Device Physics of Thin-Film Polycrystalline Cells and Modules: Phase I Annual Report, February 1998 - January 1999

This report describes work done by Colorado State University (CSU) during Phase I of this subcontract. CSU researchers continued to make basic measurements on CI(G)S and CdTe solar cells fabricated at different labs, to quantitatively deduce the loss mechanisms in these cells, and to make appropriate comparisons that illuminate where progress is being made. Cells evaluated included the new recordCIGS cell, CIS cells made with and without CdS, and those made by electrodeposition and electroless growth from solution. Work on the role of impurities focused on sodium in CIS. Cells with varying amounts of sodium added during CIS deposition were fabricated at NREL using four types of substrates. The best performance was achieved with 10/sup -2/--10 /sup -1/ at% sodium, and the relative meritsof proposed mechanisms for the sodium effect were compared. Researchers also worked on the construction and testing of a fine-focused laser-beam apparatus to measure local variations in polycrystalline cell performance. A 1-?m spot was achieved, spatial reproducibility in one and two dimensions is less than 1 ?m, and photocurrent is reliably measured when the 1-?m spot is reduced as low as 1-sunin intensity. In elevated-temperature stress tests, typical CdTe cells held at 100 deg. C under illumination and normal resistive loads for extended periods of time were generally very stable; but those held under reverse or large forward bias and those contacted using larger amounts of copper were somewhat less stable. CdTe cell modeling produced reasonable fits to experimental data, includingvariations in back-contact barriers. A major challenge being addressed is the photovoltaic response of a single simple-geometry crystallite with realistic grain boundaries.