Thermally stable inverted organic light-emitting diodes using Ag-doped 4,7-diphenyl-1,10-phenanthroline as an electron injection layer

The thermal stability of organic functional materials affects the performance and lifetime of organic light-emitting diodes (OLEDs). We have developed a thermally stable inverted OLEDs (IOLEDs) by employing silver (Ag) doped into 4,7-diphenyl-1,10-phenanthroline (Bphen) as an n-type doped electron injection layer (EIL). We found that the formation of Ag complexes by coordination reaction could enhance the thermal stability and produce an asymmetric diffraction pattern based on an analysis of grazing incidence small angle X-ray scattering. Interestingly, with the annealing temperature increasing to 100 °C, the electrical properties of electron-only cells show differentiated phenomenon that the current density based on Ag dopant remains basically unchanged, which is opposite to Cs₂CO₃ dopant. In addition, at the high temperature of 100 °C, the IOLEDs with Cs₂CO₃ doped Bphen as an EIL was damaged completely, while the Ag dopant-based devices still maintained good photoelectrical characteristics. Finally, we have demonstrated that the optimized IOLEDs achieved a 40.3% enhancement in current efficiency compared to the conventional device. This work provides a new strategy to increase the thermal stability and performance for the application of IOLEDs operated under high temperature.     

Blue top-emitting organic light-emitting devices using Alq3 as phase shift adjustment layer

Blue top-emitting organic light-emitting devices (TEOLEDs) are demonstrated by employing Alq3 as phase shift adjustment layer (PSAL) to increase the phase shift on reflection of the top electrode within a range, which also improves the light out-coupling. By adjusting the thickness of PSAL, the CIEx,y of devices, which utilize 2, 7-Di-pyrenyl-9, 9-spiro-bifluorene (DPSF) as emitting layer, changes from (0.16, 0.50) to (0.18, 0.37). The maximum current efficiency of 7.1 cd/A is acquired under 4.5 V with an increasing luminance of 139 cd/m2. Compared with adjusting the total thickness of organic layer, it is more beneficial for achieving blue TEOLEDs with high efficiency.     

Effective hole-injection layer for non-doped inverted top-emitting organic light-emitting devices

Non-doped inverted top-emitting organic light-emitting diode with high efficiency is demonstrated through employing an effective hole-injection layer composed of MoO_x. One reason for high efficiency lies on the energy-level matching between MoO_x and hole-transport, and another is due to the Ohmic contact formed between MoO_x and Ag. Both of them lead to an improvement of the hole-injection capability from Ag top anode. Moreover, the symmetrical current of “hole-only” device with MoO_x shows better hole-injection capability, which is independent of the deposition sequence. The optimized device with MoO_x hole-injection layer exhibits maximum current efficiency of 3.7 cd/A at a raised luminance level of 14,900 cd/m² and a maximum luminance of 47,000 cd/m² under 18 V.     

Percolation dominated electron transport in Tetracyanoquinodimethane mixed 4,7-diphenyl-1,10-phenanthroline thin films

Highly conducting and electron transporting organic semiconductor tetracyano-quinodimethane (TCNQ) has been added to the well known electron transporting material 4,7-diphenyl-1,10-phenanthroline (BPhen) in various mixing percentages. Electron only devices exhibited a distinctive transition from low to high conductivity. The results have been found to be in good agreement with classical percolation theory with a percolation threshold of 3.8 wt.% and a critical exponent value of 1.3. This demonstrates the formation of a morphologically stable 2D percolating network that has been supported by AFM analysis. The composition thus formed indicates a promising electron injecting material for optoelectronic devices.     

具有微腔结构的有机顶发射电致发光器件的光谱分析

摘要: 制做了具有微腔结构的蓝色和白色有机顶发射电致发光器件。利用TBADN:3%DSAPh和Alq3:DCJTB/TBADN:TBPe/Alq3:C545材料为发光层,在玻璃基片上,依次制备薄膜：Ag为阳极反射层, CuPc作为空穴注入层,NPB作为空穴传输层,ITO为光程调节层; Al/Ag作为半透明阴极,电极的透射率在30%左右。通过改变ITO层的厚度,TBADN:3%DSAPh器件获得了深蓝色发光光谱,色坐标为(0.141, 0.049),半高宽为17nm发光光谱,实现了窄带发射,Alq3:DCJTB/TBADN:TBPe/Alq3:C545器件得到了不同颜色（红、蓝、绿）的发光光谱,实现了对光谱的调节作用。文章对微腔顶发射器件的发射强度和发光光谱半高宽的结果进行了分析。     

Spectrum study of top-emitting organic light-emitting devices with micro-cavity structure

