Physical characterization of Ag:WO3 cermet films used as top electrode for stable and high contrast organic light-emitting diodes

A low reflectance green top-emitting OLED using a cermet Ag:WO₃ thin film as a top cathode has been fabricated and fully characterized. The cermet film has been made by co-evaporating Ag and WO₃ materials. Depending on the Ag concentration in the WO₃ matrix, the optical and electrical properties vary a lot, as this will be investigated by optical simulations as well as by studying single film properties deposited on silicon and glass wafers. A plasmonic absorption of Ag occurs for metal concentrations around 60–70%, which confirms the “nano” nature of the cermet material, with Ag nanocrystals embedded in an amorphous WO₃ oxide matrix (also confirmed by TEM). Using an optimal (optical, electrical) Ag:WO₃ (70%) cermet composition as a top electrode in the OLED allows to strongly reduce the device mean reflectance in the visible while its contrast ratio becomes far better (maximum + 76% at 1000 lux) than a reference green device (with Ag as top electrode instead of Ag:WO₃). The device with the cermet shows very good lifetimes when driven at constant current drive: an extrapolated lifetime of 90 000 h at 100 cd/m² has been estimated, based on accelerating ageing tests done for 1000 cd/m² to 20 kcd/m² initial luminance values.     

Highly efficient ITO-free polymer solar cells based on metal resonant microcavity using WO3/Au/WO3 as transparent electrodes

Indium tin oxide (ITO)-free polymer solar cells (PSCs) with the structure of Glass/tungsten trioxide (WO₃)/Au/WO₃/PCDTBT: PC₇₀BM/LiF/Al was fabricated and studied. The multilayer structure of WO₃/Au/WO₃ is used as the potential transparent electrode to replace ITO. Metal resonant microcavity, which can enhance light harvesting of active layers, was constructed between Au and Al electrodes. According to the J–V and IPCE characterization with 70 nm active layer, power conversion efficiency (PCE) of the ITO-free microcavity device is approaching 4.55%, which is higher than that of the ITO-based device. However, PCE of the ITO-free device is much lower than that of the ITO-based device when the thickness of active layer increases to 130 nm. The opposite experimental tendency leads to theoretical research toward the simulation of light absorption and optical electric field and the calculation of maximum short circuit current density (J_{sc max}) as a function of active layer thickness based on ITO-free and ITO-based devices. The research results show that microcavity effect is closely linked to intrinsic absorption of active layers.     

Transparent Al/WO3/Au film as anode for high efficiency organic light-emitting diodes

A transparent Al/WO₃/Au anode is introduced to fabricate high efficiency organic light-emitting devices (OLEDs). By optimizing the thicknesses of each layers of the Al/WO₃/Au structure, the transmittance of Al(7 nm)/WO₃(3 nm)/Au(13 nm) has reached over 55%. Concerning the performance of OLEDs using the optimized anode, the electroluminescence (EL) current efficiency and brightness are enhanced and the EL spectrum is greatly narrowed as compared to the OLEDs using indium-tin-oxide (ITO) as the anode. The results indicate that the metal/metal oxide/metal transparent electrode is a good structure for the anode of high performance OLEDs. In addition, Al/WO₃/Au can function as a composite transparent electrode for top-emitting OLEDs.     

Highly Transparent and Flexible Organic Light‐Emitting Diodes with Structure Optimized for Anode/Cathode Multilayer Electrodes

