关键词: 钙钛矿, 第一性原理, 光电特性, 能带结构, 吸收光谱

Formamidinium lead halide perovskites have attracted wide attention as photoelectronic conversion materials due to the high photoelectronic conversion efficiency (PCE), low cost and simple synthetic process. The structural, electronic and optical properties of mixed formamidinium lead halide perovskites FAPbI_xCl_3-x (FA=NH₂CH=NH₂⁺, x=0～3) have been investigated by the first-principles theory. Our results show that FA cations lie along [001] direction in the trigonal FAPbX₃ (X=Cl, Br, I). However, the direction is slightly shifted owing to the distortion of PbX₆ (X=Cl, I) octahedrons in the mixed FAPbI_xCl_3-x. The Pb-I bond distances (0.315～0.334 nm) are larger than Pb-Cl bond distances (0.282～0.302 nm). With the increase of I/Cl ratio, the lattice parameters and volumes of FAPbI_xCl_3-x increase. The FA cations play a crucial role in balancing the crystal structure, but they do not participate into the process of frontier orbital transition directly. They just play the role of charge donors to contribute ca. 0.76 e to PbI₃ framework. FAPbI_xCl_3-x are direct band-gap semiconductors, with the direct bandgap nature at Z (0, 0, 0.5) symmetry point. The valence band maximum (VBM) is composed of antibonding orbitals of I 5p (Cl 3p) and a few Pb 6s orbitals, and the conduction band minimum (CBM) is composed of Pb 6p orbital. There exists a combined covalent and ionic bonding mechanism between Pb and I (Cl) ions. As the I/Cl ratio increases, the band gaps decrease and the absorption spectra are red shifted. FAPbI₃ has an ideal band gap of 1.53 eV. It exhibits the superior absorption spectrum especially in the range of 300 nm to 500 nm, which elucidates that FAPbI₃ has great potential as the photoelectronic conversion material. Our results could provide theoretical guidance for the experimental design and synthesis of perovskite solar cells.

Key words: perovskite, first-principles theory, photoelectronic property, band structure, absorption spectrum