Enhancing the Photon Absorption and Charge Carrier Dynamics of BaSnO₃ Photoanodes via Intrinsic and Extrinsic Defects

Barium stannate (BaSnO3) crystallizes in the cubic perovskite-type structure and typically exhibits a wide band gap of >3.0 eV; thus, it is often considered unsuitable as a photo-absorber material for solar energy conversion. We present a spray-pyrolysis method for the fabrication of BaSnO3 photoanodes, with a smaller optical gap of ∼2.2 eV. By annealing the photoanodes in 5% hydrogen sulfide (H2S) gas, the optical gap is further reduced to ∼1.7 eV, with an ∼20-fold increase in photocurrent density and an improved onset potential of ∼0 VRHE. To understand the reasons behind this performance enhancement, we utilize a combination of spectroscopy techniques, including photoluminescence, wavelength-dependent time-resolved surface photovoltage analysis, and photoconductivity measurements. We find that H2S annealing of BaSnO3 generates a set of filled defect states associated with oxygen vacancies (VO••), Sn2+ centers (SnSn″), and sulfur substitutions (SO×), which are situated ∼1.4 to 1.9 eV below the conduction band minimum and exhibit a degree of orbital overlap with the valence band maximum. Increasing the density of these defects shifts the optical onset of photocurrent generation to ∼1.7 eV and enables holes to transport via a hopping mechanism. Resultantly, the charge carrier mobility is shown to increase by 20-fold, reaching ∼0.04 cm2 V–1 s–1.