Light-emitting Didoes Research Articles

Unravelling the Electrical Field Induced Ion Migration in Flexible OLEDs with PEDOT:PSS Electrodes.

The development of flexible organic light-emitting didoes (FOLEDs) has spurred the research on flexible transparent electrodes (FTEs). Poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) is one of the most attractive FTEs due to its adjustable conductivity and compatibility with low-cost and large-scale solution processing techniques. Significantly, highly efficient FOLEDs have been achieved with modified PEDOT:PSS FTEs. However, the intrinsic mechanisms that contribute to device degradation of FOLEDs utilizing PEDOT:PSS FTEs have not yet been fully elucidated. In this work, three ionic liquids (ILs) are used to enhance the electrical conductivity and mechanical flexibility of PEDOT:PSS FTEs. Simultaneously, the influence of the electric field induced ion migration from PEDOT:PSS FTEs on the operational stability of FOLEDs is unraveled. We find that the ILs with larger ionic radii and higher steric hindrance are beneficial to suppressing the electrical field induced ion migration and improving the operational stability of FOLEDs. Finally, large-area and high-performance FOLEDs are achieved based on the IL of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide modified PEDOT:PSS FTEs, which demonstrate a high current efficiency of 98.1 cd/A and a longer lifetime of 66.7 min. This finding may promote the practical application of PEDOT:PSS FTEs in flexible optoelectronics.

Highly Efficient All-Solution-Processed Quantum Dot Light-Emitting Diodes Using MoOx Nanoparticle Hole Injection Layer.

This paper presents a study that aims to enhance the performance of quantum dot light-emitting didoes (QLEDs) by employing a solution-processed molybdenum oxide (MoOx) nanoparticle (NP) as a hole injection layer (HIL). The study investigates the impact of varying the concentrations of the MoOx NP layer on device characteristics and delves into the underlying mechanisms that contribute to the observed enhancements. Experimental techniques such as an X-ray diffraction and field-emission transmission electron microscopy were employed to confirm the formation of MoOx NPs during the synthesis process. Ultraviolet photoelectron spectroscopy was employed to analyze the electron structure of the QLEDs. Remarkable enhancements in device performance were achieved for the QLED by employing an 8 mg/mL concentration of MoOx nanoparticles. This configuration attains a maximum luminance of 69,240.7 cd/cm2, a maximum current efficiency of 56.0 cd/A, and a maximum external quantum efficiency (EQE) of 13.2%. The obtained results signify notable progress in comparison to those for QLED without HIL, and studies that utilize the widely used poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) HIL. They exhibit a remarkable enhancements of 59.5% and 26.4% in maximum current efficiency, respectively, as well as significant improvements of 42.7% and 20.0% in maximum EQE, respectively. This study opens up new possibilities for the selection of HIL and the fabrication of solution-processed QLEDs, contributing to the potential commercialization of these devices in the future.

Boron-Dipyrromethene-Based Fluorescent Emitters Enable High-Performance Narrowband Red Organic Light-Emitting Didoes.

Narrowband organic light-emitting diodes (OLEDs) are receiving significant attention and have demonstrated impressive performance in blue and green OLEDs. However, developing high-performance narrowband red OLEDs remains a highly desired yet challenging task. Herein, we have developed narrowband red fluorescent emitters by utilizing a boron-dipyrromethene (BODIPY) skeleton in combination with a methyl-shield strategy. These emitters exhibit small full-width at half-maxima (FWHM) ranging from 21 nm (0.068 eV) to 25 nm (0.081 eV) and high photoluminescence quantum yields (ΦPL) ranging from 88.5% to 99.0% in toluene solution. Using BODIPY-based luminescent materials as emitters, high-performance narrowband red OLEDs have been assembled with external quantum efficiency as high as 18.3% at 623 nm and 21.1% at 604 nm. This work represents, to our knowledge, the first successful case of achieving NTSC pure-red OLEDs with the Commission Internationale de l'Éclairage (CIE) coordinates of [0.67, 0.33] based on conventional fluorescent emitters.

