The role of cesium fluoride as an n-type dopant on electron transport layer in organic light-emitting diodes

We report on the role of cesium fluoride (CsF) doping on the enhanced electron transport properties of tris-(8-hydroxyquinolin) aluminum (Alq₃) for organic light-emitting diodes. The electronic structures of CsF-doped Alq₃ layers with various doping concentration are characterized by in situ ultraviolet and X-ray photoelectron spectroscopies, showing an n-type electrical doping effect with Fermi level shift towards unoccupied molecular orbital and the formation of chemistry-induced gap-states. The increase in conductivity and reduction in electron injection barrier in CsF-doped Alq₃ layer with optimal doping concentration lead to the enhanced electron injection and transport, which are consistent with the improved electrical characteristics of OLEDs.     

Highly efficient and foldable top-emission organic light-emitting diodes based on Ag-nanoparticles modified graphite electrode

Top-emission flexible organic light-emitting devices (TE-FOLEDs) are highly suitable for next generation display due to their numerous assets including top-emitting configuration and mechanical flexibility. One major challenge in TE-FOLEDs is to prepare a deformable and reflective bottom electrode capable of effective carrier injection. In this paper, a new strategy for efficient and foldable TE-FOLEDs is demonstrated. It is based on a highly conductive Ag-nanoparticles (Ag-NPs) modified graphite that is used as a flexible bottom electrode. The good reflectance to full-color emission (>59% over the whole visible wavelength range), ultralow sheet resistance (<5 Ω/sq), and high tolerance to mechanical bending (almost unchanged in resistance after bending 1000 times with an angle of ±90°) of the modified graphite synergistically constitute a breakthrough in the domain of TE-FOLEDs. The maximum current efficiencies reach 15.0, 50.2, 16.8 cd/A for red, green, blue emissions, respectively. Colorimetric gamut increased by 99.6% compared to bottom emission structure with the corresponding Commission Internationale de L'Eclairage (CIE) coordinates of the red/green/blue (R/G/B) devices. In particular, the TE-FOLEDs incorporating highly flexible graphite electrodes offer great mechanical durability and the initial brightness of 5000 cd/m can be maintained over 90% after bending for 1000 bending cycles. This approach is expected to open a new avenue for developing foldable displays.     

High efficiency ITO-free flexible white organic light-emitting diodes based on multi-cavity technology

The technology of white organic light-emitting diodes (WOLEDs) is attracting growing interest due to their potential application in indoor lighting. Nevertheless the simultaneous achievement of high luminous efficacy (LE), high color rendering index (CRI), very low manufacturing costs and compatibility with flexible thin substrates is still a great challenge. Indeed, very high efficiency devices show usually low values of CRI, not suitable for lighting applications, and use expensive indium tin oxide (ITO) electrodes which are not compatible with low cost and/or flexible products. Here we show a novel low cost ITO-free WOLED structure based on a multi-cavity architecture with increased photonic mode density and still broad white emission spectrum, which allows for simultaneous optimization of all device characteristics. Without using out-coupling optics or high refractive index substrates, CRI of 85 and LE as high as 33 lm W⁻¹ and 14 lm W⁻¹ have been demonstrated on ITO-free glass and flexible substrates, respectively.     

Enhancement of light extraction of green top-emitting organic light-emitting diodes with refractive index gradually changed coupling layers

By means of refractive index gradually changed coupling layers, a highly efficient green top-emitting OLED (TOLED) with enhanced light coupling efficiency and stable colors over angles has been realized. The refractive index transition of the coupling layers including the doping layer smoothes light extraction from the semitransparent cathode metal to the air, which is the reason for the enhancement of light coupling efficiency. The doping layer in the coupling layers also acts as a microparticle diffuser to eliminate the shift in EL spectra with viewing angles. A universal simulation has also been carried out, and the result suggests that the light coupling efficiency will be enhanced further if the refractive index transition of the coupling layers is continuous.     

