Introducing plasmonic resonant scatterers in photovoltaic devices is a promising way to increase energy conversion efficiencies by trapping incoming light in ultra-thin solar cells. Colloidal plasmonic oligomers are obtained following a cost-effective self-assembly strategy and incorporated in organic-based cells produced using spin-coating techniques in ambient air conditions. An interesting increase is observed of both external quantum efficiency (EQE) and short-circuit current for solar cells loaded with plasmonic oligomers compared with reference organic cells with and without isolated gold nanoparticles. Theoretical calculations demonstrate that the wavelength-dependent EQE enhancement is a resonant process due to the increased scattering efficiency in plasmonic antennas allowed by a chemically controlled 1 nm nanogap. This method opens the way towards roll-to-roll fabrication of efficient plasmonic ultra-thin photovoltaic devices.