Fabrication of efficient polymer light-emitting diodes using water/alcohol soluble poly(vinyl alcohol) doped with alkali metal salts as electron-injection layer

An effective electron-injection layer (EIL) is crucial to efficient polymer light-emitting diodes (PLEDs) with high work-function metal as cathode. This work presents the use of water/alcohol soluble poly(vinyl alcohol) (PVA), especially doped with alkali metal salts, as a highly effective EIL to fabricate efficient multilayer PLEDs, allowing the use of stable aluminum as the cathode. Using neat PVA as EIL, the maximum brightness and maximum current efficiency of the device [ITO/PEDOT:PSS/SY/PVA/Al(90 nm)] were significantly enhanced to 5518 cd/m² and 2.64 cd/A (from 395 cd/m² and 0.06 cd/A without the EIL) due to promoted electron-injection and hole-blocking. The device performance is further enhanced by doping the PVA with alkali metal salts (M₂CO₃ or CH₃COOM; M: Na, K, Cs), and the enhancement is increased with increasing dopant concentration. Particularly, the PVA doped with 30 wt% alkali metal carbonates revealed the best performance (20214–25163 cd/m², 5.83–6.83 cd/A). This has been attributed to improved electron-injection from aluminum cathode, which has been confirmed by the corresponding increase in the open-circuit voltages (V _oc) obtained from photovoltaic measurements. Current results indicate that commercially available PVA are promising electron-injection layer for PLEDs when doped with appropriate alkali metal salts.     

Hybrid solar cells based on CuInS2 and organic buffer-sensitizer layers

Hybrid solar cells on the basis of CuInS₂ (CIS) photoabsorber on Cu-tape (CISCuT) in combination with organic buffer layers of Zn-phthalocyanine (ZnPc), ZnPc:fullerene (ZnPc:C₆₀) composite and conductive polymer buffer layers of poly(3,4-ethylenedioxythiophene) (PEDOT) doped with polystyrenesulfonate (PSS) were prepared using vacuum evaporation and spin-casting techniques. To prepare solar cells with an active area of 2 cm², the appropriate deposition parameters and thickness of ZnPc, ZnPc:C₆₀ and PEDOT-PSS layers were selected experimentally. For preparation of semitransparent contact-window layers, chromium and gold were evaporated on the surface of ZnPc, ZnPc:C₆₀ and PEDOT-PSS films. It was found that an intermediate chromium layer improves PV properties of the structures with organic buffer layers. The photosensitivity at small illumination intensities of complete structures with ZnPc and ZnPc:C₆₀ layers increased more than one order of magnitude in comparison with the structures where the PEDOT-PSS buffer layer was deposited. The presence of C₆₀ in the composite-buffer layer results in increased photoconductivity. The best structure with composite ZnPc:C₆₀ buffer layer showed an open-circuit voltage of 560 mV, a short-circuit current density of around 10 mA/cm² and a photoconversion efficiency of around 3.3% under the light illumination with an intensity of 100 mW/cm² from a tungsten-halogen lamp. The low transmission of the semitransparent chromium-gold window layer is the reason for relatively low current density.     

Synthesis and blue electroluminescent properties of zinc (II) [2-(2-hydroxyphenyl)benzoxazole]

This study reports on the properties of organic light-emitting diodes (OLEDs) with zinc (II) [2-(2-hydroxyphenyl)benzoxazole] as a hole-blocking layer. OLEDs devices are prepared in a conventional OLEDs structure (i.e., anode/HTL/EL/HBL/cathode and anode/HTL/HBL/EL/cathode). The luminescence efficiencies and the turn-on voltage are significantly affected by the existence of the hole-blocking layer. This is attributed to an excellent hole-blocking property, which is in turn due to the high HOMO energy level (6.5 eV). The device showed luminous efficiency 2.46 lm/W at 5 V. The maximum luminance of about 10,000 cd/m² is obtained, and the turn-on voltage (2.5 V) is affected by the existence of the hole-blocking layer.     

