Supporting Information for: Iodine Migration and Degradation of Perovskite Solar Cells Enhanced by. Metallic Electrodes

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1 Supporting Information for: Iodine Migration and Degradation of Perovskite Solar Cells Enhanced by Metallic Electrodes Cristina Besleaga +, Laura Elena Abramiuc +#, Viorica Stancu +, Andrei Gabriel Tomulescu +, Marian Sima +, Liliana Trinca +, Neculai Plugaru +, Lucian Pintilie +, George Alexandru Nemnes #, Mihaiela Iliescu, Halldor Gudfinnur Svavarsson, Andrei Manolescu and Ioana Pintilie *+ + National Institute of Materials Physics, Magurele, Ilfov, Romania # University of Bucharest, Faculty of Physics, Materials and Devices for Electronics and Optoelectronics Research Center, Magurele, Ilfov, Romania OPTOELECTRONICA 2001 S.A., Magurele, Ilfov, Romania School of Science and Engineering, Reykjavik University, Menntavegur 1, IS-101 Reykjavik, Iceland Corresponding Author: * ioana@infim.ro Contents: 1. Device fabrication 2. Atomic Force Microscopy (AFM) characterization 3. Perovskite solar cells in the presence of pinholes AFM and XRD 4. General and individual HRXPS spectra on fresh, pinhole-free surface of PSC 5. Photographs of cells with Ag and Mo/Ag counter electrodes showing the faster degradation in the absence of the Mo buffer layer S1

2 1. Device fabrication. The perovskite solar cells were fabricated starting from commercial glass substrate covered with FTO (resistivity 7Ω/sq, Solaronix TCO22-7). A compact TiO2 blocking layer of 150 nm thickness was deposited by spin-coating on the glass/fto substrate, using bis(acetylacetonate) solution (Aldrich), followed by thermal annealing at 450 C for 30 min. Then, a mesoporous TiO2 film (mp-tio2), composed of 20 nm size particles was deposited by spincoating using a solution of TiO2 commercial paste (Solaronix Ti-Nanoxide N/SP) diluted in ethanol (1:3, weight ratio). The mp-tio2 structures were first annealed at 150 C for 5 min and then crystallized at 500 C for 1 h. The CH3NH3PbI3-xClx mixt halide perovskite was fabricated using the modified one-step method 36 : a precursor solution containing 369 mg lead iodide, 56 mg lead chloride, 78 mg methyl sulfoxide, 159mg methyl ammonium iodide (Dysol) and 600 mg dimethylformamide, homogenized for one hour, was spin coated with 2000 rpm/25 s. After 9 s from the start of the spin cycle 150 μl of diethyl ether was dripped on top of the layer, enabling the solvent extraction process to take place. The final perovskite film is obtained after annealing at 65 C for 1 min and 100 C for 2 min. The total thickness of mp-tio2 and CH3NH3PbI3-xClx layers was estimated to be ~ 420 nm in average. All the above described processes were performed in normal laboratory conditions, at 24 C and humidity between 30% and 40%. The spiro-ometad, of ~ 200 nm thickness, was deposited by spin-coating at 1500 rpm for 30 s, in N2 enriched atmosphere, at 24 C and humidity less than 10%. The solution used for this deposition was obtained by mixing 80 mg spiro-ometad (Borun Chemical), 28 μl 4-tert-butylpyridine and 18 μl of bis(trifluoromethane)sulfonimide lithium salt in acetonitrile solution (520 mg ml -1 ). As counter electrodes Au, Ag and Mo/Ag having 100 nm thickness and area of 0.09 cm 2, have been deposited using a magnetron RF sputtering technique. S2

3 2. Atomic Force Microscopy (AFM) characterization: The topographic image of a 76 days aged FTO/TiO2/CH3NH3PbI3-xClx /Spiro-MeOTAD SC sample showing a pinhole-free surface of the cell is shown in Figure S1. The image was acquired by Atomic Force Microscopy in non-contact mode using an NT-MDT NTE-GRA Probe NanoLaboratory system (NT-MDT NSG01 cantilever with tip radius of 10 nm). Figure S1. Topographic AFM image of the spiro-ometad surface in a solar cell aged for 76 days in dry nitrogen atmosphere showing a pinhole-free surface. S3

