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In this study, the space charge limited current transport model was applied to simulate the current density–voltage characteristics of two-layer NPB/Alq3 organic light emitting diodes (OLEDs). Several parameters for simulations were moderately adjusted, and the best set of parameters were obtained. Upon increasing the applied voltage, the junction temperature of the OLED increased, further influencing the transport of the carriers. When the junction temperature was used as the reference temperature in analysis of the relationship between voltage and temperature, the simulated current density–voltage characteristics were found to be very consistent with the experimental data.

Most small molecule organic light emitting diode (SM-OLED) device structures are made in one layer using solution-based processing because the solution is usually a high dissolvent material that easily attacks the layer below it. We demonstrate a simple and reliable stamping technique for fabricating multi-solution process flexible white SM-OLEDs. The structure is anode/spin-hole injection layer/spin-emitting layer/stamping-electron transport layer/cathode. Poly(di-methyl silane) (PDMS) stamp is used for transferring electron transport layer. An intermediate ultraviolet-ozone surface treatment is introduced to temporarily modify the PDMS stamp surface. Then, the solution-based electron transport layer film can therefore be uniformly formed on top of the PDMS surface. After that the electron transport layer film on the PDMS stamp is transfer-printed onto the emitting layer with suitable heating and pressing. A solution-based processing is successfully established to efficiently fabricate flexible white SM-OLEDs. The SM-OLEDs were obtained at the current density of 20 mA/cm 2 , luminance of 1062 cd/m 2 , current efficiency of 5.57 cd/A, and Commission internationale de l'eclairage coordinate of (0.32, 0.35).

In this paper, we adopt solution/evaporation hybrid processes to fabricate white organic light emitting diodes (WOLED) with a light-emitting area of 1.5 cm * 1.5 cm. The hole injection layer (PEDOT:PSS), p-type doped hole transport layer (NPB:F4-TCNQ), and emitting layer (UBH-215:UBD-07:DBP) are coated using the solution process while the electron transport layer (TPBI), LiF, and Al are coated through vacuum thermal evaporation. The light-emitting layer is a fluorescent material (UBH-215 as host) that combines the blue fluorescent material UBD-07 and the red fluorescent material DBP for a dual band light-emitting diodes. We found that if the red DBP doping concentration is too high, the overall emitting color tends toward reddish, and the luminance efficiency decreases. For WOLEDs whose red DBP doping concentration is 0.25%, the luminance reaches 192 cd/m2, and the current efficiency 0.94 cd/A at 9V voltage. Introducing a hole injection layer between the ITO and emitting layer can enhance both the luminance and efficiency of a WOLED. In addition to the hole injection layer, it is found that inserting p-type doped HTL (NPB:F4-TCNQ) between HIL and EML using the spin- coating method can also improve luminance and efficiency. Furthermore, by optimizing and adjusting the dopant ratio and thickness of p-type doped HTL, the luminance reaches 393.1 cd/m2, the current efficiency reaches 1.32 cd/A under 9V voltage, and the OLED lifetime (half-value period) increases from the original 0.8 hours to 2 hours.

In this paper, the flexible white organic light-emitting diodes (WOLED) was fabricated on polyethylene naphthalate (PEN) with structure of ITO/EHI608 (75 nm)/HTG-1 (10 nm)/3% EB502:0.8% EY53 (5 nm)/3% EB502 (35 nm)/Alq3 (10 nm)/LiF (0.8 nm)/Al (150 nm) and was compared with glass substrate the same structure. It was seen that the performances of flexible and glass substrate are almost the same. The luminance, current efficiency, and CIE coordinates of flexible device is 6351 cd/m2, 12.7 cd/A, and (0.31, 0.38) at 50 mA/cm2, respectively. Then, an Al2O3/HfO2 film on polyethylene terephthalate (PET) was deposited using atomic layer deposition (ALD) as a thin film encapsulation layer have been described and compared, such as the characteristics of water permeability and lifetime of flexible WOLED. The results show that the PET/ALD film low value of about 0.04 g/m2d, and the PET film shows WVTR of about 3.8 g/m2/d. The lifetimes of PET/ALD and PET encapsulations are 840 min and 140 min, respectively. Simultaneous deposition of ALD film on PET film gave the lifetime of flexible WOLED is six times longer than device without ALD encapsulation.

