Energy transfer in (PEA)₂FA_n−1Pb_nBr_3n+1quasi-2D perovskites

Quasi-two dimensional perovskites demonstrate unique excitonic properties due to their multilayer structure making them attractive for various optoelectronic applications. However, the thickness of individual perovskite sheets in wet cast quasi-2D layers tends to randomly fluctuate giving rise to a specific type of disorder, which impacts on the carrier dynamics and is rather complex and remains understudied. Here, we present a study of carrier transport in Ruddlesden-Popper type (PEA)₂FA_n−1Pb_nBr_3n+1layers of ordernfrom one to four, and in the bulk FAPbBr₃layer. We use a light induced transient grating technique to measure the carrier diffusion coefficient directly, and the transient absorptionviaphotoluminescence to investigate the energy relaxation pathways. We observe two distinct energy transfer processes on different time scales. Fast energy funnelling in thicker (n≥ 3) layers is observed up to 10 ps after excitation; we attribute this to short-distance transfer of excitons to neighbouring perovskite sheets of higher order. On the longer timescale of hundreds of picoseconds, carrier in-plane transport is governed by exciton diffusion inn= 1 and 2 layers and by free carrier plasma in thicker ones. Within the carrier density range of (0.5-4) × 10¹⁹cm⁻³, the exciton diffusion coefficient inn= 1, 2 increases slowly from 1 to 2.8 cm²s⁻¹, whereas in thicker layers the dependence is much stronger and the diffusivity grows from 0.09 to 1.9 cm²s⁻¹. We explain these dependencies by a higher structural order in the thinner samples and the stronger localization of carriers in thicker ones. Also, amplified spontaneous emission (ASE) is observed in thicker (n≥ 3) layers in electron-hole plasma, as evidenced by the typical ASE line redshift upon excitation.