Research teams at the French Center for Nanosciences and Nanotechnologies (C2N) and Germany's Fraunhofer ISE have reported achieving 19.9% of power conversion efficiency for solar cells made with an ultrathin absorbing layer of 205 nm-thick gallium arsenide (GaAs) on a nanostructured back mirror.
Thinning the absorber from the usual 1 micron thick layers of semiconductor material or even 40 microns or more in case of silicon that's needed for solar cells with 20% efficiency, has its own challenges as it reduces sunlight absorption and eventually conversion efficiency.
The research at C2N and Fraunhofer ISE dealt with the problem by controlling the fabrication of patterned mirrors at the nanometer scale and created multiple overlapping resonances in the solar cell which the team identify as Fabry-Perot and guided-mode resonances. Numerous such resonances helped the spectrum to be absorbed by the visible to the infrared.
Fabrication of patterned mirrors was done by using nanoimprint lithography to directly emboss a sol-gel derived film of titanium dioxide which they say is a rapid and scalable technique.
The team believes there is potential to achieve 25% efficiency in the short term for ultrathin solar cells.
The research work has been published in the journal Nature Energy.