ITO-free down-conversion white organic light-emitting diodes with structured color conversion layers for enhanced optical efficiency and color rendering

We report our study on white organic light-emitting diodes (WOLEDs) implemented in a down-conversion scheme based on an ITO-free, cavity-enhanced blue phosphorescent OLED and a micro-structured color conversion layer (CCL) containing red and green phosphors. Cavity resonance induced by a ZnS/Ag/MoO₃ anode structure enables both efficiency enhancement/spectral refinement of blue phosphorescent OLED. In accordance with the resonance-induced effect, outcoupling assistance provided by micro-structuring of CCLs works to yield WOLEDs with both high efficiency and illumination-quality color rendering. Highly flexible WOLEDs are also demonstrated in the proposed scheme and tested at a radius of curvature of 10.8 mm to illustrate its advantages in realizing versatile next-generation light sources.     

Tunable microcavities in organic light-emitting diodes by way of low-refractive-index polymer doping

A method for enhancing the light out-coupling efficiency of organic light-emitting devices (OLEDs) has been demonstrated by blending a low-refractive-index polymer, poly(2,2,3,3,3-pentafluoropropyl methacrylate) (PPFPMA), into the emission layer. The resonant wavelength of the weak microcavity devices blueshifted accompanied with a decrease in refractive indices of the light-emitting layers after the addition of PPFPMA. Stronger directed emission toward the surface normal was obtained when the resonant wavelength became closer to the peak wavelength of intrinsic emission spectrum of the organic emitters. The luminous efficiency of the devices was enhanced by more than 20%. The results suggest that the microcavity properties of the OLEDs can be tunable through blending low-refractive-index materials.     

Effect of bulk and planar heterojunctions based charge generation layers on the performance of tandem organic light-emitting diodes

Tandem organic light-emitting diodes (OLEDs) were fabricated using organic planar and bulk heterojunctions based charge generation layers (CGLs), which were composed of cobalt phthalocyanine (CoPc) and fullerene (C₆₀). The electroluminescent (EL) characteristics of these two kinds of devices were systematically studied. The results showed that, compared to the corresponding devices with planar heterojunction (PHJ) based CGL, the tandem OLEDs with bulk heterojunction (BHJ) based CGL exhibited a dramatic improvement of performance. By investigating the electrical characteristics of CGLs, it was found that more hetero-interfaces introduced in the BHJ blend were beneficial for generating more interfacial dipoles and charge carriers, and the optimized charge transport pathways were favorable to promote both electron and hole mobilities. As a result, the improved charge carrier balance led to the efficiency enhancement of device performance. The results demonstrated the advantageous effect of BHJ blend film for the rational design of CGLs on the realization of high OLEDs performance.     

Effect of n-type barrier doping on steady and dynamic performance of InGaN light-emitting diodes

The steady and dynamic properties are comparatively simulated results show that the n-doped LED exhibits is mainly attributed to the higher carrier radiative experimental results perfectly. investigated for the n-doped and non-doped InGaN LEDs. The the superior luminescence and modulation performance, which rate of n-doped LED. The results can explain the reported     

Small-sized Al nanoparticles as electron injection hotspots in inverted organic light-emitting diodes

Al nanoparticles, with small size and ultralow coverage on ITO, can play a key role as the electron injection hotspots in both the inverted fluorescent and phosphorescent organic light-emitting diodes. The presence of the hotspots greatly reduces the operational voltage and improves the current efficiency of the devices, which are strongly dependent on the hotspot size. The microscopic and spectroscopic characterization demonstrate that the small-sized hotspots have a minor influence on the surface roughness, transparency and work function of ITO. The hotspot effect is ascribed to the highly efficient electron injection at the Al nanoparticles enhanced by the local electric field, and a physical model is proposed to clarify this mechanism. The finding indicates a promising strategy by design and craft of the injection hotspots in nanoscale to facilitate carrier injection in organic thin film devices.     

