Enhanced Hot‐Phonon Bottleneck Effect on Slowing Hot Carrier Cooling in Metal Halide Perovskite Quantum Dots with Alloyed A‐Site

Abstract A deep understanding of the effect of the A‐site cation cross‐exchange on the hot‐carrier relaxation dynamics in perovskite quantum dots (PQDs) has profound implications on the further development of disruptive photovoltaic technologies. In this study, the hot carrier cooling kinetics of pure FAPbI 3 (FA + , CH(NH 2 ) 2 + ), MAPbI 3 (MA + , CH 3 NH 3 + + ), CsPbI 3 (Cs + , Cesium) and alloyed FA 0.5 MA 0.5 PbI 3 , FA 0.5 Cs 0.5 PbI 3 , and MA 0.5 Cs 0.5 PbI 3 QDs are investigated using ultrafast transient absorption (TA) spectroscopy. The lifetimes of the initial fast cooling stage (<1 ps) of all the organic cation‐containing PQDs are shorter than those of the CsPbI 3 QDs, as verified by the electron‐phonon coupling strength extracted from the temperature‐dependent photoluminescence spectra. The lifetimes of the slow cooling stage of the alloyed PQDs are longer under illumination greater than 1 sun, which is ascribed to the introduction of co‐vibrational optical phonon modes in the alloyed PQDs. This facilitated efficient acoustic phonon upconversion and enhanced the hot‐phonon bottleneck effect, as demonstrated by first‐principles calculations.