Abstract
This study is for developing new polymeric electroluminescent materials with high efficiency and brightness. It contains: (1) syntheses and identification of highly electronegative 1,3,4-oxadiazole modified alkoxy substituted poly(p-phenylene vinylene)s (OXD-RO-PPV and POPD-MEH-PPV) and ether-type 1,3,4-oxadiazole modified phenylene vinylene copolymer (POPPPVCn), and (2) investigation of structure/property relationship and the influence of OXD moiety on PPVs by use of various spectroscopies (UV-Vis, PL and X-ray), electrochemical analysis (CV), thermal analysis (TGA, DSC) and surface analysis (AFM/LFM, NSOM). In addition, the effects of OXD moiety on device characteristics of the above materials are further explored by simultaneous measurements of electrical behavior (I-V) and electroluminescent properties (EL spectra, brightness and efficiency) in order to obtain a highly bright and efficient single layer light emitting diode.(1) OXD-RO-PPV and POPD-MEH-PPVRO-PPV and OXD-RO-PPV possess similar optical properties as yellow luminescent materials with bandgaps of 2.22 and 2.25 eV, respectively. Although the introduction of OXD moiety on side chain of PPVs does not affect the emissive color of main chain, the interaction of OXD and main chain leads to an increase in the oxidative potential of the material, and also the IP and EA. This makes the OXD-RO-PPV more electronegative than RO-PPV.The turn-on electric field, maximum brightness and maximum luminous efficiency of ITO/POPD-PPV (OXD-C10O-PPV)/Al are 1.8 MV/cm, 200 cd/m2, and 0.215 cd/A. In comparison with PDPV (C10O-PPV) single layer device, the luminous efficiency of POPD-PPV is improved by a factor of 18, but the turn-on electric filed is higher than that of PDPV (1.1 MV/cm), which can be attributed to the hole blocking and low conductivity of OXD moiety. Due to the leakage current of injected electrons without recombination with holes, the improvement for the POPD-PPV device with Ca as the cathode is limited and its efficiency is close to that of the RO-PPV device.In order to improve the efficiency of POPD-PPV device with Ca, we adopt the copolymerization method to adjust the content of OXD moiety in emissive materials. The onset of oxidative potential of the statistical copolymer, POPD-MEH-PPV, increases with the increase of OXD moiety content, being concomitant with a gradual decrease in the reversibility in cyclic voltammetry. As the composition is 80/20, the oxidative potential is higher than that of PDPV. The UV-Vis and PL spectra of POPD-MEH-PPV thin films show the increase in blue shift with the increase of POPDPV content, resulting from that the number of electron-donating oxygen attached to the phenylene ring in the main chain is reduced. The POPD-MEH-PPVs emit orange-red to yellow fluorescence light with the bandgaps between 2.1 to 2.25 eV at various compositions.For the single layer devices, ITO/POPD-MEH-PPV/Al, the brightness of above 200 cd/m2 can be achieved for each composition except for MEH-PPV and the turn-on electric field decreases with the decrease of POPDPV content. The optimal composition is 80/20, for which the maximum external quantum efficiency is 0.3 %. The use of Ca as cathode can facilitate the injection of electron, so the content of OXD moiety can be lowered. The optimal composition is now 40/60, with which the maximum brightness and maximum efficiency can reach 15000 cd/m2 at 1.5 MV/cm and 1.52 %, respectively, about 30 times brighter and 8.4 times more efficient than those of MEH-PPV. In comparison with those of DO (C10O)-MEH-PPV copolymer (having no OXD), the introduction of OXD can improved the brightness and efficiency by factors of 10 and 7, respectively. Further introducing a thin layer of polymeric conducting material (PEDOT-PSS) between the ITO and emissive layer of the devices, the turn-on voltage and the power consumption can be reduced, and the devices are more stable.The study of the blend systems, which simulate the POPD-MEH-PPV (40/60), shows that: (a) the blend film of POPD-PPV/MEH-PPV has obvious phase separation, showing the limited miscibility of the two homopolymers. The turn-on electric field of the blend device with Ca is higher, and the brightness and efficiency are lower than those of POPD-MEH-PPV (40/60); (b) APO (2-(p-Anisyl) 5-phenyl 1,3,4-oxadiazole) could form exciplex with DO-MEH-PPV in the blend film of DO-MEH-PPV/APO, in which the exciplex does not emit in the device ITO/blend/Ca/Al, due to the recrystallization of APO causing the damage of device in the exciplex-forming region. The device efficiency is poor as compared with that of DO-MEH-PPV. Thus, it is evident that the blend systems can not attain the achievement comparable to the statistical copolymers.(2) POPPPVCnPOPPPVCn can be dissolved in common organic solvents. Their solutions have UV-Vis and PL spectra similar to the corresponding oligomer, OXDPV, and possess intense blue fluorescence. In the solid state, they emit blue-green light with the bandgaps of 2.89 eV and the PL spectra show red shift as the length of the ether spacer decreases, due to the increased interaction of the chromophores at excited state. In addition, the interaction between chromophores can reduce the resistance from oxidation of the OXD moiety, such that the oxidation of shorter spacer POPPPVCn occurs at lower applied voltage.The turn-on electric field of ITO/POPPPVCn/Al decreases with the length of ether spacer. POPPPVC5 has the highest device efficiency among the POPPPVCn. The turn-on electric field, maximum brightness and luminous efficiency for the device ITO/POPPPVC5/Al are 1.85 MV/cm, 70 cd/m2 and 0.161 cd/A, respectively. For the copolymer with the shorter spacer, the EL spectrum has strong dependence on the operating voltage. For ITO/POPPPVC3/Al, it emits blue-green light at lower voltage and becomes green-white at higher voltage. This phenomenon can be enhanced by annealing of the emissive polymer and a white LED with the CIE (0.3, 0.37) can be obtained. The thermal treatment causes little effect for the copolymer with longer spacer. The results of time resolved fluorescence show that POPPPVCn with longer spacer have higher fluorescence quantum yield. In addition, there are several emissive species with different energy in the POPPPVC3 solid film, which has fluorescence even at room temperature, so its EL spectrum will change with the applied voltage.POPPPVC12 can be used as ETL in the double layer device ITO/PPV/POPPPVC12/Al. As the PPV layer thickness is 30 ~ 200 nm, the turn-on voltage is about 12 V and the brightness can reach 150 ~ 300 cd/m2. The efficiency increases with increasing the thickness of PPV and the luminous efficiency is improved by factors of 14 to 167 as compared with the single layer PPV devices. The maximum efficiency of 0.49 cd/A can be obtained, but the turn-on voltage is higher than that of the ITO/PPV/Al (about 9 V).