Electroluminescence Enhancement Research Articles

Circularly polarized thermally activated delayed fluorescence OLEDs with nearly BT.2020 red emission

Thermally activated delayed fluorescence (TADF) materials with saturated red emission anda large x CIE coordinate hold significant potential for application in high-efficiency, high-contrast and wide-gamut display techniques. Recently, circularly polarized organic light-emitting diodes (CP-OLEDs) have attracted increasing attention for the enhancement of electroluminescence (EL) performance via minimizing the light loss caused by the integrated polarizer. However, the efficient red CP-OLEDs have been rarely reported by now. In this work, a simple approach to CP-OLEDs towards BT.2020 red emission was applied via the integration of polycyclic aromatic chromophore and chiral octahydrobinaphthol, which consequently resulted in the intense circularly polarized luminescence (CPL), long-wavelength emission and excellent photoluminescence quantum yields (PLQY). The CP-OLEDs based on (R)-OBN-3CN demonstrated the state-of-the-art EL performance with the maximum external quantum efficiencies (EQEmax) of 19.7 % at 646 nm and the dissymmetry factor (gEL) of −1.1 × 10−3. Furthermore, CP-OLEDs doped with 12 wt% (R/S)-OBN-3CN exhibited efficient red electroluminescence with the CIE coordinates of (0.70, 0.30), which was very close to the saturated red emission specified in BT.2020.

Ge0.92Sn0.08/Ge multi-quantum-well LEDs operated at 2-μm-wavelength on a 12-inch Si substrate

The development of an efficient group-IV light source that is compatible with the CMOS process remains a significant goal in Si-based photonics. Recently, the GeSn alloy has been identified as a promising candidate for realizing Si-based light sources. However, previous research suffered from a small wafer size, limiting the throughput and yield. To overcome this challenge, we report the successful growth of GeSn/Ge multiple-quantum-well (MQW) p-i-n LEDs on a 12-inch (300-mm) Si substrate. To the best of our knowledge, this represents the first report of semiconductor LEDs grown on such a large substrate. The MQW LED epitaxial layer is deposited on a 12-inch (300-mm) (001)-oriented intrinsic Si substrate using commercial reduced pressure chemical vapor deposition. To mitigate the detrimental effects of threading dislocation densities on luminescence, the GeSn/Ge is grown pseudomorphically. Owing to the high crystal quality and more directness in the bandgap, enhanced electroluminescence (EL) integrated intensity of 27.58 times is demonstrated compared to the Ge LED. The MQW LEDs exhibit EL emission near 2 μm over a wide operating temperature range of 300 to 450 K, indicating high-temperature stability. This work shows that GeSn/Ge MQW emitters are potential group-IV light sources for large-scale manufacturing.

Silver nanoparticle-induced enhancement of light extraction in two-dimensional light-emitting diodes.

Monolayer transition metal dichalcogenides (TMDCs) with direct bandgaps are considered promising candidates for building light-emitting diodes (LEDs). One crucial indicator of their performance is the brightness of electroluminescence (EL). In this study, we fabricate WS2-based LEDs that make full use of the assistance of effective transient-mode charge injection. By introducing self-assembled silver nanoparticles (NPs) on top of the LED, the extraction efficiency is significantly improved, with a 2.9-fold EL enhancement observed in the experiment. Full-wave simulations further confirm that the improvement comes from the scattering capability of silver NPs, with results qualitatively fitting the experiment. This approach, with its compatibility with van der Waals heterostructures, can be further promoted to enhance the brightness of 2D monolayer TMDC-based LEDs.

