Communication pubs.acs.org/cm
Direct Crystallization Route to Methylammonium Lead Iodide Perovskite from an Ionic Liquid David T. Moore,† Kwan W. Tan,† Hiroaki Sai,†,§ Katherine P. Barteau,† Ulrich Wiesner,*,† and Lara A. Estroff*,†,‡ †
Department of Materials Science and Engineering and ‡Kavli Institute at Cornell for Nanoscale Science, Cornell University, Ithaca, New York 14853, United States S Supporting Information *
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to be hindered by a lack of complete understanding of the composition of the precursor, as the transformation A → B is more difficult to control if A is unknown. Additionally, previous reports show that the temperature range in which crystallization can occur is narrow (e.g., 80−130 °C for a 1:3 molar ratio of PbCl2:methylammonium iodide (MAI) in dimethylformamide) and that the lifetime of the pure perovskite film, in the same temperature range, is only ∼2−20 min before decomposition begins;4,5 i.e., the processing time available for growth is a function of the processing temperature. This coupling of the time and temperature, as well as their limited ranges, is an additional roadblock to better control of the crystal growth. Control of perovskite crystal growth would be substantially enhanced by two main factors: (1) comprehensive knowledge of the starting state and (2) decoupling of the temperature from the crystallization time. In this communication we report the crystallization of thin films of MAPbI3 from an ionic liquid
he recent, unprecedented increase in photovoltaic device efficiencies of the methylammonium lead tri-iodide perovskites (MAPbI3) has been, in large part, the product of improved film coverage and processing methods.1−4 Work along these lines has provided two significant lessons: first, crystal quality, in terms of grain size and crystal orientation, is a factor in achieving high performance devices,2,3 and second, perovskite formation is a two-step process in which a crystalline precursor undergoes a solid-state transformation into the desired perovskite.5 These lessons infer that controlling the crystal growth is a path to better devices, and understanding the nature of the precursor is an important component of that control. In the past year several studies have focused on understanding the nature of the precursor,6 the mechanism and kinetics of how it transforms to the perovskite,7,8 and the disposition of excess reagents and solvents used during processing.8,9 Controlling perovskite crystal growth continues
Figure 1. In situ WAXS data for films prepared from a 1:1 molar ratio of PbI2:MAI in MAFa annealed at 50 °C in N2. (a) Integrated 1D plots as a function of time after film spin coating (t = 0 min is the end of the spin cycle); black stick markers represent the known peak locations for MAPbI3; (b) 2D image of the same film taken at t = 10 min showing the film texture; inset shows the 2D image of a similarly prepared film from 1:1 PbI2:MAI in DMF for reference. Received: December 24, 2014 Revised: April 7, 2015
© XXXX American Chemical Society
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DOI: 10.1021/cm5047484 Chem. Mater. XXXX, XXX, XXX−XXX
Communication
Chemistry of Materials (IL), methylammonium formate (MAFa). We show that this processing route addresses both of these key factors thereby paving the way to advanced engineering of perovskite thin films for photovoltaic applications. Thin films were prepared by dissolving a 1:1 molar ratio of PbI2:MAI in MAFa at 30 wt %. The resulting solution was deposited by spin coating or drag coating using conditions to achieve a final film thickness of approximately 500−700 nm. It should be noted that typical dewetting behaviors associated with casting thin films from volatile organic compounds (VOC) are altered when using an IL; these differences are briefly discussed in the Supporting Information. The evolution of the crystal growth was determined by in situ synchrotron wideangle X-ray scattering (WAXS) during the postdeposition annealing process. Figure 1 shows integrated 1D scattering plots as well as a