with perovskite layers than Ag or Al, even Au [37]. Based on above report and our research results, we used Cu thin layer as top electrode to maximize air-stability. Furthermore, because very thin metal layer acts as bottom and top electrode to obtain suitable transparency (AVT of ∼20%), electrical properties of electrodes are poor comparing to conventional thick ITO based opaque device and thus which are unfavorable for charge extraction & collection to each electrode. Hence, control of interfaces of transparent Cu or Ni bottom electrode/perovskite and PCBM/Cu top electrode is also systematically carried out to minimize the contact barrier and improve long-term stability.
Fig. 1. (a) Schematic layout of ITO-free semitransparent PeSCs and (b) energy level diagram of each layer.
PEDOT:PSS (VPAI 4083) or NiOX were deposited as HTL via spincoating process. Typical PEDOT:PSS was spin-coated by two-step (at 500 rpm for 5 s and 5000 rpm for 40 s) and annealed at 120 °C for 10 min in atmospheric condition. NiOX was spin-coated at 3000 rpm for 40 s using a solution of 0.1 M nickel acetate (Sigma-Aldrich) in ethanol with 6 vol % ethanolamine followed by annealing at 350 °C for 30 min. Perovskite active layers were fabricated by spin-coating of a 25 wt% CH3NH3PbI3 solution in 2-methoxyethanol prepared by dissolving methyl ammonium iodide and PbI2 (1:1 M ratio) with N-cyclohexyl-2pyrrolidone (CHP) additive as described previous report [38]. 20 μl CH3NH3PbI3 solution was spin-coated at 7000 rpm for 90 s and dried on a hot plate at 140 °C for 20 s in N2-filled glove box to obtain optimal thickness and morphology. PCBM dissolved in chlorobenzene (20 mg/ ml) was spin-coated at 1000 rpm for 60 s on the perovskite layers. Afterward, 0.5 wt% polyethylenimine (PEIE) in methanol was subsequently spin-coated at 5000 rpm for 40 s. Finally, thin Cu (8 nm) was thermally evaporated as transparent top metal electrode in a vacuum of 10 −7 Torr. After calibration with reference Si solar cell (SRC-1000-TC-KG5-N, VLSI standards, Inc.), current density-voltage (J-V) curves were measured in an atmospheric condition (23 °C, humidity of ∼45–55%) using a Keithley 2400 equipment under standard solar irradiation (AM 1.5 G 1sun condition). The film morphologies of various HTLs were observed using a field emission scanning electron microscope (FE-SEM, Hitachi S4800) and optical transparency and AVT was evaluated using a UV–vis spectrophotometer (VarianAU/DMS-100S). The electrical properties of transparent electrodes were studied using a 4-point-probe measurement (FPP-RS9, Dasol Eng.). The work function values of electrodes and HTLs were measured using ultraviolet photoelectron spectroscopy (UPS, ESCALAB 201) with a He I (21.2 eV) source. Change in efficiency was recoded as a function of exposed time in air without any encapsulation.
2. Experimental Prior to deposition of transparent Cu and Ni bottom electrodes, glass substrates (area 1.5 X 1.5 cm2, AMG Tech.) were cleaned by ultra-sonication in deionized water, acetone, and isopropyl alcohol followed by drying in an oven at 100 °C overnight. Cu and Ni with a thickness of 15 nm and 6 nm, respectively, were evaporated at a pressure of 10 −7 Torr and then substrates were treated with UV-O3 for 20 min. Because there is a trade-off between optical transparency for more efficient use of incident light and electrical conductivity for sufficient charge collection, optimization of metal thickness was carried out and optimal thickness of Cu and Ni was decided to 15 nm and 6 nm, respectively. For careful study, we check “tooling factor” expressing the ratio between monitored thickness and actual thickness deposited on substrate measured by FE-SEM cross-sectional images. The tooling factor is calculated as follows:
Fm = Fi
Tm Ti
where, Fi is the initial tooling factor, Ti is the film thickness indicated by the instrument, and Tm is the actual, independently measured thickness of the deposited film. We calibrated the real thickness (Tm) of very thin metal layers (Cu of 15 nm and Ni of 6 nm) by using calculated Fm.
