Confinement-induced ultrafast conductivity in 2D perovskites resolved by correlative terahertz–NIR spectroscopy

Quantum wells made of quasi two-dimensional organic-inorganic hybrid perovskites (2D-PKs) offer a high degree of flexibility in tailoring optoelectronic properties through carrier confinement and functional interlayers. Compared to their 3D counterparts, 2D-PKs exhibit tunable photoluminescence, excitonic binding at room temperature, and enhanced structural stability. However, the dynamics of photoinduced charge carriers and their transport properties are highly intertwined due to the interplay of diverse excitation species, charge carrier cooling, transport, and radiative and non-radiative recombination. In this study, we employ optical-pump terahertz-probe spectroscopy (OPTP) to analyze the local conductivity dynamics of 2D-PK (n = 5) 2-(9H-carbazol-9-yl)ethan-1-ammonium [(MA)5(CzEA)2(PbI3)5] and 3D-MAPI methylammonium lead iodide (MeNH3PbI3) perovskites on picosecond timescales. Remarkably, we observe an intensity-dependent, 2D-specific buildup of an ultrafast, few-picosecond decay in local THz-conductivity. By combining OPTP with transient absorption and picosecond time-resolved photoluminescence (tr-PL), we correlate photoconductivity and carrier population. Thus, we can attribute the 2D-specific ultrafast THz response to delayed hot-carrier cooling and subsequent exciton formation, which effectively reduces the free-carrier conductivity. This intensity-dependent, ultrafast THz response appears as a signature of the recently identified hot-carrier bottleneck in bulk perovskites, and this effect manifests itself in a unique form in the 2D material. These results encourage further investigations on the impact of functional organic interlayers and provide insights into designing tunable carrier responses for ultrafast devices via adapted heterostructures and confinements.