The development of nanoscale morphology in polymer: fullerene photovoltaic blends during solvent casting

The power conversion efficiency in a conjugated polymer-functionalized fullerene bulk heterojunction organic photovoltaic (OPV) device is dependent both on the electronic properties of the constituent materials and on the nanoscale morphology of the active semiconductor layer thin-film. Here we use in situ ellipsometry and grazing incidence X-ray scattering (GI-XS) to study molecular self-organization in poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) blend films in real time, during the drying process as they are cast from solution. We illustrate the evolution of the extinction coefficient from a solution to a solid, semi-crystalline state. We show that once the solvent fraction in the film falls below 50%, the P3HT undergoes rapid crystallization via heterogeneous nucleation; a process that is complete in seconds. We also evidence a rapid, dynamic self-annealing process that reduces the characteristic lamella spacing in the P3HT crystallites. The mechanistic understanding of film-formation demonstrated here is an important component in optimizing deposition processes suitable for large-area OPV manufacture.