Highly-oriented Low-dimensional Tin Halide Perovskites with Enhanced Stability and Photovoltaic Performance Supplementary Information

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1 Highly-oriented Low-dimensional Tin Halide Perovskites with Enhanced Stability and Photovoltaic Performance Supplementary Information Yuqin Liao 1,3,Hefei Liu 1,Wenjia Zhou 1,Dongwen Yang 2, Yuequn Shang 1,Zhifang Shi 1, Binghan Li 1, Xianyuan Jiang 1, Lijun Zhang 2 *,Li Na Quan 4, Rafael Quintero-Bermudez 4, Brandon R. Sutherland 4, Qixi Mi 1, Edward H. Sargent 4, and Zhijun Ning 1 * 1 School of Physical Science and Technology, ShanghaiTech University, 100 Haike Road, Shanghai , China 2 Key Laboratory of Automobile Materials of MOE, State Key Laboratory of Superhard Materials, and College of Materials Science, Jilin University, Changchun , China 3 Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, 865 Changning Road, Shanghai , China 4 Department of Electrical and Computer Engineering, University of Toronto, 10 King s College Road, Toronto, Ontario M5S 3G4, Canada These authors contributed equally to this work * lijun_zhang@jlu.edu.cn; ningzhj@shanghaitech.edu.cn Film Characterizations. Films characterizations were carried out in ambient environment unless otherwise stated. X-ray diffraction (XRD) was performed with a Bruker D8 Advance powder diffractometer using Cu Kα 1/2 source in θ-θ model. Photoluminescence (PL) spectra were recorded by exciting the perovskite films at 500 nm with a standard 450-W Xenon CW lamp. The signal was recorded with a spectrofluorometer (Fluorolog; HORIBA FL-3) and analyzed with the software FluorEssence. Time-resolved fluorescence (TRF) spectroscopy at 840 nm was measured using same Fluorolog with a pulsed source at 504 nm DeltaDiode DD- 510L; approximately 1 mm 2 spot size) exciting at the glass side. The absorption (Abs) spectra of the perovskite films on NiO x were measured by UV-vis spectrophotometer (Agilent cary5000) using an integrating sphere accessory. The in-situ transmittance measurements were carried out every 5 mins (30 mins in total) using double-beam model (22 ± 1ºC, 45 ± 2% RH). Nuclear magnetic resonance (NMR) test was carried out by dissolving the perovskite films in Deuterated S1

2 DMSO and characterizing with Bruker-AVANCE III HD 500MHz. Space charge limited current (SCLC) measurements were conducted on electron-only (ITO/ZnO/perovskite/PCBM/Al) devices and hole-only devices (ITO/NiO x /Perovskite/spiro/Au) separately. A Keithly 2400 source meter was used to measure the relevant J-V curves. In ideal conditions, when the electric field is not strong, the J D -V characteristics obey Ohm s law as V J D = σ E = qnµ L Where q is elementary charge, n is the carrier density, μ is the mobility, V is the applied voltage, and L is the film thickness. When the electric field is strong enough, the injected current density can be derived by the Mott-Gurney law: 9 V J D = ε 0εµ 8 L Where J D is current density, ε is the static permittivity, ε 0 is the permittivity of free space, μ is the mobility, V is the applied voltage, and L is the film thickness. By linearly fitting the log(j D) - Log(V) curve, the mobility can be obtained. Films were imaged with a high-resolution field emission scanning electron microscope (SEM) (Carl Zeiss Microscopy GmbH Supra 55) and exposed to air for a short time while being transferred to the vacuum chamber. X-ray photoelectron spectroscopy (XPS) was performed for films on NiO x coated ITO substrates using a Thermo K-ALPHA Surface Analysis. Films were unavoidably exposed to air during the preparation process for approximately 1 min. XPS measurements were carried out in a vacuum system with a base pressure of mbar. The samples were etched with a beam of 1keV Ar + ions for 15 s, 30 s, and 45 s. Curve fitting was performed using the Thermo Avantage software. Given the possible drift of the spectra, the curves were corrected based on the C 1s peak at 284.8eV. 2 3 Device Characterization. Device tests were performed in the same glovebox used for device fabrication. Current-voltage (J-V) curves were measured using a Keithley 2400 source unit under simulated AM1.5G solar illumination at 100 mw/cm 2 (1 sun). The light intensity was calibrated by means of a KG-5 Si diode with a solar simulator (Enli Tech, Taiwan). The S2

