Thickness Of Emitting Layer Research Articles

Carrier transfer and luminescence characteristics of thickness‐dependent organic light‐emitting diodes using transporting material as the host of emitting layer

We have systematically investigated hole/electron distribution and exciton dynamics in blue fluorescent and yellow phosphorescent organic light‐emitting diodes (OLEDs). We demonstrate that the importance of appropriate charge carrier confinement and exciton management for the realization of highly efficient and stable OLEDs. For yellow phosphorescent device based on Ir(MePQ)2(acac), we have studied the behavior of various intrinsic emission zones on the characteristics of OLEDs with the hole/electron‐transporting matrix. The current efficiency increases from 15.7 to 31.7 cd A−1 with the rise in electron‐transporting matrix from 20 to 80 nm. We attributed the improved performance to the strong hole‐transfer capabilities and relatively weak electron‐donating abilities of Ir‐complex doped transporting matrix. In contrast, the efficiencies of blue devices decreased with increasing emitting layer (EML), suggesting that the rate of hole‐electron pair recombination continuously decrease with the increasing EML thickness due to blue fluorescent dopant hindering hole transfer. In addition, the complementary‐color white OLEDs (WOLEDs) with various EML thickness were also demonstrated. Based on their performance, we have found that the use of thin EML in WOLEDs can significantly improve optoelectronic conversion efficiency, whereas deteriorated device properties reversely.

Analysis of the relation between spectral response and absorptivity of GaAs photocathode

In order to study the relation between spectral response and absorptivity of GaAs photocathode, two kinds of GaAs photocathodes are prepared by molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD), respectively. The samples grown by the MBE include varying doping GaAs photocathodes with different values of emission layer thickness from A to E. The thickness of GaAs emission layer is 1.6 μm or 2 μm. The Al component is 0.5 or 0.63. The samples grown by the MOCVD include varying doping or various component GaAs photocathodes with different values of emission layer thickness and different window layer components from F to J. The thickness values of GaAs emission layer are 1.4 μm, 1.6 μm or 1.8 μm, respectively. The Al component is 0.7 or varies from 0.9 to 0. The doping concentration of the GaAs emission layer is divided into 8 sections between 1×1018 cm-3 and 1×1019 cm-3. The experimental spectral response curves for all samples are obtained by the optical spectrum analyzer. And the experimental reflectivity and transmittivity curves are measured by the ultraviolet visible near infrared spectrohootometer. Based on the law of energy conservation, the absorptivity curves are obtained according to the experimental reflectivity and transmittivity. In the same coordinate system, both the curves are obtained by unitary processing according to the max. A similar surface barrier can be given by dividing the normalized absorptivity by the normalized spectral response, and those are termed the similar I barrier and the similar Ⅱ barrier, respectively. The results indicate that for both the GaAs photocathodes, the experimental spectral response curves both tend to move to the infrared band compared with the experimental absorptivity curves. The average energy differences between absorptivity and spectral response are calculated to be 0.3101 eV for the MBE sample, and 0.3025 eV for the MOCVD sample, respectively. The red-shifts of the photocathodes grown by MBE are a bit bigger than those of the photocathodes grown by MOCVD. In the shortwave region, the absorptivity is very large, but the spectral response cuts off nearby 500 nm. In the visible wavelength region, the peak position of the spectral response curve shifts toward the infrared band for several hundred meV in comparison with the absorptivity curve. In the near infrared region, a red shift of several meV appears at the cut-off position of the spectral response curve in comparison with the absorptivity curve. The results have the guiding significance for improving the photoemission performance of wide-spectrum GaAs photocathode by optimizing the optical performance.

