Overcoming Degradation Mechanisms in CdTe Solar Cells: Second Annual Report, August 1999--August 2000

The scientific literature concerning degradation and atomic diffusion in CdTe/CdS cells has been reviewed. This led to the conclusion that there appear to be unexplained differences between module and cell stability, possibly due to the difference between encapsulated and air-exposed systems. A comprehensive model for Cu-assisted cell degradation is suggested, which should now be testedexperimentally by the appropriate R&D teams. Both LBIC and EBIC have been used to distinguish between stressed and unstressed cells. From these we conclude that decrease in charge separation efficiency is correlated with increase in electrical heterogeneity of the cell. Cells without back contacts were treated with a series of simple organic compounds, where the dipole is varied systematically.We find, grosso modo, such systematics in the electronic behavior and electrical characteristics of these cells, suggesting significant porosity of the CdTe film. NiTe2 was found to be the product of electroless Ni:P back contact deposition from baths of low reductant concentrations. Such back contacts were further optimized and characterized, with special attention being given to cell stabilityconsiderations.