New phenyl-substituted PPV derivatives for polymer light-emitting diodes - Synthesis, characterization and structure-property relationship study

Three new PPV derivatives with dialkoxyphenyl substituents, BEH2P-PPV, BEH3P-PPV, and BEH4P-PPV have been synthesized. The polymers were characterized by FT-IR, H-1 NMR, and elemental analysis. The polymers possess excellent solubility, high molecular weights, high photoluminescence efficiencies and good thermal stability. The influence of substitution pattern on the formation of structural defects has been investigated by measuring the signal due to tolane-bisbenzyl moieties (TBB) in the proton NMR spectra. BEH2P-PPV with a steric phenyl group at the ortho-position on the side phenyl ring shows the lowest amount of TBB, which indicates suitable steric hindrance can be applied to suppress the formation of irregular head-to-head and tail-to-tail linkage in the polymer chains. In addition, the polarity of solvents used for the Gilch polymerization will also affect the amount of irregular structure in the polymers. Polar solvents such as THF will result in polymers with low TBB content. Energy level measurement from cyclic voltammetry revealed that the influences of the substitution pattern on the HOMOs and LUMOs are different. The three polymers possess similar HOMO energy levels while the LUMO of BEH4P-PPV is much higher than that of the other two polymers. Polymer light-emitting diodes fabricated from BEH2P-PPV, BEH3P-PPV, and BEH4P-PPV with the configuration of ITO/PEDOT/polymer/Ba/Al, emitted bright blue-green to green light with the maximum peaks at 496, 488, and 525 nm, respectively. The turn-on electric field and maximum external quantum efficiencies of the diodes are 0.30, 0.50, and 0.42 MV/cm and 0.37%, 0.66%, and 0.25% respectively. The quantum efficiency is mainly determined by the electron injection from the cathode. With the highest luminance, lowest turn-on electric field, and good quantum efficiency as well as negligible structural defects, BEH2P-PPV is the most promising material among the three polymers for polymer light-emitting diodes.