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.
Rapid Cu(In,Ga)Se/sub 2/ growth using "end point detection"
Thin film Cu(InGa)Se/sub 2/ (CIGS) is grown using a two stage co-evaporation process where all of the Cu is evaporated in the first stage. Near the end of the second stage, a Cu-rich to Cu-poor transition occurs, where the power delivered to the substrate heater in order to sustain a constant substrate temperature, changes as a result of a change in the radiative behavior of the CIGS film. The output power signal is shown to respond quickly to, and be characteristic of the film composition near the transition. Using this signal to monitor the deposition process results in excellent control of the final Cu content, even when the evaporation rates are poorly known and poorly controlled. High quality devices result, even at high evaporation rates. Solar cells with efficiencies close to 15 % have been produced from CIGS deposition times below 15 minutes and are only marginally better for deposition times of up to 45 minutes, and this at constant substrate temperatures of 500/spl deg/C.
Properties of Chalcopyrite-Based Materials and Film Deposition for Thin-Film Solar Cells
The development of chalcopyrite-based solar cells started in the early 1970s, when Wagner et al. [1] realized a 12% efficient solar cell based on a CuInSe2 single crystal. A few years later, Kazmerski et al. [2] were able to demonstrate the first thin-film solar cell by evaporation of CuInSe2 as a compound. Even though these results were very promising it took two decades until the first commercial CIGS modules were produced. The full potential of the material was only reached after some important modifications of the growth and composition of the absorber film. The addition of Ga stabilized p-type conductivity and increased the bandgap. Sodium-containing glass, or the deliberate addition of Na to the growth process, was the key for further controlling film growth and free carrier concentration. These developments were mostly empirical and sometimes serendipitous. However, due to the involvement of more research teams world-wide basic understanding has been improved as a prerequisite for reproducible manufacturing processes.