Effect of macromolecular parameters and processing conditions on supramolecular organisation, morphology and electrical transport properties in thin layers of regioregular poly(3-hexylthiophene)

The effects of molecular weight and the processing conditions on the polymer supramolecular organisation, its morphology and charge carriers mobility have been investigated for regioregular poly(3-hexylthiophene) (RR-P3HT) thin layers used for the fabrication of field-effect transistors (FETs). The application of three distinctly different processing techniques (dip coating, spin coating and drop casting) together with polymers exhibiting different molecular weights, including previously unexplored molecular weight value (120 kDa by SEC equal to 27 kDa after the MALDITOF correction) enabled the separation of the influence of purely macromolecular factors from the morphological ones. It has been demonstrated that the chain length of the deposited polymer is the determining factor in the fabrication of layers with high carriers mobility which changes from 10(-5) cm(2)/(V s) for the shortest chains (Mn corrected= 1 kDa) to 0.04 cm(2)/(V S) for the longest ones (Mn corrected = 27 kDa). The changes of the film morphology cannot explain the dependence of the mobility on Mn. The observed relationship can be rationalized by considering the principal factors, intervening on three different levels: (i) on the molecular level the increase of the conjugation length with Mn is observed which leads to a higher mobility of the carriers along a single chain; (ii) on the supramolecular level the probability of the interchain charge carriers hopping is higher for longer chains since the number of low activation energy pathways for the crossing between chains grows with the increase of the chain length; (iii) on the mesoscopic level the connectivity between aggregations of higher mobility, for example, nanorods observed by AFM, is better assured for longer polymer chains. The product of these contributions results in an enhanced carriers mobility for layers fabricated from high molecular weight polymer fractions. The morphology of the RR-P3HT layers is strongly dependent on the processing method used. For high rate deposition techniques (spin coating) nanorod-type morphologies are obtained for low molecular weight polymers whereas fractions of high molecular weight give films with a granular morphology. For low deposition rate techniques (dip coating) the rod-like morphology persists even for films fabricated from the highest molecular weight fraction. Moreover, for layers obtained from intermediate polymer fractions (from 1.9 to 10.8 kDa) the individual rod width is, within the experimental error, equal to the length of the polymer chain if the all trans conformation is assumed. The above obsenation implies that polymer chains are oriented perpendicularly to the rod long axis. GIXD investigations fully corroborate this hypothesis. The rod diameter in layers deposited using the highest molecular weight fraction (27 kDa) is significantly lower than the length of the chain in the all trans conformation, suggesting chain folds via all trans-all cis-all trans sequence of conformation changes which would limit the rod diameter. (c) 2006 Elsevier BX All rights reserved.