Evidence of auger heating in hot carrier cooling of CsPbBr3 nanocrystals

Abstract

Hot Carrier (HC) Dynamics in Lead halide perovskites shows a unique hindered hot carrier cooling. In the present study, we use femtosecond transient absorption technique to interrogate CsPbBr3 to study the role of different carrier cooling mechanisms in HC cooling process. This is accomplished here by considering fitting of the high-energy region of photoinduced bleach with a Fermi-Dirac distribution instead of Maxwell-Boltzmann Distribution. This approach reveals important time dependent parameters like quasi fermi level, apart from carrier temperature. Under low carrier densities with N < <1, the dynamics revealed bleach recovery in time scales > 1 ns revealing dynamics is governed by electron-hole radiative recombination. The HC cooling dynamics under low carrier density (2 *1017/cm3) revealed the cooling is predominantly dominated by short lifetime component of ~0.26 ps. This time is significantly elongated as compared to lifetime of longitudinal optic (LO) phonon lifetime in perovskites, clearly indicative of Froehlich interaction in play along with large polaron formation. However, under higher carrier densities > 1018/cm3, additional lifetime component contributes to HC cooling with a time constant of ~17 ps. This time constant could be assigned to Auger heating. The role of LO phonon filling and Auger heating was ascertained also on the basis energy loss rate vs carrier temperature fitting with these individual cooling models. To ascertain the role of Auger heating we also used different sized samples which showed the scaling of Auger heating times in agreement with size dependent Auger recombination times as reported in the literature. Such long HC cooling opens avenues to possibly extract hot carriers before cooling leading to development of HC photovoltaics.