Investigation on photoexcited state dynamics in Cs_1−xFA_xPbI₃ perovskite quantum dots: A nanosecond transient absorption spectroscopy analysis

This study provides a perceptive analysis of the excited-state properties of Cs_1−xFA_xPbI₃ perovskite quantum dots (PQDs), which were synthesized using a facile and scalable low-temperature, open-air method. In the obtained transient absorption spectra of all examined PQD films, a noticeable photo-induced absorption (PIA) signal at 500 nm, coupled with a ground-state bleaching (GSB) signal ranging from 685 to 745 nm, is observed. The progressive elevation of formamidinium (FA) content within the perovskite quantum dots (PQDs) resulted in a faster non-radiative recombination, pointing towards a decreased density of trap states. This phenomenon finds validation in the observed attenuation of the PIA signal. Simultaneously, this led to a slower direct recombination, a fact substantiated by the observed strengthening of the GSB signal. Notably, the observed carrier lifetime increases markedly until the FA content reached 0.75, extending to over 2 μs. However, higher FA contents are found to have a negative impact on the carrier lifetime and, consequently, the device performance. Furthermore, the observed red-shifted GSB peak in PQDs is ascribed to band-gap reduction in the PQDs, resulting from an increased FA content. These findings provide insights into the intrinsic photo-physics of mixed-cation halide PQDs, with direct implications for applications in optoelectronics.