Technology support for high-throughput processing of thin-film CdTe PV modules: Annual technical report, Phase 1, 1 April 1998--31 March 1999

This report describes work performed by First Solar, L.L.C., during Phase 1 of this 3-year subcontract. The research effort of this subcontract is divided into four areas: (1) process and equipment development, (2) efficiency improvement, (3) characterization and analysis, and (4) environmental, health, and safety. As part of the process development effort, the output of the pilot-production facility was increased. More than 6,200 8-ft{sup 2} CdS/CdTe plates were produced during Phase 1--more than double the total number produced prior to Phase 1. This increase in pilot-production rate was accomplished without a loss in the PV conversion efficiency: the average total-area AM1.5 efficiency of sub-modules produced during the reporting period was 6.4%. Several measurement techniques, such as large-area measurement of CdS thickness, were developed to aid process improvement, and the vapor-transport deposition method was refined. CdTe thickness uniformity and reproducibility were improved. From a population of more than 1,100 plates, the mean standard deviation within a plate was 7.3% and the standard deviation of individual-plate averages was 6.8%. As part of the efficiency-improvement task, research was done on devices with thin-CdS and buffer layers. A cell with 13.9% efficiency was produced on a high-quality substrate, and higher than 12% efficiency was achieved with a cell with no CdS layer. A number of experiments were performed as part of the characterization and analysis task. The temperature dependence of CdTe modules was investigated; the power output was found to be relatively insensitive (<5%) to temperature in the 25 to 50 C range. As part of the characterization and analysis task, considerable effort was also given to reliability verification and improvement. The most carefully monitored array, located at the NREL, was found to have unchanged power output within the margin of error of measurement (5%) after 5 years in the field. The first round of National CdTe Team stability tests were concluded. One back-contact formulation resulted in cells that increased in efficiency as a result of 9,700 hours of light soaking. As part of the environmental, health, and safety task, an emissions survey was performed for the pilot-production facility. For production of 360 modules/day, it was predicted that the cadmium emissions would be only 0.015% of the level that would require any permitting; however, methanol emissions may require permitting if anticipated process changes are not implemented. Process improvements in edge delete, CdS material preparation, waste compaction, CdCl{sub 2}-vapor collection, and wastewater treatment were made, resulting in reduced costs, reduced emissions, and improved operator safety