Organic Light-emitting Diodes Research Articles

Organic Circularly Polarized Room-Temperature Phosphorescence: Strategies, Applications and Challenges.

Organic circularly polarized luminescence (CPL) plays crucial roles in chemistry and biology for the potential in chiral recognition, asymmetric catalysis, 3D displays, and biological probes. The long-lived luminescence, large Stokes shift, and unique chiroptical properties make organic circularly polarized room-temperature phosphorescence (CPP) a new research hotspot in recent years. Nevertheless, achieving high-performance organic CPP is still challenging due to the sensitivity and complexity of integrating triplet excitons and polarization within organic materials. This review summarizes the latest advances in organic CPP, ranging from design strategies and photophysical properties to underlying luminescence mechanisms and potential applications. Specifically, the design strategies for generating CPP are systemically categorized and discussed according to the interactions between chiral units and chromophores. The applications of organic CPP in organic light-emitting diodes, sensing, chiral recognition, afterglow displays, and information encryption are also illustrated. In addition, we present the current challenges and perspectives on developing organic CPP. We expect this review to provide some instructive design principles to fabricate high-performance organic CPP materials, offering an in-depth understanding of the luminescence mechanism and paving the way toward diverse practical applications.

An OLED display fabricated through side‐by‐side pixel patterning via photolithography

AbstractWe developed a patterning technology using photolithography that eliminates the need for a fine metal mask, thereby achieving high resolution across all red‐blue‐green pixels of an organic light‐emitting diode (OLED) display. Traditional methods using fine metal masks typically restrict the resolution and size of a display. However, the proposed technology largely bypasses these constraints. Suitable for OLED applications ranging from microdisplays in augmented reality and virtual reality to large TVs exceeding 80 in., our technology overcomes limitations on alignment accuracy, fine metal mask washing, and running costs and paves the way for mass production in OLED manufacturing.

Efficient Organic Light-Emitting Diodes Obtained by Introducing Gadolinium (Gd) Complexes Based on Pyrazolone Derivative Ligands as Hole Trappers.

The utilization of lanthanide (Ln) complexes in the realm of organic light-emitting diodes (OLEDs) has garnered extensive interest, particularly in their role as luminescent materials or electron trappers. A series of gadolinium (Gd) complexes with energy levels of high HOMO/LUMO and different triplet state energies were designed and synthesized by introducing substituents with different electronic effects onto the pyrazolone derivative ligands. Subsequently, these complexes were precisely purified by vacuum sublimation and codoped into the light-emitting layer (EML) of the OLEDs. This process was facilitated through the well-matched HOMO/LUMO levels and triplet energies among various functional materials. Consequently, the maximum external quantum efficiencies of blue, red, and green OLEDs were simultaneously enhanced with the ratios of 119%, 28%, and 71%, respectively. This improvement can be credited to the introduction of Gd(III) complex molecules within EMLs, which helps to capture excess holes and improve carriers' balance.

Interfacial Metal Chlorides as a Tool to Enhance Charge Carrier Dynamics, Electroluminescence, and Overall Efficiency of Organic Optoelectronic Devices.

Interface engineering is the key to optimizing optoelectronic device performance, addressing challenges like reducing potential barriers, passivating interface traps, and controlling recombination of charges. Metal fluorides such as lithium fluoride are employed in interface modification within organic devices due to their strong dipole characteristics but carry health risks, high processing costs, and minimal impact on interface traps in organic electronics. Hence, this study investigates alternative metal chloride (MC) nanocrystals (sodium, cesium, rubidium, and potassium chlorides) that exhibit a strong dipole moment and are readily processable with the aim of reducing the influence of interface traps. Interfacial properties are assessed via various techniques, including electron paramagnetic resonance, X-ray/ultraviolet photoelectron spectroscopy, capacitance-voltage measurements, and density functional theory calculations. In organic light-emitting diodes (OLEDs), the influence of MC on charge transfer, trap density, and light emission properties is evaluated. MCs in ZnO:PEIE nanocomposites (NCs) show improved charge transport, accelerated trapping/detrapping in ZnO:PEIE NCs, and a 50% reduction in active traps in NaCl-based devices versus the reference without MCs. RbCl-, CsCl-, and NaCl-based OLEDs exhibit substantial reductions in the potential barrier between the electron injection layer and the metal contact (Al) from 4.43 to 2.93, 3.02, and 4 eV, respectively, accompanied by enhancements of 35, 27, and 25% in electroluminescence intensity.

Plasticizer Design Principle of "Like Dissolves Like": Semiconductor Fluid Plasticized Stretchable Fully π-Conjugated Polymers Films for Uniform Large-Area and Flexible Deep-Blue Polymer Light-Emitting Diodes.

