Halide perovskites show unusual thermalization kinetics for above-bandgap photoexcitation. We explain this as a consequence of excess energy being deposited into discrete large polaron states. The crossover between low-fluence and high-fluence “phonon bottleneck” cooling is due to a Mott transition where the polarons overlap (n ≥ 1018 cm–3) and the phonon subpopulations are shared. We calculate the initial rate of cooling (thermalization) from the scattering time in the Fröhlich polaron model to be 78 meV ps–1 for CH3NH3PbI3. This rapid initial thermalization involves heat transfer into optical phonon modes coupled by a polar dielectric interaction. Further cooling to equilibrium over hundreds of picoseconds is limited by the ultralow thermal conductivity of the perovskite lattice.
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机译：卤化物钙钛矿对带隙以上的光激发显示出异常的热动力学。我们解释这是由于多余的能量沉积到了离散的大极化子态中的结果。低通量和高通量“声子瓶颈”冷却之间的交叉是由于莫特跃迁所致，其中极化子重叠（n≥10 18 cm –3 ），并且声子子群是共享的。根据Fröhlich极化子模型中的散射时间，我们计算出CH3NH3PbI3的初始冷却（加热）速率为78 meV ps –1 。这种快速的初始热化涉及将热量传递到通过极性电介质相互作用耦合的光学声子模式。钙钛矿晶格的超低热导率限制了数百皮秒内的进一步冷却至平衡。
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