Material solubility and molecular compatibility effects in the design of fullerene/polymer composites for organic bulk heterojunction solar cells

We report a systematic study of more than 100 bicomponent systems composed of 19 different fullerene derivatives blended with 9 different conjugated polymers (including previously investigated poly(3-hexylthiophene)). It was shown that short circuit current density (JSC) and light power conversion efficiency (η) of the fullerene/polymer photovoltaic devices depend on the solubility of the fullerene components in the solvent used for the blend film deposition (chlorobenzene). The revealed dependences have unusual “double branch” character because many fullerene derivatives possessing similar solubilities showed different photovoltaic performances. This behavior was related to the peculiarities of the molecular structures of the fullerene derivatives. Substituents attached to the cyclopropane ring fused with the fullerene cage in methanofullerenes affected both the morphology of their composites with conjugated polymers and their photovoltaic performance. It was demonstrated that variation of the fullerene component blended with a conjugated polymer might easily change its photovoltaic performance by a factor of 3–4. The obtained results proved that design of appropriate fullerene derivatives and novel conjugated polymers are equally important tasks on the way towards highly efficient organic photovoltaics.