representative 2D pattern after a completed 10 min heating cycle at 50 °C under nitrogen. The trace at 6 min shows the first indication of a crystalline structure with the first peak appearing at q = 10 nm−1, the known location of the (110) peak of MAPbI3. Although we cannot discount the possibility of an amorphous, solid-state intermediate, on the time scale of our X-ray experiments no crystalline intermediate was detected suggesting a one-step route to the direct thin film crystallization of MAPbI3. The lack of an intermediate structure would satisfy the first step toward better control of the desired perovskite film structure noted in the introduction as the starting materials and conditions for this process are known: a solution of dissolved ions in an ionic liquid. This would allow researchers to leverage a large body of work on templating the nucleation, and controlling the growth, of crystals from solution.10 An added benefit of this process, as is evident from Figure 1B, is the high degree of orientation of the crystal grains, which has been correlated to improved device performance;2 the inset of Figure 1B shows the more powder-like scattering patterns of films made from PbI2:MAI dissolved in dimethylformamide (DMF) for reference. The reason that MAFa eliminates, at the resolution of our experiments, the solid-state precursor, which exists when organic solvents are used, is not known at this time; however, atypical crystallization in ILs has been commonly reported.11 An additional property of ILs, which is important for a crystallization medium, is their thermal stability; specifically, ILs exhibit negligible vapor pressure until very near their boiling point.12 These properties of ILs may provide a key to decoupling the processing temperature from the time available for crystal/film growth. We tested for this decoupling by preparing several different films and subjecting them to a variety of substantially different postdeposition annealing protocols. Figure 2A shows the XRD scans for three different temperature protocols; 10 min at 50 °C, 75 °C, and 130 °C, plus results for longer annealing times of 100 and 1000 min (∼16 h) at 75 °C. Including the sample from Figure 1, we have temperature extremes of 50 to 130 °C for 10 min and time extremes of 10 to 1000 min at 75 °C. As the data in Figure 2A demonstrates, in all cases the resulting film is composed of crystalline MAPbI3; most notably, no sample shows any indication of either a precursor or decomposition to PbI2. The XRD plots do show differences in the peak ratios for the different samples, inferring different textures. The cause, and the evolution, of the texture is beyond the scope of this communication but is the subject of ongoing studies. In previous work, the maximum amount of time we could anneal a film, before the onset of decomposition, was
Figure 2. (a) XRD plots for several time/temperature annealing profiles. Times and temperatures used are marked on each trace. Black stick markers indicate known peak locations for MAPbI3. (b) SEM micrographs of a film annealed at 75 °C for 16 h; cross-section (top) and plan (bottom) views.
approximately 50% longer than the time required for complete perovskite crystallization.5 The results for the 75 °C sample show that complete crystallization can occur in as little as 10 min, and yet no decomposition is evident after 16 h. To the extent that the decomposition of MAPbI3 to PbI2 is accelerated (or may only occur) by the removal of the solvent,9,13 finding solvents that will not evaporate, at temperatures of interest for crystallization, would aid in decoupling the processing time from the temperature. Furthermore, by using an IL in which one of the ions is methylammonium, we can suppress the diffusion of MAI units from within the perovskite lattice by Le Chatelier’s principle and reduce the B
DOI: 10.1021/cm5047484 Chem. Mater. XXXX, XXX, XXX−XXX
Communication
Chemistry of Materials
