Layer optimization in wight organic light emitting diodes (WOLEDs) to reduce the portion of guided waves in ITO/Glass interface

In this paper, our main attempt was to reduce Total Internal Reflection (TIR) happening at Indium Tin Oxide (ITO, n_ito = 1.8 + 0.01i) and Glass (n_glass = 1.51) interface, which is due to ITO's higher index in comparison with Glass's, that makes light guided in ITO layer, 50% of generated light in Wight Organic Light Emitting Diodes (WOLED) are trapped in ITO layer; here we tried to reduce this portion of trapped light by implying 12-fold quasi-photonic crystal to the mentioned interface. With some gentle changes in 12-fold's structure we could reduce TIR in this interface to less than 9%. Also, far field results before and after adding the structure to WOLED were studied, which represents suitability of using this structure for lighting applications.     

A novel proposal for enhancement of light extraction efficiency in WOLEDs based on optimized photonic crystal structures

In this paper we propose a framework to enhance light extraction efficiency in white organic light emitting diodes (WOLED) using photonic crystal (PhC) structures sandwiched between indium tin oxide (ITO, n_ITO = 1.8+0.01i) and glass (n_glass = 1.51) substrate, according to the high refractive index contrast of these two layers almost 50% of the generated light inside WOLED gets trapped in the mentioned interface. The main purpose of this article is to suggest a method to intentionally optimize PhC structures to reduce total internal reflections (TIR) happening at ITO/glass interface. Here three different patterns are considered including rectangular, hexagonal and circular lattices. Using Finite Difference Time Domain (FDTD) method and the presented framework for choosing structural parameters the portion of 50% trapped light in ITO was reduced to 20% which is a large enhancement in extraction efficiency of WOLED. Also far-field results before and after adding PhCs are investigated.     

Light extraction efficiency enhancement in organic light emitting diodes based on optimized multilayer structures

The main focus of this study is to improve the light extraction efficiency, as well as directionality of organic light emitting diodes (OLEDs) using multi-layer structures between Indium tin Oxide (ITO) and glass layers in a typical OLED. In conventional OLEDs, only about half of the light generated in the emission zone can reach to the glass substrate due to refractive index mismatch in ITO (n = 1.8?i0.01)/glass (n = 1.51) interface. The main attempt is to reduce the share of total internal reflection (TIR) and hence, the effect of different structures such as Thue-Morse and Fibonacci have been investigated and optimized with suitable layer thickness and materials based on Transfer Matrix Method (TMM). The most effective Multi-layer structures have been added to conventional OLED and have been analyzed the extraction efficiency using Finite Difference Time Domain (FDTD) method. Results show large enhancement of extraction efficiency (about 40%) in ITO/glass interface. Using this idea and applying micro-lenses array to glass substrate at the same time, one can get even higher extraction efficiency in OLED. The interesting aspect of this project is its easy fabrication process in order to commercialize the product with highest extraction efficiency and low fabrication cost.     

Effects of hole injection layer thickness on the luminescent properties of white organic light-emitting diodes

This work investigates how the thickness of the hole injection layer (HIL) influences the luminescent characteristics of white organic light-emitting diodes (WOLED). Experimental results indicate that inserting a thin HIL (<200 Å) into a WOLED without an HIL reduces the brightness and clearly changes the chromaticity because the surface of the 4,4′,4″-tris{N,-(3-methylphenyl)-N-phenylamino}-triphenylamine) (m-MTDATA) film is extremely rough. In contrast, a dense film structure and the fine surface morphology of m-MTDATA of moderate thickness (350-650 Å) provides a uniform conducting path on which holes cross the indium tin oxide (ITO)/HIL interface, improving luminescent performance, associated with the relatively stable purity of the color of the emission, with Commission Internationale 1′Eclairage (CIE) coordinates of (x = 0.40, y = 0.40). However, inserting a thick HIL (>650 Å) reduces the luminescent performance and causes red-shift, because the holes and electrons in the effective emissive confinement region become less optimally balanced. Moreover, optimizing the device structure enables a bright WOLED with CIE coordinates of (x = 0.34, y = 0.33) to reach a luminance of 7685 cd/m² at a current density of 100 mA/cm², with a maximum luminous efficiency of 1.72 lm/W at 5.5 V.     