In this study, a dielectric layer/metal/dielectric layer (multilayer) electrode is proposed as both anode and cathode for use in the fabrication of transparent and flexible organic light‐emitting diodes (TFOLEDs). The structure of multilayer electrodes is optimized by systematic experiments and optical calculations considering the transmittance and efficiency of the device. The details of the multilayer electrode structure are [ZnS (24 nm)/Ag (7 nm)/MoO₃ (5 nm)] and [ZnS (3 nm)/Cs₂CO₃ (1 nm)/Ag (8 nm)/ZnS (22 nm)], as anode and cathode, respectively. The optimized TFOLED design is fabricated on a polyethylene terephthalate (PET) substrate, and the device shows high transmittance (74.22% around 550 nm) although the PET substrate has lower transmittance than glass. The TFOLEDs operate normally under compressive stress; degradation of electrical characteristics is not observed, comparable to conventional OLEDs with ITO and Al as electrodes. In addition, because the fabricated TFOLEDs show a nearly Lambertian emission pattern and a negligible shift of Commission International de l'Eclairage (CIE) coordination, it is concluded that the fabricated TFOLEDs are suitable for use in displays.     

Blade coating of Tris(8-hydroxyquinolinato)aluminum as the electron-transport layer for all-solution blue fluorescent organic light-emitting diodes

Organic light-emitting diodes (OLEDs) with a low driving voltage and efficient blue fluorescence were fabricated through blade coating. Tris(8-hydroxyquinolinato)aluminum (Alq₃) is a relatively stable electron-transporting material commonly used in evaporation. However, depositing Alq₃ through a solution process is difficult because of its extremely low solubility organic solvents, a result of its symmetrical molecular structure. In this study, Alq₃ was successfully deposited through blade coating at a very low concentration below 0.1wt%. The OLEDs contained co-dopants BUBD-1 and p-bis(p-N,N-diphenyl-aminostyryl)benzene (DSA-Ph), and a high-band-gap host 2-methyl-9,10-bis(naphthalen-2-yl)anthracene (MADN) as the emission layer with the following structure: ITO/PEDOT:PSS (40 nm)/VB-FNPD (30 nm)/MADN:2% BUBD-1:1% DSA-Ph (50 nm)/TPBI (30 nm)/LiF (0.8 nm)/Al (100 nm)or ITO/PEDOT:PSS (40 nm)/VB-FNPD (30 nm)/MADN:3% BUBD-1 (50 nm)tris(8-hydroxyquinolinato)aluminum (Alq₃; 10 nm)/LiF (0.8 nm)/Al (100 nm). 2,7-disubstituted fluorene-based triaryldiamine(VB-FNPD)is the cross-linking transporting material. The device exhibited a peak current efficiency of 5.67 cd/A for Alq₃ and 5.76 cd/A for TPBI. The device with Alq₃ has operated lifetime seven times higher than the device with TPBI.     

Simplified phosphorescent organic light-emitting diodes with a WO3-doped wide bandgap organic charge transport layer

The effects of p-type doping of wide bandgap ambipolar 4,4′-N,N′-dicarbazolebiphenyl (CBP) with WO₃ were investigated through detailed electrical device characterization. It was found that, to achieve effective doping for improved hole injection and transport, the doping level should be greater than 20 mol% and the doped layer should be at least 10 nm thick. A large downward shift of the Fermi level in WO₃-doped CBP causes band bending and depletion at the doped/undoped CBP interface, resulting in an additional energy barrier which hampers hole transport. Simplified green phosphorescent organic light-emitting diodes (PhOLEDs) with CBP as the hole transport and host material were fabricated. With a WO₃-doped hole transport layer, the PhOLEDs attained brightness of 11,163 cd/m² at 20 mA/cm², and exhibited an improved reliability under constant-current stressing as compared to undoped PhOLEDs.     

Flexible organic light-emitting diodes with ZnS/Ag/ZnO/Ag/WO3 multilayer electrode as a transparent anode

We investigated the highly flexible, transparent and very low resistance ZnS/1st Ag/ZnO/2nd Ag/WO₃ (ZAZAW) multilayer electrodes on PET substrate as an anode in flexible organic light-emitting diodes (OLEDs). A theoretical calculation was first conducted to obtain the optimal thickness of the ZAZAW multilayer for high transparency. Its measured luminous transmittance was over 80% in the visible range with a very low sheet resistance of 2.17 Ω/sq., and it had good mechanical flexibility due to the ductility of Ag. Ag’s effect on optical and electrical properties was also studied. Flexible OLEDs devices that were fabricated on ZAZAW multilayer anode showed good hole injection properties comparable to those of ITO-based OLEDs due to the use of WO₃ as a hole injection layer. However, the electroluminescent properties of the ZAZAW-based OLEDs varied depending on WO₃ thickness. Although the transmittance of the ZAZAW electrode was reduced by tuning the WO₃ thickness to adjust the microcavity effect, the device efficiency could be enhanced above that of ITO-based OLEDs.     

