Maximizing Short Circuit Current Density and Open Circuit Voltage in Oxygen Vacancy-Controlled BiCaFeTiO Thin-Film Solar Cells.

Designing solid-state perovskite oxide solar cells with large short circuit current () and open circuit voltage () has been a challenging problem. Epitaxial BiFeO (BFO) films are known to exhibit large (>50 V). However, they exhibit low (≪μA/cm) under 1 Sun illumination. In this work, taking polycrystalline BiFeO thin films, we demonstrate that oxygen vacancies (V) present within the lattice and at grain boundary (GB) can explicitly be controlled to achieve high and simultaneously. While aliovalent substitution (Ca at Bi site) is used to control the lattice V, Ca and Ti cosubstitution is used to bring out only GB-V. Fluorine-doped tin oxide (FTO)/BiCaFeTiO/Au devices are tested for photovoltaic characteristics. Introducing V increases the photocurrent by four orders ( ∼ 3 mA/cm). On the contrary, is found to be <0.5 V, as against 0.5-3 V observed for the pristine BiFeO. Ca and Ti cosubstitution facilitate the formation of smaller crystallites, which in turn increase the GB area and thereby the GB-V. This creates defect bands occupying the bulk band gap, as inferred from the diffused reflection spectra and band structure calculations, leading to a three-order increase in . The cosubstitution, following a charge compensation mechanism, decreases the lattice V concentration significantly to retain the ferroelectric nature with enhanced polarization. It helps to achieve (3-8 V) much larger than that of BiFeO (0.5-3 V). It is noteworthy that as Ca substitution maintains moderate crystallite size, the lattice V concentration dominates GB-V concentration. Notwithstanding, both lattice and GB-V contribute to the increase in ; the former weakens ferroelectricity, and as a consequence, undesirably, is lowered well below 0.5 V. Using optimum and , we demonstrate that the efficiency ∼0.22% can be achieved in solid-state BFO solar cells under AM 1.5 one Sun illumination.