Display Omitted Investigating PhOLEDs with different solvents and dopants concentrations.High efficiency white PhOLEDs induced by the electron transport/hole blocking material.To achieve efficient and low-cost small-molecule-based white PhOLEDs. In this study, white phosphorescent organic light-emitting diodes (PhOLEDs) have been demonstrated with small molecule by using solution processes. Spin-coating is employed as the way for deposition method. 4,4'-N,N'-dicarbazole-biphenyl (CBP) which is typical small molecule based ink is used for spin-coating method, and three phosphorescent dopants materials, Firidium(III) bis(4,6-difluoro phenyl)-pyridinato-N,C2'] picolinate (FIrpic), bis1-(phenyl) isoquinoline) iridium(III) acetyl acetonate (Ir(piq)2 (acac))], and iridium(III) tris(2-phenylpy-ridine) (Ir(ppy)3) were prepared. For spin-coating processes, various parameters properties were investigated including materials concentrations, solvents effect, and electron transporting materials to get high efficiency and white color of white PhOLED device. As a result, the precisely controlled solution processes could be a promising technology for solution process can be a good alternative of vacuum deposition technology.

In this research, complex emitting layers (EML) were fabricated using TCTA doping hole-transport material in the front half of a bipolar 26DCzPPy as well as PPT doping electron-transport material in the back half of 26DCzPPy. Blue dopant FIrpic was also mixed inside the complex emitting layer to produce a highly efficient blue phosphorescent organic light emitting diode (OLED). The hole and electron injection and carrier recombination rate were effectively increased. The fabricated complex emitting layers exhibited current efficiency of 42 cd/A and power efficiency of 30 lm/W when the luminance was 1000 cd/m 2 , driving voltage was 4.4 V, and current density was 2.4 mA/cm 2 . A white OLED component was then manufactured by doping red dopant [Os(bpftz) 2 (PPh 2 Me) 2 ] (Os) in proper locations. When the Os dopant was doped in between the complex emitting layers, excitons were effectively confined within, increasing the recombination rate and therefore reducing the color shift. The resulting Commission Internationale de L’Eclairage (CIE) coordinates shifted from 4 to 10 V is (Δ x =−0.04, Δ y =+0.01). The component had a current efficiency of 35.7 cd/A, a power efficiency of 24 lm/W, driving voltage of 4.6 V and a CIE x , y of (0.31,0.35) at a luminance of 1000 cd/m 2 , with a maximum luminance of 15,600 cd/m 2 at 10 V. Attaching an outcoupling enhancement film was applied to increase the luminance efficiency to 30 lm/W.

This study uses a synthetic graphite fiber as the heat dissipation substrate for top-emission organic light emitting diode (TEOLED) to reduce the impact from joule heat. UV glue (YCD91) was spin coated onto the substrate as the insulation layer. The TEOLED structure is (glass; copper; graphite) substrate/YCD91 glue/Al/Au/EHI608/TAPC/Alq3/LiF/Al/Ag. The proposed graphite fiber substrate presents better luminous performance compared with glass and copper substrate devices with luminance of 3055 cd/m2and current efficiency of 6.11 cd/A at 50 mA/cm2. When lighting period of different substrates TEOLED, the substrate case back temperature was observed using different lighting periods. A glass substrate element operating from 5 to 25 seconds at 3000 cd/m2luminance produced a temperature rate of 1.207°C/sec. Under 4000 cd/m2luminance the copper and graphite substrate temperature rates were 0.125°C/sec and 0.088°C/sec. Graphite component lifetime was determined to be 1.875 times higher than the glass components and 1.125 times higher than that of copper.

