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

Abstract

The use of metal complexes fac-tris(1-phenylpyrazolato-N,C(2)('))cobalt(III) [fac-Co(ppz)(3)], fac-tris(2-phenylpyridinato-N,C(2)(') 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 P2(1)/c, with a = 13.6121(12) A, b = 15.5600(12) A, c = 22.9603(17) A, beta = 100.5 degrees, V = 4781.3(7) A(3), 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 pmaH(3) ligand precursor in methanol. Ga(pma) crystallizes in the cubic space group I3d with cell parameters a = 20.2377(4) A, b = 20.2377(4) A, c = 20.2377(4) A, beta = 90.0 degrees, V = 8288.6(3) A(3), 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/m(2) 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/Alq(3) and Co(ppy)(3)/NPD/Alq(3) 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.

Cobalt Polypyridyl Complexes as Transparent Solution‐Processable Solid‐State Charge Transport Materials

“…Fully printed electronic circuits can thus be created. Seminal examples include the thin copper phthalocyanine absorber layer used in Tang's archetypal two-layer organic solar cell [10] and highly luminescent iridium [9] and cobalt [11] complexes used in organic light-emitting diodes (OLEDs). [7] Few examples are known where transition metal complexes have found application as CTMs in electronic devices.…”

“…Traditionally these materials have been chosen for their superior light absorption and emission properties. Seminal examples include the thin copper phthalocyanine absorber layer used in Tang's archetypal two‐layer organic solar cell and highly luminescent iridium and cobalt complexes used in organic light‐emitting diodes (OLEDs). Typically these thin layers are produced via vacuum deposition techniques rather than via solution processing methods .…”

Polymer light-emitting devices based on europium(III) complex with 11-bromo-dipyrido[3,2-a:2′,3′-c]phenazine

“…The turn-on voltages of the devices increase from 11.2 to 16.8 V with increasing doping level from 1 wt% to 8 wt%. This further indicates that the devices mainly operate by the carriertrapping mechanism rather than the energy-transfer mechanism [17,38]. A maximum brightness of 829 cd/m 2 at 153.5 mA/cm 2 and maximum EQE of 1.7% at 2.1 mA/cm 2 were obtained for the device with 1 wt% Eu(DBM) 3 (BrDPPz).…”

An Earth-abundant cobalt based photocatalyst: visible light induced direct (het)arene C–H arylation and CO2 capture

“…17 Despite remarkable advances, 7,8,18 only a few or limited examples involving cobalt complexes as photocatalysts toward organic transformations under ambient reaction conditions have been reported so far, [19][20][21] among which Co(PPz) 3 and Co(BPy) 3 have been successfully employed in the field of solar cells. 22,23 Although homogeneous photocatalysts exhibit superior catalytic performance, the cost associated with them, difficulty in recovering the catalyst from the reaction mixture, their toxicity profiles and complicated synthetic procedures present major barriers to applicability on a commercial scale. 8 It is well established that the 3Rs of green chemistry, i.e.…”