Inverted Top-emitting Organic Light-emitting Diodes Research Articles

Highly Efficient Red, Green, and Blue Inverted Top‐Emitting Organic Light‐Emitting Diodes with Microstructured TiN Substrate

The morphological and optical properties of microstructured silicon substrates (M–Si) and inverted top‐emitting organic light‐emitting diodes (ITOLEDs) based on the M–Si/Ti/TiN/Al composite electrodes are studied. The developed M–Si/Ti/TiN/Al composite electrodes function well in improving the performance of red, green, and blue ITOLED. The Al thin film deposited on TiN silicon substrate could significantly improve the reflectivity of the electrode, and the microstructured substrate could further increase the reflective path for lights inside OLEDs. Meanwhile, by adopting a light output‐coupling layer on top of the device and using the localized surface plasmon resonance effect of the top‐emitting structure, the angle‐dependent effect of ITOLEDs based on the composite electrodes is reduced. Through employing the M–Si/Ti/TiN/Al composite electrodes, all the red, green, and blue OLEDs achieve an improved performance, and the red OLED shows the best performance enhancement, with the luminance of 26 247 cd m−2, maximum current efficient of 30.7 cd A−1, and maximum external quantum efficiency of 17.4%. This provides a new strategy to achieve highly efficient ITOLEDs.

P‐161L: Late‐News Poster: High‐Efficiency Inverted Top‐Emitting Organic Light‐Emitting Diodes with Random Corrugated Structure

AbstractIn this study, we show that high‐efficiency Inverted Top‐Emitting Organic Light‐Emitting Diodes (ITEOLEDs) by using nanosized random structure (nRS). The nRS is fabricated by Reactive ion etching (RIE) process. This process enables simple and inexpensive manufacturing. Compared to a conventional device, ITEOLEDs with nRS showed improved power efficiency (+19.87%), Current efficiency (+32.65%) and external quantum efficiency (+29.24%) at a luminance 500cd/m2.

Optimizing structure for constructing a highly efficient inverted top-emitting organic light-emitting diode with stable electroluminescent spectra

We demonstrate a highly efficient inverted top-emitting organic light-emitting diode (TOLED) having stable electroluminescent spectra and color coordination with variation of viewing angles by simply tuning the resonance wavelength corresponding to the free emission of the emitter. Using a doped fluorescent emitting system, the inverted TOLED exhibits an enhanced maximum current efficiency of 19 cd/A and a power efficiency of 17 lm/W, which are much higher than those (11 cd/A and 5 lm/W) of the counterpart with normal structure, although both TOLEDs behave with similar stable electroluminescent spectra characteristics. The results indicate that we provide a simple and effective method of constructing an excellent inverted TOLED for potentially practical applications.

Stacked inverted top-emitting green electrophosphorescent organic light-emitting diodes on glass and flexible glass substrates

Stacked inverted top-emitting green electrophosphorescent organic light-emitting diodes (OLEDs) are demonstrated on glass and flexible glass substrates. A single-unit OLED is shown to have a current efficacy of 46.8cd/A at a luminance of 1215cd/m2. When two of these OLEDs are stacked, the double-unit OLED exhibits a current efficacy more than twice that of the single-unit OLED, with a current efficacy of 97.8cd/A at a luminance of 1119cd/m2. With the addition of an optical outcoupling layer of N,N′-Di-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl)-4,4′-diamine (α-NPD) on top of the semitransparent gold anode, the double-unit stacked OLED achieves a maximum current efficacy of 205cd/A at a luminance of 103cd/m2, maintaining a high current efficacy of 200cd/A at a luminance of 1011cd/m2. These stacked inverted OLED combine the advantages of inverted OLEDs with the benefits of having a stacked architecture.

High-performance inverted top-emitting green electrophosphorescent organic light-emitting diodes with a modified top Ag anode

Green electrophosphorescent inverted top-emitting organic light-emitting diodes with a Ag/1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN) anode are demonstrated. A high current efficacy of 124.7cd/A is achieved at a luminance of 100cd/m2 when an optical outcoupling layer of N,N′-di-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl-4,4′-diamine (α-NPD) is deposited on the anode. The devices have a low turn-on voltage of 3.0V and exhibit low current efficacy roll-off through luminance values up to 10,000cd/m2. The angle dependent spectra show deviation from Lambertian emission and color change with viewing angle. Hole-dominated devices with Ag/HAT-CN electrodes show current densities up to three orders of magnitude higher than devices without HAT-CN.