Bright,efficient, deep blue-emissive polymer light-emitting diodes of suitable hole-transport layer and cathode design

This study reports the fabrication of efficient deep blue-emissive polymer light-emitting diodes (PLEDs), incorporating a polyfluorene derivative of nonsymmetric and bulky aromatic groups at C-9 position as the light-emissive layer. Another poly(fluorene-co-triphenylamine) (PFO-TPA) derivative of the highest occupied molecular orbital level, −5.3 eV, is used as the hole-injection and -transport layer in the anode part. The thermally crosslinking of styryl groups in PFO-TPA inhibits the solvation of an interlayer in constructing the multilayer device architecture of PLEDs. While applying a cesium carbonate (Cs₂CO₃)/Aluminum (Al) cathode rather than Calcium (Ca)/Al, the device has the superior performance (i.e. one order of magnitude higher). Experimental results indicate that the interfacial reactions at the polymer/Ca junction, as characterized in this study, significantly degrade the luminescence properties and the device performance. Moreover, Cs₂CO₃/Al is a highly favorable cathode in fabricating polyflourene-based PLEDs. The device of the optimal configuration has a decent deep blue emission centered at 430–450 nm of the Commission Internationale de l’Eclairage chromaticity coordinates, (0.15, 0.14), with a maximum brightness of 35054.2 cd/m² and luminous efficiency of 14.0 cd/A (at 2975.0 cd/m²).     

Efficient inverted polymer solar cells incorporating doped organic electron transporting layer

An efficient inverted polymer solar cell is enabled by incorporating an n-type doped wide-gap organic electron transporting layer (ETL) between the indium tin oxide cathode and the photoactive layer for electron extraction. The ETL is formed by a thermal-deposited cesium carbonate-doped 4,7-diphenyl-1,10-phenanthroline (Cs₂CO₃:BPhen) layer. The cell response parameters critically depended on the doping concentration and film thickness of the Cs₂CO₃:BPhen ETL. Inverted polymer solar cell with an optimized Cs₂CO₃:BPhen ETL exhibits a power conversion efficiency of 4.12% as compared to 1.34% for the device with a pristine BPhen ETL. The enhanced performance in the inverted device is associated with the favorable energy level alignment between Cs₂CO₃:BPhen and the electron-acceptor material, as well as increased conductivity in the doped organic ETL for electron extraction. The method reported here provides a facile approach to optimize the performance of inverted polymer solar cells in terms of easy control of film morphology, chemical composition, conductivity at low processing temperature, as well as compatibility with fabrication on flexible substrates.     

Improved property in organic light-emitting diode utilizing two Al/Alq3 layers

We reported on the fabrication of organic light-emitting devices (OLEDs) utilizing the two Al/Alq₃ layers and two electrodes. This novel green device with structure of Al(110 nm)/tris(8-hydroxyquinoline) aluminum (Alq₃)(65 nm)/Al(110 nm)/Alq₃(50 nm)/N,N′-dipheny1-N, N′-bis-(3-methy1phyeny1)-1, 1′-bipheny1-4, 4′-diamine (TPD)(60 nm)/ITO(60 nm)/Glass. TPD were used as holes transporting layer (HTL), and Alq₃ was used as electron transporting layer (ETL), at the same time, Alq₃ was also used as emitting layer (EL), Al and ITO were used as cathode and anode, respectively. The results showed that the device containing the two Al/Alq₃ layers and two electrodes had a higher brightness and electroluminescent efficiency than the device without this layer. At current density of 14 mA/cm², the brightness of the device with the two Al/Alq₃ layers reach 3693 cd/m², which is higher than the 2537 cd/m² of the Al/Alq₃/TPD:Alq₃/ITO/Glass device and the 1504.0 cd/m² of the Al/Alq₃/TPD/ITO/Glass. Turn-on voltage of the device with two Al/Alq₃ layers was 7 V, which is lower than the others.     

