Direct crystallization route to methylammonium lead iodide perovskite from an ionic liquid

Transcription

1 Direct crystallization route to methylammonium lead iodide perovskite from an ionic liquid David T. Moore, a Kwan W. Tan, a Hiroaki Sai, a, Katherine P. Barteau, a Ulrich Wiesner, a,* Lara A. Estroff, a,b,* a Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853, USA. b Kavli Institute at Cornell for Nanoscale Science, Cornell University, Ithaca, New York 14853, USA. Current address: Center for Bio-Inspired Energy Science, Northwestern University, Evanston, Illinois 60208, USA Corresponding Authors * ubw1@cornell.edu, lae37@cornell.edu Supporting Information Experimental Methods Materials All materials were used as received. N,N-dimethylformamide (DMF, anhydrous), ethanol (absolute), methanol (99.8%), butanol (99.4%), formic acid (88% in water), methylamine (33 wt% in ethanol), titanium isopropoxide (>97%), and lead (II) iodide (PbI 2 ) were obtained from Sigma-Aldrich (St. Louis, MO); 37% hydrochloric acid was obtained from Mallinckrodt Baker. Methylammonium formate (MAFa) synthesis MAFa was synthesized by the following procedure. 25 ml of methylamine at 33% in ethanol and 10 ml absolute ethanol were added to a round bottom flask in an ice bath and placed under nitrogen flow on a schlenk line. 6 ml of 88% formic acid was mixed with 25 ml of methanol and loaded into a 60 ml syringe. The formic acid solution was added dropwise to the methylamine solution using a syringe pump at a rate of 2-3 drops per second through a septum to maintain the seal in the round bottom flask. The reaction solution was stirred slowly during the addition of the formic acid solution. After the complete addition of the formic acid, the solution was stirred for an additional 1 hour, then the flask was put under mild vacuum (~100 mtorr) for 24 hours. During the vacuum period the ice bath was replenished periodically to maintain the temperature near 0 C. At the end of 24 hours the temperature was raised to room temperature by removing the ice bath while maintaining vacuum; slow stirring was restored once the viscosity of the reaction product would allow for it. The reaction product was slowly stirred at room temperature for 60 minutes at which point the solution was a clear, viscous liquid. The MAFa was stored in a nitrogen glovebox and used within two weeks of synthesis to avoid decomposition.

2 Salt and solution preparation Methylammonium iodide (MAI) was prepared as previously published and stored in a desiccator. 1 A 30 wt% solution of PbI 2 and MAI in a 1:1 molar ratio was prepared in air by dissolving in MAFa and stirring at low speed overnight. Substrate Preparation Films were made on silicon or glass substrates that had been cleaned by sequential sonication in acetone then isopropyl alcohol (IPA) for 5 minutes followed by rinsing with IPA and deionized (DI) water and then UV-ozone (UVO) cleaning for 5 minutes. Just prior to spin coating the substrates were rinsed with IPA and DI water, dried under nitrogen flow and cleaned with UVO for 1-2 minutes. Film Preparation Spin cast films were made by depositing ~40 μl of 30 wt% solution and spinning at 2000 rpm for 30 s. Drag coated films were made by depositing 3-5 μl of 30 wt% solution on a substrate then doctor blading, by hand, with a razor blade; film thickness was controlled by using Kapton tape of various thicknesses to control the blade height. Upon completion of the deposition, the substrates were immediately placed on a preheated stage, under nitrogen flow, at the annealing temperature noted in the text. After annealing, the films were allowed to cool for several minutes and residual MAFa rinsed away by submersion in butanol for 2 minutes followed by rinsing with fresh, dried butanol. After rinsing, the bulk excess of butanol was removed by nitrogen flow followed by heating on a hot plate at 130 C for 30 seconds. Note: the films used for in situ WAXS characterization did not undergo this step as the data was collected during film formation. We verified that there were no changes to the films due to the rinsing step by characterizing films, before and after the rinse, by XRD, SEM and UV-Vis absorption; see below for results. Wide-Angle X-ray Scattering (WAXS) Characterization Samples were spin-coated at the Cornell High Energy Synchrotron Source (CHESS) and loaded on a custom-built temperature-controlled grazing incidence stage at the D1 beamline, 2 with a typical transfer period of 2-3 minutes. Detector was a Pilatus 300 K high speed pixel array detector, pixels with 172 μm/pixel resolution, at a distance of 92.5 mm from the sample. The x-ray wavelength was nm and the incident beam angle was approximately 0.25, well above the substrate critical angle. Typical exposure times were < 2 seconds. To avoid beam induced damage in the perovskite films at higher beam flux, the sample was moved perpendicular to the beam in 0.5 mm increments (the nominal beam width), and data collected from 16 different locations on the same substrate depending on the exact dimensions of the substrate used. X-ray diffraction (2-theta scans) X-ray diffraction patterns were collected using a Scintag Theta-Theta X-ray Diffractometer using Cu-Kα radiation (λ= Å). Scan speeds were adjusted such that the complete scan time was no more then 12 minutes to minimize X-ray damage to the films.

