High aspect ratio plasmonic nanocones for enhanced light absorption in ultrathin amorphous silicon films

Strategies for enabling high light absorption in ultrathin solar cell layers may contribute importantly to more viable photovoltatic. To this end, we investigate the effect of the enhanced near-field, associated with nanoparticle plasmon resonances, on the light absorption in ultrathin (20 nm) hydrogenated amorphous silicon (a-Si:H) films. In order to maintain the dipolar plasmon resonance above the a-Si:H optical gap, we employ high aspect ratio Ag nanocones coated with the a-Si:H by chemical vapor deposition. Experiments were performed for Ag/aSi:H nanocomposites on glass and on a spacer-reflector resonant cavity, used to boost and tailor the optical response. Finite element calculations were employed to model and extract the absorption rates of the different components of the samples, and to help explain the origin of the spectral features. The highest intergrated absorption in the a-Si:H film, corresponding to an ideal photocurrent of 12.5 mA/cm(2), was observed for a nanocone/a-Si:H system on a 40 nm thick TiO2 spacer placed on an Al reflector. Due to the rather high (217 nm) structures employed, the a-Si:H absorptance was however, not very sensitive to the spacer thickness. Numerical comparison with systems wherer the core Ag cones were substituted by dielectric cones, demonstrated that the effect of the plasmon resonance added significantly to the benefits of the increased amount of absorber material and enhanced light interaction imposed by other geometrical effects