What ails the photovoltaic performance in single-layered unpoled BFO? - The role of oxygen annealing in improving the photovoltaic efficiency

Abstract

We report enhanced photovoltaic efficiency in optimized monolayer BiFeO3 (BFO) thin-film based solar cells prepared by inexpensive spin-coating technique. The short circuit current density and open-circuit voltage of the single-layer BFO cell were measured to be 1.91 mA/cm(2) and 0.92 V, respectively, which is much higher than the reported values so far in monolayer BFO based photovoltaic systems. The influence of oxygen annealing on photovoltaic response is investigated, and a further improvement in photo-current density is reported. The phase purity and average crystallite size are examined by X-ray diffraction studies, which confirmed the single-phase formation of rhombohedrally distorted perovskite BFO structured layer deposition. The average crystallite size, as determined by the Williamson-Hall method, is found to be in the range of 14 - 17 nm. The micrographs reveal a uniform and crack-free deposition of both the thin-films. The cross-sectional images reveal the smooth film deposition of thicknesses in the range of 360 - 370 nm. The average bandgap values of the air and oxygen annealed films are found to be 2.57 +/- 0.02 eV and 2.64 +/- 0.02 eV, respectively. The oxygen annealing of these films show a significant reduction in Urbach energy tail values from 0.37 +/- 0.02 eV to 0.28 +/- 0.03 eV, which suggests a lower density of defect states in the oxygenated films. The X-ray photoelectron analysis showed a noticeable reduction in Fe2+ ions, hence supporting the Urbach energy studies. We also demonstrate a metal -semiconductor-metal model to explain the enhancement in short-circuit current density, suggesting that the main driving force for the photo-generated charge carriers is the Schottky field at the interfaces rather than the inherent polarization field in the as-deposited (unpolled) BFO films.