Improvement of performance of tetraphenylporphyrin-based red organic light emitting diodes using WO3 and C60 buffer layers

One of the porphyrin derivatives, meso-tetraphenylporphyrin (TPP), has been synthesized and examined as an emitter material (EM) for efficient fluorescent red organic light-emitting diodes (OLEDs). By inserting a tungsten oxide (WO₃) layer into the interface of anode (ITO) and hole transport layer N,N′-Di-[(1-napthyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4,4′-diamine (NPB) and by using fullerene (C₆₀) in contact with a LiF/Al cathode, the performance of devices was markedly improved. The current density–voltage–luminance (J–V–L) characterizations of the samples show that red OLEDs with both WO₃ and C₆₀ as buffer layers have a lower driving voltage and higher luminance compared with the devices without buffer layers. The red OLED with the configuration ITO/WO₃ (3 nm)/NPB (50 nm)/TPP (60 nm)/BPhen (30 nm)/C₆₀ (5 nm)/LiF (0.8 nm)/Al (100 nm) achieved the high luminance of 6359 cd/m² at the low driving voltage of 8 V. At a current density of 20 mA/cm², a pure red emission with CIE coordinates of (0.65; 0.35) is observed for this device. Moreover, a power efficiency of 2.07 lm/W and a current efficiency of 5.17 cd/A at 20 mA/cm² were obtained for the fabricated devices. The study of the energy level diagram of the devices revealed that the improvement in performance of the devices with buffer layers could be attributed to lowering of carrier-injecting barrier and more balanced charge injection and transport properties.     

The effect of Ag layer thickness on the properties of WO3/Ag/MoO3 multilayer films as anode in organic light emitting diodes

Transparent conductive WO₃/Ag/MoO₃ (WAM) multilayer electrodes were fabricated by thermal evaporation and the effects of Ag layer thickness on the optoelectronic and structural properties of multilayer electrode as anode in organic light emitting diodes (OLEDs) were investigated using different analytical methods. For Ag layers with thickness varying between 5 and 20 nm, the best WAM performances, high optical transmittance (81.7%, at around 550 nm), and low electrical sheet resistance (9.75 Ω/cm²) were obtained for 15 nm thickness. Also, the WAM structure with 15 nm of Ag layer thickness has a very smooth surface with an RMS roughness of 0.37 nm, which is suitable for use as transparent conductive anode in OLEDs. The current density?voltage?luminance (J?V?L) characteristics measurement shows that the current density of WAM/PEDOT:PSS/TPD/Alq₃/LiF/Al organic diode increases with the increase in thickness of Ag and WO₃/Ag (15 nm)/MoO₃ device exhibits a higher luminance intensity at lower voltage than ITO/PEDOT:PSS/TPD/Alq₃/LiF/Al control device. Furthermore, this device shows the highest power efficiency (0.31 lm/W) and current efficiency (1.2 cd/A) at the current density of 20 mA/cm², which is improved 58% and 41% compared with those of the ITO-based device, respectively. The lifetime of the WO₃/Ag (15 nm)/MoO₃ device was measured to be 50 h at an initial luminance of 50 cd/m², which is five times longer than 10 h for ITO-based device.     

Highly efficient and stable organic light-emitting diodes employing MoO3-doped perylene-3, 4, 9, 10-tetracarboxylic dianhydride as hole injection layer

We report highly efficient and stable organic light-emitting diodes (OLEDs) with MoO₃-doped perylene-3, 4, 9, 10-tetracarboxylic dianhydride (PTCDA) as hole injection layer (HIL). A green OLED with structure of ITO/20 wt% MoO₃: PTCDA/NPB/Alq₃/LiF/Al shows a long lifetime of 1012 h at the initial luminance of 2000 cd/m², which is 1.3 times more stable than that of the device with MoO₃ as HIL. The current efficiency of 4.7 cd/A and power efficiency of 3.7 lm/W at about 100 cd/m² have been obtained. The charge transfer complex between PTCDA and MoO₃ plays a decisive role in improving the performance of OLEDs.     

