Macromolecules, Vol.44, No.11, 4204-4212, 2011
2,7-Carbazole-1,4-phenylene Copolymers with Polar Side Chains for Cathode Modifications in Polymer Light-Emitting Diodes
Alcohol-soluble 2,7-carbazole-1,4-phenylene copolymers PCP-NOH and PCP-EP, comprising surfactant-like diethanolamino and phosphonate end groups on the side chains, respectively, were synthesized and utilized as electron injection layer (EIL) in combination with high work function Al electrode in polymer light-emitting diodes (PLEDs). The UV-vis absorption and photoluminescence properties of the PCP-NOH and PCP-EP are mainly determined by the conjugated 2,7-carbazole-1,4-phenylene main chain. The PCP-NOH and PCP-EP possess comparable HOMO levels of -5.20 eV and LUMO levels arround -2.35 eV. Multilayer PLEDs with a device configuration of ITO/PEDOT:PSS (40 nm)/emissive layer (70 nm)/EIL (15 nm)/Al (100 am) were successfully fabricated. With fluorescent PFO-DBT15 as the emissive layer, the PLEDs using the PCP-NOH and PCP-EP as the EIL displayed luminous efficiency.(LE) of 1.01 and 0.88 cd/A, respectively, all obviously higher than 0.015 cd/A for sole Al cathode and 0.58 cd/A for the well-known Ba/Al cathode. With a phosphorescent Ir(mppy)(3)-doped polymer blend (PVK:PBD:Ir(mppy)(3) = 100:30:1) as the emissive layer, the PLEDs with the EIL showed high maximum LE of 39.3 cd/A for PCP-NOH and 42.5 cd/A for PCP-EP, in comparison to 0.35 cd/A for sole Al cathode and 32.2 cd/A for the Ba/Al cathode. Particularly, the PLED with PCP-EP as EIL exhibited an excellent LE of 33.3 cd/A at a current density of 61.1 mA/cm(2) (luminance = 20 300 cd/cm(2)), showing weak roll-off of efficiency at high current density. Inserting the polar alcohol-soluble polymers as interlayer can significantly increase built-in potentials in the PLEDs, from which electron injection barrier from the Al electrode is decreased. The results indicate that the PCP-NOH and PCP-EP are excellent electron injection polymers for high-performance PLEDs with high work function Al electrode.