Enhanced brightness of organic light-emitting diodes based on Mg:Ag cathode using alkali metal chlorides as an electron injection layer

Different thicknesses of cesium chloride (CsCl) and various alkali metal chlorides were inserted into organic light-emitting diodes (OLEDs) as electron injection layers (EILs). The basic structure of OLED is indium tin oxide (ITO)/N,N′-diphenyl-N,N′-bis(1-napthyl-phenyl)-1.1′-biphenyl-4.4′-diamine (NPB)/tris-(8-hydroxyquinoline) aluminum (Alq₃)/Mg:Ag/Ag. The electroluminescent (EL) performance curves show that both the brightness and efficiency of the OLEDs can be obviously enhanced by using a thin alkali metal chloride layer as an EIL. The electron injection barrier height between the Alq₃ layer and Mg:Ag cathode is reduced by inserting a thin alkali metal chloride as an EIL, which results in enhanced electron injection and electron current. Therefore, a better balance of hole and electron currents at the emissive interface is achieved and consequently the brightness and efficiency of OLEDs are improved.     

Optimized Performances of Thick Film Organic Lighting-Emitting Diodes

The performance of organic light-emitting diodes (OLEDs) with thick film is optimized. The alternative vanadium oxide (V₂O₅) and N,N'-di(naphthalene-1-yl)-N,N'-diphenyl-benzidine (NPB) layers are used to enhance holes in the emissive region, and 4,7-dipheny-1,10-phenanthroline (Bphen) doped 8-tris-hydroxyquinoline aluminium (Alq₃) is used to enhance electrons in the emissive region, thus ITO/V₂O₅ (8nm)/NPB (52nm)/V₂O₅ (8nm)/NPB (52nm)/Alq₃ (30 and 45nm)/Alq₃:Bphen (30wt%, 30 and 45nm)/LiF (1nm)/Al (120nm) devices are fabricated. The thick-film devices show the turn-on voltage of about 3V and the maximal power efficiency of 4.5lm/W, which is 1.46 times higher than the conventional thin-film OLEDs.     

Bond cutting in K‐doped tris(8‐hydroxyquinoline) aluminium

A series of Al 2p, K 2p, O 1s and N 1s core‐level spectra have been used to characterize the interaction between potassium (K) and tris(8‐hydroxyquinoline) aluminium (Alq₃) molecules in the K‐doped Alq₃ layer. All core‐level spectra were tuned to be very surface sensitive in selecting various photon energies provided by the wide‐range beamline at the National Synchrotron Radiation Research Center, Taiwan. A critical K concentration (x = 2.4) exists in the K‐doped Alq₃ layer, below which the K‐doped atoms generate a strained environment near the O and N atoms within 8‐quinolinoline ligands. This creates new O 1s and N 1s components on the lower binding‐energy side. Above the critical K coverage, the K‐doped atoms attach the O atoms in the Al—O—C bonds next to the phenoxide ring and replace Al—O—C bonds by forming K—O—C bonds. An Alq₃ molecule is disassembled into Alq₂ and Kq by bond cutting and bond formation. The Alq₂ molecule can be further dissociated into Alq, or even Al, through subsequent formations of Kq.     

Observation of ferromagnetic ordering in Mn‐doped 8‐hydroxy‐quinoline aluminum

The ferromagnetic property of Mn‐doped 8‐hydroxy‐quinoline aluminum (Alq₃), synthesized by thermal co‐evaporation of pure Mn metals and Alq₃ powders, was investigated. The weak ferromagnetic property was observed in 5%‐doped Alq₃, with saturation magnetization of around 0.05μ_B/Mn. The doped Mn chemically interacted with O atoms, producing a new gap state at 0.34 eV above the highest occupied molecular orbital and reducing the effective electron concentration. This led to the decrease of the electron affinity and increase of the optical bandgap, resulting in the reduction of the hole‐injection barrier in comparison with the electron‐injection barrier to the Alq₃ layer. From these, the origin of the observed ferromagnetism is suggested. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Bond cutting in Cs‐doped tris(8‐hydroxyquinoline) aluminium

