Correlating Structure with Optoelectronic Functionality in Polymer:Fullerene Blend Films

In this thesis the influence of processing conditions on the structure and optoelectronic properties of conjugated polymer:methanofullerene blend thin-films has been investigated. These conditions strongly impact upon the efficiency with which blend films may produce a photocurrent when fabricated into an Organic Photovoltaic (OPV) device. Using the model system P3HT:PCBM, it is shown that films undergo a three-stage drying process upon casting. Heterogeneous growth of P3HT crystallites occurs once the solid fraction in the film exceeds 50 wt%. Measurements from spectroscopic ellipsometry (SE) and grazing-incidence wide-angle X-ray scattering (GIWAXS) suggest a correspondence between enhanced polymer crystallinity and the strength of the π-π* electronic transition in the polymer absorption spectrum. In-situ measurements of a blend during thermal annealing evidence the evolution of residual solvent loss upon heating, volume relaxation, phase separation and increased electronic conjugation of P3HT upon cooling. The glass transition of P3HT:PCBM blend films, measured in a thin-film geometry, is found to correlate with the minimum effective annealing temperature for improving the power conversion efficiency of thermally annealed OPVs. As-cast films with 20 to 60 wt% PCBM exhibit two glass transitions, an observation that may indicate the existence of two compositionally distinct amorphous phases. Studies on a different polymer:fullerene blend system (PCDTBT:PC71BM), indicate a greater miscibility between materials compared to blends of P3HT:PCBM. In this system, thermal annealing is found to result in increased disorder in the polymer phase of the film, and also to drive excessive phase separation of PC71BM. It is argued that thermal annealing is unlikely to be an appropriate treatment for optimising the efficiency of OPVs based on PCDTBT:PC71BM blends. Finally, Helium Ion Microscopy (HeIM) is used to image the chemical composition of OPV applicable blend films with nanometer resolution, providing a powerful technique to correlate film morphology with device functionality in a range of organic opto-electronic devices.