Properties of 2,6-di-tert.-butyl-4-(2,5-diphenyl-3,4-dihydro-2H-pyrazol-3-yl)-phenol as hole-transport material for life extension of organic light emitting diodes

We studied structural and optical properties of 5′ replaced pyrazoline by hindered phenol 2,6-di-tert.-butyl-4-(2,5-diphenyl-3,4-dihydro-2H-pyrazol-3-yl)-phenol (HPhP) films for application in organic light-emitting diode (OLED) as a hole transport layer (HTL). Analysis of impedance and current-voltage characteristics of ITO/HPhP/Al structure has shown that the current is limited by a space charged region with exponential distribution of traps near Fermi level. Characteristics of electroluminescence structure ITO/HPhP/Alq₃/poly(ethylene glycol) dimethyl ether/Al was studied and analyzed. We performed the comparative analysis of luminescence time decay in two types of electroluminescent devices with HTL from pyrazolines derivative with hindered phenol and without it. We showed that hindered phenol in HTL slows down the degradation processes in OLED.     

Laser-induced forward transfer of polymer light-emitting diode pixels with increased charge injection

Laser-induced forward transfer (LIFT) has been used to print 0.6 mm × 0.5 mm polymer light-emitting diode (PLED) pixels with poly[2-methoxy, 5-(2-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) as the light-emitting polymer. The donor substrate used in the LIFT process is covered by a sacrificial triazene polymer (TP) release layer on top of which the aluminium cathode and functional MEH-PPV layers are deposited. To enhance electron injection into the MEH-PPV layer, a thin poly(ethylene oxide) (PEO) layer on the Al cathode or a blend of MEH-PPV and PEO was used. These donor substrates have been transferred onto both plain indium tin oxide (ITO) and bilayer ITO/PEDOT:PSS (poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) blend) receiver substrates to create the PLED pixels. For comparison, devices were fabricated in a conventional manner on ITO substrates coated with a PEDOT:PSS hole-transporting layer. Compared to multilayer devices without PEO, devices with ITO/PEDOT:PSS/MEH-PPV:PEO blend/Al architecture show a 100 fold increase of luminous efficiency (LE) reaching a maximum of 0.45 cd/A for the blend at a brightness of 400 cd/m(2). A similar increase is obtained for the polymer light-emitting diode (PLED) pixels deposited by the LIFT process, although the maximum luminous efficiency only reaches 0.05 cd/A for MEH-PPV:PEO blend, which we have attributed to the fact that LIFT transfer was carried out in an ambient atmosphere. For all devices, we confirm a strong increase in device performance and stability when using a PEDOT:PSS film on the ITO anode. For PLEDs produced by LIFT, we show that a 25 nm thick PEDOT:PSS layer on the ITO receiver substrate considerably reduces the laser fluence required for pixel transfer from 250 mJ/cm(2) without the layer to only 80 mJ/cm(2) with the layer.     

Carbazole/triarylamine based polymers as a hole injection/transport layer in organic light emitting devices

This study examined the influence of the charge injection barriers on the performance of organic light emitting diodes (OLEDs) using polymers with a stepwise tuned ionization potential (I(p) approximately -5.01 - -5.29 eV) between the indium tin oxide (ITO) (phi approximately -4.8 eV) anode and tris(8-hydroxyquinolinato) aluminium (Alq3) (I(p) approximately -5.7 eV) layer. The energy levels of the polymers were tuned by structural modification. Double layer devices were fabricated with a configuration of ITO/polymer/Alq3/LiF/Al, where the polymers, Alq3, and LiF/Al were used as the hole injection/transport layer, emissive electron transport layer, and electron injection/cathode, respectively. Using the current density-voltage (J-V), luminescence-voltage (L-V) and efficiencies in these double layer devices, the device performance was evaluated in terms of the energy level alignments at the interfaces, such as the hole injection barriers (phi(h)(iTO/polymer) and phi(h)(polymer/Alq3)) from ITO through the polymers into the Alq3 layer, and the electron injection barrier (phi(e)(polymer/Alq3) or electron/exciton blocking barrier) at the polymer/Alq3 interface.     

Improved performance of the single-layer and double-layer organic light emitting diodes by nickel oxide coated indium tin oxide anode

The electrical and optical properties of the NiO films deposited under various conditions were first characterized. An ultra-thin layer of nickel oxide (NiO) was then deposited on the indium-tin oxide (ITO) anode to enhance the hole injection in the organic light-emitting diode (OLED) devices. A very low turn-on voltage (3 V) was actually observed for the device with the ITO/NiO anode in the conventional double layer heterojunction OLEDs. The enhancement of hole injection by the ITO/NiO anode was further verified by the hole-only device and by the device with a patterned NiO layer on the ITO anode. The luminance and the current density of the single-layer OLED device were also significantly improved by using the ITO/NiO anode to enhance the hole injection. Although the luminescence efficiency was low, the reasons of low efficiency were studied and the improvement method was proposed. Our results suggest that the NiO/ITO anode is an excellent choice to enhance the hole injection in OLED devices.     

