A milestone in the development of organic photovoltaics (OPVs) has been the introduction of processing additives (a small amount of high boiling point solvent, typically diiodooctane, DIO), which may significantly increase the device performance in many photovoltaic blends. One of the major ways by which additives improve OPV device performance is by helping to optimise the active layer morphology. In addition to morphology, another parameter which is vitally important to OPV performance is the energetic disorder. However, little is known concerning the effect of additives on the energetic disorder in OPVs.
We investigate how additives affect the energetic disorder in a benzodithiophene-based copolymer (PBDTTT-C-T), a model system because of the widespread use of the benzodithiophene unit in highly efficient devices. Based on temperature-dependent mobility measurements, we demonstrate that the additive (DIO) lowers energetic disorder in the blend. We show that the reduction in energetic disorder occurs primarily for electrons in acceptor domains, while the disorder for holes is relatively unaffected by DIO. The ability of DIO to decrease the energetic disorder is confirmed by highly sensitive measurements of the weak charge-transfer state emission. Wide-angle (WAXS) and small-angle X-ray scattering (SAXS) measurements suggest the origin of this reduced energetic disorder is due to increased aggregation and a larger average fullerene domain size upon addition of DIO.