Formation of Europium Chelate Complexes by Vacuum Co-Deposition and Their Application in Organic Light-Emitting Diodes

摘要

Europium (Eu) rare-earth complexes, which show intense red photoluminescence (PL) resulting from the f-f transition of Eu~(3+) 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). Research on the use of Eu complexes to fabricate OLEDs began in 1991 by Kido and colleagues. 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 (η_(ext)) of 0.01 %. In 1994, Tsutsui et al. reported a η_(ext) of 0.3 % obtained with Eu(TTA)_3phen (phen: 1,10-phenanthroline) used as an EML. Kido et al. also obtained a η_(ext) of 0.5 % with an OLED with a Eu(DBM)_3phen (DBM: dibenzoylmethanato):2-(4-biphe-nyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) (1:3) co-deposited film as an EML. Recently, Adachi et al. reported a η_(ext) of 1.29 % obtained with a 1 wt.-% Eu(TTA)_3phen film doped into a wide-energy-gap 4,4'-N,N'-dicarbazole-biphenyl (CBP) host used as an EML. Most recently, Noto et al. obtained a η_(ext) of 3.5 % with a 10 wt.-% Eu(DCNP)(DBM)_2-phen (DCNP: l,3-dicyano-l,3-propanedione) film doped into a CBP host, and Liang et al. reported a η_(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)benzi-dine (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.