Improved electron injection from silver electrode for all solution-processed polymer light-emitting diodes with Cs2CO3:conjugated polyelectrolyte blended interfacial layer

This study demonstrates the incorporation of a Cs₂CO₃:conjugated polyelectrolyte blended interfacial layer between the emissive layer and a silver (Ag) cathode, for realizing all-solution processed polymer light-emitting diodes. For a device with poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) as the emissive layer, this approach improves the maximum luminance of approximately 80,000 cd/m² and maximum current efficiency of 10.6 cd/A. It is clarified that the interfacial layer prevents Ag nanoparticles from penetrating into the emissive layer, resulting in yellow–green emission from F8BT. We also demonstrate the possibility of all-solution processed polymer light-emitting diodes utilizing solution-processed Cs₂CO₃:conjugated polyelectrolyte interfacial layer and Ag nano-ink.     

基于Alq3光耦合层的新型顶发射蓝光有机电致发光器件

研制了一种结构为Ag/Glass/ITO/TAPC/mCP/mCP∶Firpic/TPBi/LiF/Al/Ag/Alq₃的顶发射有机电致发光器件,通过在ITO玻璃衬底背面生长一层Ag反射膜,使器件发出的蓝光被反射膜反射到顶电极出射。利用顶电极表面的Alq₃光耦合层有效地提升了金属复合阴极的透射率,降低了器件的微腔效应。实验结果表明,当光耦合层厚度为30nm时,获得了最大电流效率和最大亮度分别为8.91cd/A和5758cd/m²的蓝光顶发射有机电致发光器件(TEOLED);同时,在10V电压下,其色坐标为(0.157,0.320),当亮度从1cd/m²变化到5000cd/m²时,其色坐标仅漂移(0.002,0.010),表现出良好的色稳定性。     

Impact of annealing treatment on structural and dc electrical properties of spin coated tungsten trioxide thin films for Si/WO3/Ag junction diode

We report on the effect of the thermal annealing on structural and electrical properties of p-Si/n-WO₃/Ag junction diode. According to the XRD pattern, the WO₃ films exposed that the crystalline phase transformation of monoclinic to orthorhombic structure for an increasing annealing temperature. The SEM images show an abrupt change in the plate like grain growth and surface morphology. From the UV–visible analysis, the band gap energy decreases for the higher annealing temperature. The dc electrical characterization shows that the conductivity (σ_dc), activation energy (E_a) and pre-exponential factor (σ₀) values vary function of temperature. The Si/WO₃/Ag contact junction diode parameters of ideality factor (n), barrier height (Φ_B), leakage current density (J₀) and series resistance (R_s) were examined by the J-V method, Cheung's and Norde functions as a function of annealing temperature according to the thermionic emission method (TE). The values of n and Φ_B decrease with increasing annealing temperature and better the device performance on an optimized annealing temperature at 873 K. The temperature dependent of experimental n and Φ_B revealed the presence of inhomogeneity at WO₃-Ag interface. This behavior is modeled by assuming the existence of Gaussian distribution (GD) of barrier heights in temperature range 303–423 K.     

