Electrostatic Potential Fluctuations in Thin-Film Photovoltaics

Increasing the conversion efficiency of photovoltaics based on thin-film absorbers materials requires minimizing the open circuit voltage (Voc) deficit (difference between the band gap and Voc) and producing films with sufficient charge carrier concentrations. However, inherent to these materials are high concentrations of intrinsic point defects and intentional extrinsic defects that can result in 1) significant charge carrier compensation thus reducing the carrier concentration and 2) fluctuations in the local electrostatic potential that define the band structure resulting in increased charge carrier recombination and low Voc values. Understanding both the magnitude and spatial distribution of such potential fluctuations is necessary in order to refine material and device fabrication processes to achieve the highest photovoltaic conversion efficiency. In this contribution, we demonstrate how cathodoluminescence microscopy can be used to monitor such changes in CdSeTe and CuInGaSe₂ thin-films with sub-100nm spatial resolution. In addition we discuss the physical mechanisms behind how atomic-scale compositional variations relate to the magnitude and spatial distribution of electrostatic potential fluctuations in these materials.