Thin film absorbers for tandem solar cells: an industrial perspective

Abstract

Tandem solar cells have received a lot attention from academia and industrial researchers as the potential next-generation PV technology, with higher efficiency above the limit of single-junction solar cells. Thin-film/thin-film (TF/TF) tandems are attractive due to similar toolset and processes producing the top and bottom cells, which improve scalability and promote cost reduction compared to TF/wafer tandem technologies. TF/TF/tandems additionally offer more absorber bandgap flexibility that promotes photovoltaic conversion efficiency optimization. Many materials not suitable for single junction solar cells can be explored as tandem top or bottom cells. To assess the practical efficiency potential of tandem solar cells limited by non-ideal material and device quality, we present a Shockley-Queisser-like efficiency calculation for tandem devices consisting of non-ideal top and bottom cells and with a range of absorber band gaps. The non-ideality is introduced through an experimentally measurable external radiative quantum efficiency (ERE). We find that a range of top and bottom cell band gaps enabling the highest tandem efficiency shifts from the ideal Shockley-Queisser case and depends on the top and bottom cell ERE. Furthermore, tandem cell efficiency greater than 37% can be achieved with very modest top/bottom cell EREs, for example of only 0.008%/0.5% which is typical for CdTe/CIS cells. Our results indicate that high efficiency tandem solar cells have good probability to be manufactured at high volume within a foreseeable future, despite non-ideal material and device quality due to early stages of development or constraint by manufacturing requirements. Finally, we review a number of mature and emerging thin film absorber material candidates for tandem applications. We discuss properties of these materials and the corresponding device performance as well as the associated technological challenges. We concludes on the promise of each of these materials for tandem applications that is expected to provide guidance to the photovoltaic research community.