| Recent years, organic field-effect transistors (OFETs) have attracted much attention because of their unique advantages, such as flexibility, light weight, low cost, large-area and low-temperature device fabrication. To obtain the high performance devices, many efforts have been made, such as developing new high-performance organic semiconductors, optimizing device fabrication process and so on. Morphological engineering of the active materials is one of the most important and feasible ways to realize the high performance device. It is well established that OFETs based on single crystals can provide a high-purity organic active channel for the excellent carrier transport properties. However, there remains limit of application for single-crystal transistors because of its complex fabrication technique, the feature of micrometer or even nanometer sized "small" crystals and their fragility. Therefore, preparing large-area highly-crystalline semiconductor films may make for practical applications of OFETs. Epitaxial crystallization provides an effective way for preparing well arranged and unique ordered thin films of materials. It can control the crystal structure and orientation of the overgrowing materials.In this work, much higher charge transport property has been achieved for thin film transistors once the crystal grain boundary is removed by optimizing the epitaxial-crystallization process. All these results suggest that it opens a new way for getting highly-crystalline organic films by epitaxial-crystallization of organic semiconductors on pre-oriented substrate to construct large-area high performance electronic devices.1. Highly oriented polymer thin films were prepared according to a melt-draw technique. In this work, we chose two usual polymer materials (HDPE and PVDF), which were reported as dielectrics in OFETs. The resultant films were around 30-50 nm in thickness and could reach 10×10 cm-2 in area. Such thin highly-oriented polymer films could be easily transferred onto SiO2/Si substrate and used as substrates for inducing the subsequent crystallization of organic semiconductor molecules. Both the polymer thin film and the SiO2 acted as the dielectrics in OFET. The capacities of oriented polymer-SiO2 composite films were tested. Besides, the theoretical capacity values were calculated.2. Epitaxial crystallization of perylo[1,12-b,c,d]selenophene (PESE) on highly oriented HDPE substrate through vapor phase deposition has been achieved. Oriented PESE crystals with different crystalline morphologies can be fabricated by changing the temperature of HDPE substrate during vacuum evaporation. Transistors based on expitaxial crystallized PESE films have been fabricated and the transistor properties were also studied. It was found that transistor based on the epitaxially crystallized PESE films shows different electrical characteristics depending on the film preparation conditions. The transistors based on the PESE/PE-SiO2/Si with PESE exhibit a field-effect mobility of 4.4×10-2 cm2V-1s-1 and on/off ratio of 1.97×105, which are about 2 orders of magnitudes higher than the PESE/OTS-SiO2/Si based transistors.3. Highly-oriented F-NDI films were achieved through an epitaxial-crystallization process by vacuum-depositing organic semiconductors on the highly-oriented HDPE substrate, where well-ordered F-NDI lathlike crystals were arranged with the long axis of the lathlike crystals, i.e., the b-axis of the F-NDI crystal, along the chain direction of HDPE substrate. Such high-crystalline oriented F-NDI films ensure its efficient charge transport property, with the highest electron-transporting mobility approaching 0.2 cm2V-1s-1 for transistors with the conducting channel along the b-axis orientation of F-NDI crystals on HDPE substrate, which is around 4 times significant increase compared with that of un-oriented F-NDI films. The transport anisotropy of the thin film transistors was determined to be ca.14. |
PDF Full Text Request