No abstract

In this paper, we demonstrate the strong influence of the regioregularity (RR) of poly-(3-hexylthiophene) (P3HT) on the optical anisotropy of hybrid P3HT/fullerene films before and after thermal annealing. We determined the conversion efficiency and characterized the optical anisotropy of P3HT/6,6-phenyl-C61-butyric acid methyl ester (PCBM) blends featuring various values of RR. Unlike grazing-incidence X-ray diffraction analysis, optical anisotropic measurement provides a clear and convenient view of the polymer orientation and the device anisotropic absorption at the same time. By calculating the in-plane and out-of-plane optical constants (extinction coefficients and refractive indices), we determined that the optical anisotropy of P3HT/PCBM films was improved in both orientations upon increasing the RR. Upon increasing the thermal annealing temperature, the main chains of high-RR P3HT were converted from an amorphous structure to an alignment parallel to the substrate, resulting in higher optical anisotropy. The degree of anisotropy of the high-RR P3HT/ PCBM blend was up to six times higher than that of the low-RR sample. This strong RR effect on optical anisotropy was also evident in the power conversion efficiency of large-area P3HT/PCBMbased organic solar cells.

The mechanism of mass transport and the related crystallization in poly(ethylene terephthalate) (PET) were studied. Solvent-induced crystallization can occur during the transport process in PET at low temperature. The important effect of changing the surrounding medium to solvent molecules is to reduce the glass transition temperature. This phenomenon is called “plasticization”. The extent of plasticization relies on the amount of solvent around the polymer molecules, i.e., the concentration of solvent which will depend on the mass transport before saturation. Evidences for the transport mechanism in the first stage of crystallization were revealed. The distinct diffusion front was determined from the measurements of optical microscope and microhardness. The differential scanning calorimeter (DSC) curves displayed crystallization exothermic peaks whose areas decreased with the amounts of amorphous regions, representing the solvent-induced crystallization (SINC) process. The observed phenomenon of multiple-stage crystallizations is associated with the variation of boundary conditions during mass transport, which was ignored in most theoretical analyses.

The mass transport and the evolution of related phase transformation in poly(ethylene terephthalate) (PET) were studied. Solvent-induced crystallization, which occurs during the transport process in PET, is mainly explained as the result of plasticization. The degree of plasticization depends on the concentration of solvent. Therefore, the behavior of crystallization can be closely related to the mechanism of mass transport. The evolution of transformation was studied by small-angle X-ray scattering (SAXS), Fourier transform infrared (FTIR) spectroscopy, optical microscopy (OM), and the density gradient column. During the process of transport, the Fickian mechanism dominated during the first stage of crystallization, and swelling was found to be responsible for the occurrence of most crystallization in the second stage. The first two processes can be classified as the “primary crystallization”. The last stage is associated with the “secondary crystallization”.