Recent Advances in the Inverted Planar Structure of Perovskite Solar

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Recent Advances in the Inverted Planar Structure of Perovskite Solar Cells Published as part of the Accounts of Chemical Research special issue “Lead Halide Perovskites for Solar Energy Conversion”. † † ‡ § † Lei Meng, Jingbi You,*, , Tzung-Fang Guo, and Yang Yang*, † Department of Material Science and Engineering, University of California, Los Angeles, Los Angeles, California 90095, United States ‡ Key Lab of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, P. R. China § Department of Photonics, Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan, Taiwan 701, ROC

CONSPECTUS: Inorganic−organic hybrid perovskite solar cells research could be traced back to 2009, and initially showed 3.8% efficiency. After 6 years of efforts, the efficiency has been pushed to 20.1%. The pace of development was much faster than that of any type of solar cell technology. In addition to high efficiency, the device fabrication is a low-cost solution process. Due to these advantages, a large number of scientists have been immersed into this promising area. In the past 6 years, much of the research on perovskite solar cells has been focused on planar and mesoporous device structures employing an n-type TiO2 layer as the bottom electron transport layer. These architectures have achieved champion device efficiencies. However, they still possess unwanted features. Mesoporous structures require a high temperature ° ff (>450 C) sintering process for the TiO2 sca old, which will increase the cost and also not be compatible with flexible substrates. While the planar ff − structures based on TiO2 (regular structure) usually su er from a large degree of J V hysteresis. Recently, another emerging structure, referred to as an “inverted” planar device structure (i.e., p-i-n), uses p-type and n-type materials as bottom and top charge transport layers, respectively. This structure derived from organic solar cells, and the charge transport layers used in organic photovoltaics were successfully transferred into perovskite solar cells. The p-i-n structure of perovskite solar cells has shown efficiencies as high as 18%, lower temperature processing, flexibility, and, furthermore, negligible J−V hysteresis effects. In this Account, we will provide a comprehensive comparison of the mesoporous and planar structures, and also the regular and inverted of planar structures. Later, we will focus the discussion on the development of the inverted planar structure of perovskite solar cells, including film growth, band alignment, stability, and hysteresis. In the film growth part, several methods for obtaining high quality perovskite films are reviewed. In the interface engineering parts, the effect of hole transport layer on subsequent perovskite film growth and their interface band alignment, and also the effect of electron transport layers on charge transport and interface contact will be discussed. As concerns stability, the role of charge transport layers especially the top electron transport layer in the devices stability will be concluded. In the hysteresis part, possible reasons for hysteresis free in inverted planar structure are provided. At the end of this Account, future development and possible solutions to the remaining challenges facing the commercialization of perovskite solar cells are discussed.

■ INTRODUCTION recombination.5 After 6 years of efforts, the power conversion efficiency (PCE) of perovskite solar cells has risen from about Inorganic−organic hybrid perovskite solar cells have attracted − 3% to over 20%,1,6 15 which is close to that of traditional solar great attention due to their solution processing and high 1,2 cell technologies such as Si, CIGS, and CdTe. performance. Organic halide perovskites have ABX3 crystal structures, where A, B, and X are organic cation, metal cation and ■ PLANAR STRUCTURE VERSUS MESOPOROUS halide anion, respectively (Figure 1a), where the band gap can be STRUCTURE (TiO SYSTEM) tuned from the ultraviolet to infrared region through varying 2 − these components (Figure 1b).2 4 This family of materials The hybrid perovskite solar cell was initially discovered in a dye 1 exhibits a myriad of properties ideal for PV such as high dual sensitized solar cell (DSSC) using a mesoporous TiO2 structure. fi electron and hole mobility, large absorption coefficients result- Miyasaka and co-workers were the rst to utilize the perovskite ing from sp antibonding coupling, a favorable band gap, a strong defect tolerance and shallow point defects, benign Received: September 3, 2015 grain boundary recombination effects, and reduced surface

