Abstract
Polymer light-emitting diodes( PLEDs) with device structure of indium tin oxide( ITO) /poly( 3,4-ethylenedioxylenethiophene) ∶ poly( styrenesulphonic acid)( PEDOT∶ PSS) /emissive layer /tris( 8-hydroxyquinolinato) aluminum( Ⅲ)( Alq3) /lithium fluoride( LiF) /Al were fabricated by solution procedures to obtain highly efficient saturated red electrophosphorescence. In the emissive layer,the blends of poly( 9,9-dioctylfluorene)( PFO) and 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole( PBD) were used as host materials( the mass fraction of PBD was 30%),and the iridium complexes Ir-1 and Ir-2 were used as the guest materials. The electroluminescent properties were investigated by changing the doping mass fraction of the iridium complexes in the emissive layer from 1% to 8%. The luminous efficiencies of PLEDs with Ir-1 and Ir-2 enhance when the dopant mass fraction increases from 1% to 2%. A further increase of the dopant mass fraction from 2% to8% results in a decrease of the maximum luminous efficiencies,as a result of the concentration quenching and triplet-triplet( T-T) annihilation. The highest luminous efficiency of 2. 72 cd·A- 1using Ir-1 as dopant is obtained at the doping mass fraction of 2%,and the luminous efficiency reduces to 2. 17 cd·A- 1at a higher current density of 100 mA·cm- 2. Contrastively,the device based on Ir-2 exhibits highest luminous efficiency of 2. 78 cd · A- 1at doping mass fraction of2%. In addition,the luminous efficiency still stays at 2. 30 cd·A- 1as the current density rises to100 mA·cm- 2. Compared to Ir-1 which containing phenyl group in its ancillary ligand,Ir-2 withtriphenylamine-modified ancillary ligand displays better electroluminescent performances,probably due to the triphenylamine group has bigger steric hindrance which can efficiently suppress concentration quenching and T-T annihilation. This indicates that it is an effective method to develop highly efficient electrophosphorescent devices by introducing big steric hindrance and good charge transport group into the ancillary ligand of the iridium complex.
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