Structural and morphological investigations of Poly(3-alkylthiophene) thin films prepared by low and room temperature casting and spin coating techniques

The regioregular poly(3-alkylthiophenes) (RR-P3ATs) have emerged as a benchmark in semiconducting polymers due to their availability, self-organization, solution processability, and promising electrical and optical properties arising from an enhanced crystalline microstructure. The mobility of P3AT organic field effect transistor (OFET) increases when the polymers are self-organized in a thermodynamically most stable "edge-on" orientation. Especially, crystallizing the P3AT polymeric thin films with only edge-on orientation is an important task because of their semicrystalline nature and anisotropic electrical conductivity. The X-ray and transmission electron microscopy analyses have been performed for various P3AT thin films in this thesis shows that the decrease of cast temperature (below room temperature (RT)) is an elegant way to fabricate highly crystalline, mostly edge-on oriented, homogeneous, and very thin films. In particular, thin films of P3ATs such as poly(3-pentylthiophene) (P3PT), poly(3-hexylthiophene) (P3HT), poly(3-heptylthiophene) (P3HeptT), and poly(3-octylthiophene) (P3OT) were prepared by casting 250 μl solution (concentration of 2 mg/ml) at various temperatures like 23°C, 8°C, -12°C, and -30°C. Temperature of -12°C was found to be the optimum casting temperature for the concentration of 2 mg/ml. The randomly oriented crystallites are started to orient along out-of-plane stacking direction while reducing the cast temperature from 23°C to -30°C i.e. forming more edge-on oriented crystallites. In addition to that, the in-plane stacking and crystallinity of P3AT crystallites are increased as the cast temperature decreases, which are most important for the charge transport. The enhancement of edge-on oriented crystallites while decreasing the cast temperature can be explained by the reduced evaporation rate of the solvent (CHCl3). The orientation and sizes of the P3AT crystallites were further enhanced by reducing the concentration of the solution. The out-of-plane stacking of P3AT crystallites increases with the alkyl side chain length due to the increased solubility. On the contrary, the in-plane ordering and crystallinity of P3AT thin films get worse as the alkyl side chain length increases, due to the stimulation of torsion in the thiophene backbone caused by the augment in steric hindrance between the grafted alkyl side chains. Subsequently, it hinders the OFET performance as well as the luminescent intensity. The field effect mobility of -30°C cast P3OT is twofold lesser than the P3PT even though the P3OT film is composed of bigger and more edge-on oriented crystallites because of its poor interchain stacking. The mobility of P3AT OFET cast at -30°C is higher than the ones cast at RT due to the growth of highly interchain stacked edge-on oriented crystallites. It concludes that the highly interchain stacked edge-on oriented P3AT crystallites are necessary for the fast charge transport where the growth along the chain axis (lc) is restricted due to thiophene backbone folding and entanglement as observed through the high resolution transmission electron microscope. The overall findings in this thesis show that the proper selection of cast temperature and concentration are mandatory for the fabrication of very thin and uniform films with enhanced crystallinity and orientation. The degree of preferential orientation and size of the nanodomains can be further increased by depositing the films on n-octadecyltrichlorosilane treated SiO2 substrates as well as by annealing the films at a proper annealing temperature for an hour.