Fig. 1: ETL preparation and characterization.

a Schematic illustration showing the synthesis process of FI–SnO₂ ETL. b, c C-AFM mapping of the FI–SnO₂ (left) and pristine SnO₂ (right) ETLs on FTO substrates. d FTIR spectra of FAI, SnO₂, and FI–SnO₂. e XPS spectra for Sn 3d of FI–SnO₂ and pristine SnO₂ ETLs.