Roll-off Phenomenon Research Articles

Investigation of improving organic light-emitting diodes efficiency using an ultra-thin ultraviolet-ozone-treated Nb-doped ZnO film as anode buffer layer

In this study, ultraviolet (UV)-ozone treated ultrathin Nb-doped ZnO (NZO) films are developed as an efficient anode buffer layer to improve the overall performance of organic light-emitting diodes (OLEDs). The results show that the UV-ozone treated NZO buffer layer containing 1 mol% of Nb2O5 possesses a higher oxygen content and a greater work function (5.22 eV) as compared to that of an indium tin oxide (ITO) film (~4.7 eV) signifying a reduction in hole injection barrier and thus an improvement in the injection efficiency. In addition, UV-ozone treatment can increase the surface energy of NZO films while reducing their surface roughness. Importantly, the UV-ozone treated 1 nm-thick NZO film with a Nb2O5 doping concentration of 1 mol% can help to lower the turn-on voltage from 3.2 V to 2.8 V, increase the luminance from 10,450 cd/m2 to 25370 cd/m2, and improve the current efficiency from 3.46 cd/A to 5.26 cd/A, (~52 % enhancement as compared to the standard OLED device). Moreover, OLEDs with the developed buffer layer reveal a significant improvement in the roll-off phenomenon under high current densities, indicating a key role of the optimized NZO film in enhancing the carrier balance of the devices. When applied to the p-i-n structure, the NZO film also lead to a superior device performance as compared to the conventional p-i-n structure using NPB:MoO3 as a hole injection layer, suggesting a widespread use of the developed thin film. These findings therefore show a promising use of the UV-ozone treated NZO ultrathin film as an effective anode buffer material for enhancing OLED overall performance.

Multiscale Janus Surface Structure of Trifolium Leaf with Atmospheric Water Harvesting and Dual Wettability Features.

Numerous fascinating hierarchical surfaces from nature, including cactus spines, rice leaves, Namib desert beetle, spider silks, and pitcher plants, have been thoroughly investigated to emulate and architect superior surfaces for capturing sustainable, clean, and safe freshwater from the atmosphere. Hitherto, the adaxial side of biological surfaces has been meticulously investigated for wettability and atmospheric water harvesting (AWH) applications. However, the abaxial face has not yet attracted much scientific scrutiny. Here, we revealed the multifunctional Janus surface traits of Trifolium pratense (i.e., red clover) leaf with extrusive atmospheric water fishing ability on both adaxial and abaxial faces. Water harvesting is performed by conical outgrowths (microhairs). The individual hair's intriguing topography comprises asymmetric shape and surface roughness, which plays synergetic roles in water deposition and directional transport. The water collection quantity on the leaf surface is a function of hair density, which varies significantly on two sides. Noticeably, instead of gravitational pull, the hairs perform water reaping competence under the collective impact of surface energy and Laplace pressure gradients. Consequently, both straight-up and upside-down water harvesting are presented. Furthermore, the leaf surface exhibits dual water wettability features. The upper side manifests the water-repelling and water roll-off phenomenon. In contrast, the lower surface displays a water-retaining/or pinning effect. Optical microscopy, scanning electronic microscopy, real-time optical visualization, and contact angle analysis were employed to characterize the natural and template specimens. The dorsiventral asymmetry of the Trifolium leaf examined in this work could be helpful for a plethora of applications, such as scalable AWH, rainwater collection, self-cleaning, and adhesive fixtures.

Photophysics behind luminescence phenomenon in conducting conjugate polymer blends for application as an emissive layer in PLEDs

