Highly Efficient All-Inorganic Planar Heterojunction Perovskite Solar

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Highly Efficient All-Inorganic Planar Heterojunction Perovskite Solar Cells Produced by Thermal Coevaporation of CsI and PbI2 Lyubov A. Frolova,† Denis V. Anokhin,†,‡ Alexey A. Piryazev,‡ Sergey Yu. Luchkin,§ Nadezhda N. Dremova,† Keith J. Stevenson,§ and Pavel A. Troshin*,§,† †

IPCP RAS, Semenov Prospect 1, Chernogolovka 142432, Russia Faculty of Fundamental Physical and Chemical Engineering, Moscow State University, Leninskie gory, Moscow 119991, Russia § Skolkovo Institute of Science and Technology, Nobel St. 3, Moscow 143026, Russian ‡

S Supporting Information *

ABSTRACT: We report here all inorganic CsPbI3 planar junction perovskite solar cells fabricated by thermal coevaporation of CsI and PbI2 precursors. The best devices delivered power conversion efficiency (PCE) of 9.3 to 10.5%, thus coming close to the reference MAPbI3-based devices (PCE ≈ 12%). These results emphasize that all inorganic lead halide perovskites can successfully compete in terms of photovoltaic performance with the most widely used hybrid materials such as MAPbI3.

ybrid organo-inorganic “perovskite” solar cells have attracted great interest of the research community due to their simple architecture and easy production technology, low cost of raw materials, and record-breaking power conversion efficiencies exceeding 20% for the best laboratory devices.1,2 Hybrid salts incorporating methylammonium (MA) and formamidinium (FA) cations and inorganic PbI3− anion show rather unique optoelectronic properties such as high extinction coefficients, very low exciton binding energies, balanced ambipolar charge carrier mobilities with long diffusion lengths, and unprecedented tolerance to the presence of defects and impurities.3−7 Yet, remarkably, perovskite solar cells, while displaying photovoltaic efficiencies >20%, have not been implemented on an industrial scale. While several factors may hinder the development, the primary obstacle is the poor stability of perovskite materials with respect to the elevated temperatures,8 moisture,8−10 oxygen, electric field, and light exposure,11 resulting in meager device operation lifetimes. This limitation provides an impetus for designing improved generations of perovskite absorbers to enable long-term operation stability of the solar cells. Replacing fragile organic moieties with robust inorganic cations can be considered a promising approach to design of more stable materials. While some inorganic leadhalide perovskites CsPbX3 (X = I, Br) have shown enhanced resistance toward moisture and improved thermal stability,12,13 their photovoltaic performances are still inferior compared with the classical hybrid perovskites (e.g., MA/FAPbI3).12,14−17 Improved efficiencies approaching 7.8 to 9.8% (depending on the scan direction) were reported recently for all inorganic perovskite CsPbIBr2 processed from solution with hydroiodic

H

© 2016 American Chemical Society

acid applied as an additive.13 More recently, planar solar cells with 4.7% efficiency were fabricated using thermal coevaporation of CsI and PbBr2.14 Surprisingly, the most promising lower band gap CsPbI3 material delivered lower power conversion efficiencies ranging from 1.7 to 4.7% depending on the device architecture and I−V measurement regimes.16,17 Here we report efficient planar heterojunction solar cells based on an all inorganic CsPbI3 material produced by thermal coevaporation of CsI and PbI2. These devices demonstrate maximal power conversion efficiencies exceeding 10%, good reproducibility, and reasonably low hysteresis in current− voltage characteristics. Structural evolution of the CsPbI3 films from a low temperature cubic phase to orthorhombic phase and, finally, to a higher-temperature cubic phase under thermal annealing is shown to have a crucial influence on the optoelectronic properties and photovoltaic performance of this material. These results demonstrate that all inorganic perovskites can deliver competitive photovoltaic performances, thus paving a route to the design of novel highly stable materials for efficient perovskite-based photovoltaics. The all inorganic perovskite CsPbI3 films investigated in this work were grown by thermal coevaporation of CsI and PbI2 precursors in vacuum with subsequent thermal annealing on a hot plate at temperatures ranging from 60 to 320 °C (see the details in the Supporting Information (SI)). Received: November 6, 2016 Accepted: December 7, 2016 Published: December 12, 2016 67