Hybrid white organic light emitting diodes (HWOLEDs) with fluorescence and phosphorescence hybrid structures are studied in this work. HWOLEDs were fabricated with blue/red emitting layers: fluorescent host material doped with sky blue material, and bipolar phosphorescent host emitting material doped with red dopant material. An electron blocking layer is applied that provides hole red guest emitter hole trapping effects, increases the charge carrier injection quantity into the emitting layers and controls the recombination zone (RZ) that helps balance the device color. Spacer layers were also inserted to expand the RZ, increase efficiency and reduce energy quenching along with roll-off effects. The resulting high efficiency warm white OLED device has the lower highest occupied molecule orbital level red guest material, current efficiency of 15.9 cd/A at current density of 20 mA/cm2, and Commission Internationale de L'Eclairage coordinates of (0.34, 0.39).

A mixed host emitting layer consisting of TCTA and 3TPYMB can greatly increase the efficiency of blue and white organic light emitting diodes (OLED). The hole transport material TCTA can be used as a buffer layer and a hole-transport host emitting layer. Combining TCTA with electron-transport 3TPYMB produces a mixed host emitting layer that can effectively confine the charge carrier within luminous layer. The electron transport layer 3TPYMB reduces electronic injection energy barriers, improves charge balance, and reduces drive voltage to 4.38 V with proper adjustment to optimum thickness. The luminous efficiency of blue light OLED mixed with FIrpic can reach 36.0 cd/A, 27.5 lm/W, and luminous efficiency of white light OLED mixed with Os tangerine adulterant can reach 36.1 cd/A, 26.4 lm/W.

White phosphorescent organic light-emitting diodes (WPHOLED) with high efficiency and low driving voltage were achieved by incorporating an electron transport material (3TPYMB) into a hole transport-type host (TCTA) as a mixed-host structure. For electrons, the emitting layer is nearly barrier-free until they reach the region of exciton formation, which keeps the driving voltage low. Therefore, improved the charge carrier balance within the emitting layer and enhanced the power efficiency of device. White PHOLED at a luminance of 1000 cd/m2 shown a driving voltage of 4.38 V, luminance efficiency of 36.1 cd/A, and power efficiency of 26.4 lm/W was observed. Furthermore, the power efficiency can be improved to 34.27 lm/W, and luminance efficiency to 46.7 cd/A by attaching a brightness enhancement film (BEF).

The effect of the phosphor doping concentration on the optoelectronic characteristics of white phosphorescence organic light emitting diodes (PHOLEDs) was studied. A high efficiency white PHOLED was achieved using the unique efficiency properties of the red-emitting Osmium complex ([fptz = 3-trifluoromethyl-5-(2-pyridyl)-1,2,4-triazole, PPh2Me = phosphineligand]; Os(fptz)2(PPh2Me)2) combined with a well known efficient blue-emitting Iridium complex (bis[(4,6-difluorophenyl)-pyridinate-N,C2′; FIrpic]). The results demonstrate that with 2 wt% Os(fptz)2(PPh2Me)2 co-doped into the blue-emitting layer (10 wt% FIrpic doped into the host), the device can achieve a luminance efficiency of up to 40.4 cd/A (25.6 lm/W) with a good corresponding CIE coordinate (0.39, 0.40). The doping concentrations of the red-emitting and blue-emitting phosphors were adjusted to obtain a pure white light with a CIE coordinate of (0.368, 0.366). A high efficiency white PHOLED was achieved by optimizing the doping concentration of the red-emitting and blue-emitting complexes.

The characteristics of blue phosphorescent organic light-emitting diodes (PHOLEDs) with hole–buffer structure were investigated by inserting a hole transport-type host (TCTA) between the hole transport layer (HTL, TAPC) and emitting layer (EML, FIrpic:26DCzPPy). The hole transport-type host has lower hole mobility than the HTL material, which could effectively control the hole injection current from the HTL into the EML. Moreover, the hole injection barrier from HTL to EML can be reduced due to the suitable HOMO level of the insertion hole buffer layer, which results in the improvement of driving voltage. Furthermore, the power efficiency was improved by controlling the hole injection current through buffer layer thickness optimization. From the experimental results, an optimal blue PHOLED structure was established: ITO/TAPC(20 nm)/TCTA(25 nm)/FIrpic:26DCzPPy(20 nm)/3TPYMB(40 nm)/LiF(0.5 nm)/Al(200 nm). At luminance of 1000 cd/m2, the driving voltage decreased from 6.3 to 5.1 V and power efficiency enhanced from 6.7 to 13.5 lm/W could be observed.

