A fullerene alloy based photovoltaic blend with a glass transition temperature above 200 °CElectronic supplementary information (ESI) available. See DOI: 10.1039/c6ta08106a

Organic solar cells with a high degree of thermal stability require bulk-heterojunction blends that feature a high glass transition, which must occur considerably above the temperatures encountered during device fabrication and operation. Here, we demonstrate for the first time a polymer : fullerene blend with a glass transition temperature above 200 °C, which we determine by plasmonic nanospectroscopy. We achieve this strong tendency for glass formation through the use of an alloy of neat, unsubstituted C 60 and C 70 , which we combine with the fluorothieno-benzodithiophene copolymer PTB7. A stable photovoltaic performance of PTB7 : C 60 : C 70 ternary blends is preserved despite annealing the active layer at up to 180 °C, which coincides with the onset of the glass transition. Rapid deterioration of the power conversion efficiency from initially above 5% only occurs upon exceeding the glass transition temperature of 224 °C of the ternary blend. Solar cells based on a glass-forming ternary blend of C 60 , C 70 and PTB7 display a high power conversion efficiency above 5% up to an annealing temperature of 180 °C. The excellent thermal stability correlates with a high glass transition temperature of 224 °C.