Charge photogeneration in neat conjugated polymers

The origin and yield of charges in neat conjugated polymers has long been controversial. In this paper, we review the body of literature that has been created over the past three decades of research in this field and provide insight from our own recent work highlighting the importance of polymer microstructure in understanding the photophysics of these materials. We focus primarily on polythiophene, poly(p-phenylene vinylene), and ladder-type poly(p-phenylene) derivatives, as these three prototypical polymer backbone structures have undergone the most complete study. We find compelling evidence that the primary photoexcitations in conjugated polymers include both intrachain excitons and excimers, that charges are produced in a secondary process, primarily from breakup of intrachain excitons, and that the locus of long-lived charge generation is at the interface between amorphous and crystalline domains of the polymer. Interestingly, the existence of interchromophore complexes that we refer to as excimers has largely been ignored in the development of organic photovoltaics based on conjugated polymers. We suggest that the prevalence of this species may help explain certain mysterious features in that body of work, in particular the excess energy offset required for efficient charge separation in donor/acceptor blends and the requirement for intimately mixed phases of the two materials for maximally efficient photocurrent generation.