Built-in voltage in organic light-emitting diodes with a use of Li2O/Al cathode

An effect of bilayer cathode Li2O/Al system was studied in Alq3 based organic light-emitting diodes with a variation of Li2O layer thickness from 0 to 10 nm. The device was made in a structure of ITO/(TPD)/Alq3/Li2O/Al. Current density-luminance-voltage (J-L-V) characteristics and a built-in voltage of the device were measured at ambient conditions. Built-in voltage in the device is generated due to a difference of work functions between the anode and cathode. From the J-L-V characteristics of the device, we observed an increase in luminance and current efficiency by more than 100 times and 2 times, respectively, for the device with 0.5 nm thick Li2O layer. The measured built-in voltage shows that the device with 0.5 nm thick Li2O layer has relatively higher built-in voltage compared to the others. Since the higher value of built-in voltage corresponds to the lower value of electron barrier height in cathode, the improvement in the efficiency for the device with 0.5 nm thick Li2O layer is thought to be due to a lowering of the electron barrier height.     

Flexible top-emitting organic light-emitting diodes using semi-transparent multi-metal layers

In this paper, the improved device performance of top-emitting organic light-emitting diodes (TEOLEDs) with a thin multi-metal layer stack of nickel/silver/nickel (Ni/Ag/Ni) and aluminum/silver/aluminum (Al/Ag/Al) that were used as the anode and cathode on a flexible substrate is discussed. In particular, Indium-Tin-Oxide (ITO) as an anode electrode has been used recently even though it has some problems for flexible devices. Therefore we suggested that a thin multi-metal layer electrode as a new anode is fabricated instead of ITO anode. It was verified that the ITO-free TEOLEDs showed an enhanced probability of the recombination of the electrons and holes through an improved electron/hole charge balance. We also analyzed the optical and electrical characteristics using the current density, luminance, luminance efficiency, external quantum efficiency (EQE), CIE x, y coordinates, and EL spectra of flexible TEOLED devices were characterized. ITO-free, flexible, green-emitting OLEDs with a low cost and a simple process were demonstrated.     

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.     

Investigation of the properties of indium tin oxide-organic contacts for optoelectronic applications

This paper presents some investigations on the electrical transport properties of ITO/single (double) layer organic semiconductor (m-DNB, benzil, PTCDA, Alq3) contacts in SIS-like (ITO/organic/Si) and MIS-like (ITO/organic/metal) heterostructures. The I-V characteristics have emphasised the injection properties of different contacts and the effect of space charge limited currents in correlation with the type and preparation conditions of the contacts. We have studied the influence of the type of contact (In/ITO; In/Al) on the electrical conduction in Alq3/PTCDA/Si/In heterostructure. In a planar grid contact configuration for In/Al/PTCDA/Al/In structure we have observed the effect of the low electric field on the shape of the I-V characteristic.     

Conductivity mechanisms in poly(p-phenylene vinylene) light-emitting diodes at high and low bias

Polymer light-emitting diodes (LEDs) based on the structure ITO conducting glass /poly(p-phenylene vinylene)/metal (Al, In, Mg) have been investigated with particular emphasis on some anomalous and poorly understood features of the conduction mechanisms. At large forward bias (above 3 V, where electroluminescence is seen) the DC current is dominated by hole injection at the bottom ITO electrode and is not very sensitive to the top electrode metal or the fabrication conditions. It always increases exponentially with voltage at 20°C, but studies on operating voltages and apparent ideality factors as a function of thickness indicate that the conduction mechanism probably involves thermally assisted tunnelling rather than a simple Schottky diode mechanism. In contrast, the current at low forward bias (0.1–3 V) is exceptionally sensitive to the top electrode material, fabrication conditions, and operating history of the device. Anomalous behaviour is often seen, particularly with new devices and with Mg or In electrodes. With Al electrodes, particularly after a top electrode annealing step, the behaviour becomes much more stable and resistive, and logJ increases linearly with V^1/4 over 3 orders of magnitude of current. The overall behaviour at low bias is controlled by the barrier to holes at the top electrode, but the role of filamentary defects, dopants, and the insulating interfacial layer is also discussed.     

