Impact of temperature on the efficiency of organic light emitting diodes

Organic light emitting devices (OLEDs) are known to heat up when driven at high brightness levels required for lighting and bright display applications. This so called Joule heating can in the extreme case lead to a catastrophic failure (breakdown) of the device. In this work, we compare the effect of Joule heated and externally heated OLEDs by their electrical and optical response. A reduction in resistance is observed at elevated temperatures, both, for Joule heating, and for externally heated samples driven at low current density. In both cases, we attribute the change in resistance to a higher mobility of charge carriers at the elevated temperatures. Additionally, we observe a quenching of the emission efficiency in heated single layers as well as in OLEDs, treated with an external heat source as well as on Joule heated samples.     

Influence of the applied charge on the electro-chemical degradation in green phosphorescent organic light emitting diodes

The influence of the current density on the chemical degradation processes of a phosphorescent OLED based on Ir(ppy)₃ emitter is investigated by laser-desorption/ionization time-of-flight mass spectrometry. Comparing the mass spectra collected for unaged and aged OLEDs, the formation of different chemical degradation products could be detected and are identified as dimer and trimer products of BPhen as well as Cs-adducts of these polymers and the well-known emitter-BPhen-adduct ([Ir(ppy)₂BPhen]⁺). In this work, we will show that the formation of [Ir(ppy)₂BPhen]⁺ depends strongly on the amount of the charge flowing through the device, where the other degradation products show a much different behaviour.     

具有凹凸界面结构的有机发光器件的性能研究

通过采用毫米尺度的凹凸界面结构实现了有机发光器件(Organic Light Emitting Device,OLED)发光效率的提高。在制备OLED器件过程中使用双狭缝模板,在发光层的界面处构建了高度为10nm的凸起,获得最大功率效率为23.9lm/W,最大电流效率为45.6cd/A,与传统的平直界面的OLED相比,分别提高了70%和36%。经过实验数据分析与理论模拟得出初步结论:OLED发光效率提高主要归因于金属界面处局域表面等离子体共振激发,提高了金属阴极界面的远场散射,从而提高OLED的出光效率;另外适当凹凸深度也改善了器件的电学性能。     

Full color organic light emitting diodes with laser-patterned optical path-length compensation layer

In this paper, we describe a promising way to fabricate micro-cavity OLEDs (organic light emitting diodes) with the concept of “super top emission” with soluble optical path-length compensation layer (OPCL) formed by Laser Induced Thermal Imaging (LITI) technology. OPCL is an additional layer for setting the cavity length corresponding to intrinsic wavelength of emitter. The resultant OLEDs gave a saturated three primary colors (RGB) after transmitting through common color filters by micro-cavity effect. By this approach, we could simplify the fabrication process of full color OLEDs by reducing conventional color patterning step, fine metal mask method. We also suggested the blending system of polymer and small molecule as transfer layer and this could improve the quality of laser patterning by controlling the surface energy adequately. Also, this system could be a solution to LITI problem of chronic contaminant because transferred materials could remain after cleaning process. Devices fabricated by soluble/LITI process showed comparable or even better performances and the device characteristics could enhance by further study of optimizing several parameters such as materials property and laser patterning condition. Also, we expect that this concept could lead to develop the fabrication process of large and high-resolution OLED displays.     

Enhancement in light extraction efficiency of organic light emitting diodes using double-layered transparent conducting oxide structure

We examined double-layered transparent conducting oxide (TCO) anode structures consisted of zinc-doped indium oxide (IZO) over the gallium-doped zinc oxide (GZO), and IZO over the GZO with electrochemical treatment. In bottom type OLEDs, power efficiency and current efficiency were enhanced by a factor of 1.50 and 1.14 at a current density of 10 mA/cm² in IZO/GZO anode structure, compared to the only IZO anode structure. Due to the reduced sheet resistance of the IZO/GZO TCO surface, the operating voltage of the OLED with IZO/GZO anode structure was lowered, leading to mostly enhance power efficiency. More enhanced in power efficiency and current efficiency by a factor of 1.21 and 1.25 at a current density of 10 mA/cm² were achieved in IZO/GZO anode structure with electrochemical treatment, compared to the IZO/GZO anode structure due to the change of the surface morphology of the GZO and the existence of the nanoporous layer beneath the GZO surface by an electrochemical treatment. In total, double-layered IZO/GZO anode structure with electrochemical treatment was revealed at an enhancement factor of 1.80 in power efficiency and 1.42 in current efficiency, compared to the only IZO anode structure.     

