Bandgap engineering of amorphous semiconductors for solar cell applications

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Abstract
Different bandgap engineering approaches are discussed with respect to their application in thin‐film solar cells based on hydrogenated amorphous silicon (a‐Si: H) and its alloys with Ge or C. After a survey of the different approaches reported so far in the literature, the main emphasis will lie on the application of a‐Si: H/a‐Si1‐xCx:H multilayers for use in the p‐doped window layer, or a graded bandgap a‐Si1‐xCx:Hintrinsic layer with varying Ge content x. But also the possibility of using an a‐Si: H/a‐Si1‐xCx:H multilayer as the i‐layer will be addressed. Intrinsic films of both multilayers and graded bandgap material have been deposited in different series, varying electronic well and barrier widths in the first case, and the amount, form or local position of a change of the bandgap in the second case. Physical properties of both classes of materials have been investigated thoroughly. the results are discussed with respect to an application in amorphous thin‐film solar cells, also taking into account results from the literature about the implementation of these heterostructures in devices. Conclusions for the future perspectives of these approaches are drawn.