UV-curable organic/inorganic hybrid composites with gas barrier and heat-dissipating capability have been successfully fast synthesized with microwave irradiation and utilized for the encapsulation of top-emitting organic light-emitting diodes (TEOLEDs). Experimental results manifest that lab-made organic/inorganic hybrid composites can effectively not only obstruct the invasion of moisture as well as oxygen in the atmosphere into the device but also lower the temperature of device. Therefore, the lifetimes of TEOLEDs with their encapsulation are 2.2 folds longer than those without encapsulation.

A high heat dissipation material (copper, Cu) was employed as the substrate for top emission organic light-emitting diodes (TEOLEDs). The UV glue was spin-coated onto the Cu substrate as the insulation layer to effectively improve Cu surface roughness and reduce process complexity. From the optoelectronic results, the optimized device with the Cu substrate shows the maximum luminance of 14110 cd/m2 and luminance efficiency of 7.14 cd/A. The surface and junction temperatures are measured to discuss the heat-dissipating effect on device performance. From the results, TEOLED fabricated on a Cu substrate has lower junction (55.34°C) and surface (25.7°C) temperatures, with the lifetime extended seven times. We employed Cu foil as the substrate for flexible TEOLED with maximum luminance of 10310 cd/m2 and luminance efficiency of 7.3 cd/A obtained.

The lifetimes of organic light emitting diodes (OLEDs) have been successfully enhanced with the modulation of LiF thickness and the utilization of encapsulating adhesives, which have been successfully and quickly synthesized with UV irradiation. Experimental results demonstrate that LiF and lab-made encapsulating adhesives can block the invasion of moisture as well as oxygen in the atmosphere into the OLEDs so that the lifetimes of devices with their encapsulation are 18-folds longer than those without encapsulation.

We dissolved hole transport materials α-NPD and NPB in THF solvent, and spin-coated the α-NPD + THF or NPB + THF solution onto ITO anode surface to improve the luminance efficiency and lifetime of flexible fluorescent and phosphorescent organic light emitting diodes. Then the BCP and TPBi were employed as hole blocking layer (HBL) of phosphorescent device and its thickness was optimized. From the experimental results, the maximum luminance efficiency is 4.4 cd/A at 9 V of fluorescent device and 24.4 cd/A of phosphorescent device, respectively. Such an improvement in the device performance was attributed to the smoother surface and good contact between the interface of spin-coated HTL/ITO, the hole were effectively injected from the anode into the organic layer. And the deposited HTL can block excitons from diffusing into the anode to quench, thus improving the luminance efficiency and lifetime greatly.

UV-curable organic/inorganic hybrid nanocomposites have been successfully synthesized with polymer monomers (tetramethoxysilane (TMOS)/3-glycidoxypropyl-trimethoxysilane (GPTMS)) nano-fillers (silica/alumina), curing agents (bisphenol A (BPA)), and photoinitiators (triaryl sulfonium hexafluoroantimonate (TSH)) by in situ polymerization. Their thermal stability, coefficient of thermal expansion (CTE), adhesion strength, and gas barrier capability have also been investigated with thermogravimetry analysis (TGA), thermomechanical analysis (TMA), micro-computer universal testing machine and moisture penetration tests. Experimental results indicate that nano-fillers are completely homogeneously dispersed in the polymer matrices and the increase of nano-fillers in the nanocomposites causes the raise of decomposition temperature (Td) as well as gas resistance and the reduction of CTE as well as adhesion strength. Because lab-made organic/inorganic hybrid nanocomposites have been discovered to exhibit excellent gas barrier properties, we have also applied them for the encapsulation of organic light-emitting diodes (OLEDs) and flexible OLEDs. With lab-made nanocomposite d, the lifetimes of OLEDs and flexible OLEDs can be successfully lengthened to 92 and 36 h, respectively, whereas those of OLEDs and flexible OLEDs without encapsulation are 11 and 7 h, respectively. © 2008 Elsevier B.V. All rights reserved.

