Fullerene Nucleating Agents: A Route Towards Thermally Stable Photovoltaic Blends

The bulk-heterojunction nanostructure of non-crystalline polymer:fullerene blends has the tendency to rapidly coarsen when heated above its glass transition temperature, which represents an important degradation mechanism. We demonstrate that fullerene nucleating agents can be used to thermally arrest the nanostructure of photovoltaic blends that comprise a non-crystalline thiophene-quinoxaline copolymer and the widely used fullerene derivative [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM). To this end, C₆₀ fullerene is employed to efficiently nucleate PCBM crystallization. Sub-micrometer-sized fullerene crystals are formed when as little as 2 wt% C₆₀ with respect to PCBM is added to the blend. These reach an average size of only 200 nanometers upon introduction of more than 8 wt% C₆₀. Solar cells based on C₆₀-nucleated blends indicate significantly improved thermal stability of the bulk-heterojunction nanostructure even after annealing at an elevated temperature of 130 °C, which lies above the glass transition temperature of the blend. Moreover, we find that various other compounds, including C₇₀ fullerene, single-walled carbon nanotubes, and sodium benzoate, as well as a number of commercial nucleating agents - commonly used to clarify isotactic polypropylene - permit to control crystallization of the fullerene phase. Fullerene nucleating agents are used to thermally arrest the nanostructure of photovoltaic blends that comprise a non-crystalline thiophene-quinoxaline copolymer and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM). In particular, C₆₀ efficiently nucleates PCBM crystallization and reduces the size of fullerene crystals to only 200 nanometers. Solar cells show significantly improved thermal stability of the bulk-heterojunction nanostructure even after annealing at an elevated temperature of 130 °C.