3 reference cell was calibrated and certified. The spectral mismatch correction factor is 0.49%. The devices are measured in reverse scan (0.7 V to 0 V, step 0.01 V) or forward scan (0 V to 0.7 V, step 0.01 V), and the delay time was 30 ms. The J-V curves for all devices were measured by masking the active area using a metal mask with an area of 0.04 cm 2. The external quantum efficiency (EQE) spectra were measured by a commercial system (Solar cell scan 100, Beijing Zolix Instruments Co., Ltd). The cells were subjected to monochromatic illumination (150 W Xe lamp passing through a monochromator and appropriate filters). The light intensity was calibrated by a standard photodetector (QE-B3/S BQ, Zolix). The light beam was chopped at 180 Hz and the response of the cell was acquired by a Stanford Research SR830 lock-in amplifier. Capacitance-voltage (C-V) measurements were conducted using an Agilent E4980A precision LCR meter. A perturbation of AC voltage of 10 mv at a frequency of 20 khz was superimposed on the DC bias to yield the final C-V curves. The measurements were carried out under dark conditions at room temperature and the carrier density was calculated from the Mott-Schottky equation. Devices were stored (without encapsulation or light blocking) and tested in the same nitrogen filled glovebox of thermal evaporation, with O 2 < 3 ppm, H 2 O < 1 ppm. S3

4 Figure S1 XRD patterns of pure FASnI 3 films (orange) and 20% PEA doped tin perovskite films (purple). The diffraction peak at 4 o provides direct evidence for the formation of lowdimensional structure. Figure S2 PL spectra of films with different PEA ratio when excited from the top surface. As the ratio of PEA increases, a constant blue shift of PL peak is observed and an excitonic peak emerges, a signature of the low-dimensional structure. S4

5 Figure S3 PL spectra of films with different PEA ratio when excited from the bottom surface. The presence of a peak at 625 nm in the 40% PEA sample and the more intense emission at 625 nm in the 80% PEA sample demonstrates that single-nanolayers are concentrated at the bottom. Figure S4 GIWAXS images of films with different PEA ratio (a) 0%, (b) 20%, (c) 40%, (d) 60%, (e) 80%, (f) 100%. Debye-Scherrer rings and Bragg spots indicate random or uniform orientation of crystal grains in the films, respectively. The spots closer to the central point compared to those in the image of the 20% film in (c), (d), (e) feature the PEA bilayers. This S5

6 illustrates their preferential orientation parallel to the substrate. (f) shows that in 100% PEA films, PEA bilayers are parallel to the substrate. Figure S5 Absorption spectra of pure FASnI 3 films (black) and 20% PEA doped tin perovskite films (red). Blue shift of the 20% PEA films can be observed, the absorption beyond the bandgap is resulted from the defect center in pure FASnI 3.. Figure S6 XPS Sn 3d spectra of films without PEA and with 20% PEA (orange) with different etching time. The two peaks deconvoluted from the spectra at 486.7eV and 487.4eV are associated with Sn 2+ (green) and Sn 4+ (purple), respectively. The Sn 4+ contents are shown in Table S1. Table S1 Sn 4+ contents (%) in films with different etching times S6

7 Etching time 0s 15s 30s 45s without PEA with PEA Table S2 Decomposition enthalpy HH dddddd with respect to possible disproportionation channels (in ev per Sn atom) of (PEA) 2 (FA) n-1 Sn n I 3n+1 calculated from first-principles DFTbased method. We considered two different decomposition pathways, i.e., the pathway involving only Sn 2+ compounds (path1) and the one involving only Sn 4+ compounds (path2). The case with n= represents 3D perovskite FASnI 3. n (001) (111) path path path path * (001) and (111) represent the calculated HH dddddd values with adoption of the (PEA) 2 (FA) n- 1Sn n I 3n+1 structure with FA molecules aligned along the (001) and (111) directions of the 3D quasi-cubic perovskite lattice. ** Path1: (PEA) 2 (FA) n-1 Sn n I 3n+1 2PEAI + (n-1)fai + nsni 2 Path2: (PEA) 2 (FA) n-1 Sn n I 3n+1 2PEAI + (n-1)fai + n/2sno 2 + n/2sni 4 n/2o 2 S7