Effect of ITO surface treatment on organic light emitting diodes

Performance of organic light emitting diodes with structure of ITO/NPB/NPB:Ir(piq)3/Alq3/Al was studied by inserting MoO3 hole injection layer (HIL) and using differently cleaned indium tin oxides (ITOs). High luminance and quantum efficiencies were obtained using HIL and highly cleaned ITO. High power efficiency was obtained using HIL and conventionally cleaned ITO, while low driving voltage was obtained using highly cleaned ITO without HIL.     

主客体掺杂和空穴阻挡对蓝色磷光OLED发光性能的影响研究

以铱配合物蓝色磷光材料Firpic作为掺杂剂,制备了基于CBP为主体的蓝色有机电致发光器件,其结构为ITO/CuPc/FIrpic:CBP(x%)/BCP/Alq3/LiF/Al,其中x%为发光层主客体掺杂浓度.分别研究了主客体掺杂浓度和空穴阻挡层BCP的厚度对器件发光性能的影响,当掺杂浓度为8%时,主客体间的能量传转移最充分,器件的启亮电压为5V,器件在20V时的亮度为7122.25cd/m2.器件电致发光(EL)光谱出现明显的红移现象,为Alq3部分参与了发光,影响了发光的色纯度,改变BCP的厚度,可以调节载流子复合区域和器件发光的色度坐标,达到改善器件发光性能的目的.     

Efficient TCO-free organic solar cells with modified poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) anodes

Organic photovoltaic cells (OPVs) with a highly conductive poly 3,4-ethylenedioxythiophene:poly styrenesulfonate (PEDOT:PSS) layer as an anode and that were modified with the addition of some organic solvents such as sorbitol (So), dimethyl sulfoxide (DMSO), N-methyl-pyrrolidone (NMP), dimethylformamide (DMF), and ethylene glycol (EG) were fabricated without the use of transparent conducting oxide (TCO). The conductivity of the PEDOT:PSS film that was modified with each additive was enhanced by three orders of magnitude. According to the atomic force microscopy (AFM) study, the conductivity enhancement might have been related to the better connections between the conducting PEDOT chains. The TCO-free solar cells with a modified PEDOT:PSS layer and an active layer composed of poly (3-hexylthiophene) (P3HT) and phenyl [6, 6] C61 butyric acid methyl ester (PCBM) performed as well as the indium-tin-oxide (ITO)-based organic solar cells. The power conversion efficiency (PCE) of the organic solar cells with a DMSO-, So + DMSO-, and EG-modified PEDOT:PSS layer reached 3.51, 3.64, and 3.77%, respectively, under an illumination of AM 1.5 (100 mW/cm2).     

The properties of heterojunction based on CuI/pentacene/Al

To improve the injection of charge carriers from ITO electrode into the molecular semiconductor in the pentacene-based photovoltaic structures, we propose introducing an additional transport copper iodide (CuI) layer with high conductivity. The organic flexible barrier based on ITO/CuI/pentacene/Al was fabricated using flexible polyethyleneterephtalate substrate with conductive ITO layer by vacuum deposition technique. CuI films, annealed at the temperature of 523 K, exhibited optical transmittance ∼80% in the wavelength range 400-900 nm, minimum resistivity about 1.8 × 10³ Ω/sq, and developed surface. They were used for fabrication of photosensitive barrier structure on the basis of pentacene.     

Study of the effect of the doped poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) polymeric anode on the organic light-emitting diode performances

Bottom-emitting organic diode devices with polymeric anode were fabricated and their performances were compared to devices with different anodes. The highly transparent (transmittance ≈ 90%) and conductive (700 S/cm) anode was poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) processed from aqueous solution and modified by addition of dimethyl sulfoxide (DMSO). The electro-optical characteristics of the DMSO-doped PEDOT:PSS based device and devices with architectures based on undoped PEDOT:PSS and/or indium tin oxide (ITO) were investigated and the effects of the different anodes were analyzed by means of electrical responses in static and dynamic regimes. The efficiency of the device with the proposed polymeric anode was comparable to that of ITO based device but reduced with respect to the device including PEDOT:PSS as hole-injection layer. These results were correlated to the film morphological properties and discussed in terms of interfacial state density modification.     

Electroluminescence of zinc selenium (ZnSe) nanocrystals co-doped with poly[2-methoxy-5-(2'-ethylhexyloxy-p-phenylenevinylene)] (MEH-PPV)

We have synthesized water soluble zinc selenium (ZnSe) nanocrystals by using mercaotoacetic acid (TGA) as the stabilizer. The synthesized ZnSe nanocrystals were co-doped with poly[2-methoxy-5-(2'-ethylhexyloxy-p-phenylenevinylene)] (MEH-PPV) to fabricate an organic/ inorganic hybrid multilayer light-emitting device (LED). The structure of the device was indium-tin-oxide (ITO)/poly (ethylene-dioxythiophene):poly(styrenesul-fonate) (PEDOT:PSS)/MEH-PPV:ZnSe/bathocuproine (BCP)/tris-(8-hydroxylquinoline)-aluminum (Alq3)/Al. We demonstrate that the device has a lower driving voltage and increased current densities and power efficiencies owing to the co-doped ZnSe quantum dots. We obtained good efficiency of the devices when the quality ratio of MEH-PPV and ZnSe quantum dots was 1:1.     

