Molecular Order in High-Efficiency Polymer/Fullerene Bulk Heterojunction Solar Cells

We report quantitative measurements of ordering, molecular orientation, and nanoscale morphology in the active layer of bulk heterojunction (BHJ) organic photovoltaic cells based on a thieno[3,4-b]thiophene-alt-benzodithiophene copolymer (PTB7), which has been shown to yield very high power conversion efficiency when blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). A surprisingly low degree of order was found in the polymer-far lower in the bulk heterojunction than in pure PTB7. X-ray diffraction data yielded a nearly full orientation distribution for the polymer pi-stacking direction within well-ordered regions, revealing a moderate preference for pi-stacking in the vertical direction (face-on). By combining molecular orientation Information from polarizing absorption spectroscopies with the orientation distribution of ordered material from diffraction, we propose a model describing the PTB7 molecular orientation distribution (ordered and disordered), with the fraction of ordered polymer as a model parameter. This model shows that only a small fraction (approximate to 20%) of the polymer in the PTB7/PC71BM blend is ordered. Energy-filtered transmission electron microscopy shows that the morphology of PTB7/PC71BM is composed of nanoscale fullerene-rich aggregates separated by polymer-rich regions. The addition of dliodooctane (DIO) to the casting solvent, as a processing additive, results in smaller domains and a more finely interpenetrating BHJ morphology, relative to blend films cast without DIO.