Derivatives of 2-phenylindole and carbazole as host materials for phosphorescent organic light emitting diodes

Abstract

Synthesis and thermal, electrochemical, photophysical and charge-transporting properties of the derivatives of carbazole and 2-phenylindole are reported. Compounds with reactive functional groups are emphasized. Two compounds form molecular glasses with the glass transition temperatures of 57 °C and 134 °C. The synthesized compounds absorb electromagnetic radiation in the range of 200–375 nm and emit in the range of 350–550 nm. Their solutions exhibit Stokes shifts up to 81 nm. Their triplet energy levels were found to be in the range of 2.88–3.04 eV. The ionization potentials of the synthesized compounds were found to range from 5.45 eV to 5.88 eV. The ability of photopolymerization in the solid state of the synthesized monomers was demonstrated by ATR-FTIR spectroscopy. The charge transporting properties were studied by the space-charge-limited current (SCLC) method. The zero-field hole mobilities reaching 1.97 × 10−5 cm2 V−1s−1 were observed. According to the results of characterization, two compounds were selected for studying them as the hosts in blue and green phosphorescent organic light emitting diodes. The best fabricated device consisting of indium tin oxide anode, hole-transporting layer, emitting layer with 3-((1-(4-vinylbenzyl)-2-phenyl-1H-indol-3-yl)methyl)-9-ethyl-9H-carbazole as a host and the green emitter, electron transporting layer and calcium layer topped with aluminum layer as cathode, exhibited the maximum current, power, and external quantum efficiencies of 10.3 cd/A, 7.2 lm/W, 2.9%, respectively, in the absence of light out-coupling enhancement. The devices based on the twin derivatives of 2-phenylindolylcarbazolylmethane demonstrated the relatively low values of the turn-on voltages of 3.7 and 3.1 V as well as the efficiency roll-offs of 12.5 and 22.7%.