(2) Saliba, M.; Tan, K. W.; Sai, H.; Moore, D. T.; Scott, T.; Zhang, W.; Estroff, L. A.; Wiesner, U.; Snaith, H. J. J. Phys. Chem. C 2014, 118, 17171−17177. (3) Xiao, Z.; Dong, Q.; Bi, C.; Shao, Y.; Yuan, Y.; Huang, J. Adv. Mater. 2014, 26, 6503−6509. (4) Zhang, W.; Saliba, M.; Moore, D. T.; Pathak, S.; Horantner, M.; Stergiopoulos, T.; Stranks, S. D.; Eperon, G. E.; Alexander-Webber, J. A.; Abate, A.; Sadhanala, A.; Yao, S.; Chen, Y.; Friend, R. H.; Estroff, L. A.; Wiesner, U.; Snaith, H. J. Nat. Commun. 2014, 6, 6142. (5) Tan, K. W.; Moore, D. T.; Saliba, M.; Sai, H.; Estroff, L. A.; Hanrath, T.; Snaith, H. J.; Wiesner, U. ACS Nano 2014, 8, 4730−4739. (6) Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Seok, S. I. Nat. Mater. 2014, 13, 897−903. (7) Tidhar, Y.; Edri, E.; Weissman, H.; Zohar, D.; Hodes, G.; Cahen, D.; Rybtchinski, B.; Kirmayer, S. J. Am. Chem. Soc. 2014, 136, 13249− 13256. (8) Unger, E. L.; Bowring, A. R.; Tassone, C. J.; Pool, V.; GoldParker, A.; Cheacharoen, R.; Stone, K. H.; Hoke, E. T.; Toney, M. F.; McGehee, M. D. Chem. Mater. 2014, 26, 7158−7165. (9) Williams, A. E.; Holliman, P. J.; Carnie, M. J.; Davies, M. L.; Worsley, D. A.; Watson, T. M. J. Mater. Chem. A 2014, 2, 19338− 19346. (10) Geissler, M.; Xia, Y. Adv. Mater. 2004, 16, 1249−1269. (11) Ahmed, E.; Breternitz, J.; Groh, M. F.; Ruck, M. CrystEngComm 2012, 14, 4874. (12) Reichert, W. M.; Holbrey, J. D.; Vigour, K. B.; Morgan, T. D.; Broker, G. A.; Rogers, R. D. Chem. Commun. 2006, 4767. (13) Moore, D. T.; Sai, H.; Tan, K. W.; Smilgies, D.-M.; Zhang, W.; Snaith, H. J.; Wiesner, U.; Estroff, L. A. J. Am. Chem. Soc. 2015, 137, 2350−2358.
possibility of decomposition. Whether the suppression of decomposition is due to the persistence of the solvent or the high concentration of MA is unknown, but the overall effect is clear: Processing times 2 orders of magnitude longer than those required for complete crystallization can be employed with no detectable decomposition. Lastly, the success at bypassing the crystalline precursor and decoupling the crystallization time from the temperature is of most value if it maintains or surpasses the level of film quality that has already been achieved through other methods. Figure 2B shows SEM plan and cross section views of a film annealed at 75 °C for 16 h. The film quality is excellent with complete coverage of the substrate, consistent film thickness, and very large domains on the order of tens of micrometers. In conclusion, using MAFa as the solvent we have demonstrated a synthesis method for producing high quality, crystalline thin films of MAPbI3 without a precursor phase or signs of perovskite decomposition for a broad range of temperatures and times. The protocol produces films with excellent coverage, uniformity, and large crystal domains that are highly oriented. The absence of a precursor structure will allow for the application of well-known, solution-based methods of controlling both nucleation and growth. These results may enable use of a much broader range of processing conditions that can be tuned to achieve optimal crystal and film morphology.
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ASSOCIATED CONTENT
S Supporting Information *
A complete list of experimental methods, UV−vis absorption, additional SEM, and NMR spectra on films before and after rinsing. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*(U.W.) E-mail:
[email protected]. *(L.A.E.) E-mail:
[email protected]. Present Address §
(H.S.) Center for Bio-Inspired Energy Science, Northwestern University, Evanston, Illinois 60208, United States. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award No. DESC0010560. K.W.T. gratefully acknowledges the Singapore Energy Innovation Program Office for a National Research Foundation graduate fellowship. This work made use of the facilities of the Cornell Center for Materials Research (CCMR) supported by NSF award DMR-1120296 and Cornell High Energy Synchrotron Source (CHESS) supported by the NSF and NIH awards DMR-0936384 and DMR-1332208.
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REFERENCES
(1) Eperon, G. E.; Burlakov, V. M.; Docampo, P.; Goriely, A.; Snaith, H. J. Adv. Funct. Mater. 2014, 24, 151−157. C
DOI: 10.1021/cm5047484 Chem. Mater. XXXX, XXX, XXX−XXX