Improving electron injection and microcavity effect for constructing highly efficient inverted top-emitting organic light-emitting diode

We demonstrated a highly efficient inverted top-emitting organic light-emitting diode (ITOLED) by using Ag/Al bilayer reflective cathode and CsOx electron-injection layer. With tris(8-hydroquinoline) aluminum as electron-transport and emitting layer, the maximum luminous efficiency and power efficiency of ITOLED with CsOx electron-injection layer reached 8.3cd/A and 4.3lm/W, respectively, which have been enhanced by ∼40% and ∼80%, respectively, in comparison with the counterpart using LiF as electron-injection layer (5.9cd/A and 2.3lm/W). The improved performance is resulted from the enhanced electron-injection ability of CsOx. The superior electron-injection ability of CsOx was further verified by ‘Electron-only’ device using current density versus voltage characteristics and impedance spectroscopy measurements. Our results indicated that CsOx electron-injection layer combined with Ag/Al bilayer reflective cathode provides a convenient approach for constructing highly efficient ITOLED.

Highly efficient inverted top-emitting green phosphorescent organic light-emitting diodes on glass and flexible substrates

Green phosphorescent inverted top-emitting organic light-emitting diodes with high current efficacy and luminance are demonstrated on glass and polyethersulfone (PES) substrates coated with polyethylene dioxythiophene-polystyrene sulfonate (PEDOT:PSS). The bottom cathode is an aluminum/lithium fluoride bilayer that injects electrons efficiently into an electron transport layer of 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TpPyPB). The cathode is found to be highly sensitive to the exposure of trace amounts of O2 and H2O. A high current efficacy of 96.3 cd/A is achieved at a luminance of 1387 cd/m2 when an optical outcoupling layer of N,N′-Di-[(1-naphthyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4,4′-diamine (α-NPD) is deposited on the anode.

Polarizer-free, high-contrast inverted top-emitting organic light emitting diodes: effect of the electrode structure

Circular polarizer (CP)-free, high-contrast inverted top-emitting organic light-emitting diodes (ITOLEDs) are demonstrated by using a semitransparent Ag electrode capped with a single dielectric layer as a top anode and Cs(2)CO(3) (1.5 nm)/Al (1.5 nm)/Cr (100nm) as a partially absorbing bottom cathode. Low luminous reflectance is achieved by combining the broadband absorption of Cr, the weak but inherent cavity structure, and the optimal thickness of the capping layer yielding a high transmittance of a top electrode. With the optimized organic capping layer, contrast-enhanced ITOLEDs exhibit a luminous reflectance as low as 3.6% with a large thickness margin. Their luminous efficiency is shown to be comparable to or even higher than that of CP-based conventional OLEDs.

Doping-Free Inverted Top-Emitting Organic Light-Emitting Diodes With High Power Efficiency and Near-Ideal Emission Characteristics

Inverted top-emitting organic light-emitting diodes (ITOLEDs) with high power efficiency and near-ideal emission characteristics are demonstrated by using the combination of the following: 1) an electron-injection layer composed of Cs2CO3, which lowers the turn-on voltage; 2) an electron-transporting layer with optimal electron mobility, which enhances the electron current and thus improves the carrier balance; and 3) a dielectric/metal/dielectric multilayer electrode that works as a damage-free top transparent anode optimized to achieve high efficiency and ideal emission characteristics. By this approach, ITOLEDs with power efficiency values of 3.8 and 30 lm W-1 are demonstrated in fluorescent and phosphorescent types, respectively, at a luminance value of 1000 cd m-2 with little distortion in spectral/angular characteristics.

碳酸铯修饰Al作为反射阴极的倒置顶发射OLED器件

Efficient inverted top-emitting organic light-emitting diode(ITOLED) using a thin Cs2CO3 layer modified Al as the reflective bottom cathode is demonstrated.With tris-(8-hydroxy-quinoline) aluminum(Alq3) as emitting layer and MoO3/Ag as semitransparent anode,the Al/Cs2CO3 based ITOLED shows a tune-on voltage of 3.6 V,luminance efficiency of 9.8 cd/A and 3.4 lm/W,which are much better than those(4.2 V,8.6 cd/A,2.85 lm/W) obtained from Mg∶Ag based ITOLED and those(5 V,5.5 cd/A,1.57 lm/W) obtained from bare Al based ITOLED.Electron-only devices research shows that a stronger electron-injection is obtained from Al/Cs2CO3 than Mg∶Ag and Al,which is attributed to the lower work function of Al/Cs2CO3 than Mg∶Ag and Al.The good performance of Al/Cs2CO3 based ITOLED is attributed to the efficient electron injection from the Al/Cs2CO3 cathode as well as a microcavity effect.

