Researchers at the French Centre de Nanosciences et de Nanotechnologies (C2N) have collaborated with researchers at Fraunhofer Institute for Solar Energy Systems (ISE) and others to efficiently capture the sunlight in a solar cell on an ultrathin absorbing layer made of 205 nm-thick gallium arsenide (GaAs) on a nanostructured back mirror. This new process of fabrication achieved an efficiency of nearly 20%.

Thinning the absorber reduces sunlight absorption and conversion efficiency. To date, the solar cells that were made from GaAs needed a one-micrometer thick layer of semiconductor material to deliver 20% efficiency. They can even be 40 micrometers (µm) or more in the instance of silicon. Reducing the thickness helps save rare materials and improves industrial throughput due to shorter deposition time. Earlier efforts of capturing the sunlight had significantly limited the performance due to optical and electrical losses.

The research team was led by Stéphane Collin at the C2N (CNRS/ Université Paris-Saclay), in collaboration with Fraunhofer ISE. The research group has arrived at a methodology that traps light in ultrathin layers made of 205 nm-thick GaAs, a semiconductor of the III-V family.

“Together with our French partners, we are very happy about this result and its publication in the renowned journal, Nature Energy. We expect to be able to even further improve the efficiency of ultrathin solar cells,” said Dr. Frank Dimroth, Head of the Department of III-V Photovoltaics and Concentrator Technology at Fraunhofer ISE.

The objective was to conceive a nanostructured back mirror to create multiple overlapping resonances in the solar cell, identified as Fabry–Perot and guided-mode resonances. They constrain light to stay longer in the absorber, resulting in efficient optical absorption despite the low quantity of material.

With many resonances, absorption is enhanced over a large spectral range that fits the solar spectrum from the visible to the infrared. Controlling the fabrication of patterned mirrors at the nanometer scale was critical in the project. The team used nanoimprint lithography to directly emboss a sol-gel derived film of titanium dioxide, an inexpensive, rapid and scalable technique.

Due to the high cost, GaAs solar cells are commercially limited to space application. However, researchers are already working to extend this concept for large-scale photovoltaics made of CdTe, CIGS, or silicon materials.

Recently we reported on Rice University scientists designing arrays of aligned single-wall the carbon nanotubes to channel mid-infrared radiation and significantly raise the efficiency of solar energy systems. According to the research, the carbon nanotube can be just the device to make solar panels that lose energy through heat far more efficiently.