In this work, color/luminance tunable organic light emitting diodes (OLEDs) (ITO glass/naphthyl phenyl benzidine (NPB; 80 nm)/4,4′-bis(diphenylvinylenyl)-biphenyl (ADS082BE; 35 nm)/1,3-bis[2-(2,2′-bipyridine-6-yl)-1,3,4-oxadiazo-5-yl]benzene (Bpy-OXD; 20 nm)/tris-[8-hydroxy-quinoline]aluminum (Alq 3 ; 50 nm)/lithium fluoride (LiF; 3 nm)/aluminum (Al, 80 nm)) with low turn-on voltage (3 V) and high luminance (4850 cd/m 2 at 9 V) have been successfully manufactured. The experimental results reveal that their electroluminescent properties (e.g. hue, luminescent intensity, etc.) can be modulated by the manipulation for the layer thickness of NPB/ADS082BE/Bpy-OXD and the applied voltages. In addition, we have also demonstrated lab-made UV-curable silicone-acrylate encapsulating resin exhibits excellent gas barrier capability so that the half-lifetimes of OLEDs reach 98 h whereas those without encapsulation are only 9 h.

This work demonstrates the use of lithium fluoride (LiF) as a passivation layer and a newly developed UV glue for encapsulation on the LiF passivation layer to enhance the stability of organic light-emitting devices (OLEDs). Devices with double protective layers showed a 25-fold increase in operational lifetime compared to those without any packaging layers. LiF has a low melting point and insulating characteristics and it can be adapted as both a protective layer and pre-encapsulation film. The newly developed UV glue has a fast curing time of only 6 s and can be directly spin-coated onto the surface of the LiF passivation layer. The LiF thin film plus spin-coated UV glue is a simple packaging method that reduces the fabrication costs of OLEDs.

In order to improve the luminance efficiency and flexibility of flexible organic light-emitting diodes (FOLEDs), a buffer layer was inserted between ITO and hole transport layer by spin coating. The buffer layer employed is poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), PEDOT:PSS/Teflon, N,N′-bis-(3-methylphenyl)-N,N′-bis-(phenyl)-benzidine (TPD) or bis[N-(1-naphthyl)-N-phenyl]benzidine (α-NPD). The TPD and α-NPD were first solved in chloroform before spin coating. When 40 nm PEDOT:PSS was employed as buffer layer, the FOLED luminance efficiency increased from 2 to 4 cd/A in comparison with the original device without any buffer layer. If the buffer layer was PEDOT:PSS/Teflon, the luminance efficiency increased slightly to 4.4 cd/A. If α-NPD buffer layer was used, the luminance yield of the FOLED could be increased to levels ⩾5.5 cd/A.

A blue small-molecular organic light-emitting diode (SM-OLEDs) based on a solution-process is investigated in this study. Design of experiment (DOE) with response surface methodology (RSM) was applied to optimize the driving voltage and current efficiency of blue SM-OLED devices. The spin-coating speed of the PEDOT: PSS as hole injection layer and the 26DCzPPy: FIrpic as emitting layer were chosen as two main process input factors. Analysis of variance (ANOVA) was adopted to identify significant factors before regression models were obtained. The optimal material structure was determined by minimizing and maximizing a desirability function relating to selected critical quality characteristics including the driving voltage and current efficiency, respectively.

