Low-refractive-index nanoparticle interlayers to reduce parasitic absorption in metallic rear reflectors of solar cells

Metal reflectors or electrodes in contact with optoelectronic devices can induce parasitic light absorption. A low-refractive-index (low-n) layer inserted between the metal reflector and the optically active layer(s) reduces this absorption. We investigate the use of porous, nanoparticulate films as low-n layers, and fabricate silicon solar cells with nanoparticle/silver rear reflectors. We vary the porosity and thus n (between 1.1 and 1.5) of the nanoparticle films, which are deposited by a controllable aerosol spray process, and investigate their effectiveness in reducing infrared parasitic absorption in the solar cells. Optical test structures incorporating films with the highest n exhibit an internal reflectance of over 99%, matching best-in-class structures; lower-n layers should in theory perform better still but their rougher surfaces appear to induce plasmonic absorption in the overlying silver layer. No loss in open-circuit voltage or fill factor is observed when applying the best nanoparticle films in silicon heterojunction solar cells, enabling efficiencies similar to those achieved with reference cells that employ a thick indium tin oxide layer between the wafer and the rear silver electrode.