Doping-free hybrid white organic light-emitting diodes with fluorescent blue,phosphorescent green and red emission layers

Industrialized white organic light-emitting diodes (OLEDs) currently require host-guest doping, a complicated process necessitating precise control of the guest concentration to get high efficiency and stability. Two doping-free, hybrid white OLEDs with fluorescent blue, and phosphorescent green and red emissive layers (EMLs) are reported in this work. An ultra-thin red phosphorescent EML was situated in a blue-emitting electron transport layer (ETL), while the ultra-thin green phosphorescent EML was placed either in the ETL (Device 1), or the hole transport layer (HTL) (Device 2). Device 2 exhibits higher efficiency and more stable spectrum due to the enhanced utilization of excitons by ultra-thin green EML at the exciton generation zone within the HTL. Values of current efficiency (CE), power efficiency (PE), and CRI obtained for the optimized hybrid white OLEDs fabricated through a doping-free process were of 23.2 cd/A, 20.5 lm/W and 82 at 1000 cd/m², respectively.     

Crosslinkable TAPC‐Based Hole‐Transport Materials for Solution‐Processed Organic Light‐Emitting Diodes with Reduced Efficiency Roll‐Off

1‐Bis[4‐[N,N‐di(4‐tolyl)amino]phenyl]‐cyclohexane (TAPC) has been widely used in xerography and organic light‐emitting diodes (OLEDs), but derivatives are little known. Here, a new series of solution‐processable, crosslinkable hole conductors based on TAPC with varying highest occupied molecular orbital (HOMO) energies from ?5.23 eV to ?5.69 eV is implemented in blue phosphorescent OLEDs. Their superior perfomance compared with the well‐known N4,N4,N4′,N4′‐tetraphenylbiphenyl‐4,4′‐diamine (TPDs) analogues regarding hole‐injection and mobility, electron and exciton blocking capabilities, efficiency, and efficiency roll‐off is demonstrated. Overall, the TAPC‐based devices feature higher luminous and power efficiency over a broader range of brightness levels and reduced efficiency roll off. A systematic broadening of the emission zone is observed as the hole‐injection barrier between the anode and the hole‐transporting layer increased.     

1‐Ethynyl Ethers as Efficient Thermal Crosslinking System for Hole Transport Materials in OLEDs

A new crosslinking concept based on a thermally activated one‐component building block with thermally initiated crosslinkable ynol ether is introduced. For polystyrene matrices with glass transition temperatures below the reaction temperature, full conversion is reached within 30 min at 160 °C without employing any catalysts or co‐reactants. The ynol ethers are chemically inert toward a variety of reaction conditions (e.g., radicals and strong bases) and consequently applicable to a wide range of materials for organic electronics. The crosslinkable solid compounds are bench‐stable over more than a year. The broad applicability is demonstrated with a liquid model compound and a specifically designed crosslinking monomer introduced successfully as building block into polystyrenes with pending hole transporting groups. A detailed study of crosslinking kinetics by infrared measurements as well as an alternative method of crosslinker content determination utilizing differential scanning calorimetry is presented. The crosslinkable polymer and the corresponding noncrosslinkable molecule tris(4‐(3,6‐dibutoxy‐9H‐carbazol‐9‐yl)phenyl)amine (BuO₆TCTA) are synthesized and investigated as hole transport layers (HTLs) in phosphorescent organic light emitting diodes (OLEDs). OLEDs with crosslinked and noncrosslinked HTLs show efficiencies around 80 cd A^?1, indicating negligible influence of the crosslinking process on the device performance while yielding better HTL durability against solvent rinsing.     

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.     

Enhancement of the current efficiency of organic light-emitting devices due to the surface plasmonic resonance effect of dodecanethilol-functionalized Au nanoparticles

The photoluminescence intensity of the dodecanethilol-functionalized Au (DDT-Au) nanoparticle (NP) layer/4,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine] (TAPC)/4,4′-bis(N-carbazolyl)-1,1′-biphenyl:tris(2-phenylpyridine)iridium (III) (CPB:Ir(ppy)₃) film was increased by about 1.15 times compared to that of the TAPC/CPB:Ir(ppy)₃ film due to the effect of coupling between the excitons in the emitting layer and a localized surface plasmonic resonance (LSPR) in the DDT-Au NPs. The current efficiency of the organic light-emitting devices (OLEDs) with the DDT-Au NP layer at 100 cd/m² was 14.9 cd/A larger than that without the DDT-Au NP layer, resulting in an enhancement of the out-coupling efficiency. The increase in the current efficiency of the OLEDs with a DDT-Au NP layer was attributed to the enhanced out-coupling efficiency due to the existence of the LSPR generated by the DDT-Au NPs.     

