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 (Tg) 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 Tg. 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 Tgs of 115 and 145 °C promise good thermal stability