Effects of Doping Concentration and Emission Layer Thickness on Recombination Zone and Exciton Density Control in Blue Phosphorescent Organic Light-Emitting Diodes

Herein, we evaluate the effects of the doping concentration and emissive layer (EML) thickness to control the recombination zone (RZ) and exciton density (ED) in blue phosphorescent organic light-emitting diodes (Ph-OLEDs). Varying the doping concentration of the guest molecule iridium-(III)-bis-[(4,6-difluorophenyl)-pyridinato-N,C2'] picolinate (FIrpic) in host materials together with different EML thicknesses induced a change in the RZ and ED and also influenced the quenching of triplet excitons. Increasing the dopant concentration in the 20 nm EML generated a wider RZ and greater ED and induced a blueshift in the electroluminescence spectrum, whereas a redshift in the electroluminescence (EL) spectrum was observed in the case of the 30 nm EML. It is proposed that the micro-cavity effect (around 500 nm), shift of the RZ and ED along with band alignment tactics were operative for improving the performance of the blue Ph-OLEDs.

Influence of Micro-Cavity Effect and Architecture in the Blue Phosphorescent Organic Light-Emitting Diodes

The performance of the blue phosphorescent organic light-emitting diodes (PHOLEDs) can be affected by the range of the recombination zone (RZ) and the architecture in the emitting layer (EML). In order to change the width and location of RZ, a series of devices with the double emitting layers are fabricated. The range of RZ can be judged by the electroluminescence spectra around 500 nm due to micro-cavity effect of RZ. Then, another series of devices with a single host EML1 (mCP:FIrpic, x nm) and a mixed host EML2 (mCP:TPBi:FIrpic, y nm) are fabricated under the same total thickness of EML (x+y = 30 nm), when x is 15 nm, the maximum luminance and power efficiency can reach 9073 cd/m2 (at 8 V) and 23.4 lm/W (at 103 cd/m2), respectively. Both the driving voltage and efficiency roll-off are also improved.

Efficient and long‐life green light‐emitting diodes comprising tridentate thiol capped quantum dots

In this paper, the green quantum dots capped with the ligand, tris(mercaptomethyl)nonane (TMMN), are fabricated as the light‐emitting layer for efficient and bright light‐emitting diodes. These TMMN‐capped quantum dots exhibit well‐preserved photoluminescence properties with quantum yields of ∼90% after ligand exchange. The light‐emitting diodes based on TMMN‐capped quantum dots are reported with a maximum external quantum efficiency of 16.5% corresponding to a power efficiency and current efficiency of 57.6 lm W–1 and 70.1 cd A–1, respectively. The devices exhibit high color stability that is not markedly affected by the increase of applied voltage, thus leading to a high color reproducibility. Most importantly, the devices exhibit high environmental stability. For the highest luminance devices (with emitting layer thickness of 25 nm) and the highest power efficiency devices (with emitting layer thickness of 38 nm), the lifetimes are > 480 000 h and > 110 000 h, respectively. image

Simple-structure organic light emitting diodes: Exploring the use of thermally activated delayed fluorescence host and guest materials

We investigate the dependence of the performance of non-doped blue light emitting devices with thermally activated delayed fluorescence (TADF) material bis[4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]sulfone (DMAC-DPS) emission layer on hole and electron transport layers as well as emission layer thickness and study the underlying device physics. On this basis, efficient green and orange devices using DMAC-DPS as host material and TADF material (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN) or 2,3,5,6-tetrakis(3,6-diphenylcarbazol-9-yl)-1,4-dicyanobenzene (4CzTPN-Ph) as emitting dopant are reported. In addition, white devices using single DMAC-DPS: 4CzTPN-Ph emission layer show the maximum external quantum efficiency of 13.4%, maximum power efficiency of 38.3 lm W−1 and current-insensitive Commission Internationale de I'Eclairage (CIE) coordinates of (0.29, 0.39). Compared to the approach of combining TADF host and fluorescent dopant, the present devices enable the utilization of all excitons for light emission and the adoption of broad dopant concentration without significantly affecting device efficiency, which is important for the realization of the desired colour purity for display applications, while maintaining the advantages of simple-structure and low-cost.

