Aggregation-Induced Morphology of Low-Dispersity, Regioregular Poly (3-Hexylthiophenes) and their Blends with Fullerene Derivatives

Regioregular poly(3-hexylthiophene) (rr-P3HT) provides a unique system for studying structure-property relationships that drive its performance as an electron donor material for organic electronics applications. Low dispersity rr-P3HT films can exhibit nanofibrillar morphology, resulting from self-assembly of the conjugated polymer chains via π-π stacking. These highlycrystalline domains include void spaces between adjacent nanofibrils that can be filled with small molecule electron acceptors such as fullerenes. This work investigates 1) the synthetic origins of the dispersity which makes such nanofibrils possible, 2) the process of rr-P3HT aggregation which begins in solution and which is affected by the presence of fullerene acceptor molecules, and 3) the consequences of rr-P3HT/fullerene active layer processing conditions on the morphology of thin films.
The first objective is achieved by modeling controlled polymerizations, including Grignard metathesis polymerization which is the synthetic procedure that produces rr-P3HT. Mechanistic steps leading to broad molecular weight distributions are identified, providing insight for future synthetic procedures which utilize similar catalyst-transfer principles. After producing polymers using Grignard metathesis polymerization, UV-vis and fluorescence spectroscopy are used to study the development of rr-P3HT nanofibrils in solution. Using these methods, PCBM, a commonly used fullerene derivative, is shown to significantly
impact the polymer aggregate structures in solution, and various rr-P3HT/PCBM blending scenarios are shown to produce different results as the blend solutions age over time. Blends of rr-P3HT and PCBM and neat polymer films are next investigated using traditional thermodynamic and wide angle x-ray scattering approaches, showing that film
processing conditions, which either enhance or suppress nanofibril formation, can be distinguished but prove difficult to quantify. Instead, grazing-incidence small-angle x-ray scattering is used to characterize rr-P3HT:PCBM blend films. Using Porod analysis, the size of nanostructured
domains can be determined, providing an alternate way to visualize the impact of processing conditions on bulk heterojunction films, which correlate well with results from organic photovoltaic devices prepared under similar conditions.