Organic light emitting diodes based on dipyridamole drug

The dipyridamole drug [DIP: 2,6-bis(diethanolamino)-4,8-dipiperidinopyrimido(5,4-d)pyrimidine] is widely used in treatment of coronary heart disease for its antiplatelet and vasodilating activities, and its high intensity photoluminescence (PL) has been widely reported. In this work, the fabrication and the characterization of a new OLED using the DIP molecule as an emitting layer is reported. The devices were assembled using a heterojunction between three organic molecular materials: the N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (NPB) or the 1-(3-methylphenyl)-1,2,3,4-tetrahydroquinoline-6-carboxyaldehyde-1,1′-diphenylhydrazone (MTCD) as hole-transporting layer, the DIP layer as an emitting layer and the tris(8-hydroxyquinoline aluminum) (Alq₃) as the electron transporting layer. All the organic layers were sequentially deposited in a high vacuum by thermal evaporation onto indium tin oxide substrates and without breaking vacuum. Continuous electroluminescence emission was obtained in all configurations upon varying the applied bias voltage from 4 to 30 V, the observed wide emission band was centered at 493 nm. The luminance of the devices was about 1500 (cd)/m² with 4.5 cd/A of efficiency for the best device. The charge transport behavior in the OLED is also discussed as a function of different carrier injection levels.     

Transparent organic light-emitting devices with LiF/Yb:Ag cathode

Highly transparent organic light-emitting devices (TOLEDs) based on LiF/Yb:Ag cathode are reported. The device with a structure of Indium tin oxide/N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′-biph-enyl-4, 4′-diamine (50 nm)/tris (8-hydroxyquinoline) aluminum (50 nm)/LiF(0.5 nm)/Yb:Ag (15 nm, volume ratio 1:1) shows about the same luminances and spectra from both sides. At a current density of 20 mA/cm², the luminance and electroluminescent efficiency of the top/bottom side are 165/167 cd/m² and 0.825/0.835 cd/A, respectively. We attribute the characteristics to the high transmittance and low reflectivity of Yb:Ag cathode. The results suggest that LiF/Yb:Ag electrode can be used as an effective and stable cathode in TOLEDs or top-emitting OLEDs.     

Effect of Ca and buffer layers on the performance of organic light-emitting diodes based on tris-(8-hydroxyquinoline) aluminum

The effects of buffer layers, including LiF, LiCl, NaF, NaCl, NaI, KI, RbF, RbCl, CsF, CsCl, MgF₂, CaF₂, BaF₂, and BaCl₂ on electron injection and device performance in organic light-emitting diodes based on tris-(8-hydroxyquinoline) aluminum, were investigated systematically. The insertion of the buffer layers at the organic/cathode interface not only reduced the operating voltage, but also enhanced the luminance and efficiency, which is attributed to the improvement of electron injection efficiency. It was found that the efficiency of the electron injection was closely related to the inherent properties of the buffer layer, such as its melting point (MP) and dielectric constant (ε), as well as with the buffer layer's interface with the metallic electrode through the effective work function (WF). Low MP, low ε and low WF values result in an effective improvement in the injection of the electrons, and thus to the device performance. The electroluminescent performance was further improved by the introduction of calcium between the buffer layer and the aluminum electrode.     

Quality improvement of organic thin films deposited on vibrating substrates

Most of the Organic Light-Emitting Diodes (OLEDs) have a multilayered structure composed of functional organic layers sandwiched between two electrodes. Thin films of small molecules are generally deposited by thermal evaporation onto glass or other rigid or flexible substrates. The interface state between two organic layers in OLED device depends on the surface morphology of the layers and affects deeply the OLED performance. The morphology of organic thin films depends mostly on substrate temperature and deposition rate. Generally, the control of the substrate temperature allows improving the quality of the deposited films. For organic compounds substrate temperature cannot be increased too much due to their poor thermal stability. However, studies in inorganic thin films indicate that it is possible to modify the morphology of a film by using substrate vibration without increasing the substrate temperature. In this work, the effect of the resonance vibration of glass and silicon substrates during thermal deposition in high vacuum environment of tris(8-quinolinolate)aluminum(III) (Alq₃) and N,N′-Bis(naphthalene-2-yl)-N,N′-bis(phenyl)-benzidine (β-NPB) organic thin films with different deposition rates was investigated. The vibration used was in the range of hundreds of Hz and the substrates were kept at room temperature during the process. The nucleation and subsequent growth of the organic films on the substrates have been studied by atomic force microscopy technique. For Alq₃ and β-NPB films grown with 0.1 nm/s as deposition rate and using a frequency of 100 Hz with oscillation amplitude of some micrometers, the results indicate a reduction of cluster density and a roughness decreasing. Moreover, OLEDs fabricated with organic films deposited under these conditions improved their power efficiency, driven at 4 mA/cm², passing from 0.11 lm/W to 0.24 lm/W with an increase in their luminance of about 352 cd/m² corresponding to an increase of about 250% in the luminance with respect to the same OLEDs fabricated in the same way and with the same conditions without substrate vibration.     

