Break of thermal equilibrium between optical and acoustic phonon branches of CsPbI_(3)under continuous-wave light excitation and cryogenic temperature

The kinetic of low-temperature carrier and lattice of lead-halide perovskite is yet to be fully understood.In this work,we investigate the steady-state photoluminescences(PLs)of CsPbI_(3)at the environmental temperature(Te)ranging from 20 K to 300 K,and observed anomalous behaviors at cryogenic temperatures:The carrier temperature(Tc)of pure CsPbI_(3)exhibits a negative correlation with Te,accompanied by an expansion in Urbach tails of absorption spectra(Abs.)and excessive red-shifts at peak energy of PLs.These phenomena are also observed in those samples containing a certain amount of Cs_(4)PbI_(6),but to a lesser extent and occurs at lower temperatures.It is attributed to the intensified hot phonon bottleneck effect(HPB)in CsPbI_(3)at cryogenic Te,which hinders the energy transfer from hot carriers,via longitudinal optics(LO)phonons to longitudinal acoustic(LA)phonons,to the ambient.For samples under continuous-wave laser excitation,in specific,the barrier induced by the enhanced HPB at low Teprevents the effective thermalization among carriers,LO and LA phonons,which,therefore,form thermally isolated ensembles with different temperatures.At cryogenic Terange,the elevated temperatures of carrier and LO phonon expand the high-energy side of PLs and the low-energy tail of Abs.,respectively.For those samples in which the CsPbI_(3)is mixed with Cs_(4)PbI_(6),the interfacial LO-LO interaction across them provides a bypass for heat dissipation,mitigating the heat accumulation in LO-phonons of CsPbI_(3).The results suggest that a strong HPB effect may break the thermal equilibrium among different branches of phonons in the lattice under certain extreme conditions.