3. Result and discussion As mentioned, planar heterojunction architecture is considered as a 2
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Fig. 2. (a) Transmittance and (b) RSheet of 15 nm Cu and 6 nm Ni.
metal oxide have been tried to replace PEDOT:PSS [44–47]. Recently, solution processed NiOX has shown their potential as a HTL for achieving high performance PeSCs due to high optical transparency, ptype conductivity resulted from two positively charged holes accompanying Ni2+ vacancy, and deep-lying VB of 5.2–5.4 eV [48–51]. Hence, we applied sol-gel derived NiOX as HTLs on thin metal TE and compared with conventional PEDOT:PSS systems. Also, PCBM/Cu interface was modified with solution processed polyethylenimine ethoxylate (PEIE) to minimize the contact barrier between PCBM and Cu top electrode [52–54]. Fig. 1 (b) illustrates the energy band diagram for each layer derived from UPS measurement and literature survey [52,55]. VB of sol-gel processed NiOX was evaluated as 5.3 eV. This value is well-aligned with VB of CH3NH3PbI3, and thus efficient hole transport from perovskite can be expected. The workfunction of Cu is decreased to ∼4 eV by the modification with PEIE due to the formation of surface dipole. It indicates that the contact barrier between the top electrode and the PCBM is improved and efficient electron extraction to the top electrode is allowed. From these results, higher device performance can be expected by the use of NiOX and PEIE. Thin metal bottom TE in solar cells requires both excellent optical transparency for more efficient use of incident light and electrical
better candidate for semitransparent applications owing to higher optical transparency and lower scattering than the mesoporous oxide based PeSCs. Among various planar heterojunction configurations and composition, p-i-n structured PeSCs consisting with transparent electrode (TE)/HTL/CH3NH3PbI3/PCBM/top electrode was investigated in this study. As shown in Fig. 1 (a), thin Cu and Ni were used as transparent bottom electrodes to replace typical ITO and Cu, which is known to high resistance to the chemical reaction with perovskite, was used as top electrode, respectively. Because the main purpose of this study is to demonstrate air-stable as well as efficient ITO-free semitransparent PeSCs, Cu top electrode was used instead of typical Ag or Al electrode. Interfaces of TE/photoactive perovskite layer and PCBM/top electrode also have a significant effect on the device efficiency and stability. Although PEDOT:PSS, which is commonly used HTL in polymer solar cell, was successfully applied to PeSCs, the acidic and hygroscopic nature of PEDOT:PSS causing device instability is still one of the big issues regarding to this type of solar cells [39–41]. Additionally, large mismatch between valence band (VB) of PEDOT:PSS (∼5 eV) and CH3NH3PbI3 (∼5.4 eV) induces insufficient hole transport and collection [42,43]. Therefore, various promising alternatives such as 2-D carbonaceous materials, polymer composite, CuSCN, and transition 3
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Fig. 3. (a) Representative J-V characteristics and (b) variation in photovoltaic parameters of PeSCs based on Cu and Ni with PEDOT:PSS and NiOx HTLs. Table 1 Important parameters for PeSCs based on ITO and metal thin films. Device
VOC (V)
JSC (mA/cm2)
FF (%)
PCE (%)
ITO-based PeSCs
Opaque (Cu 40 nm) Semitransparent (Cu 8 nm)
0.98 ± 0.01 0.96 ± 0.02
16.46 ± 0.44 13.45 ± 0.50
72.45 ± 2.39 65.02 ± 1.43
11.64 ± 0.27 (12.27) 8.85 ± 0.19 (9.19)
ITO-free semitransparent PeSCs
Cu 15 nm/PEDOT:PSS Cu 15 nm/NiO Ni 6nm/PEDOT:PSS Ni 6nm/NiO
0.81 1.04 0.91 1.00
4.71 ± 0.32 7.13 ± 0.20 8.47 ± 0.44 12.20 ± 0.15
67.05 49.87 67.64 69.66
2.18 3.20 5.19 8.47
± ± ± ±
0.02 0.03 0.02 0.08
± ± ± ±
2.05 3.35 5.95 0.19
± ± ± ±
0.18 0.26 0.43 0.14
(2.15) (3.68) (5.92) (8.65)
*Best PCE is presented in parenthesis.
