The Role of Surface Recombination in Halide Perovskite Nanoplatelets.

Halide perovskites are an extremely promising material platform for solar cells and photonic devices. The role of surface carrier recombination - well known to detrimentally affect the performance of devices - is not well understood thin samples, however, where the thickness is comparable to, or less than, the carrier diffusion length. Here, using time-resolved micro-spectroscopy along with modelling, we investigate charge carrier recombination dynamics in halide perovskite CH3NH3PbI3 nanoplatelets with thicknesses from ~ 20 - 200nm, ranging from much less to comparable to the carrier diffusion length. We show that surface recombination plays a stronger role in thin perovskite nanoplatelets, significantly decreasing photoluminescence (PL) efficiency, PL decay lifetime, and photo-stability. Interestingly, we find that both thick and thin nanoplatelets exhibit a similar increase in PL efficiency with increasing excitation fluence, well described by our excitation saturation model. We also find that the excited carrier distribution along the depth impacts the surface recombination. Using the diffusion-surface recombination model we determine the surface recombination velocity and carrier diffusion coefficients. This work provides a comprehensive understanding of the role of surface recombination and charge carrier dynamics in thin perovskite platelets and reveals valuable insights useful for applications in photovoltaics and photonics.