Optical monitoring and control of three-stage coevaporated Cu(In1−xGax)Se2 by real-time spectroscopic ellipsometry
Real-time spectroscopic ellipsometry (RTSE) has been applied for in situ monitoring and control of thin-film copper-indium-gallium-diselenide, i.e., Cu(In1−xGax)Se2 (CIGS), deposition by high vacuum coevaporation in the three-stage process used for efficient photovoltaic devices. Initial studies have been performed on a ∼0.7-µm CIGS layer deposited on crystal silicon to minimize surface roughness and to develop an accurate structural/ optical model of the Cu-poor-to-Cu-rich and Cu-rich-to-Cu-poor transitions that define the ends of the second (II) and third (III) stages of growth, respectively.With a better understanding of the surface achieved through this model, correlations can be made between the surface state and the unprocessed RTSE data {ψ(t), Δ(t)}. During deposition in the solar cell configuration with 2- µm-thick CIGS on a Mo-coated glass substrate, indications of the Cu poor-to-rich and Cu rich-to-poor transitions appear clearly in {ψ(t), Δ(t)}, enabling direct control of stage II and III transitions. The transition times deduced optically are in good agreement with those identified from the film/substrate emissivity by tracking the substrate heater power. It is clear, however, that RTSE can provide higher sensitivity to these transitions and is, therefore, suitable for improved control of three-stage CIGS deposition.
Cu(In,Ga)Se2 thin films grown with a Cu‐poor/rich/poor sequence: growth model and structural considerations
Thin films of Cu(In,Ga)Se2 are grown by a co‐evaporation process in which the In, Ga, and Se fluxes, as well as the substrate temperature, are constant and the only variable is the Cu flux. This Cu flux varies in three steps in such a way that the growing film evolves from Cu‐poor to Cu‐rich and then back to Cu‐poor. The film growth is monitored by the ‘end point detection’ method, and film thicknesses of the order of 2 μm are deposited in less than 20 min. Quality devices (efficiencies above 15%) are produced in our baseline processes for all of the other synthesis steps. The Cu(In,Ga)Se2 layers are studied from a (112) versus (220) (204) orientation and recrystallization point of view. Including the results from a previous study on the influence of the substrate temperature to the present X‐ray diffraction and scanning as well as tunneling electron microscopy data, a five‐stage growth model for the films is described. The specific features of these films are that they are weakly (220) (204) oriented and exhibit crevices in their top fractions. The growth model hypothesizes about the origins of these crevices and on how to avoid them. Copyright © 2003 John Wiley & Sons, Ltd.
Prediction of net pressure on pressure equalized cavities
Abstract A numerical model has been developed for prediction of net pressure on pressure equalized cavities. The results based on this model show good agreement with those obtained from mock-up tests. Parametric studies verify that reductions in cladding load are possible, with proper design of curtain walls.