Electron-Hole Binding Governs Carrier Transport in Halide Perovskite Nanocrystal Thin Films

TL;DR

This study investigates how exciton and free electron-hole pair dynamics influence carrier transport in halide perovskite nanocrystal thin films, revealing temperature-dependent diffusion behaviors critical for optoelectronic applications.

Contribution

It provides the first detailed spatial and temporal mapping of exciton diffusion in perovskite nanocrystal films across a temperature range, highlighting the coexistence of excitons and free carriers.

Findings

Diffusion length inversely correlates with nanocrystal thickness at room temperature.

Exciton diffusion declines with increasing temperature, with specific turnover points.

Thinner nanocrystals exhibit higher diffusion lengths at elevated temperatures.

Abstract

Two-dimensional halide perovskite nanoplatelets (NPLs) have exceptional light-emitting properties, including wide spectral tunability, ultrafast radiative decays, high quantum yields (QY), and oriented emission. To realize efficient devices, it is imperative to understand how exciton transport progresses in NPL thin films. Due to the high binding energies of electron-hole pairs, excitons are generally considered the dominant species responsible for carrier transfer. We employ spatially and temporally resolved optical microscopy to map exciton diffusion in perovskite nanocrystal (NC) thin films between 15 {\deg}C and 50 {\deg}C. At room temperature (RT), we find the diffusion length to be inversely correlated to the thickness of the nanocrystals (NCs). With increasing temperatures, exciton diffusion declines for all NC films, but at different rates. This leads to specific temperature turnover points, at which thinner NPLs exhibit higher diffusion lengths. We attribute this anomalous diffusion behavior to the coexistence of excitons and free electron hole-pairs inside the individual NCs within our temperature range. The organic ligand shell surrounding the NCs prevents charge transfer. Accordingly, any time an electron-hole pair spends in the unbound state reduces the FRET-mediated inter-NC transfer rates and consequently the overall diffusion. These results clarify how exciton diffusion progresses in strongly confined halide perovskite NC films, emphasizing critical considerations for optoelectronic devices.