A top-emission inverted organic light-emitting diode (TEIOLED) was fabricated by using Al/AlN x layer as the cathode in the device structure of glass/Al/AlN x /AlQ 3 /NPB/MTDATA/Au/Ag, where AlN x ultra-thin layer was obtained from Al layer under 90 W microwave plasma treatments in Ar and N 2 mixed-gas environment. The N 2 /Ar ratio and plasma treatment time were adjusted to obtain the maximum luminance and efficiency of 1206 cd/m 2 and 0.51 cd/A, respectively, both at 17 V. The AlN x layer surface after plasma treatment was examined by atomic force microscope (AFM) to study the effects of surface roughness on the electroluminescent (EL) characteristics. The thickness of AlN x layer also affected EL results apparently.

In this study, we employed an ultrathin separately doped organic light-emitting diode (OLED) structure to achieve the lowest turn-on voltage, highest luminance efficiency, and highest electroluminescence. In the simulation part, the spectrum intensity of the main emitting layer (EML) tris(8-hydroxy-quinoline) aluminum (Alq3) photoluminescence (PL) and the full width at half maximum (FWHM) of its Gaussian distribution were modulated to obtain the luminescence spectrum of an ultrathin separately doped device. It is found that as doping concentration increases, the extent of intensity modulation in simulation must be decreased in order to simulate the Alq3 peak drop. This result confirms that when the concentration of a red dopant is high, the red luminance is purer, but the brightness is weaker than that at a lower doping concentration; conversely, at a higher intensity a lower concentration of a red dopant results in more intense luminance, but an orange light, instead of a pure red light is emitted. This study aims to clarify how Alq3 intensity changes in, correspondence to different doping concentrations. For simulation results to coincide with the experimental data, it is deduced that the PL spectrum of 4-(dicuanomethylene)-2-methyl-6-(1,1,7,7-tetramethyljulol-idyl-9-enyl)-4H-pyran, DCJT (red dye), exhibits a red shift as doping concentration increases. Finally, we also adjust the thickness of organic layers (hole transport layer/EML) to more accurately approximate simulation results with respect to experimental data.

In this study, we investigated the optimum thicknesses of organic light-emitting diode (OLED) layers using a high-reflectivity metal, Al, coupled with high-work-function alloys, e.g., AuSn, AuGe, and AuGeNi, as anodes, and LiF/Al/Ag multilayers as cathodes. The work function of a double-metal-layer, Al/AuGeNi (60/2 nm), was measured to be 4.7 eV, which is higher than that of a single Al layer of 4.2 eV. A high-work-function anode increased the number of holes injected from the anode into a hole transport layer and hence increased luminescence efficiency. After the optimization of each organic layer thickness, the device produced a maximum luminance of 2930 cd/m2 and a maximum luminance yield of 1.4 cd/A, which are the significant improvements of the performance characteristics of 537 cd/m2 and 0.28 cd/A obtained before the optimization of organic thickness, when the single Al layer was used as an anode, the device only exhibited a luminescence of 190 cd/m2 and a short lifetime.

This study employed poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS), 4,4'-bis[N-(1-naphthyl)-N-phenylamino]-biphenyl (α-NPD) and α-NPD/Teflon as buffer layers between the organic layer and indium tin oxide (ITO) of a flexible organic light-emitting diode (FOLED) to enhance the luminance efficiency of the device. PEDOT:PSS and α-NPD were spin-coated onto the ITO substrate, but α-NPD was first dissolved in chloroform. With the deposition of PEDOT:PSS of 40 nm thickness, the surface roughness of ITO decreased from 2.768 to 1.415 nm, whereas the luminance yield increased from 2.0 to 4.0 cd/A; the maximum luminance was 1500 cd/m2 at 44 mA/cm2. With the deposition of α-NPD of optimum thickness as buffer layer, the device had a luminance yield of 5.2 cd/A and a maximum luminance of 1220 cd/m2 at 25 mA/cm2. When the α-NPD buffer layer was further coated with a thin layer of Teflon by evaporation, i.e., α-NPD/Teflon (55/1 nm), the luminance yield of the device increased to 6.5 cd/A.

