Efficient Light-emitting Diodes Research Articles

Thermally Induced Phase Separation of the PEDOT:PSS Layer for Highly Efficient Laminated Polymer Light-Emitting Diodes.

Among various conductive polymers, the poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) film has been studied as a promising material for use as a transparent electrode and a hole-injecting layer in organic optoelectronic devices. Due to the increasing demand for the low-cost fabrication of organic light-emitting diodes (OLEDs), PEDOT:PSS has been employed as the top electrode by using the coating or lamination method. Herein, a facile method is reported for the fabrication of highly efficient polymer light-emitting diodes (PLEDs) based on a laminated transparent electrode (LTE) consisting of successive PEDOT:PSS and silver-nanowire (AgNW) layers. In particular, thermally induced phase separation (TIPS) of the PEDOT:PSS film is found to depend on the annealing temperature (Tanneal) during preparation of the LTE. At Tanneal close to the glass transition temperature of the PSS chains, a PSS-rich phase with a large number of PSS- molecules enhances the work function of the PEDOT:PSS on the glass-side surface relative to the air side. By using the optimized LTEs, bidirectional laminated PLEDs are obtained with a total external quantum efficiency of 2.9% and a turn-on voltage of 2.6 V, giving a comparable performance to that of the reference bottom-emitting PLED based on a costly evaporated metal electrode. In addition, an analysis of the angular characteristics, including the variation in the electroluminescence spectra and the change in luminance according to the emission angle, indicates that the laminated PLED with the LTE provides a more uniform angular distribution regardless of the direction of emission. Detailed optical and electrical analyses are also performed to evaluate the suitability of LTEs for the low-cost fabrication of efficient PLEDs.

Efficient green light-emitting diodes based on alloy quantum dots with an organic/inorganic hybrid surface passivation

As we know, the ligands on the surface of core/shell structured quantum dots (QDs) play an important role. They not only bind with metal ions to passivate surface defects but also have a great influence on colloidal stability. All-organic or all-inorganic ligands afford QDs with some disadvantages such as low conductivity, poor dispersibility and undesirable stability. By combining with the advantages of organic and inorganic ligands, herein, we achieve an organic/inorganic hybrid surface passivation through exchanging partial oleic acid ligands with chlorine ions (Cl−), affording QD with a core/shell structure of CdZnSe/ZnSe/ZnS/OACl. It exhibits an excellent nanostructure with a mean size of around 11.6 nm. According to X-ray photoelectron spectroscopy (XPS) spectra, around 30% oleid acid ligands are removed by Cl−, resulting in the hybrid surface passivation. The CdZnSe/ZnSe/ZnS/OACl QD exhibit an efficient green emission at the wavelength of around 535 nm with a quantum yield of over 90%. The QD light-emitting diode (QLED) based on CdZnSe/ZnSe/ZnS/OACl exhibits a peak EQE of 15.6% and a brightness of over 1.1 × 105 cd m−2 at 6 V, corresponding to an efficient green emitting device. Therefore, we believe that the hybrid surface passivation could provid a reliable avenue to efficient QD materials.

Vertically Concentrated Quantum Wells Enabling Highly Efficient Deep-Blue Perovskite Light-Emitting Diodes.

Deep-blue perovskite light-emitting diodes (PeLEDs) based on quasi-two-dimensional (quasi-2D) systems exist heightened sensitivity to the domain distribution. The top-down crystallization mode will lead to a vertical gradient distribution of quantum well (QW) structure, which is unfavorable for deep-blue emission. Herein, a thermal gradient annealing treatment is proposed to address the polydispersity issue of vertical QWs in quasi-2D perovskites. The formation of large-n domains at the upper interface of the perovskite film can be effectively inhibited by introducing a low-temperature source in the annealing process. Combined with the utilization of NaBr to inhibit the undesirable n=1 domain, a vertically concentrated QW structure is ultimately attained. As a result, the fabricated device delivers a narrow and stable deep-blue emission at 458 nm with an impressive external quantum efficiency (EQE) of 5.82 %. Green and sky-blue PeLEDs with remarkable EQE of 21.83 % and 17.51 % are also successfully achieved, respectively, by using the same strategy. The findings provide a universal strategy across the entire quasi-2D perovskites, paving the way for future practical application of PeLEDs.

