Identification of high-temperature exciton states and their phase-dependent trapping behaviour in lead halide perovskites†
While lead halide perovskites have rapidly emerged as lucrative material candidates for optoelectronic applications, the crucial energy states and charge-carrier nature of these systems are yet to be fully understood, resulting in ongoing debates on the fundamental optical and material characters. In this work we probe the band-edge and sub-gap energy states within polycrystalline and single crystal perovskites to better understand their photophysical origins. Through temperature-, excitation intensity- and time-dependent optical measurements, we reveal the existence of both free and bound exciton contributions to the optoelectronic properties across a wide temperature region up to 300 K. The low-energy absorption and multiple-peak emission phenomena, whose physics origins have been highly controversial before, are caused by these exciton states. Furthermore, the trapping and recombination dynamics of these excitons is shown to be strongly dependent on the structural phase of the perovskite. The orthorhombic phase exhibits an ultrafast exciton trapping and distinct trap emissions, while the tetragonal phase gives low monomolecular recombination velocity and capture cross-sections (∼10−18 cm2). Within the multiphonon transition scenario, this suppression in charge trapping is caused by the increase in the charge capture activation energy due to a reduction in electron–lattice interactions within the inorganic framework, which can reasonably interpret the unexpectedly long carrier lifetime in these material systems. These findings provide a clearer understanding of the origins of the photophysical properties of perovskite materials, and the discovery of a high-temperature-stable bound exciton would bring the excitonic feature to be a concern for the perovskite energy conversion and utilization applications.