Emissive Layer Research Articles

Mild oxygen plasma-treated nickel oxides for performance enhancement of hybrid perovskite light-emitting diodes

Nickel oxide (NiOx), a representative transparent p-type semiconductor, has been a promising candidate for hole transport layer (HTL) in perovskite-based photovoltaic and light-emitting diode (LED) devices due to its high chemical stability and high transparency. However, the interface between the NiOx and perovskite emission layers shows a relatively large electrical barrier for the holes to be transported, which degrades the interfacial electrical properties. To overcome these problems, we suggest a mild oxygen plasma treatment on the NiOx surface to introduce Ni vacancies while retaining its advantages. By controllably forming the Ni vacancies, the hole concentrations could be controlled, which modulated the valence band edge level. This increases the work function of NiOx from 4.979 to 5.364 eV, which facilitates the hole transport at the interface between NiOx HTL and the perovskite emission layer. Ultimately, the overall process leads to improved electroluminescence intensity, reduced driving voltage reduction, and low charge trap density at the interface between the NiOx HTL and the perovskite emission layers. These eventually improved the external quantum efficiency of the perovskite light-emitting diodes.

External magnetic field-induced circularly polarized luminescence and electroluminescence from optically inactive thermally activated delayed fluorescence material 4CzIPN.

An achiral optically inactive organic luminophore, 4CzIPN, exhibits circularly polarized thermally activated delayed fluorescence when photoexcited under an external magnetic field. By embedding this luminophore in an active emission layer, an external-magnetic-field-induced circularly polarized electroluminescent device is developed in this study. The Faraday geometry of the applied magnetic field completely controls the direction of rotation of 4CzIPN-derived circularly polarized luminescence and electroluminescence.

Enhanced performance of solution processed OLED devices based on PFO induced TADF emission layers

The internal quantum efficiency (IQE) of the OLED devices based on thermally activated delayed fluorescence (TADF) materials was thought to nearly 100 % by assuming the light outcoupling efficiency of 20–25 %. However, the external quantum efficiency (EQE) is still limited by the ηout. One crucial factor that affects the light output coupling efficiency is the molecular transition dipole moment of the emission molecule. In this study, poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO) was chosen as an orientation inducer to enhance the horizontal orientation ratio of a TADF emitter, 2-Phenyl-4,6-bis[4-(9,9-dimethyl-9,10-dihydroacridine) phenyl] pyrimidine (DMAc-PPM), prepared using a solution method. With doping PFO in weight ratios ranging from 1 wt% to 9 wt%, the orientation factor of TADF emitter increased, which testified the inducing function of PFO. The TADF-OLED devices using PFO as an inducer exhibited an EQE increasing with the PFO doping weight ratio. The maximum EQE of 7.3 % was achieved at the doping weight ratio of 7 wt%, which increase about 1.7 times compared to the device without the PFO inducer. This observed variation of EQE differs from the trend observed in maximum luminance intensity. The increase of EQE can be attributed to the improvement of light output efficiency caused by the variation of orientation factor. The lower carrier mobility lead to the lower maximum luminance intensity. The synergistic effect of light output efficiency and carrier mobility enhanced the performance of OLED devices.

Tuning the Shades of Red Emission in InP/ZnSe/ZnS Nanocrystals with Narrow Full Width for Fabrication of Light-Emitting Diodes.

While Cd-based luminescent nanocrystals (NCs) are the most mature NCs for fabricating efficient red light-emitting diodes (LEDs), their toxicity related limitation is inevitable, making it necessary to find a promising alternative. From this point of view, multishell-coated, red-emissive InP-based NCs are excellent luminescent nanomaterials for use as an emissive layer in electroluminescent (EL) devices. However, due to the presence of oxidation states, they suffer from a wide emission spectrum, which limits their performance. This study uses tris(dimethylamino)phosphine (3DMA-P) as a low-cost aminophosphine precursor and a double HF treatment to suggest an upscaled, cost-effective, and one-pot hot-injection synthesis of purely red-emissive InP-based NCs. The InP core structures were coated with thick layers of ZnSe and ZnS shells to prevent charge delocalization and to create a narrow size distribution. The purified NCs showed an intense emission signal as narrow as 43 nm across the entire red wavelength range (626-670 nm) with an emission quantum efficiency of 74% at 632 nm. The purified samples also showed an emission quantum efficiency of 60% for far-red wavelengths of 670 nm with a narrow full width of 50 nm. The samples showed a relatively long average emission lifetime of 50-70 ns with a biexponential decay profile. To demonstrate the practical ability of the prepared NCs in optoelectronics, we fabricated a red-emissive InP-based LEDs. The best-performing device showed an external quantum efficiency (EQE) of 1.16%, a luminance of 1039 cd m-2, and a current efficiency of 0.88 cd A-1.

