21% efficient silicon heterojunction solar cells with MoOx based front contacts, that are compatible with screen-printing as high-throughput grid metallization technique, were developed. It was found, that hydrogen effusion occurring at temperatures as low as 200°C alters the electrical properties of the used a-Si:H/MoOx/TCO front stack and thus disturbs the carrier transport and reduces the fill factor in the solar cell. This effect can be mitigated by the use of hydrogen lean buffer and conduction layers [S.Essig et al.,under review, 2017].
In collaboration with UC Berkeley and ANU, innovative silicon solar cells with dopant-free asymmetric heterocontacts (DASH) were developed and optimized. DASH solar cells (Figure 1) with MoOx based hole-selective front contact and LiF based electron-selective rear contacts achieved a record efficiency over 20% (AM1.5g).
III-V/Si multi-junction solar cells were developed and optimized in collaboration with NREL (Golden, Colorado) and CSEM (Neuchâtel, Switzerland). The mechanically stacked 4-terminal devices achieved one-sun dual-junction efficiencies up to 32.8% and triple-junction efficiencies up to 35.9%. The results were included in version 50 of the famous solar cell efficiency tables [M. Green et al., Prog. in Photov. 25(7), pp.668-676 (2017)] and published in Nature Energy [S. Essig et al., Nature Energy 2, 17144,(2017)].
Furthermore, a techno-economic analysis of >30% efficient III-V/Si tandem solar cells was performed together with NREL in order to evaluate the cost competitiveness of over 30% efficient III-V/Si tandem solar cells. It revealed a factor of ~15 disparity between the $/Watt costs for III-V/Si tandem cells and conventional Si single-junction solar cells using current process costs, but highlights a path to cost competitiveness if two technological challenges can be solved: development of low-cost III-V growth techniques and new substrate materials.
