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Our group studied the effects of conduction band offset of window/Cu(In,Ga)Se2 (CIGS) layers on CIGS-based solar cell performance. To control the conduction band offset, we considered the use of a window layer of Zn1−xMgxO thin film with a controllable band gap as an alternative to the conventional window layer using CdS film. From the measurement of valence band offset between Zn1−xMgxO/CIGS layers and the band gap of each layer, we confirmed that the conduction band offset of Zn1−xMgxO/CIGS layers could be controlled by changing the Mg content of the Zn1−xMgxO film. The CIGS-based solar cells prepared for this study consisted of an ITO/Zn1−xMgxO/CIGS/Mo/soda-lime glass structure. When the conduction band minimum of Zn1−xMgxO was higher than that of CIGS, the performance of CIGS-based solar cells with a Zn1−xMgxO window layer was equivalent to that of CIGS-based solar cells with CdS window layers. We confirmed that the control of the conduction band offset of the window/CIGS layers decreases the majority carrier recombination via the Zn1−xMgxO/CIGS interface defects.

A series of experimental investigations on optical and optoelectronic properties of methane- and ethylene-based a-SiC:H films has been made. The chemical bonding structure of two kinds of a- SiC:H films has also been explored from infrared (IR) absorption structural analysis. An experimental verification for the wide gap window material in the amorphous silicon solar cell is shown on methane- and ethylene-based a-SiC:H. The methane-based a-SiC:H film shows one or two orders of magnitude larger photoconductivity recovery effect of doping than the ethylene- based one. IR absorption analysis shows that the methane-based a-SiC:H film is recognized as a rather ideal amorphous SiC alloy as compared with the ethylene-based one. It has been found through these investigations that the methane-based a-SiC:H film is superior to the ethylene- based one as a window material. Utilizing the methane-based a-SiC:H, an 8% efficiency barrier has been broken through with an a-SiC:H/a-Si:H heterojunction structure.

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