Novel Surface Passivation Technique for Low-Temperature Solution

Publication Date (Web): January 29, 2016 ... Devices with PFN-P1 showed the best efficiency (13.2%), with a small standard deviation (0.40), out of 60...
0 downloads 0 Views 3MB Size
Download PDF
Research Article www.acsami.org

Novel Surface Passivation Technique for Low-Temperature SolutionProcessed Perovskite PV Cells Neeti Tripathi,† Yasuhiro Shirai,*,†,‡ Masatoshi Yanagida,†,‡ Akiya Karen,† and Kenjiro Miyano† †

Global Research Center for Environment and Energy based on Nanomaterials Science (GREEN), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ‡ Photovoltaic Materials Unit, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan

Downloaded via UNIV OF TOLEDO on June 29, 2018 at 13:16:25 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: Low-temperature solution-processed perovskite solar cells are attracting immense interest due to their ease of fabrication and potential for mass production on flexible substrates. However, the unfavorable surface properties of planar substrates often lead to large variations in perovskite crystal size and weak charge extractions at interfaces, resulting in inferior performance. Here, we report the improved performance, reproducibility, and high stability of “p-i-n” planar heterojunction perovskite solar cells. The key fabrication process is the addition of the amine-polymer poly[(9,9-bis(3′-(N,Ndimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN-P1) to a simple spin-coating process. The PFN-P1 works as a surfactant and helps promote uniform crystallization. As a result, perovskite films with PFN-P1 have a uniform distribution of grain sizes and improved open circuit voltage. Devices with PFN-P1 showed the best efficiency (13.2%), with a small standard deviation (0.40), out of 60 cells. Moreover, ∼90% of the initial efficiency was retained over more than 6 months. Additionally, devices fabricated from PFN-P1 mixed perovskite films showed higher stability under continuous operation at maximum power point over 150 h. Our results show that this approach is simple and effective for improving device performance, reproducibility, and stability by modifying perovskite properties with PFN-P1. Because of the simplicity of the fabrication process and reliable performance increase, this approach marks important progress in low-temperature solution-processed perovskite solar cells. KEYWORDS: perovskite, solar cell, PFN, stability, low temperature, solution process

■

referred to as an “inverted structure” with respect to the n/i/p configuration. Devices with the p/i/n configuration usually consist of [poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid)] (PEDOT:PSS)/perovskite/phenyl-C61butyric acid methyl ester (PC61BM), corresponding to the p, i, and n layers, respectively; light enters the p side first.8,13,14 To date, substantial progress on the optimization of perovskite properties and interface conditions has enabled us to achieve an average efficiency of >16% in the mesoporous n-i-p configuration.10−12,16−18 On the other hand, in the p-i-n configuration, the average efficiency limit lies below 15% owing to the unfavorable properties of planar substrates.8,14,19−23 Nevertheless, the p-i-n configuration is often more attractive because of its low-temperature processing for flexible architectures, hysteresis-free J−V characteristics, and easily explainable device behavior.9,13,14 Recently, Nie et al.14 achieved a high efficiency of ∼18% by increasing the perovskite grain size in the millimeter range. However, despite a smaller device area (0.03 cm2), the large variation in average efficiency (2−4% lower than that of the champion device; 2−4% refer to

INTRODUCTION Lead halide perovskite materials have shown great potential to solve energy problems because of their unique features, such as good charge carrier mobility (∼20 cm2 V−1 s−1), large absorption coefficient (1.5 × 104 cm−1 at 550 nm), direct band gap (∼1.5 eV), small excitation binding energy (∼30 meV), and long diffusion length (100−1000 nm).1−5 Over the last 5 years, a rapid boost in power conversion efficiency (PCE) has been documented through modification of the structural or optical properties of metal-halide perovskite by using various methodologies.6−10 To date, the ambipolar semiconducting nature of perovskite has given a tremendous boost in PCE.11−15 From the perspective of a device’s architecture, perovskite solar cells can be broadly classified in two categories. One is based on the structure with n-type metal oxides used as an n-type layer (in the form of either a planar or a mesoporous layer) to construct an n-i-p configuration (n, i, and p refer to electron transporting layer, light absorber layer, and hole transporting layer, respectively). The structure normally consists of TiO2/ perovskite/spiro-MeOTAD, corresponding to the n/i/p layers, respectively; light enters the n side first.3,6,7,10−12 Another class of devices follows the concept of organic photovoltaics (OPV), wherein thin layers of organic materials are used as the hole and electron transporting layers in a p/i/n configuration, also © 2016 American Chemical Society