A series of Cs 4d and Al 2p spectra associated with valence‐band and cut‐off spectra have been used to characterize the interaction between caesium and tris(8‐hydroxyquinoline) aluminium (Alq₃) molecules in a Cs‐doped Alq₃ layer. The Cs 4d and Al 2p spectra were tuned to be very surface sensitive by selecting a photon energy of 120 eV at the National Synchrotron Radiation Research Center, Taiwan. A critical Cs concentration exists, above which a new Al 2p signal appears next to the Al 2p peak of Alq₃ in the lower binding‐energy side. The Al 2p signal was analyzed and assigned as being contributed from a mixture of Alq₂, Alq and Al. Experimental data supported the observation that bond cutting of Alq₃ by the doped Cs atoms occurred at high Cs doping concentration.     

The roles of zinc selenide sandwiched between organic layers and its applications in white light-emitting diodes

In this paper, the roles of zinc selenide (ZnSe) sandwiched between organic layers, i.e. organic/ZnSe/aluminum quinoline (Alq₃), have been studied by varying device structure. A broad band emission was observed from ITO/poly(N-vinylcarbazole)(PVK)(80 nm)/ZnSe(120 nm)/ Alq₃(15 nm)/Al under electric fields and it combined the emissions from the bulk of PVK, ZnSe and Alq₃, however, emission from only Alq₃ was observed from trilayer device ITO/N,N^′-bis-(1-naphthyl)-N,N^′-diphenyl-1, 1^′-biphenyl-4, 4^′-diamine (NPB) (40 nm)/ZnSe(120 nm)/ Alq₃(15 nm)/Al. Consequently the luminescence mechanism in the ZnSe layer is suggested to be charge carrier injection and recombination. By thermal co-evaporating Alq₃ and 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB), we get white light emission with a Commission Internationale de l’E clairage (C.I.E) co-ordinates of (0.32, 0.38) from device ITO/PVK(80 nm)/ZnSe(120 nm)/ Alq₃:DCJTB(0.5 wt% DCJTB)(15 nm)/Al at 15 V and the device performs stably with increasing applied voltages.     

1.53 μm electroluminescence from ErF3-doped organic light-emitting diodes

Near-infrared (NIR) organic light-emitting devices (OLEDs) are demonstrated by employing erbium fluoride (ErF₃)-doped tris-(8-hydroxyquinoline) aluminum (Alq₃) as the emitting layer. The device structure is ITO/N,N′-di-1-naphthyl-N,N′-diphenylbenzidine (NPB)/Alq₃: ErF₃/2,2′,2″-(1,3,5-phenylene)tris(1-phenyl-1H-benzimidazole) (TPBI)/Alq₃/Al. Room-temperature electroluminescence around 1530 nm is observed due to the ⁴I_13/2–⁴I_15/2 transition of Er³⁺. Full width at half maximum (FWHM) of the electroluminescent (EL) spectrum is ～50 nm. NIR EL intensity from the ErF₃-based device is ～4 times higher than that of Er(DBM)₃Phen-based device at the same current. Alq₃–ErF₃ composite films are investigated by the measurements of X-ray diffraction (XRD), absorption, photoluminescence (PL) and PL decay time. Electron-only devices are also fabricated. The results indicate that energy transfer mechanism and charge trapping mechanism coexist in the NIR EL process.     

Investigation of an efficient YbF3/Al cathode for tris-(8-hydroxyquinoline)aluminum-based small molecular organic light-emitting diodes

Efficient tris-(8-hydroxyquinoline)aluminum (Alq₃)-based organic light-emitting diodes (OLEDs) using YbF₃ as the electron injection layer have been investigated. With an YbF₃ (3.0 nm)/Al cathode, the device with Alq₃ as the emitting layer achieved a better performance than the control device with a LiF (0.5 nm)/Al cathode. The release of the low-work-function metal Yb is responsible for the performance enhancement. From the analysis by atomic force spectroscopy and X-ray photoemission spectroscopy, it is observed that the Alq₃-cathode interface could be well covered by YbF₃ at an optimum thickness of 3.0 nm, which helps to prevent the contact between Alq₃ and Al, and to reduce the destruction of Alq₃ by Al.     

