Correlating lattice-distortion-induced ferroelectricity with solar-cell performance in all-inorganic mixed-halide perovskites

Among all-inorganic metal-halide perovskites, mixed halides yield superior photovoltaic parameters. In this direction, we considered ${\mathrm{Cs}\mathrm{Pb}\mathrm{I}}_{3}({\mathrm{Br}}_{3})$ and observed that the ferroelectricity in the compounds appeared due to an overall distortion in the $[\mathrm{Pb}{X}_{6}{]}^{4\ensuremath{-}}$ octahedra and the consequent off-centered cesium ions. Apart from employing piezoresponse force microscopy to investigate the ferroelectric properties, we have shown that Kelvin probe force microscopy (KPFM), after applying suitable biases to pole the materials, can also be considered for probing the ferroelectricity of thin films. The band diagrams of the p-i-n heterojunctions were drawn by combining scanning-tunneling spectroscopy and KPFM results, and the devices were thereby cast and characterized. ${\mathrm{Cs}\mathrm{Pb}\mathrm{I}}_{2}\mathrm{Br}$ and ${\mathrm{CsPbIBr}}_{2}$ have provided evidence of a piezoelectric modulus (${d}_{33}$), and the devices based on the mixed-halide perovskites yielded better solar-cell parameters compared to those with the end members, namely, ${\mathrm{Cs}\mathrm{Pb}\mathrm{I}}_{3}$ and ${\mathrm{Cs}\mathrm{Pb}\mathrm{Br}}_{3}$. The ferroelectricity in the compounds and the solar-cell parameters based on them were thereby found to be correlated.