In-situ orientation and crystal growth kinetics of P3HT in drop cast P3HT:PCBM films

Abstract

Effect of casting solvents on drop cast thin films of conductive conjugated polymers is largely studied by characterizing post processed films. However, the results have often been inconclusive due to the complexity of the in-situ evolution of structures. In this research we implement in-situ grazing incidence wide angle x-ray scattering (GIWAXS) approach to extracting morphological evolution information during film formation in model Poly(3-hexylthiophene) (P3HT): [6,6]-phenyl C61-butyric acid methyl ester (PCBM) blend films that have otherwise been widely studied. Casting solvents include chloroform, benzene and tetrahydrothiophene (THT), carefully selected for their relative solubilities of P3HT and PCBM. Individual casting solvent studies show that the casting solvents' solubility for P3HT and pure solvent boiling point, along with residual solvent content in the films have significant implications on final thin film morphology and crystallization of its constituent components. For example, the orientations of P3HT in P3HT:PCBM films, cast from different solvents, are largely affected by the individual solubilities of P3HT and PCBM, and substrate surface energy. On the other hand PCBM crystal growth from different PCBM solutions predominantly depends on the solubilities of PCBM in the solvents and boiling points of solvents. In this study we correlate and distinguish the drying behavior of the blend films with respect to the drying behavior of its constituent components. These results have important ramifications for controlling desired morphology for polymer electronics, such as organic photovoltaics (OPV), organic field effect transistor (OFET) and photo-detectors.