Photo-energy conversion efficiency of CH3NH3PbI3/C60 heterojunction perovskite solar cells from first-principles

Halide perovskites have emerged as the most potential candidate for the next-generation solar cells. In this work, we conduct a comprehensive first-principles study on the photo-energy conversion efficiency (PCE) of the CH3NH3PbI3/C-60 heterojunction. Since perovskite solar cells (PSCs) are generally exposed to substantial temperature variations that affect the photo-physics and charge separation in the perovskites, finite slabs of both the tetragonal- and cubic-phases of CH3NH3PbI3 (MAPbI(3)) are constructed with different orientations of the methylammonium (MA) cation with respect to the perovskite surface. A C-60 molecule, acting as an electron acceptor and transport material, is introduced at various positions on the MAPbI(3) surface. Using the detailed balance approach, the PCE of the heterojunction is determined by examining the Kohn-Sham energies and orbitals located either in MAPbI(3) or in C-60. Our study reveals that the stability, the exciton dissociation efficiency, and the PCE of the tetragonal MAPbI(3)/C-60 heterojunctions strongly depend on the MA orientation and the C-60 position on the MAPbI(3) surface. This is attributed to the polar behavior of MAPbI(3). Using the tetragonal-phase heterojunction, a high PCE of eta similar to 19% can be achieved, if certain surface conditions are met. Built-in electric field originating from the surface dipoles of the MA cation facilitates the dissociation of electron-hole pairs and the electron transfer to fullerene. On the other hand, the heterojunction with a high-temperature cubic MAPbI(3) structure exhibits a PCE of eta similar to 10%, regardless of the MA orientation and C-60 on the MAPbI(3) surface.