Charge Recombination Dynamics in Organic Photovoltaic Systems with Enhanced Dielectric Constant

Increasing the dielectric constant of organic photovoltaic materials to reduce recombination rates has long been pursued, however, material modification often results in the modification of multiple device characteristics, making system comparison difficult. In this study, a fullerene derivative with an increased blend dielectric constant is examined by the addition of a triethylene glycol appendage to the fullerene (TEG-PCBM). Density functional theory calculations show a small change to the permanent dipole moment between TEG-PCBM and [6,6]-phenyl-C-61-butyric acid methyl ester (PC60BM) resulting in similar solubility, morphology, and device performance. TEG-PCBM is blended with donors P3HT and PTB7-Th and a comparable performance to PC60BM is found. This model system shows the rarely reported characteristic of an increase in the dielectric constant while leaving its other properties unaltered. Looking at light intensity effects on open-circuit voltage (V-oc), short-circuit current (J(sc)), and fill factor (FF) along with exciton dissociation efficiency, it is observed that when switching to the TEG- modified fullerene derivative, geminate recombination is not reduced, and Shockley-Read-Hall recombination is increased. While triethlyene glycol appendages may prove to be ineffective in improving recombination through increased dielectric constant, an approach for studying recombination in future high dielectric systems is provided.