Electron Injection Material Research Articles

Phenanthroline diimide as an organic electron-injecting material for organic light-emitting devices

We report a diimide-type organic electron-injecting material, bis-[1,10]phenanthrolin-5-yl-pyromellitic diimide (Bphen-PMDI), for organic light-emitting devices (OLEDs), which was synthesized from its monomers, pyromellitic dianhydride (PMDA) and 1,10-phenanthrolin-5-amine (PTA). The vacuum-purified Bphen-PMDI powder showed high glass transition (∼230 °C) and thermal decomposition (∼400 °C) temperatures, whereas neither melting point nor particular long-range crystal nanostructures were observed from its solid samples. The optical band gap energy and the ionization potential of the Bphen-PMDI film were 3.6 eV and 6.0 eV, respectively, leading to the lowest unoccupied molecular orbital (LUMO) energy of 2.4 eV. Inserting a 1 nm thick Bphen-PMDI layer between the emission layer and the cathode layer improved the device current density by 10-fold and the luminance by 6-fold, compared to the OLED without the Bphen-PMDI layer. The result suggests that an effective electron tunnel injection process occurs through the Bphen-PMDI layer.

Affect of the electrical characteristics depending on the hole and electron injection materials of red organic light-emitting diodes

This study examined the electrical and optical properties of red OLEDs (organic light-emitting diodes) with a four-layer structure, ITO/amorphous fluoropolymer (AF)/N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1-biphenyl-4,4′-diamine (TPD)/R-H:R-D/lithium fluoride (LiF)/Al, containing a hole injection material, AF (amorphous fluoropolymer) and an electron injection layer material, LiF. Compared to the basic structure (two-layer structure), the brightness and luminous efficiency of the four-layer structure, ITO/TPD/R-H:R-D/Al, increased approximately 100 times (30,000 lm/m2) and 150 times (51 lm/W), respectively, with an applied voltage. The excellent efficiency of the external proton was also increased 150 times (0.51%). That is, the hole and electron injection layers improved the surface roughness of ITO and Al, and the interfacial physical properties. In addition, these layers allowed the smooth injection of holes and electrons. The luminance, luminous efficiency and external quantum efficiency were attributed to an increase in the recombination rates.

Low sublimation temperature cesium pivalate complex as an efficient electron injection material for organic light-emitting diode devices

Cesium pivalate ((CH 3) 3CCOOCs) has been synthesized and applied as an electron injection material for organic light-emitting diodes, which showed low sublimation temperature of 180 °C. Typical bilayer structure of ITO/NPB (60 nm)/Alq 3 (50 nm)/EIL/Al was used to evaluate the electron injection efficacy of (CH 3) 3CCOOCs, the results showed (CH 3) 3CCOOCs/Al exhibits better electron injection than LiF/Al cathode and the power efficiency was improved by about 19% at current density of 50 mA/cm 2. More interestingly, in the typical three layer OLED structure ITO/2-TNATA (60 nm)/NPB (10 nm)/Alq 3:2% C545T (40 nm)/MADN (15 nm)/(CH 3) 3CCOOCs (2 nm)/Al, the maximum current efficiency is up to 20 cd/A with Commission Internationale d’Eclairage (CIE x , y ) color coordinates of ( x = 0.30, y = 0.65) at current density of 140 mA/cm 2, which indicates that the non-aromatic alkali metal complex can also have good match with the chemically stable compound and exhibit good electron injection properties.

Sodium−Quinolate Complexes as Efficient Electron Injection Materials for Organic Light-Emitting Diode Devices

Improvement of electron injection is one of the most effective ways for obtaining highly efficient low-voltage organic light-emitting diodes (OLEDs). Here, we report the study on the unidentate sodium−quinolate complexes as the electron injection layer for OLEDs. Sodium−quinolate complexes, 8-hydroxyquinolinolatosodium (Naq), 2-methyl-8-hydroxyquinolinolatosodium (NaMeq), and 2-(diphenylmethyl)-8-hydroxyquinolatosodium (Ph2Naq) were synthesized and characterized by 1H NMR, 13C NMR, FTIR, UV−vis, and photoluminescence (PL) spectroscopy. The produced sodium−quinolate complexes are thermally stable amorphous compounds with high glass transition and decomposition temperatures. These complexes were used as the electron injection materials for multilayer OLEDs. In the case of OLEDs using 2-methyl-9,10-bis(naphthalene-2-yl)anthracene (MADN) as the electron transport layer (ETL), Naq and NaMeq show much better performance than LiF in OLED devices.

