Models for low energy electron beam induced current experiments in polycrystalline thin film photovoltaics

Electron beam induced current (EBIC) is a powerful technique which measures the charge collection efficiency of electron-hole pairs generated by an electron beam. EBIC offers sub-micron spatial resolution and is naturally suited to study polycrystalline materials. Ideally, an EBIC measurement reflects the spatially resolved quantum efficiency of the solar cell. However, critical analysis of low energy EBIC data obtained on CdTe-CdS solar cells indicates that the EBIC signal is not simply proportional to the collection probability of a working device. As a step towards more quantitative analysis of EBIC measurements of these materials, we develop models of the collection efficiency which account for surface recombination in depletion regions and screening of built-in fields from carrier accumulation. We discuss how these models can be applied to obtain quantitative data on grain boundaries in polycrystalline materials.