Conjugated Small Molecule for Efficient Hole Transport in High‐Performance p‐i‐n Type Perovskite Solar Cells

The π‐conjugated organic small molecule 4,4′‐cyclohexylidenebis[N,N‐bis(4‐methylphenyl) benzenamine] (TAPC) has been explored as an efficient hole transport material to replace poly(3,4‐ethylenedio‐xythiophene):poly(styrenesulfonate) (PEDOT:PSS) in the preparation of p‐i‐n type CH₃NH₃PbI₃ perovskite solar cells. Smooth, uniform, and hydrophobic TAPC hole transport layers can be facilely deposited through solution casting without the need for any dopants. The power conversion efficiency of perovskite solar cells shows very weak TAPC layer thickness dependence across the range from 5 to 90 nm. Thermal annealing enables improved hole conductivity and efficient charge transport through an increase in TAPC crystallinity. The perovskite photoactive layer cast onto thermally annealed TAPC displays large grains and low residual PbI₂, leading to a high charge recombination resistance. After optimization, a stabilized power conversion efficiency of 18.80% is achieved with marginal hysteresis, much higher than the value of 12.90% achieved using PEDOT:PSS. The TAPC‐based devices also demonstrate superior stability compared with the PEDOT:PSS‐based devices when stored in ambient circumstances, with a relatively high humidity ranging from 50 to 85%.     

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.     

Solution‐Processed Thermally Activated Delayed Fluorescent OLED with High EQE as 31% Using High Triplet Energy Crosslinkable Hole Transport Materials

Solution‐processed organic light‐emitting diodes (OLEDs) with thermally activated delayed fluorescent (TADF) material as emitter have attracted much attention because of their low cost and high performance. However, exciton quench at the interface between the hole injection layer, poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), and emitting layer (EML) in devices can lead to low device performance. Here, a novel high triplet energy (2.89 eV) and crosslinkable hole‐transporting material grafted with oxetane groups, N,N‐bis(4‐(6‐((3‐ethyloxetan‐3‐yl)methoxy)hexyloxy)phenyl)‐3,5‐di(9H‐carbazol‐9‐yl)benzenamine (Oxe‐DCDPA)), as crosslinked hole transport layer (HTL) into the interface of PEDOT:PSS layer and EML is proposed for prevention of exciton quenching, and among the reported devices with single HTL in solution‐processed TADF‐OLED, the highest external quantum efficiency (EQE)/luminous efficiency (η_L) of 26.1%/94.8 cd A^?1 and 24.0%/74.0 cd A^?1 are achieved for green emission (DACT‐II as emitter) and bluish‐green emission (DMAC‐TRZ as emitter), respectively. Further improvement, using double HTLs, composed of N,N′‐bis(4‐(6‐((3‐ethyloxetan‐3‐yl)methoxy))‐hexylphenyl)‐N,N′‐diphenyl‐4,4′‐diamine with high hole mobility and Oxe‐DCDPA with high triplet energy, leads to the highest EQE/η_L of 30.8%/111.9 cd A^?1 and 27.2%/83.8 cd A^?1 for green emission and bluish‐green emission, respectively. These two devices show the high maximum brightness of 81 100 and 70 000 cd m^?2, respectively.     

Ultrahigh Efficiency Fluorescent Single and Bi‐Layer Organic Light Emitting Diodes: The Key Role of Triplet Fusion

A new family of anthracene core, highly fluorescent emitters is synthesized which include diphenylamine hole transport end groups. Using a very simple one or two layer organic light emitting diode (OLED) structure, devices without outcoupling achieve an external quantum efficiency of 6% and photonic efficiencies of 20 cd/A. The theoretical maximum efficiency of such devices should not exceed 3.55%. Detailed photophysical characterization shows that for these anthracene based emitters 2T₁≤T_n and so in this special case, triplet fusion can achieve a singlet production yield of 0.5. Indeed, delayed electroluminescence measurements show that triplet fusion contributes 59% of all singlets produced in these devices. This demonstrates that when triplet fusion becomes very efficient, fluorescent OLEDs even with very simple structures can approach an internal singlet production yield close to the theoretical absolute maximum of 62.5% and rival phosphorescent‐based OLEDs with the added advantage of much improved stability.     