Influence of the Performance of Organic Light Emitting Devices by Chemical Reaction at the Interface between Electron Transport Material and Ba Cathode

Red electrophosphorescent organic light-emitting devices using red phosphorescent dopant, and tris-(8-hydroxyquinoline)-aluminum (Alq3) and 2,9-dimethyl-4,7 diphenyl-1,10-phenanthroline (BCP) electron injecting have been fabricated. A device with Alq3 as electron transport layer shows high turn-on voltage (@ 1 cd/m 2 ) of 6.25 V, but a device with BCP does lower turn-on voltage of 5.0 V. Alq3 and BCP have silimar lowest unoccupied molucular obital, 3.0 eV and 2.9 eV, which are the higher values than work function of Ba (2.7 eV). So, Ba cathode does not form energy barrier to inject electron to organic layer from simple energy band of view. However, these devices represent different injection properties, therefore we analyze the interface between organic layer and cathode. X-ray photoelectron spectroscopy depth profiling measurements reveal the strong chemical reaction and bonding of Ba with the C and O present in the Alq3 molecules. Results show that the performance of the electrophosphorescent devices is mainly governed by the structure and the chemical reaction at the interface between Ba metal and electron-injecting material. [doi:10.2320/matertrans.M2010381]

Solution-Processed Small Molecular Organic Light-Emitting Devices with a Mixed Single Layer

We investigated the characteristics of solution-processed mixed-single-layer organic light-emitting devices (OLEDs) by mixing an electron injection material, a hole transport material, and a dopant material based on 5,6,11,12-tetraphenylnaphthacene (rubrene). The mixed-single-layer OLEDs showed better performance by optimizing the solution concentration and mixing ratio of organic materials. The performance was further improved by mixing chloroform (95 wt %) and toluene (5 wt %) as a solvent. The maximum luminance and power efficiency obtained were 12,400 cd/m2 and 1.1 lm/W, respectively. The mixed-single-layer OLEDs by solution process can be expected as an alternative route to the fabrication of small-molecular OLEDs with reduced cost of devices and avoiding the complexities of the co-evaporation of multiple organic materials in the vacuum deposition process.

Solution-Processed Small Molecular Organic Light-Emitting Devices with a Mixed Single Layer

We investigated the characteristics of solution-processed mixed-single-layer organic light-emitting devices (OLEDs) by mixing an electron injection material, a hole transport material, and a dopant material based on 5,6,11,12-tetraphenylnaphthacene (rubrene). The mixed-single-layer OLEDs showed better performance by optimizing the solution concentration and mixing ratio of organic materials. The performance was further improved by mixing chloroform (95 wt %) and toluene (5 wt %) as a solvent. The maximum luminance and power efficiency obtained were 12,400 cd/m2 and 1.1 lm/W, respectively. The mixed-single-layer OLEDs by solution process can be expected as an alternative route to the fabrication of small-molecular OLEDs with reduced cost of devices and avoiding the complexities of the co-evaporation of multiple organic materials in the vacuum deposition process.

Conjugated Zwitterionic Polyelectrolyte as the Charge Injection Layer for High-Performance Polymer Light-Emitting Diodes

A new zwitterionic conjugated polyelectrolyte without free counterions has been used as an electron injection material in polymer light-emitting diodes. Both the efficiency and maximum brightness were considerably improved in comparison with standard Ca cathode devices. The devices showed very fast response times, indicating that the improved performance is, in addition to hole blocking, due to dipoles at the cathode interface, which facilitate electron injection.

Study on the Electron Injection Mechanism of Thermally Decomposable Cs2CO3

Thermally evaporated Cs2CO3 has been reported to be an efficient electron injection material for organic light-emitting diodes (OLEDs) although its electron injection mechanism remains under debate. In this study, we reported a thermodynamic calculation of the decomposition reactions of Cs2CO3, indicating that it is thermodynamically reasonable for Cs2CO3 to decompose into metallic Cs. A thermal decomposition experiment on Cs2CO3 was carried out and the decomposition temperature of Cs2CO3 was found to be close to the calculated temperature at which Cs2CO3 decomposes into metallic Cs. The in situ X-ray photoemission spectroscopy (XPS) profiles showed an agreement between the deposited Cs2CO3 and the standard metallic Cs. The interaction between N and Cs atoms in the Cs2CO3-doped tris(8-hydroxyquinoline) aluminum (Alq3) film was verified by XPS, indicating the existence of Cs. The effect of the oxidizing atmosphere on the electron-only devices has been investigated, and the thermal decomposition of Cs2CO3 into metallic Cs was further confirmed.

