Organometallic Complexes as Hole-Transporting Materials in Organic Light-Emitting Diodes

The use of metal complexes fac-tris(1-phenylpyrazolato-N,C2‘)cobalt(III) [fac-Co(ppz)3], fac-tris(2-phenylpyridinato-N,C2‘)cobalt(III) [fac-Co(ppy)3], and {tris[2-((pyrrole-2-ylmethylidene)amino)ethyl]amine}gallium(III) [Ga(pma)] as
materials for hole-transporting layers (HTL) in organic light-emitting diodes (OLEDs) is reported. Co(ppz)3 and
Co(ppy)3 were prepared by following literature procedures and isolated as mixtures of facial (fac) and meridional
(mer) isomers. The more stable fac isomers were separated from the unstable mer forms via column chromatography
and thermal gradient sublimation. Crystals of fac-Co(ppz)3 are monoclinic, space group P21/c, with a =
13.6121(12) Å, b = 15.5600(12) Å, c = 22.9603(17) Å, β = 100.5°, V = 4781.3(7) Å3, and Z = 8. {Tris[2-((pyrrol-2-ylmethylidene)amino)ethyl]amine}gallium [Ga(pma)] was prepared by the reaction of gallium(III) nitrate
with the pmaH3 ligand precursor in methanol. Ga(pma) crystallizes in the cubic space group I4̄3d with cell parameters
a = 20.2377(4) Å, b = 20.2377(4) Å, c = 20.2377(4) Å, β = 90.0°, V = 8288.6(3) Å3, and Z = 16. These cobalt
and gallium complexes are pale colored to colorless solids, with optical energy gaps ranging 2.6−3.36 eV. A
two-layer HTL/ETL (ETL = electron-transporting layer) device structure using fac-Co(ppz)3 and fac-Co(ppy)3 as
the HTL does not give efficient electroluminescence. However, the introduction of a thin layer of a hole-transporting
material (N,N‘-bis(1-naphthyl)-N,N‘-diphenylbenzidine, NPD) as an energy “stair-step” and electron/exciton-blocker
dramatically improves the device performance. Both fac-Co(ppz)3 and fac-Co(ppy)3 devices give external quantum
efficiencies higher than 1.0%, with brightness 5000 and 7000 Cd/m2 at 10 V, respectively. Ga(pma) also functions
as an efficient interface layer, giving device performances very similar to those of analogous devices using NPD
as the interface layer. Stability tests have been carried out for Co(ppz)3/NPD/Alq3 and Co(ppy)3/NPD/Alq3 devices.
While fac-Co(ppy)3 gave stable OLEDs, the fac-Co(ppz)3-based devices had very short lifetimes. On the basis of
the experimental results of chemical oxidation of fac-Co(ppz)3, the major cause for the fast decay of the fac-Co(ppz)3 device is proposed to be the decomposition of fac-Co(ppz)3+ in the HTL layer during the device operation.