Luminescence quenching by polarons is an important loss mechanism in polymer light-emitting diodes (PLEDs). Steady-state and time-resolved photoluminescence spectroscopy of polyaniline (PAni) thin films with varying polaron doping has helped us to realize polaron density-dependent photoluminescence quenching mechanisms inside the thin films. A sharp reduction in photoluminescence emission spectra has been observed at doping densities between 1017 and 1019 cm−3. This doping concentration-dependent photoluminescence phenomenon in PAni is modeled quantitatively using quenching of excitons by polarons through long-range Förster resonance energy transfer (FRET) and short-range charge transfer (CT) mechanisms. The results match well with the experimental findings that demonstrate that both models need to be considered to explain the mechanisms of luminescence quenching. FRET and CT phenomena inside an emissive layer of PLEDs have been demonstrated to play a pivotal role in the quantum efficiency roll-off phenomenon at high current density using experimentally obtained and theoretically calculated kinetic quenching parameters. To get rid of low luminescence in PAni, it has been blended with poly(methyl) methacrylate (PMMA) that enhances its luminescence manifold. The blending of PMMA leads to the introduction of a new photophysical phenomenon—donor PMMA concentration-dependent FRET contrary to original FRET theory proposed by Förster. Concentration-dependent FRET leads to a sharp drop in luminescence from the polymer blend after reaching a critical concentration of PMMA. Therefore, the present study explores the reason behind low luminescence in conducting polymers and demonstrates ways to mitigate it along with providing an account of the photo-physics of the conducting polymer as an emissive layer in PLEDs.

Fabrication and characterization of metal insulator semiconductor Ag/PVA/GO/PVA/n-Si/Ag device

In this work, Metal Insulator Semiconductor (MIS) device is fabricated by depositing Graphene Oxide (GO) layer between the top and bottom Polyvinyl Alcohol (PVA) layers to act as an insulating layer. Topographical study of Ag/PVA/GO/PVA/n-Si/Ag device is done by Field Emission-Scanning Electron Microscopy (FE-SEM) technique. The image clearly showed the multilayered sandwiched structure having GO embedded between PVA layers. The comparative study of the effect of interface trap states, series resistance (Rs) and an insulating layer on the different layers of device performance is done using capacitance-conductance characteristics. The presence of peaks in capacitance-conductance curves revealed the presence of interface trap states in the fabricated device. The C–V measurements were taken in the frequency range of 5 kHz to 1 MHz. The interfacial state density (Nss) is found to be of the order of 1011 cm−2 eV−1 which is calculated using Hill-Coleman's method. The increase in leakage current through residual native oxide is verified by calculating the series resistance. The deviation from the ideal behavior of capacitance and conductance characteristics is observed which is attributed to Nss and Rs. The device fabricated unfolded the existence of the capacitance roll-off phenomenon.

Fine tuning of bipolar side group on dual anthracene core derivatives for highly efficient blue emitters

Three bipolar blue fluorescent materials were newly synthesized by attaching diphenylamine (DPA), triphenylamine (TPA), and 9-phenylcarbazole amine (PCA) as an electron donating group and 2,4- diphenyl-1,3,5-triazine (DPT) as an electron withdrawing group to the dual anthracene core moiety. Compounds are [10'-(4,6-Diphenyl-[1,3,5]triazin-2-yl)-[9,9']bianthracenyl-10-yl]-diphenyl-amine (DPA-AA-DPT), {4-[10'-(4,6-Diphenyl-[1,3,5]triazin-2-yl)-[9,9']bianthracenyl-10-yl]-phenyl}-diphenyl- amine (TPA-AA-DPT), and 9-(4-(10'-(4,6-diphenyl-1,3,5-triazin-2-yl)-[9,9′-bianthracen]-10-yl)phenyl)- 9H-carbazole (PCA-AA-DPT), respectively. The optical, thermal, and electroluminescence (EL) properties of the synthesized materials were measured. Newly bipolar materials were found to have blue emission in the solution state and to acquire high PLQY values. A device doped with PCA-AA- DPT showed a very high efficiency of 6.86 cd/A and EQE of 5.53% at a high current density without roll-off phenomenon.