Electroplex emission at PVK/Bphen interface for application in white organic light-emitting diodes

White organic light-emitting diode (WOLED) with a structure of ITO/poly(N-vinylcarbazole) (PVK)/4,7-diphenyl-1, 10-phenanthroline (Bphen)/tris(8-hydroxyquinoline)aluminum (Alq₃)/LiF/Al has been fabricated via the thermal evaporation technique. The electroluminescence (EL) spectrum of the as-fabricated WOLED covers from 380 to 700 nm of the visible light region with a wide blue emission from PVK and an interesting new red emission. The red emission at 613 nm in EL spectra of the WOLED was attributed to electroplex emission at PVK/Bphen interface since it was not observed in photoluminescence spectra. The WOLED showed a Commission International De l'Eclairage coordinate of (0.31, 0.32), which is very close to the standard white coordinate (0.33, 0.33).     

Pockels effect of water in the electric double layer at the interface between water and transparent electrode

We have obtained the first experimental evidence for the Pockels effect of water, which is induced by a high electric field in the electric double layer (EDL) on the water-transparent electrode interface. The electric-field induced energy shift of the visible interference fringes of a 300 nm indium-tin-oxide (ITO) electrode layer is observed, indicating a negative refractive index change at the interface. Numerical calculation reproduces well the experimental observation, showing that the signal mainly originates from water in the EDL. The Pockels constants of water are estimated to be r₃₃ = 5.1 × 100 pm/V and r₁₃ = 1.7 × 100 pm/V. The large anisotropy of the Pockels effect of water is deduced from the incidence angle dependence of the p-polarization signal. At the same time, the ITO shows a blue shift of the band gap in the UV due to the band population effect in the space charge layer. The plasma frequency in the near IR is also expected to increase due to the band population effect, since the ITO has a high doped carrier population close to metal. A negative refractive index change in the ITO space charge layer is induced from both effects, but its effect on the signal is estimated to be much smaller than that of the negative refractive index change of water in the EDL.     

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.     

Enhanced electrical stability of flexible indium tin oxide films prepared on stripe SiO2 buffer layer-coated polymer substrates by magnetron sputtering

The electrical stability of flexible indium tin oxide (ITO) films fabricated on stripe SiO₂ buffer layer-coated polyethylene terephthalate (PET) substrates by magnetron sputtering was investigated by the bending test. The ITO thin films with stripe SiO₂ buffer layer under bending have better electrical stability than those with flat SiO₂ buffer layer and without buffer layer. Especially in inward bending text, the ITO thin films with stripe SiO₂ buffer layer only have a slight resistance change when the bending radius r is not less than 8 mm, while the resistances of the films with flat SiO₂ buffer layer and without buffer layer increase significantly at r = 16 mm with decreasing bending radius. This improvement of electrical stability in bending test is due to the small mismatch factor α in ITO-SiO₂, the enhanced interface adhesion and the balance of residual stress. These results indicate that the stripe SiO₂ buffer layer is suited to enhance the electrical stability of flexible ITO film under bending.     

Ultraviolet photoelectron spectroscopy investigation of interface formation in an indium-tin oxide/fluorocarbon/organic semiconductor contact

It has been demonstrated that hole-injection in organic light-emitting devices (OLEDs) can be enhanced by inserting a UV-illuminated fluorocarbon (CF_x) layer between indium-tin oxide (ITO) and organic hole-transporting layer (HTL). In this work, the process of interface formation and electronic properties of the ITO/CF_x/HTL interface were investigated with ultraviolet photoelectron spectroscopy. It was found that UV-illuminated fluorocarbon layer decreases the hole-injection barrier from ITO to α-napthylphenylbiphenyl diamine (NPB). Energy level diagrams deduced from the ultraviolet photoelectron spectroscopy (UPS) spectra show that the hole-injection barrier in ITO/UV-treated CF_x/NPB is the smallest (0.46 eV), compared to that in the ITO/untreated CF_x/NPB (0.60 eV) and the standard ITO/NPB interface (0.68 eV). The improved current density-voltage (I-V) characteristics in the UV-treated CF_x-coated ITO contact are consistent with its smallest barrier height.     

