Fullerene alloy formation and the benefits for efficient printing of ternary blend organic solar cells

Composition average dependent properties for blends of the conjugated polymer P3HT and the fullerenes [60]PCBM, [60]ICBA and their mixtures were studied using cross-polarization magic-angle-spinning solid-state NMR techniques. We found that the blended fullerenes form an alloy and that when mixed with a third polymer component, the system exhibits pseudo-binary phase behaviour instead of the expected ternary phase behaviour. Our results experimentally confirm the earlier hypothesis that the unexpected composition average dependent IV-behaviour for these supposed ternary mixtures are indeed due to them behaving as pseudo-binary mixtures due to alloying of the fullerene components. This finding has vast implications for the understanding of polymer-fullerene mixtures and quite certainly also their application in organic solar cells where performance hinges critically on the blend behaviour which is also investigated in this study.