Recent Advances in the Inverted Planar Structure of Perovskite Solar

Article pubs.acs.org/accounts

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 (>450 °C) sintering process for the TiO2 scaffold, which will increase the cost and also not be compatible with flexible substrates. While the planar structures based on TiO2 (regular structure) usually suffer 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.

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recombination.5 After 6 years of efforts, the power conversion efficiency (PCE) of perovskite solar cells has risen from about 3% to over 20%,1,6−15 which is close to that of traditional solar cell technologies such as Si, CIGS, and CdTe.

INTRODUCTION Inorganic−organic hybrid perovskite solar cells have attracted great attention due to their solution processing and high performance.1,2 Organic halide perovskites have ABX3 crystal structures, where A, B, and X are organic cation, metal cation and halide anion, respectively (Figure 1a), where the band gap can be tuned from the ultraviolet to infrared region through varying these components (Figure 1b).2−4 This family of materials exhibits a myriad of properties ideal for PV such as high dual electron and hole mobility, large absorption coefficients resulting from sp antibonding coupling, a favorable band gap, a strong defect tolerance and shallow point defects, benign grain boundary recombination effects, and reduced surface © XXXX American Chemical Society

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PLANAR STRUCTURE VERSUS MESOPOROUS STRUCTURE (TiO2 SYSTEM) The hybrid perovskite solar cell was initially discovered in a dye sensitized solar cell (DSSC) using a mesoporous TiO2 structure.1 Miyasaka and co-workers were the first to utilize the perovskite Received: September 3, 2015

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

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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, Sn; X site could be Cl, Br or I. (b) Absorption of FAPbIyBr3‑y with different ratios of Br and I. Reprinted with permission from ref 4. Copyright 2014 Royal Society of Chemistry

Figure 2. (a) Plot of exciton diffusion 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.

structure was feasible. The first successful demonstration of the planar structure can be traced back to the perovskite/fullerene structure reported by Guo et al.,19 showing a 4% efficiency. The low efficiency reported at that time was due to the inferior film quality and inadequate absorption of the perovskite film.19 The breakthrough of the planar perovskite structure was obtained by using a dual source vapor deposition, providing dense and high quality perovskite films that achieved 15.4% efficiency.20 Recently, the efficiency of the planar structure was pushed over 19% through interface engineering.12 These results showed that the planar structure could achieve similar device performance as the mesoporous structure. The evolution of the structure of perovskite is shown in Figure 3. The planar structure can be divided into two categories depending on which selective contact is used on the bottom, that is, regular (n-i-p) and inverted (p-i-n); the device structures are shown in Figure 3b and c, respectively. The regular n-i-p structure

(CH3NH3PbI3 and CH3NH3PbBr3) nanocrystal as absorbers in DSSC structure, achieving an efficiency of 3.8% in 2009.1 In 2012, Park and Gratzel et al. reported a solid state perovskite solar cell by using the solid hole transport layer to improve stability.7 After that, several milestones in device performance have been achieved by using mesoporous structure.6−15 However, these mesoporous devices need a high temperature sintering of TiO2 layer that could increase the processing time and cost of cell production. Sum et al. and Snaith et al. independently reported that the methylammonium-based perovskites own long charge carrier diffusion lengths (∼100 nm for CH3NH3PbI3 and ∼1000 nm for CH3NH3PbI3−xClx, Figure 2).16,17 Recently, single crystals of CH3NH3PbI3 were found to reach diffusion lengths larger than 175 um.18 Further studies demonstrated that perovskites exhibit ambipolar behavior, indicating that the perovskite materials themselves can transport both of electrons and holes between the cell terminals.16 All of these results indicated that a planar B


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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 (60 days

ref 19 21 22 23 26 11 27 28 24 31 29 39 41 42 33 35 36 30

poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) and fullerene derivative, were directly implemented as the HTL and ETL in a perovskite device. Through choosing a proper fullerene derivative and optimizing the processing conditions of the perovskite film, a PCE of 3.9% was delivered. Later, Sun et al. succeeded in making a thicker and denser perovskite film by introducing two-step sequential deposition into the planar device that increased the device performance to 7.41% (Figure 4c).21 After 2013, several attempts were made to improve the efficiency, including film formation and interface engineering, which will be discussed in the next sections. The main development of the inverted structure perovskite solar cells are summarized in Table 1.

has been extensively studied and could be traced back to dye sensitized solar cells. The p-i-n structure is derived from the organic solar cell, and usually, several charge transport layers used in organic solar cells were successfully transferred into perovskite solar cells.11 In this Account, we will focus on the inverted planar structure of perovskite solar cells, including their working mechanism, methods for improving efficiency, stability, and hysteresis effects.

