Organic photovoltaic (OPV) composites have attracted considerable attention as functional materials for flexible, lightweight, and low-cost devices that can serve as photodetectors and solar-driven power generators. To this end, the power conversion efficiencies (PCE) of OPV devices have reached the level of 8% approaching the target efficiency of 10% that would allow for commercialization.
Despite the impressive progress on the PCE values, the exact mechanism that dictates the efficiency of photocurrent generation in OPV devices is not fully understood. The photoactive layers of OPVs essentially consist of binary organic composites that form solid state bulk heterojunctions able to facilitate the process of photo-induced charge separation.
An agreed model that generally describes photocurrent generation in OPVs highlights the following sequence of events: photon absorption in either component of the blend film; migration of the resulting neutral excited electronic state to the heterojunction; dissociation of this state to yield a geminate, electrostatically bound charge pair; separation of the geminate pair into mobile charges, avoiding prompt recombination; and collection of the free chares at the electrodes, avoiding bimolecular recombination. Each of these stages may be influenced by the blend microstructure, i.e., by the degree of phase segregation and by the order in molecular packing within each phase.
This talk will address the effect of OPV layer microstructure on i) the efficiency of free charge generation, as probed by the technique of transient absorption in the $,1'<(Bs time range and ii) the process of emissive bimolecular charge recombination as monitored by the technique of delayed luminescence. Model OPV systems of poly(thiophene):fullerene and fluorene copolymer:perylene diimide blend films will be used as the subjects of this study.