Electroluminescence enhancement of ZnO nanorods array determined by Au-nanoparticles position under external electric field

In this work, n-ZnO nanorod arrays (NRA)/MgZnO/p-GaN heterojunction light-emitting diodes (LEDs) were successfully prepared. Au nanoparticles (NPs) were introduced in a simple and stable method to improve ultraviolet electroluminescence efficiency of the ZnO NRA. Although Au NPs are commonly seen as a factor to form localized surface plasmons resonance coupling effect in the visible region, ultraviolet electroluminescence enhancement of ZnO NRA was observed in this work. Importantly, the enhancement performance can be dominated by the position of Au NPs relative to ZnO nanorods driven by the external electric field. Au NPs located at the top of ZnO nanorods promise the additional exciton relaxation channels. As a result, the ZnO ultraviolet near-band-edge (NBE) emission efficiency is improved by 4 times in the Au NPs modified devices. This enhancement process provides a new approach to design efficient UV ZnO-based LEDs.

Control and Enhancement of Single-Molecule Electroluminescence through Strong Light–Matter Coupling

The energetic positions of molecular electronic states at molecule/electrode interfaces are crucial factors for determining the transport and optoelectronic properties of molecular junctions. Strong light-matter coupling offers a potential for manipulating these factors, enabling a boost in the efficiency and versatility of these junctions. Here, we investigate electroluminescence from single-molecule junctions in which the molecule is strongly coupled with the vacuum electromagnetic field in a plasmonic nanocavity. We demonstrate an improvement in the electroluminescence efficiency by employing the strong light-matter coupling in conjunction with the characteristic feature of single-molecule junctions to selectively control the formation of the lowest-energy excited state. The mechanism of efficiency improvement is discussed based on the energetic position and composition of the formed polaritonic states. Our findings indicate the possibility to manipulate optoelectronic conversion in molecular junctions by strong light-matter coupling and contribute to providing design principles for developing efficient molecular optoelectronic devices.

CdS/CdSe/CdS Spherical Quantum Wells with Near-Unity Biexciton Quantum Yield for Light-Emitting-Device Applications

Compared to zero-dimensional (0D) semiconductor quantum dots, 2D semiconductor nanoplatelets (NPLs) offer a spectrally narrow luminescence and superior absorption coefficients, which makes this geometry an attractive candidate for optoelectronic applications. However, optical devices based on NPLs still suffer from nonradiative Auger decay of multiple excitons (MX), which limits the efficiency of the processes, including MX luminescence, electroluminescence, and optical gain. Here, we demonstrate that Auger recombination is strongly suppressed in spherically shaped nanoplatelets, called quantum shells (QSs), where a relaxed confinement of charges leads to diminished exciton–exciton interactions. In particular, we use single photon counting and photon correlation spectroscopy to show that two-dimensional CdS/CdSe/CdS core/shell/shell spherical QSs reach near-unity biexciton emission yield. The Auger suppression was found most prominent in QSs with the largest shell diameter. A combination of ultralong (>15 ns) biexciton (BX) emission lifetimes and strong exciton–exciton repulsion in these QS samples allowed demonstrating low-threshold amplified spontaneous emission (ASE), large modal gain, and microcavity lasing featuring sharp emission modes at the BX and MX transitions. Finally, by introducing QSs within perovskite matrices, we achieved a strong electroluminescence enhancement in light-emitting devices, yielding devices as bright as 213 W/m2 and a 2.3-fold enhancement of the maximum external quantum efficiency. These results represent a major step toward realizing solution-processed colloidal lasers and LEDs.

Sol-gel-derived zinc oxide nanoparticles for electroluminescence enhancement of MEH-PPV OLEDs

The work aims to evaluate the impact of zinc oxide nanoparticles (ZnO-NPs) in the electroluminescence enhancement of organic light-emitting diodes (OLEDs). ZnO-NP was synthesized by using the sol-gel method. XRD, optical, and photoluminescence analysis indicate the average crystallite size of the ZnO-NP is 8.19 nm, exhibits a high energy band gap of 4.15 eV, and high luminescence at 530 nm. OLED performance analysis revealed that the ZnO-NP significantly improves the luminescence brightness and device efficiency by acting as an electron transport and hole-blocking layer, which promotes a high number of electron-hole accumulation in the emissive layer.