High-efficiency blue fluorescent devices with simple structure

Thermally activated delayed fluorescence (TADF) materials can harvest both singlet and triplet excitons for light emission without the utilization of heavy metal elements such as iridium and platinum and become the third generation organic optoelectronic materials after fluorescent and phosphorescent materials. TADF material bis[4-(9,9-dimethyl-9, 10-dihydroacridine)phenyl]sulfone (DMAC-DPS) shows high photoluminescence quantum yield in solid films, rendering it as a suitable emitter for non-doped organic light emitting diodes. We optimize the hole transport layer, electron transport layer and emission layer thickness for the none-doped DMAC-DPS devices to pursuit high luminance efficiency. We compare the properties of the devices using indium tin oxide (ITO)/MoO3 hole injection contact with either a 40 nm mCP or 30 nm NPB/10 nm mCP hole transport layer and the results indicate (1) the former devices show larger current than the latter devices for there is a large energy barrier of ca. 0.7 eV for hole injection from NPB into mCP in the latter devices; (2) Both of the devices show similar luminance efficiency and the underlying reason is that the small modulation of the dominant hole current slightly affects the device efficiency. In order to investigate the influence of different electron transport layers on the device performance, we have prepared the devices with either 10 nm DPEPO/40 nm TmPyPB, 50 nm TmPyPB or 50 nm SPPO13 electron transport layer and find out (1) the devices with DPEPO electron transport layer show the highest EQE of 16.2% due to the fact that DPEPO with triplet energy of 3.3 eV can confine excitons inside the emisson layer effectively; (2) The devices with SPPO13 possess the highest current density at certain voltage among the devices and the maximum external quantum efficiency and power efficiency for the SPPO13 based devices are 14.3% and 26.8 lm W - 1, which are higher than 13.8% and 22.2 lm W - 1 for the TmPyPB based devices. The possible reason is that highly polarized phosphorus oxygen groups of SPPO13 greatly improve the electron injection and as a result improve the carrier balance in the devices. Moreover, we compare the properties of the devices with a 15 and 30 nm emission layer and find that the devices with a 30 nm emission layer have slightly better luminance efficiency. High efficiency of the DMAC-DPS devices can be attributed to the effective conversion of triplet excited states to singlet excited states via reverse intersystem crossing (RISC) process. TADF sensitized fluorescence devices can achieve high luminance efficiency by using the well-established fluorescent materials with the advantages such as the superior stability, versatility and abundance. We report highly efficient blue TADF sensitized devices using DMAC-DPS as the host material and the traditional fluorescence blue-emitting material 2,5,8,11-tetra-tert-butylperylene (TBPe) as the guest material. The devices with 1.0% TBPe show the maximum external quantum efficiency and power efficiency of 12.7% and 22.9 lm W - 1, which are about 3 times higher than those of the conventional 2-methyl-9,10-bis(naphthalen- 2-yl)anthracene (MADN): TPBe devices. This can be attributed to the effective use of triplet excited states for light emission via RISC process and highly efficient Forster energy transfer process from DMAC-DPS to TBPe. Utilization of a TADF material as host material and sensitizer and a traditional fluorescence material as emitting dopant serves as a novel approach to achieve high-efficiency organic light emitting diodes.

Influence of confinement layers in the emitting layer of the blue phosphorescent organic light-emitting diodes

The electrical and optical properties of the blue phosphorescent organic light-emitting diodes (PHOLEDs) can be affected by the various structure of confinement layer in the emitting layer (EML). A series of devices with different electron or hole confinement layer (TCTA or Bphen) are fabricated, it is more effective to balance charge carriers injection for the device with the double electron confinement layers structure, the power efficiency and luminance can reach 17.7 lm/W (at 103 cd/m2) and 3536 cd/m2 (at 8 V). In case of the same double electron confinement layers, another series of devices with different profile of EML are fabricated by changing the confinement layers position, the power efficiency and luminance can be improved to 21.7 lm/W (at 103 cd/m2) and 7674 cd/m2 (at 8 V) when the thickness of EML separated by confinement layers increases gradually from the hole injection side to the electron injection side, the driving voltage can also be reduced.