DOI: 10.1021/acs.jpclett.6b02594 J. Phys. Chem. Lett. 2017, 8, 67−72

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Figure 3. SEM images (a,b) and AFM topography (c,d) of the unannealed (a,c) and annealed at 320 °C (b,d) CsPbI3 films.

brown samples were looking very similar to those of the hightemperature black perovskite phase previously reported in the literature.17−20 However, this low-temperature brown phase is not stable and undergoes an irreversible transformation to a yellow phase under heating to temperatures above 60 °C. The yellow phase is stable up to a temperature of 320 °C, then undergoes a transition to a black phase. The calculated optical band gaps (Eg) for low-temperature brown and yellow and higher temperature black phases of CsPbI3 were 1.73, 2.66, and 1.67 eV, respectively. We want to emphasize that the formation of the lowtemperature brown perovskite phase of CsPbI3 is a particular feature of the vacuum thermal coevaporation process. We failed to obtain this low-temperature perovskite phase of CsPbI3 by

Figure 1. Visual and UV−vis−NIR data showing the temperaturedependent phase transition of CsPbI3 films. (a) Colorimetric observation of the color change from brown to yellow and then to higher temperature black phase. (b) Evolution of the absorption spectra of the films as a function of the annealing temperature.

Figure 1 shows the evolution of the absorption spectra of the coevaporated CsPbI3 films as a function of the annealing temperature. Surprisingly, the spectra of the nonannealed

Figure 2. 2D diffraction patterns of CsPbI3 films at 25 (a), 120 (b), 320 (c) and 350 °C (d). Schematic representations of room temperature (RT) cubic, orthorhombic, and high temperature (HT) cubic lattices with respect to the horizontal substrate (e). 68

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incidence wide-angle X-ray scattering (GIWAXS) was used. The thermal diagram of the performed experiment is given in Figure S1, SI). Figure 2 shows 2D GIWAXS diffractograms of the freshly deposited perovskite film heated to different temperatures. The room-temperature pattern is interpreted as a mixture of yellow orthorhombic (a = 10.71 Å, b = 5.31 Å, c = 17.35 Å) (Table T1, SI) and black cubic (a = 7.00 Å) (Table T2, SI) lattices. This combination of black and yellow phases results in a brown color of these samples. The revealed parameters of orthorhombic and cubic CsPbI3 phases were shown to be slightly different compared with that previously reported,19−21 which might be due to a moderate structural disorder in the low-temperature films and the presence of a certain amount of additional cesium atoms in the lattice acting as point defects. Azimuthal distribution of 100 and 101 peaks of cubic and orthorhombic phases, respectively, shows that the a axis of both unit cells is oriented normally to the substrate (Figure 2e). The first heating of the sample to 120 °C was accompanied by a change in the peak positions (Figure S2, SI), which can be attributed to dehydration of the top layer of the film and improvement of its crystal structure upon annealing at higher temperature. Heating of the CsPbI3 films above 120 °C leads to a considerable decrease in the relative content and the degree of ordering of the RT cubic phase (Figure 2b). The temperature-dependent 1D spectra (Figure S2, SI) show that the RT cubic phase disappears at 140 °C. The yellow orthorhombic phase undergoes transition to the high-temperature cubic phase at 315 °C (Figure 2c). We note that the a-parameter of the high-temperature cubic phase (7.36 Å) is considerably bigger than that of the lowtemperature cubic phase (Table T3, SI). The 2D pattern obtained at 320 °C shows that the cubic structure is oriented with (100) vector normal to the substrate, which suggests that the orthorhombic to cubic phase transition leads to a significant disorder due to a strong mismatch in the structure of layers parallel to the substrate (Figure 2e). Additional dotted reflections presented on the diffractograms can be attributed to the traces of individual phases of CsI and PbI2. The influence of thermal annealing on the structure and morphology of the CsPbI3 films was examined using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Figure 3 shows that nonannealed films are composed of tiny grains with an average lateral size of ∼50 nm. Thermal annealing of the films leads to growth of the grains up to ∼150 nm at 120 °C and up to 1−3 μm at 320 °C (Figure 3 and Figure S3, SI). The 3D AFM images shown in Figure 3 demonstrate also significant increase in the film roughness from ca. 80 to >200 nm. A combination of the SEM and AFM data suggests an increase in the crystallinity of the films upon annealing. Good connectivity between the grains and complete uniformity of the film can also be noticed. Scanning Kelvin Probe Microscopy (SKPM) showed that the measured surface potential also strongly depends on the crystal structure of CsPbI3 films produced at different annealing temperatures (Figure S4, SI). The average surface potential of the nonannealed films and after heating at 120 and 320 °C were −20, 550, and 645 mV, respectively, against highly oriented pyrolytic graphite (HOPG). The variation in measured surface potentials suggests that subtle morphological changes in crystallinity and phases are accompanied by a redistribution of charge within or between the domains. Higher values of the surface potential for annealed films might also be attributed to