Red dopants were doped in different emitters, blue and green, respectively, to fabricate white organic light emitting diodes on flexible substrates. The competitive emission between blue and red emitters with various doped-zones was studied. When the DCJT doped zone was located far away from the hole-injection layer, both the blue and red color can be emitted. An appropriate red-dopant position in the device enhanced the green emission from 8-hydroxyquinoline aluminum (Alq3) which was combined with the red and blue emission to generate a white light. Finally, a white emission with the CIE value, (0.30, 0.32), independent of the applied voltage, was obtained with the optimum doped width and location.

In this research, the optimization of device structures of blue fluorescent organic light emitting diode (OLED) with WBH-301 doped with WBD-701 was fabricated. By adjusting the thickness of each layer in OLED structure as well as total thickness of device, the position of recombination zone was controlled and located in the central region of emitting layer (EML) that significantly increases device efficiency. The device showed the current efficiency of 8.7 cd/A at current density 50 mA/cm2 and with Commission Internationale de L’Eclairage (CIE) coordinates (x = 0.17, y = 0.34). This efficiency enhancement is important for understanding and further improving high-performance fluorescent OLEDs.

The use of numerical simulation method to study the current–voltage (I–V) of organic light emitting diode (OLED) has always been an effective method to upgrade the luminous efficiency of OLED. As the I–V theoretical simulation equations are based on injected carrier passing through Schottky barrier, and considering that carrier capturing defect and carrier mobility rate might be generated within the inner organic layer of Pool–Frenkel model, the study had made a comparison between the I–V theoretical model with a double-layer device and the experimental data, and proposed the best parameters for the theoretical model after careful adjustment and comparison to establish an optimal simulated numerical model with a double-layer OLED current–voltage. Finally, a study was made on the carrier capturing defect and mobility rate affected by electric field and temperature.

The organic light emitting diodes (OLEDs) [1] is a new-generation flat panel display with the advantages of self-luminescence, wide viewing angle (> 160°), prompt response time (~1 μs), low operating voltage (3~10 V), high luminance efficiency, high color purity, and easy to be made on various substrates. Therefore, it’s an important topic that how to improve the luminance efficiency, lifetime and the adhesion characters of ITO/organic interface of flexible OLEDs. Zugang Liu et al. reported that the NPB (HTL) is suitable in contact with the emission layer and when they form an energy ladder structure, the driving voltage decreased and the electroluminescent output increased [2]. Thus it can be seen, the hole transport layer [3-6] is very important to balance the injection of hole and electron, to increase the luminance efficiency and lifetime. In recent years, the hole buffer layer of device typically employs LiF [7], CuPc [8], Pani:PSS [9-10] or PEDOT:PSS [9-11] to improve the hole injection efficiency. In addition, a flexible substrate (PET, metal foil, etc.) surface is not completely smooth and will usually have spikes. After the organic thin film evaporates onto the ITO substrate surface the spikes will still exist. When the device is operated under high voltage or high current density, a heavy amount of electric current will concentrate at the spikes and damage the device by causing the device to short circuit, creating Joule heat. The luminance efficiency of the device will therefore be reduced producing shorter device lifetime. Thus, the PEDOT:PSS fabrication process uses spin-coating to obtain a thin film with a smoother surface than that produced by thermal deposition. Spin-coating enhances the organic material adhesion in subsequent processes, thereby directly affecting the performance of flexible OLED. For the above reason, this research dissolved hole transport material N,N’-diphenyl-N,N’-bis(1-naphthyl)1,1’biphenyl-4,4’’diamine (α-NPD), N,N’Bis(naphthalenel-yl) -N,N’-bis(phenyl)-benzidine (NPB) or α-NPD:NPB in tetrahydrfuran (THF) solvent and spin-coated the buffer layer onto ITO surface of flexible OLEDs. Phosphorescent dye gains energy from the radiative recombination of both singlet and triplet excitons [12], improving the internal quantum efficiency of fluorescent OLEDs (FOLEDs) typically 25% at maximum to nearly 100% [13]. Enhancing the luminance 8

