Recent advances in the application of two-dimensional materials as charge transport layers in organic and perovskite solar cells

FLATCHEM xxx (2017) xxx–xxx

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FLATCHEM j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / fl a t c

Recent advances in the application of two-dimensional materials as charge transport layers in organic and perovskite solar cells Quyet Van Le a, Jae-Young Choi b, Soo Young Kim a,⇑ a b

School of Chemical Engineering and Materials Science, Chung-Ang University, 84 Heukseok-ro, Dongjak-gu, Seoul 06974, Republic of Korea School of Advanced Materials Science and Engineering, 2066 Seobu-ro, Jangan-gu, Suwon, Gyeonggi-do 440-746, Republic of Korea

a r t i c l e

i n f o

Article history: Received 9 March 2017 Revised 15 April 2017 Accepted 15 April 2017 Available online xxxx Keywords: 2D materials Hole transport layer Electron transport layer Organic solar cells Perovskite solar cells

a b s t r a c t Two-dimensional (2D) materials such as graphene and transition metal chalcogenides (MX2, M: transition metal, X: S, Se, Te) have emerged as a new class of materials due to their high carrier mobility, high transparency, tunable band gap, low cost, and solution-processable properties. These materials can be fabricated into single layers or few layers through facile processes such as chemical vapor deposition or mechanical exfoliation to unlock their superior electrical and optical properties. The ability to tune the work function enables their application as hole transport layers and electron transport layers in optoelectronic devices. In this review, we focus on recent progress in the application of 2D materials as hole transport layers and electron transport layers in organic solar cells and perovskite solar cells. Ó 2017 Published by Elsevier B.V.

Contents Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The application of 2D materials as buffer layers in organic solar cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Graphene oxide (GO) and its derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Transition metal dichalcogenide nanosheets. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MoS2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . WS2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . TaS2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NbSe2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bi2Se3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Black phosphorus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The application of 2D materials as buffer layer in perovskite solar cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . GO and its derivatives. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Transition metal dichalcogenide nanosheets. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary and outlook. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

00 00 00 00 00 00 00 00 00 00 00 00 00 00

Abbreviations: 2D, Two dimension; OSC, Organic solar cell; PSC, Perovskite solar cell; HTL, Hole transport layer; ETL, Electron transport layer; PCE, Power conversion efficiency; Jsc, Current density; Voc, Open circuit voltage; PEDOT, PSS, Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate; P3HT, Poly(3-hexylthiophene-2,5-diyl); PBDTTPD, Poly[[5-(2-ethylhexyl)-5,6-dihydro-4,6-dioxo-4H-thieno[3,4-c]pyrrole-1,3-diyl][4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b0 ]dithiophene-2,6-diyl]]; PCDTBT, Po ly[N-90 -heptadecanyl-2,7-carbazole-alt-5,5-(40 ,70 -di-2-thienyl-20 ,10 ,30 -benzothiadiazole)]; PTB7, Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b’]dithiophene-2,6-diyl][3fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]; PCBM, [6,6]-Phenyl C61 butyric acid methyl ester; SAM, Self-assembly material; ITO, Indium tin oxide; GO, Graphene oxide; rGO, Reduced graphene oxide; pr-GO, p-TosNHNH2 reduced graphene oxide; SWCN, Single wall carbon nanotube; KMnO4, Potassium permanganate; K2S2O8, Potassium persulfate; P2O5, Phosphorus pentoxide; H2O2, Hydrogen peroxide; TCNQ, Tetracyanoquinodimethane; VOx, Vanadium oxide; MoOx, Molybdenum oxide; e-MoO3, evaporated molybdenum oxide; TMD, transition metal dichalcogenide; MoS2, Molybdenum disulfide; WS2, Tungsten disulfide; TaS2, Tantalum disulfide; NbSe2, Niobium diselenide; Bi2Se3, Bismuth selenide; Al, Aluminum; Ag, Silver; LiF, Lithium fluoride; Ca, Calcium; ZnO, Zinc oxide; Ar, Argon; H2, Hydrogen; Cs, Cesium; Li, Lithium; TiOx, Titanium oxide; TFT, Thin film transistor; SiO2, Silicon dioxide; FTIR, Fourier transform infrared spectroscopy; AFM, Atomic force microscopy; SEM, Scanning electron microscopy; TEM, Transmission electron microscopy. ⇑ Corresponding author. E-mail address: [email protected] (S.Y. Kim). http://dx.doi.org/10.1016/j.flatc.2017.04.002 2452-2627/Ó 2017 Published by Elsevier B.V.