Effect of fabrication process on characteristics of phosphorescence organic light emitting diodes with methoxy-substituted starburst low-molecule as a host

The effect of dry process and wet process on the characteristics of phosphorescence organic light-emitting devices (OLEDs) employing a phosphorescent dye fac-tris(2-phenylpyridine) iridium(III) (Ir(ppy)₃) doped into a methoxy-substituted starburst low-molecule material methoxy-substituted 1,3,5-tris[4-(diphenylamino) phenyl]benzene (TDAPB) are investigated. The FT-IR and absorption spectra of TDAPB films fabricated by a dry process, and a wet process are almost same, and the PL spectra of those films are different. The carrier transport capability of TDAPB by a dry process is lower than that by a wet process. The photoluminescence intensity of Ir(ppy)₃ doped in TDAPB fabricated by a wet process is higher than that by a dry process. A maximum external current efficiency of more than 20 cd/A and luminance of more than 10,000 cd/m² were obtained. Maximum luminance of devices monotonously decreases with increasing the thickness of a dry-processed emitting layer. The main emission zone of the OLED was located in almost at the center of the emitting layer. The improvement of device performance in the OLED fabricated by a wet process was achieved due to the high efficient energy transfer from TDAPB to Ir(ppy)₃, high carrier transporting capability and the formation of homogeneous film, compared with that fabricated by a dry process.     

Highly efficient organic light-emitting diodes fabricated utilizing nickel-oxide buffer layers between the anodes and the hole transport layers

The electrical and the optical properties of organic light-emitting diodes (OLEDs) fabricated utilizing nickel-oxide (NiO) buffer layers between the anodes and the hole transport layers were investigated. The NiO layer was formed by using a thermally evaporated nickel thin film and a subsequent oxidation process. The tunneling holes in the OLED were increased due to the existence of the NiO layer between the anode and the hole transport layer, resulting in enhanced efficiency for the OLED. These results indicate that OLEDs with NiO buffer layers hold promise for potential applications in highly-efficient flat-panel displays.     

Electric field tunable light emitting diodes containing europium β-diketonates with [2.2]paracyclophane moiety

The synthesis and electroluminescent (EL) properties of two europium complexes with unsymmetrical β-diketonates and 1,10-phenanthroline are reported. The molecules are substituted by functional groups with different donor–acceptor properties and contain [2.2]paracyclophane moiety. They were used to fabricate the organic light emitting diodes (OLEDs). A large emission wavelength tunability by the applied electric field is observed for OLED containing europium β-diketonate substituted by phenyl group, with the maximum of luminance of 8 cd/m². Such tunability disappears for OLED based on europium β-diketonate substituted by CH₃ group, for which the luminance decreases to ca 2.5 cd/m². Also in that case an emission band in UV disappears. The OLED stability is lower in the latter case too, showing the importance of the substitution on the OLED operation. It shows also a high potential for the electroluminescent properties control and improvement of these Eu based macromolecules through a simple β-diketonate ligand chemical structure modification.     

White OLED using β-diketones rare earth binuclear complex as emitting layer

In this work, the fabrication and the characterization of a white triple-layer OLED using a β-diketones binuclear complex [Eu(btfa)₃phenterpyTb(acac)₃] as the emitting layer is reported. The devices were assembled using a heterojunction between three organic molecular materials: the N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (NPB) as hole-transporting layer, the β-diketones binuclear complex and the tris(8-hydroxyquinoline aluminum) (Alq₃) as the electron transporting layer. All the organic layers were sequentially deposited under high vacuum environment by thermal evaporation onto ITO substrates and without breaking vacuum. Continuous electroluminescence emission was obtained varying the applied bias voltage from 10 to 22 V showing a wide emission band from 400 to 700 nm with about 100 cd/m² of luminance. The white emission results from a combined action between the binuclear complex, acting as hole blocking and emitting layer, blue from NPB and the typical Alq₃ green emission. The intensity ratio of the peaks is determined by the layer thickness and by the bias voltage applied to the OLED, allowing us to obtain a color tunable light source.     

