Imaging the Reduction of Electron Trap States in Shelled Copper Indium Gallium Selenide Nanocrystals Using Ultrafast Electron Microscopy

Insertion of alkali metal ions, especially Na, is a well-established method to significantly increase the power conversion efficiency of copper indium gallium selenide (CIGSe)-based photovoltaic devices. However, although it is known that Na ions mostly reside on the surface of CIGSe layer following diffusion, the exact mechanism of how Na affects the carrier dynamics of CIGSe still remains ambiguous. This is mainly due to the unavailability of suitable surface-sensitive techniques. Herein, we employ four-dimensional scanning ultrafast electron microscopy (4D S-UEM), which has the unique capability of mapping the charge carrier dynamics in real time and space selectively on the materials surfaces, to directly observe the effect of Na insertion on the carrier dynamics of shelled CIGSe film. It is found that an additional layer of NaF to the thin film of ZnS-shelled CIGSe nanocrystals not only increases the grain size and improves the texture of the film but, more importantly, reduces fast electron trap channels on the surface of the material, as observed from the secondary electron dynamics in 4D S-UEM. Our density functional theory calculations further confirm that Na ions can occupy Cu vacancies and reduce the interfacial charge carrier-defect scatterings. Removal of such undesirable electron trapping channels results in increased photoconductivity of the material, thereby serving as one of the critical parameters that lead to enhancement of the efficiency of CIGSe for light harvesting purposes.