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

Abstract

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 effi cient 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 affi nity (2.55-2.8 eV), and deep HOMO energy level ( < -6.08 eV) with electron mobility as high as 3.3 × 10 − 3 cm 2 V − 1 s − 1 . Polymer-based PhOLEDs with iridium (III) bis(4,6-(di-fl uorophenyl)pyridinato-N ,C 2 ′ )picolinate (FIrpic) blue triplet emitter and solution-processed oligoquinolines as the electrontransport layers (ETLs) gave luminous effi ciency of 30.5 cd A − 1 at a brightness of 4130 cd m − 2 with an external quantum effi ciency (EQE) of 16.0%. Blue PhOLEDs incorporating solution-deposited ETLs were over two-fold more effi cient 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.

Solution-Processed Blue/Deep Blue and White Phosphorescent Organic Light-Emitting Diodes (PhOLEDs) Hosted by a Polysiloxane Derivative with Pendant mCP (1,3-bis(9-carbazolyl)benzene)

“…48 that an additional electron-transport layer enhances efficiency in FIrpic devices. 59 The E T values of PmCPSi and Tm3PyPB (2.75 eV) 15,60 are sufficiently high to confine triplet excitons in the emitting layer and to prevent luminescence quenching. The electroluminescence spectra of the blue PhOLEDs are shown in Figure 4d.…”

20% External Quantum Efficiency in Solution‐Processed Blue Thermally Activated Delayed Fluorescent Devices

“…[ 3,4 ] Typically, phosphorescent emitting materials have been used as the high effi ciency soluble materials for solution-processed OLEDs and provided high quantum effi ciency (QE) in red, green, and blue soluble OLEDs. [5][6][7][8][9][10][11][12][13][14][15] Modifi cation of chemical structure of the phosphorescent emitters was successful to solubilize the phosphorescent emitters and to obtain high QE. Great progress of the QE in the solution-processed phosphorescent OLEDs has been made for the last decade using small molecules and current QE level of the phosphorescent OLEDs is above 20%.…”

Cyclometalated Ir(III) Complexes for High-Efficiency Solution-Processable Blue PhOLEDs

“…31 12 In our single-emissive-layer the low solubility of OXD-7 may increase the device leakage current and reduce device efficiency; also the close triplet energy states of OXD-7 (2.57-2.7 eV) 37 and the emissive triplet ( 3 MLCT) energy states of the blue Ir complex (2.6 eV) may quench the emissive excitons and reduce the device efficiency.…”

“…7,8 Based on this strategy, fluorine is the most utilized substituent and the benchmark sky-blue emitter for phosphorescent OLEDs (PhOLEDs) is iridium(III) bis [4,6-(di-fluorophenyl)pyridinato-N,C 2′]picolinate (FIrpic 1) (Figure 1). 9,10,11,12,13 However, FIrpic suffers two major drawbacks. (i) It has poor solubility in common organic solvents; this limits the concentration at which it can be used as a dopant in solution-processed layers.…”