In-situ Determination of Interface Dipole Energy between Tris(8-hydroxyquinoline) Aluminum and MgO Coated Al in Inverted Top-Emitting Organic Light-Emitting Diodes

The interface dipole energies between tris(8-hydroxyquinoline) aluminum (Alq3) and Al/MgO (or bare Al) were in-situ measured using synchrotron radiation photoemission spectroscopy. The work function of Al/MgO is higher by 0.6 eV than that of Al. The interface dipole energies were determined to be -0.3 and 0.2 eV for Al/MgO and bare Al, respectively. Therefore, the barrier height of Al/MgO is higher by 0.3 eV than that of Al. The operating voltage at a current density of 50 mA/cm2 of inverted top-emitting organic light-emitting diode (ITOLED) using Al/MgO cathode decreased from 14.3 to 11.7 V, and the luminance value at the current density of 222 mA/cm2 of ITOLED increased from 1830 to 1950 cd/m2. Therefore, it is considered that MgO interfacial layer played a role in reducing the effective barrier height of electron injection by tunneling mechanism, leading to a reduction in turn-on voltage and luminance enhancement.

In-situDetermination of Interface Dipole Energy between Tris(8-hydroxyquinoline) Aluminum and MgO Coated Al in Inverted Top-Emitting Organic Light-Emitting Diodes

<i>In-situ</i>Determination of Interface Dipole Energy between Tris(8-hydroxyquinoline) Aluminum and MgO Coated Al in Inverted Top-Emitting Organic Light-Emitting Diodes

P‐216L: Late‐News Poster: Optical Properties of WO3/Ag/WO3 Multilayer as Transparent Electrode in Top Emitting OLEDs

AbstractWe demonstrate inverted top‐emitting organic light emitting diodes (ITOLEDs) using a WO3/Ag/WO3 multilayer as a transparent top electrode formed by thermal evaporation. Due to the multiple reflections and interferences, the Ag layer sandwiched between WO3 with a high value of n (&gt;2.0) can show improved transmittance property of ∼ 90 %.

The effect of Ar plasma bombardment upon physical property of tungsten oxide thin film in inverted top-emitting organic light-emitting diodes

This study examined the mechanism of performance degradation induced by the plasma sputtering process for the formation of the ITO top anode in inverted top-emitting organic light-emitting diodes (ITOLEDs) with WO 3 thin film as the hole injection layer (HIL) and the buffer layer. The degradation is mainly caused by the changes in the physical properties of the WO 3 thin film as well as the damages of organic layers, which are caused by the bombardment of energetic particles induced by the plasma sputtering process. The number of oxygen vacancies in the WO 3 thin film is increased with the increase of the bombardment energy of energetic particles such as Ar ions, resulting in the decrease in the work function and optical band gap and the dramatic increase in conductivity. Above all, the increase in the energy potential difference between WO 3 HIL and NPB HTL, which was attributed to the reduced work function of the WO 3 HIL, reduces the efficiency of carrier injection and dominantly affects the device performance. These results show that the plasma damage effect, induced by the sputtering process for ITO deposition, could not be effectively removed, even when the metal oxide thin film is used as the buffer layer in ITOLEDs.

Manipulating the Microcavity Structure for Highly Efficient Inverted Top-Emitting Organic Light-Emitting Diodes: Simulation and Experiment

A comprehensive theoretical and experimental study on inverted top-emitting organic light-emitting diodes (ITOLEDs) with a microcavity structure is demonstrated. In the ITOLEDs, tris-(8-hydroxyquinoline) aluminum (Alq3) is used as the emitting material and the outcoupling capping layer, and the 2, 9-dimethyl-4, 7-diphenyl-1, 10-phenanthroline is used as the hole/exciton-blocking layer to confine most emitting dipoles exactly at the desired resonant position. A classical optical model is adopted to simulate the electroluminescence intensity and the spectral characteristics as a function of a viewing angle. The factors that influence optical interference effects and light outcoupling are discussed systematically. The optimized ITOLED shows an enhanced outcoupling efficiency and highly saturated colors compared with that of the corresponding conventional bottom-emitting light-emitting diode while the angular dependence of the emission wavelength is minimized.

