Photovoltaic Effect Related to Methylammonium Cation Orientation and Carrier Transport Properties in High-Performance Perovskite Solar Cells.

ABSTRACT

Solar cells based on organic-inorganic hybrid halide perovskites (OIHHPs) have been widely studied because of their increasing power conversion efficiency. Extensive research has been conducted in electrical and optical properties and device fabrication. However, in terms of material science, the photovoltaic effects of OIHHP are still not well understood. Here, we theoretically investigate the photovoltaic phenomena of MAPbI3 (MA = CH3NH3+) under standard AM 1.5G sunlight illumination, considering the MA cation orientation, light incident angle, polarization, and photon energy, using Keldysh nonequilibrium Green's function formalism combined with density functional theory calculations. It is found that the short-circuit current density Jsc has a maximum value of 383.149 A/m2 when the MA orientation is parallel to the transport direction, whereas it is negligible when the MA orientation is orthogonal to the transport direction. In addition, full consideration is also given to the direction of incidence of sunlight and its polarization state. Nevertheless, of all factors considered, MA orientation plays the decisive role, for Jsc still has a respectable value of 364.112 A/m2 even for a 90° sunlight incident angle relative to the transport direction, so long as the MAs are aligned in the transport direction. The increase in the photocurrent is attributed to an increase in the transmission coefficient of low-energy holes, as well as improvement of the velocities and mobilities of electrons and holes in the MAPbI3-based device with [001] MA orientation. The results suggest that during the designing of high-performance OIHHP-based solar cells and photoelectronic devices, the crystal orientation and MA cation orientation relative to the transport direction should be carefully considered as they directly affect carrier transport properties.