Improved Performance of Organic Light-Emitting Diodes Using a Metal-Phthalocyanine Hole-Injection Layer (original) (raw)
Related papers
The role of copper phthalocyanine for charge injection into organic light emitting devices
Chemical Physics Letters, 2001
We investigate the charge injection eciency of plasma treated indium tin oxide (ITO) anodes into copper phthalocyanine (CuPc) in single-layer diodes fabricated under inert conditions. Using electroabsorption and Kelvin p`robe surface potential measurements, we demonstrate that the eective ITO work function is pinned at the energy level of the highest occupied molecular orbital of CuPc. We ascribe this eect to oxygen doping from the ITO electrode. Such doping results in high-eciency hole injection from ITO as inferred from the current±voltage characteristics. We ®nd evidence for a time-dependent modi®cation of the device characteristics particularly in reverse bias that we attribute to oxygen diusion from ITO into bulk CuPc. Oxygen plasma treatment of ITO produces an oxide surface that is stable with respect to oxygen diusion.
Improving the performance of organic light-emitting diodes
SPIE Newsroom, 2008
Organic light-emitting diodes (OLEDs) are promising candidates for large-area full-color flat panel displays due to their ease of fabrication and convenience for many applications. 1 OLEDs work through the passage of an electric current across a fluorescent or phosphorescent organic layer resulting in an excitation/emission profile of the material used. With OLEDs, the injection efficiency of electrons is a critical parameter and depends to a great extent on the work function (the minimum energy needed to move an electron out of a substance) of the electrode. A thin hole-injection layer (HIL) or an anode buffer layer (ABL) between the indium tin oxide (ITO) anode and the organic emitting layer are usually adopted to enhance the performance of the hole-injection process. 2-6 Thus, current electroluminescent devices typically have the following layered configuration: ITO anode/HIL or ABL/organic emitting layer/tris(8-hydroxyquinoline) aluminum (Alq 3 )/lithium fluoride (LiF)/aluminum cathode. Our recent work suggests that either an HIL composed of metal phthalocyanine (MPc) or an ABL of Li-doped zinc oxide (LZO) should improve the holeinjection efficiency. The organic, inorganic, and Al layers of our test device were successively deposited using vacuum vapor evaporation at room temperature. The LZO powders with a doped concentration of 5% Li were prepared by sintering a mixture of ZnO and Li 2 CO 3 powders in air. Various MPc layers were tested for their effect on injection efficiency (see ). The turn-on voltage of the devices decreases from 5.3V to 4.3V when CoPc or CuPc layers are inserted: see (b). Compared to the non-MPc device, higher emission efficiency was observed in all MPc devices. The CuPc device achieved the highest efficiency as shown in . For the same emission intensity, the higher efficiency suggests that a much lower current density is required. Figure 1.
Optical Materials, 2009
Red and near-infrared (NIR) organic light-emitting devices (OLEDs) were fabricated based on Tetra (2-Isopropyl-5-methylphenoxyl) substituted metal-free phthalocyanine (Tetra-H 2 Pc). The devices were fabricated by vacuum deposition and spin coating methods. The electroluminescence (EL) intensity at about 910 nm in the devices based on Tetra-H 2 Pc was increased by about 14 times compared with the intensity at about 930 nm in the devices based on unsubstituted metal-free phthalocyanine (H 2 Pc) in the same device structures. The improvement in the EL intensity was attributed to the large steric hindrance of non-peripheral phenoxyl substituent of Tetra-H 2 Pc. It was found that the NIR EL spectra of the doped devices exhibited a strong dependence on the concentration of Tetra-H 2 Pc. The emission at 740 nm was from Tetra-H 2 Pc monomer, while the emissions around 910 nm and 870 nm were from excimer or higher aggregated species.
Advances in Materials Physics and Chemistry
Semiconductor films of organic, doped dimetallophthalocyanine M 2 Pcs (M = Li, Na) on different substrates were prepared by synthesis and vacuum evaporation. Tetrathiafulvalene (TTF) and tetracyanoquinodimethane (TCNQ) were used as dopants and the structure and morphology of the semiconductor films were studied using IR spectroscopy, X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Spectroscopy (EDS). The absorption spectra recorded in the ultraviolet-visible region for the deposited films showed the Q and Soret bands related to the electronic π-π* transitions in M 2 Pc molecules. Optical characterization of the films indicates electronic transitions characteristic of amorphous thin films with optical bandgaps between 1.2 and 2.4 eV. Finally, glass/ITO/doped M 2 Pc/Ag thin-film devices were produced and their electrical behavior was evaluated by using the four-tip collinear method. The devices manufactured from Na 2 Pc have a small rectifying effect, regardless of the organic dopant used, while the device manufactured from Li 2 Pc-TCNQ presents ohmic-like behavior at low voltages, with an insulating threshold around 19 V. Parameters such as the hole mobility (µ), the concentration of thermally-generated holes (p 0), the concentration of traps per unit of energy (P 0) and the total trap concentration (N t(e)) were also determined for the Li 2 Pc-TTF device.