Figure 4. Architecture of the investigated solar cells (a). Histogram of the efficiencies for a batch of 12 solar cells derived from the current density−voltage (J−V) curves measured with the scan rate of 0.2 V/s in a forward direction (b). Current−voltage characteristics of the champion device measured at the low scan rate of 0.02 V/s (c) and its EQE spectrum (d).

solution processing of CsI+PbI2 system under a variety of experimental conditions, which fully agrees with other reports describing the formation of an orthorhombic nonperovskite yellow phase at room temperature.17,18 To more specifically quantify the structural evolution of CsPbI3 films as a function of temperature, in situ grazing 69

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Table 1. Best (Average) Power Conversion Efficiency (η), Open-Circuit Voltage (VOC), Short-Circuit Current Density (JSC), and Fill Factor (FF) of the Devices Extracted From J−V Measurements under Different Sweep Conditions material CsPbI3 (coevaporated, this work)

a

scan rate (V/s)

scan direction

VOC (mV)

JSC (mA/cm2)

FF (%)

η (%)

0.02

forward backward forward backward forward backward forward backward forward backward backward backward forward backward forward backward forward backward backward forward backward forward backward

990 1063 1002 1019 985 (920 ± 79) 1012 990 995 995 997 800 1230 720 740 738 750

13.9 13.8 12.8 13.0 12.1 (11.8 ± 1.3) 12.4 11.5 11.8 10.5 10.6 12.0 13.47 9.9 10.5 12.0 12.3

67.7 71.6 74.0 71.7 69.7 (65.0 ± 4.7) 68.2 70.5 72.5 63.9 73.0 30.2 65.0 53.0 61.0 33.0 42.0

1100 (850) 1100 818 959 991 998

11.9 (11.82) 10.8 8.7 8.7 16.8 16.7

75.0 (57.0) 68.9 52 56 71.8 72.9

9.3 10.5 9.5 9.5 8.3 (7.2 ± 0.6) 8.6 8.0 8.5 6.7 7.7 2.9 10.77 3.8 4.7 2.9 3.7 7.8 9.8 (6.02) 8.18 3.7 4.7 12.0 12.1

0.10 0.20 0.30 0.40

a

CsPbI3 (solution, ref 17) CsPbI3 (quantum dots, ref 23) CsPbI3 (solution, ref 16)

0.1 0.225 0.10

CsPbI3 (solution, this work)

0.10

CsPbIBr2(solution, ref 13)b

0.38

CsPbI2Br (solution, ref 23) CsPbIBr2 (coevaporated, ref 14)

0.225 1.20

MAPbI3, this work

0.10

Average values (±standard deviation from the mean) obtained for a batch of 12 devices are given in brackets. bAverage values given in brackets.