Optical properties of white organic light-emitting devices fabricated with three primary-color emitters by using organic molecular-beam deposition

White organic light-emitting devices (WOLEDs) with Mg:Ag/Alq₃/Alq₃:DCJTB/Alq₃/DPVBi/α-NPD/ITO and Mg:Ag/Alq₃/DPVBi:DCJTB/Alq₃/DPVBi/α-NPD/ITO structures were fabricated with three primary-color emitters of red, green, and blue by using organic molecular-beam deposition. Electroluminescence spectra showed that the dominant white peak for the WOLEDs fabricated with host red-luminescence Alq₃ and DPVBi layers did not change regardless of variations in the current. The Commission Inernationale de l'Eclairage (CIE) chromaticity coordinates for the two WOLEDs were stable, and the WOLEDs at 40 mA/cm² with luminances of 690 and 710 cd/cm² showed an optimum white CIE chromaticity of (0.33, 0.33). While the luminance yield of the WOLED fabricated with a host red-luminescent Alq₃ emitting layer below 30 mA/cm³ was larger than that of the WOLED fabricated with a DPVBi layer, above 30 mA/cm², the luminance yield of the WOLED fabricated with the DPVBi layer was higher than that of the WOLED with the Alq₃ layer and became more stable with increasing current density. These results indicate that WOLEDs fabricated with a host red-luminescence DPVBi layer without any quenching behavior hold promise for potential applications in backlight sources in full-color displays.     

Optical properties of ITO/TiO2 single and double layer thin films deposited by RPLAD

Indium tin oxide (ITO) and titanium dioxide (TiO₂) single layer and double layer ITO/TiO₂ films were prepared using reactive pulsed laser ablation deposition (RPLAD) with an ArF excimer laser for applications in dye-sensitized solar cells (DSSCs). The films were deposited on SiO₂ substrates either at room temperatures (RT) or heated to 200-400 °C. Under optimized conditions, transmission of ITO films in the visible (vis) range was above 89% for films produced at RT and 93% for the ones deposited at higher temperatures. Increasing the substrate temperature from RT to 400 °C enhances the transmission of TiO₂ films in the vis-NIR from about 70% to 92%. High transmission (≈90%) was observed for the double layer ITO/TiO₂ with a transmission cut-off above 900 nm. From the transmission data, the energies gaps (E_g), as well as the refractive indexes (n) for the films were estimated. n ≈ 2.03 and 2.04, respectively for ITO films and TiO₂ film deposited at 400 °C in the visible region. Post-annealing of the TiO₂ films for 3 h at 300 and 500 °C was performed to enhance n. The refractive index of the TiO₂ films increases with the post-annealing temperature. The direct band gap is 3.6, 3.74 and 3.82 eV for ITO films deposited at RT, 200, and 400 °C, respectively. The TiO₂ films present a direct band gap of 3.51 and 3.37 eV for as deposited TiO₂ films and when annealed at 400 °C, respectively. There is a shift of about 0.1 eV between ITO and ITO/TiO₂ films deposited at 200 °C. The shift decreases by half when the TiO₂ film was deposited at 400 °C. Post-annealing was also performed on double layer ITO/TiO₂.     

New double graded structure for enhanced performance in white organic light emitting diode

This study presents a new design that uses a combination of a graded hole transport layer (GH) structure and a gradually doped emissive layer (GE) structure as a double graded (DG) structure to improve the electrical and optical performance of white organic light-emitting diodes (WOLEDs). The proposed structure is ITO/m-MTDATA (15 nm)/NPB (15 nm)/NPB: 25% BAlq (15 nm)/NPB: 50% BAlq (15 nm)/BAlq: 0.5% Rubrene (10 nm)/BAlq: 1% Rubrene (10 nm)/BAlq: 1.5% Rubrene (10 nm)/Alq₃ (20 nm)/LiF (0.5 nm)/Al (200 nm). (m-MTDATA: 4,4′,4″ -tris(3-methylphenylphenylamino)triphenylamine; NPB: N,N′-diphenyl-N,N′-bis(1-naphthyl-phenyl)-(1,1′-biphenyl)-4,4′-diamine; BAlq: aluminum (III) bis(2-methyl-8-quinolinato) 4-phenylphenolate; Rubrene: 5,6,11,12-tetraphenylnaphthacene; Alq₃: tris-(8-hydroxyquinoline) aluminum). By using this structure, the best performance of the WOLED is obtained at a luminous efficiency at 11.8 cd/A and the turn-on voltage of 100 cd/m² at 4.6 V. The DG structure can eliminate the discrete interface, and degrade surplus holes, the electron-hole pairs are efficiently injected and balanced recombination in the emissive layer, thus the spectra are unchanged under various drive currents and quenching effects can be significantly suppressed. Those advantages can enhance efficiency and are immune to drive current density variations.     

