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Kinetic Control of Recombination in Organic Photovoltaics: The Role of Spin

In photovoltaic diodes recombination of photogenerated electrons and holes is a major loss process. Biological light harvesting complexes (LHCs) prevent recombination via the use of cascade structures, which lead to spatial separation of charge-carriers1. In contrast, the nanoscale morphology and high charge densities in organic photovoltaic cells (OPVs) give a high rate of electron-hole encounters, which should result in the formation of spin triplet excitons, as in organic light emitting diodes (OLEDs)2. OPVs would have poor quantum efficiencies if every encounter lead to recombination, but state-of-the-art OPVs demonstrate near-unity quantum efficiency3. Here we show that this suppression of recombination can be engineered through the interplay between spin, energetics and delocalisation 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 lying molecular triplet exciton (T1) (on the polymer) lies below the intermolecular charge transfer state (CT). We observe the formation of T1 states following bimolecular recombination, indicating that encounters of spin-uncorrelated electrons and holes generate CT states with both spin singlet (1CT) and spin triplet (3CT) characters. We show that triplet exciton formation can be the major loss mechanism in OPVs. However, we find that even when energetically favoured, the relaxation of 3CT to T1 can be strongly suppressed, via control over wavefunction delocalisation, allowing for the dissociation of 3CT back to free changes, thereby reducing recombination and enhancing device performance. Our results point towards new design rules for artificial photo-conversion systems, enabling the suppression of electron-hole recombination, and also for OLEDs, avoiding the formation of triplet states and enhancing fluorescence efficiency.