Above-bandgap emission properties of CsPbBr₃ as probed by fine-scale photoluminescence excitation spectroscopy

Halide perovskites are next-generation materials for solar cells and light emitting diodes. Despite rapid progress in applied perovskite technology, understanding of their basic properties is not complete yet. Especially, it is necessary to clarify the bandgap and the nature of the photoluminescence (PL) process (whether excitonic or electron-hole recombination) for the high performance of the devices. As a case study, we investigated unusual bandgap and recombination behaviors of CsPbBr₃ using PL excitation spectroscopy. Significant variation of both shape and intensity of the PL was observed when the excitation wavelength was tuned across the bandgap. Intriguingly, PL emission occurs above the bandgap, which arises presumably from radiative recombination at halide vacancies. Excitation power dependence of the PL shows that the power exponent continuously varies from 2 to 1 and to sublinear with increasing the laser intensity. The rate equation for pulsed excitation was modeled to account for this anomalous effect. Our model implies that the PL arises from electron-hole recombination at room temperature.