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 CsGeX 3 (X = I, Br, Cl). The polar structures of CsGeI 3, CsGeBr 3, and CsGeCl 3 with a space group of R 3 m possess a spontaneous polarization of 7.7, 8.5, and 8.2 μC/cm 2, respectively. The polarization switching induced by the opposite displacement of Ge cations under an electric field implies proper ferroelectric behavior in CsGeX 3 (X = I, Br, Cl). For CsGeI 3 and CsGeBr 3, 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 CsGeI 3 by applying an electric field and strain. Our results open new possibilities for ferroelectric halide perovskite applications in high-performance solar cells.