(303q) Source Scale-up for Physical Vapor Deposition of Cu(Inga)Se2 on Flexible Substrates

Elemental in-line evaporation on flexible substrates in a roll-to-roll configuration is a commercially attractive process for the manufacture of large-area CuInSe2-based photovoltaics. At the University of Delaware's Institute of Energy Conversion (IEC), such a process at the pilot scale is being investigated for a polyimide web substrate. The process works well for 6-inch wide substrates and for short deposition runtimes. However, it is suspected that scale-up of the Cu(InGa)Se2 Physical Vapor Deposition (PVD) process onto wider webs and longer runtimes will lead to unacceptable Cu(InGa)Se2 film qualities due to issues associated with the thermal characteristics of the elemental sources, and the effect of melt level reduction with time. In addition, a commercially viable high throughput process for Cu(InGa)Se2 film deposition requires fast web speed, which entails higher material transport rates. Consequently, either the source temperature or the nozzle diameter and/or number has to be increased. Higher operating temperatures introduce problems associated with material robustness, reduced heater efficiency, and stability and reproducibility of thermocouples. Therefore, the source design problem is reduced to achieving nozzle effusion rates that give required film thickness and uniformity for as low an operating temperature as possible.

To design such a source, we need models that predict the vapor flow rates as well as vapor flux profiles for various nozzle geometries and melt temperatures. Direct Simulation Monte Carlo (DSMC) is a technique that can be used to study rarefied gas flows over a wide range of flow conditions, from molecular flow (Kn>10) to near continuum flow (Kn<0.01). The typical operating temperatures of copper, gallium and indium sources are such that their vapor flows are in transition flow regime (0.1