 Figure 1. (a) Crystal structure of the halide perovskite material: A site is typically CH3NH3I (MAI), NH2CH NH2I (FAI), Cs; B site is commonly Pb, ﬀ Sn; X site could be Cl, Br or I. (b) Absorption of FAPbIyBr3‑y with di erent ratios of Br and I. Reprinted with permission from ref 4. Copyright 2014 Royal Society of Chemistry

ﬀ Figure 2. (a) Plot of exciton di usion length versus PL lifetime quenching ratios for CH3NH3PbI3. Reprinted with permission from ref 16. Copyright 2013 American Association for the Advancement of Science. (b) Time-resolved PL of the mixed halide perovskite (CH3NH3PbI3−xClx) with quenching.Reprinted with permission from ref 17. Copyright 2013 American Association for the Advancement of Science.

Figure 3. Three typical device structures of perovskite solar cells: (a) mesoporous, (b) regular planar structure, and (c) inverted planar structure. fi (CH3NH3PbI3 and CH3NH3PbBr3) nanocrystal as absorbers in structure was feasible. The rst successful demonstration of the DSSC structure, achieving an efficiency of 3.8% in 2009.1 In 2012, planar structure can be traced back to the perovskite/fullerene Park and Gratzel et al. reported a solid state perovskite solar cell structure reported by Guo et al.,19 showing a 4% efficiency. The by using the solid hole transport layer to improve stability.7 After low efficiency reported at that time was due to the inferior film that, several milestones in device performance have been achieved quality and inadequate absorption of the perovskite film.19 The − by using mesoporous structure.6 15 However, these mesoporous breakthrough of the planar perovskite structure was obtained devices need a high temperature sintering of TiO2 layer that could by using a dual source vapor deposition, providing dense and increase the processing time and cost of cell production. high quality perovskite films that achieved 15.4% efficiency.20 Sum et al. and Snaith et al. independently reported that the Recently, the efficiency of the planar structure was pushed over methylammonium-based perovskites own long charge carrier 19% through interface engineering.12 These results showed that ff ∼ ∼ di usion lengths ( 100 nm for CH3NH3PbI3 and 1000 nm for the planar structure could achieve similar device performance 16,17 CH3NH3PbI3−xClx, Figure 2). Recently, single crystals of as the mesoporous structure. The evolution of the structure of ff CH3NH3PbI3 were found to reach di usion lengths larger than perovskite is shown in Figure 3. 175 um.18 Further studies demonstrated that perovskites exhibit The planar structure can be divided into two categories ambipolar behavior, indicating that the perovskite materials depending on which selective contact is used on the bottom, that themselves can transport both of electrons and holes between is, regular (n-i-p) and inverted (p-i-n); the device structures are the cell terminals.16 All of these results indicated that a planar shown in Figure 3b and c, respectively. The regular n-i-p structure

B DOI: 10.1021/acs.accounts.5b00404 Acc. Chem. Res. XXXX, XXX, XXX−XXX Accounts of Chemical Research Article

Figure 4. (a) Device structure of the first inverted planar structure of perovskite solar cell. (b) band alignment. Reprinted with permission from ref 19. Copyright 2013 Wiley-VCH. (c) Thin perovskite film (<100 nm) as the absorber. Reprinted with permission from ref 21. Copyright 2014 Royal Society of Chemistry. (d) Thicker film (about 300 nm) as the absorber. Reprinted with permission from ref 22. Copyright 2014 American Chemical Society.

Table 1. Several Representative Devices Performances of Inverted Planar Structured Perovskite Solar Cells

