Optical Properties of Ferroelectric Halide Perovskites: Effects of Electric Field and Epitaxial Strain

The remarkable photovoltaic effect of inorganic halide perovskites has aroused wide research interest in combating global energy crises and serious environmental problems. Here, we performed symmetry analysis and first-principles calculations to study the structural, electronic, and optical properties of halide perovskite CsGeX3 (X = I, Br, Cl). The polar structures of CsGeI3, CsGeBr3, and CsGeCl3 with a space group of R3m possess a spontaneous polarization of 7.7, 8.5, and 8.2 μC/cm2, respectively. The polarization switching induced by the opposite displacement of Ge cations under an electric field implies proper ferroelectric behavior in CsGeX3 (X = I, Br, Cl). For CsGeI3 and CsGeBr3, the narrow band gaps and high optical absorption in the visible region indicate promising applications in ferroelectric photovoltaics. Moreover, an indirect-to-direct bandgap transition and ideal band gap for efficient solar cells can be obtained in CsGeI3 by applying an electric field and strain. Our results open new possibilities for ferroelectric halide perovskite applications in high-performance solar cells.