AFM-based microelectrical characterization of grain boundaries in Cu(In,Ga)Se2 thin films

We report on a direct measurement of two-dimensional potential distribution on the surface of Cu(In,Ga)Se₂ thin films using a nanoscale electrical characterization of scanning Kelvin probe microscopy both in air and in ultra-high vacuum. The potential measurement reveals a higher surface potential or a smaller work function on grain boundaries (GBs) of the film than on the grain surfaces. This demonstrates the existence of a local built-in potential on GBs, and the GB is positively charged. The role of the built-in potential in device performance was further examined and found to be positive, by tuning Ga content or band gap of the film. With increasing Ga content, the potential drops sharply in a Ga range of 28%∼38%. Comparing the change in the built-in potential to the theoretical and experimental photoconversion efficiencies, we conclude that the potential plays a significant role in the device conversion efficiency of NREL´s three-stage Cu(In,Ga)Se₂ device.