Позитронная эмиссионная томография с рубидием-82 в исследованиях перфузии миокарда

This review considers literature sources on myocardial perfusion studies using positron emission tomography with rubidium-82. The history of the development of the method, the protocols of the study, the dissymmetric data are analyzed, and comparisons are made with other positron emitters that are used in clinical practice and research to study myocardial blood supply. The use of PET/CT with rubidium-82 makes it possible to obtain valuable diagnostic information and it allows to measure myocardial blood directly and make a separate assessment of the coronary arteries function. Due to the fact that the production of rubidium-82 does not require an on-site cyclotron and a radiochemical laboratory, this method of imaging is more accessible than other positron emitters used for the same purpose. Also, the study is not associated with significant discomfort for the patient, since the full stress/rest imaging protocol requires less than half an hour. However, the use of rubidium-82 has a number of drawbacks, including the relatively low sharpness of the resulting image due to the high energy of the emitting positrons. Also there is a necessity for a mathematical correction of the roll-off phenomenon, which is a decrease in radiopharmaceutical extraction with an increase in myocardial blood flow. Due to the short half-life period, the provision of stress tests with ergometers is difficult. It needed to use pharmacological stress tests. In addition, usage of rubidium-82 is characterized by a high cost both due to the expensive production of the parent isotope, strontium-82, and the need for frequent replacement of generators – on average, 11 to 13 times a year.

Improving Charge-Imbalanced Problem of Quantum Dot Light-Emitting Diodes with TPBi/ZnO Electron Transport Layer.

To solve charge-imbalanced problem caused by excessive electron injection into the emitting layer (EML) of quantum dot light emitting diodes (QLEDs) with ZnO electron transport layer (ETL), we proposed QLEDs with TPBi((2,2',2''-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)))/ZnO ETL layered design. Spin coated TPBi demonstrated lower value of the lowest unoccupied molecular orbital (LUMO) than conduction band maximum (CBM) of ZnO, resulting in effective prevention of excessive injection of electrons into the EML even under excessive stress conditions. Experimental results demonstrated that QLEDs with TPBi/ZnO ETL not only could minimize charge imbalanced problem under high current density operation, but also could increase the maximum luminance of QLEDs by up to 156% (i.e., from 10,320 to 16,081 cd/m²). In addition, the new design with TPBi resulted in low roll-off phenomenon in external quantum efficiency (EQE)-current density characteristics.

Characterization studies of NiO:K anode buffer layer in Alq3 and TADF-based organic light-emitting diodes by secondary ion mass spectrometry and admittance spectroscopy

In this paper, we used thermal evaporation method to deposit metal oxide films as inorganic buffer layer in Organic light-emitting diodes (OLEDs), and investigated how the K2CO3-doped NiO (KNO) buffer layers affect OLEDs’ performances. The mechanisms of enhanced hole injection into the hole transport layer NPB (α-naphthylphenylbiphenyl diamine) via NiO:K2CO3 anode buffer layers in Alq3-based and TADF-based OLEDs were studied by means of J-V-L characteristics, secondary ion mass spectroscopy (SIMS), and admittance spectroscopy measurements. For Alq3-based OLEDs, devices’ opto-electrical performances showed increased of luminance from 8780 cd/m2 to 22940 cd/m2, the decreased of turn-on voltage from 3.4 V to 3.1 V at 1 mA/cm2, and increased of current efficiency from 4.3 cd/A to 3.48 cd/A when 1-nm-thick K2CO3-doped NiO film with doping concentration of 1 mol% was inserted into OLEDs. TADF-based OLEDs’ current efficiency increased from 21.7 cd/A to 29.7 cd/A with similar anode buffer layer parameters. SIMS depth profiles on OLEDs suggested higher indium (In) diffusion from ITO anode onto TADF-based OLEDs upon device operation leading to higher roll-off phenomenon. Deposition of NiO:K2CO3 as anode buffer layer onto OLEDs device reduced the diffusion of In in organic layers during device operation. Investigation of the capacitance as function of voltage and frequency yield information about carrier injection characteristics for both Alq3 and TADF-based OLEDs. Due to better carrier recombination capability, TADF-based OLEDs’ slope in V > Vbi area has fewer shifts and more stable response, leading to higher amplitude in current efficiency enhancement in comparison with fluorescents (Alq3) material.

