The enhanced performance of organic light-emitting diodes by using PCBM as the anode modification layer

[6,6]-Phenyl C₆₁ butyric acid methyl ester (PCBM) is used to modify an indium tin oxide (ITO)-coated substrate. Organic light-emitting diodes (OLEDs) using PCBM as the anode modification layer are fabricated. The dependence of performance on different PCBM thicknesses is also investigated. When the thickness of the PCBM film is appropriate, the brightness and efficiency of OLEDs are enhanced, which is attributed to an enhanced hole injection and an improved carrier balance. The enhancement of hole injection was ascribed to the formation of a dipole layer at the anode/organic interface.     

Surface modification of indium tin oxide anode with self-assembled monolayer modified Ag film for improved OLED device characteristics

Modification of electrodes has attracted much attention in the study of organic semiconductor devices. A self-assembled monolayer (SAM) of 4-fluorothiophenol is employed to modify the Ag film on the surface of indium tin oxide (ITO) to improve the hole injection and the surface morphology. The modified anode was characterized by X-ray photoelectron spectroscopy (XPS), atomic force microscope (AFM), and UV–vis transmittance spectra. To investigate the effect of the modification on the device characteristics, typical double layer devices with the structure of anode/-naphthylphenylbiphenyl diamine (NPB, 60 nm)/tris-(8-hydroxyquinoline) aluminum (Alq₃, 60 nm)/LiF(0.7 nm)/Al(100 nm) were fabricated using the modified anode and the bare ITO. The effect of Ag layer thickness on the device performance is also investigated. The results revealed that SAM modified ultra-thin Ag film is an effective buffer layer for organic light emitting diode. The device using the ITO/Ag (5 nm)/SAM as anode show improved device characteristics than that of using bare ITO as anode. The enhancements in luminance and efficiency are attributed to enhanced hole injection and smooth surface between anode and the organic material. The Ag thickness of 5 nm is chosen as an acceptable compromise between substrate transparency and the device performance.     

Enhancing the performance of polymer light-emitting diode via methanol treatment

Enhanced performance of polymer light-emitting diodes (PLEDs) was realized by performing methanol treatment on the emissive layer. Experimental results show that with methanol treatment the electron current increased owing to the rougher surface topography of the emissive layer. Therefore, the balance between electron and hole current was improved. The maximum luminous efficiency and external quantum efficiency of PLEDs increased from 19.7 cd/A and 6.89% to 22.5 cd/A and 7.87%, respectively.     

The performance enhancement in organic light-emitting diode using a semicrystalline composite for hole injection

A semicrystalline composite, 3, 4, 9, 10 perylenetetracarboxylic dianhydride （PTCDA） doped N,N＇-di（1-naphthyl）- N,N＇-diphenylbenzidine （NPB）, has been fabricated and characterized. An organic light-emitting diode using such a composite in hole injection exhibits the improved performance as compared with the reference device using neat NPB in hole injection. For example, at a luminance of 2000 cd/m2, the former device gives a current efficiency of 2.0cd/A, higher than 1.6cd/A obtained from the latter device. Furthermore, the semicrystalline composite has been shown thermally to be more stable than the neat NPB thin film, which is useful for making organic light emitting diodes with a prolonged lifetime.     

Efficiency control of organic light-emitting diode for high contrast ratio performance in active matrix display applications

We demonstrate the possibility of controlling organic light-emitting diodes (OLEDs) efficiency with an optimized hole-delay layer (HDL) in order to achieve very high contrast ratio needed for active matrix display applications. No triplet exciton confinement structure and a molybdenum trioxide (MoO₃) HDL at ITO/hole transport layer interface were used for a careful placing and adjusting of the exciton recombination zone. An optimized thickness of 50 nm thick MoO₃ allows to cut off the efficiency at the black level of active matrix driving mode while it gets greater at higher luminance at high current density region over 20 mA/cm² compared to conventional device. Our suggested efficiency control way is believed to be very useful to future active matrix OLED displays.     