Influence of emissive layer thickness on electrical characteristics of polyfluorene copolymer based polymer light emitting diodes

Polymer light emitting diodes (PLEDs) with a device configuration of ITO/PEDOT:PSS/PFONPN01 [Poly [2,7-(9,9’-dioctylfluorene)-co-N-phenyl-1,8-naphthalimide (99:01)]/LiF/Al have been fabricated by varying the emissive layer (EML) thickness (40/65/80/130 nm) and the influence of EML thickness on the electrical characteristics of PLED has been studied. PLED can be modelled as a simple combination of resistors and capacitors. The impedance spectroscopy analysis showed that the devices with different EML thickness had different values of parallel resistance (RP) and the parallel capacitance (CP). The impedance of the devices is found to increase with increasing EML thickness resulting in an increase in the driving voltage. The device with an emissive layer thickness of 80nm, spin coated from a solution of concentration 15 mg/mL is found to give the best device performance with a maximum brightness value of 5226 cd/m2.

Effects of Recombination Zone Formation on Optical Path Length and Device Performance in Blue Phosphorescent Organic Light-Emitting Diodes with Quantum Well Structure

The effects of location and density in triplet exciton formation on the device performance were systematically investigated by the variation of quantum well (QW) structure in blue phosphorescent organic light-emitting diodes. The hole transporting material of 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) as an interlayer was sandwiched between front emitting layer (EML) and rear EML for the construction of QW structure. The variation of the number of the QW unit induces the different exciton formation in recombination zone (RZ) in which the change of exciton density in the QW structure gives rise to the different optical path length and micro-cavity effect of the EL spectra. As the number of the QW unit increases, the corresponding EL spectrum is blue-shifted. It is originated that the exciton density in the front EML increases and the corresponding optical path length is shorten from the EML to the anode. However, the RZ formation in the single EML without QW structure is widened and red-shifted with increasing the EML thickness because the width of the RZ increases toward to the cathode and the length of optical path is expanded from the anode. In addition, the device having 2 units of QW structure in the EML shows the highest device performance of current efficiency, due to the good charge balance and avoiding the serious triplet exciton quenching by the interlayer. The current efficiency of 11.7 cd/A (from 11.7 to 7.3), power efficiency of 5.5 lm/W (from 5.5 to 3.4), and the operation voltage of 9.7 V at 5000 cd/m2 were presented for 2 unit QW, while the current efficiency of 9.4 cd/A (from 9.4 to 4.7), power efficiency of 5.9 lm/W (from 5.9 to 1.6) and the operation voltage of 7.1 V at 5000 cd/m2 were suggested for 1 unit QW, both showing relatively large efficiency roll-off.

Quantum Yield of Reflection Mode Varied Doping GaN Photocathode

Using the NEA photocathode activation and evaluation experiment system, the varied doping GaN photocathode has been activated and evaluated. According to the diffusion and orientation drifting equation, the quantum yield formula of reflection mode varied doping NEA GaN photocathode was gotten. The factors affecting the quantum efficiency of varied doping GaN photocathode were studied. For the varied doping GaN photocathode, the quantum efficiency is mainly decided by the escape probability of electron P, he absorption coefficient α, the electron diffuse length L D , the reflectance of cathode materials for incident light R, emission layer thickness Te and the inside electric field E . The experiment and analysis results show: With the directional inside electric field in the bulk, the varied doping NEA GaN photocathode has better photoemission performance than uniform doping photocathode.

Confinement of holes and electrons in blue organic light-emitting diodes with additional red emissive layers

We used various emissive layer (EML) structures with ultrathin red EMLs to enhance the charge carrier balance and carrier recombination rate in blue PHOLED devices. These EML materials have different energy gaps between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels. The ultrathin red EMLs, which were inserted in between the blue EMLs, effectively confined the charge carriers in EML, and increased the carrier recombination rate. The thickness of the individual EML was optimized, under 30nm of the total thickness of EML. The blue PHOLEDs with ultrathin red EMLs achieved a luminous efficiency of 19.24cd/A, which was 28.7% higher than those without ultrathin red EMLs, and the maximum external quantum efficiency was 11.81% at 500cd/m2.