Synthesis,characterization and high-efficiency blue electroluminescence based on coumarin derivatives of 7-diethylamino-coumarin-3-carboxamide

A novel tripodal compound, tris[2-(7-diethylamino-coumarin-3-carboxamide)ethyl]amine (tren-C), and a model compound, N-butyl-7-(diethylamino)-coumarin-3-carboxamide, were synthesized and characterized by elemental analysis, infrared and ¹H NMR spectra. The structure of the model compound was characterized by single crystal X-ray crystallography. The electroluminescence devices of ITO/2-TNATA (5 nm)/NPB (40 nm)/CBP: tren-C or model compound (wt%, 30 nm)/Bu-PBD (30 nm)/LiF (1 nm)/Al (100 nm) were fabricated and characterized. The EL spectra of the devices comprising vacuum vapour-deposited films using tren-C as a dopant are similar to the PL spectrum of tren-C in chloroform solutions. At the concentration of 0.5 wt% tren-C, a blue-emitting OLED with an emission peak at 464 nm, a maximum external quantum efficiency (EQE) of 1.39% and a maximum luminous efficiency of 2 cd/A at the current density of 20 mA/cm², and a maximum luminance of 1450 cd/m² at 12 V are achieved.     

Fabrication of WO3 nanostructures by anodization method for visible-light driven water splitting and photodegradation of methyl orange

Visible light-responsive WO₃ nanostructures were synthesized by anodization in a NH₄F/Na₂SO₄ electrolyte solution. Applied potential and anodization time play an important role in the formation of self-organized WO₃ nanostructures, further developed upon anodization. The average pore diameter of ～80 nm with thickness of～300 nm of WO₃ nanoporous layer was successfully synthesized at 50 V for 15 min. The uniform and regular WO₃ nanoporous layer exhibited better photocurrent density of～0.18 mA/cm² at 0.7 V vs. SCE and better photodegradation of MO solution of ～50% after 5 h of visible-light illumination. The larger active surface area of WO₃ nanoporous layer played a significant role to generate more electron-hole pairs, which triggered the PEC water splitting reaction and photodegradation reaction much more effectively.     

Solution‐Processed,Alkali Metal‐Salt‐Doped,Electron‐Transport Layers for High‐Performance Phosphorescent Organic Light‐Emitting Diodes

High‐performance, blue, phosphorescent organic light‐emitting diodes (PhOLEDs) are achieved by orthogonal solution‐processing of small‐molecule electron‐transport material doped with an alkali metal salt, including cesium carbonate (Cs₂CO₃) or lithium carbonate (Li₂CO₃). Blue PhOLEDs with solution‐processed 4,7‐diphenyl‐1,10‐phenanthroline (BPhen) electron‐transport layer (ETL) doped with Cs₂CO₃ show a luminous efficiency (LE) of 35.1 cd A^?1 with an external quantum efficiency (EQE) of 17.9%, which are two‐fold higher efficiency than a BPhen ETL without a dopant. These solution‐processed blue PhOLEDs are much superior compared to devices with vacuum‐deposited BPhen ETL/alkali metal salt cathode interfacial layer. Blue PhOLEDs with solution‐processed 1,3,5‐tris(m‐pyrid‐3‐yl‐phenyl)benzene (TmPyPB) ETL doped with Cs₂CO₃ have a luminous efficiency of 37.7 cd A^?1 with an EQE of 19.0%, which is the best performance observed to date in all‐solution‐processed blue PhOLEDs. The results show that a small‐molecule ETL doped with alkali metal salt can be realized by solution‐processing to enhance overall device performance. The solution‐processed metal salt‐doped ETLs exhibit a unique rough surface morphology that facilitates enhanced charge‐injection and transport in the devices. These results demonstrate that orthogonal solution‐processing of metal salt‐doped electron‐transport materials is a promising strategy for applications in various solution‐processed multilayered organic electronic devices.     