Nanosurface-reconstructed perovskite for highly efficient and stable active-matrix light-emitting diode display.

Perovskite quantum dots (QDs) are promising for various photonic applications due to their high colour purity, tunable optoelectronic properties and excellent solution processability. Surface features impact their optoelectronic properties, and surface defects remain a major obstacle to progress. Here we develop a strategy utilizing diisooctylphosphinic acid-mediated synthesis combined with hydriodic acid-etching-driven nanosurface reconstruction to stabilize CsPbI3 QDs. Diisooctylphosphinic acid strongly adsorbs to the QDs and increases the formation energy of halide vacancies, enabling nanosurface reconstruction. The QD film with nanosurface reconstruction shows enhanced phase stability, improved photoluminescence endurance under thermal stress and electric field conditions, and a higher activation energy for ion migration. Consequently, we demonstrate perovskite light-emitting diodes (LEDs) that feature an electroluminescence peak at 644 nm. These LEDs achieve an external quantum efficiency of 28.5% and an operational half-lifetime surpassing 30 h at an initial luminance of 100 cd m-2, marking a tenfold improvement over previously published studies. The integration of these high-performance LEDs with specifically designed thin-film transistor circuits enables the demonstration of solution-processed active-matrix perovskite displays that show a peak external quantum efficiency of 23.6% at a display brightness of 300 cd m-2. This work showcases nanosurface reconstruction as a pivotal pathway towards high-performance QD-based optoelectronic devices.

Ca2+ induced highly fluorescent CsPb(Br/Cl)3 perovskite quantum dots via fast Anion-Exchange & Cation-Doping Inter-Promotion strategy for efficient deep-blue light-emitting diodes

Despite the impressive development of perovskite light-emitting diodes (PeLEDs), it is still challenging to achieve high-efficiency deep-blue PeLEDs using colloid perovskite quantum dots (PQDs). The efficiency of PQDs with a wavelength below 460 nm, which meets the requirements for deep-blue emission in the Telecommunication Union UHD television standard (ITU REC. 2020), lags far behind those of their sky-blue counterparts. To address this issue, a novel strategy of fast anion-exchange & cation-doping inter-promotion (FAECDIP) is proposed to achieve highly efficient deep-blue PQDs by introducing CaBr2 into the CsPbCl3 PQDs. Owing to the presence of Ca2+, the speed of ion exchange is increased, driven by the smaller cation, Ca2+, improving the preparation efficiency. Additionally, Ca2+ was doped on the surface of PQDs. Based on studies of fast anion-exchange and theoretical calculations, Ca2+ improves the optical performance by decreasing the number of traps and increasing the crystallinity of target PQDs, facilitating the stability of treated films and PeLEDs by enhancing the formation energy of halogen vacancies. Here, a high PLQY of 80.3 % CaBr2-induced CsPb(Cl/Br)3 deep-blue PQDs (∼446 nm) was achieved. The correspondent PeLEDs (∼447 nm) achieved a superior EQE of 5.88 %, which is the state-of-the-art among the reported deep-blue PeLEDs. Our strategy provides a potential route to achieve deep-blue PeLEDs, which differs from the previous tedious-complex methods.

Enhanced efficiency of deep ultraviolet light-emitting diodes utilizing full-coverage Al reflector and highly reflective Ni/Rh p-electrode

Increasing the reflection of p-side is an effective way to improve the optoelectronic performance of flip-chip light-emitting diodes (FCLEDs). Here, we propose a full-coverage Al reflector (FAR) and a highly reflective Ni/Rh p-electrode to enhance the performance of deep ultraviolet (DUV) FCLEDs. The physical mechanism for the impact of the FAR and Ni/Rh electrode on the light extraction efficiency (LEE) is discussed theoretically. Simulations demonstrate that the combination of the FAR and Ni/Rh electrode improves the LEEs of transverse electric- and transverse magnetic-polarized light by 13.62% and 27.08%, respectively. At an injection current of 100 mA, the fabricated DUV FCLEDs with FAR and Ni/Rh electrode exhibits an external quantum efficiency of 4.01% and a wall plugging efficiency of 2.92%, which are 16.85% and 13.18% higher than those of conventional DUV FCLEDs, respectively. These results support the promise of the FAR and Ni/Rh electrode for high-power DUV LED applications.

