Experimental Evidence Relating Charge-Transfer-State Kinetics and Strongly Reduced Bimolecular Recombination in Organic Solar Cells

Significantly reduced bimolecular recombination relative to the Langevin recombination rate has been observed in a limited number of donor-acceptor organic semiconductor blends. The strongly reduced recombination has been previously attributed to a high probability for the interfacial charge-transfer (CT) states (formed upon charge encounter) to dissociate back to free charges. However, whether the reduced recombination is due to a suppressed CT-state decay rate or an improved dissociation rate has remained a matter of conjecture. Here we investigate a donor-acceptor material system that exhibits significantly reduced recombination upon solvent annealing. On the basis of detailed balance analysis and the accurate characterization of CT-state parameters, we provide experimental evidence that an increase in the dissociation rate of CT states upon solvent annealing is responsible for the reduced recombination. We attribute this to the presence of purer and more percolated domains in the solvent-annealed system, which may, therefore, have a stronger entropic driving force for CT dissociation.