2 perovskite processing HTL ETL Voc (V) Jsc (mA/cm ) FF (%) PCE (%) stability ref

one-step PEDOT:PSS PC61BM/BCP 0.60 10.32 63 3.9 19

two-step PEDOT:PSS PC61BM 0.91 10.8 76 7.4 21

one-step (Cl) PEDOT:PSS PC61BM 0.87 18.5 72 11.5 22

one-step (Cl) PEDOT:PSS PC61BM/TiOx 0.94 15.8 66 9.8 23

solvent engineering PEDOT:PSS PC61BM/LiF 0.87 20.7 78.3 14.1 26

one-step (moisture, Cl) PEDOT:PSS PC61BM/PFN 1.05 20.3 80.2 17.1 11 one-step (hot-casting, Cl) PEDOT:PSS PCBM 0.94 22.4 83 17.4 27

one-step (HI additive) PEDOT:PSS PC61BM 1.1 20.9 79 18.2 28

coevaporation PEDOT:PSS/Poly-TPD PC61BM 1.05 16.12 67 12.04 24

coevaporation PEDOT:PSS/PCDTBT PC61BM/LiF 1.05 21.9 72 16.5 31 two-step spin-coating PTAA PCBM/C60/BCP 1.07 22.0 76.8 18.1 29

one-step solvent PEDOT:PSS C60 0.92 21.07 80 15.44 39

one-step (Cl) PEDOT:PSS PC61BM/ZnO 0.97 20.5 80.1 15.9 140 h 41

one-step (Cl) PEDOT:PSS PC61BM/ZnO 1.02 22.0 74.2 16.8 60 days 42

one-step NiOx PC61BM/BCP 0.92 12.43 68 7.8 33

one-step NiOx:Cu PC61BM/C60-bis surfcant 1.11 19.01 73 15.4 244 h 35

solvent engineering NiOx PC61BM/LiF 1.06 20.2 81.3 17.3 36

two-step NiOx ZnO 1.01 21.0 76 16.1 >60 days 30 has been extensively studied and could be traced back to dye poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) sensitized solar cells. The p-i-n structure is derived from the (PEDOT:PSS) and fullerene derivative, were directly imple- organic solar cell, and usually, several charge transport layers used mented as the HTL and ETL in a perovskite device. Through in organic solar cells were successfully transferred into perovskite choosing a proper fullerene derivative and optimizing the 11 solar cells. processing conditions of the perovskite film, a PCE of 3.9% was In this Account, we will focus on the inverted planar structure delivered. Later, Sun et al. succeeded in making a thicker and of perovskite solar cells, including their working mechanism, denser perovskite film by introducing two-step sequential ffi ff methods for improving e ciency, stability, and hysteresis e ects. deposition into the planar device that increased the device performance to 7.41% (Figure 4c).21 After 2013, several attempts ■ THE INVERTED PLANAR STRUCTURE were made to improve the efficiency, including film formation The first inverted planar structure of perovskite solar cells and interface engineering, which will be discussed in the next adopted a similar device structure to the organic solar cell sections. The main development of the inverted structure (Figure 4a and b).19 The traditional organic transport layers, perovskite solar cells are summarized in Table 1.

C DOI: 10.1021/acs.accounts.5b00404 Acc. Chem. Res. XXXX, XXX, XXX−XXX Accounts of Chemical Research Article