Application of ultra-thin CdS film as buffer layer in non-doped blue organic light-emitting diodes

The device inserted 0.5 nm thick cadmium sulfide (CdS) as buffer layer, prepared by vacuum thermal evaporation method, has been studied on the non-doped blue organic light-emitting diode. Compared to the device without ultra-thin CdS film, the maximum luminance of the device with ultra-thin CdS film was 11,370 cd/m² at 11 V, and the maximum current efficiency reached 3.10 cd/A, increased 1.5 times and 1.2 times, respectively. In the optimized devices with the structure of ITO/MoO₃ (10 nm)/TPABBI (35 nm)/Bphen (40 nm)/CdS (0, 0.1, 0.3 and 0.5 nm)/LiF (0.5 nm)/Al (100 nm), the effects of CdS layer on the photoelectric performance of the devices were investigated in detail. When the CdS thickness was 0.3 nm, the maximum luminance was 13,590 cd/m² at 9 V and the turn on voltage was only 3 V. The maximum current efficiency of 3.42 cd/A was obtained. It is indicated that the simple structure of the device with inserted ultra-thin CdS film, cheap and stable inorganic photoelectric material, may be a promising way to fabricate hybrid organic–inorganic LEDs with high performances.     

Enhanced performance in organic light-emitting diodes by sputtering TiO2 ultra-thin film as the hole buffer layer

Organic light-emitting diodes were prepared using titanium oxide (TiO₂) ultra-thin film by RF magnetron sputtering as the hole buffer layer. The device configuration is ITO/TiO₂/N-N′-diphenyl-N-N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine/tris(8-quinolinolato)-aluminum/LiF/Al. The maximum luminous efficiency for the 1.2 nm TiO₂ device is increased by approximately 46% (6.0 cd/A), in comparison with that of the control device (4.1 cd/A). The atomic force microscopy shows that with the insertion of TiO₂ buffer layer, the roughness of ITO surface decreases, which is favorable to improve the device luminance and increase the device lifetime. The mechanism behind the enhanced performance is that the TiO₂ layer enhances most of the holes injected from the anode and improves the balance of the hole and electron injections.     

High efficiency green phosphorescent organic light emitting device with (TCTA/TCTA0.5TPBi0.5/TPBi): Ir(ppy)3 emission layer

A new high efficiency green light emitting phosphorescent device with an emission layer consisting of {4,4',4'-tris(N-carbazolyl)-triphenylamine[TCTA]/TCTA_0.5TPBi_0.5/1,3,5-tris(N-phenylbenzimiazole-2-yl)benzene[TPBi]}:tris(2-phenylpyridine)iridium(III)[Ir(ppy)₃] was fabricated and its electroluminescence characteristics were evaluated in comparison with those of devices with emission layers made of (TCTA_0.5TPBi_0.5):Ir(ppy)₃ and (TCTA/ TPBi):Ir(ppy)₃.The device with the emission layer consisting of (TCTA/TCTA_0.5TPBi_0.5/TPBi):Ir(ppy)₃ showed a luminance of 11,000 cd/m² at an applied voltage of 8 V and maximum current efficiency of 63 cd/A under a luminance of 500 cd/m². The peak wavelength in the electroluminescent spectral and color coordinate on the Commission Internationale de I'Eclairage(CIE) chart were 513 nm and (0.31, 0.62) in this device, respectively. Under a luminance of 10000 cd/m², the current efficiency of this device was 55 cd/A, which is 1.4 and 1.1 times better than those of the devices with the emission layers made of (TCTA_0.5TPBi_0.5):Ir(ppy)₃ and (TCTA/TPBi):Ir(ppy)₃, respectively.     

Semitransparent passive matrix organic light-emitting displays

There has been an increased interest in developing top emission organic light-emitting diodes (OLEDs) that are able to emit light from both sides of the OLED display. One important application of the top emission device structure is to achieve monolithic integration of a top-emitting OLED on a polycrystalline or amorphous silicon thin film transistors used in active matrix displays. A high performance dual-sided top-emitting polymer OLED developed in this work exhibited a total luminous efficiency of ∼5.0 cd/A at 4.0 V, which is comparable to that observed for a control device having bottom emission structure. A laser ablation technology was developed to define the pixels. The cathode separation was achieved without using the conventional reverse trapezoid type separators that are normally used for pixellated OLED displays. A prototype of semitransparent polymer light-emitting passive matrix display has a matrix of 100 × 32 with a display area of 32.25 mm by 11.15 mm.     