4,4′,4″-三(N-3-甲基苯基-N-苯基氨基)三苯胺掺杂MoOx作为空穴传输层对有机太阳电池性能的影响

通过采用4,4′,4″-三(N-3-甲基苯基-N-苯基氨基)三苯胺 (m-MTDATA)掺入MoO_x作为器件的空穴传输层来提高酞菁铜(CuPc)/C₆₀小分子 有机太阳电池的效率. 采用真空蒸镀的方法制备了一系列器件, 其中结构为铟锡氧化物 (ITO)/m-MTDATA:MoO_x(3:1)(30 nm)/CuPc(20 nm)/C₆₀(40 nm)/4,7-二苯 基-1,10-菲罗啉 (Bphen)(8 nm)/LiF(0.8 nm)/Al(100 nm)的器件, 在AM1.5 (100 mW/cm²)模拟太阳光的照射条件下, 开路电压V_oc=0.40 V, 短路电流J_sc=6.59 mA/cm², 填充因子为0.55, 光电转换效率达1.46%, 比没有空穴传输层的器件ITO/CuPc(20 nm)/C₆₀(40 nm)/Bphen(8 nm)/LiF(0.8 nm)/Al(100 nm) 光电转换效率提高了38%. 研究表明, 加入m-MTDATA:MoO_x(3:1)(30 nm)空穴传输层减小了有机层和ITO电极之间的接触电阻, 从而减小了整个器件的串联电阻, 提高了器件的光电转换效率.     

Radiation-stimulated modification of C60 films on Si-oxide surfaces

The electronic structure of C₆₀/SiO_x/Si(1 0 0) interface was studied by photoemission (valence-bands, C 1s, and Si 2p core levels) and near-edge X-ray absorption fine structure (C 1s threshold) spectroscopies. It was concluded that the SiO_x/Si surface is non-reactive with respect to the interaction with C₆₀. The exposure of the C₆₀/SiO_x/Si system under non-monochromatic synchrotron radiation causes modification of the electronic structure of this system. It is explained by polymerization of the C₆₀ molecules and arising a strong ionic-like interaction of the polymerized C₆₀ with the SiO_x surface. Annealing of this system up to temperatures of 550–625 °C leads to complete desorption of the C₆₀ molecules from the non-irradiated sample areas while the modified by radiation fullerenes remain attached to the substrate.     

Efficiency control of organic light-emitting diode for high contrast ratio performance in active matrix display applications

We demonstrate the possibility of controlling organic light-emitting diodes (OLEDs) efficiency with an optimized hole-delay layer (HDL) in order to achieve very high contrast ratio needed for active matrix display applications. No triplet exciton confinement structure and a molybdenum trioxide (MoO₃) HDL at ITO/hole transport layer interface were used for a careful placing and adjusting of the exciton recombination zone. An optimized thickness of 50 nm thick MoO₃ allows to cut off the efficiency at the black level of active matrix driving mode while it gets greater at higher luminance at high current density region over 20 mA/cm² compared to conventional device. Our suggested efficiency control way is believed to be very useful to future active matrix OLED displays.     

富勒烯掺杂NPB空穴传输层的有机电致发光器件

报道了不同掺杂浓度NPB：C₆₀（富勒烯）作为空穴传输层对有机电致发光器件性能的影响.采用真空热蒸镀方法,制作了ITO/ NPB：C₆₀ （x % ）/Alq₃/LiF/Mg:Ag结构的四种有机电致发光器件.当NPB：C₆₀的掺杂浓度是15％时,器件的启亮电压是4 V,最大亮度是11000 cd/m².然而,当NPB：C₆₀的掺杂浓度是20％时,器件的最大亮度降     

Photoluminescence characterization of a novel red-emitting phosphor In2(MoO4)3:Eu3+ for white light emitting diodes

The red-emitting phosphor In₂(MoO₄)₃:Eu³⁺ with cubic crystal structure was synthesized by a conventional solid-state reaction technique and its photoluminescence properties were investigated. The prepared phosphor can be efficiently excited by ultraviolet (395 nm) and blue (466 nm) light. The emission spectra of the phosphor manifest intensive red-emitting lines at 612 nm due to the electric dipole ⁵D₀→⁷F₂ transitions of Eu³⁺. The chromaticity coordinates of x=0.63, y=0.35 (λ_ex=395 nm) and x=0.60, y=0.38 (λ_ex=466 nm) are close to the standard of National Television Standard Committee values (NTSC) values. The concentration quenching of In₂(MoO₄)₃:Eu³⁺ is 40 mol% and the concentration self-quenching mechanism under 466 nm excitation was the dd intereaction. As a result of the strong emission intensity and good excitation, the phosphor In₂(MoO₄)₃:Eu³⁺ is regarded as a promising red-emitting conversion material for white LEDs.     

