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

Barium stannate (BaSnO3) crystallizes in the cubic perovskite-typestructure and typically exhibits a wide band gap of >3.0 eV; thus, it is oftenconsidered unsuitable as a photo-absorber material for solar energy conversion. Wepresent a spray-pyrolysis method for the fabrication of BaSnO3photoanodes, witha smaller optical gap of similar to 2.2 eV. By annealing the photoanodes in 5% hydrogensulfide (H2S) gas, the optical gap is further reduced to similar to 1.7 eV, with an similar to 20-foldincrease in photocurrent density and an improved onset potential of similar to 0VRHE.Tounderstand the reasons behind this performance enhancement, we utilize acombination of spectroscopy techniques, including photoluminescence, wave-length-dependent time-resolved surface photovoltage analysis, and photo-conductivity measurements. Wefind that H2S annealing of BaSnO3generates aset offilled defect states associated with oxygen vacancies (VO center dot center dot), Sn2+centers (SnSn ''), and sulfur substitutions (SOx), which aresituated similar to 1.4 to 1.9 eV below the conduction band minimum and exhibit a degree of orbital overlap with the valence bandmaximum. Increasing the density of these defects shifts the optical onset of photocurrent generation to similar to 1.7 eV and enables holes totransport via a hopping mechanism. Resultantly, the charge carrier mobility is shown to increase by 20-fold, reaching similar to 0.04 cm2V-1s-1.