In this study, we dissolved hole transport layer (HTL) material NPB in THF (tetrahydrofuran) solvant, and spin-coated the N,N'-Bis(naphthalene-l-yl)-N,N'-bis(phenyl)-benzidine (NPB) solution on the surface of Indium Tin Oxide (ITO) anode to enhance the luminance efficiency and lifetime of flexible phosphorescent organic light emitting diodes (POLEDs), where the 2,2',2''-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) was employed as hole blocking layer (HBL) and its thickness was optimized. Such an improvement in the device performance was attributed to the improved hole-electron balance. Finally, we employed 2,9-Dime-thyl-4,7-dphenyl-1,10-phenanhroline (BCP) or TPBi as hole blocking layer. The maximum luminance efficiency reaching 24.4 cd/A can be obtained.

Solution-based processing was applied to fabricate green phosphorescent organic light-emitting diodes (OLEDs). EPH31 was used as a phosphorescent host, doped with guest dopant green phosphorescent Ir(ppy)3, and dissolved in chlorobenzene solvent to form the emitting layer. Device structural parameters were controlled by changing the spin coating speed of the emitting layer and hole injection layer [poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate), PDOT:PSS] to adjust the thickness of the electron transport layer [tris(8-hydroxyquinolinato)aluminum, Alq3]. In addition, the differences in using CsF and LiF materials as the electron injection layer were investigated. A maximum current efficiency of 13.6 cd·A-1 was obtained at a high emitting layer spin coating speed. Despite the close resemblance in both the luminance intensity and current efficiency when using CsF and LiF as the electron injection layer, CsF devices had a low driving voltage. Smooth and stable films resulting from the spin coated hole injection layer, along with the control of the thickness of the electron transport layer (Alq3) and electron injection layer (CsF), effectively improved the performance of green OLEDs. The emitting layer host material (CBP) and three guest dopants [Firpic, Ir(ppy)3, and Ir(piq)2] were dissolved in toluene solvent during solution preparation to fabricate white OLEDs. The properties of the resulting solution-processed white PHOLEDs are a current efficiency of 2.4 cd·A-1 at 20 mA·cm-2 and CIE coordinates of (0.33, 0.33) at 9 V. Results of these experiments demonstrate that solution processing can be used as an alternative to and in conjunction with thermal evaporation.

In this study N,N'-bis(naphthalen-1-yl)-N,N'-bis(phenyl)-benzidine (NPB) was dissolved in tetrahydrofuran (THF) solvent, and the resulting solution was spin-coated onto an indium tin oxide (ITO)-coated plastic substrate to form the first hole transport layer (HTL). A second hole transport layer of NPB was then deposited by thermal evaporation on the first HTL to produce a flexible organic light-emitting diode (OLED) with a double HTL structure. The luminance efficiency of a device with a double HTL structure is thereby improved. Utilizing spin-coated (NPB+THF)/evaporated NPB as the double HTL structure, the efficiency can reach 3.86 cd/A, and the lifetime is extended by 14.4% compared with that of the conventional single thermally evaporated HTL.

This paper reports on the separately-doped structures of organic light-emitting diodes (OLED) in which red dye 4-(dicyanomethylene)-2-methyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJT) was doped into the host emitting layer of tris(8-hydroxyquinoline)aluminum (Alq3) by means of ultrathin separate doping (like quantum wells). The parameters in the ultrathin layer, including its width, position, number and spacing between separately doped layers, were changed to study their effects on electroluminescence (EL) characteristics. Thin doped layers increase the luminance efficiency. When the ultrathin separately doped layers were in the vicinity of the NPB/Alq3 interface, where N,N'-diphenyl-N,N'-bis(1-naphthyl)-(1,1'-biphenyl)-4,4'-diamine (NPB) is used as a hole transport layer, luminance efficiency also increased. The highest luminance efficiency reaching 4 cd/A was achieved using a double ultrathin separately doped structure with optimum doping thickness and 2 nm of space between the two layers. The study also found that double ultrathin separately doped structures achieved better EL intensity than single or triple ultrathin separately doped structures.