Fig. 4: Effect of TFSI additives doping and surface treatment of perovskite films.
a SEM images tracking the morphological evolution of a KTFSI-doped perovskite film during a two-step process within 10 min. Scale bar: 1 µm. b Schematic illustration of the proposed mechanism for KTFSI mediated perovskite grain growth. (i) FAI solution spin-coated on the porous, KTFSI-incorporated PbI2 layer. (ii) TFSI⁻ anions inhibited further reaction of FAI with unreacted PbI2 and perovskite intermediate phase. (iii) Larger perovskite grains upon heating from further reaction of FAI with PbI2. Green circles: FAI; Violet polygons: TFSI- ions; Yellow polygons: PbI2 and unreacted perovskite intermediate phase. c Three-dimensional excitation-emission matrix (EEM) fluorescence spectra of a KTFSI-doped perovskite film during different annealing times, mapping the PL intensity evolution in the 750-900 nm emission range under excitation from 400 to 500 nm. Dark current–voltage (I–V) characteristics of the hole-only devices with FTO/PTAA/PVSK/spiro/Au configuration for d control (PVSK with potassium iodide, KI) and e PVSK with KTFSI (molar equivalent of KI). The trap-filled limit voltages (VTFL) derived from the I-V curves are 1.008 V (control) and 0.752 V (KTFSI), respectively. Combined UPS and LEIPS spectra of the f pristine perovskite and g with KTFSI. WF and EVBM: 4.16 and −1.46 eV for pristine PVSK; 4.57 and −0.88 eV for KTFSI added PVSK, respectively; ECBM are obtained by adding optical bandgap of Eg (1.53 eV) to EVBM (0.07 and 0.65 eV for pristine and KTFSI) and calculated from LEIPS (in brackets, 0 eV and 1 eV, respectively). In situ grazing incidence X-ray diffraction (GIXRD, ω = 1°) monitoring the evolution of diffraction intensity as a function of angle every minute during annealing for FAPbI3 perovskite films post-treated with h PEATFSI and i OATFSI for 20 min at 70 °C in air.