Modification effect of hole injection layer on efficiency performance of wet-processed blue organic light emitting diodes

We examined the performance of solution-processed organic light emitting diodes (OLEDs) by modifying the hole injection layer (HIL), poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS). Atomic force microscopy (AFM) showed morphological changes with surface roughness (R_RMS) of 1.47, 1.73, and 1.37 nm for pristine PEDOT: PSS, PEDOT: PSS modified with a 40 v% deionized water and with a 30 v% acetone, respectively. The surface hydrophobicity of the acetone modified PEDOT:PSS HIL layer was decreased by 34% as comparing with the water modified counterpart. Electrical conductivity was increased to two orders of magnitude for the water and acetone modified PEDOT:PSS as compared to pristine. We observed a low refractive index and high transmittance for the modified HILs. We fabricated and explored electroluminescent properties of bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) (FIrpic) based sky blue device by utilizing HIL with and without modification. The changes in electrical conductivity, surface roughness, refractive index, and transmittance of the modified HILs strongly influenced the performance of devices. By utilizing a 30% acetone modified HIL, the power efficiency was increased from 14.2 to 24.2 lm/W, an increment of 70% and the EQE from 8.5 to 13.1% at 100 cd/m², an increment of 54%. The maximum luminance also increased from 11,780 to 18,190 cd/m². The findings revealed herein would be helpful in designing and fabricating high efficiency solution processed OLEDs.     

Correlations of impedance-voltage characteristics and carrier mobility in organic light emitting diodes

The correlation of accumulation charges at the interfaces of organic layers and carrier mobility in organic light emitting devices (OLEDs) were investigated through the impedance versus voltage (Z-V) characteristics of devices. The properties of devices with various combinations of cathode structures, HTLs and ETLs were investigated to understand the impedance transition in Z-V characteristics of OLEDs. It was observed that there is an extra impedance transition before devices turn on when the hole mobility in the HTL is much greater than the electron mobility in the ETL in the devices, which makes the Z-V characteristics a potential tool to compare the electron mobility in ETL and hole mobility in HTL.     

Chemical degradation processes of highly stable red phosphorescent organic light emitting diodes

We investigate the chemical degradation processes of highly stable red organic light emitting diodes (OLEDs) based on the triplet emitter tris(1-phenylisoquinoline)iridium(III) by laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF-MS). The analysis of LDI-TOF spectra, collected on OLEDs driven at different current densities, shows a direct correlation between the lifetime of the devices and the formation of the three different reaction products: a BPhen dimer, an adduct of BPhen dimer with cesium, and the complex [BAlq₂ + Al(Me-q)₂]⁺ as well. Additionally it was possible to identify another degradation product, whose chemical structure is related to the α-NPD molecule as well to the fluorine of the used p-dopant. This product is only observable in devices aged at very high current densities.     

Highly efficient white transparent organic light emitting diodes with nano-structured substrate

Enhancing outcoupling efficiency and stabilizing emission spectra are of high technical importance in realizing high quality white transparent organic light emitting diodes (TOLEDs). In this work, we demonstrate a random nano-scattering layer (RSL) as a structure which can effectively address those tasks. The RSL contributes to bottom and top emissions by scattering and reflection, respectively. With the use of RSL, we achieved remarkable total efficiency enhancement of 101%. Also, a viewing angle independent stable white spectrum with a color rendering index of 79 was achieved. With its straight forward processing, our RSL can be readily applied to deal with various photonic applications to enhance both efficiency and emission spectra.     