Highly efficient green phosphorescent organic light emitting diodes with simple structure

A series of simple structures is investigated for realization of the highly efficient green phosphorescent organic light emitting diodes with relatively low voltage operation. All the devices were fabricated with mixed host system by using 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) and 1,3,5-tri(p-pyrid-3-yl-phenyl)benzene (TpPyPB) which were known to be hole and electron type host materials due to their great hole and electron mobilities [μ_h(TAPC): 1 × 10^?2 cm²/V s and μ_e(TpPyPB): 7.9 × 10^?3 cm²/V s] [1]. The optimized device with thin TAPC (5–10 nm) as an anode buffer layer showed relatively high current and power efficiency with low roll-off characteristic up to 10,000 cd/m². The performances of the devices; with buffer layer were compared to those of simple devices with single layer and three layers. Very interestingly, the double layer device with TAPC buffer layer showed better current and power efficiency behavior compared to that of three layer device with both hole and electron buffer layers (TAPC, TpPyPB, respectively).     

Phosphorescent organic light emitting diodes with a cross-linkable hole transporting material

A cross-linkable hole transporting material PLEXCORE® HTL was incorporated in phosphorescent organic light emitting diodes. This hole transporting material is based on an arylamine derivate. The device performance in terms of efficiency and lifetime was compared to the same devices with a thermally evaporated 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB)-based hole transporting layer. The resulting devices with the cross-linkable HTL gave higher efficiency, smaller roll-off and longer lifetime compared with devices with the NPB-based devices. This new hole transporting material paves the road toward solution processed multilayer light emitting devices.     

Multifunctional Synthesis Approach of In:CuCrO2 Nanoparticles for Hole Transport Layer in High‐Performance Perovskite Solar Cells

While there are very limited studies of doped ternary metal oxide based hole transport materials, a multifunctional synthesis approach of In doped CuCrO₂ nanoparticles (NPs) as efficient hole transport layers (HTLs) including simplifying the synthesis requirements is proposed, enabling doping and achievement of treatment‐free HTLs. Remarkably, compared with conventional methods for synthesizing CuCrO₂ NPs, the newly proposed azeotropic promoted approach dramatically reduces the reaction time by 90% and the calcination temperature by one‐third, which not only promotes high throughput production but also reduces power consumption and cost in synthesis. Equally important, indium is successfully doped into CuCrO₂, which is fundamentally difficult in low temperature processes. The In doping offers less d–d transition of Cr³⁺ and p‐type doping characteristics for improving HTL transmittance and conductivity, respectively. Interestingly, In doped CuCrO₂ HTL with these improvements can be achieved by a simple ambient‐condition process and exhibits thermal stability up to 200 °C, which allows perovskite solar cells (PSCs) to achieve a power conversion efficiency of 20.54%. Meanwhile, the devices show good repeatability and photostability. Consequently, the work contributes to establishing a simple approach to realize pristine and doped multinary oxides based HTL for the development of practical and high performing PSCs.     

Water‐Soluble Lacunary Polyoxometalates with Excellent Electron Mobilities and Hole Blocking Capabilities for High Efficiency Fluorescent and Phosphorescent Organic Light Emitting Diodes

High performance solution‐processed fluorescent and phosphorescent organic light emitting diodes (OLEDs) are achieved by water solution processing of lacunary polyoxometalates used as novel electron injection/transport materials with excellent electron mobilities and hole blocking capabilities. Green fluorescent OLEDs using poly[(9,9‐dioctylfluorenyl‐2,7‐diyl)‐co‐(1,4‐benzo‐{2,1′,3}‐thiadiazole)] (F8BT) as the emissive layer and our polyoxometalates as electron transport/hole blocking layers give a luminous efficiency up to 6.7 lm W^?1 and a current efficiency up to 14.0 cd A^?1 which remained nearly stable for about 500 h of operation. In addition, blue phosphorescent OLEDs (PHOLEDs) using poly(9‐vinylcarbazole) (PVK):1,3‐bis[2‐(4‐tert‐butylphenyl)‐1,3,4‐oxadiazo‐5‐yl]benzene (OXD‐7) as a host and 10.0 wt% FIrpic as the blue dopant in the emissive layer and a polyoxometalate as electron transport material give 12.5 lm W^?1 and 30.0 cd A^?1 power and luminous efficiency, respectively, which are among the best performance values observed to date for all‐solution processed blue PHOLEDs. The lacunary polyoxometalates exhibit unique properties such as low electron affinity and high ionization energy (of about 3.0 and 7.5 eV, respectively) which render them as efficient electron injection/hole blocking layers and, most importantly, exceptionally high electron mobility of up to 10^?2 cm² V^?1 s^?1.     