Received: November 21, 2015 Accepted: January 29, 2016 Published: January 29, 2016 4644

DOI: 10.1021/acsami.5b11286 ACS Appl. Mater. Interfaces 2016, 8, 4644−4650

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Perovskite formation by using a successive spin-coating method with PFN-P1. CB, chlorobenzene. IPA, isopropyl alcohol. (b) Schematic of device configuration. Small needle-like items represent PFN-P1 polymers in the perovskite film. Chemical structures of (c) PFN-P2 and (d) PFN-P1.

PCE percentages and not to percent of the ∼18%) is indicative of the unmitigated challenges in device reproducibility in this configuration. Moreover, the stability of perovskite devices is a serious issue, regardless of the particular device architecture. The main challenge with perovskite devices is the large morphological deviations caused by the uncontrolled growth of perovskite crystals, which is responsible for large efficiency fluctuations. A wide range of methods have been proposed to solve this problem, including vacuum deposition, a one-step solution process, a two-step solution process, and vaporassisted solution processes.6−9,24 The vapor deposition method is cost-intensive and time-consuming, whereas solution processes have become more popular owing to their low cost and ease of fabrication. In particular, for planar devices, a twostep deposition method6,9 has been largely successful for fabricating homogeneous and void-free perovskite films. Recent reports have revealed that, in solution-based processes, crystallization kinematics can be substantially affected by the solvents and additives used. For example, Kim et al.25 successfully controlled the morphology and crystal domains by using a mixture of γ-butyrolactone (GBL) and N,Ndimethylformamide (DMF). Moreover, additives such as 1,8diiodooctance (DIO),26 1,3-bis[3,5-di(pyridin-3-yl)phenyl]benzene (BmPyPhB),27 1-chloronaphthalene (CN),28 and polyethylene glycol (PEG)29 have been used to promote spreading of the perovskite precursor to create homogeneous nucleation sites for the uniform growth of perovskite crystals. Although there are plenty of reports on perovskite growth and morphology in the literature, few detailed studies of the distribution of these additives and their correlation with longterm stability have been reported.30−32 We have recently demonstrated the creation of hysteresisfree devices with durability up to nearly 2 months by incorporating a small amount of chorine using an interdiffusion method.33 To improve the device properties further, we extended our study to investigate the effect of a neutral surfactant polymer (poly[9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)]) (PFN-P1) in the perovskite film. It has been well documented that the morphological quality of the PbI2 film in the two-step method of fabricating perovskite PV cells is extremely important. There are two factors that improve performance: (1) a flat film surface and (2) uniform crystallite nucleation. We hypothesized that adding some kind of capping layer during evaporation of the PbI2 solution and film formation might serve this purpose, and a surfactant layer would be a natural solution. However, adding

a surfactant to the spreading solution (in our case, PbI2 in N,Ndimethylformamide (DMF)) is not effective because, being surface active, surfactant molecules may not stay in the bulk solution and it would be difficult to control their precise concentration in the spreading solution. With this in mind, we devised a novel way to dynamically spread surfactant molecules on the drying PbI2 film (Figure 1a). The method is simple and can be quantitatively precise because of its high reproducibility. Our results suggest that the surfactant molecules of PFN-P1 are indeed adsorbed on the top surface at monomolecular thickness. Comparing to the samples without PFN-P1, the surface morphology is smooth and the crystallite size distribution is more uniform; therefore, the fabrication process is highly reproducible. The added insulatoralbeit monomolecularhas an adverse effect of increasing the series resistance slightly. This disadvantage is, however, easily outweighed by the benefits: this technique could be used to fabricate efficient and reproducible perovskite devices just by spreading a surfactant layer over a drying solution film, without the need to worry about the solubility or micellar formation of the surfactant molecules. The PFN-P1-based devices that we created in this study had the best efficiency of 13.2% (average efficiency, ∼12%; standard deviation ±0.40) among a total of 60 cells, with more than 85% reproducibility. Interestingly, devices with PFN-P1 had far superior durability of more than 6 months.

