High-Efficiency Thin-Film Cadmium Telluride Photovoltaic Cells: Annual Technical Report, 20 January 1996 - 19 January 1997

The University of Toledo (UT) photovoltaics group was instrumental in developing radio-frequency (rf) sputtering for CdS/CdTe thin-film solar cells. During the third phase of the present contract, the work focused on efforts to determine factors that limit the efficiency in 'all- sputtered' thin-film CdTe solar cells on soda-lime glass. Researchers found that all-sputtered cells, which aredeposited at substantially lower temperature than those by sublimation or vapor deposition, require less aggressive CdCl2 treatments than do other deposition techniques and this is presumably related to CdS/CdTe interdiffusion. The CdS/CdTe interdiffusion has been studied by several methods, including photoluminescence and capacitance-voltage measurements. Researchers also deposited special thinbilayer films on quartz and borosilicate glass. Also, to better understand the properties of the ternary alloy material, they used laser physical vapor deposition to prepare a series of CdSxTe1-x films on borosilicate glass. In addition to efforts focused on factors that may be unique to rf- sputtered cells, researchers participated in activities emphasizing common issues in CdTe thin-film solarcells. Researchers prepared cells on both 7059 borosilicate glass with conducting tin oxide layers deposited at the University of South Florida (USF) and on soda- lime glass with textured SnO2 from LOF. Including the work from the previous phase, researchers have now fabricated seven types of cells with CdS thicknesses of 300, 200, 100, 80, 60, 20, and 0 nm. For each CdS thickness, cells on the7059 and soda-lime superstrates were prepared side-by-side through all steps in the preparation to facilitate direct comparisons. The 7059/USF SnO2 superstrates had much better performance for CdS thicknesses of 100 nm and below, with the major differences occurring in the open- circuit voltage. One set of cells prepared without CdS by sputtering CdTe directly on the 7059 USF SnO2 superstratesshowed an initial Voc of 760 mV and initial efficiency of 8.7%. Other characterization measurements in use at UT include temperature-dependent Hall effect and electrical conductivity, current-voltage (I-V), spectral quantum efficiency (SQE), and frequency-dependent capacitance- voltage (C-V) measurements.