Improved efficiencies of hybrid organic light‐emitting diodes using efficient electron injection layer

Hybrid organic‐inorganic light‐emitting diodes were developed with pristine ZnO (2.0 wt%) and Cu‐doped ZnO (2.0 wt%) as electron injection layer and iridium(III)‐bis‐2‐(4‐fluorophenyl)‐1‐(naphthalen‐1‐yl)‐1H‐phenanthro[9,10‐d]imidazole (acetylacetonate) [Ir(fpnpi)₂ (acac)] as green emissive layer (521 nm). The pristine ZnO and Cu‐doped ZnO are deposited at indium tin oxide cathode and emissive layer interface. The electroluminescent performances increased by electron injection layer–Cu‐doped ZnO compared with ZnO‐based device because Cu‐doped ZnO injects electron efficiently result in balanced h⁺ ? e^? recombination in emissive layer than ZnO‐based device. The Cu‐doped ZnO (2.0 %) device shows luminance (L) of 10 982 cd/m² at 23.0 V (ZnO, 1450 cd/m² at 23.0 V).     

Efficient white organic light-emitting devices using 4,7-diphenyl-1,10-phenanthroline as block layer

A white light-emitting device has been fabricated with a structure of ITO/m-MTDATA (45 nm)/NPB (10 nm)/DPVBi (8 nm)/DPVBi:DCJTB 0.5% (15 nm)/BPhen (x nm)/Alq₃ [(55−x) nm]/LiF (1 nm)/Al, with x=0, 4, and 7. BPhen was used as the hole-blocking layer. This results in a mixture of lights from DPVBi molecules (blue-light) and DCJTB (yellow-light) molecules, producing white light emission. The chromaticity can be readily adjusted by only varying the thickness of the BPhen layer. The CIE coordinates of the device are largely insensitive to the driving voltages. When the thickness of BPhen is 7 nm, the device exhibits peak efficiency of 6.87 cd/A (3.59 lm/W) at the applied voltage of 6 V, the maximum external quantum efficiency η_ext=2.07% corresponding to 6.18 cd/A, and the maximum brightness is 18494 cd/m² at 15 V.     

Highly efficient greenish blue-emitting organic diodes based on pyrene derivatives

We report the synthesis of pyrene derivatives as the light emissive layer for highly efficient organic electroluminescence (EL) diodes. Multilayer devices were fabricated with pyrene derivatives (ITO/NPB (50 nm)/blue material (30 nm)/BCP (10 nm)/Alq₃ (30 nm)/LiF (1 nm)/Al). By using 1,1′-dipyrene (DP) and 1,4-dipyrenyl benzene (DPB), the devices produced the blue EL emissions with 1931 Commission International de L’Eclairage coordinates of (x=0.21, y=0.35) and (x=0.19, y=0.25), respectively. The device with DPB shows a maximum brightness of 42,445 cd/m² at 400 mA/cm² and the luminance efficiency of 8.57 cd/A and 5.18 lm/W at 20 mA/cm².     

Suppression of bias stress-induced degradation of pentacene-TFT using MoOx interlayer

The bias stress effect in pentacene thin-film transistors (TFTs) with and without MoO_x interlayer was characterized. The device without MoO_x interlayer showed a large threshold voltage shift of 5.1 V after stressing with a constant gate-source voltage of −40 V for 10000 s, while at the same condition, the device with MoO_x interlayer showed a low threshold voltage shift of 1.9 V. The results can be attributed to the stable interface between MoO_x/pentacene and small contact resistance change for the device with MoO_x/Cu electrode. Pentacene-TFTs with MoO_x interlayer showed a high field-effect mobility of 0.61 cm²/V s and excellent bias stability, which could be a significant step toward the commercialization of OTFT technology.     

Reduced hole loss in organic light emitting diode incorporating two p-doped hole transport layers

The hole-injecting structure of 15 nm MoO₃-doped 4,4-N,N-bis [N-1-naphthyl-N-phenyl-amino]biphenyl (NPB:MoO₃)/5 nm MoO₃-doped 4,4′-N,N′-dicarbazole-biphenyl (CBP:MoO₃) has been used in organic light emitting diodes (OLEDs). It was found that a device using the 15 nm NPB:MoO₃/5 nm CBP:MoO₃/NPB combination was superior to one adopting the 20 nm NPB:MoO₃/NPB combination due to two major causes: the NPB:MoO₃/CBP:MoO₃ interface is a quasi-ohmic contact, and the hole transport barrier from CBP:MoO₃ to NPB is smaller than that from NPB:MoO₃ to NPB; moreover, it outperformed the device employing the 20 nm CBP:MoO₃/NPB combination, mostly because of the higher conductivity of NPB:MoO₃ compared to CBP:MoO₃. We demonstrate that using a structure resulting from uniting two p-doped hole transporters is a beneficial, simple method of achieving an improved trade-off between high conductivity and small hole transport barrier, thereby leading to a significantly reduced ohmic loss in the hole current conduction in the OLEDs, relative to the single p-doped layers.     