Low-temperature cross-linkable hole transporting materials through chemical doping for solution-processed green PHOLEDs

Since the inter-layer mutual solubility is an obstacle to the development of solution-processed OLED, cross-linking is considered to be the best method to obtain solvent resistance. Vinyl is the most widely reported crosslinking group, but a problem raised that crosslinking usually need a high temperature. Here, two vinyl-crosslinked hole transporting materials, 3,3'-(1,3,4-oxadiazole-2,5-diyl)bis (N-phenyl-N-(4-vinylphenyl)aniline) (OXZ-VPAN), 3,3'-(4-phenyl-4H-1,2,4-triazole-3,5-diyl)bis (N-phenyl-N-(4-vinylphenyl)aniline) (TRZ-VPAN) were designed and synthesized. The introduction of pentaerythritol tetra(3-mercaptopropionate) (PETMP) and vinyl groups by thiol-ene reaction to reduce the crosslinking temperature. As a result, crosslinking can be achieved at 120 °C with the solvent resistance higher than 99%. The surface morphology of the films before and after crosslinking were characterized by atomic force microscope, and it was found that the roughness of the film was improved after dopped with PETMP. The solution-processed green phosphorescent OLEDs devices based on the obtained HTM exhibit excellent performance. Maximum current efficiency of 57.1 cd A⁻¹ and external quantum efficiency of 16.0% (Ir (mppy)₃) are obtained when OXZ-VPAN served as HTL. This low temperature feasible cross-linking process to prepare HTLs promotes the development solution-processed OLEDs.     

Highly Stable and Efficient Perovskite Solar Cells with 22.0% Efficiency Based on Inorganic–Organic Dopant‐Free Double Hole Transporting Layers

Most of the high performance in perovskite solar cells (PSCs) have only been achieved with two organic hole transporting materials: 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenylamine)‐9,9‐spirobifluorene (Spiro‐OMeTAD) and poly(triarylamine) (PTAA), but their high cost and low stability caused by the hygroscopic dopant greatly hinder the commercialization of PSCs. One effective alternative to address this problem is to utilize inexpensive inorganic hole transporting layer (i‐HTL), but obtaining high efficiency via i‐HTLs has remained a challenge. Herein, a well‐designed inorganic–organic double HTL is constructed by introducing an ultrathin polymer layer dithiophene‐benzene (DTB) between CuSCN and Au contact. This strategy not only enhances the hole extraction efficiency through the formation of cascaded energy levels, but also prevents the degradation of CuSCN caused by the reaction between CuSCN and Au electrode. Furthermore, the CuSCN layer also promotes the formation of a pinhole‐free and compact DTB over layer in the CuSCN/DTB structure. Consequently, the PSCs fabricated with this CuSCN/DTB layer achieves the power conversion efficiency of 22.0% (certified: 21.7%), which is among the top efficiencies for PSCs based on dopant‐free HTLs. Moreover, the fabricated PSCs exhibit high light stability under more than 1000 h of light illumination and excellent environmental stability at high temperature (85 °C) or high relative humidity (>60% RH).     

Highly Efficient Hole Injection Using Polymeric Anode Materials for Small‐Molecule Organic Light‐Emitting Diodes

A novel, highly efficient hole injection material based on a conducting polymer polythienothiophene (PTT) doped with poly(perfluoroethylene‐perfluoroethersulfonic acid) (PFFSA) in organic light‐emitting diodes (OLEDs) is demonstrated. Both current–voltage and dark‐injection‐current transient data of hole‐only devices demonstrate high hole‐injection efficiency employing PTT:PFFSA polymers with different organic charge‐transporting materials used in fluorescent and phosphorescent organic light‐emitting diodes. It is further demonstrated that PTT:PFFSA polymer formulations applied as the hole injection layer (HIL) in OLEDs reduce operating voltages and increase brightness significantly. Hole injection from PTT:PFFSA is found to be much more efficient than from typical small molecule HILs such as copper phthalocyanine (CuPc) or polymer HILs such as polyethylene dioxythiophene: polystyrene sulfonate (PEDOT‐PSS). OLED devices employing PTT:PFFSA polymer also demonstrate significantly longer lifetime and more stable operating voltages compared to devices using CuPc.     