3 Scanning electron microscopy Scanning electron microscopy (SEM) images were acquired on uncoated perovskite thin films on silicon substrates using a Tescan Mira3 SEM equipped with an in-lens detector. TiO 2 Substrate Preparation The silicon substrates were rinsed with DI water and isopropanol sequentially, dried under nitrogen flow, and exposed to oxygen plasma for ~3 min (100 W). The TiO 2 compact layer spin-coating solution was prepared by adding 0.8 ml titanium isopropoxide and 0.07 g 2 M HCl into 8 g of ethanol. The TiO 2 solution was spin-coated on the substrates at 2000 rpm (45 s) in a nitrogen drybox and thermally annealed in a tube furnace at 450 C (2 h) in air with a 5 C/min ramp rate. 1H NMR 1 H NMR spectra were recorded on a Varian INOVA 400 spectrometer in deuterated dimethyl sulfoxide (DMSOd 6 ) and referenced to residual DMSO-d 5 (δ=2.47 ppm). Annealed films and single crystals of perovskite were dissolved in DMSO-d 6 and NMR performed on the resulting solutions. Evaluation of Rinsing Procedure on Film Structure To ensure that the rinsing protocol does not substantially alter the film or crystal properties, SEM, XRD and UV- Vis data was collected on films before and after the rinsing protocol. The data presented in Figure S1 shows no substantial differences in composition or film structure due to the rinsing protocol.

4 Figure S1: Data collected on films before and after the rinsing protocol. (a) XRD scans before (blue) and after (red) rinsing with the known peak locations for MAPbI3 (black) and MAI (red) shown as stick markers along the x axis. (b) UV-Vis absorption data before (blue) and after (red) rinsing. (c) SEM micrograph of annealed film prior to rinsing, inset shows detail of the crystal surface, (d) SEM micrograph of the same film in (c) after rinsing. Film Deposition on TiO 2 Most of the data reported was taken on films made on bare Si wafers. To ensure that the use of relevant substrates for solar cells would not substantially alter the results, additional films were prepared on TiO 2 coated Si. Figure S2 shows that the grain size of the film grown on TiO 2 is actually increased. We also note that in this film larger gaps remain between grains. It should be noted, however, that while we optimized annealing conditions for Si, we did not for TiO 2. We therefore expect that further optimization for this specific substrate will eliminate these differences.

5 Figure S2: SEM micrographs for films made on bare Si (a) and TiO2 coated Si (b). Films were prepared by drag coating and annealed at 75 C; both films were made during the same experimental session and using identical processing conditions. Removal of MAFa The use of an IL solvent differs from that of VOC solvents in the method by which the solvent is removed; the IL must be rinsed from the film after the annealing is complete. To quantify the amount of MAFa remaining in the films, we performed 1 H NMR on rinsed films by dissolving the films in deuterated DMSO. Figure S3 shows the resulting spectra of rinsed films along with several standards. As methylammonium is common to both the perovskite and MAFa we look specifically for the signal from the carbonyl bound hydrogen, δ 8.40 (s, 1H), Figure S3 clearly shows no significant MAFa or butanol remains in the rinsed films. The perovskite reference was taken from single crystals of MAPbI 3.

6 Figure S3: 1 H NMR spectra of (from top to bottom) a film rinsed with butanol, single-crystal MAPbI3, methylammonium formate (MAFa), and butanol. While the signals for MAPbI3 [δ 7.44 (s, 3H, NH3), 2.34 (s, CH3 3H)] were present in the rinsed film, the signals for butanol [δ 4.09 (s, 1H), (m, 2H), (m, 4H), 0.83 (t, 3H)] were undetected. A small peak at 2.29 ppm in the rinsed film is consistent with MAFa [δ 8.64 (s, 3H), 8.40 (s, 1H), 2.30 (s, 3H)] as a residual at less than 2 % (by weight). Signals at 3.30 and 2.47 ppm correspond to residual water and residual DMSO-d5, respectively, and their stronger signals in the rinsed film and MAPbI3 spectra compared to those in the MAFa and butanol spectra are due to concentration differences.

7 Dewetting Behavior of IL Films Burlakov et al. note in their work on controlling surface coverage of MAPbI 3 thin films that the causes of dewetting fall into two principle classes: (1) those that are controllable by manipulation of the interfacial energies of the air, film, and substrate; and (2) those that are largely uncontrollable due to largely inevitable imperfections in the film or film-substrate interface structures. 3 For films using MAFa as IL solvent, we see no evidence of typical dewetting behavior of the first class while the latter stays, essentially, unchanged and care must be taken during processing to ensure the substrates are as physically and energetically homogeneous as possible. References (1) Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Science 2012, 338, (2) Bian, K.; Choi, J. J.; Kaushik, A.; Clancy, P.; Smilgies, D.; Hanrath, T. ACS Nano 2011, 5, (3) Burlakov, V. M.; Eperon, G. E.; Snaith, H. J.; Chapman, S. J.; Goriely, A. Appl. Phys. Lett. 2014, 104,