Relaxation Between Bright Optical Wannier Excitons in Perovskite Solar Absorber CH\(_3\)NH\(_3\)PbI\(_3\)

Abstract

We study the light-absorbing states of the mixed-halide perovskite CH\(_{3}\)NH\(_{3}\)PbI\(_2\)Cl and tri-iodide perovskite CH\(_{3}\)NH\(_{3}\)PbI\(_3\) with density functional and many-body calculations to explain the desirable photovolatic features of these materials. The short-lived electron-hole bound states produced in this photovoltaic material are of halide to lead electron transfer character, with a Wannier-type exciton. Bethe-Salpeter (GW+BSE) calculations of the absorption cross section reveal strong screening of the electron-hole interaction. The atomic character of the exciton retains ligand-to-metal character within the visible spectrum, with differing degrees of localization outside the unit cell. The average electron-hole separation in the lowest exciton is found to be about 5\(A^{\circ}\), slightly larger than the Pb-I bond length. Finally, we determine the role of methylammonium's dipole in the ultrafast relaxation by preparing an atomistic model of the picosecond electronic dynamics in the tri-iodide, PbI\(_3\). Our model allows us to identify phonon modes which couple strongly to the electronic excitations, and explain the picosecond timescale intra-band relaxation dynamics seen in recent transient absorption experiments. We largely substantiate the conjectured three band model for the dynamics, but also identify other possible relaxation channels in the tri-iodide.