Please cite this article in press as: Q. Van Le et al., Recent advances in the application of two-dimensional materials as charge transport layers in organic and perovskite solar cells, FlatChem (2017), http://dx.doi.org/10.1016/j.flatc.2017.04.002

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Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

Introduction In recent years, the need for renewable and sustainable energy resources has dramatically increased due to depleting resources of fossil fuels such as coal and oil. Among methods that produce clean and sustainable energy such as wind energy, hydraulic energy, and bio energy, turning solar energy to electric energy is most promising due to the unlimited energy provided by the Sun. Specifically, Si-based solar cells have been developed and applied in practice since 1970 [1]. However, the expensive fabrication method of Sibased solar cells has prohibited their wide usage. Thus, the replacement of Si-based solar cells with other types including dyesensitized solar cells, organic solar cells (OSCs), quantum dot solar cells, and perovskite solar cells (PSCs) has been intensively investigated [2–5]. Emerging as a new class of solar cells, OSC and PSC have received tremendous attention due to their high power conversion efficiency, flexibility, and possibility of low-cost fabrication via a roll-to-roll process [6–8]. The power conversion efficiency (PCE) has been reported to be over 10% for OSCs and over 20% for PSCs [9–11]. PSCs have been predicted to beat commercial Sibased solar cells in the market due to their ease of fabrication and extremely high PCE, which significantly reduces the production cost. Advancements in OSCs and PSCs are still underway through investigations on the active layer, electrodes, and interlayers, to boost their efficiency and to realize the stability needed for commercial use. One of the strategies to improve device performance is the insertion of interlayers between the active layers and electrodes to facilitate charge collection [12]. Generally, high-workfunction polymers and metal oxide have been utilized as hole transport layers (HTLs) [13–15], whereas low-work-function polymers and metal complexes have been employed as electron trans-

port layers (ETL) [16,17]. Poly(3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS) is widely used as an HTL, but it has several problems including high acidity of suspension (pH  1), hygroscopic properties, and inhomogeneous electrical properties, resulting in poor long-term stability [18–20]. In addition, the metal oxide compounds for HTL or ETL are normally fabricated via low vacuum deposition, which increases the fabrication cost [14,15]. Therefore, abundant and solution-processable materials with low cost and high stability for HTL and ETL in OPVs and PSCs need to be identified. Since a single layer of graphene was successfully fabricated by Novoselov et al. in 2004, the electrical and optical properties of graphene have been significantly enhanced [21,22]. The twodimensional (2D) structures of transition metal dichalcogenides have also been explored, opening a universe of applications for 2D materials in the field of optoelectronic devices [23–25]. It is reported that the work functions of 2D materials are tunable through functionalization or specific treatment methods [26–28]. Further, 2D materials can be fabricated easily via solutionprocessable techniques [29,30]. Thus, 2D materials are highly promising alternatives to the traditional HTL and ETL materials in OSCs and PSCs. The recent progress in applying 2D materials to OPVs and PSCs are summarized and discussed in this review. The application of 2D materials as buffer layers in organic solar cells Graphene oxide (GO) and its derivatives GO was used as an HTL in 2010 by Li et al. GO showed PCE comparable to that of PEDOT:PSS-based OSCs, demonstrating its potential in the field of optoelectronic devices, as shown in Figs. 1 and 2

Fig. 1. (a) Schematic of the photovoltaic device structure consisting of the following: ITO/GO/P3HT:PCBM/Al. (b) Energy level diagrams of the bottom electrode (ITO); interlayer materials (PEDOT:PSS, GO), P3HT (donor), and PCBM (acceptor); and the top electrode Al. Adapted with permission from Ref. [31].

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same device in 2012 [38]. Notably, the efficiency of the GO- and Csdoped GO-based OSC was significantly increased (PCE = 3.67%) superior to PEDOT:PSS and LiF-based OSCs (PCE = 3.15%), as shown in Fig. 5 [41]. These reports indicated that GO and its derivatives could replace conventional HTL and ETL in the near future. Recent reports of GO and its derivatives as buffer layers in OSCs are listed in Table 1.

