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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - COLIBRI (Carrier-selective contacts for silicon photovoltaics based on broadband-transparent oxides)

Teaser

Summary

This project aimed at a cost-effective efficiency enhancement of silicon-based solar cells and thereby an increase of the competiveness and profitability of photovoltaic systems. Within this project, two strategies were investigated: (1) the use of metal oxide based carrier-selective contacts to reduce losses in Si solar cells caused by both highly-doped regions and defective metal/semiconductor interfaces; and (2) the development of Si-based tandem solar cells.

(1) Since decades, crystalline Si (c-Si) solar cells are the most established photovoltaic technology. Their main advantages are long lifetime (>25 years), non-toxicity and the high abundance of Si. However, for full competitiveness with traditional sources of electricity, important new steps need to be taken to increase their performance. An innovative contacting scheme has been investigated which eliminates the main loss mechanisms in c-Si solar cells arising from doped pn-junctions and the direct contact of metal with Si. The novel contacts generate a highly passivating and carrier-selective interface to Si and enable solar cells without doped pn-junctions. No cost-intensive patterning technique is required for the device fabrication and parasitic optical absorption, as present in Si heterojunction solar cells, can be minimized.

(2) The theoretical efficiency limitation of Si single-junction solar cells of 29.4% can be overcome by their application in multi-junction solar cells. Adding a wide-bandgap top cell to a Si cell allows better exploitation of the blue part of the solar spectrum and thus theoretical one-sun efficiencies over 40%. For optimum tandem cell performance, the top cell needs to feature a bandgap energy between 1.6 and 1.9 eV and operate close to its thermodynamic limit. Therefore, Si-based multi-junction solar cells were developed and optimized within this project and their cost competiveness was evaluated by a techno-economic analysis.

Work performed

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 200Â°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 (NeuchÃ¢tel, 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.

Final results

Our experiments show that carrier-selective contacts can be used for the fabrication of >20% efficient wafer based silicon solar cells, which is an important step towards the departure from the impurity-doping paradigm that dominates since decades crystalline silicon photovoltaics. Those contact structures are of increasing interest and are currently investigated by all leading silicon PV research institutes throughout the world. It can be expected that solar cells with dopant-free asymmetric heterocontacts will soon reach efficiencies close to 25% and thereby become more relevant for commercial cell manufacturers.
This project also showed that the efficiency limitation of Si photovoltaics can be overcome by combining them with wide-bandgap top cells. The new record one-sun efficiencies of >32% achieved with mechanically stacked III-V/Si tandem solar cells display a breakthrough and raised attention in the photovoltaic community. Our techno-economic analysis showed that the fabrication costs of III-V top cells need to be further reduced â€“especially those for growth substrates and MOCVD deposition- to reach cost competitiveness. If the future development of reliable but inexpensive top-cells is successful, the next generation of photovoltaic systems will provide nearly twice the power of todayâ€™s installations enabling significantly reduced balance of systems costs and thus lowering the costs of renewable energy.