compared with the solution processing.24 Additionally, we want to emphasize high reproducibility of the solar cell parameters within the batch of 12 devices (Figure 4b and Table T4, SI), which also seems to be a consequence of high quality of the vacuum-processed CsPbI3 films. The best device parameters were extracted from the J−V curves measured at low voltage sweep rate of 20 mV/s (Figure 4c). The short-circuit current density obtained in this measurement was also reconfirmed by integration of the EQE spectrum, as shown in Figure 4d. Thus the best devices based on CsPbI3 delivered power conversion efficiency of 9.3 to 10.5% depending on the J−V scan direction. It is worth noting that the reference planar heterojunction devices based on hybrid MAPbI3 perovskite showed quite comparable characteristics (Table 1). A steady-state photocurrent measurement was performed for the CsPbI3-based perovskite solar cells under constant load near the device maximal power point (Figure S5). The current at the maximal power point decreased from ∼11.80 to ∼11.04 mA/cm2 over time, which corresponds to the relatively small loss in the performance (93.6% of the initial efficiency is preserved). In conclusion, we have demonstrated that all inorganic lead halide perovskites can successfully compete in terms of photovoltaic performance with the most widely used hybrid materials such as MAPbI3. Excellent intrinsic photochemical and thermal stability of inorganic perovskite materials, in general, presents a real promise to reach acceptable solar cell operation lifetimes required for successful commercialization of perovskite-based photovoltaics. In the present work, we have obtained a power conversion efficiency of 9 to 10% for planar heterojunction solar cells based on vacuum-processed CsPbI3 films, which is, to the best of our knowledge, one of the highest values ever reported for the all inorganic lead halide

reduced density of surface defects acting as trap centers for electrons.22 We note that the surface potential also depends on the size and orientation of the grains relative to the substrate, as it can be concluded from the SKPM image shown in Figure S3f, SI. The architecture of the solar cells based on the CsPbI3 films is shown schematically in Figure 4a. Semitransparent ITO glass is covered with a compact TiOx (c−TiOx) electron-transport layer, thus forming an electron-collecting electrode. Perovskite active layer of CsPbI3 is grown by vacuum coevaporation of CsI and PbI2 with subsequent annealing at 320 °C for 15 min in a nitrogen atmosphere inside the glovebox. Hole transport layer of poly(3-hexylthiopehne) (P3HT) is deposited above the photoactive layer by spin-coating, and the device structure is completed by evaporation of the top gold electrode. To reach the best photovoltaic performance, we optimized the thicknesses of the TiOx, perovskite, and P3HT layers as well as the stoichiometric composition of the active layer material. It is well known that J−V characteristics of the perovskite solar cells demonstrate significant dependence on the voltage sweep rate and direction. Figure S5 (SI) shows a series of J−V curves obtained for a CsPbI3-based device under different measurement conditions. One might notice that there is indeed some effect of the scan rate and direction on the photovoltaic characteristics of the solar cells. The efficiency of a single device measured under different conditions changes from 6.7% (fast scan, forward direction) to 10.5% (slow scan, reverse direction, Table 1). The observed “hysteresis” effect is much less pronounced than in the case of previously reported solutionprocessed CsPbI3 and CsPbIBr2 solar cells15,17 as well as very recently reported devices based on CsPbI3 quantum dots.23 Most likely, the reduced hysteresis in the CsPbI3-based solar cells is related to a higher quality of the perovskite films produced by vacuum coevaporation of the precursor iodides as 70