AlGaInP系LED的表面纳米级粗化以及光提取效率提高

分析了常规AlGaInP系发光二极管（LED）光提取效率低的主要原因,半导体的折射率与空气折射率相差很大,导致全反射使有源区产生的光子绝大部分不能通过出光面发射到体外。通过在LED出光层采用纳米压印技术引入表面纳米结构,以改变光子的传播路径,从而使得更多的光子能够发射到体外。理论分析与实验结果表明,与常规平面结构相比,...     

Device performances of third order micro-cavity green top-emitting organic light emitting diodes

Results on the device performances of 3rd order micro-cavity effects in green top emitting organic light emitting diodes (TEOLEDs) with optical spacer layer are reported. The performances of fabricated 3rd order micro-cavity green devices using fac-tris(2-phenylpyridine)iridium [Ir(ppy)₃] green phosphorescent emitter and magnesium silver alloy semi-transparent cathode are systematically compared with the 2nd order cavity device. The optical and electrical simulation results of the 3rd order cavity device with emissive layer (EML) located near the semi-transparent cathode shows a current efficiency (CE) of 71.0 cd/A (70% value of the 2nd order device) and more serious color coordinates shift as compared to the 2nd order cavity device. With the center EML positioned 3rd order cavity device similar color variation behavior is observed but the CE value is 73.8 cd/A (73% value of the 2nd order device). The fabricated 3rd order TEOLEDs exhibit about 65–75% CE values of the 2nd order device as the simulation. Additionally, the viewing angle characteristics are significantly improved in the 3rd order center EML TEOLEDs as the simulation. The performances of fabricated TEOLEDs with 3rd order micro-cavity conditions are in quite good agreement with the optical and electrical simulation results.     

Analysis of alternating current driven electroluminescence in organic light emitting diodes: A comparative study

The alternating current (AC) responses of double-injection and double-insulated organic light-emitting diodes (OLEDs) were investigated and compared. To reveal the electroluminescent (EL) processes in these devices, the AC voltage and frequency dependence of the EL intensity and capacitive current were studied in the time domain with a focus on phase difference analysis. It was found that the voltage-dependent transit time and frequency-dependent carrier distribution were important for the AC-driven performance of the double-injection OLEDs. In contrast, although the double-insulated OLEDs shared some similarities with the double-injection OLEDs, they had some unique characteristics, which were the absence of resistive current and phase shift of EL profiles. It was revealed that the EL in the double-insulated OLEDs was driven by the displacement current generated by the ionization of the doped layers, which, however, formed space charge regions and undermined the EL emission. The space charge redistributed the electric field across the devices after the initiation of EL, making the EL maintain for a limited time interval. This effect was significant under low frequency and high AC voltage. Comparing the phase difference between both devices, it was indicated that the space charge effect was responsible for the observed EL phase shift and the asymmetric EL profiles at low frequency and high AC voltage in the double-insulated OLEDs. The proposed model was also of help to understand the EL saturation phenomena with AC frequency and voltage in those devices.     

Improved efficiency roll-off at high brightness in simplified phosphorescent organic light emitting diodes with a crossfading-host

We report a high efficiency and low efficiency roll-off green phosphorescent organic light emitting diode using both hole- and electron-transporting host materials in a crossfading profile. To eliminate the energy barrier and reduce the charge carrier accumulation, the host materials are used as transporting layers as well, which also simplifies the device fabrication. It is found out that the recombination zone of gradient doping host sample is not only wider but also extended at high current density, which contributes to the suppressed efficiency roll-off at high luminance. An external quantum efficiency of 21.0% at 1000 cd/m² is obtained, and maintains to 19.3% at 10,000 cd/m².     

