Resonant dielectric high-contrast gratings as spectrum splitting optical elements for ultrahigh efficiency (>50%) photovoltaics

Resonant dielectric gratings are explored as low cost spectrum-splitting optical elements in a photovoltaic device architecture that incorporates many independently connected subcells of different bandgaps for ultrahigh efficiency (>50%). Gratings can have broadband reflectivity by appropriately offsetting grating resonances. Subwavelength feature sizes suppress diffraction, and the high-refractive index of the grating layer leads to relatively angle-insensitive reflectance. Gratings can be fabricated by nanoimprint lithography, making them a scalable and economical option for photovoltaic applications. Using these gratings to create a series of dichroic filters to separate incident solar light into a series of bands which can then be coupled into independent subcells has potential for high efficiency by overcoming losses due to lack of absorption of subbandgap photons and thermalization of excited carriers which account for loss of over 40% of incident power on single-junction solar cells. Independent connection and physically separated subcells permit freer bandgap selection and relieve the current-matching constraint which limits conventional monolithic tandem multijunction cell efficiency.