Superior white electroluminescent devices using nitrogen-doped carbon dots/TiO2 nanorods heterostructures

Nitrogen-doped carbon dots (NCDs), exhibiting strong yellow emission in aqueous solution and solid matrices, have been utilized for fabricating heterostructure white electroluminescence devices. These devices consist of nitrogen-doped carbon dots as an emissive layer sandwiched between an organic hole transport layer (PEDOT:PSS) and an array of rutile TiO2 nanorods, acting as an electron transport layer. Under an applied forward bias of 5 V, the device exhibits broadband electroluminescence covering the wavelength range of 390–900 nm, resulting in pure white light emission characteristics at room temperature. The result demonstrates the successful fabrication of all solution-processed, low-cost, eco-friendly NCDs-based LEDs with CIE (Commission Internationale d’Éclairage) coordinate of (0.31, 0.34) and color rendering index (CRI) > 90, which are close to ideal white light emission characteristics. The device functionalities are achieved based on defect-related NIR emission from TiO2 nanorods array and visible emission from nitrogen-doped carbon dots. This result paves a new opportunity to develop low-cost, solution-processed nitrogen-doped carbon dots based on warm White light emitting diodes with high CRI for large-area display and lighting applications.

Simultaneously Realizing High Efficiency and High Color Rendering Index for Hybrid White Organic Light-Emitting Diodes by Ultra-Thin Design of Delayed Fluorescence Sensitized Phosphorescent Layers.

In consideration of energy economization and light quality, concurrently attaining high external quantum efficiency (ηext ) and high color rendering index (CRI) is of high significance for the commercialization of hybrid white organic light-emitting diodes (WOLEDs) but is challenging. Herein, a blue luminescent molecule (2PCz-XT) consisting of a xanthone acceptor and two 3,6-diphenylcarbazole donors is prepared, which exhibits strong delayed fluorescence, short delayed fluorescence lifetime, and excellent electroluminescence property, and can sensitize green, orange, and red phosphorescent emitters efficiently. By employing 2PCz-XT as sensitizer and phosphorescent emitters as dopants, efficient two-color and three-color WOLED architectures with ultra-thin phosphorescent emitting layers (EMLs) are proposed and constructed. By incorporating a thin interlayer to modulate exciton recombination zone and reduce exciton loss, high-performance three-color hybrid WOLEDs are finally achieved, providing a high ηext of 26.8% and a high CRI value 83 simultaneously. Further configuration optimization realizes a long device operational lifetime. These WOLEDs with ultra-thin phosphorescent EMLs are among the state-of-the-art hybrid WOLEDs in the literature, demonstrating the success and applicability of the proposed device design for developing robust hybrid WOLEDs with superb efficiency and color quality.

Self-assembled monolayers as hole transport layers for efficient thermally evaporated blue perovskite light-emitting diodes