Lead(IV) dioxide: an effective electron injection material to realize high-efficiency inverted top-emitting organic light-emitting diodes

Lead(IV) dioxide (PbO2) has been used as the electron injection layer (EIL) to realize high-efficiency inverted top-emitting organic light-emitting diodes (I-TOLEDs). It can be seen that the inserting of the PbO2 EIL significantly reduces operational voltage, thus greatly improving the current efficiency and power efficiency of fabricated I-TOLEDs. The 10-(2-benzothiazolyl)-1, 1, 7, 7-tetramethyl-2, 3, 6, 7-tetrahydro-1H, 5H, 11H-[1] benzopyrano [6, 7, 8-ij] quinolizin-11-one (C545T)-based I-TOLEDs with the PbO2 EIL exhibit a maximum current efficiency of 31.6 cd A−1 and a maximum power efficiency of 14.3 lm W−1, which are both higher than 22.5 cd A−1 and 5.4 lm W−1 of the I-TOLEDs with LiF as the EIL respectively. A detailed analysis with respect to the role mechanism of PbO2 in electron injection has been presented. The improvement in EL performance is attributed to the formation of the interfacial dipoles at the electrode interface due to charge transfer between PbO2 and Alq3.

Thermally Decomposable KBH4 as an Efficient Electron Injection Material for Organic Light-Emitting Diodes

In this work, KBH4 was studied as a novel electron injection material (EIM) for organic light-emitting diodes (OLEDs). Optimized OLEDs using KBH4 as the EIM showed a high efficiency of 11.0 cd/A at 1000 cd/m2, comparable to that of a control device using LiF (9.2 cd/A). The lifetime of the device with KBH4 was also comparable to that of the control device. Thermal evaporation behavior was studied by thermodynamic calculation and X-ray photoemission spectroscopy (XPS), demonstrating that the improved electron injection was attributable to the low-work-function metallic K released by the thermal decomposition of KBH4.

Thermally Decomposable Lithium Nitride as an Electron Injection Material for Highly Efficient and Stable OLEDs

Improvement of electron injection is essential for highly efficient low-voltage organic light-emitting diodes (OLEDs). In this work, we report our study on thermally decomposable Li3N as the electron injection layer (EIL) for OLEDs. We use a quartz crystal microbalance method to investigate the decomposition process of Li3N during vacuum thermal evaporation in situ. Thermodynamics study also proves that Li3N decomposes in vacuum to form metallic lithium and nitrogen. OLEDs with Li3N as the EIL outperform those with conventional LiF in every respect of view. An optimized green OLED with a Li3N EIL shows 35% higher efficiency and more than doubled lifetime compared with the control device with a LiF EIL.

High efficiency and long lifetime in organic light-emitting diodes using bilayer electron injection structure

A lithium quinolate-based electron injection structure was developed to improve luminance efficiency and lifetime of organic light-emitting diodes (OLEDs). A electron injection material based on Li complex, 8-hydroxyquinolinato lithium (Liq), was introduced as an electron injection material for OLEDs and the efficiency and lifetime of OLEDs were investigated according to the structure of the electron injection layer. A bilayer electron injection structure, a mixed layer of tris(8-hydroxyquinoline) aluminium (Alq 3) and Liq and a Liq layer, showed high efficiency of 11.6 cd/A compared with 9.8 cd/A for lithium fluoride (LiF). In addition, the extrapolated lifetime of OLED with the bilayer electron injection structure was improved by 40% at 1000 cd/m 2.

Effect of electron injection and transport materials on efficiency of deep-blue phosphorescent organic light-emitting devices

We investigate the performance of FIr6-based deep-blue phosphorescent organic light-emitting devices (PHOLEDs) with three different electron transport materials, bathocuproine (BCP), 4,7-diphenyl-1,10-phenanthroline (BPhen), and tris[3-(3-pyridyl)mesityl]borane (3TPYMB), and study the effect of doping alkaline metals (Li and Cs) into these charge transport materials. External quantum efficiency ( η EQE ) of (20 ± 1)% and peak power efficiency ( η P ) of (36 ± 2) lm/W were achieved maintaining Commission Internationale de L’Eclairage (CIE) coordinates of ( x = 0.16, y = 0.28) in p-i-n dual-emissive-layer (D-EML) deep-blue PHOLEDs with 3TPYMB as the electron transport material and 3TPYMB:Cs as the electron injection layer. The high efficiencies are attributed to the high triplet energy of 3TPYMB as well as the increased conductivity of 3TPYMB:Cs.