Synergistic Enhancement of Luminescent and Ferroelectric Properties through Multi‐Clipping of Tetraphenylethenes

AbstractSynergistically enhancing luminescent and ferroelectric (SELF) properties are observed from a tetraphenylethene (TP) substituted with clipping groups (C), where the C is consisting of a 4‐[3,5‐bis‐(3‐decyloxy‐styryl)‐styryl]‐phenyl (DOS) unit. The DOS units of TPCn are self‐assembled via intermolecular interaction to clip themselves and induce TP aggregation, as evidenced by clip‐induced quenching of emission at DOS units (Eclip) accompanied by aggregation‐induced emission enhancement of TPs (EAIE). TPC4 demonstrates strong photoluminescence in a dilute chloroform solution and large EAIE in aqueous (>50%) THF solution. TPCn demonstrates SELF properties in film state, with high quantum yields of photoluminescence (>80%) and ferroelectric switching. Due to the introduction of four clips, TPC4 has a higher remnant polarization (Pr = 2.27 µC cm−2) at room temperature than TPC1. TPC4 is successfully employed in a light‐emitting electrochemical cell to achieve over 1290 cd m−2 under pulsed current conditions. The TPC4 film on a flexible substrate produced a piezoelectric output voltage of up to 0.13 V and a current density of 1.14 nA cm−2 upon bending. These results indicate that the side chain clipping and TP aggregation resulted in unprecedented flexible SELF properties in a single compound, offering simultaneous enhancement of electroluminescence, mechanical sensitivity, and energy harvesting capacity.

Fabrication of Flexible High‐Temperature Film Thermometers and Heat‐Resistant OLEDs Using Novel Hot Exciton Organic Fluorophores

AbstractThe development of organic thermosensitive fluorophores for use in heat‐resistant organic light emitting diodes (OLEDs) and large‐area and flexible high‐temperature sensing remains challenging due to the susceptibility of such materials to thermally facilitated nonradiative decay. A series of “hot exciton” materials (“C1” and “C2”) based on pyrrole‐substituted triarylphosphine oxides that exhibit high heat resistance have been developed. At a temperature of 260 °C, the films retain 42% (C1) and 29% (C2) of their room temperature fluorescence. This is thanks to thermally facilitated reverse intersystem crossing (RISC) from a high‐lying triplet to a singlet state. By combining the novel fluorophores with a yellow emitter with an extremely large Stokes shift, flexible and large‐area ratiometric film thermometers are fabricated that demonstrate naked‐eye high‐temperature sensing. The relative sensitivity, Sr, of the film thermometer is higher than 1% K–1 in the high‐temperature region (393 to 470 K), with the maximum Sr reaching 1.26% K−1 at 430 K. Using these blue emitters, heat‐resistant cyan and white OLEDs are also fabricated. With thermally populated singlets and nearly 100% exciton harvesting via fast RISC, the C1‐based cyan OLED exhibits a nearly 12‐fold enhancement in electroluminescence on heating from room temperature to 530 K, while the corresponding white OLED displays a 5.7‐fold electroluminescence enhancement.

Efficiency Roll-Off Free Electroluminescence from Monolayer WSe2.

Exciton-exciton annihilation (EEA) is a nonradiative process commonly observed in excitonic materials at high exciton densities. Like Auger recombination, EEA degrades luminescence efficiency at high exciton densities and causes efficiency roll-off in light-emitting devices. Near-unity photoluminescence quantum yield has been demonstrated in transition metal dichalcogenides (TMDCs) at all exciton densities with optimal band structure modification mediated by strain. Although the recombination pathways in TMDCs are well understood, the practical application of light-emitting devices has been challenging. Here, we demonstrate a roll-off free electroluminescence (EL) device composed of TMDC monolayers tunable by strain. We show a 2 orders of magnitude EL enhancement from the WSe2 monolayer by applying a small strain of 0.5%. We attain an internal quantum efficiency of 8% at all injection rates. Finally, we demonstrate transient EL turn-on voltages as small as the band gap. Our approach will contribute to practical applications of roll-off free optoelectronic devices based on excitonic materials.

