Growth and carrier transport properties of highly oriented films of the semiconducting polymers via solution dip-casting

Abstract

Effective control of molecular orientation and packing as well as the film texture of organic semiconductor plays a crucial role in achieving high performance of the electronic device such as high carrier mobility. Development of facile and scalable solution processing method for film deposition is one of the important routes to such a goal. In this paper, we report on the successful preparation of the large area, macroscopically aligned film of the semiconducting polymer P(NDI2OD-T2) and PTHBDTP via an improved solution dip-coating process in which a tilted substrate is immersed in the dilute solution. Polarized optical microscopy images reveal the parallel stripe structures of both kinds of the deposited films. The chain backbones of both P(NDI2OD-T2) and PTHBDTP are highly aligned along the descending direction of solution level in the dip-coating process as indicated from polarized UV-vis spectra and X-ray diffraction measurements. Furthermore, the atomic force microscopy images of the oriented films of both kinds of polymers clearly exhibit the highly preferentially oriented nanofibril-like domains, parallel to the alignment direction of chain backbone. We elucidate the dip-coating growth process in our experiment in terms of the surface tension-and solvent evaporation-guided self-assembly of chain backbones at the substrate-solution interface near the solution surface. The influence of film texture on carrier transport property is examined by fabricating field effect transistor (FET) based on the aligned film of semiconducting polymer. The FET device of the aligned P(NDI2OD-T2) exhibits a remarkable enhancement of electron mobility by a factor of four compared with the unaligned devices, as well as a large mobility anisotropy of 19. Such a transport behavior is proposed to be attributed to the characteristic charge conducting pathways induced by chain backbone alignment in the polymeric film. In this case, fast intra-chain transport contributes to the majority of device current when the channel current is parallel to the alignment direction of the film, while charge transport will be limited severely by the inter-chain hopping within the fibrous domain and across the disordered domain boundary when the current is perpendicular to alignment direction. The facile method developed here presents a promising approach to fabricating the low-cost, high-performance organic electronic devices.