High power efficiency solution‐processed double‐layer blue phosphorescent organic light‐emitting diode by controlling charge transport at the emissive layer and heterojunction

We have demonstrated an effective method of enhancing the power efficiency of double–emissive solution‐processed blue phosphorescent organic light‐emitting diode (PHOLED) by controlling the charge transport in the heterojunction and emissive layer. The first emissive layer consists of poly(vinylcarbazole) (PVK) and bis(4,6 difluorophenylpyridinato‐N,C2)picolinatoiridium (FIrpic) mixed with 4,4′,4″‐tris(N‐carbazolyl)‐triphenylamine (TCTA) or 1,3‐bis[(4‐tert‐ butylphenyl)‐1,3,4 oxidiazolyl] phenylene (OXD‐7). The second layer consists of an alcohol‐soluble 2,7‐bis(diphenylphosphoryl)‐9,9′‐spirobi[fluorene] (SPPO13) and FIrpic blend. The incorporation of OXD‐7 into PVK blurs the interface between the emissive layers and widens the recombination zone while blending TCTA into PVK reduces the hole‐ injection barrier from PEDOT:PSS to PVK. By adding TCTA or OXD‐7 into the first emissive layer, we have achieved a power efficiency of 10 lm/W and 11 lm/W, respectively, at 1000 cd/m².

     

Highly efficient yellow organic light-emitting diodes based on a hole-dominant host layer co-doped with yellow emitting and electron transporting guests

We fabricated high efficiency yellow-color organic light-emitting diodes (OLEDs) by co-doping 2,8-di(t-butyl)-5,11-di[4-(t-butyl)phenyl]-6,12-diphenylnaphthacene (TBRb) and tris(8-hydroxyquinolinato)aluminum (Alq₃) guests in a N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) host. Co-doping of electron-transporting Alq₃ with yellow dopant TBRb in hole-dominant NPB layers resulted in substantial luminance-yield improvement compared to TBRb-only counterparts without color contamination from Alq₃ green emission. The luminance yield of 11.4 cd/A, corresponding to the co-doping of 8% TBRb and 2% Alq₃, is larger than previously reported luminance-yield values of conventional TBRb-based yellow OLEDs. Another advantage of the TBRb-Alq₃ co-doped OLEDs is an insignificant roll-off of efficiency at high current-density and/or brightness levels.     

Studies on influence of light on fluorescence of Tris-(8-hydroxyquinoline)aluminum thin films

Tris-(8-hydroxyquinoline)aluminum (Alq₃) thin films, the most widely used electron transport/emissive material in the organic electroluminescent (EL) devices, have been deposited on glass substrates by thermal evaporation process. Alq₃ thin films were exposed to light for various time periods under normal ambient. The fluorescence of as-prepared and light exposed Alq₃ thin films and formation of luminescent quencher have been studied using fluorescence, Mass, MALDI-ToF-MS, ¹H & ¹³C NMR, and FT-IR spectroscopy. It is observed that among the three 8-hydroxyquinoline (HQ) units in Alq₃ molecule, one HQ unit is affected during the light exposure in the normal ambient. It is found that the affected resultant Alq₃ molecule containing the carbonyl group acts as fluorescent quencher and the energy of excitons in the Alq₃ molecule in the light exposed Alq₃ thin films can be non-radiatively transferred to the neighboring fluorescent quencher, quenching the fluorescence of light exposed Alq₃ thin films in the normal ambient.     

新型苯乙烯衍生物掺杂的蓝色及白色有机电致发光器件

研究了新型高效蓝色掺杂剂EBDP的电致发光性能. 分别以EBDP为掺杂剂制备了结构为氧化铟 锡(ITO)/酞菁铜(CuPc)/N,N′-二(1-萘基)-N,N′-二苯基-1,1′-联苯-4-4′-二胺(NPB)/2- 叔丁基-9,10-二-(2-萘基)蒽(TBADN):EBDP/8-羟基喹啉铝(Alq₃)/LiF/Al 与ITO /CuPc/NPB/TBADN:EBDP: 4-二氰亚甲基-2-叔丁基-6-(1,1,7,7-四甲基久咯呢定基-9-烯基)- 4H-吡喃/Alq_{3 关键词： 有机电致发光 蓝色掺杂剂 蓝色电致发光器件 白色电致发光器件}     

Development of organic light‐emitting diodes for electro‐optical integrated devices

