Research On Novel Blue Electroluminescent Polymers And Alignment Method

As a novel kind of functional polymer, polymeric electroluminescent materials have attracted much attention due to the low turn-on voltage, ease to realize full color display, wide viewing angles, light weight, and simple manufacture process of their PLEDs. So far, among the conjugated polymers that emit three basic colors, red and green light emitting materials have fulfilled the requirement for EL devices. However, blue light emitting materials have become the key problem due to their wide energy region, relatively low color purity and electroluminescence efficiency.On the other hand, liquid crystalline conjugated polymers (LCCP) that incorporated the liquid crystalline properties and light-emitting properties have unique long range order, optical anisotropy, and can be used to make new light emitting devices with polarized emitting properties and controllable view angle. When used as the backlights for the conventional LCDs, the polarized light emitted from the EL device based on aligned conjugated polymers would improve the brightness, contrast, light emitting efficiency of the LCDs. However, there is relatively less study on liquid crystalline poly (p-phenylene) derivatives, and moreover, the alignment methods used nowadays are not sufficient for large scale manufacture.In this paper, two series of blue light emitting copolymers have been designed and synthesized, and their thermal properties, liquid crystalline properties, photoluminescence properties, electrochemical properties, and the electroluminescence properties of their single-layered PLEDs are studied. In addition, first step study on the alignment method of the conjugated copolymers has been made.1. A series of biphenyl-co-p-phenylenevinylene conjugated copolymers containing saturated aliphatic segments in the main chain have been synthesized through Wittig polycondesation reaction. The copolymers show good thermal stability with weight-loss temperatures higher than 385℃and glass-transition temperatures higher than 98℃. The copolymers in chloroform solution and in thin films all emit pure blue light, and the quantum yields in chloroform solution are in the range of 0.40-0.48. PBPV-H6PV and PBPV-H8PV with no methoxyl group on the side chain show typical thermotropic nematic liquid-crystalline phase as revealed by DSC, POM and WAXD. The HOMO energy levels of the copolymers are 5.69-5.76 eV, which are higher than those of poly(p-phenylene), and the LUMO energy levels of the copolymers are 2.81-2.94 eV, which are lower than those of poly(p-phenylene). It implied that the charge injection barrier of the biphenyl-co-p-phenylenevinylene copolymers are more balanced than poly(p-phenylene).2. A series of fluorene-co-p-phenylenevinylene conjugated copolymers containing saturated aliphatic segments in the main chain have been synthesized through Wittig polycondesation reaction. Different length of non-conjugated segments with/without methoxyl group on the side chain have been introduced into the fluorene-co-p-phenylenevinylene main chain to control the effective conjugated segments, confining the emitting light in the pure blue light region. Meanwhile, introduction of soft segments might reduce the tendency of chain aggregation in the solid state. The weight loss temperatures of the four amorphous copolymers are higher than 380℃, and the glass transition temperatures are in the range of 65-83℃. The single-layered PLEDs based on the copolymers with the configuration of ITO/PEDOT/copolymer/Ca/A1 all emit pure blue light under low driving voltage. Especially, the PLED based on PFV-H6PV shows good performance. The maximum brightness is reached to 1130cd/m2 with a current-luminescence efficiency of 0.16 cd/A.3. A novel alignment technology has been adopted to make alignment layers with PBPV-H8PV (liquid crystalline) and PFV-M6PV (amorphous). The polarized UV spectra and polarized PL spectra of the copolymers show low alignment ratio. This is probably because for the relatively low molecular weight and wide-spreaded nanopores, the copolymers could only partly align in the relatively small nanopores. Further research on increasing the molecular weight of the copolymers and decreasing the aperture of the nanoalumina membrane are next works.