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.

卤化铅钙钛矿的缓慢热载流子弛豫特性显示出热载流子太阳能电池的巨大前景。在这里，我们发现卤化铅钙钛矿的热载流子弛豫可以延长两个数量级。通过超快光谱，我们揭示了由高于导带最小值 (CBM) 的中间状态介导的缓慢热载流子弛豫过程。这种中间状态是通过结合极性甲基铵 (MA) 阳离子形成的。第一性原理计算进一步证实了 MA 阳离子的加入引入了从 CBM + 1 到 CBM 状态的延长弛豫过程，这是由于强自旋轨道耦合导致 CBM 的分裂状态。延长效应被解释为 MA 阳离子在很大程度上增加了晶格畸变并使其状态间耦合失谐。