Figure 5. High quality of perovskite films obtained by various methods: (a) Interdiffusion. Reprinted with permission from ref 25. Copyright 2014 Royal Society of Chemistry. (b) Solvent engineering. Reprinted with permission from ref 26. Copyright 2014 Royal Society of Chemistry. (c) Moisture assisted. Reprinted with permission from ref 11. Copyright 2014 AIP Publishing. (d) Hot spin coating. Reprinted with permission from ref 27. Copyright 2015 American Association for the Advancement of Science. ■ FILM GROWTH FOR IMPROVING EFFICIENCY OF improve the film quality. This produced an 18.1% efficiency INVERTED PLANAR SOLAR CELLS without hysteresis.28 Although the inverted planar of perovskite solar cells based on Initial studies adopted a thin perovskite absorbing layer of less PEDOT:PSS hole transport layer showed promising efficiency, than 100 nm, which limited the light harvesting and short circuit there are still several challenges need to overcome for achieving current of the devices.19,21After learning that the perovskite higher efficiencies.3,11 The growth of perovskite film on materials possess diffusion lengths over 100 nm,16,17 thicker PEDOT:PSS often leads to pinhole generation and incomplete absorption layers were possible without sacrificing the charge 3,11 surface coverage resulting in low device performance. Recent transport properties. Yang et al. and Snaith et al. first results show that perovskite growth is strongly dependent on independently used a thicker mixed halide perovskite film ∼ ffi the bottom substrates. We have showed that NiOx could yield a ( 300 nm) as the absorber (Figure 4d), showing an e ciency fi 22,23 better perovskite lm than on PEDOT:PSS and give higher of about 11.5% and 9.8%, respectively. Bolink et al. showed 30 ff ffi Voc. Meredith et al. also observed di erent crystal quality 12.04% e ciency by thermal deposition of 285 nm thick of ff 31 24 when perovskite was deposited on di erent polymer surfaces. perovskite and adopting an organic charge transport layer. Perovskite appears to show better crystallization when a more Later, several processing methods for growth of high quality crystalline bottom substrate is used, indicating that the of perovskite film have been invented. Huang et al. showed a fi ff fi crystallinity of the substrate could contribute to the lm quality di usion approach to form high quality CH3NH3PbI3 lms, of the perovskite layer.31 Wetting properties between the solvent which could be considered as modified two-step process, where fi with the bottom substrate was found to be another issue rst PbI2 was deposited onto PEDOT:PSS and followed by spin- effecting perovskite film growth. Huang et al. showed crystal size coating of MAI onto the PbI2 surface and annealing to form the fi 25 of the perovskite lm was much larger than that of the thickness perovskite film (Figure 5a). The perovskite film was formed by ffi ff ffi and demonstrated a higher e ciency by using PTAA as the hole interdi usion of CH3NH3I into PbI2, producing e cient devices 25 transport layers, which was could severe as a nonwetting surface of 15% that are highly reproducible. Seok et al. transferred the for the solvent used for perovskite, such as DMF.29 solvent engineering to get pinhole free film and adopted it into the inverted structured perovskite solar cells, yielding 14.1% efficiency (Figure 5b).26 You et al. discovered that moisture- ■ INTERFACE ENGINEERING FOR EFFICIENT SOLAR assisted perovskite film growth could improve the film quality CELL-HOLE TRANSPORT LAYER during annealing of the perovskite precursor film and showed The n-i-p regular structure usually shows an open voltage over − as high as 17.1% efficiency.14 Nie et al. reported millimeter 1V,12 15 while the inverted structure shows a slight open circuit − sized perovskite gains by coating the hot precursor film onto voltage drop (0.9−1 V).19,21 23,25,27,30 In addition to the inferior the hot substrate (Figure 5d), for a champion device close crystal film quality on PEDOT:PSS as mentioned above, the to 18% efficiency.27 Recently, Im reported a high quality band alignment between perovskite and the traditional hole perovskite film on PEDOT:PSS using HI as an additive to transport layer PEDOT:PSS could be another issue. The work

D DOI: 10.1021/acs.accounts.5b00404 Acc. Chem. Res. XXXX, XXX, XXX−XXX Accounts of Chemical Research Article

