In Situ Monitoring the Uptake of Moisture into

Transcription

1 Supporting Information to: In Situ Monitoring the Uptake of Moisture into Hybrid Perovskite Thin Films Johannes Schlipf, Lorenz Bießmann, Lukas Oesinghaus, Edith Berger #, Ezzeldin Metwalli, Johannes A. Lercher #, Lionel Porcar, and Peter Müller-Buschbaum * Technische Universität München, Physik-Department, Lehrstuhl für Funktionelle Materialien, James-Franck-Str. 1, Garching, Germany Technische Universität München, Physics-Department and ZNN, Physics of Synthetic Biological Systems, Am Coulombwall 4a, Garching, Germany # Technische Universität München, Department of Chemistry and Catalysis Research Center, Lichtenbergstraße 4, Garching, Germany Institut Laue-Langevin (ILL), 6 Jules Horowitz, Grenoble, France Corresponding Author *muellerb@ph.tum.de S-1

2 Experimental Section. Perovskite film deposition. For synthesis of hybrid perovskite thin films we choose a common two-step method, that was used for efficient solar cells. 1 3 In our case, we adapted the method by Docampo et al. with slight changes, as neutron scattering experiments require large samples and many methods are optimized for solar cell samples on small substrates. We used p-doped silicon (Si) wafers (Si-Mat Silicon Materials e.k.) with 100 mm diameter and polished on the (100) side where we deposited the perovskite layer. The Si native oxide layer was important for proper surface coverage of the perovskite. Cleaning was done by subsequent rinsing with acetone and 2- propanol. Before spin-coating a 1 M solution of PbI 2 (Sigma Aldrich, TCI Chemicals) in dimethylformamide (DMF) the silicon surface was further cleaned and functionalized with O 2 plasma (0.4 mbar, 250 W, 10 min). In contrast to other methods, the solution was dropped onto the wafer at room temperature and rotation was started afterwards. If this step would have been done at 60 C as usual, it would have resulted in inhomogeneous perovskite films. Furthermore, not spin-casting the solution dynamically resulted in thicker films as required for GISANS experiments. Finally, the lower temperature also resulted in slightly more porous films which lead to faster degradation kinetics and made the experiment feasible for in situ neutron scattering experiments. 4 The films were then heated to 60 C and dipped into a mixed solution of MAI (9.5 mg/ml or M) and MACl (0.5 mg/ml or M) for 10 min to fully convert them to MAPI perovskite, which was slightly longer than usual as the films were thicker. Afterwards, the films were washed in a rinsing bath of 2-propanol at room temperature and dried on a spin-coater at 3000 rpm for 30 s. The experiments were conducted in ambient atmosphere while humidity was generally between %RH. S-2

3 In situ GISANS measurements. In situ grazing incidence small angle neutron scattering (GISANS) experiments were conducted at the Institut Laue-Langevin (ILL) in Grenoble (France) at the beamline D22. The aperture opening was 40 mm and a neutron wavelength of 6 Å was used. The detector at a sample-detector distance of 11 m was a 3 He multi-tube detector (Reuter- Stokes tubes) with 128 x 256 pixels à 8 mm x 4 mm. The sample chamber was built from extremely pure aluminum (Al 99.5) to exclude activation. The samples were placed vertically to have a better resolution in the out-of-plane scattering signal on the detector. The incident angle was α i = and typical exposure time was 10 min. To achieve better statistics e.g. for the fitting, several frames were summed up as noted in the text 5. Humidity experiments. For the in situ GISANS and the ex situ XRD experiments, saturated salt solutions in D 2 O (Deutero) were employed. After injection of 5 ml into a basin inside the chamber the D 2 O started to saturate the L volume. The established saturated relative humidity levels (at ~26 C) were as follows: LiCl:Mg(NO 3 ) 2 (41 %RH), Mg(NO 3 ) 2 (58 %RH), NaCl (73 %RH). With pure D 2 O humidity levels of 93 %RH and 96 %RH were achieved at ~26 C and 41 C, respectively. The water basin was located at the backside of the sample as to not accidentally splash the perovskite film during injection. Neutrons enter and exit the chamber through two windows covered with thin aluminum foil practically transparent to neutrons. The chamber was temperature controlled with a Julabo Heating Circulator thermostat running a water-polyethyleneglycol mixture through the chamber base. For monitoring temperature and relative humidity, a Sensirion Evaluation Kit EK-H5 and a SHT2x was used. Before the subsequent experiment, the chamber was disassembled and blow-dried with a fan. Additionally, in order to have defined low humidity starting conditions, it was put to mild underpressure with a vacuum pump. At elevated temperatures, it takes time until a stable atmosphere is established S-3

