| Polymer light-emitting diodes(PLEDs) have been attracting much attention recently as a new technology for full-color displays and light sources.Polarized electroluminescence (EL) from polymer is researched in view of potential application as the backlight of liquid crystal display(LCD).The power efficiency of LCD will be significantly improved and the structure of device will be simplified if polarized PLEDs are employed.Based on the research of traditional PLEDs,polarized EL is realized from oriented light-emitting polymers on an alignment layer.With the main thread of the orientation of light-emitting polymer thin films,this thesis involves in the theory of polarized emission,the optical anisotropy characterization of oriented films and the devices fabrication,based on which a novel orientation technique is developed.1.The physical mechanism of polarized EL from polymers can be discussed through electromagnetic theory and liquid crystal state of polymers.The former demonstrates the consistency between the emission polarization and the direction of molecular backbone based on the dipole oscillation model.The latter indicates that the rigid rods structure plays a decisive role on the liquid crystal state of polymer,and therefore the polarized EL.An order parameter is introduced to character the orientation level of polymer films,from which the express of the dichroic ratio for oriented films is derived.2.The optical anisotropy caused by the aligned polymer molecular is an important characteristic for oriented films.Using the measured data of reflectance and transmittance, a photometric fitting method based on Forouhi-Bloomer dispersion model is developed to determine the anisotropic optical constants(including refractive index and extinction coefficient) and thickness of polymer thin films commonly.When the test incident light for spectra measurement is linearly polarized,the anisotropic optical constants of in-plane oriented polymer thin films can be determined.Oriented polymer films of three polyfluorene derivatives discussed in this thesis were measured.The calculated results indicate that PFBT and PFO-BT-DBT show the maximal birefringence of 0.86 and 0.67, respectively,at the peak wavelength.3.Four fluorene-based light-emitting polymers were used to fabricate polarized PLEDs of blue-,green-,red- and white-light emitting for their thermotropic liquid crystal characteristics.The device structure is ITO/Alignment layer/Active layer/Cathode,and the hole injecting layer poly(3,4-ethylene dioxythiophene):poly(styrenesulfonate)(PEDOT: PSS) is mechanically rubbed as the alignment layer.Polarized white-light EL from a single copolymer poly(9,9-dioctylfluorene-4,7-bis(2-thienyl)-2,1,3-benzothiadiazole-2,1,3-benzo-thiadiazole) (PFO-BT-DBT) can be realized for the first time through intrachain energy transfer.The optimized rubbing strength and thermal treatment are employed,by measuring the phase transition temperatures as well as the polarized absorption and photoluminescence spectra of the oriented films.With the optimized thermal treatment,an EL polarized ratio of up to 24 was achieved,which is relatively high in the world.The PLED exhibits a good stability of efficient white emission with a CIE coordinate of(0.33, 0.35),which is almost the perfect white light.4.In order to avoid the effect on the device performance by the mechanically rubbed alignment layer,micro-imprint was employed.An thermal cured elastic polymer film of Polydimethylsiloxane(PDMS) served as the stamp,channels with a minimal period of 1.6μm were micro-imprinted on the PEDOT:PSS layer as the alignment layer.The light-emitting layer can also be oriented and shows obviously optical anisotropic by this method, which also indicates that a higher resolution of the microstructure helps to reach a more effective orientation.
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