In spite of the successful enhancement of the power-conversion efficiency (PCE) in organic bulk heterojunction solar cells by surface plasmon resonance (SPR), the incorporation of several tens of nm-sized (25–50 nm) metal nanoparticles (NPs) has some limitations to further enhancing the PCE due to concerns related to possibly transferring non-radiative energy and disturbing the interface morphology. Instead of tens of nm-sized metal NPs, here, we have incorporated dodecanethiol stabilized Au nanoclusters (Au:SR, R=the tail of thiolate) with sub-nm-sized Au38 cores on inverted bulk heterojunction (BHJ) solar cells. Although metal NPs less than 5 nm in size do not show any scattering or electric field enhancement of incident light by SPR effects, the incorporation of emissive Au:SR nanoclusters provided effects that were quite similar to those of tens of nm-sized plasmonic metal NPs. Due to effective energy transfer, based on the protoplasmonic fluorescence of Au:SR, the highest performing solar cells fabricated with Au:SR clusters yielded a PCE of 9.15%; this value represents an ~20% increase in the efficiency compared to solar cells without Au:SR nanoclusters.