Improved out-coupling efficiency of organic light-emitting diodes using micro-sized perovskite crystalline template

In this study, we demonstrated that the out-coupling efficiency of organic light-emitting didoes (OLEDs) is enhanced using external light extraction film obtained from the CH 3 NH 3 PbBr 3 (MAPbBr 3 ) perovskite template. We found that the perovskite crystalline is very suitable to be used as a template because by changing the process conditions such as the solvent type, its micro-scale crystal growth can be achieved. In addition, the perovskite template can be used repeatably, allowing multiple films to be imprinted on one template. By attaching the imprinted polydimethylsiloxane (PDMS) films on the bottom-emitting OLED, we found that the light-emitting performances are improved. The surface morphology of the perovskite template as well as the imprinted PDMS light extraction film were investigated with an optical microscope and a scanning electron microscope (SEM). OLEDs equipped with PDMS light extraction films imprinted on 20 wt% and 30 wt% MAPbBr 3 templates show improved external quantum efficiency by about 30.9% and 11.1%, respectively. It is demonstrated that the transmittance and haze characteristics of films are not significantly changed by imprinting from templates with different concentrations. Rather, the improvement in the out-coupling efficiency of the OLEDs is largely attributed to the fill factor of the crystals of the film and not the light scattering effect from the film. We demonstrated that the out-coupling efficiency of organic light-emitting didoes (OLEDs) is enhanced using external light extraction film obtained from the CH3NH3PbBr3 (MAPbBr3) perovskite template. We found that the perovskite crystalline is very suitable to be used as a template because by changing the process conditions such as the solvent type, its micro-scale crystal growth can be achieved. In addition, the perovskite template can be used repeatably, allowing multiple films to be imprinted on one template. By attaching the imprinted polydimethylsiloxane (PDMS) films on the bottom-emitting OLED, we found that the light-emitting performances are improved. The surface morphology of the perovskite template as well as the imprinted PDMS light extraction film were investigated with an optical microscope and a scanning electron microscope (SEM). OLEDs equipped with PDMS light extraction films imprinted on 20 wt% and 30 wt% MAPbBr3 templates show improved external quantum efficiency by about 30.9% and 11.1%, respectively. It is demonstrated that the transmittance and haze characteristics of films are not significantly changed by imprinting from templates with different concentrations. Rather, the fill factor is reduced when the PDMS light extraction film is imprinted on templates with high concentration. Thus, the improvement in the out-coupling efficiency of the OLEDs is largely attributed to the fill factor of the crystals of the film and not the light scattering effect from the film. The low-cost, and simple fabrication process for the light extraction films pave the way for commercial application in advance OLEDs. • External light-extraction film is obtained with perovskite template. • Micro-scale crystal template is prepared and PDMS is imprinted. • The EQE enhancement of 30.9% is demonstrated. • Fill factor of template is investigated.

Cathode made by silver-precursor ink for all-solution processed quantum dots light-emitting diodes

To realize all-solution processed quantum dots (QDs) light-emitting didoes (QLEDs) with every functional layer processed via wet coating, silver-precursor ink converted cathode is introduced for the first time. One challenge to utilize solution-processed cathode is that the silver-precursor ink reacts with the electron transport layer zinc oxide (ZnO). Another one is that the silver ions permeate through ZnO to quench the QDs' fluorescence. To overcome the challenges, the annealing temperature of ZnO is optimized to 180 °C, while the silver-precursor ink's annealing condition of 180 °C/5 min is selected. As a result, the all-solution processed QLEDs achieve a maximum brightness of 1.32 × 10 4 cd m −2 , a maximum current efficiency of 8.98 cd A −1 , and a maximum external quantum efficiency of 6.92% which reaches 61% of the efficiency of QLEDs with evaporated silver cathode. - Cathode is made by silver-precursor ink to realize all-solution processed QLEDs. - ZnO is annealed at 180 °C to enhance the corrosion resistance. - ZnO can't fully prevents silver ions' permeation from quenching fluorescence. - Silver-precursor ink is converted to silver at high temperature with short time. - Efficiency with solution cathode reach 61% of that with evaporated cathode.