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(4) Filip, M. R.; Eperon, G. E.; Snaith, H. J.; Giustino, F. Steric engineering of metal-halide perovskites with tunable optical band gaps. Nat. Commun. 2014, 5, 5757−9. (5) Baikie, T.; Fang, Y.; Kadro, J. M.; Schreyer, M.; Wei, F.; Mhaisalkar, S. G.; Graetzel, M.; White, T. J. Synthesis and crystal chemistry of the hybrid perovskite (CH3NH3)PbI3 for solid-state sensitised solar cell applications. J. Mater. Chem. A 2013, 1, 5628− 5641. (6) Even, J.; Pedesseau, L.; Katan, C. Analysis of Multivalley and Multibandgap Absorption and Enhancement of Free Carriers Related to Exciton Screening in Hybrid Perovskites. J. Phys. Chem. C 2014, 118, 11566−11572−11572. (7) Tanaka, K.; Takahashi, T.; Ban, T.; Kondo, T.; Uchida, K.; Miura, N. Comparative study on the excitons in lead-halide-based perovskitetype crystals CH3NH3PbBr3 CH3NH3PbI3. Solid State Commun. 2003, 127, 619−623. (8) Conings, B.; Drijkoningen, J.; Gauquelin, N.; Babayigit, A.; D’Haen, J.; D’Olieslaeger, L.; Ethirajan, A.; Verbeeck, J.; Manca, J.; Mosconi, E.; et al. Intrinsic Thermal Instability of Methylammonium Lead Trihalide Perovskite. Adv. Energy Materials 2015, 5, 1500477. (9) Noh, J. H.; Im, S. H.; Heo, J. H.; Mandal, T. N.; Seok, S. I. Chemical Management for Colorful, Efficient, and Stable Inorganic− Organic Hybrid Nanostructured Solar Cells. Nano Lett. 2013, 13, 1764−1769. (10) Christians, J. A.; Miranda Herrera, P. A.; Kamat, P. V. Transformation of the excited state and photovoltaic efficiency of CH3NH3PbI3 perovskite upon controlled exposure to humidified air. J. Am. Chem. Soc. 2015, 137, 1530−1538. (11) Misra, R. K.; Aharon, S.; Li, B.; Mogilyansky, D.; Visoly-Fisher, I.; Etgar, L.; Katz, E. A. Temperature- and Component-Dependent Degradation of Perovskite Photovoltaic Materials under Concentrated Sunlight. J. Phys. Chem. Lett. 2015, 6, 326−330. (12) Kulbak, M.; Gupta, S.; Kedem, N.; Levine, I.; Bendikov, T.; Hodes, G.; Cahen, D. Cesium Enhances Long-Term Stability of Lead Bromide Perovskite-Based Solar Cells. J. Phys. Chem. Lett. 2016, 7, 167−172. (13) Sutton, R. J.; Eperon, G. E.; Miranda, L.; Parrott, E. S.; Kamino, B. A.; Patel, J. B.; Hörantner, M. T.; Johnston, M. B.; Haghighirad, A. A.; Moore, D. T.; Snaith, H. J. Bandgap-Tunable Cesium Lead Halide Perovskites with High Thermal Stability for Efficient Solar Cells. Adv. Energy Mater. 2016, 6, 1502458. (14) Ma, Q.; Huang, S.; Wen, X.; Green, M. A.; Ho-Baillie, A. W. Y. Hole Transport Layer Free Inorganic CsPbIBr2 Perovskite Solar Cell by Dual Source Thermal Evaporation. Adv. Energy Mater. 2016, 6, 1502202. (15) Kulbak, M.; Cahen, D.; Hodes, G. How Important Is the Organic Part of Lead Halide Perovskite Photovoltaic Cells? Efficient CsPbBr3 Cells. J. Phys. Chem. Lett. 2015, 6, 2452−2456. (16) Ripolles, T. S.; Nishinaka, K.; Ogomi, Y.; Miyata, Y.; Hayase, S. Efficiency enhancement by changing perovskite crystal phase and adding a charge extraction interlayer in organic amine free-perovskite solar cells based on cesium. Sol. Energy Mater. Sol. Cells 2016, 144, 532−536. (17) Eperon, G. E.; Paternò, G. M.; Sutton, R. J.; Zampetti, A.; Haghighirad, A. A.; Cacialli, F.; Snaith, H. J. Inorganic caesium lead iodide perovskite solar cells. J. Mater. Chem. A 2015, 3, 19688−19695. (18) Møller, C. K. Crystal structure and photoconductivity of caesium plumbohalides. Nature 1958, 182, 1436−1436. (19) Møller, C. K. The structure of CsPbI3. Mater. Fys. Medd. Dan. Vid. Selsk. 1959, 32, 1. (20) Trots, D. M.; Myagkota, S. V. High-temperature structural evolution of caesium and rubidium triiodoplumbates. J. Phys. Chem. Solids 2008, 69, 2520−2526. (21) Stoumpos, C. C.; Malliakas, C. D.; Kanatzidis, M. G. Semiconducting Tin and Lead Iodide Perovskites with Organic Cations: Phase Transitions, High Mobilities, and Near-Infrared Photoluminescent Properties. Inorg. Chem. 2013, 52, 9019−9038. (22) Adhikari, N.; Dubey, A.; Khatiwada, D.; Mitul, A. F.; Wang, Q.; Venkatesan, S.; Iefanova, A.; Zai, J.; Qian, X.; Kumar, M.; Qiao, Q.

