Modification of PCBM crystallization via incorporation of C₆₀ in polymer/fullerene solar cells

The morphological effects of the incorporation of C₆₀ into blended thin-films of poly(3-hexylthiophene) and [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) are investigated. The results show that addition of C₆₀ readily alters the growth-rate and morphology of PCBM crystallites under different environmental conditions. The effect of C ₆₀ 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 crystallite size at C₆₀ loadings of ≈50 wt% in the fullerene phase. Organic field-effect transistors (OFETs) are prepared to evaluate the electron mobility of PCBM/C₆₀ films 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 micrometer-sized fullerene crystals.