Enhancement of electrical property by oxygen doping to copper phthalocyanine in inverted top emitting organic light emitting diodes

We reported the evidence of oxygen doping to copper-phthalocyanine (CuPc) by O2-plasma treatment to inverted top-emitting organic light-emitting diodes (ITOLEDs). In situ synchrotron-radiation photoelectron spectroscopy results showed that a new Cu–O bond appeared and the energy difference between the highest-occupied molecular orbital and EF is lowered by 0.15 eV after plasma treatment. The oxygen ions chemically interacted with Cu atoms and transferred charges to the CuPc. Thus the hole injection barrier was lowered, enhancing the electroluminescent property of ITOLEDs.

Efficient inverted top-emitting organic light-emitting diodes using ultrathin MoO3/C60 bilayer structure to enhance hole injection

Efficient inverted top-emitting organic light-emitting diodes with aluminum (Al) as both the cathode and semitransparent anode are investigated. It is found that introduction of the ultrathin molybdenum trioxide (MoO3)/fullerene (C60) bilayer structure between the low work function Al top anode and the hole-transporting layer dramatically enhances the device performance as compared to the devices with sole MoO3 or C60 buffer layer. The ultraviolet photoemission spectroscopy and x-ray photoelectron spectroscopy indicate that the hole injection barrier between Al anode and hole-transporting layer is effectively reduced via strong dipole effect at Al/MoO3/C60 interfaces with its direction pointing from Al to C60.

Lead(IV) dioxide: an effective electron injection material to realize high-efficiency inverted top-emitting organic light-emitting diodes

Lead(IV) dioxide (PbO2) has been used as the electron injection layer (EIL) to realize high-efficiency inverted top-emitting organic light-emitting diodes (I-TOLEDs). It can be seen that the inserting of the PbO2 EIL significantly reduces operational voltage, thus greatly improving the current efficiency and power efficiency of fabricated I-TOLEDs. The 10-(2-benzothiazolyl)-1, 1, 7, 7-tetramethyl-2, 3, 6, 7-tetrahydro-1H, 5H, 11H-[1] benzopyrano [6, 7, 8-ij] quinolizin-11-one (C545T)-based I-TOLEDs with the PbO2 EIL exhibit a maximum current efficiency of 31.6 cd A−1 and a maximum power efficiency of 14.3 lm W−1, which are both higher than 22.5 cd A−1 and 5.4 lm W−1 of the I-TOLEDs with LiF as the EIL respectively. A detailed analysis with respect to the role mechanism of PbO2 in electron injection has been presented. The improvement in EL performance is attributed to the formation of the interfacial dipoles at the electrode interface due to charge transfer between PbO2 and Alq3.

Highly efficient inverted top-emitting organic light-emitting diodes using a lead monoxide electron injection layer

By introducing an effective electron injection layer (EIL) material, i.e., lead monoxide (PbO), combined with the optical design in device structure, a high efficiency inverted top-emitting organic light-emitting diode (ITOLED) with saturated and quite stable colors for different viewing angles is demonstrated. The green ITOLED based on 10-(2-benzothiazolyl)-1, 1, 7, 7-tetramethyl-2, 3, 6, 7-tetrahydro-1H, 5H, 11H-[1] benzopyrano [6, 7, 8-ij] quinolizin-11-one exhibits a maximum current efficiency of 33.8 cd/A and a maximum power efficiency of 16.6 lm/W, accompanied by a nearly Lambertian distribution as well as hardly detectable color variation in the 140 degrees forward viewing cone. A detailed analysis on the role mechanism of PbO in electron injection demonstrates that the insertion of the PbO EIL significantly reduces operational voltage, thus greatly improving the device efficiency. More importantly, the optically optimized device structure by setting the resonant wavelength at the peak wavelength of the intrinsic emission of the emitter and adding an effective outcoupling layer further enhances the device efficiency, at the same time, also reduces the color shift with viewing angles, leading to the simultaneous optimization in efficiency and angular emission characteristics in the fabricated ITOLEDs.

Inverted top-emitting organic light-emitting diodes by whole device transfer

We present a method for improving the efficiency of charge carrier injection at both cathode and anode of inverted top-emitting organic light-emitting diode (TOLED). In this method, a bottom-emitting OLED (BOLED) is first fabricated on a flexible substrate, which is then transferred as a whole to a glass substrate for the fabrication of the inverted TOLED (ITOLED). The electrode engineering that is responsible for the improvement, which is not possible in the traditional fabrication of ITOLED, is made realizable by the whole device transfer.