Thermo-cleavable poly(fluorene-benzothiadiazole) to enable solution deposition of multi-layer organic light emitting diodes

The design of solution processable multi-layer organic light emitting diodes (OLEDs) is often hampered by the choice of solvents. To avoid the dissolution of the emission layer upon the subsequent deposition of further functional layers, we synthesize and investigate thermo-cleavage in poly[2,7-(3-(9-methyl-9H-fluorene-9-yl)propyl (2-methylhexane-2-yl) carbonate)-alt-4,7-(benzo[c][1,2,5]thiadiazole)] (c-F8BT). Employed in OLEDs, the non-cleaved polymer yields about the same current efficiency as state-of-the-art F8BT. During pyrolysis at 200 °C, the polymer releases its solubility groups, fully maintaining the device efficiency but becoming insoluble. This feature allows to enhance the OLED performance by applying an additional bathophenanthroline hole blocking layer from solution or by incorporating a low-molecular weight electron transport moiety into the affixing polymer matrix.     

Enhancing the efficiency of simplified red phosphorescent organic light emitting diodes by exciton harvesting

High efficiency red phosphorescent organic light emitting diode (PHOLED) employing co-doped green emitting molecule bis(2-phenylpyridine)(acetylacetonate)iridium(III) [Ir(ppy)₂(acac)] and red emitting molecule bis(2-methyldibenzo[f,h]quinoxaline)(acetylacetonate)iridium(III) [Ir(MDQ)₂(acac)] into 4,4′-bis(carbazol-9-yl)biphenyl (CBP) host in a simplified wide-bandgap platform is demonstrated. The green molecule is shown to function as an exciton harvester that traps carriers to form excitons that are then efficiently transferred to the Ir(MDQ)₂(acac) by triplet-to-triplet Dexter energy transfer, thereby significantly enhancing red emission. In particular, a maximum current efficiency of 37.0 cd/A and external quantum efficiency (EQE) of 24.8% have been achieved without additional out-coupling enhancements. Moreover, a low efficiency roll-off with the EQE remaining as high as 20.8% at a high luminance of 5000 cd/m² is observed.     

Correlation between interlayer thickness and device performance in blue phosphorescent organic light emitting diodes with a quantum well structure

We systematically examined the effects of interlayer (ITL) thickness variation in an emission layer (EML) on electrical and optical characteristics of blue phosphorescent organic light-emitting diodes. The EML consisted of a quantum well structure using a hole transport material 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) as an ITL. This ITL facilitated the confinement of charge carriers in the recombination zone (RZ), adjusted the charge carrier balance in the EML, and prevented the triplet exciton loss to adjacent transport layers. The thickness variation in the ITL greatly influenced the size and location of the RZ and the exciton density (ED), which is related to charge balance and exciton diffusion in the EML. A micro-cavity effect around 500 nm and the corresponding redshift/blueshift in the electroluminescent spectrum arose from different ITL thicknesses. Remarkably, the device having a 5-nm-thick TAPC ITL showed better current and power efficiencies than those of any other devices because of the rearrangement of the locations of excitons and ED through control of the hole/electron charge density.     

Improved light coupling efficiency of organic light- emitting diode and polymer optical waveguide integrated device by grating coupler

In this work, the light coupling efficiency of organic light-emitting diode (OLED) and polymer optical waveguide integrated device was improved by the grating coupler. To maximize light coupling efficiency, the grating coupler was optimized by finite-difference time-domain (FDTD) method. Based on the simulation results, the grating coupler was fabricated via laser interference lithography process and an OLED was integrated on the surface of it. Comparing the integrated devices without and with grating coupler, light coupling efficiency of the grating-based integrated device was improved by about 5%. The proposed integrated device has the potential application for low-cost and flexible monolithic optical sensors.     

