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

Abstract

Colloidal chemistry methods have made quasi 2D perovskites readily accessible. Ultrathin perovskites exhibit charge transport properties which are beneficial for solar cells and the combination of layers with different thicknesses directs charge carriers 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, 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) is characterized by femtosecond transient absorption spectroscopy and time-resolved photoluminescence. It is found that second- and third-order processes dominate in MAPbBr₃ nanosheets, indicating exciton-exciton annihilation (EEA) and Auger recombination. Long-lived excitons in thin layers (e.g., n  =  5, E_b =  136 meV) funnel into high n within 10–50 ps, which decreases their exciton binding energy below k_BT and leads to radiative recombination. Parallel and consecutive funneling compete with 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.