Structure And Electrical Properties Of Discotic Liquid Cystals

Discotic liquid crystal with electron-rich cores can be self-assembled into columuar mesophase via intermolecular interaction. The columns arrange in order, which causes a high charge transporting property. It has found their potentials to be used in photoconductive devices, organic light emitting diodes, solar cells, etc.Phthalocyanines (Pcs) are aromatic compounds having semiconducting properties. They are thermally and chemically stable, having promising application in optoelectronic devices. The research on the electrical properties of Pcs can be traced back to 1970s. Although they have been widely developed as photoconductors, their applications in other optoelectronic devices were rarely reported. In recent years the practical use of phthalocyanine compounds have received increasing attention because of their importance as prototype semiconductors and their uses in technological applications in which Pcs can be used as hole transport layers, or injection layers.In this work, we used Copper Phthalocyanines derivatives doped with polymers. Our research focused on the controlling of the processing and structure of such materials, and their application as optoelectronic devices. The details are:The crystal structures of side chained CuPc derivatives, oo-CuPc and tb-CuPc, and their mixed films with polymers were studied by X-ray diffraction. Both the powder tb-CuPc and powder oo-CuPc have oriented in a certain degree. The XRD patterns of mixed thin films of tb-CuPc and PMMA show that a hexagonal stack formed in the molecules aggregation. With the increasing PMMA content, the long-range order in the films decreases. Then comparing several different heat treatments on the films, the results indicate that keeping the samples at 125℃ for two hours and decreasing the temperature slowly to the room temperature are propitious to making the hexagonal stack perfect. The oo-CuPc does not form the hexagonal stack during the film making process. But after heat treatment, it forms hexagonal stack.The electric property of the doped polymer films are prepared by spin-coating were characterized by direct current-voltage and AC impedance measurements. The contact between electrode and materials is ohmic contact. With the charge injection, it changes to the hopping conduction. The introduction of PC has an effect on the alignment of oo-CuPc molecules. AC conductivity changes with frequency. In the LF region, it reflects the DC properties of materials. In the HF region, the dependence of ac conductivities to the frequency is σ = 1/R + Aω". The index n is about 0.96-0.998. The transport mechanism is hopping conduction. The introduction of PC does not change the transport mechanism of materials. The whole system equals to a parallel resistance and capacitance.