Nano-antenna arrays for next-generation solar cells – NANOCELL

Reducing the absorber thickness is one of the main issues for most photovoltaic technologies, because of the material cost or scarcity, and potential efficiency improvements with higher carrier concentration. Hence, highly efficient energy conversion in ultra-small volumes of semiconductors is a major challenge for next-generation solar cells. However, it requires the development of novel light-trapping strategies, semiconductor growth techniques and nanofabrication processes scalable to large surface areas.

Following these guidelines, an ideal solar cell would be made of semiconductor nanostructures (nanocell photovoltaic devices) and can be drawn as follows: it is composed of very small volume absorber (typical dimensions 100 nm), with high light absorption and carrier collection efficiencies, and low parasitic losses. In principle, this strategy should result in significant efficiency improvement due to less bulk recombination and higher carrier concentration (higher Voc), and a strong semiconductor material saving. Moreover, this ideal solar cell would be fabricated by bottom-up processes on reusable substrates and transferred on host substrates, in order to guaranty large scale and low cost fabrication.

In this project, we aim at developing new architectures, technologies and methods for next-generation solar cells with ultra-low absorber volumes. The goals of this project are twofold:
(1) to pave the way towards fully optimized (optically, electronically, technologically) nanocell photovoltaics devices made of semiconductor nanostructures fabricated by bottom-up approaches (localized growth, in particular),
(2) to have short-term impact on thin-film photovoltaics by developing new technological processes (nanopatterning, heterostructure epitaxial growth, and transfer processes with scalable and potentially low cost fabrication techniques) and tools (characterization and modeling).

As an intermediate step towards the final design, we will fabricate a GaAs nanowire solar cell on inactive silicon substrate, before the development of the final goal of the project: a GaAs nanowire-based solar cell with fully optimized light trapping, transferred on a metallic back contact-mirror on glass. Nanocell solar cells will be based on the localized growth of III-V nanostructures (GaAs) on Si substrates, with the aim of reaching small volume solar cells, at the frontier of the possible volume reduction. The optimization of the nanophotonic absorption enhancement with an efficient material system will lead to concepts transferable to other polycristalline thin film technologies. Importantly, we propose to use small aspect ratios nanowires, leading to less transport issues and less interface recombination than in hitherto proposed nanowire solar cells. High efficiencies can only be achieved if this approach is coupled to advanced nanophotonics to avoid a strong photocurrent loss penalty.

The selective growth of well-organized nanowire arrays on Si substrates, that will be transferred on host substrates, and combined with optimized nanophotonic/plasmonic designs and efficient passivation, are the main issues that will be addressed in this project. The high-level expertise gathered by the partners of this consortium covers the wide scope of activities required, and will enable to tackle these issues. Each of these achievements will put our results at the highest level as compared to current state-of-the-art, and will pave the way towards next-generation solar cells. We target 20% conversion efficiency.