Room Temperature Crystallized Phase‐Pure α‐FAPbI3 Perovskite with In‐Situ Grain‐Boundary Passivation

Abstract Energy loss in perovskite grain boundaries (GBs) is a primary limitation toward high‐efficiency perovskite solar cells (PSCs). Two critical strategies to address this issue are high‐quality crystallization and passivation of GBs. However, the established methods are generally carried out discretely due to the complicated mechanisms of grain growth and defect formation. In this study, a combined method is proposed by introducing 3,4,5‐Trifluoroaniline iodide (TFAI) into the perovskite precursor. The TFAI triggers the union of nano‐sized colloids into microclusters and facilitates the complete phase transition of α‐FAPbI3 at room temperature. The controlled chemical reactivity and strong steric hindrance effect enable the fixed location of TFAI and suppress defects at GBs. This combination of well‐crystallized perovskite grains and effectively passivated GBs leads to an improvement in the open circuit voltage (Voc ) of PSCs from 1.08 V to 1.17 V, which is one of the highest recorded Voc without interface modification. The TFAI‐incorporated device achieved a champion PCE of 24.81%. The device maintained a steady power output near its maximum power output point, showing almost no decay over 280 h testing without pre‐processing.


Synthesis:
3,4,5-trifluoroaniline iodide (TFAI): TFAI is prepared by mixing 4321 mg 3,4,5-Trifluoroaniline and 8.14 ml hydroiodic acid in 40 ml ethanol, followed by ice bathing and stirring for 3 hours.The brown precipitates is obtained after the solution is dried and filtrated.To purify the product, diethyl ether is used to wash the precipitates and remove the excess ligands three times.After that, The precipitates is fully dissolved into ethanol to recrystallize.The wash and redissolve flow is repeated three times.Finally, the collected white TFAI powder is dried in the vacuum oven at 60℃ overnight.

Device fabrication:
The ITO glasses are sequentially cleaned with detergent, deionized (DI) water, acetone and IPA in the ultrasonic tank for 15 min and then dried by compressed air.
After baking at 150℃ for 10 min, Oxygen plasma treatment is applied to remove the surface contamination.The SnO2 layer from the diluted 2 % dispersion colloid is spin-coated on the substrate at 3000 rpm for 30 s followed by annealing at 80℃ for 50 min in a vacuum oven.The prepared substrate is then transferred into the N2-filled glove box after UV-zone treatment for further perovskite layer deposition.The perovskite solution prepare from FAI (1.4 M), PbI2 (1.4 M), MACl (0.5 M), MDACl2 (0.05 M) and TFAI (0, 0.5 mol%, 1 mol%, 2 mol% and 5 mol%) in anhydrous DMF/DMSO (8:1 (v:v)) is deposited on the SnO2 substrate at 1000 rpm for 10 s and 5000 rpm for 10 s. 1 ml anti-solvent chlorobenzene is drop on the spinning perovskite film with or without TFAI at the last 20 s in the second step.The as-deposited perovskite film is sequentially annealed at 150 ℃ for 15 min for complete crystallization.Spiro-OMeTAD hole transport solution (72.3 mg ml -1 ) in 1.5 ml chlorobenzene is prepared by mixing 25 μl Li-TFSI (520 mg ml -1 in ACN), 27.5 μl Co(III) TFSI (300 mg ml -1 in ACN) and 37.5 μl 4-tBP additives.The doped Spiro-OMeTAD solution is spin-coated on the perovskite layer at 3000 rpm for 30 s.
For the SCLC testing, PCBM solution (15 mg ml -1 in chlorobenzene) is deposited on the film at 1000 rpm for 60 s and 5000 rpm 2s to form a hole blocking layer.Finally, 85 nm gold electrode is thermally evaporated on the top of the device at 10 -5 Pa to complete the fabrication process.
Photoelectron Spectroscopy (UPS) spectra are measured by Thermo Scientific Escalab Xi+.-5 V bias is applied on the film during testing.The energy step is fixed at 0.05 eV.X-ray Photoelectron Spectroscopy (XPS) measurements are carried out using Thermo Fisher Scientific K-Alpha+.The irradiation spot (200 × 200 μm) is focused by an Al K Alpha source gun.The analyzer mode is selected as CAE: Pass Energy 80.0 eV with an energy step of 0.1 eV.Conductive Atomic Force Microscopy (C-AFM) images of perovskite films are conducted from Bruker instrument.Surface topographies, current mapping images and 1D patterns along scan direction are analyzed by NanoScopeAnalysis software.Absorption spectra of perovskite films are measured using an ultraviolet spectrophotometer (SHIMADZU UV-1750) from 550 nm to 850 nm.Steady-state PL spectra are carried out by an F20-UV thin-film analyzer (FILMETRICS) with 405 nm laser excitation.PL mapping of perovskite films on glass and FTO/glass substrates are examined by MicroRaman spectroscopy (WITEC Alpha300R) with a 532 nm excitation source.Voltage-Capacitance (C-V)plots and Electrochemical Impedance Spectroscopy (EIS) of the devices are measured by an electrochemical workstation Zennium-pro (Zahner).For the C-V testing, the AC frequency is set at 10 kHz with an amplitude bias of 10 mV under dark.For the EIS measurement, the applied DC bias is fixed at Vbi with an AC amplitude of 50 mV

Figure S2 .
Figure S2.Statistical distributions of perovskite grain size with and w/o TFAI incorporation.

Figure S7 .
Figure S7.ToF-SIMS of 3D render reconstruction of depth profile from (a) F -and (b) SnO2 signal.(c) 2D mapping of F -in perovskite layer.

Figure S8 .
Figure S8.The SEM images of (a) pristine (b) TFAI 1% and (c) TFAI 5% perovskite films captured with an accelerated electron beam at 20 kV.The red curves represent the perovskite GBs.Spots 1 to 3 represent the positions from GB to grain.The spots 4 to 6 represent the positions from grain to GB. EDX results of (d) pristine (e) TFAI 1% and (f) TFAI 5% films measured at corresponding positions 1 to 6. Spot model is applied to obtain the elemental information at points of SEM images.

Figure S14 .
Figure S14. the incidence dependent penetration depth of X ray applied in GIWAXS technique.

Figure S15 .
Figure S15.cross-section SEM images of pristine and TFAI-incorporated perovskite films.The red dash circles show the pin holes lying at the interface between perovskite and SnO2/FTO substrates.

Figure S16 .
Figure S16.The C-AFM current mapping of pristine and TFAI-incorporated perovskite films at 1.0 V bias.The spots from X1 to X4 represent the GBs in line.

Figure S21 .
Figure S21.The PL mapping of pristine, TFAI 1% and TFAI 5% films on (a) ITO and (c) glass substrates.The corresponding PL spectra are plotted on (b) ITO and (d) glass substrates.

Figure S23 .
Figure S23.(a) Mott-Schottky plots of the pristine and TFAI-incorporated perovskite PSCs.(b)The J-V curves of pristine and TFAI-incorporated PSCs under forward and reverse scan directions.

Figure S25 .
Figure S25.Prolonged SPO test of pristine and TFAI-incorporated PSCs under constant applied bias near the maximum power output points.

Table S1 .
Performance parameters of pristine and TFAI-incorporated PSCs