Gold Nanowire and Nanorod Plasmonic Mechanisms for Increasing Ultra-Thin Organic Photovoltaic Active Layer Absorption

We theoretically investigate the effect of incorporating gold cylindrical- and ellipsoidal-shaped nanowires and gold nanorods situated centrally within the active layer of organic bulk-heterojunction photovoltaic devices, on the optical absorption performance using finite element electromagnetic simulations. Gold cylindrical nanowire-embedded devices show increased active layer absorption enhancement with increasing radius; however, this effect decreases with the introduction of a polystyrene dielectric capping layer around the nanowires. Active layer absorption, with respect to changes in the orientation, aspect ratio, periodicity, and spacing between ellipsoidal nanowires were optimized. A maximum absorption enhancement weighted by AM 1.5 solar spectrum of 17 % is predicted for gold ellipsoidal nanowires of aspect ratio of 1.167 with in-plane horizontal orientation and arranged with periodicity of 35 nm within a 30-nm thin active layer. We attribute this enhancement primarily to interparticle electromagnetic coupling between adjacent nanowires, where, a maximum spatial and spectral overlap of the electromagnetic field with the absorption band of the active layer material is achieved. This effect increases with decreasing aspect ratio as well as decreasing periodicity with a trade-off observed between nanowire packing density and the active layer absorption enhancement. For gold nanorod-embedded organic photovoltaic devices, the inter-particle electromagnetic coupling effects are weaker and longitudinal surface–plasmon resonances supported by the nanorods are more pronounced. However, since the longitudinal surface–plasmon resonances occur at wavelengths greater than the absorption edge of the photovoltaic active layer, a mere 3.4 % increase in absorption enhancement is achieved for the photovoltaic device incorporating gold nanorods with aspect ratio of 1.167 and periodicity of 35 nm.