Regulating Surface-Passivator Binding Priority for Efficient Perovskite Light-Emitting Diodes.

Suppressing trap-assisted nonradiative losses through passivators is a prerequisite for efficient perovskite light-emitting diodes (PeLEDs). However, the complex bonding between passivators and perovskites severely suppresses the passivation process, which still lacks comprehensive understanding. Herein, the number, category, and degree of bonds between different functional groups and the perovskite are quantitatively assessed to study the passivation dynamics. Functional groups with high electrostatic potential and large steric hindrance prioritize strong bonding with organic cations and halides on the perfect surface, leading to suppressed coordination with bulky defects. By modulating the binding priorities and coordination capacity, hindrance from the intense interaction with perfect perovskite is significantly reduced, leading to a more direct passivation process. Consequently, the near-infrared PeLED without external light out-coupling demonstrates a record external quantum efficiency of 24.3% at a current density of 42mA cm-2. In addition, the device exhibits a record-level-cycle ON/OFF switching of 20000 and ultralong half-lifetime of 1126.3h under 5mA cm-2. An in-depth understanding of the passivators can offer new insights into the development of high-performance PeLEDs.

Strategically constructed AlGaN doping barriers for efficient deep ultraviolet light-emitting diodes.

Here, we propose a sandwich-like Si-doping scheme (undoped/Si-doped/undoped) in Al0.6Ga0.4N quantum barriers (QBs) to simultaneously promote the optoelectronic performances and reliability of deep ultraviolet light-emitting diodes (DUV-LEDs). Through experimental and numerical analyses, in the case of DUV-LEDs with conventional uniform Si-doping QB structure, severe operation-induced reliability degradation, including the increase of reverse leakage current (IR) and reduction of light output power (LOP), will offset the enhancement of optoelectronic performances as the Si-doping levels increase to an extent, which hinders further development of DUV-LEDs. According to a transmission electron microscope characterization and a numerical simulation, an improved interfacial quality in multiple quantum wells (MQWs) and more uniform carrier distribution within MQWs are demonstrated for our proposed Si-doping structure in comparison to the uniform Si-doping structure. Consequently, the proposed DUV-LED shows superior wall-plug efficiency (4%), IR at -6 V reduced by almost one order of magnitude, and slower LOP degradation after 168-h 100 mA-current-stress operation. This feasible doping scheme provides a promising strategy for the high-efficiency and cost-competitive DUV-LEDs.

Onion-like multicolor thermally activated delayed fluorescent carbon quantum dots for efficient electroluminescent light-emitting diodes

Carbon quantum dots are emerging as promising nanomaterials for next-generation displays. The elaborate structural design is crucial for achieving thermally activated delayed fluorescence, particularly for improving external quantum efficiency of electroluminescent light-emitting diodes. Here, we report the synthesis of onion-like multicolor thermally activated delayed fluorescence carbon quantum dots with quantum yields of 42.3–61.0%. Structural, spectroscopic characterization and computational studies reveal that onion-like structures assembled from monomer carbon quantum dots of different sizes account for the decreased singlet-triplet energy gap, thereby achieving efficient multicolor thermally activated delayed fluorescence. The devices exhibit maximum luminances of 3785–7550 cd m−2 and maximum external quantum efficiency of 6.0–9.9%. Importantly, owing to the weak van der Waals interactions and adequate solution processability, flexible devices with a maximum luminance of 2554 cd m−2 are realized. These findings facilitate the development of high-performance carbon quantum dots-based electroluminescent light-emitting diodes that are promising for practical applications.

Formation of homogeneous pyridinic emission states in carbon dots via solid-state carbonization of aminoquinoline: Optical design principles and light-emitting diode applications