poor transmittance in the visible region between 400 and 800 nm compared to a typical ITO bottom electrode. A Ni film has a higher sheet resistance of ∼63 Ω/cm2 than that of Cu of ∼5 Ω/cm2 due to relatively lower thickness. To evaluate the potential of Cu and Ni as TE, ITO-free semitransparent PeSCs were fabricated and their performance was compared with typical ITO based devices. Fig. 3 (a) shows representative current density-voltage (J-V) curves of PeSCs with PEDOT:PSS and NiOX HTL. All devices showed negligible hysteresis behavior at all scan directions. The ITO based reference opaque PeSC with NiOX HTL and
conductivity for sufficient charge collection, but there is a trade-off between them. Hence, optimization of metal thickness is important to obtain desirable device performance. Based on the relationship between metal thickness and performance, we decided the optimal thickness of Cu and Ni to 15 nm and 6 nm, respectively, and their optical and electrical properties were characterized. Although thinner Cu has enough electrical conductivity, 15 nm is required to successful operation of devices. Fig. 2 (a) and (b) show transmittance spectra and sheet resistance of metal films deposited on glass substrates. As can be seen from photographs and transmittance spectra, both Cu and Ni exhibited
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Fig. 4. (a) Transmittance spectra of Cu and Ni with NiOx and (b) FE-SEM surface images of Cu and Ni, Cu and Ni with NiOx before and after thermal annealing, and perovskite layers on each layer.
obtained due to higher RS in thinner Cu electrode system, but still high FF of 66.77% indicates that 8 nm Cu is enough to collect electrons from PCBM. Thick Cu in the opaque PeSC serves as a rear reflector inducing increase in path length of incident light in the perovskite layer. It indicates more harvesting of photons. In contrast, the transmittance of thin Cu permits passing through of nonaborbed photons, resulting in loss of JSC. ITO-free semitransparent PeSCs showed lower PCE than ITO based control devices. Both Cu and Ni based devices exhibited improved performance by replacing typical PEDOT:PSS to NiOX HTL, but Cu based device still showed poor performance of less than 4%. In case of Ni based device, performance was prominently improved by using NiOX
40 nm Cu top electrode showed open-circuit voltage (VOC) of 0.94 V, short-circuit current density (JSC) of 16.73 mA/cm2, fill factor (FF) of 77.82%, and PCE of 12.27%. As described above, adjustment of thickness and surface morphology of perovskite is essential to demonstrate proper performance as well as transparency and ∼120 nm perovskite layer with smooth surface is selected as optimal condition for our semitransparent PeSCs. Here, PCE of ∼12% is reasonable values considering relatively thin perovskite layer of ∼120 nm. Comparing to opaque device, semitransparent ITO based PeSC with 8 nm Cu top electrode exhibited lower PCE of 9.19% with lower FF and JSC. Series resistance (RS) values of opaque and semitransparent PeSCs were calculated to 3.12 Ω cm2 and 13.06 Ω cm2, respectively. Lower FF was 5
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PEEOT:PSS. However, because Cu and Ni electrodes with NiOX HTL have still poor transmittance comparing to ITO/NiOX (AVT of ∼81%), all ITO-free devices showed lower photocurrent than that of ITO based devices. The higher VB of NiOX of 5.30 eV than that of PEDOT:PSS (4.95–5.00 eV) makes better energy matching with VB of CH3NH3PbI3 (5.4 eV) inducing higher VOC than PEDOT:PSS based devices. Furthermore, since NiOX is known to have small electron affinity, effective blocking of electrons can minimize recombination with the holes at the front electrode [56,57]. Consequently, Ni based device showed significantly improved PCE of 8.20% by replacing PEDOT:PSS to NiOX. On the other hand, Cu/NiOX based devices still showed poor performance of less than 4%, despite increased transmittance and better energy matching with CH3NH3PbI3. To find the reason of poor performance in Cu based systems, surface morphologies of metal thin layer with and without NiOX, which strongly affect the quality of perovskite layer and overall performance of PeSCs, were characterized. As shown in Fig. 4 (b), surfaces of Ni without and with NiOX were smooth and dense and perovskite film grown on Ni/NiOX exhibited homogeneous pinhole free surface morphology. On the other hand, smooth and dense Cu surface changed to porous layer after NiOX formation due to cracking of Cu films during thermal annealing of spin-coated NiOX at ∼350 °C. Cracking of Cu leads to degradation of electrical conductivity of Cu front electrode. Moreover, rough surface results in non-uniform surface morphology and pinholes on perovskite film. For both of these reasons, Cu/NiOX based devices still showed poor performance of less than 4%. As mentioned, the purpose of this study is to demonstrate semitransparent PeSCs with high stability as well as high efficiency. To evaluate the stability of the ITO-free semitransparent PeSCs, devices were kept in atmospheric environment (at ∼25 °C with a humidity of 45–60%) without any encapsulation and change in PCE was recorded. As shown in Fig. 5, Cu based devices showed poor stability compared to Ni based system. In particular, the PCE of Cu/PEDOT:PSS device was rapidly decreased and plummeted to ∼0% only after 6 h. On the contrary, PCE of the Ni/NiOX based device remains 60% of the initial efficiency after 144 h exposure in air.
