High charge-separation efficiency combined with the reduced fabrication costs associated with solution processing and the potential for implementation on flexible substrates make 'plastic' solar cells a compelling option for tomorrow's photovoltaics 1 . Attempts to control the donor/acceptor morphology in bulk heterojunction materials as required for achieving high powerconversion efficiency have, however, met with limited success 2-4 . By incorporating a few volume per cent of alkanedithiols in the solution used to spin-cast films comprising a low-bandgap polymer and a fullerene derivative, the power-conversion efficiency of photovoltaic cells (air-mass 1.5 global conditions) is increased from 2.8% to 5.5% through altering the bulk heterojunction morphology 5 . This discovery can potentially enable morphological control in bulk heterojunction materials where thermal annealing is either undesirable or ineffective.Bulk heterojunction 'plastic' solar cells are based on phaseseparated blends of polymer semiconductors and fullerene derivatives . Because of self-assembly on the nanometre length scale, mobile carriers and excitons formed after absorption of solar irradiation diffuse to a heterojunction before recombination and are dissociated at the polymer/fullerene interface . Ultrafast charge transfer from semiconducting polymers to fullerenes guarantees that the quantum efficiency for charge transfer at the interface approaches unity , with electrons on the fullerene network and holes on the polymer network. After breaking the symmetry by using different metals for the two electrodes, electrons migrate towards the lower work-function metal and holes migrate towards the higher work-function metal. Despite high chargeseparation efficiency, a significant fraction of carriers recombine at donor/acceptor interfaces before extraction from the device owing, in part, to the inherently random interpenetrating network morphology formed after spin-casting. Carrier recombination before reaching the electrodes and low mobility limit both the device fill factor (FF) and the overall photon harvesting by reducing the optimum active-layer thickness 12 . The carrier lifetime is largely controlled by the phase morphology between the donor and acceptor materials . Although significant advances in the performance of polymer-based photovoltaic devices have been made during the past few years 3,14-16 , the ability to control the morphology of the donor/acceptor network is critical to optimizing efficiency.While investigating the use of alkanethiol-stabilized gold nanoparticles for polymer solar-cell applications, we discovered that by incorporating small concentrations of alkanethiols Absorption (a.u.