In contrast to the non-crystallinity of amorphous silicon, thin-film copper indium gallium diselenide (CIGS) is a polycrystalline material consisting of small crystallites approximately 0.5 - 1.0 μm in size. CIGS has several characteristics that make it a valuable PV material. One is its absorption coefficient, which is among the highest for semiconductor materials. Ninety-nine percent of the light incident on CIGS is absorbed in the first micrometer of the device. Thus, cells with a thickness of that order of magnitude are possible. Another favorable characteristics is that copper indium gallium diselenide has one of the highest current densities of any semiconductor material, with the potential to produce high current outputs. Third, these films retain their performance properties better than most semiconductors. And last, CIGS is amenable to large-area, automated production.

Efficiencies of 19% and higher have been reported for small-area, experimental cells made of thin-film CIGS. A principle problem with the material is its low open-circuit voltage. However, this deficiency seems to be correctable by improving compositional uniformity by for example, removing oxygen.

The CIS is usually formed on a base electrode of molybdenum (Mo), chosen for its refractory nature and good electrical conductivity. Thin-film CIGS is a p-type semiconductor, and a junction is formed at the surface by deposition of a very thin layer of CdS. This creates an n-p homojunction just inside the CIGS, rather than a simple heterojunction. It is also possible to form effective junction without the use of CdS. The device is completed by deposition of a transparent conductor such as zinc oxide on top of the junction to help collect the light- generated current. Figure below shows typical CIGS solar module. In a manner similar to the definition and monolithic integration of thin-film a-Si cells, individual CIGS cells are defined and serially interconnected via three patterning steps. The first scribe (in the molybdenum - Mo) is performed by a laser beam, while the second and third scribes (to remove CIGS and separate the ZnO) can be performed mechanically or by laser. Again, metal foils are bonded to the first and last cells, and the module is encapsulated using a top cover glass laminated with encapsulant.

CIGS module cross section

Cross-Section of Tandem Junction CIGS Device

Monolithic Integration of CIGS Plate

Glass-Glass Laminated Module