Scaleup of Cu(InGa)Se₂Thin Film Coevaporative Physical Vapor Deposition Process, 1. Evaporation Source Model Development

Abstract

Even though rapid advances have been made in improving Cu(InGa)Se 2 thin-film-based solar cell efficiencies, the breakthroughs have been limited to the laboratory scale. Most commercially viable thin-film technologies reside at the premanufacturing development stage and scaleup has proven to be much more difficult than expected. Elemental in-line evaporation on flexible substrates in a roll-to-roll configuration is a commercially attractive process for the manufacture of large-area CuInSe 2 -based photovoltaics. At the University of Delaware's Institute of Energy Conversion (IEC), such a process is being investigated, at the pilot scale, for a polyimide web substrate. The process works well for 6-in.-wide substrates and for short deposition runtimes. However, a commercially viable process is required to produce large-area, high-quality films at a muchhigher throughput. Specifically, the desired film thickness (∼2 µm) and composition uniformity must be achieved continuously and reproducibly on large-area (12-in.-wide) substrates at translation speeds (∼1 ft/ min) much higher than those currently used in pilot-scale processes. Although achievement of the desired film thickness and composition setpoints is best addressed by proper control system design, the film thickness uniformity is determined by the source design and individual nozzle effusion rates. The nozzle effusion rates, in turn, are dependent on the melt surface temperature profile and, thus, on the source design itself. Therefore, proper source design is critical in achieving film thickness uniformity. We have identified two modeling requirements for effective thermal evaporation source design: (i) a detailed three-dimensional thermal model of the evaporation source, to predict the melt surface temperature accurately, and (ii) a nozzle effusion model, to predict the effusion rate and the vapor flux distribution for a given nozzle geometry (length and diameter), melt surface temperature, and evaporant. To meet the first requirement, we have developed a first-principles three-dimensional electrothermal model using COMSOL Multiphysics software; the Direct Simulation Monte Carlo (DSMC) method has been used for effusion modeling. In this paper, which is the first of a two-part series, we present the details of the two models and their experimental validation.

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