Polymorphism in organic electronic materials: PCBM as a case study

A large part of organic electronics relies on the assembly of (macro)molecular materials into structures with different degrees of organization at different spatial length scales. Understanding and controlling these delicate self-assembly phenomena is clearly crucial for future developments. Our group has been active in this general area through a combination of structural and computational studies on well-defined model systems. Our recent work has concerned the PCBM fullerene derivative, which is by far the most widespread electron-transporting material in organic photovoltaic devices. Due to its structural complexity and competitive interactions, this compound has a natural tendency to polymorphism. Some of the questions which come up naturally are: What is the actual size of the “polymorph space” of PCBM? How can these polymorphs be accessed or avoided? To what extent is this relevant for this material’s electronic properties and performance in “real life” applications? In an attempt to tackle some of these questions, we have determined from powder X-ray data the first structure of solvent-free PCBM crystals [1] and developed models for its packing in the amorphous phase [2]. More recently, we have used electron transport calculations to discriminate between these and other exiting crystal structures, which include solvent molecules[3]. These calculations have been performed within the Marcus scheme, despite the known limitations of the localized picture for fullerene-based materials[4]. If time permits, I will briefly discuss our recent work on a coarse-grained quantum-chemical model of the electronic states in molecular conjugated materials, which attempts to overcome this limitation