Transition metal dichalcogenide nanosheets Similar to GO, two-dimensional transition metal dichalcogenides (2D-TMDs) can be synthesized in single layer or few layer morphologies through a mechanical or chemical process [61,62]. Owing to their excellent electrical and optical properties, 2DTMD nanosheets have been widely investigated and applied to a variety of optoelectronic devices such as photodetectors, fieldeffect transistors, gas sensors, the hydrogen evolution reaction, and solar cells [23–25,63–65]. Their superior carrier mobilities are caused by ballistic transport, enhanced photoluminescence that results from quantum confinement effects, and larger band gaps than those observed in the bulk material. It is believed that the presence of lone pairs of electrons at the surfaces of 2DTMDs, the presence of non-covalent interaction and the absence of dangling bonds could enhance their resistances to reactions with other chemical species [66]. Therefore, 2D-TMD nanosheets are expected to be applied in OSCs to improve stability. The role of 2D-TMDs as a buffer layer in OSC is investigated in this review.

Fig. 2. (a) Current–voltage characteristics of photovoltaic devices with no hole transport layer (curve labeled as ITO), with a 30 nm PEDOT:PSS layer and 2 nm thick GO film. (b) Current–voltage characteristics of ITO/GO/P3HT:PCBM/Al devices with different GO thicknesses. All measurements were under simulated A.M. 1.5 illumination at 100 mW/cm2. Adapted with permission from Ref. [31].

[31]. However, GO was reported as an insulating material due to the presence of saturated sp3 bonds (shown in Fig. 3), making the performance of GO-based devices sensitive to the thickness of the interlayer [32]. This hinders the possibility of GO replacing PEDOT:PSS in optoelectronic devices. In 2009, Eda et al. reported that the properties of GO can be tuned from insulator to semimetal with reduction, providing a guideline for its application in electronic devices [33]. After this report, many groups have tried to develop more reliable routes for fabricating HTLs derived from GO. In 2011, Yun et al. replaced GO with p-TosNHNH2-reduced GO (pr-GO) as HTL in OSCs, resulting in PCE of 3.63%, which is comparable to that of PEDOT:PSSbased OSCs, as shown in Fig. 4. Interestingly, the pr-GO-based OSC is less sensitive to the thickness of the HTL layer than the GO-based OSC. Additionally, it was shown that the air stability of the pr-GO-based OSC is higher than that of the PEDOT:PSS-based OSC [34]. Many approaches have been pursued to improve the electrical characteristics and uniformity of GO films using thermal reduction [35], p-type doping [36], GO composite [37], functionalization [38], and chemical reduction [39]. Interestingly, GO was not only applied as an HTL but also as an ETL in OSCs by performing ntype doping or blending with other ETLs [40–42]. Liu et al. reported the possibility of using GO and Cs-doped GO as HTL and ETL in the

MoS2 In 2013, Yun et al. reported the application of MoS2 as an HTL in OSC [28]. The work function of MoS2 nanosheets is around 4.5 eV, which is not suitable for HTL or ETL in OSCs. Hence, they modified the work function of the MoS2 nanosheets through p-type doping and n-type doping. As a result, PCE of 3.38 and 2.73% were achieved with p-type MoS2 as HTL and n-type MoS2 as ETL, respectively [28]. Gu et al. used ultrathin MoS2 nanosheets as HTL in inverted OSCs in 2013. PCE of 8.11% was obtained, which is slightly higher than the PCE of OSCs based on thermally evaporated MoO3 (PCE = 7.54%) [67]. Instead of using expensive noble metal doping, Le et al. utilized ultraviolet-ozone (UVO) treatment for 15 min to tune the work function of MoS2 from 4.4 to 4.9 eV. The efficiency of OSC increased from 1.08 to 2.44% [68]. Yun et al. used H2O2 to partially oxidize MoS2 to form MoS2/MoO3 composites, increasing the work function to 4.8 eV and resulting in PCE of 6.9% [69]. In another report, Liu et al. synthesized MoS2 using chemical exfoliation (ce-MoS2) and used a hydrophilic surfactant to modify the properties of ce-MoS2 (m-MoS2). As a result, the PCE of OSC reached 7.26% with m-MoS2 as the HTL. Notably, m-MoS2-based OSCs maintained PCE over 7% after 100 days, confirming the role of MoS2 in improving the stability of OSC, as shown in Fig. 6 [70]. These results indicated that ultrathin MoS2 nanosheets could be a promising candidate for HTL in OSCs. The details of device performance in recent reports are summarized in Table 2.

