(153d) Modification of PCBM Crystallization Via Incorporation of C60 in Polymer/Fullerene Solar Cells

Polymer solar cells are a field of active research in the drive to create low-cost, flexible organic photovoltaics. Key to the development of these devices is the optimization of the active layer morphology, which has been extensively studied in a number of polymer/fullerene systems. Equally important to achieving an optimum morphology, however, is the ability to stabilize it against degradation at operating conditions that are typical of working devices. This is an issue because the continued demixing of polymer/fullerene blends leads to growth of large-scale fullerene crystallites that eventually destroy the optimum thin-film structure. This work evaluates the morphological and performance effects of the incorporation of C₆₀ into blended thin-films of poly(3-hexylthiophene) and [6,6]-Phenyl C₆₁ butyric acid methyl ester (PCBM). The results show that addition of C₆₀ readily alters the growth-rate and morphology of PCBM crystallites under different environmental conditions. The effect of C₆₀ on the growth of large PCBM crystallites is thoroughly characterized using optical microscopy, electron microscopy and UV-Visible absorption spectroscopy. Results show that C₆₀ incorporation modifies fullerene aggregation and crystallization and greatly reduces the average crystal size at C₆₀ loadings of ~50 wt% in the fullerene phase. Organic field-effect transistors (OFETs) and organic solar cells (OSCs) are fabricated from mixed-fullerene active layers to evaluate their performance. It is demonstrated that the use of fullerene mixtures in organic electronic applications is a viable approach to produce more stable devices and to control the growth of fullerene crystals.