Developing carbon dots (CDs) for practical applications faces significant hurdles due to non-homogeneity in their chemical structure, leading to broad spectral distribution and the excitation-energy-dependent behavior of their fluorescence (FL) emission properties. To address this challenge, we implemented solid-state carbonization of 6-aminoquinoline (6-AQ) devoid of any additives that could induce chemical non-homogeneity. This process yielded pyridinic-nitrogen-doped CDs (PN-CDs) characterized by homogenous, uniform emission states arising from pyridine-incorporated heteroatomic structures. Notably, these PN-CDs exhibited excitation-energy-independent FL at 520 nm with a full-width-half-maximum of 70 nm. To elucidate the structure-property relationship in PN-CDs, a comprehensive analysis encompassing time-resolved spectroscopy and ab initio calculations was conducted. Furthermore, we performed a comparative study by manipulating synthesis parameters and utilizing six AQ isomers, offering valuable insights to optimize the optical properties of PN-CDs. Finally, we successfully demonstrated high-color-purity and efficient green light-emitting diodes utilizing PN-CDs, highlighting their potential for future display applications. This paves the way for advancement in optical materials with enhanced performance and reduced environmental impact.

Efficient non-doped cluster light-emitting diodes based on semiconducting copper iodide hybrids

Copper iodide hybrid clusters have attracted lots of interest in solution-processed light-emitting diodes (LEDs) for solid-state lighting and display in recent years, benefiting from their simple synthesis, structural diversity, high photoluminescence quantum yield (PLQY), and non-toxicity. However, most efficient copper iodide cluster LEDs are made by host-guest doped emitters and thus suffer from cumbersome synthesis and increased costs from the expensive hosts. Here, we report a non-doped cluster LED based on a novel copper iodine hybrid [35DPPy]4Cu2I2 (35DPPy: 3,5-diphenylpyridine) with high PLQY and decent charge transport characteristic. The non-doped [35DPPy]4Cu2I2 film synthesized by a one-step spin-coating approach possess the flat and compact morphology, high PLQY of 59.6%, and excellent heat/ultraviolet (UV) light/moisture/oxygen stability. The efficient green PL of [35DPPy]4Cu2I2 is proved to originate from the synergistic contribution from thermally activated delayed fluorescence and phosphorescence. A non-doped green LED based on [35DPPy]4Cu2I2 emitters is fabricated, achieving a peak external quantum efficiency of 4.8% and a maximum luminance of 3895 cd/m2. More importantly, large-area LEDs with an emitting area of 4.00 cm2 are realized with homogeneous and bright electroluminescence, showing their attractive potentials as novel electroluminescent emitters in the application of non-doped cluster LEDs.

High-Efficiency Pure-Red CsPbI3 Quantum Dot Light-Emitting Diodes Enabled by Strongly Electrostatic Potential Solvent and Sequential Ligand Post-treatment Process.

Efficient pure-red emission light-emitting diodes (LEDs) are essential for high-definition displays, yet achieving pure-red emission is hindered by challenges like phase segregation and spectral instability when using halide mixing. Additionally, strongly confined quantum dots (QDs) produced through traditional hot-injection methods face byproduct contamination due to poor solubility of metal halide salts in the solvent octadecene (ODE) at low temperatures. Herein, we introduced a novel method using a benzene-series strongly electrostatic potential solvent instead of ODE to prevent PbI2 intermediates and promote their dissolution into [PbI3]-. Increasing methyl groups on benzene yields precisely sized (4.4 ± 0.1 nm) CsPbI3 QDs with exceptional properties: a narrow 630 nm PL peak with photoluminescence quantum yield (PLQY) of 97%. Sequential ligand post-treatment optimizes optical and electrical performance of QDs. PeLEDs based on optimized QDs achieve pure-red EL (CIE: 0.700, 0.290) approaching Rec. 2020 standards, with an EQE of 25.2% and T50 of 120 min at initial luminance of 107 cd/m2.

Optimizing Ag films towards efficient flexible quantum-dot light-emitting diodes

Metal film electrodes are frequently adopted in flexible quantum-dot light-emitting diodes (FQLEDs) to form microcavity structures, which accelerate the recombination of excitons and thereby enhancing the efficiency of devices, especially under high driving current density. However, metal films prepared by thermal evaporation grow according to the Volmer-Weber nucleation mechanism, thereby resulting in the formation of numerous metal islands on the target substrate. The serious leakage current and light scattering (hence weakened microcavity effect) caused by these islands lead to inefficient FQLEDs. Here we fabricated a highly uniform and smooth Ag-based flexible electrode by utilizing an ultrathin polyethyleneimine as the nucleation-inducing seed layer. A smooth surface with a root-mean-square of approximately 3.7 nm and an ultralow sheet resistance of 3.65 Ω sq−1 have been obtained for the Ag film with a thickness of only 16 nm. Thus, a high-efficiency FQLED is achieved with the maximum external quantum efficiency up to 25.6 %. Furthermore, this device exhibits ultralow efficiency roll-off, characterized by the external quantum efficiency of over 20 % across an extremely wide luminance range, from 300 to 160,000 cd m−2, meeting all display and lighting requirements.