Perovskite light-emitting diodes (PeLEDs) based on thermal evaporation technique have demonstrated great potential in newly-developed wide color gamut display applications. However, it is still a challenging task to obtain high efficiency thermally evaporated blue PeLEDs. One of the key issues is the lack of desirable functional materials with both hole transport function and perovskite buried interface modification function, which can alleviate the accumulation of high density of deep-level defects between hole transport layer (HTL) and perovskite emitting layer (EML) and weaken the interfacial nonradiative recombination process. In this study, we developed a new multifunctional wide-bandgap self-assembled monolayers (SAMs) suitable for thermally evaporated blue PeLEDs, named (2-(3-bromo-6-(4-formylphenyl)-9H-carbazol-9-yl)ethyl)phosphonic acid (CZPC). The interactions between the carbonyl groups (C = O) contained in CZPC and uncoordinated Pb2+ are critical for improving crystallinity and suppressing nonradiative losses at the buried perovskite-hole transporter interface. In addition, the phosphonic acid groups bonding with indium tin oxide (ITO) can form a self-assembled monolayer to facilitate carrier injection and transport. Finally, simplified thermally evaporated sky-blue (488 nm) PeLEDs utilize CZPC as HTL and no additional functional layers such as hole injection layer are used show excellent device performance with a maximum external quantum efficiency (EQE) of 5.42 %, accompanied by the CIE color coordinates of (0.07, 0.25) and a full width at half maximum (FWHM) of 24 nm. This work lays a foundation for the further exploration of SAMs on high-performance thermally evaporated blue PeLEDs towards ultra-high resolution display applications.

Performance Signature of the Best Candidate-Graded Bandgap Materials for Solar Cells with Steady-State Conversion Efficiency

This is a comprehensive research endeavor focused on enhancing the efficiency of the proposed solar cell design. The integration of the simulation techniques, judicious material selection, and meticulous performance metrics showcase a methodical approach toward creating a solar cell capable of achieving high efficiency across a wide spectrum of light in the AM 1.5 G1 sun solar cell illumination spectrum. Having said this, many researchers are still working on the efficiency potential—based on external radiative efficiency (ERE), open-circuit voltage loss, and fill factor loss—of high-efficiency solar cells. The solar cell is built on aluminum-doped zinc oxide (ZnO) as a transparent conductive oxide layer; aluminum nitride (AlN) as the window layer (emitter); an SWCNT layer as the absorber layer; gallium phosphide (GaP) as the contact layer; and silicon as the substrate. The proposed solar cell transmission, reflection, and absorption relative to the variations in wavelength band spectrum are studied. The conduction and valence band energy diagrams of the solar cell design structure are simulated against the layer thickness variations for the suggested solar cell structure. Short-circuit current density and maximum power variations are clarified versus the bias voltage. Light current density is simulated versus the bias voltage (J/V characteristics curve) of the suggested solar cell design structure. The carrier generation–recombination rate is also simulated by the COMSOL simulation program versus the layer thickness of the suggested solar cell structure. The solar cell circuit design has a fill factor (FF) value of 74.31% and a power conversion efficiency value of 29.91%.

Ligand-induced Cation-Π Interactions Enable High-Efficiency, Bright And Spectrally Stable Rec. 2020 Pure-Red Perovskite Light-Emitting Diodes.

Achieving high-performance perovskite light-emitting diodes (PeLEDs) with pure-red electroluminescence for practical applications remains a critical challenge because of the problematic luminescence property and spectral instability of existing emitters. Herein, we report high-efficiency Rec. 2020 pure-red PeLEDs simultaneously exhibiting exceptional brightness and spectral stability based on CsPb(Br/I)3 perovskite nanocrystals (NCs) capping with aromatic amino acid ligands featuring cation-π interactions. We prove that strong cation-π interactions between the PbI6 -octahedra of perovskite units and the electron-rich indole ring of tryptophan (TRP) molecules not only chemically polish the imperfect surface sites and induce crystal reconstruction of the NCs, but also markedly increase the binding affinity of the ligand molecules, leading to high photoluminescence quantum yields and greatly enhanced spectral stability of the CsPb(Br/I)3 NCs. Moreover, the incorporation of small-size aromatic TRP ligands ensures superior charge transport properties of the assembled emissive layers. The resultant devices emitting at around 635nm demonstrate a champion external quantum efficiency of 22.8%, a max luminance of 12,910cd m-2 and outstanding spectral stability, representing one of the best-performing Rec. 2020 pure-red PeLEDs achieved so far. This article is protected by copyright. All rights reserved.