Optimization of hybrid blue organic light-emitting diodes based on singlet and triplet exciton diffusion length

The hybrid blue organic light-emitting diodes (HB OLEDs) with triplet harvesting (TH) structures within an emitting layer (EML) are fabricated with fluorescent and phosphorescent EMLs. The TH is to transfer triplet excitons from fluorescence to phosphorescence, where they can decay radiatively. Remarkably, the half-decay lifetime of a hybrid blue device with fluorescent and phosphorescent EML thickness of 5 and 25 nm, measured at an initial luminance of 500 cd/m2, has improved twice than that of using a conventional structure. Additionally, the blue device’s efficiency improved. We attribute this improvement to the efficient triplet excitons energy transfer and the optimized distribution of the EML which depends on singlet and triplet excitons diffusion length that occurs within each the EML.

Green light-emitting diode from bromine based organic-inorganic halide perovskite

Organic-inorganic halide perovskites have attracted considerable attention owing to their outstanding solar cell efficiency. Meanwhile, these halide perovskites exhibit good light emitting in visible and near-infrared range with high fluorescence quantum yield, resulting in electroluminescence. However, it remains challenging for lighting and display due to the low luminance and poor long-term stability. Herein, high performance green light-emitting diodes are fabricated from bromine based perovskite (CH3NH3PbBr3) by systematically adjusting the preparation conditions and optimizing the emitting layer thickness. A high luminance up to 1500 cd m−2 (one of the highest values for perovskites-based light-emitting diodes) was achieved with 80 nm perovskites-emitting layer, due to the well-crystallized, full-coverage property of the films. This result further confirms the great prospect of organic-inorganic perovskites in optoelectronics.

Solution processed multi-color organic light emitting diodes for application in telecommunications

In this work we present an all solution processing scheme for the fabrication of the three primary colors, (R–G–B), emitting organic light-emitting diodes (OLEDs) via efficient color tuning of a blue organic semiconducting (OSC) thin film, in particular the poly[9,9-di-(2′-ethylhexyl)fluorenyl-2,7-diyl] (PF), in which different color fluorescent emitters are dispersed to define the final emitting color and thus to simplify the different color device fabrication. The transmission speed of the fabricated OLEDs was also examined for possible application in interactive telecommunications. To increase the response speed of the different color devices we altered both the device geometry and the electron injection efficiency. To this end we increased the emissive layer thickness and decreased the device emissive area and we also performed engineering of the cathode interfaces through the incorporation of solution processed porphyrin interlayers in order to lower the electron injection barrier height. The final devices exhibited improved operational characteristics and, consequently, modulation speeds.

Exciplex formation and electroluminescent absorption in ultraviolet organic light-emitting diodes**Project supported by the National Natural Science Foundation of China (Grant Nos. 61136003 and 61275041) and the Guangxi Provincial Natural Science Foundation, China (Grant No. 2012GXNSFBA053168).

We investigated the formation of exciplex and electroluminescent absorption in ultraviolet organic light-emitting diodes (UV OLEDs) using different heterojunction structures. It is found that an energy barrier of over 0.3 eV between the emissive layer (EML) and adjacent transport layer facilitates exciplex formation. The electron blocking layer effectively confines electrons in the EML, which contributes to pure UV emission and enhances efficiency. The change in EML thickness generates tunable UV emission from 376 nm to 406 nm. In addition, the UV emission excites low-energy organic function layers and produces photoluminescent emission. In UV OLED, avoiding the exciplex formation and averting light absorption can effectively improve the purity and efficiency. A maximum external quantum efficiency of 1.2% with a UV emission peak of 376 nm is realized.

