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

Abstract

Barium stannate (BaSnO 3 ) 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 BaSnO 3 photoanodes, with a smaller optical gap of ∼2.2 eV. By annealing the photoanodes in 5% hydrogen sulfide (H 2 S) 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 V RHE . 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 H 2 S annealing of BaSnO 3 generates a set of filled defect states associated with oxygen vacancies (V O ••), Sn 2+ centers (Sn Sn ″ ), and sulfur substitutions (S O × ), 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 cm 2 V −1 s −1 .

Carrier Localization on the Nanometer‐Scale limits Transport in Metal Oxide Photoabsorbers

“…This reduction of carrier localization strength rationalizes our previously reported increase of TRMC-derived mobility. [67] Future work will clarify if an increasing XRD domain size is responsible for this reduced localization as in CuFeO 2 , or if the annealing in a reducing atmosphere introduces extended defect bands of oxygen vacancies as suggested in references, [67][68][69][70] which would allow relatively fast transport. In the latter case, the localization length may correspond to the size of the defect clusters.…”

Cation‐Radius‐Controlled Sn−O Bond Length Boosting CO2 Electroreduction over Sn‐Based Perovskite Oxides

“…Because the radii of Ba 2 + and Sr 2 + conformed to the tolerance-factor condition for the generation of cubic Sn-based perovskite oxides, the Ba 1À x Sr x SnO 3 series could exhibit the same crystal structure sharing the space group of Pm-3m. [22,23,30] Whereas the crystal structures of other Snbased perovskite oxides composed of smaller A-site cations, such as CaSnO 3 with orthorhombic phase, were quite different from that of the Ba 1À x Sr x SnO 3 series. [31] Hence, in our work, to investigate the effect of SnÀ O bond length as a single factor on CO 2 RR property, the Ba 1À x Sr x SnO 3 series were selected as the proof-of concept catalysts, instead of other Sn-based perovskite oxides.…”

Cation‐Radius‐Controlled Sn−O Bond Length Boosting CO2 Electroreduction over Sn‐Based Perovskite Oxides

“…Because the radii of Ba 2 + and Sr 2 + conformed to the tolerance-factor condition for the generation of cubic Sn-based perovskite oxides, the Ba 1À x Sr x SnO 3 series could exhibit the same crystal structure sharing the space group of Pm-3m. [22,23,30] Whereas the crystal structures of other Snbased perovskite oxides composed of smaller A-site cations, such as CaSnO 3 with orthorhombic phase, were quite different from that of the Ba 1À x Sr x SnO 3 series. [31] Hence, in our work, to investigate the effect of SnÀ O bond length as a single factor on CO 2 RR property, the Ba 1À x Sr x SnO 3 series were selected as the proof-of concept catalysts, instead of other Sn-based perovskite oxides.…”

“…As a note, the reason why we chose Ba 1−x Sr x SnO 3 series as proof‐of‐concept catalysts was as follows. Because the radii of Ba 2+ and Sr 2+ conformed to the tolerance‐factor condition for the generation of cubic Sn‐based perovskite oxides, the Ba 1−x Sr x SnO 3 series could exhibit the same crystal structure sharing the space group of Pm‐ 3 m [22,23,30] . Whereas the crystal structures of other Sn‐based perovskite oxides composed of smaller A‐site cations, such as CaSnO 3 with orthorhombic phase, were quite different from that of the Ba 1−x Sr x SnO 3 series [31] .…”