Conjugated polymers are the one of the most promising candidates for the active layer of low-cost Organic-field-effect transistors (OFETs). The charge carrier mobility of these conjugated polymers is the key material property, limiting the performance of the devices. The structural ordering along the whole thickness of film, especially near to interface region play an important role of controlling such electrical properties which ultimately gives high device performance. In the present work using complementary techniques I have studied structural properties of P3HT films as a function of molecular weight and film thickness. Detailed structural studies have been done to understand this polythiophene families internal structural ordering and consequently its correlation with charge carrier mobility. This work focussed on the dependence of the charge transport on morphology of the best known polythiophene group member i.e. regioregular poly(3-hexylthiophene). P3HT films (thin as well as bulk) with different molecular weights provided an ideal system for correlating morphological changes in conjugated polymers to resulting changes in charge transport seeing that the charge carrier mobility in OFETs was found to increase up to four orders-of-magnitude as the molecular weight (Mw) of P3HT is increased from 2500 g/mol to 30,000 g/mol.
Grazing incidence X-ray scattering (GIXS) with combination of its complementary techniques like Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM) were used to measure changes in the crystallinity and crystal orientation associated with varying the annealing conditions, substrate surface treatment at a constant Mw. In particular, X-ray grazing-incidence diffraction (GID) used for depth-resolved structural analysis. We have noticed the several diffraction peaks which are associated with crystalline ordering within the films for low as well as HMW fractions (LMW, HMW) of P3HT besides the amorphous scattering from disordered part of the polymer sample. Both molecular weight fractions display a well pronounced periodicity normal to the surface due to stacking of main chains. The inter-planar distances for such stacked sheets were found to be 1.5 and 1.6 nm for low and high molecular weight fractions, respectively. These films are formally amorphous having the crystalline domains dispersed in amorphous matrix. Here, a textured and strong orientation effects are observed. On the other hand, for the low molecular weight fraction, the X-ray diffraction data are different at the surface and for the bulk samples, where the CCD images show a randomly orientated powder without any preferential orientation of nanocrystallites.
Our results for LMW P3HT indicate the overall higher crystallinity, better in-plane stacking and the concentration of highly oriented crystals, but the mobility is more than a factor of 100 lower than HMW P3HT. These counterintuitive results indicate the charge carrier mobility of conjugated polymers is coupled to several different aspects of the morphology. In the case of the LMW films, the ordered regions are embedded in amorphous matrix which isolates the crystallites from their next neighbours. Whereas in HMW films, the long chains connect the small ordered regions and provide a smooth pathway for charges to move through the film. The molecular structure and morphology of an organic semiconductor are important key factors which control the properties of the interface between the organic film and the insulator, thus a part of research has also focused on interface engineering. We have modified the interface layer interaction by varying the dielectric layers (HMDS, OTS). Our GIXS results on samples with a chemically modified surface showed highly oriented crystals that were nucleated from the substrate and correlate with variations in charge transport for the first 5-10 layers. Correlation between the structural, thermal properties with the OFET performance gives strong evidence that the transport properties of layers prepared from both fractions of poly(3-hexylthiophene) is largely determined by the crystallinity of the samples and in part, responsible for the strong dependence of the OFET mobilities for polymer OFETs on the preparation conditions.
Regarding the comparison of electronic and structural properties for HMW fraction, we have found that the mobility remains constant over a wide range of film thicknesses. A clear interface region found for thick HMW P3HT films and it is responsible for charge transport in the OFET measurements. The existence of amorphous regions in between highly-crystalline lamellae (or nanofibrils) for both short and long chain P3HT samples, controls the charge carrier mobility of such semi-crystalline system. Therefore control of this amorphous region could hold the answer for high charge carrier mobility of any semicrystalline polymers.