The hole in the bucky: structure–property mapping of closed- vs. open-cage fullerene solar-cell blends via temperature/composition phase diagrams

The morphology development of polymer-based blends, such as those used in organic photovoltaic (OPV) systems, typically arrests in a state away from equilibrium – how far from equilibrium this is will depend on the materials chemistry and the selected assembly parameters/environment. As a consequence, small changes during the blend assembly alter the solid-structure development from solution and, in turn, the final device performance. Comparing an open-cage ketolactam fullerene with the prototypical[6,6]-phenyl-C 61 -butyric acid methyl ester in blends with poly[2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2- b ]thiophene] (PBTTT), we demonstrate that experimentally established, non-equilibrium temperature/composition phase diagrams can be useful beyond rationalization of optimum blend composition for OPV device performance. Indeed, they can be exploited as tools for rapid, qualitative structure–property mapping, providing insights into why apparent similar donor:acceptor blends display different optoelectronic processes resulting from changes in the phase-morphology formation induced by the different chemistries of the fullerenes.