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THE INVERTED PLANAR STRUCTURE The first inverted planar structure of perovskite solar cells adopted a similar device structure to the organic solar cell (Figure 4a and b).19 The traditional organic transport layers, C


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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.

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improve the film quality. This produced an 18.1% efficiency without hysteresis.28 Although the inverted planar of perovskite solar cells based on PEDOT:PSS hole transport layer showed promising efficiency, there are still several challenges need to overcome for achieving higher efficiencies.3,11 The growth of perovskite film on PEDOT:PSS often leads to pinhole generation and incomplete surface coverage resulting in low device performance.3,11 Recent results show that perovskite growth is strongly dependent on the bottom substrates. We have showed that NiOx could yield a better perovskite film than on PEDOT:PSS and give higher Voc.30 Meredith et al. also observed different crystal quality when perovskite was deposited on different polymer surfaces.31 Perovskite appears to show better crystallization when a more crystalline bottom substrate is used, indicating that the crystallinity of the substrate could contribute to the film quality of the perovskite layer.31 Wetting properties between the solvent with the bottom substrate was found to be another issue effecting perovskite film growth. Huang et al. showed crystal size of the perovskite film was much larger than that of the thickness and demonstrated a higher efficiency by using PTAA as the hole transport layers, which was could severe as a nonwetting surface for the solvent used for perovskite, such as DMF.29

FILM GROWTH FOR IMPROVING EFFICIENCY OF INVERTED PLANAR SOLAR CELLS Initial studies adopted a thin perovskite absorbing layer of less than 100 nm, which limited the light harvesting and short circuit current of the devices.19,21After learning that the perovskite materials possess diffusion lengths over 100 nm,16,17 thicker absorption layers were possible without sacrificing the charge transport properties. Yang et al. and Snaith et al. first independently used a thicker mixed halide perovskite film (∼300 nm) as the absorber (Figure 4d), showing an efficiency of about 11.5% and 9.8%, respectively.22,23 Bolink et al. showed 12.04% efficiency by thermal deposition of 285 nm thick of perovskite and adopting an organic charge transport layer.24 Later, several processing methods for growth of high quality of perovskite film have been invented. Huang et al. showed a diffusion approach to form high quality CH3NH3PbI3 films, which could be considered as modified two-step process, where first PbI2 was deposited onto PEDOT:PSS and followed by spincoating of MAI onto the PbI2 surface and annealing to form the perovskite film (Figure 5a).25 The perovskite film was formed by interdiffusion of CH3NH3I into PbI2, producing efficient devices of 15% that are highly reproducible.25 Seok et al. transferred the 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 moistureassisted perovskite film growth could improve the film quality during annealing of the perovskite precursor film and showed as high as 17.1% efficiency.14 Nie et al. reported millimeter sized perovskite gains by coating the hot precursor film onto the hot substrate (Figure 5d), for a champion device close to 18% efficiency.27 Recently, Im reported a high quality perovskite film on PEDOT:PSS using HI as an additive to

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INTERFACE ENGINEERING FOR EFFICIENT SOLAR CELL-HOLE TRANSPORT LAYER The n-i-p regular structure usually shows an open voltage over 1 V,12−15 while the inverted structure shows a slight open circuit voltage drop (0.9−1 V).19,21−23,25,27,30 In addition to the inferior crystal film quality on PEDOT:PSS as mentioned above, the band alignment between perovskite and the traditional hole transport layer PEDOT:PSS could be another issue. The work D


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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

hole transport layer is CuSCN. Significant progress has been made with CuSCN in the n-i-p perovskite solar cell, showing 12.4% efficiency using a 600−700 nm thick of CuSCN layer.37 Recently, using electrodeposited CuSCN as a hole transport layer in inverted structure, as high as 16.6% efficiency have been obtained.38 Several other metal oxides such as MoO3, V2O5, and WO3 seem to be suitable for high stability hole transport layers; however, it seems these metal oxide materials are not resistive enough to the acidity of CH3NH3I.