Organic light‐emitting diodes (OLEDs) are discussed for electro‐optical integrated devices that are used for optical signal transmission. Organic optical devices including polymeric optical fibers are used for optical communication applications to realize polymeric electro‐optical integrated devices. The OLEDs were fabricated by vacuum process, i.e. the organic molecular beam deposition (OMBD) technique or a solution process on a polymeric or a glass substrate, for comparison. Optical signals faster than 100 MHz have been created by applying pulsed voltage directly to the OLED utilizing rubrene doped in 8‐hydoxyquinolinum aluminum (Alq₃), as an emissive layer. OLEDs fabricated by solution process utilizing rubrene doped in carrier‐transporting materials have also discussed. OLEDs utilizing polymeric materials by solution process are also fabricated and discussed. Moving‐picture signals are transmitted utilizing both vacuum‐ and solution‐processed OLEDs, respectively.     

Probing electric field distribution in organic double-layer diode by electric field induced optical second harmonic generation

Analyzing spectroscopic optical properties of an organic double-layer diode comprised of α-NPD and Alq₃ layers, we studied the selectively probing of electric field distribution in one of the two layers by using the microscopic electric field induced optical second harmonic generation (EFISHG) measurement. Spectroscopic SHGs from Indium–Zinc-Oxide/N,N-Di(naphthalene-1-yl)-N,N′-diphenyl-benzidine/tris(8-quinolinolato) aluminium/Al (IZO/α-NPD/Alq₃/Al) diodes were measured. Results showed that the SHG peaks were generated at 940 and 1050 nm from the α-NPD and Alq₃ layers, respectively, due to the EFISHG process, and the electric field in each layer can be selectively probed. The contribution of the accumulated charge at the double-layer α-NPD and Alq₃ interface was also identified by the d.c. voltage dependence on the EFISHG intensity.     

Simple near‐infrared photodetector based on charge transfer complexes formed in molybdenum oxide doped N,N′‐di(naphthalene‐1‐yl)‐N,N′‐diphenyl‐benzidine

We have demonstrated a simple near‐infrared (NIR) photodetector (PD) based on charge transfer complex (CTC) formed in molybdenum trioxide (MoO₃) doped N,N′‐di(naphthalene‐1‐yl)‐N,N′‐diphenyl‐benzidine (NPB), which shows a photocurrent of about 0.35 A/cm² at –3 V under 980 nm illumination. The existence of CTC formation promotes photocurrent generation which is investigated by comparison with MoO₃ doped 2‐methyl‐9,10‐di(2‐naphthyl)anthracene (MADN) film which has no CTC absorption. It can be evolved that this kind of simple‐structure photodetector has potential application in the near‐infrared (NIR) detection area. It is shown in this Letter that although both MoO₃ and NPB have larger energy gaps of about 3 eV and weak absorption in the NIR region, the charge transfer complexes formed by mixing the two materials show an extra absorption band and good photoelectric response in the NIR region.

     

Color-tunable organic light-emitting devices using a thin N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine layer at heterojunction interface

Novel types of multilayer color-tunable organic light-emitting devices (OLEDs) with the structure of indium tin oxide (ITO)/N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB)/aluminum (III)bis(2-methyl-8-quinolinato)4-phenylphenolato (BAlq)/tris-(8-hydroxyquinolate)-aluminum (Alq₃)/Mg:Ag were fabricated. By inserting a thin layer with different thickness of a second NPB layer at the heterojunction interface of BAlq/Alq₃, the emission zone of devices shifted greatly and optoelectronic characteristics underwent large variation. Although BAlq was reported as a very good hole-blocking and blue-light-emission material, results of measurements in this paper suggested that a certain thickness of NPB layer between BAlq and Alq₃ plays an important role to modify device characteristics, which can act as recombination-controlling layer in the multilayer devices. It also provides a simple way to fabricate color-tunable OLEDs by just changing the thickness of this “recombination-controlling” layer rather than doping by co-evaporation.     

