New Solution-Processable Electron Transport Materials for Highly Efficient Blue Phosphorescent OLEDs

Several new solution-processable organic semiconductors based on dendritic oligoquinolines were synthesized and were used as electron-transport and hole-blocking materials to realize highly efficient blue phosphorescent organic light-emitting diodes (PhOLEDs). Various substitutions on the quinoline rings while keeping the central meta-linked tris(quinolin-2-yl)benzene gave electron transport materials that combined wide energy gap (>3.3 eV), moderate electron affinity (2.55-2.8 eV), and deep HOMO energy level (<-6.08 eV) with electron mobility as high as 3.3 x 10(-3) cm(2) V-1 s(-1). Polymer-based PhOLEDs with iridium (III) bis(4,6-(di-fluorophenyl)pyridinato-N,C-2')picolinate (FIrpic) blue triplet emitter and solution-processed oligoquinolines as the electron-transport layers (ETLs) gave luminous efficiency of 30.5 cd A(-1) at a brightness of 4130 cd m(-2) with an external quantum efficiency (EQE) of 16.0%. Blue PhOLEDs incorporating solution-deposited ETLs were over two-fold more efficient than those containing vacuum-deposited ETLs. Atomic force microscopy imaging shows that the solution-deposited oligoquinoline ETLs formed vertically oriented nanopillars and rough surfaces that enable good ETL/cathode contacts, eliminating the need for cathode interfacial materials (LiF, CsF). These solution-processed blue PhOLEDs have the highest performance observed to date in polymer-based blue PhOLEDs.