Blue and white top-emitting organic light-emitting devices OLEDs with cavity effect have been fabricated.TBADN:3%DSAPh and Alq3:DCJTB/TBADN:TBPe/Alq3:C545 were used as emitting materials of microcavity OLEDs.On a patterned glass substrate,silver was deposited as reflective anode,and copper phthalocyanine (CuPc)layer as HIL and 4'-bis[N-(1-Naphthyl)-N-phenyl-amino]biphenyl(NPB)layer as HTL were made.Al/Ag thin films were made as semi-transparent cathode with a transmittance of about 30%.By changing the thickness of indium tin oxide ITO,deep blue with Commission Internationale de L'Eclairage chromaticity coordinates(CIEx,y)of(0.141,0.049)was obtained on TBADN:3%DSAPh devices,and different color(red,blue and green)was obrained on Alq3:DCJTB/TBADN:TBPe/Alq3:C545 devices,full width at half maxima(FWHM)was only 17 nm.The spectral intensity and FWHM of emission in cavity devices have also been studied.     

Metal-anode-dependent spectra and efficiency in blue top-emitting organic light-emitting devices

In this study, the blue top-emitting organic light-emitting devices (TEOLEDs) with different metal anodes are fabricated. The effect of different anode materials on the spectra and efficiency of blue TEOLEDs is studied. We demonstrate that Al is a more suitable anode material for blue TEOLEDs due to its larger phase shift on reflectance (PSR) than the other common metal materials, such as Ag and Au. The influence of light outcoupling layer (LOL) on the transmittance and PSR of cathode is also investigated to obtain the optimum condition for devices. Angle-independent electroluminescence (EL) spectra are obtained in blue TEOLEDs for each metal anode but the device with Al anode possesses higher efficiency and much thicker organic layers, which is beneficial to the lifetime of the device. These results offer a practicable platform for the realization of TEOLEDs based full-color displays and lightings.     

Highly efficient green top-emitting organic light-emitting devices with metal electrode structure

In this work, the electrical and optical characteristics of top-emitting organic light-emitting device (TEOLED) using metal Ag as anode with different thicknesses have been investigated. The emission peak of fabricated TEOLED is 512 nm for a full-width at half-maximum (FWHM) of 48 nm in forward direction. The TEOLED turns on at 3 V with luminance of 2.38 cd/m² and reaches 16,300 cd/m² at 9 V. The maximum of current efficiency is 5.2 cd/A at 7 V, corresponding to the external quantum efficiency of 1.72%.     

Radiation simulations of top-emitting organic light-emitting devices with two- and three-microcavity structures

We demonstrate the simulation results of the radiation properties from top-emitting organic light-emitting devices (top-emitting OLEDs) with two- and three-microcavity structures based on the general electromagnetic theory. The parameters of the layer thickness and complex refractive index of each layer, the locations and density of the oscillating dipoles, and the emission photoluminescence spectrum are varied to optimize the device performance. In evaluating the deice performances, the output spectrum, the intensity distribution, and the viewing-angle characteristics of a top-emitting OLED are concerned. The simulation results are consistent with the Fabry-Perot cavity equation, which can be used as a guideline for designing a two-cavity top-emitting OLED. In such a design process, the dipole position is chosen first. Then the thicknesses of the whole organic layer, the semitransparent cathode, and the dielectric layer are adjusted for optimizing the device performance. In a three-cavity top-emitting OLED, not only the emission intensity and the viewing angle can be optimized at the same time, but also the emission wavelength can be independently tuned. Besides, the use of a three-cavity structure helps to narrow the spectral width and increase the color purity.     

High performance top-emitting organic light-emitting diodes with copper iodide-doped hole injection layer

Efficient top-emitting organic light-emitting diodes were fabricated using copper iodide (CuI) doped 1,4-bis[N-(1-naphthyl)-N′-phenylamino]-4,4′-diamine (NPB) as a hole injection layer and Ir(ppy)₃ doped CBP as the emitting layer. CuI doped NPB layer functions as an efficient p-doped hole injection layer and significantly improves hole injection from a silver bottom electrode. The top-emitting device shows high current efficiency of 69 cd/A with Lambertian emission pattern. The enhanced hole injection is originated from the formation of the charge transfer complex between CuI and NPB.     

High efficiency green phosphorescent top-emitting organic light-emitting diode with ultrathin non-doped emissive layer

Ultrathin non-doped emissive layer (EML) has been employed in green phosphorescent top-emitting organic light-emitting diodes (TOLEDs) to take full advantages of the cavity standing wave condition in a microcavity structure. Much higher out-coupling efficiency has been observed compared to conventional doped EML with relatively wide emission zone. A further investigation on dual ultrathin non-doped EMLs separated by a special bi-layer structure demonstrates better charge carrier balance and improved efficiency. The resulting device exhibits a high efficiency of 125.0 cd/A at a luminance of 1000 cd/m² and maintains to 110.9 cd/A at 10,000 cd/m².     