Figure 6. Device structure and performance of modified PEDOT:PSS based devices. (a) Device structure; poly-TPD was used as a typical layer. Several other p-type materials could be also used. (b) Band alignment. (c) Device performance using PEDOT:PSS/poly-TPD bilayer as hole transport layer under different light intensity. Reprinted with permission from ref 24. Copyright 2014 Nature Publishing Group. (d) Device performance using PEDOT:PSS/PCDTBT bilayer as hole transport layer. Reprinted with permission from ref 31. Copyright 2015 Nature Publishing Group function of the generally used hole transport layer PEODT:PSS hole transport layer is CuSCN. Significant progress has been is about 4.9−5.1 eV, which is shallower than that of the valence made with CuSCN in the n-i-p perovskite solar cell, showing band of perovskite layer (5.4 eV), leading to an imperfect ohmic 12.4% efficiency using a 600−700 nm thick of CuSCN layer.37 contact between perovskite and the p-type transport layer and a Recently, using electrodeposited CuSCN as a hole transport consequent Voc loss (Figure 6). To address this problem, layer in inverted structure, as high as 16.6% efficiency have been fi 38 PEDOT:PSS must be modi ed or replaced by another material obtained. Several other metal oxides such as MoO3,V2O5,and with a high work function. The most common polymers, such as WO3 seem to be suitable for high stability hole transport layers; poly-TPD,24 PCDTBT,31 and PTAA,29 with deep HOMO levels however, it seems these metal oxide materials are not resistive ∼− ( 5.4 eV) have been used to modify the PEDOT:PSS surface, enough to the acidity of CH3NH3I. and the devices based on the bilayer hole transport layer 24,29,31 (PEDOT:PSS/polymer) showed an enhanced Voc (>1 V). ■ INTERFACE ENGINEERING OF ELECTRON Unfortunately, these polymers are usually hydrophobic, and TRANSPORT LAYER thus, the perovskite precursor cannot be coated onto these polymer surfaces. Alternatively, evaporation processes of perov- For p-i-n perovskite solar cells, fullerenes are usually used as the skite film must be adopted.24,31 Recently, p-type water-soluble electron transport layer, where PCBM is the most popular n-type polyelectrolyte with deep work function (>5.2 eV) showed good charge transport layer. Recent studies show that C60 should be 32 ff device performance when it is used as hole transport layers, more e ective than that of PCBM as an electron transport layer 39 which could be a good candidate of replacing PEDOT:PSS if it is due to the higher mobility and conductivity of C60. Jen et al. air stable. showed that the PCE of fullerene-derived perovskite solar cells Transition metal oxides such as NiO, MoO3,V2O5 and WO3 improves with increasing electron mobility in the fullerene layer, own higher work function than PEDOT:PSS, and these metal indicating the critical role of bulk transport through fullerene 39 oxides could be a good candidate of replacing of PEDOT:PSS in promoting charge dissociation/transport (Figure 8a). As for fi to get a larger VOC. Guo et al. rst reported use of solution improving the charge transport properties of the fullerene, processed NiOx as the hole transport layer, demonstrating an Li et al. attempted to dope the PCBM using graphdiyne (GD) efficiency of about 8% (Figure 7a−c).33 Han et al. adopted a to improve the coverage and the conductivity of PCBM, and hybrid interfacial layer of NiO/Al2O3 to improve the device also improve the device performance from 10.8% to 13.9% efficiency to 13% with a high fill factor.34 Jen et al. used Cu-doped (Figure 8b).40 In addition, it was confirmed that the fullerene NiOx as a hole transport layer and achieved open circuit voltages itself cannot fully form a prefect ohmic contact with a metal such as high as 1.1 V with an efficiency of 15.4% .35 You et al. adopted a as Al or Ag. Several buffer layers, including BCP,19 PFN,11 LiF,26 − fi 35 sol gel process for high quality NiOx lms, demonstrating an and self-assembled C60 derivatives, can further improve the efficiency of 16.1%.30 Recently, Seok et al. reported an inverted ohmic contact. Furthermore, metal oxides such as ZnO and fi structure using a NiOx lm deposited via pulsed laser deposition TiO2, combined with fullerene as an electron transport layer, (PLD), and pushed the device efficiency as high as high as not only improve the ohmic contact but also improve device 36 23,30,41,42 17.13% (Figure 7d). In addition to NiOx, another promising stability.

E DOI: 10.1021/acs.accounts.5b00404 Acc. Chem. Res. XXXX, XXX, XXX−XXX Accounts of Chemical Research Article

Figure 7. Device structure and performance using NiOx as hole transport layer. (a) Device structure. (b) Band alignment. (c) Device performance of the ﬁ rst demonstration using NiOx as hole transport layers. Reprinted with permission from ref 33. Copyright 2014 Wiley-VCH. (d) Present champion device performance using NiO as hole transport layers. Reprinted with permission from ref 36. Copyright 2015 Wiley-VCH.

