  Ultrathin solar cells reduce material usage and allow the use of
lower-quality materials thanks to their one order of magnitude smaller
thickness than their conventional counterparts. However, efficient photonic
light-trapping is required to harvest the incident light efficiently for an
otherwise insufficient absorber thickness. Quasi-random photonic crystals are
predicted to have high efficient light-trapping while being more robust under
angle and thickness variations than simple photonic crystals. Here we
experimentally demonstrate a light-trapping solution based on quasi-random
photonic crystals fabricated by polymer blend lithography. We control the
average lattice parameter by modifying the spin-coating speed. We demonstrate
an ultrathin GaAs cell of 260 nm with a rear quasi-random pattern with
submicron features, and a Jsc =26.4 mA/cm2 and an efficiency of 22.35% under
the global solar spectrum.