function of the generally used hole transport layer PEODT:PSS is about 4.9−5.1 eV, which is shallower than that of the valence band of perovskite layer (5.4 eV), leading to an imperfect ohmic contact between perovskite and the p-type transport layer and a consequent Voc loss (Figure 6). To address this problem, PEDOT:PSS must be modified or replaced by another material with a high work function. The most common polymers, such as poly-TPD,24 PCDTBT,31 and PTAA,29 with deep HOMO levels (∼ −5.4 eV) have been used to modify the PEDOT:PSS surface, and the devices based on the bilayer hole transport layer (PEDOT:PSS/polymer) showed an enhanced Voc (>1 V).24,29,31 Unfortunately, these polymers are usually hydrophobic, and thus, the perovskite precursor cannot be coated onto these polymer surfaces. Alternatively, evaporation processes of perovskite film must be adopted.24,31 Recently, p-type water-soluble polyelectrolyte with deep work function (>5.2 eV) showed good device performance when it is used as hole transport layers,32 which could be a good candidate of replacing PEDOT:PSS if it is air stable. Transition metal oxides such as NiO, MoO3, V2O5 and WO3 own higher work function than PEDOT:PSS, and these metal oxides could be a good candidate of replacing of PEDOT:PSS to get a larger VOC. Guo et al. first reported use of solution processed NiOx as the hole transport layer, demonstrating an efficiency of about 8% (Figure 7a−c).33 Han et al. adopted a hybrid interfacial layer of NiO/Al2O3 to improve the device efficiency to 13% with a high fill factor.34 Jen et al. used Cu-doped NiOx as a hole transport layer and achieved open circuit voltages as high as 1.1 V with an efficiency of 15.4% .35 You et al. adopted a sol−gel process for high quality NiOx films, demonstrating an efficiency of 16.1%.30 Recently, Seok et al. reported an inverted structure using a NiOx film deposited via pulsed laser deposition (PLD), and pushed the device efficiency as high as high as 17.13% (Figure 7d).36 In addition to NiOx, another promising

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INTERFACE ENGINEERING OF ELECTRON TRANSPORT LAYER For p-i-n perovskite solar cells, fullerenes are usually used as the electron transport layer, where PCBM is the most popular n-type charge transport layer. Recent studies show that C60 should be more effective than that of PCBM as an electron transport layer due to the higher mobility and conductivity of C60.39 Jen et al. showed that the PCE of fullerene-derived perovskite solar cells improves with increasing electron mobility in the fullerene layer, indicating the critical role of bulk transport through fullerene in promoting charge dissociation/transport (Figure 8a).39 As for improving the charge transport properties of the fullerene, Li et al. attempted to dope the PCBM using graphdiyne (GD) to improve the coverage and the conductivity of PCBM, and also improve the device performance from 10.8% to 13.9% (Figure 8b).40 In addition, it was confirmed that the fullerene itself cannot fully form a prefect ohmic contact with a metal such as Al or Ag. Several buffer layers, including BCP,19 PFN,11 LiF,26 and self-assembled C60 derivatives,35 can further improve the ohmic contact. Furthermore, metal oxides such as ZnO and TiO2, combined with fullerene as an electron transport layer, not only improve the ohmic contact but also improve device stability.23,30,41,42 E


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Figure 7. Device structure and performance using NiOx as hole transport layer. (a) Device structure. (b) Band alignment. (c) Device performance of the first 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.

Figure 8. (a) Inverted planar structure of perovskite solar cells using different 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.

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STABILITY OF INVERTED STRUCTURE The power conversion efficiency of perovskite solar cells has been pushed up to 20% and 18% for regular and inverted structures, respectively. For practical application, a reliable and stable performance is strongly needed. Present results show that the stability of perovskite is a critical issue, the main problem in stability of perovskite come from the instability of perovskite material itself, which decomposes into PbI2 and CH3NH3I in a high humidity environment. Another concern is interfacial stability, several organic charge transport layers that have been commonly used in perovskite solar cells can react with oxygen and water in the ambient air, thus promoting device degradation. In addition, several electrode materials such as Ag, Al could react with perovskite when they were directly contact.30