Realizing high efficiency and luminance in green, yellow and blue organic light emitting diode by deoxyribonucleic acid

Although the effect of the electron blocking layer (EBL) material, deoxyribonucleic acid (DNA), on the electroluminescence (EL) performance of organic light-emitting diodes (OLEDs) has been studied, the process of DNA regulation of exciton recombination region within the device is still unclear. Herein, devices with and without EBL were fabricated using different DNA spin-coating speeds, and the photoelectric performance of device were measured. By using DNA compounded with cetyltrimethyl ammonium (CTMA) as the EBL and hole buffer layer, so-called BioLEDs. The DNA-based green Alq3 BioLEDs achieve higher luminance (39 000 cd m−2) and higher current efficiency (8.4 cd A−1), which are increased by 49% and 54%, respectively, compared to the reference OLEDs without the addition of DNA. Similarly, the maximum luminance and efficiency of yellow Rubrene BioLEDs is increased by 64% (from 12 120 to 19 820 cd m−2) and 74% (from 1.36 to 2.36 cd A−1), the luminance and efficiency of blue TCTA BioLEDs is increased by 101% and 245%. Specifically, we found that as the thickness of DNA-CTMA increases, the exciton recombination region moves towards the interface between the emitting layer (EML) and the hole transport layer (HTL). By better confining excitons within the EML, the current efficiency of the BioLEDs is effectively improved. Accordingly, we provide a possible idea for achieve high performance DNA-based BioLEDs by adding DNA-CTMA EBL and hole buffer layers. Meanwhile, as the DNA thickness increases, the exciton recombination region moves towards the EML/HTL interface, thereby enhancing the efficiency of the DNA-based BioLEDs.

Phosphorescent Dye Sensitized Quantum-Dot Light-Emitting Diodes with 37% External Quantum Efficiency.

Exciton harvesting is of paramount importance for quantum-dot light-emitting diodes (QLEDs). Direct exciton harvesting by quantum dots (QDs) emitting layer suffers from poor hole injection due to the low conduction bands and valence bands of QDs, leading to unbalanced electron-hole injection and recombination. To address this issue, here we report an exciton sensitizing approach, where excitons form on a phosphorescent dye doped layer which then transfer their energies to adjacent QDs layer for photon emission. Due to the very efficient exciton formation and energy transfer processes, higher device performance could be achieved. To demonstrate the above strategy, red QLEDs with a phosphorescent dye Iridium (III) bis (2 - methyldibenzo - [f, h] quinoxaline) (acetylacetonate), Ir(MDQ)2 (acac) doped hole transporting layer were fabricated and studied. At a doping concentration of 10wt.%, the best device achieves record high current efficiency, power efficiency and external quantum efficiency of 37.3cd A-1 , 41.0lm W-1 and 37.0%, respectively. Simultaneously, the efficiency roll-off characteristic was greatly improved that 35.0% EQE can be well remained at high luminance level of 450 000cd m-2 . Moreover, the devices also exhibit good stability and reproducibility. This article is protected by copyright. All rights reserved.

Solution Processed Light Emitting Diode Based on InP Quantum Dots with Hybrid Emissive Layer

Solution-processable Cadmium-based Quantum dots light emitting diodes (QLEDs) are regarded as a good candidate for good performance display devices due to excellent color purity and inexpensive production. However, the toxicity still poses a severe risk. Although the most effective cadmium-free substance is thought to be InP for producing efficient QLEDs, its efficiency falls far short of its cadmium-based counterparts. We made a homogeneous thin film by mixing the organic compound 4, 4-bis (N-carbazyle)-1, 1-biphenyl (CBP) with InP/ZnSe quantum dots (QDs). Then, in our QLED design, we utilized this film as an emissive layer. The new QDs-based device showed a higher luminance of 14600 cd/m2 and having higher external quantum efficiency (EQE) of 11.6%. We discovered that the mixed QDs without any phase separation had a consistent distribution of CBP, which facilitated transfer of energy to QDs and injection of holes into the emissive layer. The blended QD was also shown to restrict injection of electrons into the hole transport/injection layers, protecting the device’s structural integrity. Developing a balanced charge in stable optoelectronic devices densities has a high potential when a homogeneous QD layer is combined with an effective charge transport material.

