In Situ X-ray Scattering Studies of the Influence of an Additive on the Formation of a Low-Bandgap Bulk Heterojunction

The evolution of the morphology of a high-efficiency, blade-coated, organic-photovoltaic (OPV) active layer containing the low band gap polymer poly[(4,8-bis[5-(2-ethylhexyl)thiophene-2-yl]benzo[1,2-b:4,5-b′]dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiophene))-2,6-diyl] (PBDTTT-C-T) is examined by in situ X-ray scattering. In situ studies enable real-time characterization of the effect of the processing additive 1,8-diiodoocatane (DIO) on the active layer morphology. In the presence of DIO, X-ray scattering indicates that domain structure radically changes and increases in purity, while X-ray diffraction reveals little change in crystallinity/local order. The solidification behavior of this active layer differs dramatically from those that strongly crystallize such as poly(3-hexylthiophene) and small molecule containing systems, exposing significant diversity in the solidification routes relevant to high-efficiency polymer–fullerene OPV processing. In the presence of DIO, we find quanti...