Supporting Information

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1 Supporting Information Robust Stability of Efficient Lead-Free Formamidinium Tin Iodide Perovskite Solar Cells Realized by Structural Regulation Weiyin Gao, a Chenxin Ran, a Jingrui Li, b Hua Dong, a Bo Jiao, a Lijun Zhang, c Xuguang Lan, d Xun Hou, a and Zhaoxin Wu *,a a Key Laboratory of Photonics Technology for Information, Key Laboratory for Physical Electronics and Devices of the Ministry of Education, School of Electronic and Information Engineering, Xi an Jiaotong University, Xi an , P. R. China b Department of Applied Physics, Aalto University, FI AALTO, Finland c State Key Laboratory of Superhard Materials, Key Laboratory of Automobile Materials of MOE, and College of Materials Science and Engineering, Jilin University, Changchun , China d Institute of Artificial Intelligence and Robotics, Xi an Jiaotong University, Xi an , China * Corresponding zhaoxinwu@mail.xjtu.edu.cn S1

2 1. Theoretical Calculation We carried out first-principles (density-functional-theory, DFT) calculations starting from the experimental α- and β-phases of FASnI 3. [1] To model the disordered structures of the cubic α-phase, we adopted the supercell model that hosts quasi-randomly distributed FA + cations in single cells. We fixed the Sn 2+ and I ions at their Wyckoff positions in the undeformed structure during the geometry optimization, thus mimicking their average locations of the vibrating I anions at high temperatures (similar approaches were applied to CsSnI [2] 3 and MAPbI [3] 3 ). The very small standard deviation (< 0.01 Å) indicates that this model can perfectly reproduce the cubic phase of FASnI 3. This approach does not allow us to analyze the inorganic-framework deformation such as the Sn I Sn angles analogously to Ref. [4]. For a systematic analysis of the deformation of the Sn I framework, we need to perform DFT-based calculations on several larger model systems (such as [3, 5] ) to include the many possible alignments of FA + ions, which is beyond the scope of this work. For β-phase we adopted the supercell model of tetragonal lattice with regularly aligned FA + ions. We modeled the Cs doping with replacing different amount of FA + by Cs + in the supercell models. For example, a supercell consisting of 5 FA + and 3 Cs + corresponds to x = 0.375, i.e., Cs FA SnI 3. All possible alignments of doped Cs + cations within these supercell models were considered. For each x composition in α-phase, the standard deviation of total energy is < 7 mev per Cs x FA 1 x SnI 3 unit. The total-energy deviation in β-phase is a bit larger, ranging between 8 and 23 mev. Note: to limit the computational costs, we did not go beyond the models. Therewith we could not include as many x values and cover as many possibilities of Cs-alignments at each x as in larger model systems. Nevertheless, our model can give a correct trend of how thermodynamic properties vary with x, and the considered structures at each x can provide sufficient information of how these properties depend on the alignment of Cs + ions in the alloy. We used the PBEsol exchange-correlation functional [6] in the DFT calculations. For hybrid perovskites such as MAPbI 3, Perdew-Burke-Ernzerhof (PBE) [7] S2

3 exchange-correlation functional augmented with long-range van der Waals (vdw) corrections based on the Tkatchenko-Scheffler (TS) method [8] was demonstrated able to produce the lattice parameters in good agreement with experiment and describe the organic-inorganic interaction properly. [9,10] However, for Cs-containing systems TS-vdW with standard parameters will somewhat underestimate the lattice constants. We thus chose the PBEsol functional, which works well for CsSnI [2] 3 and was estimated behaving similarly with PBE+vdW(TS) based on a recent density-functional analysis. [11] In addition, scalar relativistic effects were included by means of the zero-order regular approximation. [12] All calculations in this work were performed with the all-electron numeric atom-centered orbital code FHI-aims. [13 15] Direct lattice-vector optimization was carried out with the analytical stress tensor implemented in FHI-aims. [16 ] For all calculations we used tier 2 basis sets and a Γ-centered k-point mesh. 2. Experimental Section 2.1 Materials. PEDOT: PSS (Clevios Al4083, Haraeus), SnI 2 (99.99%, Sigma Aldrich), CsI (99.99%, Sigma Aldrich), SnF 2 (99.5%, Sigma Aldrich), FAI (99%, MaterWin New Material), BCP (99%, Nichem), chlorobenzene (Anhydrous 99.8%, Sigma Aldrich), and DMSO (Anhydrous 99.8%, Alfa Aesar) were used as received. 2.2 Film deposition To prepare Cs x FA 1-x SnI 3 film, FASnI 3 and PPASnI 3 precursor solutions were first prepared by dissolving mg SnI 2 (0.75 mmol), 129 mg FAI (0.75 mmol), 11.9 mg SnF 2 (0.75 mmol), and mg SnI 2 (0.8 mmol), mg CsI (0.8 mmol), 11.9 mg SnF 2 (0.08 mmol) in 1 ml DMSO, respectively, and stirred for 2 h under room temperature to form a 0.75 M solution. Then different volume ratios of the FASnI 3 and CsSnI 3 solutions were mixed to prepare the Cs x FA 1-x SnI 3 precursor solution. To fabricate Cs x FA 1-x SnI 3 films, the precursor solution was spin-coated on the substrate at 5000 rpm (5000 rpm/s) for 35 s, and 100 μl chlorobenzene was dropped at the 25th s after spin started. Finally, the film was dried at 40 C for 2 min and 100 C for 20 S3

