(709c) Semiconducting Triblock Terpolymers for Microstructured Organic Photovoltaics

Semiconducting polymers have become common electron-donating materials in organic photovoltaics (OPVs) and recent advances in the fabrication and post-processing of polymer?fullerene bulk heterojunction solar cells have allowed for devices with power conversion efficiencies of up to 7% to be realized. This increase is highly reliant on the thin film microphase separation of the active layer components, which significantly impacts the efficiency of charge separation for the bound electron-hole pair (exciton). An understanding of how exciton dissociation and the internal morphology of the active layer affects device performance would facilitate cell optimization and ultimately lead to higher efficiencies. Because block copolymers self-assemble on the same length scale as the exciton dissociation length, block copolymers containing a rigid, semiconducting moiety (rod) covalently linked to a flexible block (coil) have attracted increasing attention. However, systematic studies of model rod-coil diblock copolymers have shown that the range of thermodynamically stable microstructures available in these systems is limited. In fact, many organic photovoltaic heterojunction models have suggested that hexagonally packed cylinders with their long axis perpendicular to electrodes may serve as the most efficient active layer morphology; however, this microstructure is not accessed easily in many rod-coil diblock copolymer systems.

It is well-known that the addition of a third chemically distinct coil block to a coil-coil diblock copolymer allows for the creation of a multitude of additional thermodynamically stable morphologies in coil-coil-coil triblock terpolymer systems. By extending this paradigm to the realm of semiconducting block copolymers, we expect to access a wider variety of microstructures in functional block terpolymers. Here, we report on the synthesis, phase behavior, and semiconducting performance of an optoelectronically active rod-coil-coil triblock terpolymer. Combining the controlled reaction schemes of Seigrist polycondensation, anionic polymerization, and ?click? chemistry, we generate triblock terpolymers with tunable molecular weights, narrow molecular weight distributions, and easily manipulated rod volume fractions. The structure and thermodynamic behavior of this triblock terpolymer system is studied for a variety of rod compositions using differential scanning calorimetry (DSC), wide and small-angle x-ray scattering (WAXS and SAXS), and transmission electron microscopy (TEM). Importantly, we demonstrate the presence of hexagonally packed structures with characteristic domain spacings on the order of 10 nm over a wide window of rod volume fractions.