Nanoscale Research Letters, 2012
Surface morphology and thermal stability of Cu-phthalocyanine (CuPc) films grown on an epitaxially grown MgO (001) layer were investigated by using atomic force microscope and X-ray diffractometer. The (002) textured β phase of CuPc films were prepared at room temperature beyond the epitaxial MgO/Fe/MgO(001) buffer layer by the vacuum deposition technique. The CuPc structure remained stable even after post-annealing at 350°C for 1 h under vacuum, which is an important advantage of device fabrication. In order to improve the device performance, we investigated also current-voltage-luminescence characteristics for the new top-emitting organic light-emitting diodes with different thicknesses of CuPc layer.
Current efficiency in organic light-emitting diodes with a hole-injection layer
2008
We have systematically investigated the effect of layer structures on the current efficiency of prototypical hole-injection layer ͑HIL͒/hole-transport layer ͑HTL͒/electron-transport layer ͑ETL͒ organic light-emitting diodes based on 4 , 4Ј ,4Љ-tris͓N-͑3-methylphenyl͒-N-phenylamino͔triphenylamine ͑MTDATA͒ as the HIL, 4 , 4Ј-bis͓N-͑1-naphthyl͒-N-phenylamino͔biphenyl ͑NPB͒ as the HTL, and tris͑8-quinolinolato͒aluminum ͑Alq͒ as the ETL. With bilayer devices, the current efficiency is limited by exciplex emissions in the case of MTDATA/Alq and quenching of Alq emissions by NPB + radical cations in NPB/Alq. The improved current efficiency in trilayer MTDATA/NPB/Alq devices can be attributed to a reduction in NPB + radical cations at the NPB/Alq interface and a strong electric field in the NPB layer.
Journal of Nanoscience and Nanotechnology, 2014
New hole injection layer (HIL) materials for organic light-emitting diodes (OLEDs) based on phenothiazine and phenoxazine were synthesized, and the electro-optical properties of the synthesized materials were examined by UV-vis and photoluminescence spectroscopy, and by cyclic voltammetry. 10,10 0 -bis(4-tert-butylphenyl)-N7,N7 0 -di(naphthalen-1-yl)-N7,N7 0 -diphenyl-10H,10 0 H-3,3 0 -biphenoxazine-7,7 0 -diamine (1-PNA-BPBPOX) showed glass transition temperatures (T g ) of 161°C, which was higher than that (110°C) of Tris(N-(naphthalen-2-yl)-N-phenyl-amino) triphenylamine (2-TNATA), a commercial HIL material. The HOMO levels of the synthesized materials were 4.9-4.8 eV, indicating a good match between the HOMO of indium tin oxide (ITO) (4.8 eV) and the HOMO of N,N 0 -bis(naphthalen-1-yl)-N,N 0 -bis(phenyl)benzidine (NPB) (5.4 eV), a common hole transfer layer (HTL) material. Because the synthesized materials showed minimal absorption at wavelengths shorter than 450 nm, they have good potential for use as effective HIL materials. The synthesized materials were used as the HIL in OLED devices, yielding power efficiencies of 2.8 lm/W (1-PNA-BPBPOX) and 2.1 lm/W (2-TNATA). These results indicate that 1-PNA-BPBPOX yields a higher power efficiency, by a factor of 33%, than the 2-TNATA commercial HIL material. Also, 1-PNA-BPBPOX exhibited a longer device lifetime than 2-TNATA.
High efficiency p–i–n organic light-emitting diodes with a novel n-doping layer
Microelectronics Reliability, 2010
This study demonstrated p-i-n organic light-emitting diodes (OLEDs) incorporating a novel n-doping transport layer which is comprised of cesium iodide (CsI) doped into tris-(8-hydroxyquinoline) aluminum (Alq 3 ) as n-doping electron transport layer (n-ETL) and a p-doping hole transport layer (p-HTL) which includes molybdenum oxide (MoO 3 ) doped into 4,4 0 ,4 00 -tris[2-naphthyl(phenyl)amino] triphenylamine (2-TNATA). The device with a 15 wt.% CsI-doped Alq 3 layer shows a turn on voltage of 2.4 V and achieves a maximum power efficiency of to 4.67 lm/W as well, which is significantly improved compared to these (3.6 V and 3.21 lm/W, respectively) obtained from the device with un-doped Alq 3 . This improvement is attributed to an increase in the number of electron carriers in the transportation layer leading to an efficient charge balance in the emission zone. A possible mechanism behind the improvement is discussed based on X-ray photoelectron spectroscopy (XPS).
Organic Electronics, 2011
The systematic variations in structural, optical and electrical properties of lead phthalocyanine (PbPc)-based solar cells with organic buffer layers were investigated. Transition of the PbPc crystal from a monoclinic phase to a triclinic one was observed when the buffer layers were changed. The structural properties of triclinic PbPc grown on the sexithiophene (6T) buffer layer was superior to those of PbPc grown on the other organic substrates. Since the crystal growth of organic layers is dominated by the anisotropic intermolecular interactions at organic hetero-interfaces, the highly oriented 6T buffer layer with the atomically flat morphology promotes the growth of well-ordered PbPc layers. The performance of the organic solar cells (OSCs) was in direct correlation with the structural and electrical properties of the PbPc single layers. The formation of the triclinic phase enhances the spectrum sensitivity at around 890 nm, suggesting the enhancement of the current density of OSCs. Thus, the control of both crystal quality and phase of PbPc layers using a proper organic buffer layer is effective in enhancing the device performance of OSCs.