4 min. 2.3 Film characterizations Field emission scanning electron microscope (SEM) (Quanta250, FEI, USA) and atomic force microscope (AFM, NT-MDT, Russia) were used to investigate the morphology of perovskite film. XRD measurements were performed with an X-ray diffractometer (D/MAX-2400, Rigaku, Japan) with Cu Kα radiation. The absorption spectra were acquired on a UV-Vis spectrophotometer (U-3010, Hitachi High-Technologies, Japan). The PL spectra were acquired on a photoluminescence spectroscopy (Fluoromax 4, HORIBA Jobin Yvon, United States). EIS experiments were carried out in under AM 1.5 illumination at 0 V with an electrochemical workstation (CHI660C, CH Instruments) in a frequency range from 0.1 Hz to 1 MHz. 2.4 Device fabrication The ITO substrate was sequentially cleaned with ultrapure water, acetone, ethanol and isopropanol. After 10 minutes of UV-O 3 treatments, PEDOT: PSS solution was filtered and spin-coated onto the substrate at 5000 rpm for 60 s and then annealed 20 min at 150 C. After the deposition of perovskite layer, C 60 (30 nm), BCP (8 nm) were sequentially evaporated at the rate of 1.0 Å/s. Ag (120 nm) was finally evaporated through a shadow mask with active area of 0.09 cm 2 to finish the device. After fabrication, all the devices were kept unencapsulated in the glovebox for further measurements. 2.5 Device Characterizations All measurements were conducted in ambient air. The photovoltaic performance was under an AAA solar simulator (XES-301S, SAN-EI), AM 1.5G irradiation with an intensity of 100 mw/cm 2. The photocurrent-voltage (J-V) curve was measured using a Keithley 2602 Source-Meter. Incident photon-to-current conversion efficiency (IPCE) spectra were collected by the solar cell quantum efficiency measurement system (SolarCellScan 100, Zolix instruments. Co. Ltd.). S4

5 For SCLC measurement, the device structure of ITO/PEDOT: PSS/perovskite/Au was used. The trap-state density in the film can be estimated by the equation: n trap = 2ε 0 εv TFL /ql 2 where ε is the vacuum permittivity, ε 0 is the static dielectric constant of FASnI 3 (~5.7), and q is the elemental charge. S5

6 Figure S1. AFM measurements of Cs x FA 1-x SnI 3 films modified by the increasing proportion of Cs. S6

7 Figure S2. (a) SEM and (b) AFM image of CsSnI 3 film. S7

8 Figure S3. J-V curves of PSCs based on FASnI 3 film with various precursor concentrations. S8

9 Figure S4. J-V curves of PSCs based on FASnI 3 film with various solvent compositions. S9

10 Figure S5. J-V curves of PSCs based on FASnI 3 film with various annealing temperatures. S10

11 Figure S6. J-V curves of PSCs based on different additives. S11

12 Figure S7. Histograms of the device parameters of 30 PSCs fabricated under the same experimental conditions using 0% CsI and 8% CsI. S12

13 Figure S8. J-V curves of PSCs based on (a) 0% CsI and (b) 8% CsI when storing in N 2 at given period. S13

14 Figure S9. J-V curves of PSCs based on (a) 0% CsI and (b) 8% CsI when storing in air at given period. S14

15 Figure S10. J-V and power output tracking curves of PSCs based on (a) 0% CsI and (b) 8% CsI that used for the steady-state photocurrent measurement. S15

16 Figure S11. J-V curves of PSCs based on (a) 0% CsI and (b) 8% CsI when annealing at 100 C at given period. S16

17 Figure S12. XRD pattern of perovskite films with (a) 0% CsI and (b) 8% CsI doping before and after thermal annealing at 100 C for 60 min. UV-Vis measurements of perovskite films with (c) 0% CsI and (d) 8% CsI doping before and after thermal annealing at 100 C for 60 min. S17

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