Low resistive transparent and flexible ZnO/Ag/ZnO/Ag/WO3 electrode for organic light-emitting diodes

Transparent electrodes cannot easily be created with high transmittance and low sheet resistance simultaneously, although some optoelectronic devices, such as large organic light-emitting diode (OLED) displays and lightings, require very low resistive transparent electrodes. Here, we propose a very low resistive transparent electrode (～1.6 Ω/sq) with a high transmittance (～75%) for OLED devices, the transmittance level of which represents the highest reported value to date given such a low sheet resistance level. It consists of a stacked silver (Ag)/zinc oxide (ZnO)/Ag multilayer covered by high refractive index dielectric layers. The proposed multilayer electrode with optimal layer thicknesses has a high and wide spectral transmittance peak due to interference. The low sheet resistance is a result of two Ag layers connected via the sandwiched ZnO layer. In addition to its low sheet resistance coupled with high transmittance, the proposed multilayer electrode has good flexibility. An OLED with an anode of the stacked Ag/ZnO/Ag multilayer shows performance comparable to that of an anode of indium tin oxide.     

Synthesis and electroluminescent properties of highly efficient anthracene derivatives with bulky side groups

Three new asymmetric light emitting organic compounds were synthesized with diphenylamine or triphenylamine side groups; 10-(3,5-diphenylphenyl)-N,N-diphenylanthracen-9-amine (MADa), 4-(10-(3,5-diphenylphenyl)anthracen-9-yl)-N,N-diphenylaniline (MATa), and 4-(10-(3′,5′-diphenylbiphenyl-4-yl)anthracen-9-yl)-N,N-diphenylaniline (TATa). MATa and TATa had a PL_max at 463 nm in the blue region, and MADa had a PL_max at 498 nm. MADa and MATa had T_g values greater than 120 °C, and TATa had a T_g of 139 °C. EL devices containing the synthesized compounds were fabricated in the configuration: ITO/4,4′,4′′-tris(N-(2-naphthyl)-N-phenyl-amino)-triphenylamine (2-TNATA) (60 nm)/N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (NPB) (15 nm)/MADa or MATa or TATa or 9,10-di(2′-naphthyl)anthracene (MADN) (30 nm)/8-hydroxyquinoline aluminum (Alq₃) (30 nm)/LiF (1 nm)/Al (200 nm). The efficiency and color coordinate values (respectively) were 10.3 cd/A and (0.199, 0.152; bluish-green) for the MADa device, 4.67 cd/A and (0.151, 0.177) for the MATa device, and 6.07 cd/A and (0.149, 0.177) for the TATa device. The TATa device had a high external quantum efficiency (EQE) of 6.19%, and its luminance and power efficiencies and life-time were more than twice those of the MADN device.     

Flexible QLED and OPV based on transparent polyimide substrate with rigid alicyclic asymmetric isomer

A series of isomeric alicyclic-functionalized polyimide with chemical imidization and thermal imidization (CPI-x and TPI-x) were prepared from a rigid alicyclic-functionalized isomerism diamine, 5(6)-amino-1-(4-aminophenyl)-1,3,3-trimethylindane (DAPI). The influences of incorporation of the isomeric rigid alicyclic structure onto the backbone of the polymers were systematically investigated in terms of optical, thermal, mechanical, dielectric and surface properties, respectively. Due to the moderate chemical imidization condition, CPI-x series retain higher glass transition temperature (Tg) in the range of 329–429 °C than the Tg of TPI-x (from 321.9 °C to 370.7 °C). Therefore, the CPI-1 film based 5-DAPI was chosen as the flexible substrate. MoO₃ was chosen as the interface layer to improve the compatibility between PI substrate and the metal electrode. Then a ultra-thin layer of MoO₃ (3 nm)/Au(2 nm)/Ag(4 nm) was utilized to be the transparent electrode. After annealing at 220 °C for 0.5 h, zinc oxide (ZnO) was deposited onto the electrode to maintain the superior electron mobility and improve the transparency of the electrode. Consequently, the flexible quantum dots light emitting diode (QLED) obtained a high luminance of 5230 (cd/m²) and EQE of 5.2%, meanwhile, a device performance of 4.36% was achieved in organic photovoltaic (OPV) devices.