Aggregation and morphology control enables polymer solar cells with efficiencies near 11.5%

Current high-efficiency (>9.0%) PSCs are restricted to materials combinations that are based on limited donor polymers and only one specific fullerene acceptor, PC71BM. Furthermore, best-efficiency PSCs are mostly based on relatively thin (100 nm) active layers. Here we first report multiple cases of high-performance thick-film (300 nm) PSCs (efficiencies up to 10.8%, fill factors up to 77%) based on conventional PCBM and many non-PCBM fullerenes. Our simple aggregation control and materials design rules allowed us to develop, within a short time, three new donor polymer, six fullerenes (including C60-based fullerenes), and over ten polymer:fullerene combinations, all of which yielded higher efficiency than previous state of art devices (~9.5%). The common structural feature of the three new donor polymers, the 2-octyldodecyl (2OD) alkyl chains sitting on quaterthiophene, causes a temperature-dependent aggregation behavior that allows for the processing of the polymer solutions at moderately elevated temperature, and more importantly, controlled aggregation and strong crystallization of the polymer during the film cooling and drying process. This results in a well-controlled and near-ideal polymer:fullerene morphology (containing highly crystalline, preferentially orientated, yet small polymer domains) that is controlled by polymer aggregation during warm casting and thus insensitive to the choice of fullerenes. The 2OD structural motif is then further applied to several other polymer backbones and produces three additional polymers with efficiencies between 10-11.5%. Our best efficiency (11.5%) is achieved via the combination of new structural designs, interface and optical engineering and optimizations on the solvents and additives of the polymer:fullerene solution.