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-2yl]benzo [1,2-b :4,5-b dithioph en e) -2,6-diyl-a lt-(4-(2-ethylh exan oyl) thieno [3,4-b]thiophene))-2,6-diy1] (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(3hexylthiophene) 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 quantitative agreement between the evolution of the phase structure revealed by small -angle X-ray scattering and the binodal phase structure of a simple Flory Huggins model.