Formation of europium chelate complexes by vacuum co-deposition and their application in organic light-emitting diodes

Europium (Eu) rare-earth complexes, which show intense red photoluminescence (PL) resulting from the f-f transition of Eu 31 ions after the harvesting of both singlet and triplet excitons produced on the surrounding ligands, have been attracting a lot of attention as light-emitting materials for organic light-emitting diodes (OLEDs).([1-28]) Research on the use of Eu complexes to fabricate OLEDs began in 1991 by Kido and colleagues.([1]) An OLED with an emitting layer (EML) made of a Eu(TTA)(3) (TAA: thenoyltrifluoroacetonato) film doped into a poly(methylphenylsilane) (PMPS) host demonstrated a maximum external quantum efficiency (eta(ext)) of 0.01%. In 1994, Tsutsui et al.([3]) reported a eta(ext) of 0.3% obtained with Eu(TTA)(3)phen (phen: 1,10-phenanthroline) used as an EML. Kido et al.([2]) also obtained a eta(ext) of 0.5% with an OLED with a Eu(DBM)(3)phen (DBM: dibenzoylmethanato):2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) (1:3) codeposited film as an EML. Recently, Adachi et al.([15]) reported a eta(ext) of 1.29% obtained with a 1 wt.-% Eu(TTA)(3)phen film doped into a wide-energy-gap 4,4'-N,N'-dicarbazole-biphenyl (CBP) host used as an EML. Most recently, Noto et al.([20]) obtained a eta(ext) of 3.5 % with a 10 wt.-% Eu(DCNP)(DBM)(2)-phen (DCNP: 1,3-dicyano-1,3-propanedione) film doped into a CBP host, and Liang et al.([16,24]) reported a eta(ext) of 5.7 cd A(-1) obtained at a J of 0.04 mA cm(-2) with a 0.6 wt.-% Eu(DBM)(3)-phen film doped into an N,N'-diphenyl-N,N'-di(m-tolyl)benzidine (TPD) host. The efficiency of OLEDs using Eu complexes has been gradually improving over the past decade, which has sparked further interest in the development of Eu-based OLEDs.