Understanding the Optoelectronic Processes in Colloidal 2D Multi-Layered MAPbBr3 Perovskite Nanosheets: Funneling, Recombination and Self-Trapped Excitons

Quasi two-dimensional (2D) colloidal synthesis made quantum confinement readily accessible in perovskites, generating additional momentum in perovskite LED research and lasing. Ultrathin perovskite layers exhibit high exciton binding energies and beneficial charge transport properties interesting for solar cells. In 2D perovskites, the combination of layers with different thickness helps to direct charge carriers in a targeted manner toward thicker layers with a smaller bandgap. However, detailed knowledge about the mechanisms by which excitons and charge carriers funnel and recombine in these structures is lacking. Here, we characterize colloidal 2D methylammonium lead bromide (MAPbBr3) Ruddlesden-Popper perovskites with a broad combination of layers (n = 3 to 10, and bulk fractions with n > 10) in one stack by femtosecond transient absorption spectroscopy and time-resolved photoluminescence, which gives comprehensive insights into the complexity of funneling and recombination processes. We find that after photoexcitation second- and third-order processes dominate in MAPbBr3 nanosheets, which indicates exciton-exciton annihilation (EEA) and Auger recombination. Long-lived excitons in thin layers (e.g., n = 5, Eb = 136 meV) funnel into high n with t = 10-50 ps, which decreases their exciton binding energy below kB T = 26 meV ( T = 300K) and leads to radiative recombination. Parallel and consecutive funneling compete with exciton trapping processes, making funneling an excellent tool to overcome exciton self-trapping when high-quality n-n interfaces are present. Free charge carriers in high n regions on the other hand facilitate radiative recombination and EEA is bypassed, which is desirable for LED and lasing applications.