Solution‐Processible Multi‐component Cyclometalated Iridium Phosphors for High‐Efficiency Orange‐Emitting OLEDs and Their Potential Use as White Light Sources

The synthesis and photophysical studies of several multifunctional phosphorescent iridium(III) cyclometalated complexes consisting of the hole‐transporting carbazole and fluorene‐based 2‐phenylpyridine moieties are reported. All of them are isolated as thermally and morphological stable amorphous solids. Extension of the π‐conjugation through incorporation of electron‐pushing carbazole units to the fluorene fragment leads to bathochromic shifts in the emission profile, increases the highest occupied molecular orbital levels and improves the charge balance in the resulting complexes because of the propensity of the carbazole unit to facilitate hole transport. These iridium‐based triplet emitters give a strong orange phosphorescence light at room temperature with relatively short lifetimes in the solution phase. The photo‐ and electroluminescence properties of these phosphorescent carbazolylfluorene‐functionalized metalated complexes have been studied in terms of the coordinating position of carbazole to the fluorene unit. Organic light‐emitting diodes (OLEDs) using these complexes as the solution‐processed emissive layers have been fabricated which show very high efficiencies even without the need for the typical hole‐transporting layer. These orange‐emitting devices can produce a maximum current efficiency of ～ 30 cd A^–1 corresponding to an external quantum efficiency of ～ 10 % ph/el (photons per electron) and a power efficiency of ～ 14 lm W^–1. The homoleptic iridium phosphors generally outperform the heteroleptic counterparts in device performance. The potential of exploiting these orange phosphor dyes in the realization of white OLEDs is also discussed.     

Stable solution processed hole injection material for organic light-emitting diodes

A new aqueous based polymer, Plexcore® OC AQ1200 (AQ1200) was used as a hole injection layer (HIL) in organic light emitting diodes (OLEDs) and the device results are compared with polyethylene dioxythiophene:polystyrene sulfonate (PEDOT:PSS) in terms of injection efficiency and stability. Dark injection transient measurements show a higher injection efficiency in hole-only devices using AQ1200 HIL compared with PEDOT:PSS. Using AQ1200 as an HIL, high efficiency phosphorescent OLEDs are demonstrated to have a long lifetime, with an estimated operational half lifetime (LT 50) of more than 8000 h from an initial luminance of 1000 cd/m².     

Self-Organized Co3O4-SrCO3 Percolative Composites Enabling Nanosized Hole Transport Pathways for Perovskite Solar Cells

Perovskite solar cells (PSCs) are expected to profoundly impact the photovoltaic society on account of its high-efficiency and cost-saving manufacture. As a key component in efficient PSCs, the hole transport layer (HTL) can selectively collect photogenerated carriers from perovskite absorbers and prevent the charge recombination at interfaces. However, the mainstream organic HTLs generally require multi-step synthesis and hygroscopic dopants that significantly limit the practical application of PSCs. Here, a self-organized percolative architecture composed of narrow bandgap oxides (e.g., Co₃O₄, NiO, CuO, Fe₂O₃, and MnO₂) and wide bandgap SrCO₃ oxysalt as efficient HTLs for PSCs is presented. The percolation of dual phases offers nanosized hole transport pathways and optimized interfacial band alignments, enabling significantly improved charge collection compared with the single phase HTLs. As a consequence, the power conversion efficiency boosted from 8.08% of SrCO₃ based device and 15.47% of Co₃O₄ based device to 21.84% of Co₃O₄-SrCO₃ based one without notable hysteresis. The work offers a new direction by employing percolative materials for efficient charge transport and collection in PSCs, and would be applicable to a wide range of opto-electronic thin film devices.     

有机/聚合物异质结电致发光器件的光电性能

器件结构是影响有机发光器件(OLED)性能的重要因素之一.采用8-hydroxyquinoline-aluminum(AlQ)作为发光层(EML)和电子传输层(ETL),polyvinylcarbazole (PVK)作为空穴传输层(HTL),制备了具有有机小分子/聚合物异质结结构的OLED器件,通过其电压-电流-发光亮度(V-J-B)特性测试,研究了HTL的引入及其膜厚对器件性能的影响.实验结果表明,HTL的引入有效地改善了OLED的光电性能,同时HTL膜厚对器件性能具有显著影响,当HTL膜厚为20 nm时,所制备的OLED器件具有最小的驱动电压和启亮电压、最大的发光亮度和发光效率.

Abstract：

The device construction plays an important role in improving the optoelectronic performance of organic electroluminescence devices (OLEDs). Heterojunction OLEDs with a configuration of glass/ITO/PVK/AlQ/Mg/Al were fabricated by using 8-hydroxyquinoline-aluminum (AlQ) as the emission layer (EML) and electron transport layer (ETL) and polyvinylcarbazole (PVK) as the hole transport layer (HTL). The effect of the HTL thickness on the performance of OLEDs was investigated with respect to the driving voltage, turn-on voltage, electroluminescence brightness and efficiency of the devices. Experimental results demonstrate that the optical and electrical properies of OLEDs are closely related to the HTL thickness. The device fabricated with the HTL thickness of 20 nm possesses the best photoelectric properties such as the minimum driving voltage and turn-on voltage, and the maximum electroluminescence brightness and efficiency.