8 Figure S7 Band gap energy (in ev) of 2D (PEA) 2 (FA) n-1 Sn n I 3n+1 perovskite films as a function of n derived from (a) the peaks of PL spectra and (b) first principles DFT-based calculations. In (b) the (001) and (111) represent the calculated values with adoption of the (PEA) 2 (FA) n- 1Sn n I 3n+1 structure with FA molecules aligned along the (001) and (111) directions of the 3D quasi-cubic perovskite lattice. We note that the calculated band gaps are generally lower than the experimental values, which is caused by the known problem of DFT-based calculations underestimating band gaps of materials. Nevertheless the tendency of the increase of calculated gaps with decreasing n shows reasonable agreement with experimental data. Table S3 Explicit band gap values (in ev) of 2D (PEA) 2 (FA) n-1 Sn n I 3n+1 perovskite films at different n values derived from the peaks of PL spectra (Exp.) and first principles DFT-based calculations (Cal.). n Exp / Cal. (001) / (111) / * The (001) and (111) represent the calculated values with adoption of the (PEA) 2 (FA) n- 1Sn n I 3n+1 structure with FA molecules aligned along the (001) and (111) directions of the 3D quasi-cubic perovskite lattice. S8

9 Figure S8 Capacitance voltage curve of 20% PEA-doped film with 10 % SnF 2 (a), FASnI 3 film with 10% SnF2 (b), and 20% PEA-doped film without SnF 2 (c). Figure S9 Electron and hole mobility of 20% PEA films with 10% SnF 2 (a, b) and FASnI 3 films with 10% SnF 2 (c, d) measured by space charge limited current (SCLC) measurements. The curve is divided into Ohmic, trap field limit (TFL), and Child regions. The measured electron mobilities (µ e ) are cm 2 V 1 s 1 (the 20% PEA films) and cm 2 V 1 s 1 (FASnI 3 ), and the hole mobilities (µ h ) are 0.14 cm 2 V 1 s 1 (the 20% PEA films) and cm 2 V 1 s 1 (FASnI 3 ). The mobilities of the 3D FASnI 3 are several-orders lower than the values of the 2D films. This is reasonably expected since the 3D Sn-based perovskites are known to suffer from S9

10 substantial deep-level defects (e.g., Sn 4+ derived ones) acting as carrier trapping centers due to their intrinsic materials instability. Such comparison further supports our finding that the fabricated 2D (PEA) 2 (FA) n-1 Sn n I 3n+1 structure enhances materials stability and thus suppress defect formation. Figure S10 Calculated band structures of (a) 2D (PEA) 2 (FA) n-1 Sn n I 3n+1 with n = 6 and (b) 3D bulk (FA)SnI 3 (n = ). The valence band maximum is set to energy zero (in dash line). The effective mass values of band-edge states are indicated; the hole mass is depicted with mm h, and the heavy and light electron masses with mm hee and mm llll. S10

11 Figure S11 Time-resolved fluorescence kinetics at 840 nm for 20% PEA films on ITO (orange) and on a hole (NiO x, green) or electron (PCBM, purple) quenching layer under exciton at 504 nm. The solid lines are fit to a double exponential decay. The carrier lifetime of the films on ITO is 6.3 ns. Lifetimes on NiO x and PCBM are 4.97 ns and 3.12 ns, respectively. Reduced lifetime in the presence of quenching layers indicates efficient carrier transport. Figure S12 Cross-section SEM image of the device based on the 20% PEA-doped film without the Al electrode. (a) uncolored and (b) colored. The thickness of perovskite, NiO x, and PCBM are around 200 nm, 50nm, and 100 nm, respectively. S11

12 Figure S13 SEM images of 20% PEA films with various SnF 2 molar concentration of 0% (a), 5% (b), 10% (c), 15% (d), and 20% (e). Relationship between PCE of the 20% PEA tin PSCs and the molar concentration of SnF 2 (f). Figure S14 J-V curves under AM1.5G illumination of the device based on 20% PEA-doped film in reverse and forward scan directions. Only slight hysteresis is observed for the devices. S12

13 Figure S15 Light-intensity dependence of the device based on 20% PEA-doped film. (a) J sc versus light intensity and (b) V oc versus light intensity. The light intensity ranges from 1 to 100 mw cm -2. Figure S16 Cross-section SEM image of the device based on the FASnI 3 film (10% SnF 2 ) without the Al electrode. S13

14 Figure S17 Representative J-V curves under AM1.5G illumination of the device based on 3D FASnI 3 film (10% SnF2). Figure S18 J-V curves under AM1.5G illumination of the larger area (0.13 cm 2 ) device based on 20% PEA-doped film in reverse and forward scan directions. S14

15 Figure S19 NMR measurement of the films with and without the addition of PEAI. As the addition of PEAI, corresponding peaks (between 7 to 7.5 ppm) are observed, and the calculated ratio of PEA to FA molecules is close to the value that was employed in films fabrication. S15