Enhancing the Photon Absorption and Charge Carrier Dynamics of BaSnO<sub>3</sub> Photoanodes via Intrinsic and Extrinsic Defects
Soniya Gahlawat, Ibbi Y. Ahmet, Patrick Schnell, Igal Levine, Siyuan Zhang, Pravin P. Ingole, Fatwa F. Abdi
Abstract
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.