Band-Gap Tuning in Perovskite-type Ferroelectric ZnSnO$_{3}$ by Doping and Core-Shell Approach for Solar Cell Applications

Ferroelectric (FE) perovskite materials are an emerging class of potential absorbers in next generation solar cells due to their spontaneous polarization which facilitates electron-hole separation and drive charge carriers at opposite ends. With a large remnant polarization of $\approx $ 59 $\mu $C/cm$^{2}$, perovskite-type LiNbO$_{3}$ (LN)-ZnSnO$_{3}$, containing earth abundant elements is of much interest as a high performance solar absorber. However, the wide band-gap in ZnSnO$_{3}$ ($\sim$ 3.7 eV) is unsuitable to absorb the broad solar range, which can be overcome by band-gap engineering. Here, we discuss band-gap tuning through substitutional doping (Sb, Cu, Ca, Ba) in LN-type ZnSnO$_{3}$ nanorods, synthesized by a facile solvothermal process. A band-gap as low as 2.5 eV was obtained in 5 at.{\%} Ca doped ZnSnO$_{3}$ nanorods showing superior FE properties. The current-voltage ($I-V)$ measurements under light revealed multiple orders of enhancement as compared to the dark. The band-gap in ZnSnO$_{3}$ is also found to be a strong function of the lattice constant which is tuned by introducing a slight strain through lattice mismatching using a core shell approach. A detailed structural, optical, and FE analyses are provided to predict the future of this technologically important material in next generation FE photovoltaics.