WS2 The use of WS2 as an HTL in OSC was reported by Le et al. [75]. In this work, they utilized UVO treatment to tune the work function of WS2, making it suitable to reduce the charge transfer barrier between the active layer and an indium tin oxide (ITO) electrode. The highest PCE achieved was 2.4% [75]. Following, Kwon et al. used WS2 nanosheets that were synthesized using the chemical evaporation method. The increase of PCE (PCE = 3.08%) in OSC compared to the previous work (PCE = 2.4%) was thought to come from the uniformity of the WS2 layer [72] (see Fig. 7 and Table 3).

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Fig. 3. Electrical and optical properties of a three-layered GO: a) conductivity as a function of exposure time to hydrazine monohydrate vapor at 80 °C; b) conductivity of GO exposed for 24 h to hydrazine monohydrate vapor at 80 °C and then annealed at increasing temperatures (solid dots), pristine GO annealed in Ar/H2 (open dots) and UHV (triangles); c) transmittance at 550 nm of GO as a function of exposure time to hydrazine monohydrate vapor at 80 °C; d) Transmittance at 550 nm of GO film exposed for 24 h to hydrazine monohydrate vapor at 80 °C and then annealed at increasing temperatures (solid dots), and pristine GO annealed in Ar/H2 (open dots); e) transfer characteristics of TFT devices based on reduced three-layered GO thin films deposited on SiO2 (300 nm)/Si with three different annealing temperatures (150, 250, 450 °C) and reduction via hydrazine. The carrier concentration as a function of annealing temperature is reported in the inset. Measurements were conducted in vacuum. Adapted with permission from Ref. [32].

Fig. 4. (a) Device structure (left) and representative current density-voltage (J–V) curves of OSCs based on different anode interfacial layers (right). Influence of the different interfacial layers of varying thicknesses on the (b) PCE, (c) fill factor (FF), (d) short-circuit current density (Jsc), and (e) open-circuit voltage (Voc) of solar cells. Adapted with permission from Ref. [34].

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Fig. 5. (a) Device structures and (b) energy level diagrams of the normal device and the inverted device with GO as the hole-extraction layer and GO–Cs as the electronextraction layer. Adapted with permission from Ref. [41].

Table 1 GO and its derivatives used as OSC buffer layers. Material

Function

Deposition method

Modification method

Device structure

PCE (%)

Ref

GO GO

HTL HTL

None None

[31] [43]

HTL HTL HTL HTL HTL

3.61 3.0 4.37 3.74 3.53 6.58

[44] [45] [46] [47] [36]

GO GO

HTL HTL

Spin-coated Spin-coated

p-Doped by chlorine GO:PEDOT:PSS

8.21

[48]

GO GO

HTL HTL

Spin-coated Spin-coated

GO: SWCNT GO/VOx GO/MoOx

GO GO rGO rGO

HTL HTL HTL HTL

Spin-coated Spin-coated Spin-coated Spin-coated

Silanization GO: rGO Reduced by vitamin C and treated by fluorine plasma Reduced by L-cysteine

rGO rGO

HTL HTL

Spin-coated Spin-coated

Reduce by p-TosNHNH2 Reduced by phenylhydrazine

rGO rGO rGO rGO rGO GO GO rGO rGO GO GO

HTL HTL HTL HTL HTL ETL ETL ETL ETL HTL, ETL HTL, ETL

Spin-coated Spin-coated Spray-coated Spin-coated Spin-coated Stamp transferred Spin-coated Spin-coated Spray-coated Spin-coated Spin-coated

Reduced by phenylhydrazine Reduced by thermal Reduced by thermal treatment Reduced by thermal treatment Reduced by thermal treatment GO/TiOx GO-Li/TiOx GO-pyrene-PCBM GO:ZnO GO-Cs GO-Cl GO-Li

