Maximizing Short Circuit Current Density and Open Circuit Voltage in Oxygen Vacancy-Controlled Bi1-xCaxFe1-yTiyO3-delta Thin-Film Solar Cells

Abstract

Designing solid-state perovskite oxide solar cells with large short circuit current (J(SC)) and open circuit voltage (V-OC) has been a challenging problem. Epitaxial BiFeO3 (BFO) films are known to exhibit large V-OC (>50 V). However, they exhibit low J(SC )(<<mu A/cm(2)) under 1 Sun illumination. In this work, taking polycrystalline BiFeO3 thin films, we demonstrate that oxygen vacancies (V-O) present within the lattice and at grain boundary (GB) can explicitly be controlled to achieve hie J(SC) and V-OC simultaneously. While aliovalent substitution (Ca2+ at Bi3+ site) is used to control the lattice V-O, Ca and Ti cosubstitution is used to bring out only GB-V-O. Fluorine-doped tin oxide (FTO)/Bi1-xCaxFe1-yTiyO3-delta/Au devices are tested for photovoltaic characteristics. Introducing V-O increases the photocurrent by four orders (J(SC) similar to 3 mA/cm(2)). On the contrary, V-OC is found to be <0.5 V, as against 0.5-3 V observed for the pristine BiFeO3. Ca and Ti cosubstitution facilitate the formation of smaller crystallites, which in turn increase the GB area and thereby the GB-V-O. 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 J(SC). The cosubstitution, following a charge compensation mechanism, decreases the lattice V-O concentration significantly to retain the ferroelectric nature with enhanced polarization. It helps to achieve V-OC (3-8 V) much larger than that of BiFeO3 (0.5-3 V). It is noteworthy that as Ca substitution maintains moderate crystallite size, the lattice V-O concentration dominates GB-V-O concentration. Notwithstanding, both lattice and GB-V-O contribute to the increase in J(SC); the former weakens ferroelectricity, and as a consequence, undesirably, V-OC is lowered well below 0.5 V. Using optimum J(SC) and V-OC, we demonstrate that the efficiency -0.22% can be achieved in solid-state BFO solar cells under AM 1.5 one Sun illumination.