The influence of annealing temperature and tellurium (Te) on electrical and dielectrical properties of Al/p-CIGSeTe/Mo Schottky diodes

p-CuIn_0.7Ga_0.3(Se_(1?x)Te_x)₂ type thin films were synthesized by thermal evaporation method on Mo coated glass substrates. To obtain Al/CuIn_0.7Ga_0.3(Se_(1?x)Te_x)₂/Mo Schottky diode structure for two compositions of x = 0.0 and 0.6, Al metal was evaporated on upper surface of CuIn_0.7Ga_0.3(Se_(1?x)Te_x)₂ as a front contact. Al/p-CuIn_0.7Ga_0.3(Se_(1?x)Te_x)₂/Mo structures were annealed temperature range from 150 °C to 300 °C for 10 min under vacuum. The electrical and dielectrical properties of Al/p-CuIn_0.7Ga_0.3(Se_(1?x)Te_x)₂ (CIGSeTe) Schottky barrier diodes (SBD) have been investigated. Capacitance–Voltage (C–V) characteristics, Conductance–Voltage (G/w–V) characteristics and interface state density were studied in order to obtain electrical and dielectrical parameters. The effects of interface state density (N_ss), series resistance (R_s), the dielectric constant (?′), dielectric loss (?″), dielectric loss tangent (tan δ), ac electrical conductivity (σ_ac) and carrier doping densities were calculated from the C–V and G/w–V measurements and plotted as a function of annealing temperature. It was observed that the values of carrier doping density N_A for annealing temperature at 150 °C decreased from 2.83 × 10⁺¹⁵ cm^?3 to 2.87 × 10⁺¹⁴ cm^?3 with increasing Te content from x = 0.0 to 0.6. The series resistance for x = 0.0 found to be between 10 and 75 Ω and between 50 and 230 Ω for x = 0.6 in the range of annealing temperature at 150–300 °C.     

White organic electroluminescence from fluorescent bis (2-(2-hydroxyphenyl) benzoxazolate)zinc doped with phosphorescent material

Efficient white electroluminescence has been obtained by using an electroluminescent layer comprising of a blue fluorescent bis (2-(2-hydroxyphenyl) benzoxazolate)zinc [Zn(hpb)₂] doped with red phosphorescent bis (2-(2′-benzothienyl) pyridinato-N,C^3′)iridium(acetylacetonate) [Ir(btp)₂acac] molecules. The color coordinates of the white emission spectrum was controlled by optimizing the concentration of red dopant in the blue fluorescent emissive layer. Organic light-emitting diodes were fabricated in the configuration ITO/α-NPD/Zn(hpb)₂:0.01 wt%Ir(btp)₂acac/BCP/Alq₃/LiF/Al. The J-V-L characteristic of the device shows a turn on voltage of 5 V. The electroluminescence (EL) spectra of the device cover a wide range of visible region of the electromagnetic spectrum with three peaks around 450, 485 and 610 nm. A maximum white luminance of 3500 cd/m² with CIE coordinates of (x, y=0.34, 0.27) at 15 V has been achieved. The maximum current efficiency and power efficiency of the device was 5.2 cd/A and 1.43 lm/W respectively at 11.5 V.     

All-in-one-type organic light-emitting diodes for color tuning using phosphor in glasses with Pb-free silicate powders

We demonstrate all-in-one-type organic light-emitting diodes (OLEDs) that are fabricated using a color converting plate as a substrate. The color converting plate is Pb-free phosphor-in-glass (PiG), which is prepared by mixing Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce³⁺) and SiO₂–B₂O₃–RO (R = Ba, Zn) glass frit by sintering at 750 °C for 30 min. The maximum luminance, luminance efficiency, and power efficiency of blue OLEDs fabricated on commercial glass are measured as 10500 cd/m², 10.18 cd/A, and 2.95 lm/W, respectively. The Commission Internationale de l'Eclairge (CIE) coordinates of blue OLEDs is (0.167, 0.325). Our obtained results show that the luminance value decreased as the PiG thickness increased, and the glass to phosphor (GTP) ratio decreased. The OLED devices fabricated on the PiG substrate (GTP ratio = 9:1, thickness: 150 μm) showed a maximum luminance, luminance efficiency, and power efficiency of 7600 cd/m², 8.76 cd/A, and 2.85 lm/W, respectively. The CIE color coordinates changed to (0.286, 0.504) at 200 mA/cm². These results proved that color coordination can be easily adjusted by varying the GTP ratio and the thickness of the PiG.     