Polymer encapsulation of flexible top-emitting organic light-emitting devices with improved light extraction by integrating a microstructure

An improved efficiency from an encapsulated flexible top-emitting organic light-emitting device (FTOLED) has been demonstrated by integrating a microstructure onto the polymer encapsulation film. Soft-nanoimprint lithography is employed to integrate the microstructure onto the polymer surface, which enables large area fabrication with high quality, low cost, and repeatable use of the poly(dimethylsiloxane) mold. The light extraction of the FTOLEDs has been improved by integrating the microstructure with two-dimensional tapered micropillars array on the polymer encapsulation film, which can suppress the reflection by enhancing the critical angle of total reflection owing to its gradually changed refractive index. Moreover, the microstructured surface exhibits a hydrophobic property owing to its high contact angle, which results in a self-cleaning ability to protect the FTOLEDs from being polluted by water droplets and dust particles in practical applications.     

Bright green organic light-emitting devices having a composite electron transport layer

A bright green organic light-emitting device employing a co-deposited Al-Alq₃ layer has been fabricated. The device structure is glass/indium tin oxide (ITO)/ N, N′-diphenyl-N, N′- (3-methylphenyl)-1, 1′-biphenyl-4, 4′-diamine (TPD)/tris(8-quinolinolato) aluminum (Alq₃)/ Al-Alq₃/Al. In this device, Al-Alq₃ is used as electron transport layer (ETL). The device shows an operation voltage of 6.1 V at 20 mA/cm². At optimal condition, the brightness of a device at 20 mA/cm² is 2195 cd/m² achieved a luminance efficiency of 5.64lm/W. The result proves that the composite Al-Alq₃ layer is suitable for the ETL of organic light-emitting devices (OLEDs).     

具有阶梯势垒的低压有机电致发光器件

在2-t-butyl-9,10-di-(2-naphthyl)anthracene(TBADN)/tris(8-hydroxyquinoline)aluminum(Alq3)界面及TBADN/4'7-diphyenyl-1,10-phenanthroline(Bphen)界面上插入Gaq薄膜作为阶梯势垒,使有机电致发光器件的电子注入得到改善.由于Gaq(2.9 eV)的LUMO(分子最低空余轨道能级)位于Alq3(3.1 eV)(或 Bphen(3.0 eV))的LUMO和TBADN的LUMO(2.8 eV)之间,形成了从Alq3(或Bphen)经Gaq到TBADN的势垒阶梯,提高了电子注入,进而提高了器件效率.实验表明:与没有阶梯势垒的器件相比,无论是单一电子器件还是完整器件,在相同电流密度下,具有阶梯势垒的器件的电压都有所下降.在电流密度为20 mA/cm2时,当电子传输层为Alq3时,单一电子器件的电压从7.9 V降到4.9 V,完整器件的电压从7 V降到5.8 V;当电子传输层为Bphen时,单一电子器件的电压从4.2 V降到3.1 V,完整器件的电压从6.2 V降到5.1 V.在电流密度为200 mA/cm2,Alq3为电子传输层时,亮度从1 992 cd/m2升到3 281 cd/m2,最高亮度达到3 420 cd/m2,Bphen为电子传输层时,亮度从1 745 cd/m2 升到2 876 cd/m2,最高亮度达到3 176 cd/m2.本文运用能级隧穿理论对上述现象进行了解释.     

BCP层对蓝光有机电致发光器件效率的影响

采用真空热沉积的方法,在常规的三层器件基础上,通过改变空穴阻挡层BCP的厚度,制备了具有结构为氧化铟锡(ITO)/N,N'-diphenyl-N,N'-bis(1-naphthyl)(1,1'-biphenyl)-4,4'-diamine(NPB)/2,9-dimethyl-4,7-diphenyl-1,10-phenan throline(BCP)/Hydroxyquinoline aluminum(Alq3)/Mg:Ag的双异质结的有机电致发光器件.结果表明,当空穴阻挡层BCP的厚度从0.1 nm逐渐增加到4.0 nm时,器件的电致发光光谱实现了从绿色到蓝绿色再到蓝色发射的转变.同时,空穴阻挡层BCP起到了调节载流子复合区域和改变器件发光颜色的作用,并且通过详细的器件结构优化证明一定厚度的BCP层显著地提高了蓝光器件的效率,达到了7.3 lm/W.     