Fine-Tuning Optoelectronic Features in Organic-Inorganic Hybrid Metal Halides: A Focus on the Phenylbutanammonium Series.

Extensive research has been dedicated to exploring the potential applications of organic-inorganic hybrid metal halides in optoelectronics. This study presents findings on three metal halides based on phenylbutanammonium (PBA). Specifically, (PBA)2MnBr4(H2O)2 and (PBA)2Sn(IV)Cl6 exhibit zero-dimensional structures with P21/c and Pnma space groups, respectively, while (PBA)2Sn(II)Br4 features a two-dimensional structure with P1̅ space group. Under UV excitation, (PBA)2MnBr4(H2O)2 exhibits double emission arising from the 4T1 → 6A1 transitions of Mn2+ in two distinct coordination environments. The emission spectrum of (PBA)2SnCl6 aligns with that of PBACl, suggesting that the luminescence originates from the organic component. The yellow emission of (PBA)2SnBr4 is attributed to the self-trapped excitons. This study introduces the PBA series of compounds, revealing that varying metal ions and halogen combinations can adjust the structural dimensions and influence optical properties. The insights gained from this work serve as a guide for the preparation of efficient white light-emitting diodes.

One-Step Extrusion Preparation for Low-Cost Multicolored Carbon Dot Fluorescent Films.

Carbon dots (CDs), an emerging class of environmentally friendly luminescent materials, have been extensively applied in full-color display of light-emitting diodes (LEDs). However, the synthesis of CDs usually requires high pressure, and the conventional preparation of CD-based LED films involves intricate manufacturing techniques. The resulting security risk and high cost restrict the further application of CD-based LEDs. Here, we present a cost-effective method to obtain CD-based LED films via in situ extrusion in a twin-screw extruder. By fine-tuning the extrusion temperature, we created blue-emitting LEDs. The resulting fluorescent films exhibited remarkable stability, retaining 94% fluorescence intensity after 180 days of storage. We also prepared yellow, pure white, and cool white LEDs with malleable shapes, validating the versatility of the in situ extrusion method. More importantly, the manufacturing cost for CD-based films amounts to only about 0.8 RMB/g, which is substantially lower than that of the reported preparation methods. This work offers a cost-effective, safe, and massive approach to preparing photoluminescent LEDs, paving the way for the development of more sustainable and efficient CD-based LEDs.

Enhancing Energy Transfer through Structure Reconstruction of Quasi-2D Perovskites for Highly Efficient Light-Emitting Diodes

Enhancing Energy Transfer through Structure Reconstruction of Quasi-2D Perovskites for Highly Efficient Light-Emitting Diodes

Enhancing Light Outcoupling Efficiency via Anisotropic Low Refractive Index Electron Transporting Materials for Efficient Perovskite Light-Emitting Diodes.

Thanks to the extensive efforts toward optimizing perovskite crystallization properties, high-quality perovskite films with near-unity photoluminescence quantum yield are successfully achieved. However, the light outcoupling efficiency of perovskite light-emitting diodes (PeLEDs) is impeded by insufficient light extraction, which poses a challenge to the further advancement of PeLEDs. Here, an anisotropic multifunctional electron transporting material, 9,10-bis(4-(2-phenyl-1H-benzo[d]imidazole-1-yl)phenyl) anthracene (BPBiPA), with a low extraordinary refractive index (ne) and high electron mobility is developed for fabricating high-efficiency PeLEDs. The anisotropic molecular orientations of BPBiPA can result in a low ne of 1.59 along the z-axis direction. Optical simulations show that the low ne of BPBiPA can effectively mitigate the surface plasmon polariton loss and enhance the photon extraction efficiency in waveguide mode, thereby improving the light outcoupling efficiency of PeLEDs. In addition, the high electron mobility of BPBiPA can facilitate balanced carrier injection in PeLEDs. As a result, high-efficiency green PeLEDs with a record external quantum efficiency of 32.1% and a current efficiency of 111.7cd A-1 are obtained, which provides new inspirations for the design of electron transporting materials for high-performance PeLEDs.