ITO/GO/P3HT:PCBM/Al ITO/GO/PTB7:PC71BM/ LiF/Al ITO/C60-SAM/P3HT:PCBM/GO/Ag ITO/GO/P3HT:PCBM/LiF/Al ITO/GO/P3HT:PCBM/Ca/Al ITO/GO/P3HT:PCBM/LiF/Al ITO/GO/P3HT:PCBM/Ca/Al ITO/GO/PTB7:PC71BM/Ca/Al ITO/GO/PCDTBT:PC71BM/ ITO/GO:PEDOT:PSS/ PTB7:PC71BM/Al ITO/GO:SWCNTs/P3HT:PCBM/Ca/Al ITO/ZnO/P3HT:PCBM/GO/VOx/ Ag ITO/ZnO/P3HT:PCBM/GO/MoOx/Ag ITO/GO/P3HT:PCBM/LiF/Al ITO/GO:rGO/P3HT:PCBM/Ca/Al ITO/rGO/P3HT:PCBM/LiF/Al ITO/rGO/PBDTTPD:PC71BM/ ZnO/Al ITO/rGO/P3HT:PCBM/Ca/Al ITO/rGO/P3HT:PCBM/Ca/Al ITO/rGO/P3HT:ICBA/Ca/Al ITO/rGO/PTB7:PC71BM/Ca/Al ITO/ZnO/P3HT:PCBM/rGO/Ag ITO/rGO/P3HT:PCBM/LiF/Al ITO/rGO/P3HT:PCBM/LiF/Al ITO/rGO/P3HT:PCBM/Al ITO/rGO/P3HT:PCBM/LIF/Al ITO/PEDOT:PSS/PCDTBT:PC71BM/GO/TiOx/Al ITO/PEDOT:PSS/PCDTBT:PC71BM/GO-Li/TiOx/Al ITO/PEDOT:PSS/P3HT:PCBM/rGO-pyrene-PCBM/Al ITO/rGO:ZnO/P3HT:PCBM/MoO3/Ag ITO/GO/P3HT:PCBM/GO-Cs/Al ITO/GO-Cl/PCDTBT:PC71BM/ GO-Li/TiOx/Al ITO/GO-Cl/PTB7:PC71BM/GO-Li/ TiOx/Al G/PEDOT:PSS/P3HT:PCBM/Ca/Al

3.5 7.5

GO GO GO GO GO

Spin-coated LangmuirBlodgett Spin-coated Spin-coated Spin-coated Spin-coated Spin-coated

G

Anode

Transferred

None Controlled KMnO4 Sulfated by H2SO4 Pre-oxidation by H2SO4, K2S2O8, P2O5 p-Doped by metal chlorine

TCNQ

4.1 3.4

[37] [49]

3.08 4.21 2.72 4.8

[38] [39] [50] [51]

3.63 3.62 4.57 6.71 3.3 2.75 3.71 3.14 3.98 7.5 6.29 3.89 4.85 3.67 7.17

[34] [52]

[53] [35] [54] [55] [56] [40] [57] [58] [42] [41] [59]

8.83 2.58

[60]

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Fig. 6. (a) Chemical structures of PTB7 and PC71BM and a schematic of the OSCs architecture: ITO/ce-MoS2 or m-MoS2/PTB7:PC71BM/PFN/Al. (b) J–V curves of devices fabricated using different MoS2 solutions with various storage times. (c) Corresponding external quantum efficiency (EQE) spectra for devices with freshly prepared ce-MoS2 and m-MoS2 solutions. (d) PCE change of the devices fabricated using ce-MoS2 or m-MoS2 solutions with various periods of time. Adapted with permission from Ref. [70].

Table 2 MoS2 used as HTL in OSCs. Material

Function

Deposition method

Modification method

Device structure

PCE (%)

Ref

MoS2

HTL

Spin-coated

Doping

3.38 2.73

[28]

MoS2

HTL

Spin-coated

None

HTL HTL HTL HTL HTL HTL HTL

Spin-coated Spin-coated Spin-coated Spin-coated Transferred Spin-coated Spin-coated

UVO H2O2 Surfactant UVO UVO UVO UVO

4,02 8.11 2.44 6.9 7.26 7.64 2.96 2.95 8.66

[67]