■

EXPERIMENTAL SECTION

Materials and Characterizations. All chemicals were purchased from commercial suppliers and used as received, unless stated otherwise. Perovskite precursor solutions were prepared by dissolving PbI2 [Sigma-Aldrich, 99% purity] and methylammonium iodide (MAI) [Wako Chemicals] in anhydrous DMF (400 mg mL−1) and ethanol (50 mg mL−1), respectively. PC61BM [Solenne, 99% purity] solution (2 wt %) dissolved in anhydrous CB was used for coating of the electron transport layer. PFN-P1 [1-Material] dissolved in anhydrous CB (3 mg in 8 mL−1), and PFN-P2 [1-Material] dissolved in anhydrous ethanol (2 mg in 16 mL−1) were used for interface engineering. All solutions were filtered through 0.45 μm syringe filters to avoid the risk of particle formation. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) analysis was performed with a TOFSIMS5 (ION-TOF GmbH) instrument. TOF-SIMS depth profiles from detection of positive secondary ions were acquired by using a pulsed 60 kV Bi32+ for analysis and 5 kV or 10 kV Ar2000+ gas cluster ion beams for sputtering. The sputtered area was 300 × 300 μm and the analysis area was 100 × 100 μm. Depth profiles were obtained with the noninterlaced mode of sputtering/analysis cycles. The X-ray diffraction patterns were collected by using a Bruker D8 advanced X4645

DOI: 10.1021/acsami.5b11286 ACS Appl. Mater. Interfaces 2016, 8, 4644−4650

Research Article

ACS Applied Materials & Interfaces ray diffractometer (Cu Kα radiation, λ= 1.54050 A). Top-surface and cross-sectional images were taken under a high-resolution scanning electron microscope (Hitachi-4800) at a 5 kV accelerating voltage carefully to avoid damages to the samples. The current density− voltage (J−V) characteristics and incident monochromatic IPCE spectra or external quantum efficiency (EQE) were measured with a CEP-200BX spectrometer (Bunkokeiki, Tokyo, Japan). For photostability testing, the device was evaluated under short-circuit and opencircuit conditions continuously for 20 min time intervals under continuous illumination (1 sun). To investigate hysteresis behavior, scan rates were varied from 0.02 to 0.4 V/s. A histogram of 60 devices were deduced from the active device area of 0.18 cm2 defined by an aperture mask. Device Fabrication. Solar cell devices were fabricated on precleaned patterned indium tin oxide (ITO)-coated glass substrates (15 Ω square−1). The ITO substrates were precleaned in an ultrasonic bath with detergent, pure water, and 2-propanol, followed by ultraviolet-ozone treatment for 5 min to remove organic residuals. A thin layer (30 nm) of PEDOT:PSS (Clevios, Al4083) was formed by spin coating at 3000 rpm and subsequently dried at 140 °C for 10 min on a hot plate under ambient conditions. Substrates were transferred to a nitrogen-filled glovebox (6 months) (Figure 8), without generating any hysteresis in J−V characteristics (Figure S9). However, devices without PFN-P1 showed a 40% loss of initial efficiency after 70 days, as reported previously.33 We further tested the performance of these aged devices at maximum power point under 1 Sun conditions (Figure 9). Devices without PFN-P1 degraded very rapidly (PCE from 8% to 3%) in less than 50 h, whereas PFN-P1 devices continued to show efficiencies >7% even after 150 h under continuous working conditions. Whereas Voc and FF remained constant,

light intensity for devices with and without PFN-P1, having the slope of nkBT/e (where n is the ideality factor, kB is the Boltzmann constant, e is the elementary charge, and T is the temperature in Kelvin). Devices with PFN-P1 had a lower slope of ∼1.02, suggesting that the bimolecular recombination process dominated during device operation.14,41 On the other hand, devices without PFNP1 showed strong dependence of Voc on light intensity, with a slope of 1.27 kBT/e, implying the presence of more trapassisted recombination.41−43 Furthermore, the dark current density of the PFN-P1 devices was significantly suppressed in 4648