Single-component exciplex hosts for OLED application

Fluorescent organic light-emitting diode (FOLED) using double-component intermolecular exciplex, i.e., the physical blend of an electron donor (D) and an electron acceptor (A), as host material will suffer from very precise and delicate control on the doping procedure. Herein, we demonstrate that the replacement of double-component intermolecular exciplex host with its single-component counterpart can not only result in simplified device fabrication processes, but also lead to significantly enhanced electroluminescence (EL) performance. The reason is that mono-component exciplex could trigger more effective intermolecular D/A interactions, and hence higher exciton utilization efficiency (ηr: 50% vs. 16%) as well as better film amorphism and thermal stability than its bi-component counterpart. Furthermore, by taking advantage of the efficient TTA process occurring on the pyrene constructive units, a single-component exciplex-hosted FOLED capable of showing quite stable and relatively high ηr in a rather large driving current density range is realized (54%–63%@0.1–550 mA cm−2). This work not only deciphers the superiority of single-component intermolecular exciplexes over their double-component counterparts in FOLED host applications, but also sheds light on the rational design of host materials for acquiring FOLEDs with stable and high ηrs under various current densities.

Carbon dots promote the carrier recombination in Poly (9-vinyl carbazole) to enhance its electroluminescence

Carbon dots (CDs) are one species of multifunctionally auxiliary materials with promising application in photoelectrocatalyst and photoelectrodevices. However, there are few reports on enhancing the electroluminescence (EL) of materials by CDs and the effects of CDs on the charge transport dynamics have not been revealed so far. Herein, CDs are firstly successful to be applied in the enhancement of EL of Poly (9-vinyl carbazole), which is set as model material for research, abbreviated as PVK with the EL wavelength of 432 nm. The brightness of CDs enhanced PVK LEDs reaches 176 cd m−2 that obviously 54.3 % higher than that of pure PVK based LEDs and other parameters are all improved. The Commission Internationale d’Eclairage coordinates of CDs enhanced PVK LEDs are situated at (0.17, 0.10). Notably, through the transient photovoltage technology and method of empirical mode decomposition, the mechanisms of charge transport dynamics between CDs and PVK molecules are determined. This work paves a new direction for the improvement of the luminescent properties of electroluminescent devices and a new mentality to understand the effects of CDs on charge transport in the EL process.

Gap Size-Dependent Plasmonic Enhancement in Electroluminescent Tunnel Junctions

Nanoscale plasmonic structures have been primarily characterized through scattering studies, but electroluminescence offers an exciting alternative from a technological standpoint by removing the need for optical excitation. In sub-nanometer biased junctions, electronic tunneling can serve as the excitation source for plasmon-coupled electroluminescence, but the gap size dependence to this plasmonic enhancement has not been characterized. Here, we simultaneously probe the electroluminescence and conductance of Au tunnel junctions. We find that plasmonic enhancement increases as the gap size is reduced for junctions biased between 1.4 and 1.8 V, consistent with the behavior of charge transfer plasmons. At biases above 1.9 V, we see decreasing plasmonic enhancement with the decreasing gap, showing quenching due to tunneling in remarkable agreement with the trends observed for high energy plasmons in scattering experiments. Critically, we find that plasmonic enhancement of electroluminescence is gap size-dependent and, furthermore, is in agreement with the nature of modes excited by scattering.