MoS2 MoS2 MoS2 MoS2 MoS2 MoSx MoS2

ITO/p-doped MoS2/P3HT:PCBM/Ca/Al ITO/n-doped MoS2/P3HT:PCBM/ PEDOT:PSS ITO/ZnO/P3HT:PCBM/MoS2/Ag ITO/ZnO/PTB7:PC71BM/MoS2/Ag ITO/MoS2/P3HT:PCBM/LiF/Al ITO/MoS2/PBDTTT-CF:PC71BM/Ca/Al ITO/MoS2/PTB7:PC71BM/PFN/Al ITO/MoS2/PTB7:PC71BM/PFN/Al ITO/MoS2/P3HT:PCBM/LiF/Al ITO/MoSx/P3HT:PCBM/LiF/Al ITO/MoS2/PTB7:PC71BM/Ca/Al

TaS2 Le et al. demonstrated the potential of TaS2 as an HTL in OSC by improving the PCE from 1.8 (no HTL) to 3.09% by utilizing UVO treatment for 30 min [76]. Interestingly, untreated TaS2 can be used as an ETL exhibiting PCE of 2.7%, which is slightly higher than that of a TiOx-based device (2.5%). Thus, it is suggested that TaS2 can be used as dual function buffer layers in OSC after proper treatment, as shown in Fig. 8. NbSe2 The use of 2D NbSe2 nanosheets as HTL in OSC were reported by Gu et al. in 2013 [77]. In this work, Gu et al. used 2D NbSe2 as HTL

[68] [69] [70] [71] [72] [73] [74]

without any further treatment. The OSC with structure of ITO/ZnO/ PTB7:PC71BM/NbSe2/Ag was fabricated resulting in PCE of 8.1%, which is much higher than that of e-beam evaporated MoO3based devices (PCE = 7.54%) [77]. The increase of device performance is thought to come from the low trap density and suitable electrical dipole for the extraction and suppression of charge recombination by 2D NbSe2 nanosheets, as shown in Fig. 9. Bi2Se3 The use of layered Bi2Se3 as HTL was reported by Yuan et al. in 2015 [78]. In this report, the authors demonstrated that the high PCE of 4.37% was achieved, as shown in Fig. 10. Further, it is shown

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Fig. 7. (a) J–V characteristics of the devices according to the device structure and UVO treatment time. Inset shows the OPV device structure. (b) Schematic band diagram of the OPV cells. Adapted with permission from Ref. [75].

Table 3 WS2, TaS2, NbSe2, Bi2Se3 and black phosphorus used as HTLs and ETLs in OSCs. Material

Function

Deposition method

Modification method

Device structure

PCE (%)

Ref

WS2 WS2 TaS2 NbSe2 Bi2Se3 BP

HTL HTL HTL HTL HTL ETL

Spin-coated Transferred Spin-coated Spin-coated Spin-coated Spin-coated

UVO UVO UVO None None None

ITO/MoS2/P3HT:PCBM/LiF/Al ITO/MoS2/P3HT:PCBM/LiF/Al ITO/TaS2/P3HT:PCBM/LiF/Al ITO/ZnO/PTB7:PC71BM/NbSe2/Ag ITO/ZnO/P3HT:PC61BM/L-Bi2Se3/Ag ITO/BP/PTB7:PC71BM/MoO3/Ag

2.4 3.08 3.09 8.1 4.37 8.18

[75] [72] [76] [77] [78] [79]

that the life time of the Bi2Se3-based OSC was prolonged for over 4 months. The origin of the superior performance in Bi2Se3-based OSC was thought be the high conductivity, crystallinity, and energy level alignment between the electrode and active layer. These results suggested that Bi2Se3 is a promising candidate as an alternative to PEDOT:PSS in OSCs. Black phosphorus 2D back phosphorous (BP) was first applied as electron transport layer by Shenghuang Lin et al. [79]. It was reported that solu-

tion exfoliated few layers black phosphorus can be used as an effective ETL in OSCs. Typically, BP was sandwich between ZnO and PTB7:PC71BM in an inverted solar cell with configuration as shown in Fig. 11(a). The use of BP increased PCE of inverted solar cells from 7.37 to 8.18% [79]. The improvement in device performance was reported to come from the cascaded energy level caused by BP. Furthermore, BP also improved the stability of OPV by maintaining its initial PCE of 94.18% after two months [79]. These results indicated that BP is an effective ETL in OSCs in terms of high performance and stability.

