Picosecond electron trapping limits the emissivity of CsPbCl₃ perovskite nanocrystals

Lead halide perovskite nanocrystals (NCs) have emerged as enabling materials for optoelectronics and photonics. A parameter essential for these applications is the photoluminescence quantum yield (PL QY) of these NCs. Despite being generally conceived as "defect-tolerant," perovskite NCs often have PL QYs significantly lower than unity, particularly for CsPbCl₃ NCs with QYs typically lower than 10%. Postsynthetic treatments by (pseudo)halide salts were found to effectively improve the PL QYs, but the exact role played by the treatments (i.e., passivating electron and/or hole trapping sites) remains unclear. Here, we performed a side-by-side comparison between as-prepared and treated CsPbCl₃ NCs using transient absorption and time-resolved PL measurements of sub-ps time resolution. We clearly identify ps electron trapping as the dominant channel impairing the PL QYs of as-prepared CsPbCl₃ NCs. Electron trapping is effectively alleviated in the halide salt treated NCs. These insights should allow for rational improvement of the emissivity of perovskite NCs for the above-mentioned applications.