Revealing the Structure-Luminescence Relationship in Robust Sn(IV)-Based Metal Halides by Sb3+ Doping.

Low-dimensional hybrid metal halides are an emerging class of materials with highly efficient photoluminescence (PL), but the problems of poor stability remain challenging. Sn(IV)-based metal halides show robust structure but exhibit poor PL properties, and the structure-luminescence relationship is elusive. Herein, two Sn(IV)-based metal halides (compounds 1 and 2) with the same constituent ((C6H16N2)SnCl6) but different crystal structures have been prepared, which however show poor PL properties at room temperature due to the absence of active ns2 electrons. To improve materials' PL properties, Sb3+ with active 5s2 electrons was embedded into the lattice of Sn4+-based hosts. As a result, efficient emissions were achieved for Sb3+-doped compounds 1 and 2 with a maximum PL efficiency of 14.28 and 62%, respectively. Experimental and calculation results reveal that the smaller distorted lattice structure of the host could result in the blueshift of the emission from Sb3+. Thus, a tunable color from red to orange was realized. Benefiting from the broadband efficient emission from Sb3+-doped compound 2, an efficient white light-emitting diode with a high color rendering index of up to 92.3 was fabricated to demonstrate its lighting application potential. This work promotes the understanding of the influence of robust Sn(IV)-based host lattice on the PL properties of Sb3+, advancing the development of environmentally friendly, low-cost, and high-efficiency Sn(IV)-based metal halides.

Charge Injection and Auger Recombination Modulation for Efficient and Stable Quasi-2D Perovskite Light-Emitting Diodes.

The inefficient charge transport and large exciton binding energy of quasi-2D perovskites pose challenges to the emission efficiency and roll-off issues for perovskite light-emitting diodes (PeLEDs) despite excellent stability compared to 3D counterparts. Herein, alkyldiammonium cations with different molecular sizes, namely 1,4-butanediamine (BDA), 1,6-hexanediamine (HDA) and 1,8-octanediamine (ODA), are employed into quasi-2D perovskites, to simultaneously modulate the injection efficiency and recombination dynamics. The size increase of the bulky cation leads to increased excitonic recombination and also larger Auger recombination rate. Besides, the larger size assists the formation of randomly distributed 2D perovskite nanoplates, which results in less efficient injection and deteriorates the electroluminescent performance. Moderate exciton binding energy, suppressed 2D phases and balanced carrier injection of HDA-based PeLEDs contribute to a peak external quantum efficiency of 21.9%, among the highest in quasi-2D perovskite based near-infrared devices. Besides, the HDA-PeLED shows an ultralong operational half-lifetime T50 up to 479h at 20mA cm‒2, and sustains the initial performance after a record-level 30000 cycles of ON-OFF switching, attributed to the suppressed migration of iodide anions into adjacent layers and the electrochemical reaction in HDA-PeLEDs. This work provides a potential direction of cation design for efficient and stable quasi-2D-PeLEDs.

Suppressing interfacial nonradiative recombination by alkali hydroxides for efficient blue perovskite light-emitting diodes

Blue perovskite light-emitting diodes (PeLEDs) provide unlimited possibility in the field of optoelectronic devices due to their excellent characteristics. However, the widely used hole-transport material, poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), has strong acidity and poor conductivity which deteriorate the efficiency and stability of perovskite devices. In this work, we modified PEDOT:PSS layer using alkalis, LiOH, NaOH, and KOH, to address these issues. Our findings indicated that the indium defect from indium tin oxide substrate was passivated after the introduction of LiOH, which resulted in the suppression of nonradiative recombination. In addition, the conductivity was improved owing to the increased phase separation of PEDOT:PSS. Taking advantage of optimized interface, the small-n phases of quasi-two-dimensional (quasi-2D) perovskite films were redistributed in quantities and dimensions. As results of these improvements, blue PeLEDs possessed the external quantum efficiencies of 5.95 % and an extended half-lifetime with emission-light wavelength of 473 nm. These findings offer an effective strategy to achieve highly efficient blue light-emitting diode.