Technology Support for Initiation of High-Throughput Processing of Thin-Film CdTe PV Modules: Phase 1 Technical Report, 14 March 1995 - 13 March 1996

During the first phase of this subcontract, we made progress in the important areas of stability, advanced deposition techniques, efficiency, the back contact, no-contact film diagnostics (photoluminescence), and cadmium waste control. The progress in stability was in both the demonstration of devices maintaining at least 90% of the initial efficiency of more than 19,000 hours of continuous lightsoak and the development of methods that can accurately predict long-term behavior based on the first 5,000-10,000 hours of life. We conducted experiments to determine if device behavior could be accelerated with thermal or voltage stresses. Notable achievements in advancing the state of the art in deposition technology include depositing CdTe on a 3600-cm2 substrate at 600 torr and designingand fabricating a new deposition feed system with a remote semiconductor source. Mathematical modeling of the deposition has permitted a more rapid development of a large-area coater. The efficiency has been increased on small-area devices to 13.3% by decreasing the thickness of the CdS devices and of the glass substrate. Work also focused on using a high resistivity SnO2 buffer layer betweenthe transparent conductive oxide and thin CdS to help preserve the open-circuit voltage while increasing the current-density. The back contacting process has been simplified by replacing the wet post-deposition etch with a vapor Te deposition step on small area devices. Results show that the devices perform comparably in efficiency but better in stability under light-soaking and open-circuitconditions. Preliminary studies of the correlation between CdS photoluminescence after the chloride treatment and the final device efficiency showed a positive correlation that may be applicable for in-line quality control. We also successfully demonstrated the prevention of at least 99.9% of all incoming Cd from leaving in an uncontrolled manner through the land, air, or water.