Device Physics of Thin-Film Polycrystalline Cells and Modules: Final Report, February 1998 -- August 2001

This report describes the basic measurements of CdTe and CI(G)S solar cells fabricated at a number of collaborating laboratories. The first area of emphasis has been to quantitatively deduce the loss mechanisms in these cells, and to make appropriate comparisons that illuminate where progress in being made. Cells evaluated include those at or near record efficiencies and those made with newprocessing strategies. A second area of emphasis, the role of impurities, has focused on sodium in CIS. Cells made with varying amounts of sodium added during CIS deposition were fabricated at NREL using four types of substrates. Best performance was achieved with 0.01 to 0.1 at% sodium. The third area of study has been small-spot measurement of micro-nonuniformities. A new facility was built tofocus a laser beam onto a solar cell with 1-um beam size, 1-um resolution and repeatability, and one-sun intensity. Specific projects to date have focused on CdTe and have included the local effect of CdCl2, local intermixing of sulfur, and the effect of temperature-induced stress. Documentation of cell changes at elevated temperatures has been the fourth area of study. Changes seen in CdTe J-Vcurves are almost certainly related to diffusion of copper from the back contact, but in most cases an activation-energy model predicts sufficient stability for a 30-year product lifetime. Transients seen in some CIS cells are primarily seen in fill-factor and are primarily driven by voltage bias rather than illumination condition. The final area of emphasis has been numerical simulations ofCdTe and CI(G)S cells. Results with CdTe are able to replicate experimental data, have explained the effects of partial overlap of the primary junction with the back contact, and have shown how variations in carrier density, carrier lifetime, and CdTe layer thickness impact cell performance.