Alternative device structures for CIGS-based solar cells with semi-transparent absorbers

In this work, we discuss two alternative structures of CIGS-based solar cells, a bi-facial configuration and a back-wall configuration. One of the critical issues in these structures is fabrication of a transparent back contact which facilitates not only ohmic contact for holes in the valance but also an effective reflector for electrons in the conduction band. In order to meet these requirements of the transparent back contact, a p-type amorphous Si (p-a-Si)/intrinsic amorphous Si (i-a-Si) bi-layer was deposited onto the ITO back contact. Absorber films were formed by a metal precursor reaction in H2Se and H2S, and their thicknesses were adjusted to about 0.25 irtm to enhance current collection from back-side illumination. The resulting devices with a bi-facial structure of SLG/ITO/p-Si/i-Si/Cu(In, Ga)(Se,S)(2)/CdS/i-ZnO/ITO/Ni-Al exhibited a comparable open-circuit voltage to a Cu(In, Ga)(Se,S)(2) cell on a Mo substrate in tests with illumination of both sides. Back-wall cells with a structure of SLG/ITO/p-Si/i-Si/Cu(In, Ga)(Se,S)(2)/CdS/i-ZnO/Ag-reflector were also fabricated and improved current collection by greater light trapping was achieved compared to the bi-facial cell with back-side illumination.