Origin of High Nonradiative Recombination and Relevant Optoelectronic Properties of Ba₂Bi_1+xNb_1−xO₆: Candidate for Photo(electro)catalysis and Photovoltaic Applications?

Abstract Recently, perovskite oxides have shown a lot of promise as active materials for photovoltaic (PV) and photoelectrochemical (PEC) devices. In this article, a series of oxides Ba₂Bi_1+xNb_1−xO₆ (0 ≤ x ≤ 0.8) is evaluated for PV/PEC applications via a combined theoretical and experimental study. Ab‐initio calculation reveals that the stoichiometric compound (Ba₂BiNbO₆) spontaneously induces the formation of Bi_Nb antisites, leading to off‐stoichiometric structure. The electronic structure of off‐stoichiometric compounds confirms mixed valence character of Bi, introducing intermediate bands in the band structure. UV–vis diffuse reflectance spectroscopy (DRS) predicts the bandgap in the visible range (1.5–1.9 eV). Simulation reveals excellent absorption properties, indicating the possibility of high photoconversion efficiency. Lack of sharp peaks in the DRS, however, raises concern about the level of nonradiative recombination. Point defect simulation suggests the possibility of few deep level donors Bi_Ba, Nb_Bi, V_O, in high concentrations (≈10¹⁸–10¹⁹ cm⁻³), which can act as nonradiative recombination centers. Ultraviolet photoelectron spectroscopy confirms majority carriers to be n‐type. Self‐consistent simulation of the Fermi level (E_F) suggests an overall cancellation of donors and acceptors, leading to E_F pinning at the mid‐gap region explaining the observed high bulk resistivity. This indicates the requirement of extrinsic doping to achieve desired properties for applications.