Light-trapping nanostructures in ultrathin GaAs solar cells : towards a record efficiency

Abstract: Producing electrical power with photovoltaic solar panels is very attractive as the sun constitutes an inexhaustible source of clean energy. However, in order to make it a better alternative to other energy sources, its drawbacks must be addressed. In particular, scientific research is very active to find technologies that increase the efficiency of the solar cells, or decrease their cost. Reducing the thickness of thin-film solar cells would serve this second objective by reducing the amount of material needed. It also paves the way towards third-generation solar cells, e.g. nanowire solar cells, because the effective lighttrapping techniques that are developed and implemented will be needed in those future devices. This master’s thesis aims at implementing a theory of multi-resonant absorption to ultrathin GaAs solar cells. The absorption is based on resonant photonic and plasmonic modes, excited via a periodically nanostructured mirror at the rear side of the cell. A process for the fabrication of solar cells implementing such a nanostructured mirror was developed, and solar cells with simpler designs have already been made, with a 200 nm thick absorber. They have been characterized and exhibit quite high Fill-Factor (82 %) and open-circuit voltage (1.03 V) that are not degraded after transfer on another substrate, which is promising for the final cell. The short-circuit current exhibited a value of 16.1 mA.cm-2 without anti-reflection coating, and with only a flat mirror at the rear side. Based on numerical simulations it is supposed to increase by 8.5 mA.cm-2 with the addition of an Anti-Reflection Coating (ARC) and with the nanostructured mirror. Taking this contribution into account, we expect a short-circuit current of 24.6 mA.cm-2 and an efficiency of 21 %, that have never been reached for such thin solar cells (200 nm). The process development is still ongoing for this final design, and we look forward to complete the fabrication of a device.