4 3. Perovskite solar cells in the presence of pinholes AFM and XRD PSC-Au with pinholes fresh 693 h 15h@70 0 C CH 3 NH 3 Pbl 3 b) Intensity (cps) Pbl 2 CH 3 NH 3 PbCl theta (deg.) Figure S2. a) Topographic AFM image of the spiro-ometad surface in a fresh solar cell showing the presence of pinholes; b) the corresponding XRD patterns recorded at different annealing stages. The curves in b) correspond to the fresh cell (black rectangles), to the cell aged for 693h at 24 0 C in dark and N2 atmosphere with humidity below 10% (red circles) and to the same cell after an additional heat treatment of 15h at 70 0 C (blue triangles). The degradation of the PSC-Au samples with pinholes cannot be connected with the decomposition of the crystalline MAPI phase, as has been already discussed in Ref. 29 in the article, but rather with Au diffusion into the perovskite producing shunts across the device and generating trapping states in the perovskite absorber able to increase the recombination rate in the device.the only significant change in the XRD patterns is the increase in the peak intensity of PbI2 and this it happens mainly during annealing at 24 0 C. This increase could be accounted for PbI2 progressive crystallization and/or to the hypothetic decomposition of possible weakly bounded perovskite clusters in amorphous state, 1 not to be seen in XRD. S4

5 4. General and individual HRXPS spectra on fresh, pinhole-free surface of PSC XPS intensity (cps) 5 SC - Mo/Ag 2 days aged samples 3.5x10 SC - Au 3.0x10 5 No-HTM, no electrodes Ag 3d SC, no electrodes 2.5x x x x10 5 Au 4s Ag 3s I 3d Au 4p 3/2 Ag 3p Au 4d C 1s C 1s Ag 4s Au 4f Ag 4p I 4d 5.0x F 1s O 1s Pb 4d N 1s C 1s Cl 2s Cl 2p I 4s Pb 4f I 4d Pb 5d Binding Energy (ev) Figure S3. HRXPS on 2 days old samples, with and without electrodes. For a better visibility, the spectra recorded on samples with Mo/Ag or Au electrodes have been translated with 7000 and cps, respectively. For samples with electrodes the HRXPS spot (110 m diameter size) scanned the electrode surface. Figure S4. Individual XPS spectra measured on the surface of the CH3NH3PbI3-xClx layer, from which the amount of Cl, x = 0.39, was estimated by dividing the XPS intensity to the XPS atomic sensitivity factors (0.61 for Cl 2p3/2 and 6 for I 3d5/2 - Ref. 43 in the article) S5

6 5. Photographs of cells with Ag and Mo/Ag counter electrodes showing the faster degradation in the absence of the Mo buffer layer Figure S5. Photographs of the front and backside of a PSC sample with Mo/Ag and Ag electrodes at different aging times at 24 0 C, in N2 atmosphere, dark, humidity below 10%. No electrical measurements have been performed on the sample. It can be observed that in the absence of the Mo buffer layer, the MAPI decomposition takes place faster, in 12 days the whole perovskite below Ag electrode changing its color to yellow. The MAPI decomposition process does not start only from the edge of the Ag electrode as in the case Mo/Ag electrode but also from regions right below the electrode. The Mo buffer layer delay this degradation process mainly because the iodine exhaustion of the MAPI caused by the formation of AgI does not start below the Mo buffer layer but from the edge of the electrodes where iodine can bypass the Mo layer and react with Ag. Considering that no electrical measurements have been performed on the sample, the migration of iodine from the perovskite can only be determined by the workfunction difference between FTO and Ag (or Mo/Ag) electrodes. S6

7 Reference (1) Luo, S.; Daoud, W.A. Crystal Structure Formation of CH3NH3PbI3-xClx Perovskite. Materials 2016, 9, 123. S7