The first group of chalcogenide solar cells that we discuss are chalcopyrite solar cells. The name of this class of materials is based on chalcopyrite (copper iron disulfide, CuFeS2). Like all the chalcopyrites, its forms tetragonal crystals.
Many chalcopyrites are semiconductors. As they consist of elements from groups I, III, and VI, they are also called I-III-VI semiconductors or ternary semiconductors. In principle, all these combinations can be used:
The most common chalcopyrite used for CHALCOGENIDE SOLAR CELLS is a mixture copper indium diselenide (CuInSe2, CIS) and copper gallium diselenide (CuGaSe2, CGS). This mixture is called copper indium gallium diselenide [Cu(InxGa1-x)Se2, CIGS], where the x can vary between 0 and 1. Several research groups and companies use a compound that also contains sulfur; it is called copper indium gallium diselenide/disulfide [Cu(InxGa1-x)(SeyS1-y)2, CIGSS], where y is a number in between 0 and 1.
The physical properties of CIGS(S) are rather complex and many different views exist on these properties among scientists. CuInSe2 has a band gap of 1.0 eV, the bandgap of CuInS2 is 1.5 eV and CuGaSe2 has a band gap of 1.7 eV. By tuning the In:Ga ratio x and the Se:S ratio y, the band gap of CIGS can be tuned from 1.0 eV to 1.7 eV. As CIGS(S) is a a direct band gap semiconductor
material, it has a large absorption coefficient, hence an absorber thickness of 1-2 μm is sufficient for absorbing a large fraction of the fraction of the light with energies above the band gap energy. Also the typical electron diffusion length is the order of a few micrometers. CIGS(S) is a p-type semiconductor, the p-type character resulting from intrinsic defects in the material that amongst others are related to Cu deficiencies. The many different types of defects in CIGS(S) and their properties are a topic of ongoing research.
below Figure (a) illustrates a typical CIGS solar cell structure deposited on glass, which acts as a substrate. On top of the glass a molybdenum layer (Mo) of typically 500 nm thick is deposited, which acts as the electric back contact. Then, the p-type CIGS absorber layer is deposited with a thickness up to 2 μm. Onto the p-CIGS layer, a thin n-CIGS layer is deposited, for example an indium/gallium rich Cu(InxGa1-x)3Se5 alloy. The pn-junction is formed by stacking a thin cadmium sulfide (CdS) buffer layer of around 50 nm thickness onto the CIGS layers. The n-type region is extended with the TCO layer, that also is of n type: First an intrinsic zinc oxide (ZnO) layer deposited followed by a layer of Al doped ZnO. The Al is used as an n dopant for the ZnO.
Similar to thin film silicon technology, the n-type TCO acts as the transparent front contact for the solar cell.