4 completely at the position of the sensor which is the reason for the later onset of the measured temperature rise to 41 C in Figure 1c as only the socket of the chamber is heated. Pictures of the setup are displayed in Figure S1. XRD measurements. X-ray diffraction (XRD) measurements were conducted using a Bruker D8 Advance with a Cu K α source. As the measurements were conducted ex-situ, i.e. in ambient atmosphere, a fast scan (~4 min) was conducted within the first 10 min after the sample was removed from the humidity chamber. Afterwards, a new measurement was taken every 47 min to follow the dehydration of the films in situ. SEM measurements. Scanning electron microscopy (SEM) images were taken with NVision40 field emission SEM (Carl Zeiss AG) with an acceleration voltage of 5 kv or 15 kv and a working distance of ~7.3 mm. Thermogravimetric H 2 O adsorption measurements. The sample, MAPI on Si substrate, was dried overnight under vacuum (10-7 mbar) to ensure the removal of pre-adsorbed gases or moisture. For reference measurements, the film was washed off by sonication in DMSO and wiping, and the blank Si substrate was additionally heated to 200 C for one hour before the start of the experiment to ensure complete removal of solvent residues. Adsorption isotherms of H 2 O were determined by dosing H 2 O vapor into the evacuated system in the pressure range between 1 and 28 mbar, analogous to 3.6 to 87 % relative humidity, on a Setaram TG-DSC 111 system at 25 C. We note that these measurements are quasi-static as the sample had time to equilibrate as opposed to the in situ GISANS measurements where the humidity was steadily increasing until it saturated. Therefore, the water content obtained from these measurements for 75 %RH is slightly S-4

5 higher than the value for 73 %RH obtained from GISANS; however, even after almost 500 min GISANS measurement time, the water content still seems to increase for this sample which suggests that it had not reached equilibrium when the measurement was stopped. Figure S1. Photographs of the setup at the D22 instrument, showing on the left the sample environment with the sample in vertical position relative to the beam (coordinate system as displayed), and the water basin at the backside. In the right image the lid is screwed on top to close the humidity chamber. A sensor for combined read-out of relative humidity and temperature is located inside the chamber. S-5

6 Figure S2. Raw 2D GISANS data for five perovskite films exposed to D 2 O vapor and saturated salt solutions from D 2 O at 26 C or 41 C as indicated and resulting different humidity levels. 1 h integration time is chosen for the 2D GISANS data of the dry state, the hydrated state (with the time noted) and the dehydrated state. S-6

7 Figure S3. Difference plots (the first frame of each set is subtracted) of the in-plane cuts for various humidity levels shown with corresponding Gaussian fits (green) to determine the Yoneda peak position. Each curve represents a 10 min measurement and is color coded from red to blue as the experiment progresses. In the bottom right, the average critical angles for the five samples as extracted from the films Yoneda peak positions are plotted with the theoretical positions of (1) MAPI, (2) monohydrate, (3) dihydrate and (4) PbI 2. The humidity levels are as follows: 41 %RH (red), 58 %RH (violet), 73 %RH (blue), 93 %RH (dark blue) and 96 %RH (black). Vertical error bars originate from the fits, horizontal ones are from the typical integration time for one frame of the GISANS data (10 min). S-7