Origins of the improved device performance in solution-processed TADF organic light-emitting devices with a polymer underneath layer

In solution-processed organic light-emitting devices (OLEDs), a polymer interface layer underneath the emission layer is essential to improving the device's performance. The typical polymer is Poly (9-vinylcarbazole) (PVK), which can dramatically increase the device efficiency whereas the mechanisms are not well understood. In this work, we explored the related mechanisms by smart experimental design and measurements. The results reveal that in addition to reducing the hole current density and balancing the carrier distribution, the PVK underneath layer also improves the film morphology and enhances the horizontal orientation of the transition dipole moment of the thermally activated delayed fluorescence (TADF) emitters. Consequently, the device's efficiency and stability are notably increased by about three times. This work provides insights into the origins of improved device performance in solution-processed TADF OLEDs with a polymer underneath layer and also highlights the effect of the underneath layer on the film formation of the solution-processed emission layer.

INVESTIGATING THE EFFECTS OF SOLVENT AND CONCENTRATION OF ZnO NANOPARTICLES ON THE PROPERTIES OF GREEN-QDLEDS FOR DISPLAY APPLICATIONS

Colloidal quantum-dot-based green light-emitting diodes (QD-LEDs) are attractive for use in display devices because of the remarkable electrical and optical characteristics of quantum dot materials. In this study, ZnO NPs were synthesized as the carrier transporting layer using a solution-mediated process. After that, all solution- multilayer QD-LEDs with a structure of ITO-Glass/PEDOT:PSS/PVK/Green-QDs/ZnO/Al/Encap was fabricated. The effect of the solvent on the emission layer was investigated, along with the effect of the ZnO NPs concentration of the electron transfer layer. The regular green-QDLED device showed a maximum luminance of 9865 cd/m2, a luminance efficiency of 13.84 cd/A, and an EQE of 3.68%, at a turn-on voltage at 6.0 V.

An In-Depth Analysis of Variation in Characteristic Performance of OLED with Respect to Position of Charge Generation Layer

In this paper, a high performance blue organic light emitting diode having a charge generation layer (CGL) is proposed and compared with the other five CGL and non-CGL based devices. The utilized CGL layer in the different structure consists of two materials; HAT-CN (hexaazatriphenylene-hexacarbonitrile) and TAPC (1,1-bis[(di-4-tolyamino)phenyl)]cyclohexane,) for electrons and holes generation, correspondingly. In the proposed novel structure, the CGL layer is incorporated outside of the emissive layer (EML) which significantly enhances the device performance in terms of current and luminescence. The device exhibits luminescence and current values as 3636.3 cd m−2 and 0.44 A, respectively. Furthermore, this paper represents in-depth internal analysis of the six devices (D1-D6). This analysis is provided by drawing horizontal and vertical cutlines inside the devices. The proposed device is analysed and compared with other mentioned devices in terms of several parameters such as Langevin recombination rate, electron concentration, hole concentration, band energy, total current density, electron affinity, hole QFL (quasi-Fermi level), conduction current density, potential distribution and electron/hole mobility. In comparison with D1, D2, D3, D4 and D5, the current of the proposed device (D6) is about 16.9, 2.2, 1.7, 3 and 1.6 times improved, correspondingly. Moreover, structural analysis is also included to understand the performance of the devices more precisely.

Recent research of materials for emissive layer of OLED

As time has gone on, a lot of changes in display technology have taken place. Organic light-emitting diode (OLED) is the one of the many types of display technology. They have attracted many interests in the academic filed and commercial worlds, and developed rapidly since their earliest introduction. They are now highly regarded and used in the display and lighting industries due to a number of promising characteristics, such as environmentally friendly, high energy efficiency, large area, light weight, and thin shape. Among various light emitting materials, polymers have higher glass transition temperature. Therefore, polymer-based OLEDs exhibit higher stability and have received wide attention. The development of OLED devices can be divided into three main generations of emitter materials based on the technique of the emissive layer of OLEDs. The first generation of OLED is fluorescent OLED (FOLED), the second generation is phosphorescent OLED (POLED), and the third generation is thermally activated delayed fluorescence (TAFD). In this article, device structure, mechanism and development of polymer-based OLEDs are introduced. Specifically, working principles and materials of the three generations are presented with a focus on TADF-based OLEDs. In addition, their device performances including quantum efficiency and lifetime are compared quantitatively.