DOI: 10.1021/acsami.5b11286 ACS Appl. Mater. Interfaces 2016, 8, 4644−4650

Research Article

ACS Applied Materials & Interfaces

the present method, EIS analyses are underway,43 and further enhancement of efficiency should be achievable by rational interface engineering.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b11286. TOF-SIMS depth profiles, exponential fitting to the PFN-P1 SIMS signal, device parameters as a function of PFN-P1, J−V hysteresis and reverse bias results, stability test, cross-sectional SEM, and hysteresis of aged devices. (PDF)

■

Figure 8. Stability data on encapsulated PFN-P1 devices stored under ambient conditions for 201 days (>6 months).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors acknowledge the support under the MEXT Program for Development of Environmental Technology using Nanotechnology.

■

REFERENCES

(1) Wang, B. H.; Xiao, X. D.; Chen, T. Perovskite photovoltaics: a high-efficiency newcomer to the solar cell family. Nanoscale 2014, 6, 12287−12297. (2) Leijtens, T.; Stranks, S. D.; Eperon, G. E.; Lindblad, R.; Johansson, E. M. J.; McPherson, I. J.; Rensmo, H.; Ball, J. M.; Lee, M. M.; Snaith, H. J. Electronic Properties of Meso-Superstructured and Planar Organometal Halide Perovskite Films: Charge Trapping, Photodoping, and Carrier Mobility. ACS Nano 2014, 8, 7147−7155. (3) Kulkarni, S. A.; Baikie, T.; Boix, P. P.; Yantara, N.; Mathews, N.; Mhaisalkar, S. Band-gap tuning of lead halide perovskites using a sequential deposition process. J. Mater. Chem. A 2014, 2, 9221−9225. (4) Savenije, T. J.; Ponseca, C. S.; Kunneman, L.; Abdellah, M.; Zheng, K. B.; Tian, Y. X.; Zhu, Q. S.; Canton, S. E.; Scheblykin, I. G.; Pullerits, T.; Yartsev, A.; Sundstrom, V. Thermally Activated Exciton Dissociation and Recombination Control the Carrier Dynamics in Organometal Halide Perovskite. J. Phys. Chem. Lett. 2014, 5, 2189− 2194. (5) Edri, E.; Kirmayer, S.; Mukhopadhyay, S.; Gartsman, K.; Hodes, G.; Cahen, D. Elucidating the charge carrier separation and working mechanism of CH3NH3PbI3-xClx perovskite solar cells. Nat. Commun. 2014, 5, 3461. (6) Burschka, J.; Pellet, N.; Moon, S. J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Gratzel, M. Sequential deposition as a route to high-performance perovskite-sensitized solar cells. Nature 2013, 499, 316−319. (7) Liu, M. Z.; Johnston, M. B.; Snaith, H. J. Efficient planar heterojunction perovskite solar cells by vapour deposition. Nature 2013, 501, 395−398. (8) You, J.; Hong, Z.; Yang, Y.; Chen, Q.; Cai, M.; Song, T.-B.; Chen, C.-C.; Lu, S.; Liu, Y.; Zhou, H.; Yang, Y. Low-Temperature SolutionProcessed Perovskite Solar Cells with High Efficiency and Flexibility. ACS Nano 2014, 8, 1674−1680. (9) Xiao, Z.; Bi, C.; Shao, Y.; Dong, Q.; Wang, Q.; Yuan, Y.; Wang, C.; Gao, Y.; Huang, J. Efficient, high yield perovskite photovoltaic devices grown by interdiffusion of solution-processed precursor stacking layers. Energy Environ. Sci. 2014, 7, 2619−2623. (10) Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Seok, S. I. Solvent engineering for high-performance inorganic-organic hybrid perovskite solar cells. Nat. Mater. 2014, 13, 897−903.

