A major source of loss in cadmium sulfide/cadmium telluride (CdS/CdTe) solar cells results from light absorbed in the CdS window layer, which is not converted to electrical current. This film can be made more transparent by oxygen incorporation during sputter deposition at ambient temperature. Prior to this work, this material has not produced high-efficiency devices on tin oxide-coated soda-lime-glass substrates used industrially. Numerous devices were fabricated over a variety of process conditions to produce an optimized device. Although the material does not show a consistent increase in band gap with oxygenation, absorption in this layer can be virtually eliminated over the relevant spectrum, leading to an increase in short-circuit current. Meanwhile, fill factor is maintained, and open-circuit voltage increases relative to baseline devices with sublimated CdS. The trend of device parameters with oxygenation and thickness is consistent with an increasing conduction band offset at the window/CdTe interface. Optimization considering both initial efficiency and stability resulted in a National Renewable Energy Laboratory verified 15.2%-efficient cell on 3.2-mm soda-lime glass. This window material was shown to be compatible with SnO 2 -based transparent conducting oxide and high resistance transparent coated substrates using in-line compatible processes.

The status of the highest efficiency CdTe solar cells is presented in the context of comparative loss analysis among the leading technologies for single-and polycrystalline photovoltaic materials. The Shockley-Queisser limit of a single-junction cell, with acknowledgement of variations from standard conditions, is used for reference. The highest CdTe currents achieved are comparable with the best single-crystal cells and superior to other thin-film cells. Voltages match those of multicrystalline Si, but lag behind those of CIGS and crystalline Si, and considerably lag behind crystalline GaAs. The potential for still higher CdTe efficiency will likely require a combination of reduced bulk recombination, smaller back-contact barriers, device structures with advantageous internal fields, and transparent emitters with minimal band offsets.

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