Impact of cadmium-rich back surfaces on cadmium chloride treatment and device performance in close space sublimation deposited CdTe solar cells

The cool-down process following close-space sublimation deposition of CdTe films has been found to induce the formation of a Cd-rich layer at the back surface. It has also been shown that this layer may subsequently block the in-diffusion of CdCl₂ used in the post-growth activation step of solar cell devices. Various routes to correct this have been investigated, with the most effective being found to be pre-etching of the as-grown surfaces prior to doping with a nitric-phosphoric acid etch. This leads to increase in the amount of chlorine and oxygen in-diffusion owing to the formation of a Te-rich layer and this improves device efficiency from ~2% to ~10%. Incorporation of the etching step into device processing for CdTe/CdS devices with a ZnO buffer layer and two-stage CdTe growth allowed for fabrication of cells having 13.6% efficiency (1 cm²). The devices were grown on NSG TEC™ C15, an SnO₂:F multilayer on low iron soda lime glass.