8 material silicon MAPI monohydrate dihydrate lead iodide structural formula Si CNH 6PbI 3 CNH 6PbI 3 D 2O (CNH 6) 4PbI 6 2D 2O PbI 2 molar weight [g mol -1 ] density [g cm -3 ] molar volume [cm 3 mol -1 ] volume ratio sum of b coherent Nb [10-6 Å -2 ] critical angle [ ] # in Figure 2 & Figure S Table S1. Material properties of MAPI perovskite, possible degradation products and the silicon substrate. Not referenced numbers are taken from the National Institute of Standards and Technology (NIST) Center for Neutron Research available online at (accessed at ) or from own calculations. Determination of water content. The determined Yoneda peak positions represent the average critical angle of the material composition at each point. The critical angle = (1) depends on the neutron wavelength and on the neutron scattering length density which depends on the dispersion coefficient of the material. 6 = (2) S-8

9 Thus, the water content, or more precise the volume content of D 2 O molecules in the compound is determined by 1 x = (3) where and are the values obtained from theoretical calculations (see Table S1). GISANS data treatment. Figure S4. (a) Exemplary snapshot of out-of-plane cuts for MAPI films exposed to 93 %RH for the dry (red), the hydrated (blue) and the dehydrated (black) film with respective model curves (green). The plotted data is summed from three frames (equivalent of 30 min integration time) to increase statistics. Green lines are the simulation results obtained from a model within the framework of the DWBA that best fit to the data (see text below). (b) Respective cuts for 73 %RH and 93 %RH in Kratky representation showing the inflation and deflation of structures during hydration and after dehydration. S-9

10 The graphs in Figure S4a shows the out-of-plane cuts for the perovskite films exposed to 93 %RH. As the cuts are performed at the Yoneda position, the scattering contrast is high and one can get an insight into the lateral film morphology. In order to translate the information hidden in the diffuse scattering signal into real space, we assume a model of spheres with two different sizes deposited on a reflecting substrate. In grazing-incidence geometry, the reflection contributions are accounted for by the distorted-wave Born approximation (DWBA), while we assume that the scattering signal from the two different sized domains does not interfere (local monodisperse approximation). As we are looking at cuts, we further assume a 1D paracrystalline distribution for the particles. 10 We determine the peak positions in the Kratky representations with Gaussian fits and following our results from modeling with the DWBA assume spherical shapes. Thus, we can calculate the crystal volumes and see that the changes translate to a volume increase of (3.8 ± 1.9) % for 73 %RH, (9.8 ± 3.4) % for 93 %RH and (9.7 ± 0.8) % for 96 %RH for the small domains. Comparison to calculations using literature values of the mono- and dihydrate material densities 8,9 shows that the changes are too small and we can exclude that significant amounts of dihydrate are formed during the timeframe of our experiments (cf. Table S1). S-10

11 q y peak position [x10-2 nm -1 ] domain radius [nm] domain volume [x10 5 nm 3 ] volume ratio 73 %RH dry ± ± ± ± hydrated ± ± ± ± dehydr ± ± ± ± MAPI, partly monohydrate 93 %RH dry ± ± ± ± hydrated ± ± ± ± dehydr ± ± ± ± MAPI, monohydrate, partly dihydrate 96 %RH dry ± ± ± ± hydrated ± ± ± ± MAPI, monohydrate, partly dihydrate Table S2. Q y positions obtained from fitting Gaussian functions to the Kratky-style plots of the out-of-plane GISANS cuts (at q z = nm -1 ) and calculated values for crystal volumes assuming spheres. Although the calculated domain sizes coincide rather well with the ones obtained from modelling with the distorted-wave Born approximation, we want to highlight that we are only looking at relative changes here. S-11

12 Scanning electron microscopy. Figure S5. Scanning electron microscopy images of dry perovskite films and ones exposed to humid atmosphere for 6 h in D2O vapor. It is apparent, that the once facetted crystals become more round-shaped. Additionally, the surface roughness seems to be increased (cf. enhanced contrast in the second image on the right). Images are taken with 15 kv to enhance contrast. S-12

