CdTe is a nearly perfect absorber material for second generation polycrystalline solar cells because the bandgap closely matches the peak of the solar spectrum, relatively high absorption coefficient and good electronic properties in the polycrystalline phase. Fabricating high-efficiency CdS/CdTe solar cells with an ultra-thin absorber layer is a challenging yet highly desirable step in improving CdTe technology. Most of today’s CdTe solar cells utilize an absorber layer which is about 2.5 µm to 8 µm thick. Thinning this layer down typically results in poorer cell performance due to shunting, incomplete photon absorption, fully depleted CdTe layer or interference between the main and the back contact junction when the CdTe layer thickness approaches a certain limit. While some of these losses are fundamental, others can be minimized by careful optimization of the fabrication steps.
In this dissertation I present the results of such optimization. Magnetron sputtered CdS/CdTe solar cells with the absorber layer thicknesses from 2.6 µm to 0.3 µm were studied. The deposition process itself and all the post-deposition parameters such as the CdCl2 treatment, thickness of the Cu layer in the thermally evaporated Cu/Au back contact, and the back contact thermal activation/diffusion time for a given CdTe layer thickness were optimized to achieve the top performance for the cell of a given thickness. 13.5 % efficiency cells with 2.5 µm CdTe, 12% cells with 1 µm CdTe, 9.7% cells with 0.5 µm CdTe and 6.7% with 0.3 µm CdTe were fabricated.