Application of CIS to High-Efficiency PV Module Fabrication: Annual Technical Progress Report, 1 April 1995 - 31 March 1996

We investigated the interactions between the soda-lime glass substrate, the Mo contact film and the CIS absorber layer. Excessive Na diffusion through the Mo layer was found to be the reason for excessive interaction between the substrate and the CIS layers obtained by the H2Se selenization technique. This chemical interaction influenced the stoichiometric uniformity of the absorbers. Addition ofGa into the CIS layers by the two-stage selenization technique yielded graded absorber structures with higher Ga content near the Mo/absorber interface. Gallium was later diffused through the absorber film by a high-temperature annealing step, and large bandgap alloys were obtained. Solar cells with active-area efficiencies of close to 12% were fabricated on these CIGS layers. Sulfur additionexperiments were also carried out during this period. By controlling the Se and S availability to the precursors during the reaction step of the process, various S profiles were obtained in high-bandgap absorber layers. The highest efficiency cell made on S-containing absorbers was about 10% efficient. A low-cost, non-vacuum technique was successfully developed for CIS film growth. Layersprepared using this novel approach were used for solar-cell and submodule fabrication. Solar cells with active-area efficiencies around 13% were demonstrated; submodules with efficiencies above 8% were also fabricated. These results represent the best PV devices ever produced on CIS layers obtained by a non-vacuum technique.