iso‐BAI Guided Surface Recrystallization for Over 14% Tin Halide Perovskite Solar Cells

Abstract Tin‐based perovskite solar cells (PSCs) are promising environmentally friendly alternatives to their lead‐based counterparts, yet they currently suffer from much lower device performance. Due to variations in the chemical properties of lead (II) and tin (II) ions, similar treatments may yield distinct effects resulting from differences in underlying mechanisms. In this work, a surface treatment on tin‐based perovskite is conducted with a commonly employed ligand, iso‐butylammonium iodide (iso‐BAI). Unlike the passivation effects previously observed in lead‐based perovskites, such treatment leads to the recrystallization of the surface, driven by the higher solubility of tin‐based perovskite in common solvents. By carefully designing the solvent composition, the perovskite surface is effectively modified while preserving the integrity of the bulk. The treatment led to enhanced surface crystallinity, reduced surface strain and defects, and improved charge transport. Consequently, the best‐performing power conversion efficiency of FASnI3 PSCs increases from 11.8% to 14.2%. This work not only distinguishes the mechanism of surface treatments in tin‐based perovskites from that of lead‐based counterparts, but also underscores the critical role in designing tailor‐made strategies for fabricating efficient tin‐based PSCs.

All the other chemical materials were purchased from Sigma-Aldrich and used as received unless stated otherwise.

Device fabrication
ITO substrates were sequentially rinsed by sonication in detergent, deionized (DI) water, acetone, and isopropanol for 30 min, respectively, and then dried under nitrogen gas before use.Cleaned ITO substrates were treated with ultraviolet-ozone for 15 min, followed by the deposition of a hole transporting layer of PEDOT:PSS by spin-coating the PEDOT:PSS solution at 4500 rpm for 30 s, and then annealed at 130 ºC for 30 min.
The SnI2 precursor was prepared by dissolving I2 (1.0 M) in a mixture of DMF/DMSO with the volume ratio of 4:1, and then excess Sn powder was added, followed by a 30min of vigorous shaking.The FASnI3 perovskite precursor was prepared by dissolving FAI (0.95 M), MAI (0.05 M), and SnF2 (0.1 M) into the SnI2 precursor, and then excess 6% 1.5 M PEAI was added into the precursor.The perovskite precursor was filtered to isolate the tin powder before use.The iso-BAI was prepared by dissolving 1mg/mL iso-BAI in 5:95 (volume ratio) MB:CB.The ICBA solution was prepared by dissolving 18mg/mL ICBA in CB.The BCP solution was prepared by dissolving 0.5mg/mL BCP in IPA.The perovskite film was deposited by spin-coating the perovskite precursor on the ITO/PEDOT:PSS substrate at 4500 rpm for 50 s, and 100 uL chlorobenzene containing 0.5 mg/ml PCBM was dripped onto the substrate at the 9th second from the start of spin-coating.The as-cast film was then annealed at 80 ºC for 30 min.For the iso-BAI treatment, 50uL was dripped onto the annealed perovskite film at 4000rpm for 20s, followed by an annealing process at 80 ºC for 10min.
ICBA and BCP solutions were subsequently spin-coated at 2500 rpm and 4000 rpm for 20 s, respectively.Finally, a 100 nm Ag electrode was deposited by thermal evaporation.

Characterizations
The crystalline structures for the perovskite films were measured by XRD on a Rigaku Smart Lab (λ = 1.54 Å).GIWAXS measurements were carried out using a Xeuss 2.0 SAXS/WAXS laboratory beamline with a Cu X-ray source (8.05 keV, 1.54 Å) and a Pilatus 3R 300K detector.In situ GIWAXS experiments for the perovskite spin-coating process and light stability tests were conducted at TLS 23A small-and wide-angle X-ray scattering (SWAXS) beamline at the National Synchrotron Radiation Research Center (NSRRC), Hsinchu, Taiwan.The J-V curves were measured by a Keysight B2901A source meter unit under an AM 1.5G solar simulator (SS-F5; Enli Technology, Taiwan), and the light intensity was calibrated using a standard silicon reference cell.X-ray photoelectron spectroscopy (XPS) characterizations were performed at BL09A2 U5 beamline at the National Synchrotron Radiation Research Centre, Taiwan.The incident photon energy is 750 eV and the data were calibrated using an Au sample.UV-Vis absorption spectra were taken on a Hitachi U-3501 ultraviolet/visible/near-infrared spectrophotometer.The perovskite surface morphology was characterized by a high-resolution field emission scanning electron microscopy (HR-FESEM) (FEI, Quanta 400).Photoluminescence measurements of perovskite films on glass were conducted by using an Edinburgh FLSP920 spectrophotometer installed with an excitation source of 485 nm picosecond pulsed diode laser with an average power of 0.15 mW.Both the 1 H-NMR and 119 Sn-NMR spectra are recorded with the Brucker AVANCE III 400 NMR, and chemical shift values(δ) are expressed in parts per million using residual solvents protons as internal standard.AFM and KPFM measurements were conducted with the Nikon Ti Inverted microscope bruker and the JPK atom force microscope with a single photon spectrometer.The probing tip was the ElectriMulti75-G probe from Budget Sensors.
Electrochemical impedance spectroscopy (EIS) measurements were conducted using Zahner Electrochemical workstation.The steady-state PL measurement is conducted with the home-made setup by Tsang's group [1].TRPL measurements are conducted using a photoluminescence spectrometer (iHR320, HORIBA).A 463 nm laser diode (DeltaDiode-470L, Horiba) as a pump source was used for photoexcitation with the frequency of 100 MHz, .The TRPL spectrum was obtained by a high-sensitivity photon counting detector (TRPL-PPD900-MICOS, horiba).PLQY measurements are conducted by mounting the perovskite films in an integrating sphere with Enlitech LQ-100X-PL.

Figure S1 .
Figure S1.(a) Perovskite film without any surface treatment (b) Perovskite film treated

Figure S2 .
Figure S2.Peak area of the (100) peak of the perovskite films treated with iso-BAI

Figure S3 .
Figure S3.(a) AFM, and (b) SEM images of the iso-BAI treated film before annealing.

Figure S4 .
Figure S4.XPS spectra of the perovskite film after etching with and without the iso-

Figure S5 .
Figure S5.GIWAXS diffraction patterns of the control film (a-d), and the iso-BAI treated film (e-h), at incident angles of 0.05 o , 0.10 o , 0.20 o , 0.40 o .

Figure S6 .
Figure S6.GIWAXS intensity profiles of the control film and iso-BAI treated film

Figure S7 .
Figure S7.Williamson-Hall analysis of the microstrain of (a) the control film, and (b)

Figure S9 .
Figure S9.(a) Voc, Jsc, FF, and PCE of the PSC devices fabricated with the control,

Figure S10 .
Figure S10.PCE comparison of the PSC devices fabricated with the control film, the

Figure S11 .
Figure S11.PCE comparison of the PSC devices fabricated with the control film, the

Figure S12 .
Figure S12.Photoluminescence quantum yield comparison of the control film and the

Figure S13 .
Figure S13.UV-vis spectra of the control film and the iso-BAI treated film, and the

Figure S14 .
Figure S14.Nyquist plots of the PSC devices fabricated with (a) the control, and (b)

Figure S15 .
Figure S15.Contact angle measurements with water of (a) the control films, and (b)