Roll-Off Displacement in Ex Vivo Experiments of RF Ablation With Refrigerated Saline Solution and Refrigerated Deionized Water

The recurrence rate in the treatment of liver tumors using radio frequency ablation (RFA) is often related to incomplete tissue necrosis and consequently the limitation in the ablation volume. This paper proposes an ablation protocol combined with the infusion of saline solution and deionized water aiming at achieving a time displacement in the roll-off occurrence and consequently increasing the volume of ablation. An infusion of saline solution and deionized water at 5 and 23 °C was performed to evaluate the influence of these liquids on the RFA procedure in ex vivo bovine liver pieces. The obtained results were used to propose a mathematical model of the roll-off phenomenon by means of the system identification techniques. The RFA combined with the infusion of saline solution 0.9% at 5 °C presented optimal results, with a time delay of the roll-off occurrence in 27.8% compared to pure RFA ( p = 0.002) and an increase in the necrotic volume of 51.2% ( p = 0.0002). Two Box-Jenkins models were obtained to describe the roll-off phenomenon: 1) pure RFA; and 2) RFA combined with the saline solution 0.9% at 5 °C. The RFA therapy combined with the saline solution 0.9% at 5 °C increases the time range to the roll-off occurrence, leading to higher necrosis volumes in ex vivo bovine liver samples. The development of a mathematical model to describe the roll-off behavior demonstrated that the transient response is improved by the infusion of the saline solution 0.9% at 5 °C.

Effects of Buffer Layer Treatments on the Characteristics and Performances of OLEDs

The authors demonstrated the performance of Organic Light Emitting Diodes (OLEDs) with hexylphosphonic acid (PA) or UV-ozone treatment on their molybdenum trioxide (MoO3) anode buffer layers. The OLEDs with a PA treated (5 mM @1 h) MoO3 layer have lower turn-on voltage and low current efficiency roll-off under high operating current. The hole-only device (ITO/MoO3 with PA or UV-ozone treatment/NPB/Al) was fabricated to calculate the active energy via temperature dependent I-V measurement. When the devices were operated at high temperatures, the activation energy of the UV-ozone treated, and untreated hole-only devices became nonlinear. However, the activation energy of the PA treated devices had a more stable performance at high temperatures. The interfacial resistance of the untreated hole-only devices and the PA and UV-ozone treated devices were calculated by Admittance Spectroscopy (AS) to be 204.5, 363, and 450 Ω. These results indicated that the PA treated devices reduced the activation energy, surface energy mismatch, and interface resistance to enhance the efficiency of carrier injection, which in turn lowers the turn on voltage. Moreover, PA treatment produces a long carbon chain and smooths the morphology of MoO3 to prevent roll-off phenomenon of OLEDs under high operating current.

Effects of the Profile of Workpiece Surface on Material Removal Rate Distribution in Polishing Process

Accurate prediction of the material removal rate (MRR) distribution is very important for the control of the polishing process. However, the widely used prediction method of MRR based on the Preston equation is still incapable of predicting the roll-off phenomenon in polishing process. One of the reasons is that many of the researchers’ neglected the effect of the surface profile of the workpiece on the MRR. In this paper, the evolutionary process of MRR distribution with the change of surface profile using two different polishing pad is studied, it is found that MRR varies gradually with the change of surface profile and tends to be uniform finally. Based on the analysis of contact pressure considering the actual surface profile of workpiece and modified Preston equation, the distribution of MRR is analyzed. It is found that the Preston coefficient distribution on workpiece surface is stable when the surface profile variation is small and shows obvious differences from the center to the edge of the workpiece. Through the comparison it is found that correlation between the regularities of Preston coefficient distribution and the type of polishing pad is significant. The research results in this paper will play an important guiding role in the quantitative prediction method research of polishing process.

