Controlling Over Molecular Alignment,Film Structure And Carrier Transport Of High-performance Semiconducting Polymers

In the past two decades ?-conjugated molecular and polymeric semiconductors have been widely utilized as key components of organic electronic devices such as organic solar cells, organic light-emitting diodes (OLED), organic field-effect transistors (OFET) and various types of sensors, and shown the bright application potentials in large-area and low-cost flexible electronics. However more extensive investigations on materials development, device fabrications as well as charge transport behaviors are required to solve the issues related to organic semiconductors for example the lower carrier mobility compared to their inorganic counterparts. One of key factors for achieving high performance of organic electronic device is the control of film structure of organic semiconductors (OS), especially molecular packing and orientation, via effective and scalable methods for thin and device fabrication. However, the approaches developed so far for film structural (e.g. film texture) control face many limitations such as generality, scalability and simplicity.Therefore, in this thesis several donor-acceptor (D-A) copolymers which exhibit high carrier mobility have been utilized as candidate materials. The large efforts will be made to develop novel methods on film (especially films with oriented structure)fabrication, as well as to control film microstructure of OS, in order to improve the device performance of the OFETs based on these materials. Furthermore,the studies will be performed to explore the relationship of film structure and charge transport properties of OS. Several important results have been achieved as follows:1. we fabricated large-area highly oriented and ordered films of an excellent electron transporter (P(NDI2OD-T2)) by improved solution-cast in high magnetic field. Microstructural characterizations reveal that the chain backbones of P(NDI2OD-T2) are highly aligned along the applied magnetic field in the films.Based on the synchrotron-based X-ray diffraction analysis of the polymer films cast from different solvents, a mechanism which controls the alignment process is proposed, which emphasizes that molecular aggregates of P(NDI2OD-T2) preformed in the solution initiate magnetic alignment and finally determine the degree of film texture. Thin film transistors based on the magnetic-aligned P(NDI2OD-T2) films exhibit an enhancement of electron mobility by a factor of four compared to the unaligned devices, as well as a large mobility anisotropy of 7. Furthermore, the time-modulated magnetic field technique is utilized to effectively control the orientation of ?-conjugated plane of the backbones, thus the degree of face-on molecular packing of P(NDI2OD-T2) is enhanced significantly. Compared to the unaligned devices, the mobility in the vertical direction has improved two orders of magnitude. This work demonstrates magnetic field as a promising approach to improve electrical and optical properties of high performance semiconducting polymers in the devices such as OFETs and organic solar cells.2.In order to improve the homogeneity in film thickness and morphology on the magnetic-aligned films. In this work, we have successfully achieved magnetic alignment of semiconducting polymers pre-deposited by a spin-casting processing.Two donor-acceptor (D-A) type polymers- P(NDI2OD-T2) and DPP2T which exhibit exceptionally high carrier mobility were utilized. The as-prepared "wet"films of the polymers via spin-casting were solvent- annealed under HML (up to 9T).The resultant films of two kinds of the D-A polymers exhibit large-area highly oriented mono-domain structure (texture) in centimeter scale from polarized optical microscopy observation. The polarized UV-vis spectra and synchrotron-based X-ray diffraction reveal that chain backbones of both P(NDI2OD-T2) and DPP2T within the films are highly aligned along the direction of applied magnetic field,and the degree of chain alignment is much enhanced compared to the films grown via solution-cast under HMF. The AFM images show that the polymer films consist of the highly oriented nanofibril-like domains with low surface roughness.Furthermore, the orientation of ?-conjugated planes of the backbones can be effectively controlled by solvent-annealing of the spin-coated wet films under a time-modulated magnetic field, which leads to a remarkably enhanced degree of face-on molecular packing of P(NDI2OD-T2). A mechanism which controls the alignment process is proposed. The film structural manipulation via the strategy presented in this work is expected to have remarkable impact on charge transport properties of the polymer-based devices for example field-effect transistors, OLEDs and solar cells.3. We have reported on successful achievement of the large area, macroscopically aligned films of the semiconducting polymers P(NDI2OD-T2) and PTHBDTP via an improved solution dip-coating process, during which the tilted substrate was immersed in the dilute chlorobenzene solution. Polarized optical microscopy (POM)images reveal the parallel stripe structures on both kinds of the deposited films. The chain backbones of both P(NDI2OD-T2) and PTHBDTP are highly aligned along the descend direction of solution level during dip-coating process, as indicated from polarized UV-vis spectra and X-ray diffraction measurements. Furthermore, the atomic force microscopy image of the oriented films of both kinds of polymers clearly exhibit the highly preferitially oriented nanofibril-like domains, parallel to the alignment direction of chain backbones. We elucidate the film growth via dip-coating in our experiments as the surface tension- and solvent evaporation-guided chain backbone self-assembly process near the substrate-solution interface on the surface of the solution. We also fabricating field effect transistors (OFET) based on the aligned films of semiconducting polymers. FET devices of the aligned P(NDI2OD-T2) exhibit a remarkable enhancement of electron mobility by a factor of four compared to the unaligned devices, as well as a large mobility anisotropy of 19. Such transport behaviors are proposed to be attributed to the characteristic charge conducting pathways induced by chain backbone alignment in the polymeric films,in which fast intra-chain charge transport is expected to contribute mainly to device current when the channel current is parallel to the alignment direction of the films.The facile method developed here presents a promising approach to fabricate low-cost, high-performance organic electronics devices.4. In this work, we extend such strategy onto state of the art donor-acceptor (D-A)semiconducting copolymers (SP) which were demonstrated as good active layers in the OTFTs and solar cells. Three kinds of the D-A SP's, PDVT-10, P(NDI20D-T2)and PBDTTT-C-T with different levels of structural order (from amorphous to semi-crystalline) in neat films, were mixed with polystyrene (PS) at different SP/PS ratios. Furthermore the higher carrier mobility and on/off ratio of three D-A polymers in the OFETs of the blends compared to their neat films, in spite of severe disruption of long-range inter-chain stacking order in the blend with low semiconductor content. The microstructural characterizations reveal that most of the P(NDI2OD-T2) is accumulated at the top part of the film and the PBDTTT nanofibrils penetrate deeply the inert matrix and are dispersed rather evenly through the blend layer.We explore the correlation between film microstructures and carrier transport properties of the OTFTs and diode devices prepared from these polymeric blends.