Correlating Emissive Non-Geminate Charge Recombination with Photocurrent Generation Efficiency in Polymer/Perylene Diimide Organic Photovoltaic Blend Films

Evidence for a correlation between the dynamics of emissive non-geminate charge recombination within organic photovoltaic (OPV) blend films and the photocurrent generation efficiency of the corresponding blend-based solar cells is presented. Two model OPV systems that consist of binary blends of electron acceptor N'-bis(1-ethylpropyl)-3,4,9,10-perylene tetracarboxy diimide (PDI) with either poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) or poly(9,9-dioctylindenofluorene-co-benzothiadiazole) (PIF8BT) as electron donor are studied. For the F8BT:PDI and PIF8BT:PDI devices photocurrent generation efficiency is shown to be related to the PDI crystallinity. In contrast to the F8BT:PDI system, thermal annealing of the PIF8BT:PDI layer at 90 degrees C has a positive impact on the photocurrent generation efficiency and yields a corresponding increase in PL quenching. The devices of both blends have a strongly reduced photocurrent on higher temperature annealing at 120 degrees C. Delayed luminescence spectroscopy suggests that the improved efficiency of photocurrent generation for the 90 degrees C annealed PIF8BT:PDI layer is a result of optimized transport of the photogenerated charge-carriers as well as of enhanced PL quenching due to the maintenance of optimized polymer/PDI interfaces. The studies propose that charge transport in the blend films can be indirectly monitored from the recombination dynamics of free carriers that cause the delayed luminescence. For the F8BT:PDI and PIF8BT:PDI blend films these dynamics are best described by a power-law decay function and are found to be temperature dependent.