Dual-functionalized poly(ethylene oxide) for interface modification and addition of rubidium-doped CsPbBr3 perovskite composite emission layer to enhance brightness of perovskite light-emitting diodes

Metal halide perovskite as an emitter has extremely high color purity, but the generally poor morphology of the perovskite layer restricts the performance of the emitter. The additive of a suitable polymer can greatly boost the uniformity of the spin-coated perovskite film. The all-inorganic perovskite layer deposited by the solution method has a multi-crystalline structure. Besides, there are a lot of defects and grain boundaries on the surface, and the carriers will undergo non-radiative recombination at these defects. In this study, Cs1-xRbxPbBr3 was prepared by introducing Rb+ cations with smaller ionic radius to partially replace Cs+ cations, and combined with poly(ethylene oxide) (PEO) and (1,3,5-benzinetriyl)tris(1-phenyl-1-H-benzimidazole) (TPBi) blended composite film as the light-emitting layer, that can increase the film coverage and radiative recombination efficiency of the perovskite film. The polymer PEO was introduced as a buffer layer deposited on the poly-TPD hole transport layer (HTL) to to enhance the hydrophilicity and passivate the poly-TPD HTL/perovskites interface in order to obtain high-efficiency perovskite light-emitting diodes. By adjusting the concentration of the PEO buffer layer, the champion luminance and external quantum efficiency are 25650 cd/m2 and 0.844 %, respectively.

Reduced Toxicity and Aqueous-Based Solvent Using Phosphoric Acid as the Dopant Source for Formation Emitter Layer of Silicon Solar Cells

Reduced Toxicity and Aqueous-Based Solvent Using Phosphoric Acid as the Dopant Source for Formation Emitter Layer of Silicon Solar Cells

High‐Performance Blue Perovskite Films and Micro‐Arrays for Light‐Emitting Diodes with Ionic Liquid Interlayer

AbstractMetal halide perovskites have attracted considerable attention for light‐emitting diode (LED) applications due to their desirable optoelectronic properties including high brightness and color purity. However, the performance of blue perovskite LEDs (PeLEDs) remains inferior to their red and green counterparts. Herein, an ionic liquid (IL), specifically 1‐butyl‐3‐methylimidazolium tetrafluoroborate is introduced as the interlayer on the hole transport layer (HTL). This IL demonstrates a strong interaction with the perovskite emissive layer, resulting in effective defect passivation and a shallower valence band maximum. Consequently, nonradiative recombination is reduced, and hole injection is enhanced. Additionally, a soft lithography method employing a transfer process is successfully developed that enables precise micropatterning of the perovskite light‐emitting layer. Through these advancements, the IL‐modified PeLED exhibits pure blue emission at 470 nm with a maximum luminance of 891 cd m−2 and an impressive maximum EQE of 8.3%. Furthermore, the micro PeLED with an IL interlayer achieves a maximum luminance of 400 cd m−2 and a maximum EQE of 3.9%.

Interfacing Lanthanide Metal-Organic Frameworks with ZnO Nanowires for Alternating Current Electroluminescence.

Alternating current electroluminescence (ACEL) devices are attractive candidates in cost-effective lighting, sensing, and flexible displays due to their uniform luminescence, stable performance, and outstanding deformability. However, ACEL devices have suffered from limited options for the light-emitting layer, which presents a significant constraint in the progress of utilizing ACEL. Herein, a new class of ACEL phosphors based on lanthanide metal-organic frameworks (Ln-MOFs) is devised. A synthesis of lanthanide-benzenetricarboxylate (Ln-BTC) thin film on a brass grid substrate seeded with ZnO nanowires (NWs) as anchors is developed. The as-synthesized Ln-BTC thin film is employed as the emissive layer and shows visible electroluminescence driven by alternating current (2.9V µm-1 , 1kHz) for the first time. Mechanistic investigations reveal that the Ln-based ACEL stems from impact excitation by accelerated electrons from ZnO NWs. Fine-tuning of the ACEL color is also demonstrated by controlling the Ln-MOF compositions and introducing an extra ZnS emitting layer. The advances in these optical materials expand the application of ACEL devices in anti-counterfeiting.