Maximilian Spies

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.

The solution‐based fabrication of reproducible, high‐quality lead iodide perovskite films demands a detailed understanding of the crystallization dynamics, which is mainly determined by the perovskite precursor solution and its processing conditions. A systematic in situ study is conducted during the critical phase before the nucleation in solution to elucidate the formation dynamics of lead iodide perovskite films. Using ultraviolet (UV) absorption spectroscopy during spin coating allows to track the evolution of iodoplumbate complexes present in the precursor solution. It is found that prior to film formation, a novel absorption signature at 3.15 eV arises. This is attributed to the emergence of a PbI ₂ ‐DMF solvated (PDS) phase. The amount of PDS phase is closely connected to the concentration of the solution layer during spin coating. It is also proposed that PDS clusters are a predecessor of crystalline perovskite phases and act as nucleation seeds in the precursor solution. In this way, this work provides insights into the early stages of perovskite crystallization.