Fabrication and Properties of DC Magnetron Sputtered Indium Tin Oxide on Flexible Plastic Substrate

Indium tin oxide (ITO) thin films deposited on flexible polyethylene terephthalate (PET) substrates at low temperature by DC magnetron sputtering from an In-Sn (90-10 wt pct) alloy target were studied. The correla-tion between deposition conditions and ITO property was systematically investigated and characterized. These as-deposited ITO films were used as the anode contact for flexible organic light-emitting diodes (FOLEDs). The fabricated FOLEDs with a structure of PET/ITO/NPB (50 nm)/Alq (20 nm)/Mg:Ag (100 nm) showed a maximum luminance of 2125 cd/m2 at 13 V.     

Organic light-emitting diodes having carbon nanotube anodes

Single-walled carbon nanotube (SWNT) films on flexible PET (polyethyleneterephthalate) substrates are used as transparent, flexible anodes for organic light-emitting diodes (OLEDs). For polymer-based OLEDs having the structure: SWNT/PEDOT-PSS:MeOH/TFB (poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine)) + TPD-Si(2) (4,4'-bis[(p-trichlorosilylpropylphenyl)phenylamino]biphenyl) /BT (poly(9,9-dioctylfluorene-co-benzothiadiazole))/CsF/Al, a maximum light output of 3500 cd/m(2) and a current efficiency of 1.6 cd/A have been achieved. The device operational lifetime is comparable to that of devices with Sn-doped In(2)O(3) (ITO)/PET anodes. The advantages of this novel type of anode over conventional ITO are discussed.     

Solution-processed flexible polymer solar cells with silver nanowire electrodes

The conventional anode for organic photovoltaics (OPVs), indium tin oxide (ITO), is expensive and brittle, and thus is not suitable for use in roll-to-roll manufacturing of OPVs. In this study, fully solution-processed polymer bulk heterojunction (BHJ) solar cells with anodes made from silver nanowires (Ag NWs) have been successfully fabricated with a configuration of Ag NWs/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/polymer:phenyl-C(61)-butyric acid methyl ester (PCBM)/Ca/Al. Efficiencies of 2.8 and 2.5% are obtained for devices with Ag NW network on glass and on poly(ethylene terephthalate) (PET), respectively. The efficiency of the devices is limited by the low work function of the Ag NWs/PEDOT:PSS film and the non-ideal ohmic contact between the Ag NW anode and the active layer. Compared with devices based on the ITO anode, the open-circuit voltage (V(oc)) of solar cells based on the Ag NW anode is lower by ~0.3 V. More importantly, highly flexible BHJ solar cells have been firstly fabricated on Ag NWs/PET anode with recoverable efficiency of 2.5% under large deformation up to 120°. This study indicates that, with improved engineering of the nanowires/polymer interface, Ag NW electrodes can serve as a low-cost, flexible alternative to ITO, and thereby improve the economic viability and mechanical stability of OPVs.     

不同空穴注入电极对有机薄膜电致发光性能的影响

成功地制备了用铝掺杂的氧化锌（ＡＺＯ）透明导电膜作阳极的有机薄膜电致发光器件，并对单层和双层结构的ＡＺＯ器件以及以两种不同ＩＴＯ作阳极器件的电致发光光谱，电流电压特性，亮度电压特性以及电致发光量子效率进行了详细的比较分析和讨论。     

Improving efficiency of organic light-emitting diodes fabricated utilizing AZO/Ag/AZO multilayer electrode

Multilayer transparent electrode based on Al-doped zinc oxide (AZO)/Ag/Al-doped zinc oxide (AZO) was fabricated by sputtering, and a green organic light-emitting diode (OLED) device utilizing AZO/Ag/AZO as anode was fabricated. The AZO/Ag/AZO multilayer film exhibited superior square resistance and optical transmittance to those of commercial indium tin oxide (ITO). In comparison with the green OLEDs based on ITO and pure AZO anode, the green OLED based on AZO/Ag/AZO showed the highest light-emitting efficiency. The results indicate that AZO/Ag/AZO multilayer electrodes are a promising low-cost, low-toxic and low-temperature processing electrode scheme for OLED application.