Study of vibration-rotation levels in formamide with high resolution FTIR spectroscopy: The ν12, 2ν12, ν11, and ν9 bands

The far infrared and infrared spectra of formamide (HCONH₂) have been recorded at high resolution (0.00125 cm⁻¹) in the region of 90-1060 cm⁻¹. Over 20,000 transitions from the out-of-plane NH₂ wagging motion (n₁₂ = 1 ← 0 fundamental, n₁₂ = 2 ← 0 overtone, n₁₂ = 2 ← 1 difference bands), torsion (n₁₁ = 1 ← 0 bands), and out-of-phase NCO/NH₂ bend (n₉ = 1 ← 0 bands) have been assigned. Molecular parameters have been obtained for the ground state and the unperturbed n₁₂ = 1 state. The least-squares fit calculations were completed with the microwave data available in the literature. The complicated resonance system between the n₁₂ = 2, n₁₁ = 1, and n₉ = 1 states has been investigated carefully. Thus, we have been able to verify almost all resonances (avoided crossing) existing in the region J, K investigated. In the coupled Hamiltonian used for the fit, all Watson’s reduced parameters, including the octic ones and 16 Coriolis coupling parameters were taken into account. The rms deviation obtained from the fit was 0.000247 cm⁻¹.     

A new laser direct etching method of indium tin oxide electrode for application to alternative current plasma display panel

For cost effective fabrication and time of alternative current plasma display panels (AC PDPs), an indium tin oxide (ITO) layer was patterned directly with a Q-switched diode pumped Nd:YVO₄ laser (λ = 1064 nm). As experimental results, 500 mm/s scan speed with 40 kHz repetition rate was suitable for the application to AC PDP ITO electrode. In comparison with the chemically wet-etched ITO patterns by photolithography method, laser-ablated ITO patterns showed the formation of shoulders at the edge of the ITO lines and a ripple-like structure of the etched bottom. By dipping the laser-ablated ITO films in the chemical etching solution for 30 s at 50 °C, the shoulders were effectively removed without affecting the discharging properties of AC PDP.     

Low-temperature growth of ZnO nanorods on PET fabrics with two-step hydrothermal method

An effective low-temperature growth method to fabricate hexagonally oriented ZnO nanorod arrays onto PET fabrics is reported. The effect of substrate pre-treatment and C₆H₁₂N₄ concentration on the structure of ZnO nanorod arrays were investigated in details by X-ray diffraction (XRD), FE-SEM and ultraviolet protection factor (UPF). The results show that substrate pre-treatment, C₆H₁₂N₄ concentration indeed have great influence on the growth of ZnO nanorod arrays. It is indispensable to introduce a ZnO seed layer on the substrate and under growth condition of n(C₆H₁₂N₄):n[Zn(NO₃)₂] = 1:1, T = 90 °C, t = 3 h, the well-aligned ZnO nanorod arrays with 40-50 nm in diameter and 300-400 nm in length were achieved on the pre-treated PET fabrics. The ZnO nanorods grown on PET fabrics possessed an ultrahigh ultraviolet protection factor of 480.52 in this study, indicating an excellent protection against ultraviolet radiation in comparison with the untreated PET fabrics.     

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.     

Effect of thickness and porous structure of SiC layers on the spectral response of the Pd/SiC-pSi Schottky photodiodes