ELECTRON TRANSPORT LAYER ON STABILITY For the inverted structure, electron transport layer is the topmost layer exposed to the ambient air except for metal electrode. It was found that the fullerene could absorb oxygen or water onto the surface, leading to a dipole moment and a large resistance as a result of degradation (Figure 9d).30 On the other hand, the small molecular fullerene layer could not be too thick due to its low conductivity. A thin fullerene layer cannot form a continuous film on the perovskite surface, and could potentially lead to physical direct contact between perovskite and the electrode. In this case, the perovskite and metal such as Al or Ag can react when in a humid environment (Figure 10).30 Therefore, the devices based on the PCBM ETL showed poor stability (Figure 9a, b).30 To improve the device stability, several metal oxides have been F


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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 PEDOT:PSS with NiOx.30

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PEROVSKITE MATERIALS STABILITY Although the interface can improve the stability of perovskite solar cells, the intrinsic properties of the perovskite material are the main issue for long-term stability. The typical perovskite material CH3NH3PbI3 shows severe moisture and light instability. The perovskite materials could be easily decomposed into PbI2 and CH3NH3I, followed by decomposition into CH3NH2 and HI when exposing the perovskite in a high level of moisture environment. It was found that perovskite can selfdecompose when exposed to light for long periods of time. Exploration of highly stable perovskite materials could be the next direction in perovskite solar cell research.43

Figure 10. Electrode stability with different electron transport layers (PCBM and ZnO). Reprinted with permission from ref 30. Copyright 2015 Nature Publishing Group.

used in the inverted structure.23,30,41,42 Snaith et al. first reported a PCBM/TiOx bilayer electron transport layer, improving the coverage of the electron transport layer as the pinholes from the PCBM could be filled.23 Similary PCBM/ZnO bilayers were found to be suitable for improving device performance and stability (Figure 11).41,42 You et al. significantly improved stability through use of ZnO as the top electron charge transport layer. It was found that the device can nearly maintain its original efficiency after 60 days of storage in ambient air with room light soaking (Figure 12).30

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HYSTERESIS IN INVERTED PLANAR SOLAR CELLS I−V Hysteresis is still a controversial topic in perovskite solar cells. This issue was first raised at the beginning of 2014.12,13 It has been found that hysteresis in the regular planar structure is more severe than that in the case of the mesoporous structure.12,13 Interestingly, most reports have shown negligible

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HOLE TRANSPORT LAYER STABILITY PEDOT:PSS has been widely used in organic solar cells as an organic charge transport layer due to its good conductivity. However, PEDOT:PSS is hydrophilic and can easily absorb G


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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


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further improved. In inverted structures, fullerenes can forbid ion movement and enhance the charge transfer to reduce the hysteresis. It seems that metal oxides introduce hysteresis, and a fullerene/metal oxide such as PCBM/ZnO bilayer system shows promise in terms of reduced hysteresis and improved highly stability. The planar structure shows a simple device structure and a promising efficiency (close to 20%). By further improving the perovskite crystallinity and also the interface, it could be anticipated that the efficiency will catch up or be over than that of traditional inorganic thin film solar cells such as CIGS and CdTe. Compared with the inorganic thin films solar cells, which needs high vacuum and temperature processing, planar structure of perovskite solar cells could be fabricated in ambient air or nitrogen-filled glovebox at low temperature. This will reduce the fabrication cost. Furthermore, perovskite solar cells are based on solution process, which is compatible with several coating techniques, such as blade-coating and roll-to-roll techniques to produce large area and flexible devices. Perovskite solar cells could be utilized in our daily life if the long stability issue could be resolved.

hysteresis in the inverted planar structure with fullerene as electron transport layer.11,22,30,44 The most popular explanation is ion movement stabilization.44−47 The fullerene penetrates/ diffuses into the perovskite layer through the pinholes/grain boundary during processing (spin-coating or annealing44,45). Mobile ions in the perovskite interact with fullerene to form a fullerene halide radical,46 which is thought to stabilize electrostatic properties, reducing the electric field-induced anion migration that may give rise to hysteresis, and thus resulting in no hysteresis.47 Huang et al. also demonstrated that the fullerene penetration into perovskite layer during the annealing and passivate the traps in the perovskite and reduce the hysteresis (Figure 13). In our opinion, except for ion movement stabilization, neglected charge accumulation and also capacitance could be another reason for hysteresis free. On one hand, while fullerene was diffused into perovskite, the device showed a similar structure as bulk heterojunction (BHJ) or a mesoporous structure (Figure 13), and there are efficient channels for charge extraction. On the other hand, fast charge transfer between perovskite and fullerene allows efficient charge extraction.30 While in the regular structure, there is limited interface contact between metal oxide and perovskite, and more importantly, the charge transfer between metal oxide and perovskite is not efficient,30 which could lead to serious charge accumulation at the interface between perovskite/metal oxide and form a large capacitance. Therefore, there is no or neglected hysteresis in inverted structure, while there is significant hysteresis in regular structure.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.Y.). *E-mail: [email protected] (Y.Y.). Notes

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The authors declare no competing financial interest.