Improving the electroluminescence of Si nanocrystal via black silicon and silver surface plasmons

In this paper, black silicon (Si) and silver (Ag) surface plasmons were used to improve the electroluminescence (EL) intensity of silicon nanocrystals (Si-nc), and the mechanism of black Si and Ag nanoparticles enhancing the EL of Si-nc LED is analyzed. The Ag nanoparticles are introduced into Si-nc LED to generate surface plasmons. The surface plasmons radiate the local electromagnetic field, which can increase the excitation probability of Si-nc and increase the EL intensity of Si-nc LED. In addition, black silicon can improve the charge injection efficiency and light emission efficiency. The atomic force microscope (AFM) and reflectivity spectra of black silicon at different etching time, as well as the AFM and absorption spectra of Ag nanoparticles at different postannealing temperatures, were measured. Three different device structures were designed. The EL intensity of Si-nc increased by 3.2 times at the etching time of 5 minutes and increased by 4.9 times at the postannealing temperature of 200 °C, respectively. A maximal 8.5-fold EL enhancement of Si-nc was readily achieved after using the black Si as substrate and incorporating Ag surface plasmons.

Improved Brightness and Color Tunability of Solution-Processed Silicon Quantum Dot Light-Emitting Diodes

Herein, an enhanced electroluminescence (EL) performance of solution-processed silicon quantum dot light-emitting diodes (Si-QLEDs) is demonstrated. We prepared colloidal inks of silicon quantum do...

One-dimensional micro-scale patterned conjugated polymer structures in bilayer architecture and light emitting diode application

To maximize luminescent brightness in organic light-emitting diode (OLED) systems, charge transport dynamics of emissive layer is needed to be controlled. Herein, we pattern aligned micrometer scale conjugated polymer structures using a solution based processing method. Super yellow (SY)-polyethylene oxide (PEO) nanofibers were electrospun using simple uniaxial electrospinning method. A hybrid bilayer of SY-PEO fiber and poly (9,9-dioctylfluorene) derivative PFO film was formed by the sequential deposition of the electrospun fiber and spin-coated film. The maximum brightness of hybrid LED devices was increased over 1.7 fold to that of film-film bilayer device possessing same average film thickness. Besides, optical properties such as absorption/luminescence of SY were modulated by the rapid crystallization and solvent evaporation during electrospinning process compared to the spin-coated film. The electroluminescence enhancement and color control was attributed to balanced charge transport and enhanced Förster resonance energy transfer (FRET). A patterned hybrid bilayer was formed by the combination of the SY-PEO fiber and PFO thin film via solution based sequential deposition methods, including electrospinning and spin-coating. Thanks to the molecular packing and increment of conjugation length in electrospun SY-PEO nanofibers, such nanofibers exhibited distinct optical properties in comparison with SY thin film. Micro-dimensional alignment of conjugated polymer fiber-film bilayer device exhibited superior electroluminescent brightness besides color tunability of constructed LED device. Such enhancements were attributed to balanced charge transport and enhanced Förster resonance energy transfer in fiber-film hybrid emissive bilayer. • Aligned conjugated polymer fibers by electrospinning process. • Electrospun super yellow -polyethylene oxide nanofibers. • One-dimensional conjugated polymer microstructures in fiber-film hybrid bilayer architecture. • Utilization of conjugated polymer nanofibers in light emitting devices.

Evidence of high-temperature exciton condensation in two-dimensional atomic double layers.

A Bose-Einstein condensate is the ground state of a dilute gas of bosons, such as atoms cooled to temperatures close to absolute zero1. With much smaller mass, excitons (bound electron-hole pairs) are expected to condense at considerably higher temperatures2-7. Two-dimensional van der Waals semiconductors with very strong exciton binding are ideal systems for the study of high-temperature exciton condensation. Here we study electrically generated interlayer excitons in MoSe2-WSe2 atomic double layers with a density of up to 1012 excitons per square centimetre. The interlayer tunnelling current depends only on the exciton density, which is indicative of correlated electron-hole pair tunnelling8. Strong electroluminescence arises when a hole tunnels from WSe2 to recombine with an electron in MoSe2. We observe a critical threshold dependence of the electroluminescence intensity on exciton density, accompanied by super-Poissonian photon statistics near the threshold, and a large electroluminescence enhancement with a narrow peak at equal electron and hole densities. The phenomenon persists above 100 kelvin, which is consistent with the predicted critical condensation temperature9-12. Our study provides evidence for interlayer exciton condensation in two-dimensional atomic double layers and opens up opportunities for exploring condensate-based optoelectronics and exciton-mediated high-temperature superconductivity13.