In this work, we study the effect of the thickness and porous structure of silicon carbide (PSC) layers on the electrical properties of Schottky photodiodes by using a palladium (Pd) layer deposited on non-porous silicon carbide (SiC) and porous-SiC (PSC) layers. The non-porous and porous-SiC layers were realized on a p-type silicon (Si(1 0 0)) substrate by pulsed laser deposition using a KrF laser (248 nm) and thermal deposition of a thin Pd layer. The porous structure of the SiC layer deposited was developed by an electrochemical (anodization) method. The electrical measurements were made at room temperature (295 K) in an air ambience. The effect of the porous surface structure and the thickness of the SiC layer were investigated by evaluating electrical parameters such as the ideality factor (n) and barrier height (?_Bp). The thickness of the porous layer significantly affects the electrical properties of the Schottky photodiodes. Analysis of current-voltage (I-V) characteristics showed that the forward current might be described by a classical thermal emission theory. The ideality factor determined by the I-V characteristics was found to be dependent on the SiC thickness a value For a thin SiC layer (0.16 μm) n was around 1.325 with a barrier height 0.798 eV, while for a thick layer (1.6 μm), n and ?_Bp were 1.026 and 0.890 eV, respectively for Pd/SiC-pSi. These results indicate Schottky photodiodes with high performance are obtained for thicker SiC layer and for thin layer of PSC. This effect showed the uniformity of the SiC layer. In the same case the ideality factor (n) decreases for Pd/PSC-pSi(1 0 0) for low SiC thickness by report of Pd/PSC-pSi(1 0 0) Schottky photodiodes, but for Pd/PSC-pSi(1 0 0) n increase for large SiC thickness layer. We notice that the barrier height (?_Bp) was reversely depend by report of ideality factor. A spectral response value of (SR) of 34 mA/W at λ = 400 nm was measured for Pd/0.16 μm SiC-pSi Schottky photodiode with low SiC thickness. On the other hand, a value of SR = 0.14 mA/W at λ = 900 nm was obtained when we used PSC layer (Pd/PSC-pSi(1 0 0)). A reverse behaviour occurs for thicker SiC layer. Finally, it was found that the thickness and surface porous structure have strong effect on sensitivity.     

Infrared guiding behavior in a 1 × N spatial soliton switch

The use of photorefractive spatial solitons in the field of telecommunication is attractive, because they enable the realization of a variety of all-optical switching and coupling devices. We suggest a new design for a switch with one input and N outputs for infrared light in the telecommunication range (1520-1630 nm). The important refractive index n₀ and its modulation Δn of our strontium-barium-niobate-crystal (SBN) in the infrared wavelength region is measured to be n₀ = 2.29 and Δn = 1.9 × 10⁻⁴. With these results the experimental observations show a good agreement to the theoretical predictions. The experimentally realization of a 1 × 8 switch demonstrates the potential of this technology.     

Periodicity and orientation dependence of electrical properties of [(Pb0.90La0.10)Ti0.975O3/PbTiO3]n (n = 1-6) multilayer thin films

The [(Pb_0.90La_0.10)Ti_0.975O₃/PbTiO₃]_n (PLT/PT)_n (n = 1-6) multilayer thin films were deposited on the PbO_x(1 0 0)/Pt/Ti/SiO₂/Si substrates by RF magnetron sputtering method. The layer thickness of PbTiO₃ in one periodicity kept unchanged, and the layer thickness of (Pb_0.90La_0.10)Ti_0.975O₃ is varied. The electrical properties of the (PLT/PT)_n multilayer thin films were investigated as a function of the periodicity (n) and the orientation. The studied results show that the PbO_x buffer layer results in the (PLT/PT)_n films’ (1 0 0) orientation, and the (1 0 0)-oriented (PLT/PT)_n multilayer thin films with n = 2 exhibit better pyroelectric properties and ferroelectric behavior than those of (PLT/PT)_n films with other periodicities and orientations. The underlying physical mechanism for the enhanced electrical properties of (PLT/PT)_n multilayer thin films was carefully discussed in terms of the periodicities and orientations.     

The luminescence and optoelectrical properties of ITO films prepared by a sputter type negative metal ion deposition

Transparent conducting indium tin oxide (ITO) thin films were prepared on glass substrates by a magnetron sputter type negative ion source which requires cesium (Cs) vapor injection for surface negative ionization on the ITO target surface. Although the film was prepared at 70 °C, it attained high optical transmittance, 88% and low resistivity, 2.03 × 10⁻⁴ Ω cm, at an optimized Cs partial pressure of P_Cs = 1.7 × 10⁻³ Pa. The as-deposited ITO films have a poly-crystalline structure with (2 1 1), (2 2 2), (4 0 0), (4 1 1) and (4 4 0) reflections.Also, ITO films prepared at P_Cs = 1.7 × 10⁻³ Pa were post-deposition vacuum annealed at 300 °C for 30 min. The films had a resistivity of 1.8 × 10⁻⁴ Ω cm and a transparency of 89.2%. The post-deposition vacuum annealed ITO film was used as an anode for a transparent organic light emitting diode (TOLED). A maximum luminance of 19,000 cd/m² was obtained.