Salen-aluminum complexes as host materials for red phosphorescent organic light-emitting diodes

Abstract

The properties of monomeric and dimeric salen-aluminum complexes, [salen(3,5-τBu)2Al(OR)], R = OC6H4- p-C6H6 (H1) and R = [salen(3,5-τBu)AlOPh]C(CH 3)2 (H2) (salen = N,N′-bis-(salicylidene) -ethylenediamine) as host layer materials in red phosphorescent organic light-emitting diodes (PhOLEDs) were investigated. H1 and H 2 exhibit high thermal stability with decomposition temperature of 330 and 370 °C. DSC analyses showed that the complexes form amorphous glasses upon cooling of melt samples with glass transition temperatures of 112 and 172 °C. The HOMO (ca. -5.2 ∼ -5.3 eV) and LUMO (ca. -2.3 ∼ -2.4 eV) levels with a triplet energy of ca. 1.92 eV suggest that H1 and H2 are suitable for a host material for red emitters. The PhOLED devices based on H1 and H2 doped with a red emitter, Ir(btp)2(acac) (btp = bis(2-(2′-benzothienyl)- pyridinato-N,C3; acac = acetylacetonate) were fabricated by vacuum-deposition and solution process, respectively. The device based on vacuum-deposited H1 host displays high device performances in terms of brightness, luminous and quantum efficiencies comparable to those of the device based on a CBP (4,4′-bis(Ncarbazolyl) biphenyl) host while the solution-processed device with H2 host shows poor performance.