Reduction of driving voltage in organic light-emitting diodes with molybdenum trioxide in CuPc/NPB interface

A novel structure of organic light-emitting diode was fabricated by inserting a molybdenum trioxide (MoO₃) layer into the interface of hole injection layer copper phthalocyanine (CuPc) and hole transport layer N,N′-diphenyl-N,N′-bis(1-napthyl-phenyl)-1,1′-biphenyl-4,4′-diamine (NPB). It has the configuration of ITO/CuPc(10 nm)/MoO₃(3 nm)/NPB(30 nm)/ tris-(8-hydroxyquinoline) aluminum (Alq₃)(60 nm)/LiF(0.5 nm)/Al. The current density-voltage-luminance (J-V-L) performances show that this structure is beneficial to the reduction of driving voltage and the enhancement of luminance. The highest luminance increased by more than 40% compared to the device without hole injection layer. And the driving voltage was decreased obviously. The improvement is ascribed to the step barrier theory, which comes from the tunnel theory. The power efficiency was also enhanced with this novel device structure. Finally, “hole-only” devices were fabricated to verify the enhancement of hole injection and transport properties of this structure.     

Tandem organic light-emitting diode with molybdenum tri-oxide thin film interconnector layer

A 10-nm thickness molybdenum tri-oxide (MoO₃) thin film was used as the interconnector layer in tandem organic light-emitting devices (OLEDs). The tandem OLEDs with two identical emissive units consisting of N,N'-bis(naphthalen-1-yl)-N,N'-bis(phenyl)-benzidine (NPB)/tris(8-hydroxyquinoline) aluminum (Alq₃) exhibited current efficiency-current density characteristics superior to the conventional single-unit devices. At 20 mA/cm², the current efficiency of the tandem OLEDs using the interconnector layers of MoO₃ thin film was about 4.0 cd/A, which is about twice of that of the corresponding conventional single-unit device (1.8 cd/A). The tandem OLED showed a higher power efficiency than the conventional single-unit device for luminance over 1200 cd/m². The experimental results demonstrated that a MoO₃ thin film with a proper thickness can be used as an effective interconnector layer in tandem OLEDs. Such an interconnector layer can be easily fabricated by simple thermal evaporation, greatly simplifying the device processing and fabrication processes required by previously reported interconnector layers. A possible explanation was proposed for the carrier generation of the MoO₃ interconnector layer.     

Characterization of two-emitter WOLED with no additional blocking layer

In this paper, white organic light emitting diodes (WOLEDs) utilizing two primary-color emitters with no additional blocking layer are fabricated. With a structure of ITO/2TNATA (20 nm)/NPB (20 nm)/NPB: rubrene (2%) (10 nm)/ADN (30 nm)/Alq₃ (20 nm)/LiF (1 nm)/Al (100 nm), a white light with CIE coordinates of (0.344, 0.372) is generated at a current density of 30 mA/cm² and the electroluminescence (EL) spectra consist of two broad bands around 456 nm (ADN) and 556 nm (NPB:rubrene). The device shows the low turn-on voltage and bright white emission with a power efficiency of 2.3 lm/W at a luminance of 100 cd/m². Through control of the location of the recombination zone and energy transfer, a stable white light emission is achieved. The maximum color shift is less than 0.02 units on the 1931 CIE x,y chromaticity diagram. Given the spectral power distribution of WOLED, the parameters of a light source (chromaticity coordinate, CCT, CRI, and the luminous efficacy) can be calculated. A MATLAB program for this purpose is developed in this paper. Based on this, the design of WOLED for an illumination and display system using a white emitter with color filter arrays is discussed.     

Luminescent properties of KGd1−x(WO4)2:Eux and KGd1−x(WO4)2−y(MoO4)y:Eux phosphors in UV-VUV regions

KGd_1−x(WO₄)_2−y(MoO₄)_y:Eu³⁺_x(0.1?x?0.75, y=0 and 0.2) phosphors are synthesized through traditional solid-state reaction and their luminescent properties in ultraviolet (UV) and vacuum ultraviolet (VUV) regions are investigated. Under 147 nm excitation, these phosphors show characteristic red emission with good color purity. In order to improve their emission intensity, the MoO₄²⁻ (20 wt%) is introduced into the anion of KGd_1−x(WO₄):Eu³⁺_x. The Mo⁶⁺ and Eu³⁺ co-doped KGd(WO₄)₂ phosphors show higher emission intensity in comparison with the singly Eu³⁺-doped KGd(WO₄)₂ in VUV region. The chromaticity coordination of KGd_0.45(WO₄):Eu³⁺_0.55 is (x=0.669, y=0.331), while that of KGd_0.45(WO₄)_1.8(MoO₄)_0.2:Eu³⁺_0.55 is (x=0.666, y=0.334) in VUV region.