Taking crystalline silicon technology to the next level
Traditional solar cells, where crystalline silicon (c-Si) is the dominant semiconducting material, have been a trustworthy photovoltaic (PV) technology for many decades. They have continuously delivered efficiency improvements and cost reduction. But today, devices perform near to their theoretical limits. This calls for tandem devices that have significantly higher efficiency potential. The EU-funded SiTaSol project worked on the production of a dual-junction solar cell, using gallium arsenide phosphide (GaAsP) and silicon (Si). The GaAsP/Si solar cell is built on a new platform that combines low-cost Si wafers and high-throughput epitaxy. The solution is based on the principle of tandem technology, that is, the division of the solar spectrum into different spectral bands which are absorbed and converted in different absorbing materials – in this case, GaAsP and Si.
Towards an affordable and highly efficient solution
“The main goal of the SiTaSol project was to identify a technology for a III-V/Si tandem solar cell that reaches 30 % efficiency with a scalable process to the GW/year scale, leading to costs which are acceptable on the PV market,” explains Frank Dimroth, project coordinator. To achieve this, several components had to be investigated: the epitaxy process for III-V crystal layers; manufacturing the Si wafer ready for III-V growth; the formation of the Si bottom cell, including light-trapping; the processing of the crystals to solar cells with contacts; and the anti-reflection coating. SiTaSol built a new epitaxy machine with the largest current throughput and produced GaAsP/Si tandem solar cells with 20.8 % efficiency using a low-cost Si substrate and high growth rates of 100 µm/h. After adding a third junction, even 25.9 % were realised. Dimroth clarifies: “Not all directions were successful, but finally, we could demonstrate III-V/Si tandem solar cells that are close to our target. Of course, the efficiency is still lower than the one of hero devices, but it is encouraging that the combination of low-cost manufacturing and high performance can be reached.”
Innovation and experimentation pay off
The team successfully investigated new technologies for making Si substrates with a nearly perfect surface without using chemical mechanical planarisation polishing. Moreover, they implemented new processes to improve light absorption in Si and tried new ways of making metal contacts on the cells using ink-jet printing – a technique that shows promise if improved in future. Different methods were also investigated for combining the III-V absorber with Si – for example, with glueing as an alternative to direct growth. It was found that the most straightforward way to do this is to grow the very thin III-V layers (2-3 µm) directly onto the Si bottom cell.
Environmental benefits of tandem solar cell technology
High conversion efficiency has an ecological advantage: it means we need less module area, and consequently, less land and fewer materials for building the modules. “The results show that higher PV conversion efficiency is a driver towards lower environmental impact. The good news is that we could not find any major barriers on the way towards realising this product on large scale,” concludes Dimroth.
Keywords
SiTaSol, solar cells, efficiency, silicon, low cost, photovoltaic, PV, c-Si, crystalline silicon