Improved near-UV electroluminescence of ZnO nanorod array LEDs by coupling with a graphene plasmon layer

Abstract The development of short-wavelength light-emitting diodes (LEDs) with high emission efficiency, a fascinating research area, is still necessary because of great scientific interest and practical significance. Here, a graphene plasmon layer treated by oxygen plasma was employed into ZnO nanorod/p-GaN LEDs for a surface plasmon effect. The graphene-decorated heterojunction exhibited an approximately 4-fold improvement of ZnO ultraviolet (UV) electroluminescence (EL) intensity relative to a primitive p-n junction device. Time-resolved spectroscopy and temperature-dependent luminescence measurement indicated that the EL enhancement resulted from the coupling of ZnO excitons with graphene surface plasmons. The current research not only provides an opportunity to construct three-dimensional architecture from a vertical array of one-dimensional nanorods and a two-dimensional graphene layer, but also proposes an effective strategy to improve near-UV emission efficiency in various devices.

Ultra-Violet Electroluminescence of ZnO Nanorods/MEH-PPV Heterojunctions by Optimizing Their Thickness and Using AZO as a Transparent Conductive Electrode

In this paper, a series of ITO/ZnO/ZnO nanorods/MEH-PPV/Al were prepared with different thicknesses of MEH-PPV that were changed from 15, 10 to 7 nm. The electric field in the devices was analyzed. An increase in the electric field on ZnO made hole injection easy and the electrons tunnel fast through thinner MEH-PPV to ZnO. This made the carriers prefer to recombine inside the ZnO layer, and the emission of ZnO was predominant under direct current (DC) bias. Furthermore, another device was fabricated with the structure of AZO (Al-doped ZnO)/ZnO/ZnO nanorods/MEH-PPV/Al. Ultra-violet (UV) electroluminescence (EL) at 387 nm from ZnO band edge emission was realized under DC bias. The turn-on voltage of the devices having AZO as the electrode is lower than that of ITO, and the EL power is enhanced. This work also studies the effect of inserting LiF underneath the Al electrode and above the layer of MEH-PPV. The LiF film inserted caused an obvious decrease in turn-on voltage of the devices and a pronounced increase in the EL power. The mechanism of electroluminescence enhancement is also discussed.

Low driving voltage and enhanced performance of solution-processed blue flexible organic light-emitting diode with PTAA/AgNWs/PTAA composite hole injection layers

The composite hole injection layers (c-HILs) PTAA/AgNWs/PTAA were introduced into solution-processed blue flexible organic light-emitting diode (FOLED) to improve the hole injection ability, further to lower the driving voltage and enhance the device’s performance. By optimizing the c-HILs, the optimal FOLED has been achieved with a maximum luminance of 6051 cd m−2 at 9 V and a maximum current efficiency of 1.76 cd A−1, which are increased by 45% and 75% compared to that of the control device without c-HILs. Meanwhile, the optimal device presents the low turn-on voltage of 2.7 V at 1 cd m−2 and driving voltage of 6.0 V at 1000 cd m−2, which are decreased by 0.4 V and 1.9 V compared to that of the control device. The electroluminescent performance enhancement of FOLED is attributed to that the introduction of c-HILs can decrease the hole injection barrier and improve the hole injection ability, which will result in the balanced carrier recombination in emitting layer. Moreover, we analysed the electrical and morphological characteristics of c-HILs, and investigated the mechanism by energy level analysis and impedance spectroscopy.