ﬀ Figure 8. (a) Inverted planar structure of perovskite solar cells using di erent fullerenes (PC61BM, IC61BA and C60) as electron transport layers. Reprinted with permission from ref 39. Copyright 2015 Wiley-VCH. (b) Device performance using PCBM with and without graphdiyne doping. Reprinted with permission from ref 40. Copyright 2015 American Chemical Society.

■ STABILITY OF INVERTED STRUCTURE ■ ELECTRON TRANSPORT LAYER ON STABILITY The power conversion eﬃciency of perovskite solar cells has For the inverted structure, electron transport layer is the topmost been pushed up to 20% and 18% for regular and inverted layer exposed to the ambient air except for metal electrode. It was structures, respectively. For practical application, a reliable and found that the fullerene could absorb oxygen or water onto the stable performance is strongly needed. Present results show that surface, leading to a dipole moment and a large resistance as a the stability of perovskite is a critical issue, the main problem in result of degradation (Figure 9d).30 On the other hand, the small stability of perovskite come from the instability of perovskite molecular fullerene layer could not be too thick due to its low ﬁ material itself, which decomposes into PbI2 and CH3NH3Iin conductivity. A thin fullerene layer cannot form a continuous lm a high humidity environment. Another concern is interfacial on the perovskite surface, and could potentially lead to physical stability, several organic charge transport layers that have been direct contact between perovskite and the electrode. In this case, commonly used in perovskite solar cells can react with oxygen the perovskite and metal such as Al or Ag can react when in a and water in the ambient air, thus promoting device degradation. humid environment (Figure 10).30 Therefore, the devices based In addition, several electrode materials such as Ag, Al could react on the PCBM ETL showed poor stability (Figure 9a, b).30 with perovskite when they were directly contact.30 To improve the device stability, several metal oxides have been

F DOI: 10.1021/acs.accounts.5b00404 Acc. Chem. Res. XXXX, XXX, XXX−XXX Accounts of Chemical Research Article

Figure 9. (a) Degradation of inverted planar structure of perovskite solar cells using ITO/PEDOT:PSS/perovskite/PCBM/Al. (b) Light J−V and dark J−V curves of the devices under storage in ambient air for diﬀerent times (c, d) ZnO and PCBM interface stability under ambient air, respectively. Reprinted with permission from ref 30. Copyright 2015 Nature Publishing Group.

water from the surrounding environment. In addition, PEDOT:PSS has acidic properties that could potentially react with the bottom metal oxide layers. Both of which would aﬀect the long-term stability of the perovskite solar cell.30,35 According to these concerns, inorganic charge transport layers have been used, such as NiOx and CuSCN. Jen et al. used Cu:NiOx as a hole transport layer, showing markedly improved air stability as compared to the PEDOT:PSS-based device. The PCE of the Cu:NiOx based device remains above 90% of the initial value even after 240 h of storage in air (light soaking has not been mentioned).35 You et al. also demonstrated that the air stability of perovskite solar cells can be improved by replacing 30 PEDOT:PSS with NiOx.

Figure 10. Electrode stability with different electron transport layers ■ PEROVSKITE MATERIALS STABILITY (PCBM and ZnO). Reprinted with permission from ref 30. Copyright Although the interface can improve the stability of perovskite 2015 Nature Publishing Group. solar cells, the intrinsic properties of the perovskite material are the main issue for long-term stability. The typical perovskite 23,30,41,42 used in the inverted structure. Snaith et al. first reported material CH3NH3PbI3 shows severe moisture and light instability. The perovskite materials could be easily decomposed a PCBM/TiOx bilayer electron transport layer, improving the coverage of the electron transport layer as the pinholes from into PbI2 and CH3NH3I, followed by decomposition into the PCBM could be filled.23 Similary PCBM/ZnO bilayers were CH3NH2 and HI when exposing the perovskite in a high level found to be suitable for improving device performance and of moisture environment. It was found that perovskite can self- stability (Figure 11).41,42 You et al. significantly improved decompose when exposed to light for long periods of time. Exploration of highly stable perovskite materials could be the stability through use of ZnO as the top electron charge transport 43 layer. It was found that the device can nearly maintain its original next direction in perovskite solar cell research. efficiency after 60 days of storage in ambient air with room light soaking (Figure 12).30 ■ HYSTERESIS IN INVERTED PLANAR SOLAR CELLS I−V Hysteresis is still a controversial topic in perovskite solar ■ HOLE TRANSPORT LAYER STABILITY cells. This issue was first raised at the beginning of 2014.12,13 PEDOT:PSS has been widely used in organic solar cells as an It has been found that hysteresis in the regular planar structure organic charge transport layer due to its good conductivity. is more severe than that in the case of the mesoporous However, PEDOT:PSS is hydrophilic and can easily absorb structure.12,13 Interestingly, most reports have shown negligible