Enhancement of top emission for organic light-emitting diode via scattering surface plasmons by nano-aggregated outcoupling layer

A stable self nano-aggregated bathocuproine film was fabricated and introduced atop of a conventional organic light emitting diode for enhancing top emission. It leads to a 2.7–2.1-fold enhancement on top emission at applied voltage from 4 to 9 V which is much larger than the 1.5–1.3-fold enhancement for a device overlaid with an amorphous bathocuproine film. The more effective outcoupling of this method probably arises from surface plasmon modes being scattered by only the nanostructured surface, and thus without phase cancellation, at the bathocuproine/air boundary. Moreover, this method nearly preserves the original electroluminescent spectra and has no damage on electrical properties.     

Improved current efficiency in organic light-emitting devices with a hole blocking layer

A hole-blocking layer (HBL) of 4,7-diphenyl-1,10-phenanthroline (BPhen) is incorporated between the emitting layer (EML) and the electron transport layer (ETL) for a tris-(8-hydroxyqunoline)aluminum based organic light-emitting device (OLED). Such a structure helps to reduce the hole-leakage to the cathode, resulting in an improved current effi-ciency. The BPhen improves the balance of hole and electron injections. The current efficiency is improved compared with that of the device without the blocking layer. The highest luminous efficiency of the device with 6 nm BPhen acting as a blocking layer is 3.44 cd/A at 8 V, which is improved by nearly 1.5 times as compared with that of the de-vice without it.     

Fourfold power efficiency improvement in organic light-emitting devices using an embedded nanocomposite scattering layer

It has been demonstrated that internal extraction structures (IES) can be introduced in OLEDs to decrease the ratio of the waveguide mode and simultaneously increase the ratios of the substrate and radiation modes. In this study, titanium oxide (TiO₂) nanoparticles (NPs) combined with transparent photoresist (TPR) were utilized to form an embedded nanocomposite scattering layer between the indium-tin-oxide (ITO) and glass substrate, leading to a significant boost in the out-coupling efficiency of the OLEDs. Inside the nanocomposite scattering layer, NPs of different sizes served distinct functions. The 250 nm-TiO₂ particles were used to induce scattering and diminish the light reflection back to the ITO layer. On the other hand, the refractive index of the TPR can be increased by increasing the concentration of the 25 nm-TiO₂, which reduced the difference in the refractive index between the ITO and TPR and thus multiplied the amount of light entering into the scattering layer. By employing nanocomposite substrate with mixed dual-sized NPs, we obtained power efficiencies of the blue phosphorescent OLEDs that were about 4.3 times higher than that of the control device at the high luminance of 5 × 10³ cd/m².     

Efficient fluorescent white organic light-emitting devices based on a ultrathin 5,6,11,12-tetraphenylnaphthacene layer

White organic light-emitting devices (OLEDs) were fabricated using a ultrathin layer 5,6,11,12-tetraphenylnaphthacene as the yellow light-emitting layer and p-bis(p-N,N-diphenyl-aminostyryl)benzene (DSA-ph) doped in 2-methyl-9,10-di(2-naphthyl)anthracene (MADN) matrix as the blue light-emitting layer. The thickness of rubrene ultrathin layer will seriously affect the device performance, and the device with 1 nm rubrene achieves the best performance, with the maximum luminance of 33,152 cd/m² at 11 V and the maximum current efficiency of 8.69 cd/A at 7 V.     

Efficient double-emitting layer inverted organic light-emitting devices with different spacer layers

Double-emitting layer inverted organic light-emitting devices (IOLEDs) with different spacer layers were investigated, where 2,20,7,70-tetrakis(carbazol-9-yl)-9,9-spirobifluorene (CBP), 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), 4,7-diphenyl-1,10-phenanthroline (Bphen) and 4,40,400-tris(N-carbazolyl)-triphenylamine (TCTA) were used as spacer layers, respectively, and GIr1 and R-4b were used as green and red guest phosphorescent materials, respectively. The results show that the device with BCP spacer layer has the best performance. The maximum current efficiency of the BCP spacer layer device reaches up to 24.15 cd.A-1 when the current density is 3.99 mA.cm-2, which is 1.23 times bigger than that of the CBP spacer layer device. The performance is better than that of corresponding conventional device observably. The color coordinate of the device with BCP spacer layer only changes from (0.625 1, 0.368 0) to (0.599 5, 0.392 8) when the driving voltage increases from 6 V to 10 V, so it shows good stability in color coordinate, which is due to the adoption of the co-doping evaporation method for cladding luminous layer and the effective restriction of spacer layer to carriers in emitting layer.