| Summary: | 博士 === 國立清華大學 === 化學工程學系 === 90 === This study is for developing new polymeric electroluminescent materials with high efficiency and brightness. It contains synthesis and identification of (1) different electronegative moieties modified PPVs, (2) green-emitting alkoxyphenyl substituted PPVs, and (3) yellow-emitting copolymers of dimethyl- and alkoxyphenyl- substituted PPVs. We investigate the effect of different side chain structure on the structure and property of polymer films and the performance of its electroluminescent devices by use of various spectroscopies, electrochemical analysis, X-ray diffraction, TOF measurements, and applied voltage-current characteristic curves of double- and single- carriers devices in order to obtain a highly bright and efficient light emitting diodes.
(1) We introduce different structure electronegative moieties, naphthyl-substituted oxadiazole (NOXD), phenyl-substituted oxadiazole (POXD), t-butylphenyl-substituted oxadiazole (BOXD), and t-butylphenyl-substituted triazole (BTAZ), in the end of the long side chain of PPVs and investigate their structure effect. The introducing of such electronegative moieties results in a dilution effect on conjugation main chain. We find the end-caped t-butyl group can reduce the polar of the electronegative moiety and increase steric hinder, which separate molecular chian effectively and promote photoluminescent efficiency of the polymer films. In addition, we find the introducing of such electronegative moieties indeed promotes its electron flux and performance of its electroluminescent device. Through adjustment of electronegative moiety content by copolymerization, we can find a suitable content to obtain maximum device efficiency. In summary, we find the incorporation of BTAZ moiety allows a tuning of the ratio of electron flux to hole flux in a broad range such that the originally hole-dominated transport in PPVs can turn to electron-dominated transport. As proper content of BTAZ is incorporated, the electron and hole fluxes can be tuned to an equivalent level for improving the efficiency and brightness of the device with the high work function metal, aluminum, as the cathode to a level equivalent to that with the low work function metal, calcium.
(2) Charge transport behaviors and performances of electroluminescent devices of the two green-emitting poly(phenylene vinylene)s, the homopolymer of 2-[3’-(3,7-dimethyloctyloxy)phenylene]-
p-phenylene-vinylene (m-DMOP-PPV) and the copolymer of 50 % by mole 2-[4’-(3,7-dimethyloctyloxy)phenylene]-p-phenylene-vinylene and 50 % 2-[3’-(3,7-dimethyloctyloxy)phenylene]-p-phenylene-vinylene (p-DMOP—co-m-DMOP-PPV) are investigated. The former is found to have more ordered chain alignment than the latter. Such ordered alignment leads to significant increases of the mobilities of charge carriers, especially for that of electron, which is promoted to a level equivalent to that of hole. The balance of electron and hole mobility could be the reason for its high brightness (100,000 cd/m2) and efficiency (12 cd/A) of the EL device.
(3) We have synthesized the yellow light emitting polymers by modifying the green emission alkoxyphenyl-substituted Poly(phenylene vinylene)s (PPV) through copolymeration with the short side chain comonomer [(2,5-dimethoxy-p-phenylene)vinylene] (DMeOPV) to allow a more locally ordered main chain structure. Through the charge carrier flux measurements on single carrier devices, it is found that the limiting factor that determines the overall rate of charge transport is the inter-chain charge transport, which is greatly affected by chain structure and morphology. The local order can promote the electron mobility and flux, which allows an adjusting of the electron and hole carriers fluxes to some extent such that an improvement in the device performance can be made. The maximum brightness of the yellow light emitting polymers with 10 and 25 % of comonomer (DMeOPV) are 57000 (at 15 V/90 nm) and 73000 (at 13 V/83 nm) cd/m2; and the maximum luminous efficiencies are 15.5 (at 7 V/90 nm) and 11.9 cd/A (at 5.5 V/83 nm), respectively.
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