Facile Monitoring of Fullerene Crystallization in Polymer Solar Cell Blends by UV- vis Spectroscopy

A simple technique is demonstrated based on UV-vis spectroscopy that permits the onset of fullerene crystallization to be monitored in polymer:fullerene bulk-heterojunction blends. The growth of fullerene crystals only occurs during thermal annealing above the glass transition temperature (T-g) of the blend. Light scattering due to the presence of micrometer-sized fullerene crystals reduces the apparent transmittance, which can serve as a clear indicator for the upper limit of the T-g. This principle is used to deduce this critical parameter for two high-performance photovoltaic blends that are based on a thiophene-quinoxaline and a thiophene-isoindigo copolymer, respectively. High T(g)s of 115 and 145 degrees C promise good thermal stability. image