Capacitance roll‐off and frequency‐dispersion capacitance–conductance phenomena in field plate and guard ring edge‐terminated Ni/SiO2/4H‐nSiC Schottky barrier diodes

In this work, field plate and guard ring edge‐terminated Ni/4H‐nSiC Schottky barrier diodes (SBD) were fabricated using standard photolithography process. Strange peaks in capacitance–conductance curves, capacitance roll‐off, and a high value of ideality factor ( = 1.3) in fabricated SBD were seen as a signature of interface trap states (Nss) at the residual oxide (2.2 nm)/4H‐nSiC interface and series resistance (Rs). Schottky capacitance spectroscopic, High–low capacitance–voltage (C–V) and forward‐bias current–voltage (I–V) techniques, in the frequency range from 100 Hz to 1 MHz, determines Nss of the order of 1012 cm−2 eV−1 and were found exponentially distributed in the bandgap of SiC. Using Hill–Coleman's method, the density Nss was calculated to be 1.15 × 1015 cm−2 eV−1 at 100 Hz and 7.81 × 1012 cm−2 eV−1 at 1 MHz, which explains the larger value of capacitance at low frequencies. Relaxation times and capture cross sections of Nss were also estimated. Calculated values of Nss were used in a Silvaco simulation that emphasize that bulk level defects present in the SiC also contributes in the experimentally observed strange peaks in C–V characteristics of fabricated SBD. At higher current levels, calculated values of Rs (V, ), confirm an increase of leakage current through residual oxide and describes the capacitance roll‐off phenomena in the fabricated SBD.

Viscoelastic plastic continuous physical model of a magnetorheological damper applied in the high speed train

In the preliminary design stage of high-speed train smart suspension, a simple, yet accurate magnetorheological (MR) damper model whose parameters have clear physical meaning is needed. Based on the working mechanism analysis and the dynamic behavior study of the MR damper, a new consecutive viscoelastic plastics (VEP) model is proposed. A methodology to find the parameters of the proposed model directly has been proposed. The comparison with experimental results indicates that the proposed model could adequately characterize the intrinsic nonlinear behavior of the MR damper, including the hysteretic behavior, roll-off phenomenon, and the variation of the hysteresis width in terms of the frequency and magnitude of excitation. The results of experimental testing prove that the accuracy of the proposed model is higher than that of the phenomenological model while only containing four undetermined parameters with clear physical meaning. Moreover, based on the proposed VEP model, a nonlinear stiffness VEP (nkVEP) model is developed with higher precision in the hysteretic region. The nkVEP model, which can reproduce the behavior of the damper with fluctuating input current, is developed. The proposed model could predict accurately the response of the MR damper in a wide range of frequency and displacement.

The impact of interface/border defect on performance and reliability of high-k/metal-gate CMOSFET

The impact of interface/border defect on performance and reliability was investigated for gate-first and gate-last high-k/metal-gate CMOSFET. For high-k/metal-gate CMOSFET, VFB roll-off is critical as effective oxide thickness (EOT) scales below 15 angstrom (Å) especially for metal gate-first device with an extra capping layer to adjust VTH. By proposing a model of metal-gate process induced interface trap releasing oxygen, the vertical and lateral interface trap distribution and the dependence of interfacial layer (IL) on VFB roll-off phenomena can be well interpreted. In this work, we found that VFB roll-off can be improved by reducing oxygen vacancy (VO) at the HfO2/IL interface with suppression of oxygen diffusion from high-k to IL. By the way, a metal gate-last process with lower interface trap was proposed to minimize VO formation by suppress oxygen releasing, thus optimize a 28nm 15Å EOT HfO2/metal-gate CMOSFET with low VFB–EOT roll-off, it can be a reference for sub 22nm CMOSFET with thin EOT (<15Å) design.

Improvement of roll-off in power efficiency for mixed single layer organic light emitting devices by co-doping

Roll-off phenomena in power efficiency for rubrene-based mixed single layer organic light emitting devices is investigated. The evaluation of operational temperature demonstrates that Joule heating is not the main reason for roll-off in power efficiency with current density. Co-doping method through rubrene co-doping with some additive dopant, C6, DCJTB and Nile red, in mixed single layer are proposed for suppressing the roll-off in power efficiency. By co-doping suitable Nile red with rubrene, the efficiency roll-off is found to be largely improved in whole range of current density. Moreover, improved device performance is also realized even if with a 3wt% fluorescent doping concentration. The improved performance is ascribed that there exists a competitive emission mechanism with respect to different dopant.