perovskites. Importantly, thermal annealing is necessary to induce phase changes in crystal structure and morphology to realize the final device performance. Even though the black perovskite phase of CsPbI3 undergoes slow back transition to the yellow orthorhombic polymorph, this process can be slowed down or completely prevented, for example, by using quantum dots, as shown in a very recent report by Luther et al.23 Our results emphasize the potential of the precursor iodide coevaporation technology (similar to the one used for hybrid perovskite systems24) producing high-purity, uniform and wellcrystalline perovskite films on the contrary to the solution processing, where the film quality is strongly affected by the solvent properties, wetting, evaporation, and other deposition parameters (e.g., spin-coating conditions). We anticipate that a broad range of other inorganic perovskite materials will soon emerge from optimized use of this vacuum coevaporation approach.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b02594. Experimental procedures, Figures S1 and S2 with the additional GIWAXS data, Figures S3 and S4 with the probe microscopy data for the CsPbI3 films, Figure S5 with the additional current−voltage characteristics measured with different voltage sweeping rates, Figure S6 with the steady-state photocurrent measurement data, Tables T1−T3 with crystallographic parameters, and Table T4 with the additional characteristics of the CsPbI3 perovskite solar cells. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +7-496-522-1418. Fax: +7-496-522-3507. ORCID

Pavel A. Troshin: 0000-0001-9957-4140 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was performed in the frame of the Next Generation Skoltech-MIT collaboration program. A.A.P. thanks the Ministry of Education and Science of the Russian Federation for support via the Scholarships of the President of the Russian Federation for Young Scientists and Graduate Students (no. SP-2238.2016.1).

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REFERENCES

(1) Jeon, N. J.; Noh, J. H.; Yang, W. S.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. Compositional Engineering of Perovskite Materials for High-performance Solar Cells. Nature 2015, 517, 476−480. (2) Li, X.; Bi, D.; Yi, C.; Decoppet, J.-D.; Luo, J.; Zakeeruddin, S. M.; Hagfeldt, A.; Gratzel, M. A vacuum flash-assisted solution process for high-efficiency large-area perovskite solar cells. Science 2016, 353, 58− 62. (3) Ishihara, T.; Takahashi, J.; Goto, T. Optical properties due to electronic transitions in two-dimensional semiconductors (CnH2n+1NH3)2PbI4. Phys. Rev. B: Condens. Matter Mater. Phys. 1990, 42, 11099−11107. 71

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The Journal of Physical Chemistry Letters Interfacial Study To Suppress Charge Carrier Recombination for High Efficiency Perovskite Solar Cells. ACS Appl. Mater. Interfaces 2015, 7, 26445−26454. (23) Swarnkar, A.; Marshall, A. R.; Sanehira, E. M.; Chernomordik, B. D.; Moore, D. T.; Christians, J. A.; Chakrabarti, T.; Luther, J. M. Quantum dot-induced phase stabilization of -CsPbI3 perovskite for high-efficiency photovoltaics. Science 2016, 354 (6308), 92−95. (24) Liu, M.; Johnston, M. B.; Snaith, H. J. Efficient planar heterojunction perovskite solar cells by vapour deposition. Nature 2013, 501, 395−398.

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