Light trapping in thin-film solar cells via scattering by nanostructured antireflection coatings

The use of nanostructured TiO2 layers fabricated on thin-film solar cells to provide, simultaneously, both antireflection functionality and light trapping via scattering of long-wavelength photons into guided optical modes is demonstrated and analyzed in thin-film quantum-well solar cells. Nanosphere lithography is used for fabrication of periodic arrays of subwavelength-scale TiO2 structures, and separation of active device layers from their epitaxial growth substrate and integration with the nanostructured TiO2 layer enables increased optical absorption via coupling to both Fabry-Perot resonances and guided lateral propagation modes in the semiconductor. The nanostructured TiO2 layer is shown to act as a graded-index coating at optical wavelengths and simultaneously to scatter incident light into guided optical modes within the device. The dependence of these effects on angle of incidence is also analyzed.