The origins of performance enhancement in hybrid plasmonic organic photovoltaic devices are often embroiled in a complex interaction of light scattering, localized surface plasmon resonances, exciton-plasmon energy transfer and even non-plasmonic effects. To clearly deconvolve the plasmonic contributions from a single nanostructure, we herein investigate the influence of a single silver nanowire (NW) on the charge carriers in bulk heterojunction polymer solar cells using spatially-resolved optical spectroscopy, and correlate to electrical device characterization. Polarization-dependent photocurrent enhancements with a maximum of ~36% over the reference are observed when the transverse mode of the plasmonic excitations in the Ag NW is activated. The ensuing higher absorbance and light scattering induced by the electronic motion perpendicular to the NW long axis lead to increased exciton and polaron densities instead of direct surface plasmon-exciton energy transfer. Finite-difference time-domain simulations also validate these findings. Importantly, our study at the single nanostructure level explores the fundamental limits of plasmonic enhancement achievable in organic solar cells with a single plasmonic nanostructure.