Enhancing the light extraction efficiency for AlGaN-based DUV LEDs with a laterally over-etched p-GaN layer at the top of truncated cones.

It is known that light extraction efficiency (LEE) for AlGaN-based deep ultraviolet light-emitting didoes (DUV LEDs) can be enhanced by using truncated cone arrays with inclined sidewalls. In this work, the air-cavity-shaped inclined sidewall is applied and the p-GaN layer at the top of the truncated cone is laterally over-etched so that more light escape paths are generated for AlGaN-based DUV LEDs. The experimental results manifest that when compared with DUV LEDs only having the air-cavity-shaped inclined sidewall, the optical power for the DUV LEDs with laterally over-etched p-GaN at the top of the truncated cone is enhanced by 30% without sacrificing the forward bias. It is because the over-etched p-GaN makes little effect on the carrier injection and does not affect the ohmic contact resistance. Moreover, the simulation results show that the truncated cone with laterally over-etched p-GaN layer can enhance the LEE because the reduced p-GaN area can suppress the optical absorption and supplies additional light paths for DUV photos. Then, more light will be reflected into escape cones at the sapphire side.

A novel red-emitting Ca2GdHf2Al3O12:Eu3+ phosphor for light-emitting didoes and field emission display

In this work, a novel garnet-based Eu3+-doped Ca2GdHf2Al3O12 phosphor was synthesized through the traditional high temperature solid-state method. The phase purity, fundamental structure of the crystal, morphology, photoluminescence and cathodoluminescence properties have been investigated in detail. Along the temperature rises to 200 °C, the Ca2GdHf2Al3O12:0.08Eu3+ remains about 70.2% according to the initial emission intensity. Under the electron bombardment, the Ca2GdHf2Al3O12:0.08Eu3+ phosphor reveals a remarkably improved performance in the anti-degradation property. All results indicating that the Ca2GdHf2Al3O12:Eu3+ could be regarded as a valuable phosphor in the multiple industrial applications, such as WLEDs and FEDs.

Room temperature synthesis of Tb3+-activated BiF3 green-emitting nanoparticles with high thermal stability for near-ultraviolet white light-emitting didoes

Room temperature synthesis of Tb3+-activated BiF3 green-emitting nanoparticles with high thermal stability for near-ultraviolet white light-emitting didoes

Enhanced flexibility and stability of PEDOT:PSS electrodes through interfacial crosslinking for flexible organic light-emitting diodes

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films have drawn extensive attention as one of the most promising flexible transparent conductive electrodes to replace traditional indium tin oxide. However, some critical issues, such as weak adhesion, vulnerability to moisture and detrimental acidic property, need to be addressed before the practical application and industrialization. Here, we propose a facile and effective strategy of interfacial crosslinking to further improve the flexibility and stability of PEDOT:PSS electrodes with high transparency and conductivity by introducing polyethyleneimine ethoxylated (PEIE) on the surface. The flexibility and stability of PEDOT:PSS electrodes with PEIE overcoating layer are significantly improved, which can be attributed to the interfacial crosslinking reaction between PEIE and PSS. Finally, flexible organic light-emitting didoes (OLEDs) are constructed based on the PEDOT:PSS electrodes modified by PEIE, and current efficiency is enhanced from 20.5 to 76.4 cd/A with a 2.7-fold enhancement, owning to the improved carrier balance. This study confirms that PEIE is effective in protecting the PEDOT:PSS films from mechanical damage and moisture attack, while maintaining the high conductive and transmittance, and illustrates a promising future in low-cost flexible optoelectronic devices employing PEDOT:PSS electrodes. Polyethyleneimine ethoxylated (PEIE) is introduced on the surface of PEDOT:PSS, which improves the flexibility and stability of the highly transparent and conductive PEDOT:PSS electrodes due to the interfacial crosslinking reaction between PEIE and PSS. This approach enhances the luminous efficiency from 20.5 to 76.4 cd/A for flexible organic light-emitting diodes, illustrating a promising future in low-cost flexible optoelectronic devices. • Polyethyleneimine ethoxylated (PEIE) is introduced on the surface of PEDOT:PSS. • The interfacial crosslinking reaction between PEIE and PSS enhanced the flexibility and stability of PEDOT:PSS electrodes. • This approach enhances the luminous efficiency from 20.5 to 76.4 cd/A of flexible organic light-emitting diodes.