使用PTB7作为阳极修饰层提高有机发光二极管的性能

采用poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b’]dithiophene-2,6-diyl][3-?uoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]](PTB7)作为有机发光二极管器件的阳极修饰层,制备了结构为indium tin oxide(ITO)/PTB7(不同浓度)/N,N’-Bis(naphthalen-1-yl)-N,N’-bis(phenyl)benzidine(NPB,40 nm)/8-hydroxyquinoline(Alq3,60 nm)/LiF(1 nm)/Al的系列器件,同时研究了不同浓度的PTB7对器件性能的影响.PTB7的最佳浓度为0.25 mg/mL,器件性能得到明显的改善,起亮电压为4.3 V.当驱动电压为14.6 V时,最大亮度为45800 cd/m2,最大电流效率为9.1 cd/A.与没有PTB7修饰的器件相比,其起亮电压降低了1.9 V,最高亮度提升了78.5%.器件性能提高归因于PTB7的插入使得空穴注入和传输能力大大改善.     

Indium tin oxide surface treatments for improvement of organic light-emitting diode performance

We have systematically investigated the influence of UV ozone and acid (HCl) treatments (separate and combined) of the surface of indium tin oxide (ITO) on the ITO parameters and the performance of organic light-emitting diodes (OLEDs) fabricated on the treated substrates. The ITO substrates were characterized by Hall measurements, Seebeck coefficient measurements and surface-probe microscopy. After ITO characterization, two types of devices (ITO/NPB/rubrene/Alq₃/LiF/Al and ITO/TPD/rubrene/Alq₃/LiF/Al) were fabricated on the differently treated substrates. It was found that in both cases the optimal treatment was HCl followed by UV ozone, which resulted in the lowest turn-on voltage and the highest luminous efficiency. The maximum luminous efficiency in the ITO/NPB/rubrene/Alq₃/LiF/Al OLED with HCl followed by UV ozone treatment was 2.15 lm/W compared to 1.46 lm/W with UV ozone treatment only. PACS 81.65Cf; 85.60.Jb     

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.     

Ultrafast laser ablation of indium tin oxide thin films for organic light-emitting diode application

Ultrafast laser ablation of ITO thin film coated on the glass has been investigated as a function of laser fluence as well as the number of laser pulses. The ablation threshold of ITO thin film was found to be 0.07 J/cm² that is much lower than that of glass substrate (about 1.2–1.6 J/cm²), which leads to a selective ablation of ITO film without damage on glass substrate. The changes in the electrical resistance and morphology of ablated trench of ITO electrode were found to be strongly dependent on the processing conditions. We present the performance of organic light-emitting diodes (OLED) fabricated with ITO electrode patterned by ultrafast laser ablation.     

Solution-processed WOx hole injection layer for efficient fluorescent blue organic light-emitting diode

Solution-processed tungsten oxide (s-WO_x) interfacial layer for efficient hole injection in fluorescent blue organic light-emitting diode (OLED) is demonstrated. The OLED using 2-methyl-9,10-bis(naphthalen-2-yl)anthracene (MADN) as emitter shows luminous efficiency of 3.3 cd/A, power efficiency of 2.5 lm/W and external quantum efficiency of 4.6% with Commission Internationale d'Eclairage (CIE) color coordinates of (0.154, 0.102). Using MADN doped 1-4-di-[4-(N,N-diphenyl)amino]styryl-benzene as emitter, luminous efficiency of 10.8 cd/A, power efficiency of 6.4 lm/W and external quantum efficiency of 7.2% with CIE color coordinates of (0.167, 0.283) are achieved. Atomic force microscopy and X-ray photoelectron spectroscopy show that s-WO_x features superior film morphology and non-stoichiometry with slight oxygen deficiency. Current-voltage characteristics and impedance spectroscopy analysis indicate that s-WO_x behaves slightly enhanced hole injection and accordingly contributes to improved device performance in comparison with conventional vacuum thermal evaporation WO_x. Our results pave an alternative way for broadening WO_x application with solution process and advancing fluorescent blue OLEDs.     

The visible and ultraviolet organic light-emitting diodes with germanium dioxide as facile solution-processed anode buffer layer

Germanium dioxide (GeO₂) aqueous solutions are facilely prepared and the corresponding anode buffer layers (ABLs) with solution process are demonstrated. Atomic force microscopy, X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy measurements show that solution-processed GeO₂ behaves superior film morphology and enhanced work function. Using GeO₂ as ABL of organic light-emitting diodes (OLEDs), the visible device with tris(8-hydroxy-quinolinato)aluminium as emitter gives maximum luminous efficiency of 6.5 cd/A and power efficiency of 3.5 lm/W, the ultraviolet device with 3-(4-biphenyl)-4-phenyl-5-tert-butylphenyl-1,2,4-triazole as emitter exhibits short-wavelength emission with peak of 376 nm, full-width at half-maximum of 42 nm, maximum radiance of 3.36 mW/cm² and external quantum efficiency of 1.5%. The performances are almost comparable to the counterparts with poly (3,4-ethylenedioxythiophene):poly (styrenesulfonate) as ABL. The current, impedance, phase and capacitance as a function of voltage characteristics elucidate that the GeO₂ ABL formed from appropriate concentration of GeO₂ aqueous solution favors hole injection enhancement and accordingly promoting device performance.     