13 Dehydration experiments with in situ XRD. Figure S6. X-ray diffraction patterns for films exposed to humid atmosphere as noted. The dry film is shown in red and the first blue film corresponds to a measurement taken of the hydrated sample about 5-10 min after being removed from the humidity chamber. Subsequent curves color-coded from blue to black are additional measurements taken every 47 min to investigate the dehydration. The films exposed to D 2 O for 1.5 h already show trace amounts of PbI 2 (peaks at ) which vanish completely within the first 3.5 h in ambient atmosphere. On the other hand, no such peaks appear for the sample stored in NaCl(aq) atmosphere for 1.5 h. This result suggests that only monohydrate is formed during hydration which can transform back to MAPI as soon as the sample is removed from the humid environment. 11 After 6 h exposure to D 2 O atmosphere, the peaks do not vanish anymore, so the PbI 2 stays inside. As PbI 2 is a degradation product of the dihydrate, this could indicate that while in the hydrated form, dihydrate was S-13

14 present in the film. 11 For the film stored in NaCl(aq) atmosphere for 6 h, however, no additional PbI 2 is detected which in turn means that no dihydrate is formed. Positions of Bragg reflexes are indicated by purple bars and labeled exemplary in the graph on the bottom right. References (1) Liang, K., Mitzi, D. B., Prikas, M. T. Synthesis and Characterization of Organic Inorganic Perovskite Thin Films Prepared Using a Versatile Two-Step Dipping Technique. Chem. Mater. 1998, 10, (2) Burschka, J., Pellet, N., Moon, S.-J., Humphry-Baker, R., Gao, P., Nazeeruddin, M. K., Grätzel, M. Sequential Deposition as a Route to High-Performance Perovskite-Sensitized Solar Cells. Nature 2013, 499, (3) Docampo, P., Hanusch, F. C., Stranks, S. D., Döblinger, M., Feckl, J. M., Ehrensperger, M., Minar, N. K., Johnston, M. B., Snaith, H. J., Bein, T. Solution Deposition-Conversion for Planar Heterojunction Mixed Halide Perovskite Solar Cells. Adv. Energy Mater. 2014, 4, (4) Oesinghaus, L., Schlipf, J., Giesbrecht, N., Song, L., Hu, Y., Bein, T., Docampo, P., Müller-Buschbaum, P. Toward Tailored Film Morphologies: The Origin of Crystal Orientation in Hybrid Perovskite Thin Films. Adv. Mater. Interfaces 2016, 3, (5) Müller-Buschbaum, P., Bießmann, L., Metwalli, E., Oesinghaus, L., Porcar, L., Schlipf, J. Impact of Moisture on the Morphology of Hybrid Organometal Halide Perovskite Films 2016, Institut Laue-Langevin (ILL), ILL-DATA S-14

15 (6) Müller-Buschbaum, P. GISAXS and GISANS as Metrology Technique for Understanding the 3D Morphology of Block Copolymer Thin Films. Eur. Polym. J. 2016, 81, (7) 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, (8) Hao, F., Stoumpos, C. C., Liu, Z., Chang, R. P. H., Kanatzidis, M. G. Controllable Perovskite Crystallization at a Gas-Solid Interface for Hole Conductor-Free Solar Cells with Steady Power Conversion Efficiency over 10%. J. Am. Chem. Soc. 2014, 136, (9) Wakamiya, A., Endo, M., Sasamori, T., Tokitoh, N., Ogomi, Y., Hayase, S., Murata, Y. Reproducible Fabrication of Efficient Perovskite-Based Solar Cells: X-Ray Crystallographic Studies on the Formation of CH 3 NH 3 PbI 3 Layers. Chem. Lett. 2014, 43, (10) Renaud, G., Lazzari, R., Leroy, F. Probing Surface and Interface Morphology with Grazing Incidence Small Angle X-Ray Scattering. Surf. Sci. Rep. 2009, 64, (11) Leguy, A. M. A., Hu, Y., Campoy-Quiles, M., Alonso, M. I., Weber, O. J., Azarhoosh, P., van Schilfgaarde, M., Weller, M. T., Bein, T., Nelson, J., Docampo, P., Barnes, Piers R. F. Reversible Hydration of CH 3 NH 3 PbI 3 in Films, Single Crystals, and Solar Cells. Chem. Mater. 2015, 27, S-15