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-<TEX>$^tBu)_2$</TEX>Al(OR)], R = <TEX>$OC_6H_4-p-C_6H_6$</TEX> (H1) and R = [salen(3,5-<TEX>$^tBu$</TEX>)AlOPh]C<TEX>$(CH_3)_2$</TEX> (H2) (salen = N,N'-bis-(salicylidene)-ethylenediamine) as host layer materials in red phosphorescent organic light-emitting diodes (PhOLEDs) were investigated. H1 and H2 exhibit high thermal stability with decomposition temperature of 330 and <TEX>$370^{\circ}C$</TEX>. DSC analyses showed that the complexes form amorphous glasses upon cooling of melt samples with glass transition temperatures of 112 and <TEX>$172^{\circ}C$</TEX>. 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, <TEX>$Ir(btp)_2$</TEX>(acac) (btp = bis(2-(2'-benzothienyl)-pyridinato-N,<TEX>$C^3$</TEX>; 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.