Improved device performance of solution-processed red-colored Cu–In–Zn–S-based quantum dot light-emitting diodes enabled by doping TCTA into the emitting layer

The working mechanisms in the cadmium-free Cu–In–Zn–S(CIZS)-based quantum dot light-emitting didoes (QD-LEDs) were reported in previous reports, which were mai nly determined by the direct charge injection or the energy transfer. In this paper, we presented an improved device performance in solution-processed red-colored CIZS-based QD-LEDs by doping a small molecule 4,4′,4″-tris-(carbazol-9-yl)- triphenylamine (TCTA) into the emitting layer, and the best peak current efficiency of 3.22 cd A−1 and maximum luminance (Lmax) of 2679 cd m−2 could be achieved by optimizing the doping percentage of TCTA in the CIZS/ZnS nanocrystals (NCs). Such red-colored QD-LEDs exhibited a 210% increase in the current efficiency and no obvious change in the electroluminescence (EL) spectra as compared with the QD-LEDs without TCTA. The improvement of the device performance could be attributed to the synergistic effects of more balanced hole injection and the effective exciton energy transfer from TCTA to the NCs. The balanced hole injection could be verified by the fact that an increased current efficiency in the QD-LEDs by doping TCTA into an organic hole-transport layer.

Photonic crystal tunnel junction deep ultraviolet light emitting diodes with enhanced light extraction efficiency.

We report on the demonstration of top emitting AlGaN tunnel junction deep ultraviolet (UV) light emitting didoes (LEDs) operating at ∼267 nm. We show, both theoretically and experimentally, that the light extraction efficiency can be enhanced by nearly a factor of two with the incorporation of AlGaN nanowire photonic crystal structures. A peak wall-plug efficiency (WPE) ∼3.5% and external quantum efficiency (EQE) ∼5.4% were measured for AlGaN LEDs directly on-wafer without any packaging. This work demonstrates a viable path for achieving high efficiency deep UV LEDs through the integration of AlGaN planar and nanoscale structures.

Mass Transfer of Microscale Light-Emitting Diodes to Unusual Substrates by Spontaneously Formed Vertical Tethers During Chemical Lift-Off

A much simplified method for transferring Gallium nitride (GaN) light emitting didoes (LEDs) to an unusual substrate, such as glass, Si, polyethylene terephthalate, or polyurethane, was demonstrated with spontaneously formed vertical tethers during chemical lift-off (CLO), without requiring a sacrificial layer or extra process steps. The LED arrays resided on a stamp that was coated with an adhesive layer. After the layer with the LEDs was transferred to the new substrates, the stamp was removed by acetone to complete the preparation. Over 3 × 3 cm2 LED arrays transferred to various substrates without any damage and misorientation. We also found that the optical and electrical characteristics improved after transfer due to decease in built-in stress. This simple and practical method is expected to greatly facilitate the development of transferrable full color GaN microLEDs on various substrates with either greatly reduced or no damage.

A Strategy for Architecture Design of Crystalline Perovskite Light-Emitting Diodes with High Performance.

All present designs of perovskite light-emitting diodes (PeLEDs) stem from polymer light-emitting diodes (PLEDs) or perovskite solar cells. The optimal structure of PeLEDs can be predicted to differ from PLEDs due to the different fluorescence dynamics and crystallization between perovskite and polymer. Herein, a new design strategy and conception is introduced, "insulator-perovskite-insulator" (IPI) architecture tailored to PeLEDs. As examples of FAPbBr3 and MAPbBr3 , it is experimentally shown that the IPI structure effectively induces charge carriers into perovskite crystals, blocks leakage currents via pinholes in the perovskite film, and avoids exciton quenching simultaneously. Consequently, as for FAPbBr3 , a 30-fold enhancement in the current efficiency of IPI-structured PeLEDs compared to a control device with poly(3,4ethylenedioxythiophene):poly(styrene sulfonate) as hole-injection layer is achieved-from 0.64 to 20.3 cd A-1 -while the external quantum efficiency is increased from 0.174% to 5.53%. As the example of CsPbBr3 , compared with the control device, both current efficiency and lifetime of IPI-structured PeLEDs are improved from 1.42 and 4 h to 9.86 cd A-1 and 96 h. This IPI architecture represents a novel strategy for the design of light-emitting didoes based on various perovskites with high efficiencies and stabilities.

