Influences of Sub-Bandgap Photons on Photogenerated Carriers in CsPbBr ₃ Nanocrystals

CsPbBr3 nanocrystals (nano-CsPbBr3) have shown extraordinary optoelectronic properties together with outstanding stability under ambient conditions. In this study, we report the influences of sub-bandgap photons on photogenerated carriers generated by the band-to-band photoexcitation in nano-CsPbBr3. We discovered that the influences of sub-bandgap photons strongly depend on temperature. Temperature-dependent photoluminescence spectra, measured in the absence and the presence of sub-bandgap photons in addition to the band-to-band photoexcitation, reveal the significance of electron–phonon (EP) interactions and their effects on photogenerated carriers in nano-CsPbBr3, together with the effects of thermal excitation. The results of temperature-dependent time-resolved photoluminescence further support the significance of EP interactions and their influences. The roles of EP interactions and thermal excitation are further elucidated by the differences in photocurrent and photovoltage (ΔJSC and ΔVOC) measured in the absence and presence of additional sub-bandgap photons as a function of temperature. The ΔJSC and ΔVOC show positive values, i.e., enhancements, but gradually turn negative with increasing temperatures due to an increase in EP interactions, which facilitate nonradiative recombination. The study assists in distinguishing the intrinsic photoabsorption of sub-bandgap photons attributed to nano-CsPbBr3 from the effects of device structures and band engineering once nano-CsPbBr3 are used for upconversion devices. Furthermore, the comprehensive understanding of the photophysical properties of nano-CsPbBr3 will help to acquire general optoelectronic properties of perovskite nanocrystals, which are still required for further intensive research.