Compact TiO2 film thickness optimization and its effect on CsPbBr3-based photoanode performance for oxygen evolution reaction.

Abstract

Compact TiO2 is an effective hole-blocking and electron transport layer (ETL) for perovskite photoanodes, but its performance depends critically on synthesis conditions and layer thickness. Here we report the thermal oxidation of electron-beam-evaporated Ti to produce pinhole-free, thickness-tunable c-TiO2 films (12–83 nm) to evaluate them as ETLs in back- illuminated n–i–p CsPbBr3 photoanodes for the oxygen evolution reaction. Cyclic voltammetry and electrochemical impedance spectroscopy confirmed that films thicker than 51 nm provided complete substrate coverage and yielded pinhole-free, compact layers with effective hole- blocking capabilities, which potentially reduce recombination and improve electron collection. Photoanodes were evaluated via linear sweep voltammetry to determine the influence of TiO2 thickness during water oxidation. An optimal 51 nm c-TiO2 layer yielded a photocurrent density of 4.2 mA/cm2 at 1.23 V vs. RHE and an ABPE of 0.93% at 0.8 V vs. RHE, maintaining >1.5 mA/cm2 over four hours in aqueous, catalyst-free OER conditions. Thicker ETLs (65–83 nm) reduce photocurrent due to increased series resistance. These results highlight the importance of quality and precise control over TiO2 thickness. Thermal oxidation of e-beam Ti offers a reproducible route to compact, transparent ETLs that balance coverage, transmittance, and resistance for stable aqueous photoelectrochemical operation.