Impact of high temperature annealing on La diffusion and flatband voltage (Vfb) modulation in TiN/LaOx/HfSiON/SiON/Si gate stacks

We present a detailed analysis of the impact of high temperature annealing on the chemical and electronic properties of TiN/HfSixOyNz/SiOxNy/Si gate stacks, where an ultra-thin LaOx capping layer (0.4–1 nm) is inserted between the TiN metal gate and the HfSixOyNz dielectric. From our experimental results, we demonstrate that La atoms diffuse through the entire nitrided hafnium silicate and reach the SiOxNy interfacial layer to form a La-silicate. In addition, hard x-ray photoelectron spectroscopy analysis highlights the band alignments’ shift of the gate stacks, which is well related to Vfb shifts based on an interfacial dipole and/or fixed charges model. Finally, this study reveals that the Vfb roll-off phenomenon is amplified with an increasing amount of La atoms near the substrate interface. A correlation between LaOx thickness and interface trap density (Dit) is observed, and a mechanism explaining the roll-off behavior is proposed.

Identification of electrically active defects in thin dielectric films by spectroscopic ellipsometry

A study of resonant photo-absorption features in high-k dielectric film stacks using vacuum ultraviolet spectroscopic ellipsometry demonstrates that all optically observable dielectric-related defects are located in the interfacial SiO2 layer rather than in the bulk high-k film. The defects, located at 2.9 eV, 3.6 eV, 3.9 eV, and 4.75 eV within the bandgap of this bottom interface, are found to be strongly affected by processing conditions. These results are supported by both electrical and physical characterization measurements that identify a consistent trend in the evolution of charge trapping defects for samples subjected to identical processing conditions. In addition, evidence is provided correlating the optically active 2.9 eV defect to positively charged oxygen vacancies in the bottom interfacial layer that have recently been proposed as contributing to the flatband voltage roll-off phenomenon. The close connection between these results and both ab initio calculations and experimental findings substantiate the use of spectroscopic ellipsometry as a unique characterization method for identifying process-induced defects during development and fabrication of dielectric film stacks.

Flat-band voltage shift in metal-gate/high-k/Si stacks

In metal-gate/high-k stacks adopted by the 45 nm technology node, the flat-band voltage (Vfb) shift remains one of the most critical challenges, particularly the flat-band voltage roll-off (Vfb roll-off) phenomenon in p-channel metal-oxide-semiconductor (pMOS) devices with an ultrathin oxide layer. In this paper, recent progress on the investigation of the Vfb shift and the origin of the Vfb roll-off in the metal-gate/high-k pMOS stacks are reviewed. Methods that can alleviate the Vfb shift phenomenon are summarized and the future research trend is described.

CONNECTING ELECTRICAL AND STRUCTURAL DIELECTRIC CHARACTERISTICS

An attempt is made to correlate electrical measurement results to specific defects in the dielectric stacks of high-k/metal gate devices. Defect characteristics extracted from electrical data were compared to those obtained by ab initio calculations of the dielectric structures. It is demonstrated that oxygen vacancies in a variety of charge states and configurations in the interfacial SiO2layer of the high-k gate stacks contribute to random telegraph noise signal, time-dependent dielectric breakdown, and the flatband voltage roll-off phenomenon.

Design of CIC roll-off compensation filter in a W-CDMA digital IF receiver

In a digital intermediate frequency (IF) receiver, decimation is performed to reduce the computational complexity and cascaded integrated comb (CIC) filter is used together with the decimation as an anti-aliasing filter. However, the CIC filter generates the roll-off phenomenon in the pass-band, which causes the receiving performance to be considerably degraded due to the distorted pass-band flatness of the receiving filter. In this paper, we propose a design method of the CIC roll-off compensation filter to reduce the performance degradation due to the roll-off characteristics of the CIC filter for a W-CDMA digital IF receiver. The performance of the proposed CIC roll-off compensation filter is confirmed through computer simulation in such a way that bit error rate (BER) is minimized by compensating the roll-off characteristics. In addition, the proposed method can be used in the design of a digital-to-analog (DAC) compensation filter and interpolator in the transmitter.