CONCLUSIONS AND FUTURE OUTLOOK Perovskite solar cells have reached efficiencies over 20% based on the regular n-i-p mesoporous structures, whereas for the inverted p-i-n structure the state art of performances are approximately 18% efficient. To further improve efficiencies of inverted solar cells, it is necessary to modify interfaces, especially the contact between perovskite and the hole charge transport layer. The commonly used PEDOT:PSS surface should be modified or replaced by another material with a higher work function, and in favor of larger crystal growth. After resolving the band alignment between hole transport layer and perovskite and also the growth of the perovskite film on the hole transport layer, the efficiency should be much improved and should better compete with the regular structure of perovskite solar cells. Several processing technologies such as solvent treatment/ annealing,26,47 moisture assisted growth,11 additives,28 and hot spin-coating27 have been confirmed as an effective way for obtaining high quality of perovskite films. Exploring new methods or combining these existing technologies and further improving the film quality should be our next direction. The stability of the perovskite is still the main issue for its commercial application. Even though more than 60 days stability under weak room light illumination has been demonstrated, it is not enough for practical applications. Further exploration of alternative perovskite material candidates and stabilizing additives may lead to improved perovskite stability in the future. Moreover, encapsulation technology is another important component necessary to enhance the lifetime of perovskite solar cells. To improve the stability at interfaces, inorganic charge transport layers show better results. At the least, the upmost layer should be robust and relatively impenetrable to moisture and oxygen. At present, metal oxides such as ZnO and TiO2 colloids have been coated onto the fullerene transport layers for improved device stability. However, these methods could be

Biographies Lei Meng is a Ph.D. candidate in the department of Materials Science and Engineering at UCLA. He obtained his bachelor degree from Shanghai Jiaotong University in China and master degree from Northwestern University. His research interests are mainly in polymer and perovskite solar cells. Jingbi You is currently a full professor in the Institute of Semiconductors, Chinese Academy of Sciences (ISCAS), and he obtained National 1000 Young Talents award in 2015. He received his Ph.D. degree in Material Sciences from ISCAS in 2010, and later joined Prof. Yang Yang’s group of UCLA as a postdoc fellow from 2010 to 2015. His present research interests are organic/inorganic semiconductor materials and their optoelectronic devices such as LEDs, solar cells, and detectors. Tzung-Fang Guo is professor and the department chair of the Department of Photonics, Advanced Optoelectronic Technology Center, National Cheng Kung University, Taiwan. He received his PhD degree in 2002 from the department of Materials Science and Engineering, UCLA. His current research interests are organic electrooptical materials and devices, polymer and organic light-emitting diodes (PLEDs and OLEDs), and advance functional materials. Yang Yang is the Tannas Jr. chair professor of Materials Science and Engineering at UCLA. He holds a Ph.D. in Physics and Applied Physics from the University of Massachusetts, Lowell in 1992, respectively. Before he joined UCLA in 1997 as an assistant professor, he served on the research staff of UNIAX (now DuPont Display) in Santa Barbara from 1992 to 1996. Yang is now the faculty director of the Nano Renewable Energy Center (NREC) of the California Nanosystem Institute of UCLA. He is a materials physicist with expertise in the fields of organic electronics, organic/inorganic interface engineering, and the development and fabrication of related devices, such as photovoltaic cells, LEDs, and memory devices. I


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ACKNOWLEDGMENTS This work was financially supported by a grant from the National Science Foundation (ECCS-1202231), Air Force Office of Scientific Research (FA9550-12-1-0074). J.Y. acknowledges the financial support from National 1000 Young Talents awards. We would like to thank all the scientists who kindly provided their original high resolution figures which are cited in this Account and also thank Mr. Nicholas De Marco for helping proof reading the manuscript.

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