Electron injection property at the organic–metal interface in organic light-emitting devices revealed by current–voltage characteristics

Abstract Electron injection and transport are key issues in organic light-emitting devices (OLEDs). In this letter, we successfully demonstrated the electron injection property which is usually characterized by electronic barrier height from aluminum cathode to tris(8-hydroquinoline) aluminum (Alq 3 ) or 4,7-diphyenyl-1,10-phenanthroline (BPhen) (the most common two electron-transport materials) by current–voltage ( J – V ) characteristics. The electronic barrier height of 0.66 eV at the Alq 3 /Al interface and that of 0.10 eV at the Alq 3 /LiF/Al interface was obtained, which is in good agreement with the experimental results investigated by ultraviolet photoelectron spectroscopy (UPS) by Mori et al. [Appl. Phys. Lett. 73 (1998) 2763]. The electronic barrier height of 0.83 eV at BPhen/Al interface and that of 0.098 eV at BPhen/Cs 2 O/Al interface was also demonstrated for the first time, suggesting Cs 2 O is a highly efficient electron-injection material (EIM) for BPhen. We proposed a simple and yet practical approach to estimate the electron-injection barrier-height at the organic–metal interface in OLEDs. The results were further explained by the dipole effect.

Isophorone-based analogues with A-π-D-π-A structure for red organic light emitting devices

New electroluminescent materials, two DCM analogues 1,4-Bis[2-[3-(1,1-dicyan ovinyl)-5,5-dimethyl-cyclohexeneyl]vinyl]benzene ( 1) and 1,4-Bis{2-[3-(1,1-dicya novinyl)-5,5-dimethyl-cyclohexeneyl]vinyl}-2,5-di-(n-butoxy)benzene ( 2) with A-π-D-π-A structure were synthesized and characterized. Optical properties (UV–vis and fluorescence) were studied for both compounds according to the spectra analysis. Theoretical calculation and cyclic voltammetry were used to analyzed the frontier molecular orbital of the two compounds. The result suggest that compound 2 can used not only as dopants but also as host emission and electron injection materials. Two kinds of red OLEDs were fabricated using 2 and all the devices achieved good CIE coordinates.

Lithium phenolate complexes for an electron injection layer in organic light-emitting diodes

We synthesized π-conjugated lithium phenolate complexes, lithium 2-(2-pyridyl)phenolate (LiPP), lithium 2-(2′, 2′′-bipyridine-6′-yl)phenolate (LiBPP), and lithium 2-(isoquinoline-1′-yl)phenolate (LiIQP). These complexes showed lower sublimation temperatures of 305–332 °C compared to 717 °C of LiF. The organic light-emitting devices (OLEDs) using these complexes as an electron injection layer exhibited high efficiencies which are comparable to that of the device using LiF. Especially, a 40-nm thick film of LiBPP or LiPP was effective as an electron injection material, providing low driving voltages, while such a thick film of LiF serves as a complete insulator, resulting in high driving voltages.

Organic cesium salt as an efficient electron injection material for organic light-emitting diodes

Cesium quinoline-8-oxide (Csq), as an organic salt, was synthesized and applied as an electron injection material for organic light-emitting diodes. A typical bilayer structure of N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine/tris-(8-hydroxy-quinolinolato) aluminum (III) (Alq3) was used to evaluate the electron injection of Csq. The results showed Csq/Al had better electron injection than LiF/Al cathode, and a maximum current efficiency of 4.13 cd/A (3.33 cd/A for LiF/Al) was obtained when the Csq layer is 2.0 nm. It is proposed that the enhanced electron injection by Csq is related to the interfacial chemical reaction between Csq and Al and the self-doping of Csq by librated Cs.

A soluble nonionic surfactant as electron injection material for high-efficiency inverted bottom-emission organic light emitting diodes

A soluble nonionic surfactant, polyethylenimine 80% ethoxylated (PEIE) solution, was used as the electron injection material in inverted bottom-emission organic light emitting diodes (OLEDs). The transparent PEIE film was formed on indium-tin-oxide cathode by simple spin-coating method and it was found that the electron injection was greatly enhanced. The devices with PEIE electron injection layer had achieved significant enhancement in luminance and efficiency. The maximum luminance reached 47 000 cd/m2, and the maximum luminance efficiency and power efficiency arrived at 19.7 cd/A and 10.6 lm/W, respectively. Our results indicate that PEIE is a promising electron injection material to realize high performance OLEDs.

Power-law-type electron injection through lithium fluoride nanolayers in phosphorescenceorganic light-emitting devices

This work reports a power-law-type electron injection model that accounts for the thicknesseffect of lithium fluoride (LiF) nanolayers used as an electron injection layer inphosphorescence organic light-emitting devices (PHOLEDs). A series of PHOLEDs werefabricated with various LiF thicknesses in order to investigate the influence of LiFnanolayer thickness on the device current. The PHOLEDs exhibited pronouncedly changedelectron currents by only 0.2 nm thickness variation. The device current as a function of LiFthickness excellently followed the power-law model proposed in this study, from which aphysical constant indicating the intrinsic nature of electron injection materials has beenextracted.