Enhanced performance in organic photovoltaic devices with KMnO4 solution treated indium tin oxide anode modification

The properties of poly(3-hexylthiophene):(6,6)-phenyl C₆₁ butyric acid methyl ester (P3HT:PCBM) organic photovoltaic devices (OPVs) with indium tin oxide (ITO) anode treated by KMnO₄ solution are investigated. The optimized KMnO₄ solution has a concentration of 50 mg/L, and ITO is treated for 15 min. The modification of ITO anode results in an enhancement of the power conversion efficiency (PCE) of the device, which is responsible for the increase of the photocurrent. The performance enhancement is attributed to the work function modification of the ITO substrate through the strong oxygenation of KMnO₄, and then the charge collection efficiency is improved.     

Improved performances in top-emitting organic light-emitting diodes based on a semiconductor zinc oxide buffer layer

Electroluminescence performances are improved by inserting a semiconductor zinc oxide (ZnO) buffer layer into the emissive tris-(8-hydroxyquinoline)aluminum (Alq₃) layer and the semitransparent Al/Ag cathode in top-emitting organic light-emitting diodes (TEOLEDs) with structures of Si/SiO₂/Ag/Ag₂O/4,4′, 4″-tris(3- methylphenylphenylamino)triphenylamine/ 4,4′-bis[N-(1-naphthyl-1-)-N-phenyl- amino]-biphenyl/Alq₃/ZnO/Al/Ag. The thermal deposition of ZnO layer onto Alq₃ results in Alq₃ anion formation, which is beneficial to electron injection by generating some new energy levels in the forbidden band of Alq₃. In addition, a large hole-injection barrier of 2 eV at the interface of Alq₃/ZnO effectively blocks hole injection into Al/Ag cathode, leading to more carrier recombination in the emissive region.     

Enhanced performance in organic photovoltaic devices with a KMnO4 solution treated indium tin oxide anode modification

<正>The properties of poly（3-hexylthiophene）:（6,6）-phenyl C₆₁ butyric acid methyl ester（P3HT:PCBM） organic photovoltaic devices（OPVs） with an indium tin oxide（ITO） anode treated by a KMnO₄ solution are investigated.The optimized KMnO₄ solution has a concentration of 50 mg/L,and ITO is treated for 15 min.The modification of ITO anode results in an enhancement of the power conversion efficiency（PCE） of the device,which is responsible for the increase of the photocurrent.The performance enhancement is attributed to the work function modification of the ITO substrate through the strong oxygenation of KMnO₄,and then the charge collection efficiency is improved.     

Improved light extraction of GaN-based light-emitting diodes with surface-textured indium tin oxide electrodes by nickel nanoparticle mask dry-etching

GaN-based light-emitting diodes (LEDs) with surface-textured indium tin oxide (ITO) as a transparent current spreading layer were fabricated.The ITO surface was textured by inductively coupled plasma (ICP) etching technology using a monolayer of nickel (Ni) nanoparticles as the etching mask.The luminance intensity of ITO surface-textured GaN-based LEDs was enhanced by about 34% compared to that of conventional LED without textured ITO layer.In addition,the fabricated ITO surface-textured GaN-based LEDs would present a quite good performance in electrical characteristics.The results indicate that the scattering of photons emitted in the active layer was greatly enhanced via the textured ITO surface,and the ITO surface-textured technique could have a potential application in improving photoelectric characteristics for manufacturing GaN-based LEDs of higher brightness.     

