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. Attempts to control the donor/acceptor morphology in bulk heterojunction materials as required for achieving high power-conversion efficiency have, however, met with limited success. 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. This discovery can potentially enable morphological control in bulk heterojunction materials where thermal annealing is either undesirable or ineffective.