Figure 9. Stability testing of aged devices with maximum power point tracking (MPPT) under continuous illumination (1 sun).

loss of PCE in the PFN-P1 devices was due mainly to the reduction in Jsc with increasing exposure time. Despite the fact that the devices were already 6 months old, the PFN-P1 devices showed far superior stability.

■

CONCLUSIONS In summary, we demonstrated the production of highly reproducible and stable planar perovskite solar cells by introducing the amine-polymer PFN-P1 into the perovskite films. Detailed analysis of the perovskite films using TOF-SIMS clearly indicated that materials such as PFN-P1, which dissolve in DMF solution, can be introduced into the perovskite films in a controlled manner by using a simple spin-coating method. On one hand, PFN-P1 was able to control the grain size distribution for high reproducibility of device performance, and on the other hand reduced recombination losses due to the presence of the PFN-P1 led to a substantial improvement in Voc. Combining these two factors resulted in PCE superior to that of devices without PFN-P1, with more than 85% reproducibility. Additionally, devices fabricated from PFN-P1 mixed perovskite films showed higher stability over more than 6 months and with continuous operation at maximum power point over 150 h. Our results show that this approach is simple and effective for improving device performance, reproducibility, and stability by modifying perovskite properties with PFN-P1. To gain more understanding on the stability enhancement by 4649