Fig. 5. Stability of PeSCs based on Cu and Ni with PEDOT:PSS and NiOx HTLs.
as follows: VOC = 0.98 V, JSC = 11.90 mA/cm2, FF = 70.02%, and PCE = 8.20%. As shown in photograph in Fig. 3 (a), ITO-free semitransparent solar cell was successfully demonstrated and the average visible transmittance (AVT) of completed Ni based solar cell, which is determined by taking an average of the transmittance in the visible region, was evaluated to ∼22%. In order to realize a versatile applications of semitransparent PeSCs, it is necessary to satisfy a required AVT of 20- 25% of the full device with proper photovoltaic performance. The performance and optical transparency reported to date is vary depending on the device structure and materials (see summarized recent reports on semitransparent PeSCs in table S1. in supporting information). PeSCs achieving AVT of ∼20% exhibited PCE of ∼10% or lower. Furthermore, ITO-free semitransparent PeSCs showed poor PCE comparing to ITO based devices. Therefore, we conclude that our ITOfree semitransparent PeSCs are comparable to the best performance reported to date. Even better, Ni/NiOX based devices exhibited extremely small fluctuations in performance as shown in Fig. 3 (b) and Table 1. Here, we recorded deviations of important parameters within 8 devices fabricated with each configuration to verify the reliability of device performance. For further understanding on the effect of HTLs on the performance, optical, electrical, and structural changes after preparation of HTL on Cu and Ni thin layers were investigated. Cu and Ni with PEDOT:PSS showed still poor transmittance in the whole wavelength region and thus low photocurrent was obtained (see Fig. S1 in supporting information). In particular, because thicker film of 15 nm was required for Cu based device operation, Cu/PEDOT:PSS showed lower transmittance than Ni/PEDOT:PSS and leaded to lower JSC of ∼4 mA/cm2. As mentioned, by replacing typical PEDOT:PSS HTL to NiOx, both Cu and Ni based devices exhibited improved performance. Improvement in VOC and JSC was observed for both cases. Fig. 4 (a) shows the transmittance of Ni and Cu with NiOX HTL. As shown in transmittance spectra and photographs in Fig. 4 (a), Ni and Cu with NiOX showed higher transmittance of ∼70% and ∼60%, respectively, than that of Ni and Cu without and with typical PEDOT:PSS HTL. Increase in transmittance is attributed to the partial oxidation of thin metal electrode during thermal annealing of sol-gel processed NiOX and therefore higher photocurrent can be obtained by using NiOX HTL instead of
4. Conclusion In summary, ITO-free semitransparent PeSCs based on ultra thin Cu and Ni films, which are well-known non-precious metal having sufficient electrical conductivity and high work-function, were studied. To demonstrate air-stable as well as efficient semitransparent PeSCs, thin Cu and Ni were characterized as bottom electrode and their effect on overall performance of biplanar PeSCs consisting with thin metal film/ HTL/perovskite/PCBM/PEIE/Cu was investigated. Both Cu and Ni based devices exhibited improved performance by replacing typical PEDOT:PSS HTL to NiOX. In particular, Ni/NiOX based PeSC exhibiting the average visible AVT of 22% showed prominently improved performance as follows: VOC = 0.98 V, JSC = 11.90 mA/cm2, FF = 70.02%, and PCE = 8.20%. Considering required AVT for the application of power-generating window of ∼25% and recent reports, this result is one of the best performances reported to date. However, Cu based device still showed poor performance of less than 4% due to lower transmittance than that of Ni system and cracking of Cu films during thermal annealing of spin-coated NiOX. Cracking of Cu caused decrease in the electrical conductivity of Cu electrode and uneven surface morphology and pinholes in the upper perovskite layer. Furthermore, Ni/NiOX based device showed superior stability to Cu based systems. Acknowledgements This research was financially supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2018R1A2B6001206) & the Kunsan National University's Long6
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term Overseas Research Program for Faculty Member in the year 2016.
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