Chalcopyrite absorbers

Klenk, M. Schenker, O. Alberts, V. Bucher, E. 2001. Properties of flash evaporated chalcopyrite absorber films and solar cells. Thin Solid Films 387 47 19. 

CuInSi (and, even more, CuInSei) are strong candidates for thin-film photovoltaic cells. For this purpose, the chalcopyrite structure (which is an ordered lattice) is preferred over the disordered, zincblende form. Due to the large absorption coefficients of these materials, a 1-iJim-thick film is more than enough to absorb almost all the suprabandgap radiation. Somewhat thicker films are generally used, due to problems of pinholes, which commonly occur in thinner films. A number of methods have been used to deposit these films. Surprisingly, very few (published) attempts have been made to deposit them by CD. 

Fig. 9.2. Scanning electron micrograph of the cross-section of a typical chalcopyrite solar cell with Cu(In,Ga)Se2 (CIGSe) absorber (substrate now shown). Reprinted with permission from [1]...

Since in the chalcopyrite module the ZnO films are the last to be deposited, the processing must be compatible with the remainder of the cell structure. This implies in particular that substrate temperatures must be limited to 200-250°C [10] even though better ZnO properties could be achieved at higher deposition temperatures. Interdiffusion at the absorber/buffer interface has been made responsible for the instability [27] but it is believed that a detailed study using current state of the art material would be required to clarify this point. 

The standard molybdenum contact is characterized by low optical reflection, which becomes relevant in efforts to reduce the absorber thickness (light trapping). A study of other metals has not identified clearly promising alternatives [78], Hence, an ohmic contact between TCO and chalcopyrite with good electrical and optical properties could also be useful in developing a cell with a high reflectivity metal/TCO back contact. 

Equation 1.20 requires that Se(0) is absorbed onto a deposition surface containing Cu2Se. When sufficient Se(0) is available it is then consumed to form CuSe2 at the surface which has been shown to be the site where In reacts to form the CuInSe2 chalcopyrite compound [109]. This proposed mechanism requires, of course, that the electrode surface should be continually refreshed in Cu2Se and Se. 

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