Shape- and excitation-dependent charge-carrier dynamics in colloidal MAPbI perovskites as nanostripes, nanosheets and nanoplatelets

Nanostructured lead halide perovskites are an attractive group of materials for highly efficient optoelectronic applications. Considering the wide variety in shape, size and crystal structures (layered, non-layered, hollow, double), many different excited-state population and relaxation processes can be present, such as exciton formation, free charge carrier generation, exciton-exciton annihilation, and Auger recombination. Here, we present the synthesis of nanostructures with different morphology to improve the optical properties of methylammonium lead iodide and to investigate the corresponding charge-carrier dynamics and recombination processes. Excitation intensity dependent time-resolved photoluminescence (PL) measurements are performed to highlight the PL decay behaviour of nanosheets, nanoplatelets, and nanostripes. In all three cases a faster decay rate is observed with increasing excitation fluence. This behaviour can be attributed to the occurrence of bimolecular recombination of electrons and holes in nanosheets and exciton-exciton annihilation in nanostripes with high carrier densities. Performing these measurements at low excitation intensities revealed unusual long charge-carrier lifetimes of a few microseconds. Additionally, the nanostripes showed the best results in terms of photoluminescence quantum yield of up to 75% and the nanosheets demonstrate improved stability. The nanoplatelets are considered as intermediate structures between nanosheets and nanostripes - balancing quantum yield and stability.