Manufacture of Thin-Film Solar Cells: Modeling and Control of Cu(InGa)Se2 Physical Vapor Deposition onto a Moving Substrate

Abstract

Absorber layers for thin-film Cu(InGa)Se2 solar cells are produced by co-evaporation from elemental effusion sources. To meet the product specifications on thickness and composition, the vapor flow rates from the sources need to be controlled tightly. The thickness and composition of the final product are measured by X-ray fluorescence (XRF). Using the cinematic technique, a discrete dynamic model of the deposition process has been developed, and together with the XRF model, it can be cast into a Wiener model structure. The nonlinear map is globally invertible, which allows for linearization and complete decoupling of the individual control loops. Two types of controllers were tested in simulation studies: model predictive control (MPC) and internal model control (IMC). For MPC, both controllers with output bias and state estimation were developed; for IMC, both first-order plus time delay and state space models were used. Controller performance for reference tracking is compared under input, parametric, and output uncertainty.