Analysis of device performance and thin-film properties of thermally damaged organic light-emitting diodes

This paper reports the variation in the optical and geometrical properties of individual organic layers to be used for thermally damaged top-emission organic light-emitting diodes (TEOLEDs). The copper deposited on the back of TEOLEDs is employed as a thermal facilitator, and a certain thermal damage occurs to the organic layers and devices. The phosphorescent host material 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) is rapidly damaged to a significant extent owing to the low glass transition temperature (T_g), which also changes its optical and geometrical surface properties. Although the optical properties of the hole transport layer, N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) were changed slightly, the surface morphology was changed significantly. Despite having a higher T_g, the exciton blocking layer, tris(4-carbazoyl-9-ylphenyl)amine (TCTA), shows notable variations in optical properties and surface morphology due to heat exposure. Surprisingly, the electroluminescence spectra and micro-cavity are affected by increasing temperature without any considerable changes in device performance. Hence, this study reveals that besides T_g, the surface morphologies and thicknesses of the organic layers are also important factors in the annealing process and play a vital role in causing thermal damage to TEOLEDs.     

Electron injection in inverted organic light-emitting diodes with poly(ethyleneimine) electron injection layers

Electron-injection mechanisms from the air-stable metal-oxide cathode to light-emitting polymer layer are studied. The device configuration is aluminum (Al) doped zinc oxide (AZO)/poly(ethyleneimine) (PEI)/poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT)/molybdenum trioxide/Al, known as an inverted organic light-emitting diode (iOLED). PEI reduces the electron injection barrier between AZO and F8BT by 0.4 eV, and blocks holes at AZO(PEI)/F8BT interface in iOLEDs. The accumulation of holes at the interface greatly enhances the electron injection because of the Fowler-Nordheim type tunneling injection, leading to high current efficiency of iOLEDs.     

Green phosphorescent organic light-emitting devices based on different electron transport layers combining with fluorescent sub-monolayer

We report a small molecule host of 4,4(-N,N)-dicarbazole-biphenyl (CBP) doped with 8% tris(2-phenylpyridine) iridium (Irppy3) for use in efficient green phosphorescent organic light-emitting devices (PHOLEDs) combined with different electron transport layers of Alq and BAlq. The PHOLEDs exhibit maximum current efficiency and power efficiency of 19.8 cd/A and 6.21 lm/W, respectively. The high performance of such PHOLEDs is attributed to the better electron mobile ability of BAlq and sub-monolayer quinacridone (QAD) as carrier trapping layer and equal charge carrier mobilities of hole and electron to form the broad carrier recombination zone in the emitting layer, which can reduce the triplet-triplet annihilation and improve the efficiency of the device. #$TABThis work has been supported by the Major Project of Science and Technology Office of Fujian Province of China (No.2014H0042), the Natural Science Foundation of Fujian Province of China (No.2015J01664), the Project of Science and Technology Research of Quanzhou in Fujian Province of China (Nos.2013Z125 and 2014Z137), and the 2016 Annual National or Ministries Preparatory Research Foundation Project in Quanzhou Normal University (No.2016YYKJ21). E-mail:yanghuishan1697@163.com      

Universal electron transporting layers via mixing two homostructure molecules with different polarities for organic light-emitting diodes

In general, electron transport layer (ETL) in organic light-emtting diodes (OLEDs) consists of single component of electron transporting material (ETM) or a mixture with n-dopant such as 8-hydroxyquinolinolato-lithium (Liq). However, there exists a limit to controlling a wide range of carrier density in OLEDs according to the required characteristics of the devices due to electrically insulating property of Liq. Here, we suggest a universal strategy to construct an efficient ETL. We synthesized two ETMs, diphenyl-[4-(10-phenyl-anthracene-9-yl)-phenyl]-amine (An-Ph) and phneyl-[4-(10-phenyl-anthracene-9-yl)-phenyl]-pyridin-3-yl-amine (An-Py) that have the same core structures with different polarities in functional groups. The electrical characteristics of electron-only-devices (EODs) were investigated by space charge limited current (SCLC) modeling and impedance spectroscopy analysis. Interestingly, the homostructure type ETL composed of An-Ph and An-Py showed not only superior electron transporting capability, but also the possibility of controlling electron injection and transporting in a wide range compared to the heterostructure type ETL of An-Ph and Liq. Compared to the An-Ph-only EOD, the electron mobility in 75% An-Py-mixed homostructure EOD increased by almost 4 orders of magnitude. Such dramatic variation of electron mobility was achieved thanks to the molecular design strategy to separate charge injection and charge transport regions within a molecule, which consequently induced the giant surface potential (GSP) effect between the ETL/cathode interface. As a result, the external quantum efficiency (EQE) of blue fluorescent and phosphorescent OLEDs with the homostructure ETLs was enhanced by 28.6% and 34%, respectively, compared to that of each control device without manipulating outcoupling effects.     