G DOI: 10.1021/acs.accounts.5b00404 Acc. Chem. Res. XXXX, XXX, XXX−XXX Accounts of Chemical Research Article

Figure 11. (a) Device structure of inverted planar structure of perovskite solar cells with bilayer (PCBM/ZnO). (b) Device stability using PCBM/ZnO bilayer structures. Reprinted with permission from ref 41. Copyright 2014 Springer.

Figure 12. (a) J−V curves of all metal oxide based perovskite solar cells stored in ambient for 60 days. (b) Comparison between organic and inorganic charge transport layers. Reprinted with permission from ref 30. Copyright 2015 Nature Publishing Group.

Figure 13. Hysteresis study. (a) Forward and reverse scan for the PCBM with and without thermal annealing. (b) Proposed mechanism of PCBM diﬀusion for grain boundaries passivation. (c) Conﬁrmation of reduced trap density. (d) Light response of the device with and without PCBM annealing. Reprinted with permission from ref 44. Copyright 2014 Nature Publishing Group

H DOI: 10.1021/acs.accounts.5b00404 Acc. Chem. Res. XXXX, XXX, XXX−XXX Accounts of Chemical Research Article hysteresis in the inverted planar structure with fullerene as further improved. In inverted structures, fullerenes can forbid electron transport layer.11,22,30,44 The most popular explanation ion movement and enhance the charge transfer to reduce the − is ion movement stabilization.44 47 The fullerene penetrates/ hysteresis. It seems that metal oxides introduce hysteresis, and diffuses into the perovskite layer through the pinholes/grain a fullerene/metal oxide such as PCBM/ZnO bilayer system boundary during processing (spin-coating or annealing44,45). shows promise in terms of reduced hysteresis and improved Mobile ions in the perovskite interact with fullerene to form a highly stability. fullerene halide radical,46 which is thought to stabilize electro- The planar structure shows a simple device structure and a static properties, reducing the electric field-induced anion promising efficiency (close to 20%). By further improving the migration that may give rise to hysteresis, and thus resulting in perovskite crystallinity and also the interface, it could be no hysteresis.47 Huang et al. also demonstrated that the fullerene anticipated that the efficiency will catch up or be over than that penetration into perovskite layer during the annealing and of traditional inorganic thin film solar cells such as CIGS and passivate the traps in the perovskite and reduce the hysteresis CdTe. Compared with the inorganic thin films solar cells, which (Figure 13). In our opinion, except for ion movement needs high vacuum and temperature processing, planar structure stabilization, neglected charge accumulation and also capacitance of perovskite solar cells could be fabricated in ambient air or could be another reason for hysteresis free. On one hand, while nitrogen-filled glovebox at low temperature. This will reduce fullerene was diffused into perovskite, the device showed a similar the fabrication cost. Furthermore, perovskite solar cells are based structure as bulk heterojunction (BHJ) or a mesoporous on solution process, which is compatible with several coating structure (Figure 13), and there are efficient channels for charge techniques, such as blade-coating and roll-to-roll techniques to extraction. On the other hand, fast charge transfer between produce large area and flexible devices. Perovskite solar cells perovskite and fullerene allows efficient charge extraction.30 could be utilized in our daily life if the long stability issue could While in the regular structure, there is limited interface contact be resolved. between metal oxide and perovskite, and more importantly, the charge transfer between metal oxide and perovskite is not ■ AUTHOR INFORMATION ffi 30 e cient, which could lead to serious charge accumulation at Corresponding Authors the interface between perovskite/metal oxide and form a large *E-mail: [email protected] (J.Y.). capacitance. Therefore, there is no or neglected hysteresis in * inverted structure, while there is significant hysteresis in regular E-mail: [email protected] (Y.Y.). structure. Notes The authors declare no competing