Polar methylammonium organic cations detune state coupling and extend hot-carrier lifetime in lead halide perovskites

The slow hot-carrier relaxation properties of lead halide perovskites show great promise for hot-carrier solar cells. Here, we discover that the hot-carrier relaxation could be prolonged by two orders of magnitude in lead halide perovskites. By ultrafast spectroscopy, we unravel a slow hot-carrier relaxation process mediated by an intermediate state above the conduction band minimum (CBM). This intermediate state is formed by incorporating polar methylammonium (MA) cations. The first-principle calculation further confirms that the incorporation of MA cations introduces a prolonged relaxation process from CBM + 1 to CBM state, which are the split-off states of the CBM due to the strong spin-orbit coupling. The prolonging effect is interpreted that MA cations largely increase the lattice distortions and detune their inter-state coupling. This work reveals a neglected way to prolong the hot-carrier relaxation processes and provides guidance for the hot-carrier photovoltaics to overcome the current Shockley-Queisser limit.