CIGSS solar cells based on CVD ZnO buffer layers

This paper describes investigations of cells based on Siemens Solar material, CIGSS, and highly resistive ZnO (i-ZnO) buffer layers grown by MOCVD. Resistive i-ZnO buffer layers are grown on CIGSS at 100°C, after heating the substrate to 250°C and using nitrogen as a carrier gas. The use of a KCN etch on the CIGSS surface prior to growth of i-ZnO buffer layers was determined to be beneficial to cell performance. XPS studies show that the etching step removes oxygen from the substrate surface that had complexed with Se to form SeO₂. Cells that do not receive a KCN etch typically have shunted I-V curves leading to relatively low open circuit voltages, fill factors and efficiencies. A tentative model is proposed for the effect of KCN. Finally, one cell (using a KCN etch) exhibited a total area efficiency of 12.7 % with an open circuit voltage of 0.577 Volts