The Need for Photovoltaic Unit Operations Experimentation

Summary

In order to develop a rational design for commercial scale manufacture of thin-film photovoltaic cells, a process engineering research effort must be coordinated with the bench scale experiments which focus on basic physics, materials science, and device engineering. From the viewpoint of manufacturing, the photovoltaic cell needs to be considered as both a chemical product and an electronic device. A process for manufacturing solar cells, which is both economical and reliable, must be capable of producing millions of square meters per year. The scale of such an operation is similar in technical, capital, and labor requirements to facilities for production of commodity chemicals such as methanol and ethylene.

Unit operations experimentation has long been a part of process research and development in the chemical process industries. In this paper, we show that unit operations research is necessary for the developing photovoltaics industry. To illustrate, we discuss the development of a commercial scale process design for deposition of the active semiconductor layers of the cadmium sulfide-copper sulfide solar cell. Coupled heat and mass balance equations were developed to model the evaporation of cadmium sulfide. After verification at the bench scale, the model was used to design a continuous reel-to-reel vacuum coater for deposition of cadmium sulfide. This vacuum deposition unit, which in its present configuration has a throughput of 120 cm² per minute, has been purchased and installed at the University of Delaware. The results of some initial experiments will be discussed.