Development of an exciplex type mixed host using a pyrrolocarbazole type material for extended lifetime in green phosphorescent organic light-emitting diodes

Abstract Lifetime improvement of green phosphorescent organic light-emitting didoes (PHOLEDs) by an exciplex type host was studied by mixing a hole transport type host and an electron transport type host. A pyrrolocarbazole type material was developed as the hole transport type host and a triazine type material was the electron transport type host. The exciplex type mixed host showed much longer lifetime and improved efficiency compared with each host material constituting the exciplex type mixed host. Hole and electron stability of the exciplex host was proposed as the key factor for the long lifetime of the green phosphorescent device.

A non-catalytic vapor growth regime for organohalide perovskite nanowires using anodic aluminum oxide templates.

In this work, a novel and facile synthesis process to fabricate single crystalline organometal halide perovskite nanowires has been successfully developed. Nanowires were grown in a high density ordered array from metal nanoclusters inside anodic aluminum oxide templates using a non-catalytic chemical vapor deposition method. Specifically, perovskite NWs were grown as a result of the reaction between methylammonium iodide (MAI) and the Pb/Sn (Pb or Sn) metal in anodic aluminum oxide templates under optimal conditions. The characterization results show that there is a reaction zone at the interface between the perovskite material and metal, at the bottom of the anodic aluminum oxide nanochannels. In order to sustain perovskite NW growth, MAI molecules have to diffuse downward through the perovskite NWs to reach the reaction zone. In fact, the reaction is facilitated by the formation of an intermediate product of the metal iodide compound. This suggests that the Pb/Sn metal is converted to PbI2/SnI2 first and then perovskite NWs are formed as a result of the reaction between MAI and PbI2/SnI2 through a vapor-solid-solid process. The optical characterization results demonstrate that the as-synthesized NWs with an ultra-high nanostructure density can serve as ideal candidates for optoelectronic devices, such as solar cells, light-emitting didoes, photodetectors, etc. And the reported growth approach here is highly versatile combining the merits of excellent controllability, cost-effectiveness and tunability on material composition and physical properties.

High Color-Purity Green, Orange, and Red Light-Emitting Diodes Based on Chemically Functionalized Graphene Quantum Dots.

Chemically derived graphene quantum dots (GQDs) to date have showed very broad emission linewidth due to many kinds of chemical bondings with different energy levels, which significantly degrades the color purity and color tunability. Here, we show that use of aniline derivatives to chemically functionalize GQDs generates new extrinsic energy levels that lead to photoluminescence of very narrow linewidths. We use transient absorption and time-resolved photoluminescence spectroscopies to study the electronic structures and related electronic transitions of our GQDs, which reveals that their underlying carrier dynamics is strongly related to the chemical properties of aniline derivatives. Using these functionalized GQDs as lumophores, we fabricate light-emitting didoes (LEDs) that exhibit green, orange, and red electroluminescence that has high color purity. The maximum current efficiency of 3.47 cd A−1 and external quantum efficiency of 1.28% are recorded with our LEDs; these are the highest values ever reported for LEDs based on carbon-nanoparticle phosphors. This functionalization of GQDs with aniline derivatives represents a new method to fabricate LEDs that produce natural color.