financial interest. ■ CONCLUSIONS AND FUTURE OUTLOOK Biographies Perovskite solar cells have reached efficiencies over 20% based on the regular n-i-p mesoporous structures, whereas for the inverted Lei Meng is a Ph.D. candidate in the department of Materials Science p-i-n structure the state art of performances are approximately and Engineering at UCLA. He obtained his bachelor degree from 18% efficient. To further improve efficiencies of inverted solar Shanghai Jiaotong University in China and master degree from cells, it is necessary to modify interfaces, especially the contact Northwestern University. His research interests are mainly in polymer between perovskite and the hole charge transport layer. The and perovskite solar cells. fi commonly used PEDOT:PSS surface should be modi ed or Jingbi You is currently a full professor in the Institute of Semi- replaced by another material with a higher work function, and in conductors, Chinese Academy of Sciences (ISCAS), and he obtained favor of larger crystal growth. After resolving the band alignment National 1000 Young Talents award in 2015. He received his Ph.D. between hole transport layer and perovskite and also the growth degree in Material Sciences from ISCAS in 2010, and later joined Prof. fi ffi of the perovskite lm on the hole transport layer, the e ciency Yang Yang’s group of UCLA as a postdoc fellow from 2010 to 2015. His should be much improved and should better compete with the present research interests are organic/inorganic semiconductor regular structure of perovskite solar cells. materials and their optoelectronic devices such as LEDs, solar cells, Several processing technologies such as solvent treatment/ and detectors. annealing,26,47 moisture assisted growth,11 additives,28 and hot spin-coating27 have been confirmed as an effective way for Tzung-Fang Guo is professor and the department chair of the obtaining high quality of perovskite films. Exploring new methods Department of Photonics, Advanced Optoelectronic Technology or combining these existing technologies and further improving Center, National Cheng Kung University, Taiwan. He received his the film quality should be our next direction. PhD degree in 2002 from the department of Materials Science and The stability of the perovskite is still the main issue for its Engineering, UCLA. His current research interests are organic electro- commercial application. Even though more than 60 days stability optical materials and devices, polymer and organic light-emitting diodes under weak room light illumination has been demonstrated, (PLEDs and OLEDs), and advance functional materials. it is not enough for practical applications. Further exploration Yang Yang is the Tannas Jr. chair professor of Materials Science and of alternative perovskite material candidates and stabilizing Engineering at UCLA. He holds a Ph.D. in Physics and Applied Physics additives may lead to improved perovskite stability in the future. from the University of Massachusetts, Lowell in 1992, respectively. Moreover, encapsulation technology is another important Before he joined UCLA in 1997 as an assistant professor, he served on component necessary to enhance the lifetime of perovskite the research staff of UNIAX (now DuPont Display) in Santa Barbara solar cells. To improve the stability at interfaces, inorganic charge from 1992 to 1996. Yang is now the faculty director of the Nano transport layers show better results. At the least, the upmost layer Renewable Energy Center (NREC) of the California Nanosystem should be robust and relatively impenetrable to moisture and Institute of UCLA. He is a materials physicist with expertise in the fields oxygen. At present, metal oxides such as ZnO and TiO2 colloids of organic electronics, organic/inorganic interface engineering, and the have been coated onto the fullerene transport layers for development and fabrication of related devices, such as photovoltaic improved device stability. However, these methods could be cells, LEDs, and memory devices.

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K DOI: 10.1021/acs.accounts.5b00404 Acc. Chem. Res. XXXX, XXX, XXX−XXX