New double graded structure for enhanced performance in white organic light emitting diode

This study presents a new design that uses a combination of a graded hole transport layer (GH) structure and a gradually doped emissive layer (GE) structure as a double graded (DG) structure to improve the electrical and optical performance of white organic light-emitting diodes (WOLEDs). The proposed structure is ITO/m-MTDATA (15 nm)/NPB (15 nm)/NPB: 25% BAlq (15 nm)/NPB: 50% BAlq (15 nm)/BAlq: 0.5% Rubrene (10 nm)/BAlq: 1% Rubrene (10 nm)/BAlq: 1.5% Rubrene (10 nm)/Alq₃ (20 nm)/LiF (0.5 nm)/Al (200 nm). (m-MTDATA: 4,4′,4″ -tris(3-methylphenylphenylamino)triphenylamine; NPB: N,N′-diphenyl-N,N′-bis(1-naphthyl-phenyl)-(1,1′-biphenyl)-4,4′-diamine; BAlq: aluminum (III) bis(2-methyl-8-quinolinato) 4-phenylphenolate; Rubrene: 5,6,11,12-tetraphenylnaphthacene; Alq₃: tris-(8-hydroxyquinoline) aluminum). By using this structure, the best performance of the WOLED is obtained at a luminous efficiency at 11.8 cd/A and the turn-on voltage of 100 cd/m² at 4.6 V. The DG structure can eliminate the discrete interface, and degrade surplus holes, the electron-hole pairs are efficiently injected and balanced recombination in the emissive layer, thus the spectra are unchanged under various drive currents and quenching effects can be significantly suppressed. Those advantages can enhance efficiency and are immune to drive current density variations.     

基于ZnSe的有机-无机异质结电致发光器件

以电子束蒸发的方法制备硒化锌(ZnSe)薄膜，研究了基于ZnSe的有机-无机异质结电致发光器件.在双层器件ITO/ZnSe(50nm)/Alq₃(12nm)/Al中看到了峰值位于578nm的ZnSe电致发光，却很难得到单层器件ITO/ZnSe(50—120nm)/Al的电致发光；在此基础上进一步引入有机空穴传输层(HTL)，通过改变器件的结构，讨论了ZnSe对有机-无机异质结器件ITO/HTL/ZnSe/Alq₃/Al电致发光特性的影响.其电致发光光谱的研究结果证实了ZnSe在器件中的作用：ZnSe既起传输电子的作用，也起到传输空穴的作用，还作为发光层.并对ZnSe的发光机理进行了讨论. 关键词： 硒化锌 有机-无机异质结 电致发光 空穴传输层     

Charge carriers bulk recombination instead of electroplex emission after their tunneling through hole-blocking layer in OLEDs

Charge carriers bulk recombination instead of forming electroplex after their tunneling through a hole-blocking layer, i.e. 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), in organic electroluminescence (EL) device ITO/poly-(N-vinyl-carbazole)(PVK)/BCP/tris(8-hydroxyquinoline) aluminum (Alq₃)/Al is reported. By changing the thickness of BCP layer, one can find that high electric fields enhance the tunneling process of holes accumulated at the PVK/BCP interface into BCP layer instead of forming “electroplex emission” as reported earlier in literatures. Our experimental data show that charge carriers bulk recombination takes place in both PVK layer and BCP layer, and even in Alq₃ layer when BCP layer is thin enough. Further, it is suggested that PVK is the origin of the emission shoulder at 595 nm in the EL spectra of trilayer device ITO/PVK/BCP/Alq₃/Al.     

Charge‐transfer‐induced doping at an electron donor (α‐sexithiophene)/acceptor (C60) interface and mobility improvement

We studied various aspects relating to surface charge‐transfer‐induced doping at an organic/organic interface using in situ electrical measurements with a field‐effect transistor (FET) during the formation of the electron donor/acceptor interface. Adsorption of the electron‐accepting molecules (C₆₀) on top of the electron donating molecules (α‐6T) led to an increase in the FET hole mobility in an α‐6T film. Under illumination, the FET hole mobility in the α‐6T film with C₆₀ deposition was significantly increased in comparison with that in the dark due to exciton dissociation at the C₆₀/α‐6T interface, resulting in a large threshold voltage shift. The origin of the mobility increase is explained by the multiple trapping and release (MTR) model in which the mobility is determined by the carrier density. Various phenomena relevant to charge transfer and charge transport at the organic/organic interface are reported and their origins are discussed. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)