Smart Carbon Fiber Bicycle Seat Post with Light and Sensor Integration

The development of high-performance sports equipment is nowadays focused on lightweight structure designs with integrated functions. Even in cycling an increasing number of manufacturers prefer to use fiber reinforced plastics (FRP) in their frames and further bicycle parts. During the manufacturing of these FRP-components it is possible to integrate flat and flexible electronics and sensors such as stitched strain gauges into semi-finished reinforcement textiles.Seat posts made of aluminum with integrated Light Emitting Didoes (LED) for taillight already exist, but the LEDs must be applied subsequent to the manufacturing process, thus damaging the structure to some degree. The new innovative seat post presented here is made of carbon fiber reinforced plastic (CFRP) and contains LED lights as well as sensors for ambient light illuminance and acceleration, which are integrated into the laminate structure during the manufacturing process. In that way a highly functional smart lightweight seat post with fully integrated taillight can be realized. The seat post can be designed with several layers of braided carbon textiles, which can drape around the single LED lights with a minimum influence on the mechanical properties and resulting in a high-quality carbon surface.Due to the integrated sensors, the seat post will detect the motion of the bike and the illuminance of the ambient light to activate the taillight when necessary. The thresholds for acceleration and illuminance can be programmed also using a timer function, so that the lighting stays constant even in discontinuous lighting conditions (for example due to traffic lights at night) but will turn on when necessary and stay off in a daytime ride. A prototype with all integrated sensors and electronics could successfully put into operation.The main innovation and advantage of the new smart and lightweight seat post is the automated activation of the taillight without intervention of the cyclist. This guarantees maximum safety and comfort especially while biking through tunnels or underpasses. A second advantage is the nearly unaffected performance because of minimum additional weight as well as no aerodynamic disadvantages resulting from external lighting, which allows the use of the new seat post in high-performance bicycle disciplines in training and competition.

Effect of nitrogen position of carboline on the device performances of blue phosphorescent organic light-emitting diodes

A δ-carboline derived compound, 5-(3′-(9-carbazolyl)-[1,1′-biphenyl]-3-yl)pyrido[3,2-b]indole, was synthesized as a high triplet energy bipolar host material for blue phosphorescent organic light-emitting didoes and it was compared with α-carboline derived host material with the same backbone structure. The δ-carboline derived host material showed better electron transport properties than the host with α-carboline due to better electron accepting properties. Therefore, the new host material reduced driving voltage and increased the power efficiency of blue phosphorescent organic light-emitting diodes compared to a standard host with α-carboline moiety. A high external quantum efficiency of 25.3% and a high power efficiency of 36.4lm/W were achieved in the blue phosphorescent organic light-emitting diodes.

Reduced junction temperature and enhanced performance of high power light-emitting diodes using reduced graphene oxide pattern

Thermal management has become a crucial area for further development of high-power light-emitting didoes (LEDs) due to the high operating current densities that are required and result in additional joule heating. This increased joule heating negatively affects the performance of the LEDs since it greatly decreases both the optical performance and the lifetime. To circumvent this problem, a reduced graphene oxide (rGO) layer can be inserted to act as a heat spreader. In this study, current–voltage and light-output-current measurements are systematically performed at different temperatures from 30 to 190 °C to investigate the effect that the embedded rGO pattern has on the device performance. At a high temperature and high operating current, the junction temperature (Tj) is 23% lower and the external quantum efficiency (EQE) is 24% higher for the rGO embedded LEDs relative to those of conventional LEDs. In addition, the thermal activation energy of the rGO embedded LEDs exhibits a 30% enhancement as a function of the temperature at a bias of −5 V. This indicates that the rGO pattern plays an essential role in decreasing the junction temperature and results in a favorable performance in terms of the temperature of the high power GaN-based LED junction.

Efficient hybrid white light-emitting diodes by organic-inorganic materials at different CCT from 3000K to 9000K.

The hybrid white light-emitting didoes (LED) with polyfluoren (PFO) polymer and quantum dot (QD) was investigated using dispensing method at the different correlated color temperature (CCT) for cool and warm color temperature. This result indicates that the hybrid white LED device has the higher luminous efficiency than the convention one, which could be attributed to the increased utilization rate of the UV light. Furthermore, the CIE 1931 coordinate of high quality white hybrid LED with different CCT range from 3000K to 9000K is demonstrated. Consequently, the angular-dependent CCT and the thermal issue of the hybrid white LED device were also analyzed in this study.