DOI: 10.1021/acsami.5b11286 ACS Appl. Mater. Interfaces 2016, 8, 4644−4650

Research Article

ACS Applied Materials & Interfaces (11) Im, J. H.; Jang, I. H.; Pellet, N.; Gratzel, M.; Park, N. G. Growth of CH3NH3PbI3 cuboids with controlled size for high-efficiency perovskite solar cells. Nat. Nanotechnol. 2014, 9, 927−932. (12) Zhou, H.; Chen, Q.; Li, G.; Luo, S.; Song, T.-b.; Duan, H.-S.; Hong, Z.; You, J.; Liu, Y.; Yang, Y. Interface engineering of highly efficient perovskite solar cells. Science 2014, 345, 542−546. (13) You, J. B.; Yang, Y. M.; Hong, Z. R.; Song, T. B.; Meng, L.; Liu, Y. S.; Jiang, C. Y.; Zhou, H. P.; Chang, W. H.; Li, G.; Yang, Y. Moisture assisted perovskite film growth for high performance solar cells. Appl. Phys. Lett. 2014, 105, 183902. (14) Nie, W.; Tsai, H.; Asadpour, R.; Blancon, J.-C.; Neukirch, A. J.; Gupta, G.; Crochet, J. J.; Chhowalla, M.; Tretiak, S.; Alam, M. A.; Wang, H.-L.; Mohite, A. D. High-efficiency solution-processed perovskite solar cells with millimeter-scale grains. Science 2015, 347, 522−525. (15) Zhou, Z.; Pang, S.; Liu, Z.; Xu, H.; Cui, G. Interface engineering for high-performance perovskite hybrid solar cells. J. Mater. Chem. A 2015, 3, 19205−19217. (16) Tao, C.; Neutzner, S.; Colella, L.; Marras, S.; Srimath Kandada, A. R.; Gandini, M.; Bastiani, M. D.; Pace, G.; Manna, L.; Caironi, M.; Bertarelli, C.; Petrozza, A. 17.6% stabilized efficiency in lowtemperature processed planar perovskite solar cells. Energy Environ. Sci. 2015, 8, 2365−2370. (17) Yang, B.; Dyck, O.; Poplawsky, J.; Keum, J.; Puretzky, A.; Das, S.; Ivanov, I.; Rouleau, C.; Duscher, G.; Geohegan, D.; Xiao, K. Perovskite Solar Cells with Near 100% Internal Quantum Efficiency Based on Large Single Crystalline Grains and Vertical Bulk Heterojunctions. J. Am. Chem. Soc. 2015, 137, 9210−9213. (18) Ahn, N.; Son, D.-Y.; Jang, I.-H.; Kang, S. M.; Choi, M.; Park, N.G. Highly Reproducible Perovskite Solar Cells with Average Efficiency of 18.3% and Best Efficiency of 19.7% Fabricated via Lewis Base Adduct of Lead(II) Iodide. J. Am. Chem. Soc. 2015, 137, 8696−8699. (19) Docampo, P.; Ball, J. M.; Darwich, M.; Eperon, G. E.; Snaith, H. J. Efficient organometal trihalide perovskite planar-heterojunction solar cells on flexible polymer substrates. Nat. Commun. 2013, 4, 2761. (20) Malinkiewicz, O.; Yella, A.; Lee, Y. H.; Espallargas, G. M.; Graetzel, M.; Nazeeruddin, M. K.; Bolink, H. J. Perovskite solar cells employing organic charge-transport layers. Nat. Photonics 2014, 8, 128−132. (21) Wang, F.; Yu, H.; Xu, H. H.; Zhao, N. HPbI3: A New Precursor Compound for Highly Efficient Solution-Processed Perovskite Solar Cells. Adv. Funct. Mater. 2015, 25, 1120−1126. (22) Heo, J. H.; Han, H. J.; Kim, D.; Ahn, T. K.; Im, S. H. Hysteresisless inverted CH3NH3PbI3 planar perovskite hybrid solar cells with 18.1% power conversion efficiency. Energy Environ. Sci. 2015, 8, 1602− 1608. (23) Xiao, Z.; Dong, Q.; Bi, C.; Shao, Y.; Yuan, Y.; Huang, J. Solvent Annealing of Perovskite-Induced Crystal Growth for PhotovoltaicDevice Efficiency Enhancement. Adv. Mater. 2014, 26, 6503−6509. (24) Chen, Q.; Zhou, H. P.; Hong, Z. R.; Luo, S.; Duan, H. S.; Wang, H. H.; Liu, Y. S.; Li, G.; Yang, Y. Planar Heterojunction Perovskite Solar Cells via Vapor-Assisted Solution Process. J. Am. Chem. Soc. 2014, 136, 622−625. (25) Kim, H. B.; Choi, H.; Jeong, J.; Kim, S.; Walker, B.; Song, S.; Kim, J. Y. Mixed solvents for the optimization of morphology in solution-processed, inverted-type perovskite/fullerene hybrid solar cells. Nanoscale 2014, 6, 6679−6683. (26) Liang, P.-W.; Liao, C.-Y.; Chueh, C.-C.; Zuo, F.; Williams, S. T.; Xin, X.-K.; Lin, J.; Jen, A. K. Y. Additive Enhanced Crystallization of Solution-Processed Perovskite for Highly Efficient Planar-Heterojunction Solar Cells. Adv. Mater. 2014, 26, 3748−3754. (27) Chen, C. C.; Bae, S. H.; Chang, W. H.; Hong, Z. R.; Li, G.; Chen, Q.; Zhou, H. P.; Yang, Y. Perovskite/polymer monolithic hybrid tandem solar cells utilizing a low-temperature, full solution process. Mater. Horiz. 2015, 2, 203−211. (28) Song, X.; Wang, W. W.; Sun, P.; Ma, W. L.; Chen, Z. K. Additive to regulate the perovskite crystal film growth in planar heterojunction solar cells. Appl. Phys. Lett. 2015, 106, 033901.

