LAUSR

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 cha...