In biological complexes, cascade structures promote the spatial separation of photogenerated electrons and holes, preventing their recombination(1). In contrast, the photogenerated excitons in organic photovoltaic cells are dissociated at a single donor-acceptor heterojunction formed within a de-mixed blend of the donor and acceptor semiconductors(2). The nanoscale morphology and high charge densities give a high rate of electron-hole encounters, which should in principle result in the formation of spin-triplet excitons, as in organic light-emitting diodes(3). Although organic photovoltaic cells would have poor quantum efficiencies if every encounter led to recombination, state-of-the-art examples nevertheless demonstrate near-unity quantum efficiency(4). Here we show that this suppression of recombination arises through the interplay between spin, energetics and delocalization of electronic excitations in organic semiconductors. We use time-resolved spectroscopy to study a series of model high-efficiency polymer-fullerene systems in which the lowest-energy molecular triplet exciton (T-1) for the polymer is lower in energy than the intermolecular charge transfer state. We observe the formation of T-1 states following bimolecular recombination, indicating that encounters of spin-uncorrelated electrons and holes generate charge transfer states with both spin-singlet ((CT)-C-1) and spin-triplet ((CT)-C-3) characters. We show that the formation of triplet excitons can be the main loss mechanism in organic photovoltaic cells. But we also find that, even when energetically favoured, the relaxation of (CT)-C-3 states to T-1 states can be strongly suppressed by wavefunction delocalization, allowing for the dissociation of (CT)-C-3 states back to free charges, thereby reducing recombination and enhancing device performance. Our results point towards new design rules both for photo-conversion systems, enabling the suppression of electron-hole recombination, and for organic light-emitting diodes, avoiding the formation of triplet excitons and enhancing fluorescence efficiency.