Synthesis and electroluminescent properties of blue emitting materials based on arylamine-substituted diphenylvinylbiphenyl derivatives for organic light-emitting diodes

This paper reports the synthesis and electroluminescent properties of a series of blue emitting materials with arylamine and diphenylvinylbiphenyl groups for applications to efficient blue organic light-emitting diodes (OLEDs). All devices exhibited blue electroluminescence with electroluminescent properties that were quite sensitive to the structural features of the dopants in the emitting layers. In particular, the device using dopant 4 exhibited sky-blue emission with a maximum luminance, luminance efficiency, power efficiency, external quantum efficiency and CIE coordinates of 39,000 cd/m², 12.3 cd/A, 7.45 lm/W, 7.71% at 20 mA/cm² and (x = 0.17, y = 0.31) at 8 V, respectively. In addition, a blue OLED using dopant 2 with CIE coordinates (x = 0.16, y = 0.18) at 8 V exhibited a luminous efficiency, power efficiency and external quantum efficiency of 4.39 cd/A, 2.46 lm/W and 2.97% at 20 mA/cm², respectively.     

“Quasi‐freestanding” Graphene‐on‐Single Walled Carbon Nanotube Electrode for Applications in Organic Light‐emitting Diode

An air‐stable transparent conductive film with “quasi‐freestanding” graphene supported on horizontal single walled carbon nanotubes (SWCNTs) arrays is fabricated. The sheet resistance of graphene films stacked via layer‐by‐layer transfer (LBL) on quartz, and modified by 1‐Pyrenebutyric acid N‐hydroxysuccinimide ester (PBASE), is reduced from 273 Ω/sq to about 76 Ω/sq. The electrical properties are stable to heat treatment (up to 200 ºC) and ambient exposure. Organic light‐emitting diodes (OLEDs) constructed of this carbon anode (T ≈ 89.13% at 550 nm) exhibit ≈88% power efficiency of OLEDs fabricated on an ITO anode (low turn on voltage ≈3.1 eV, high luminance up to ≈29 490 cd/m², current efficiency ≈14.7 cd/A). Most importantly, the entire graphene‐on‐SWCNT hybrid electrodes can be transferred onto plastic (PET) forming a highly‐flexible OLED device, which continues to function without degradation in performance at bending angles >60°.     

MoO3 buffer layer effect on photovoltaic properties of interpenetrating heterojunction type organic solar cells

Photovoltaic cells, with a conducting polymer/fullerene (C₆₀) interpenetrating heterojunction structure fabricated by spin-coating a conducting polymer onto a C₆₀ thin film, have been investigated and demonstrated a high efficiency as solar cells based on organic materials. The photovoltaic properties of the solar cells with a structure of indium-tin-oxide (ITO)/C₆₀/poly(3-hexylthiophene) (PAT6)/Au have been improved by the insertion of a molybdenum trioxide (VI) (MoO₃) layer as a cathode buffer layer. In the solar cells with the structure of ITO/C₆₀/PAT6/MoO₃/Au, the energy conversion efficiency has been improved to 1.15% under AM1.5 (100 mW/cm²) illumination.     

Bright electroluminescence from single-layer organic light-emitting diodes comprising an ambipolar carrier transport layer of phenyldipyrenylphosphine oxide

We investigated the carrier transport and recombination characteristics of single-layer organic light-emitting diodes (SLOLEDs) composed of a phenyldipyrenylphosphine oxide (POPy₂) layer doped with orange fluorescent molecules of 2,5-bis-[{bis-(4-methoxy-phenyl)-amino}-styryl]-terephthalonitrile (BST). The SLOLEDs achieved a high external quantum efficiency of 1.6% and a high luminance of 24,000 cd/m² at a low driving voltage of 8 V. These very good electroluminescence characteristics originate from factors that include our use of the following: (1) the ambipolar POPy₂ layer, which can transport balanced amounts of electrons and holes, (2) a high BST-doping concentration that traps injected carriers on BST molecules, and (3) insertion of an undoped POPy₂ layer next to a metallic cathode to prevent exciton quenching.     

New oxadiazole complex with bipolar ligand for organic light-emitting devices

New oxadiazole complex with bipolar ligand, Zn(POTPA)₂, was designed and synthesized, and used as an emitter material in single-layer organic electroluminescent (EL) devices (OLED). The UV absorbance of Zn(POTPA)₂ is caused by electron π–π^? transition. Zn(POTPA)₂ exhibited strong blue luminescence in solution and film. Compared with triphenylamine and 2,5-diphenyl-1,3,4-oxadiazole, cyclic voltammograms exhibited that Zn(POTPA)₂ has bipolar properties, and the optical band gap energy, Eg, calculated based on the cyclic voltammograms is nearly equal to that deduced from the absorption spectrum. Single-layer device with the structure of ITO/Zn(POTPA)₂/Mg:Ag were fabricated and blue electroluminescence was observed with a maximum luminance of 271 cd/m² and efficiency of 0.46 cd/A.