Phase Behavior and Crystallization Kinetics of Polymer:Fullerene Solar Cell Blends - A Thermal Characterization Study based on Fast Scanning Calorimetry Techniques

Conjugated polymer - fullerene blends form the active layer in bulk heterojunction solar cells, in which the polymer acts as the donor and the fullerene as an acceptor. These materials are potentially an interesting technological alternative for inorganic solar cells to efficiently and cost-effectively convert solar power into electrical power. The main goal of this work is to get insight in the fundamentals of the phase behaviour of conjugated polymer - fullerene blends. For an optimal efficiency of bulk heterojunction solar cells, a co-continuous, nano-structured phase-separated morphology is aimed at. As this morphology is intrinsically unstable, the blends need to be designed in such a way that the desired morphology is stabilized or frozen out. The phase behavior, crystallization kinetics, and long-term stability of frequently used organic solar cell blends was studied using thermal and surface analysis techniques. Due to the intrinsic thermal properties of the organic materials used as donor/acceptors, their glass transition temperature and their melting and crystallization behaviour, the morphology of these blends is in most cases not stable at the desired maximum operation temperature. The results from this work indicate possible directions for controlling and stabilizing the nanomorphology.