A cast seed-mediated growth method for preparing gold nanoparticle-attached indium tin oxide surfaces

An effective approach to attach gold nanoparticles (AuNPs) directly on indium tin oxide (ITO) surface with higher density is reported. The attachment was carried out using a cast seed-mediated growth method, which was revised from our previous seed-mediated growth approach. The cast seeding with three-cycles of the drop of the seed solution containing Au nano-seed particles and the evaporation at 30 °C followed by the treatment in the growth solution containing HAuCl₄, cetyltrimethylammonium bromide (CTAB) and ascorbic acid was found to be suitable to prepare the AuNPs attached ITO surfaces having higher density and narrower size distribution. The 10-cycles cast seeding formed the connected or networked nanostructures of AuNPs, though the optical properties were different from those of the dispersed AuNP-attached ITO. The present cast seeding approach provides a facile and useful strategy to attach AuNPs on the surface without the use of certain organic binder molecules.     

Titania surface modification and photovoltaic characteristics with tungsten oxide

WO₃-coated TiO₂ film was prepared by depositing TiO₂ suspension containing small amounts of ammonium tungstate solution. The morphology and structure of the samples were characterized with high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and photoluminescence (PL) emission spectrum. The results showed that WO₃ formed a coating layer on surface of TiO₂ and significantly reduced the surface traps of TiO₂ nanoparticles. Transient photovoltage and electrochemical impedance measurements (EIS) were employed to study the charge separation/recombination process. The results revealed that the charge recombination was greatly retarded and the electron lifetime was increased due to the coating layer of WO₃. These observations showed good correlation with current-voltage analyses of dye-sensitized solar cell fabricated from these films, with WO₃ overlayer resulting in an increase in open-circuit voltage of up to 37 mV and 11% improvement in overall device efficiency.     

An investigation into the effects of oxygen plasma discharge on tin-doped indium oxide thin films

The surfaces of tin-doped indium oxide (ITO) thin films for polymer light-emitting electrochemical cells (LECs) were modified by oxygen plasma discharge. The properties of the ITO surfaces were evaluated by means of the measurements of X-ray photoelectron spectroscopy (XPS), contact angle, surface free energy and polarity. The influence of surface properties of the ITO thin films on the performance of polymer LECs was investigated in terms of the turn-on voltage, injection current and luminance. When oxygen plasma discharge was employed to modify the ITO surfaces, the surface properties of ITO are optimized due to the improvement of surface stoichiometry and the enhancement of wettability. And the improved surface properties benefited from the oxygen plasma discharge is observed to decay with the time after the plasma discharge. The difference in chemical composition, surface free energy and polarity between the non-treated and treated ITO surfaces appears to become smaller with the increase of the time after plasma discharge. In addition, the electrical and optical performance of the devices is found to become worse with the increasing time after plasma discharge on ITO substrates. The results demonstrate that the device performance strongly depends on the ITO surface properties and the ITO/organic interface characteristics.     

Electroless deposition of silver on synthesized calcite via surface modification

Metallic silver was deposited on the surface of synthesized calcite via a simple electroless deposition method. Calcite with cubic morphology was prepared first by homogeneous precipitation and it was subsequently surface modified using ammonium oxalate. The electroless deposition was carried out using formaldehyde as the reducing agent and silver nitrate as the silver source. Both calcite and the silver deposited calcite were characterized by different techniques. Surface modification of calcite with ammonium oxalate is necessary for the deposition of silver and the size of the deposited silver particles could be controlled by changing the deposition parameters such as concentration of the reagents and the deposition time. Lower concentration of silver ions (e.g. 0.01 M AgNO₃) and shorter deposition times (e.g. 30 min) lead to the formation of silver nanoparticles on calcite.     

Hydrophilic modification of polyethersulfone porous membranes via a thermal-induced surface crosslinking approach

A thermal-induced surface crosslinking process was employed to perform a hydrophilic surface modification of PES porous membranes. Difunctional poly(ethylene glycol) diacrylate (PEGDA) was used as the main crosslinking modifier. The addition of trifunctional trimethylolpropane trimethylacrylate (TMPTMA) into the reaction solutions accelerated the crosslinking progress of PEGDA on PES membranes. The membrane surface morphology and chemical composition were characterized by scanning electron microscopy (SEM) and FTIR-ATR spectroscopy. The mass gains (MG) of the modified membranes could be conveniently modulated by varying the PEGDA concentration and crosslinking time. The measurements of water contact angle showed that the hydrophilicity of PES membranes was remarkably enhanced by the coating of crosslinked PEGDA layer. When a moderate mass gain of about 150 μg/cm² was reached, both the permeability and anti-fouling ability of PES membranes could be significantly improved. Excessive mass gain not only contributed little to the anti-fouling ability, but also brought a deteriorated permeability to PES membranes.