Optimization of carrier transport layer: A simple but effective approach toward achieving high efficiency all-solution processed InP quantum dot light emitting diodes

High performance quantum dot light emitting diodes (QD-LED) are being considered as a next-generation technology for energy efficient solid-state lighting and displays. In recent years, cadmium (Cd)-based QLEDs have made great progress in performance, which is close to commercial applications. However, the performance of environmentally friendly Cd-free QD-LED still needs to be improved. In this letter, using InP/ZnS quantum dots (QDs), an environmentally friendly red QDs material, as the light emitting layer, low-cost all-solution processed red InP/ZnS QD-LED are fabricated. The optimized device with a hybrid multilayered structure employing an organic double hole transport layer (HTL) with doping small molecules (TFB/PVK:TAPC) and an inorganic ZnMgO nanoparticles (NPs) electron transport layer (ETL), here TFB, PVK and TAPC represent poly [(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4’-(N-(p-butylphenyl))-diphenylamine)], poly (9-vinlycarbazole) and 1,1-bis [4-[N,N′-di (p-tolyl)amino]phenyl]-cyclohexane, respectively. The best device exhibits a peak current efficiency (CE) of 7.58 cd A⁻¹, which is 2.4 times higher than the control device using PVK (HTL) and ZnO (ETL). At the same time, turn-on voltage dropped from 2.8 V (control devices) to 2.4 V. These superb QD-LED performances originate not only from the improved hole injection by the introduction of a double hole layer and the reduced the quenching of excitons by using ZnMgO NPs ETL but also from increasing the hole mobility with doping of small molecule materials in PVK to balance the carrier transportation. This work provides a simple and feasible idea with optimization the carrier transport for realizing high-efficiency QD-LED devices.     

Improving the flexibility of large-area transparent conductive oxide electrodes on polymer substrates for flexible organic light emitting diodes by introducing surface roughness

In this study, transparent conductive oxide (TCO) electrodes with highly enhanced flexibility were developed on polymer substrates for application in flexible organic emitting diodes (OLEDs). TCOs, particularly indium tin oxide (ITO), have superior functional properties as electrodes compared to other materials but are inherently brittle, which significantly limits the bendability of the flexible devices. To improve the fracture strength of ITO on a polymer substrate under bending, we investigated the effect of expanding the film surface areas on the reduction of the stresses induced by an external bending force. Regularly spaced channels were imprinted at an elevated temperature onto polymer substrate surfaces using Teflon^®. Then, both amorphous (a-ITO) and crystalline ITOs (c-ITO) were dc magnetron sputter deposited. As the channel patterns on the substrate surfaces were reflected into the growing film surfaces, the ITO surfaces became unidirectionally wavy, which increased the surface area by approximately 500%. The electrical and optical properties of the wavy ITOs were measured using a four-point probe and a UV–visible spectrophotometer, respectively, and the flexibility was evaluated with cyclic bending tests. For comparison, flexible OLEDs were also fabricated on both wavy ITO and conventional ITO. Our results revealed that the functional properties of ITOs with expanded surfaces are equivalent to those of conventional ITOs on the polymer substrates. However, their cyclic bending stability was significantly improved. After 10,000 cycles at a bending radius of 10 mm, the electrical resistivity change was less than half of the conventional ITO. The current density–voltage (J–V) characteristics of the flexible OLEDs on the wavy ITOs were also nearly equal to those on conventional ITOs.     

Bulk-like Al/Ag bilayer film due to suppression of surface plasmon resonance for high transparent organic light emitting diodes

We demonstrate the enhanced optical and electrical properties of an ultrathin silver (Ag) film by applying an aluminum (Al) seed layer between LiF and Ag as a transparent cathode for higher-transparency organic light-emitting diodes (OLEDs). Although the thickness ranges from 4 to 8 nm, the ultrathin Ag film is a continuous and uniform bulk-like film with an Al seed layer, which suppresses the surface plasmon absorption. Compared to an Ag-only cathode, the measured transmittance spectra were considerably increased, comparable with the theoretical calculations of a bulk Al/Ag bilayer film. The Al/Ag bilayer cathode has a transmittance of 87% at a 550 nm wavelength and a sheet resistance of 19.5 Ω/sq with a 4-nm-thick Ag layer. The transparent OLED devices that employed the Al/Ag cathode showed a transmittance of 72% at a 550 nm wavelength for an Ag thickness of 6 nm.