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Fig. 8. (a) Changes of secondary electron spectra of TaS2 with UVO treatment time. Schematic band diagram of (b) normal OSC (N-OPV) and (c) inverted OSC (I-OPV). (c) J–V characteristics of the N-OPVs for UVO irradiation times on TaS2 as HEL. (d) J–V characteristics of the I-OPVs using TaS2 as the electron extraction layer. Adapted with permission from Ref. [76].

Fig. 9. (a) Schematic illustration of the inverted OSCs. A photoactive layer is located between a ZnO-modified ITO cathode and an NbSe2 layer-modified Ag-based anode. The active layer is a blend of (PTB7:PC71BM). (b, c) Schematic illustration of the monolayer flake of NbSe2 along the [0001] plane. (d) J–V curves and (e) EQE spectra of the devices based on PTB7:PC71BM with different HTLs. Adapted with permission from Ref. [77].

The application of 2D materials as buffer layer in perovskite solar cells GO and its derivatives Organometal halide PSC is an emerging candidate in the field, which can possibly replace Si-solar cells in the market due to its high efficiency and low cost of fabrication via solutionprocessable methods [4]. Similar to OSC, the HTL and ETL play a

crucial role in producing high performance PSCs. To avoid the use of an unstable organic HTL in PSC, GO and its derivatives have been investigated by many groups. In 2014, Wu et al. improved the PCE of PSC from 2.64 (no HTL) to 11.11% using GO as HTL, which was even higher than that of the PEDOT:PSS-based device, as shown in Fig. 12 [80]. Yeo et al. demonstrated that rGO can improve the efficiency from 8.8 (PEDOT:PSS based PSC) to 9.95%, and also the stability of the PSC [81]. Meanwhile, Palma et al. used rGO to replace

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Fig. 10. (a) UV–vis absorbance spectrum of layered Bi2Se3 (L-Bi2Se3) solution; the inset shows an image of the L-Bi2Se3 suspension. (b) TEM images of Bi2Se3 nanoplates (the scale bar is 100 nm; the inset image is a fast Fourier transform pattern of the lattice image). (c) High resolution TEM image of Bi2Se3 nanoplates (the scale bar is 4 nm; the insert image shows the magnified TEM image of Bi2Se3). (d) GIXD spectrum of exfoliated L-Bi2Se3 on ITO substrate. (e) Scheme structure of the organic solar cell (ITO/ZnO/ P3HT:PC61BM/L-Bi2Se3/Ag). (f) J–V curves of the devices based on L-Bi2Se3 measured with different storing time under ambient conditions. Adapted with permission from Ref. [78].

Fig. 11. (a) Conventional and inverted architectures of OSCs based on PTB7/PC71BM; J–V characteristics of (b) conventional, and (c) inverted OSCs with BP incorporation under different conditions, (d) Energy band structure of the OSC based on PTB7/PC71BM with the incorporation of BP (three times coating). Adapted with permission from Ref. [79].

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Fig. 12. (a) Schematic of the inverted photovoltaic device configuration of ITO/GO/CH3NH3PbI3xClx/PCBM/ZnO/Al. (b) Cross-sectional SEM image of the optimized inverted device configuration. (c) J–V curve and (d) EQE spectrum of champion device employing 2 nm GO as the hole conductor. (e) Histogram of PCEs measured from 30 devices. Adapted with permission from Ref. [80].

Fig. 13. (a) Schematic of the PSC and energy band alignment. (b) J–V curves of solar cells selected for the 1987 h endurance test. As-prepared rGO (dark blue), 1987 h rGO (blue), as-prepared spiro-OMeTAD (dark red), and 1987 h spiro-OMeTAD (red). The arrows indicate the variation of the J–V parameters with time. Adapted with permission from Ref. [81].

Fig. 14. (a) Structures of tested devices, without A) GO-Li layer as reference and with B) GO-Li layer on m-TiO2. (b) Schematic energy band diagram of different functional layers in PSC relative to vacuum. The band alignments are not to scale and show only the relative positions. (c) J–V curves of the best performing devices without encapsulation and under 1 SUN illumination. The GO-Li based cell (curve b) showed an increased JSC (+10.5%) and a slightly reduced VOC (6%) when compared to the reference one (curve a), leading to an increased efficiency (+12%). Adapted with permission from Ref. [86].