(29) Chang, C. Y.; Chu, C. Y.; Huang, Y. C.; Huang, C. W.; Chang, S. Y.; Chen, C. A.; Chao, C. Y.; Su, W. F. Tuning Perovskite Morphology by Polymer Additive for High Efficiency Solar Cell. ACS Appl. Mater. Interfaces 2015, 7, 4955−4961. (30) Leijtens, T.; Eperon, G. E.; Pathak, S.; Abate, A.; Lee, M. M.; Snaith, H. J. Overcoming ultraviolet light instability of sensitized TiO2 with meso-superstructured organometal tri-halide perovskite solar cells. Nat. Commun. 2013, 4, 2885. (31) Mei, A. Y.; Li, X.; Liu, L. F.; Ku, Z. L.; Liu, T. F.; Rong, Y. G.; Xu, M.; Hu, M.; Chen, J. Z.; Yang, Y.; Gratzel, M.; Han, H. W. A holeconductor-free, fully printable mesoscopic perovskite solar cell with high stability. Science 2014, 345, 295−298. (32) Guarnera, S.; Abate, A.; Zhang, W.; Foster, J. M.; Richardson, G.; Petrozza, A.; Snaith, H. J. Improving the Long-Term Stability of Perovskite Solar Cells with a Porous Al2O3 Buffer Layer. J. Phys. Chem. Lett. 2015, 6, 432−437. (33) Tripathi, N.; Yanagida, M.; Shirai, Y.; Masuda, T.; Han, L.; Miyano, K. Hysteresis-free and highly stable perovskite solar cells produced via a chlorine-mediated interdiffusion method. J. Mater. Chem. A 2015, 3, 12081−12088. (34) He, Z. C.; Zhang, C.; Xu, X. F.; Zhang, L. J.; Huang, L.; Chen, J. W.; Wu, H. B.; Cao, Y. Largely Enhanced Efficiency with a PFN/Al Bilayer Cathode in High Efficiency Bulk Heterojunction Photovoltaic Cells with a Low Bandgap Polycarbazole Donor. Adv. Mater. 2011, 23, 3086−3089. (35) He, Z. C.; Zhong, C. M.; Su, S. J.; Xu, M.; Wu, H. B.; Cao, Y. Enhanced power-conversion efficiency in polymer solar cells using an inverted device structure. Nat. Photonics 2012, 6, 591−595. (36) Wu, Y. Z.; Islam, A.; Yang, X. D.; Qin, C. J.; Liu, J.; Zhang, K.; Peng, W. Q.; Han, L. Y. Retarding the crystallization of PbI2 for highly reproducible planar-structured perovskite solar cells via sequential deposition. Energy Environ. Sci. 2014, 7, 2934−2938. (37) Kim, H. S.; Park, N. G. Parameters Affecting I-V Hysteresis of CH3NH3PbI3 Perovskite Solar Cells: Effects of Perovskite Crystal Size and Mesoporous TiO2 Layer. J. Phys. Chem. Lett. 2014, 5, 2927− 2934. (38) Xiao, Z.; Yuan, Y.; Shao, Y.; Wang, Q.; Dong, Q.; Bi, C.; Sharma, P.; Gruverman, A.; Huang, J. Giant switchable photovoltaic effect in organometal trihalide perovskite devices. Nat. Mater. 2015, 14, 193−198. (39) Chen, H. W.; Sakai, N.; Jena, A. K.; Sanehira, Y.; Ikegami, M.; Ho, K. C.; Miyasaka, T. A Switchable High-Sensitivity Photodetecting and Photovoltaic Device with Perovskite Absorber. J. Phys. Chem. Lett. 2015, 6, 1773−1779. (40) Shao, Y.; Xiao, Z.; Bi, C.; Yuan, Y.; Huang, J. Origin and elimination of photocurrent hysteresis by fullerene passivation in CH3NH3PbI3 planar heterojunction solar cells. Nat. Commun. 2014, 5, 5784. (41) Cowan, S. R.; Roy, A.; Heeger, A. J. Recombination in polymerfullerene bulk heterojunction solar cells. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 82, 245207. (42) Shi, J. J.; Dong, J.; Lv, S. T.; Xu, Y. Z.; Zhu, L. F.; Xiao, J. Y.; Xu, X.; Wu, H. J.; Li, D. M.; Luo, Y. H.; Meng, Q. B. Hole-conductor-free perovskite organic lead iodide heterojunction thin-film solar cells: High efficiency and junction property. Appl. Phys. Lett. 2014, 104, 063901. (43) Miyano, K.; Yanagida, M.; Tripathi, N.; Shirai, Y. Simple characterization of electronic processes in perovskite photovoltaic cells. Appl. Phys. Lett. 2015, 106, 093903.

4650

DOI: 10.1021/acsami.5b11286 ACS Appl. Mater. Interfaces 2016, 8, 4644−4650