Metallic Nanoparticles and Nanostructure for Light Trapping

One major challenge for photovolatics solar cell is to improve its light-to-electrical efficiency while reducing its cost and weight. Thin film solar cells satisfy the demand of the high efficiency, low cost, and low weight. The hydrogenated alloy of amorphous silicon (a-Si:H) has high optical absorption than many other materials, a-Si:H thin film solar cells are able to improve the absorption efficiency, since the minority carrier diffusion length is less than or around 300nm for amorphous silicon. However, there is a limitation in all thin film solar cell technologies, the limitation is that red spectrum absorbance is too low. Excitation of surface plasmons via scattering from noble metal nanostructures was proposed to overcome the limitation, to enhance light-trapping efficiency, and to increase light absorption in red spectrum.  We investigated a sandwich-like solar cell: a-Si:H thin film between a top layer of nano-particles and a bottom layer of metallic structures. We studied the optical absorption efficiency of the a-si:H thin film with nano-metallic particles, and investigate the size and shape of these nano-particles. Our investigations show that for a 100nm thick a-Si:H thin film, deposit an array of nano-metallic cubes or cylinders will increase the optical absorption in the red light (e.g. 650nm) dramatically. We also see the strong light trapping effect of the bottom metallic nano structures as a back reflector.