Features of the current-voltage characteristics in thin conductive layers of organic light-emitting diodes

The current-voltage characteristic of a thin organic layer located between conductive electrodes is analytically modeled. To this end, a theoretical model is developed which considers not only the interaction of an injected carrier with its mirror charge “reflected” in the nearest electrode, but also the effect of multiple reflections and the injection current from the opposite electrode. The current-voltage characteristics at various temperatures and barrier heights are compared to the model previously developed by Arkhipov et al. The limits of applicability of this model are determined. At low temperatures and voltages, the effect of multiple reflections becomes significant, which cause an increase in the current. These results should be considered when testing individual thin layers constituting multilayer organic light-emitting diodes.     

High triplet energy electron transport type exciton blocking materials for stable blue phosphorescent organic light-emitting diodes

High triplet energy electron transport materials with dibenzothiophene and dibenzofuran cores modified with a diphenyltriazine unit were investigated as electron transport type exciton blocking materials for stable blue phosphorescent organic light-emitting diodes. The two exciton blocking materials showed high triplet energy above 2.80 eV and enhanced quantum efficiency of the blue phosphorescent devices by more than 40% while maintaining stability of the pristine blue devices without the high triplet energy exciton blocking layer.     

利用CsN3n型掺杂电子传输层改善OLED器件性能的研究

为了能够有效地提高电子的注入和传输能力,改善有机电致发光器件的性能,本文利用CsN₃作为n型掺杂剂,对有机电子传输材料Bphen进行n型电学掺杂,制备了结构为ITO/MoO₃（2 nm）/NPB（50 nm）/Alq₃（30 nm）/Bphen（15 nm）/Bphen：CsN₃（15 nm,x%,x=10,15,20）/Al（100 nm）的器件。实验结果表明,CsN₃是一种有效的n型掺杂剂,以掺杂层Bphen：CsN₃ 作为电子传输层,可以有效地降低电子的注入势垒,改善器件的电子注入和传输能力,从而降低器件的开启电压,同时提高了器件的亮度和发光效率。在掺杂浓度为10%时器件的性能最优,开启电压仅为2.3 V,在7.2 V的驱动电压下,达到最大亮度29 060 cd/m²,是非掺杂器件的2.5倍以上。当驱动电压为6.6 V时,达到最大电流效率3.27 cd/A。而当掺杂浓度进一步提高时,由于Cs扩散严重,发光区形成淬灭中心,造成器件的效率下降。     

Cadmium-free quantum dots based violet light-emitting diodes: High-efficiency and brightness via optimization of organic hole transport layers

Bright and efficient violet quantum dot (QD) based light-emitting diodes (QD-LEDs) with heavy-metal-free ZnSe/ZnS have been demonstrated by choosing different hole transport layers, including poly(4-butyl-phenyl-diphenyl-amine) (poly-TPD), poly[9,9-dioctylfluorene-co-N-[4-(3-methylpropyl)]-diphenylamine] (TFB), and poly-N-vinylcarbazole (PVK). Violet QD-LEDs with maximum luminance of about 930 cd/m², the maximum current efficiency of 0.18 cd/A, and the peak EQE of 1.02% when poly-TPD was used as HTL. Higher brightness and low turn-on voltage (3.8 V) violet QD-LEDs could be fabricated when TFB was used as hole transport material. Although the maximum luminance could reach up to 2691 cd/m², the devices exhibited only low current efficiency (∼0.51 cd/A) and EQE (∼2.88%). If PVK is used as hole transport material, highly efficient violet QD-LEDs can be fabricated with lower maximum luminance and higher turn-on voltages compared with counterpart using TFB. Therefore, TFB and PVK mixture in a certain proportion has been used as HTL, turn-on voltage, brightness, and efficiency all have been improved greatly. The QD-LEDs is fabricated with 7.39% of EQE and 2856 cd/m² of maximum brightness with narrow FWHM less than 21 nm. These results represent significant improvements in the performance of heavy-metal-free violet QD-LEDs in terms of efficiency, brightness, and color purity.     

