Publication date: 28th August 2024
Hybrid metal halide perovskites have given rise to some of the most exciting developments in optoelectronics over the last decade, and are the most promising candidates for high-performance multi-junction solar cells that surpass the fundamental efficiency limits of traditional devices. Furthermore, their bandgap tunability, high radiative quantum yields, and defect tolerance also make them excellent light emitters. Two-dimensional (2D) perovskite semiconductors have promising prospects for enhancing the stability of perovskite-based photovoltaic devices. In addition, these low-dimensional materials with electronic confinement offer further opportunities in light emission and quantum technologies. However, their technological applications still require a comprehensive understanding of the nature of charge carriers and their transport mechanisms. This talk will show how time-resolved optical spectroscopy can be employed to investigate charge-carrier dynamics, exciton formation dynamics, charge-carrier mobilities, and charge-phonon coupling in perovskite semiconductors. I will discuss the impact of heterogeneities and low dimensionality, and the peculiar softness of the lattice. Our work reveals band transport with high in-plane mobilities that give rise to efficient long-range conductivity in 2D perovskites. We show how the organic cation moderates the coupling of charge carriers to optical phonon modes, impacting the charge-carrier mobilities. Furthermore, we demonstrate a new experiment for simultaneously recording the terahertz and optical transmission transients, thus allowing us to monitor the exciton formation dynamics over the picosecond timescale. The observed dynamics reveal a long-living population of free charge-carriers that greatly surpasses the theoretical predictions of the Saha equation even at temperatures as low as 4K. Our findings provide new insights into the temperature-dependent interplay of exciton and free charge carriers in 2D Ruddlesden-Popper perovskites. Furthermore, the sustained free charge-carrier population and high mobilities revealed by this work demonstrate the potential of these semiconductors for applications that require efficient charge transport, such as solar cells, transistors, and electrically driven light sources.