高度取向的半导体聚合物薄膜的溶液浸涂法生长及其电荷传输特性研究

有效地控制有机半导体分子取向和堆积特性对实现高性能电子器件具有非常重要的意义,而发展简便高效的溶液相成膜技术是实现这一目的的重要途径.本文采用改进的溶液浸涂法,成功地成长出大面积宏观取向的半导体聚合物P(NDI2OD-T2)和PTHBDTP薄膜.偏光显微镜和极化的紫外-可见光吸收谱测量显示,薄膜中聚合物分子主链骨架沿成膜时液面下移方向择优取向.原子力显微镜观察到聚合物薄膜由纳米尺度的取向有序晶畴构成,畴的取向与分子链的取向一致.采用衬底-溶液界面处表面张力和溶剂蒸发诱导的分子自组织过程来解释浸涂法生长聚合物取向薄膜的微观机理.使用取向的P(NDI2OD-T2)薄膜制备场效应晶体管,显著地提高了电子迁移率(可达4倍),并实现高达19的迁移率各向异性度.这可归因于共轭的聚合物主链骨架择优取向引起电荷传导通路的变化.

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

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.