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spiro-OMeTAD to improve the life time of PSC [82]. In the prepared state, the PCE of the PSC based on rGO was inferior to that of the spiro-OMeTAD-based PSC. However, the PCE of the rGO-based device increased from 4.87 to 6.62% after 1987 h in air. On the other hand, the PCE of the spiro-OMeTAD-based device decreased from 11.06 to 6.5%, as shown in Fig. 13 [82]. The authors hypothesized that the increase of PCE in the rGO-based PSC is due to the reduction of density-charged bulk defects within the perovskite layer caused by light illumination during measurement. Further, the decrease of PCE in spiro-OMeTAD-based PSC comes from the negative effect of additives that corroded the perovskite materials. In other work, Luo et al. employed the rGO/dopant-free spiroOMeTAD as an HTL for PSC to eliminate the use of additives in

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spiro-OMeTAD, resulting in a PCE of 10.6% [83]. Surprisingly, the PCE of rGO/dopant-free spiro-OMeTAD-based PSC retained 85% of its initial value even after 500 h, whereas the PCE of pyridinedoped spiro-OMeTAD-based PSC retained only 36% of its initial value. More recently, Lee et al. demonstrated that a blend of GO/ PEDOT:PSS could also improve the efficiency and lifetime of PSC [84]. Specifically, the PCE increased from 8.23% (PEDOT:PSSbased PSC) to 9.74%. In addition, the result showed that the PEDOT:PSS-based PSCs were completely diminished after 300 h, while the GO/PEDOT:PSS-based PSC still operated after 500 h under ambient conditions. The use of layered GO/PEDOT:PSS as HTL in PSC was suggested by Li et al., resulting in the improvement of PCE from 10 to 13.1% [85]. To apply GO as ETL in PSC, Agresti et al. employed lithiumneutralized graphene oxide (GO-Li) as an interlayer to facilitate electron transport from a perovskite layer to a TiO2 layer. The work function of GO reduced to 4.3 eV through Li-modification, matching the TiO2 conduction band. Thus, the PCE of GO-Li/m-TiO2based device increased from 10.3 to 11. 8%, as shown in Fig. 14 [86]. Transition metal dichalcogenide nanosheets The use of MoS2 and WS2 as HTL in PSCs has also been demonstrated by several groups [87,88]. Matteocci et al. inserted a thin layer of MoS2 between CH3NH3PbI3 and spiro-OMeTAD as HTL and as a protective layer. An efficiency of 13.3% was obtained, which is similar to that of a spiro-OMeTAD-based device (PCE = 14.2%), as shown in Fig. 15 [87]. Interestingly, the MoS2based PSC was more stable in air than the reference cells without MoS2. Specifically, the PCE of the MoS2-based device retained 93% of its initial value after 550 h, while the PCE of the device without MoS2 retained only 66%. Kim et al. used only MoS2 or WS2 as HTLs in inverted PSCs. PCEs of 9.53 and 8.02% were recorded with MoS2 and WS2 as HTLs, respectively, which is comparable to that of PEDOT:PSS-based devices [88]. These suggested that MoS2 and WS2 are promising candidates as HTLs in PSCs to replace unstable organic HTLs. Summary and outlook High-efficiency OSCs and PSCs with 2D HTL and ETL materials have been demonstrated. It is shown that the PCE of OSCs and PSCs with 2D materials is higher than that with commercial PEDOT:PSS. The replacement of PEDOT:PSS with 2D materials is feasible in the near future. However, a more reliable and low-cost scalable fabrication process for the uniform layer is needed to bring 2D materials to market. Thus, there is scope for engineering 2D materials with low cost and high efficiency that are suitable for use in OSCs and PSCs. It is considered that 2D-TMD materials such as MoS2, WS2, TaS2, and NbSe2 need more attention and investigation. Acknowledgment This research was supported by a National Research Foundation of Korea (NRF) grant provided by the Korean government (MSIP) (No. 2014R1A2A1A11051098). References

Fig. 15. (a) J–- curve of the cell fabricated with MoS2 HTL and without HTL. (b) J-V curve of the cells measured after fabrication (0.1 cm2 active area). (c) Stability of PSCs after 550 h. Adapted with permission from Ref. [87].

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Please cite this article in press as: Q. Van Le et al., Recent advances in the application of two-dimensional materials as charge transport layers in organic and perovskite solar cells, FlatChem (2017), http://dx.doi.org/10.1016/j.flatc.2017.04.002