Efficient non-doped monochrome and white phosphorescent organic light-emitting diodes based on ultrathin emissive layers

Efficient red, orange, green and blue monochrome phosphorescent organic light-emitting diodes (OLEDs) with simplified structure were fabricated based on ultrathin emissive layers. The maximum efficiencies of red, orange, green and blue OLEDs are 19.3 cd/A (17.3 lm/W), 45.7 cd/A (43.2 lm/W), 46.3 cd/A (41.6 lm/W) and 11.9 cd/A (9.2 lm/W). Moreover, efficient and color stable white OLEDs based on two complementary colors of orange/blue, three colors of red/orange/blue, and four colors of red/orange/green/blue were demonstrated. The two colors, three colors and four colors white OLEDs have maximum efficiencies of 30.9 cd/A (27.7 lm/W), 30.3 cd/A (27.2 lm/W) and 28.9 cd/A (26.0 lm/W), respectively. And we also discussed the emission mechanism of the designed monochrome and white devices.     

Manipulating the LUMO distribution of quinoxaline-containing architectures to design electron transport materials: Efficient blue phosphorescent organic light-emitting diodes

A series of new quinoxaline-containing compounds, namely, 2,3,6,7-tetrakis(3-(pyridin-3-yl)phenyl)quinoxaline (Tm3PyQ), 2,3,6,7-tetrakis(3-(pyridin-4-yl)phenyl)quinoxaline (Tm4PyQ), 1,4-bis(2,3-dimethyl-7-(pyridin-3-yl)quinoxalin-6-yl)benzene (3PyDQB), and 1,4-bis(2,3-dimethyl-7-(pyridin-4-yl)quinoxalin-6-yl)benzene (4PyDQB) were designed and synthesized as electronic transporting materials. The lowest unoccupied molecular orbital (LUMO) distributions of these compounds vary with the locations of quinoxaline moieties, which result in adjustable intermolecular charge-transfer integrals. All the compounds exhibit favorable electron affinity (2.73–2.88 eV) and good thermostability (glass transition temperatures in the range of 112–148 °C). Using these compounds as electron transport layers, the bis(4,6-(difluorophenyl)pyridinato-N,C^2′)picolinate iridium(Ⅲ) (Firpic)-based blue phosphorescent organic light emitting diodes (PhOLEDs) achieve good performances with a maximum current efficiency (η_c,max) of 30.2 cd A⁻¹ and a maximum external quantum efficiency (η_ext,max) of 14.2%. Moreover, these efficiencies reveal small roll-offs at high luminance.     

阴极蒸镀和隔离层对有机发光二极管性能的影响

制备了简单结构的有机发光二极管(OLED)ITO/NPB/Alq3/Al/Ag。实验结果表明,快速蒸镀法制备的Ag阴极越厚,器件性能越差,而慢速蒸镀200nmAg阴极时器件性能也较差。在Alq3与Al阴极之间插入BCP/C60/LiF隔离层后,即使快速蒸镀法制备的Ag厚达280nm,器件的最大电流密度、最大亮度和最大电流效率仍分别高达248.6mA/cm2、5380.7cd/m2和3.52cd/A。隔离层不仅保护NPB和Alq3基本不被玻璃化,还很好地与Alq3和Al阴极匹配,大大提高了器件性能。