ИССЛЕДОВАНИЕ ХАРАКТЕРИСТИК ИЗЛУЧЕНИЯ ПРОДУКТОВ СГОРАНИЯ ГАЗОГЕНЕРАТОРА

One of the main tasks of oil producing industry is the increase of producible oil index and the oil-field development rate. In the context of hydrocarbon crude stock reduction and the increasing energy consumption, the issue of hard to recover reserves development occurs and the high-viscosity heavy oil rates are considered to be such reserves. Chemical agents (resolvents) addition, mechanical effects, magnetic and electrostatic fields imposing, and thermal treatment are the basic methods of oil viscosity reduction. The main method of heating and viscosity reduction of hard to recover oil rates is the application of gas producers, which operate using solid fuel. The use of ballistite fuels in gas producers with the possibility of adding various coolants of different concentrations allows regulating temperature and emission characteristics of homogeneous combustion products to the level specified by the recovery methods. The paper presents the methods and the results of heat emission calculation of the combustion products of a gas producer that operates using solid fuel. Water is used as a coolant. The combustion products are represented by the aggregation of monodisperse system of sootflakes and gas phase. Using the method of computing experiment, the authors studied the influence of a coolant concentration (0 %, 10 % and 25 %) on the temperature and spectral and integral emission parameters (fluence rate and emissivity factor) of combustion products at different pressures (0,1 MPa and 7 MPa). It was established that the cooling agents addition influences on the fluence rate value more than the emissivity factor when the combustion products have only gas phase. The growth of soot content causes the increase of spectral and integral emission parameters even when adding agents lowering combustion products temperature. The authors present the characteristic curve of integral parameters on the emitting layer thickness.

Enhanced Hybrid Blue Organic Light Emitting Diodes with a Multi-Emitting Layer Using Fluorescent and Phosphorescent Emitters

We investigated efficient blue organic light emitting diodes (OLEDs) containing fluorescent and phosphorescent emitters in the emitting layer (EML). We fabricated several hybrid blue OLEDs with different thicknesses or switching positions for the fluorescent and the phosphorescent EMLs using the fluorescent deep blue emitter 1,6-Bis(N-phenyl-p-CN-phenylamino)-pyrenes and the phosphorescent blue emitter iridium(III)bis[(4,6-difluorophenyl)-pyridinato-N,C2]picolinate. The fabricated device E stacked a 20 nm thickness of phosphorescent EML and a 10 nm thickness of fluorescent EML. Hybrid blue OLEDs have many advantages in terms of their electrical characteristics, as they have a lower efficiency roll-off than phosphorescent OLEDs and a higher efficiency than fluorescent OLEDs.

Resolution properties of transmission-mode exponential-doping Ga0.37Al0.63As photocathodes.

Using the modulation transfer function obtained by establishing and solving the two-dimensional continuity equation, we have calculated and comparatively analyzed the resolution characteristics of transmission-mode exponential-doping and uniform-doping Ga0.37Al0.63As photocathodes. The calculations show that compared with a uniform-doping Ga0.37Al0.63As photocathode, the exponential-doping structure can significantly improve not only the resolution, but also the quantum efficiency of the photocathode. This improvement is different from the approach for high resolution by reducing the emission layer thickness Te and electron diffusion length LD, or by increasing the recombination velocity of the back-interface, SV, which results in low quantum efficiency. Furthermore, the improvement in resolution and quantum efficiency for the transmission-mode exponential-doping Ga0.37Al0.63As photocathodes is the result of the effect of the built-in electric field on electron transport and lateral diffusion.

Influence of the Emission Layer Thickness on the Optoelectronic Properties of Solution Processed Organic Light-Emitting Diodes

We investigated the optoelectronic properties of solution processed organic light emitting diodes (OLEDs) as a function of their active layer thickness. By using a horizontal dipping technique and by accelerating the coating bar during wet film deposition, we fabricated OLED arrays with different emission layer thicknesses but identical process records in a single process step. The comparison of the optoelectronic device parameters allows for conclusions on injection limitation, the optimization of the layer thickness, and, in conjunction with optical simulations of the weak cavity effect, to promote a deeper understanding of the emission profile. To show the universality of this method, we investigated purely polymeric emitters, blends of polymers and small molecules as well as all-small molecule material systems.