Effect of Capping Ligand Engineering on Transport Properties and Carrier Dynamics in Cubic CsPbI3 Nanocrystal Film

Abstract

Cubic (α) phase CsPbI3 perovskite nanocrystals (NCs) are highly sought-after solar cell materials due to their appropriate bandgap of 1.73 eV. Nevertheless, the NCs are capped by insulating organic ligands. For utilizing these NCs in device fabrication, dot-to-dot electronic transport is essential, which can be achieved by removing the capping ligands by washing with methyl acetate and lead acetate. Time-resolved terahertz spectroscopy has been used to study the charge carrier dynamics and the transport properties of α-CsPbI3 NC film, methyl acetate, and lead acetate treated α-CsPbI3 NC films on a nonconducting substrate to evaluate the effect of ligand removal. Ligand treatment enhances inter-NC coupling, leading to a 5-fold increase in photoconductivity and mobility and a 2-fold increase in diffusion length. However, ligand-treatment of the NC film also introduces new defect states confirmed by the presence of strong excitation-energy-dependent transport properties and a new faster carrier recombination pathway in the ligand-treated films. Overall, the resultant effect of ligand treatment is beneficial as the newly introduced traps are within the valence and conduction bands, not in the bandgap.