Physical blending of fully π-conjugated polymers (FπCPs) is an effective strategy to achieve intrinsically stretchable films for the fabrication of flexible optoelectronic devices, but easily causes phase separation, nonuniform morphology and uncontrollable photo-electronic processing. This may cause low efficiency, unstable and nonuniform emission, and poor color purity, which are undesirable for deep-blue flexible polymer light-emitting diodes (FPLEDs). Herein, a "Like Dissolves Like" design principle to prepare semiconductor fluid plasticizers (SFPs) is established and intrinsically stretchable FπCPs films via external plasticization for high-performance deep-blue FPLEDs are developed. Three fundamental requirements are proposed, "similar conjugated skeleton, similar molecular polarity, and similar electronic structures," to prepare model-matched nonpolar M1 and polar M2 plasticizers for poly(9,9-dioctylfluorene) (PFO). Large-area plasticized PFO films exhibit an efficient, narrowband, and stable ultra-deep-blue electroluminescence (FWHM < 40nm, CIE: 0.12, 0.04), uniform morphology, and excellent intrinsic stretchability (fracture strain >20% and crack-onset strain >120%). Efficient and uniform deep-blue FPLEDs based on stretchable PFO films are fabricated with a high brightness of ≈3000cd cm-2. Finally, blended PFO films exhibit outstanding stretch-deformation cycling stability of their deep-blue electroluminescent behavior, confirming the effectiveness of the "Like Dissolves Like" principle to design matched SFPs for stretchable FπCP films in flexible electronics.

Understanding and Controlling the Formation of Nonradiative Defects in Blue Organic Triplet Emitters

Phosphorescent organic light-emitting devices (PHOLEDs) suffer from destructive molecular processes due to triplet-polaron and triplet-triplet annihilation. These processes are energetically driven and hence are particularly active in decreasing the lifetime of blue PHOLEDs. It has recently been shown that increasing triplet radiative rates via the Purcell effect effectively extends the device operational lifetime by reducing the triplet radiative lifetime, thus decreasing their density and the probability of triplet-annihilation reactions. We provide an analytical framework using Marcus theory to explain the observed, approximately exponential relationship between exciton energy and device lifetime. From transient drift-diffusion dynamics, we show that the Purcell effect reduces the exciton density in the steady state and increases the photoluminescent yield, thereby reducing defect generation rates and extending the device lifetime. We control the radiative rate of excitons in microcavities, thereby connecting the exciton energy and decay rates with the observed device lifetime. The device lifetime is shown to follow a power-law dependence on the Purcell factor (PFm) with m=1.5 to 2.5, dependent on the TTA-to-TPA ratio and photoluminescence quantum yield. From our analysis, a fivefold increase in PF has the potential to extend the blue PHOLED lifetime by up to 2 orders of magnitude, making the blue PHOLED lifetime comparable to that of state-of-the-art green PHOLEDs. Published by the American Physical Society 2024

Long-Lived Room-Temperature Phosphorescence from Silica Nanoparticles with In Situ Generated Carbonaceous Defects for Blue Organic Light-Emitting Diodes.

In this study, in situ formed silica nanoparticles (SNPs) emitting second-level phosphorescence at room temperature without a phosphorescent dopant have been achieved for the first time. This phosphorescence is achieved through the simple in situ formation of carbonaceous defects (CDs) within the SNPs, followed by passivation of the CDs by a robust silica matrix. The CD in the SNPs, termed CD@SNPs, are synthesized by cross-linking tetraethyl orthosilicate (TEOS) and (3-aminopropyl)triethoxysilane (APTES), and these cross-linked components create a porous structure within the silica matrix. Upon calcination, the carbon-related structures within the pores deform, leading to the formation of CDs. Confined within a robust silica matrix, the molecular motion of the CD is restricted, facilitating the generation of a stable triplet state and suppressing nonradiative decay. Moreover, the robust silica matrix passivates the CD confined in the SNPs at a nanoscale. These comprehensive effects enable prolonged phosphorescence emission from the SNPs. In addition, these phosphorescence-emitting SNPs have been applied as dopants in the emissive layer of organic light-emitting diodes to realize blue light emission. This feature suggests the possibility of utilizing luminescent SNPs as light emitters in display technologies.

Origins of Narrowband Emission in Nitrogen/Carbonyl Multiresonance Thermally Activated Delayed Fluorescence Emitters: Steric Locks and Vibrational Coupling Effects.

The incorporation of tert-butyl groups and spiro-functionalization into C═O/N-embedded multiresonance thermally activated delayed fluorescence (MR-TADF) systems has yielded materials with superior narrowband emission and excellent color purity. To elucidate the mechanisms underlying the enhanced properties, we present a theoretical study of a series of fused nitrogen/carbonyl derivatives with narrower emission profiles. The key steric factors that contribute to narrowband emission were identified through energy decomposition analysis, induced by structural relaxation in states S0 and S1. Additionally, we achieved potential narrower-band and deep-blue emission by targeting the suppression of vibrational coupling effects. This work provides compelling evidence that a 1-tert-butyl substitution, acting as an end lock, offers minimal reorganization energy and optimal structural stability when combined with a fused lock. Furthermore, new compounds such as 1tBuCZQ and 1tBuDQAO have been identified as promising MR-TADF emitters, delivering ultranarrowband emission as high-quality organic light-emitting diodes.

Regulating Optoelectronic and Thermoelectric Properties of Organic Semiconductors by Heavy Atom Effects.

Heavy atom effects can be used to enhance intermolecular interaction, regulate quinoidal resonance properties, increase bandwidths, and tune diradical characters, which have significant impacts on organic optoelectronic devices, such as organic field-effect transistors (OFETs), organic light-emitting diodes (OLEDs), organic photovoltaics (OPVs), etc. Meanwhile, the introduction of heavy atoms is shown to promote charge transfer, enhance air stability, and improve device performances in the field of organic thermoelectrics (OTEs). Thus, heavy atom effects are receiving more and more attention. However, regulating heavy atoms in organic semiconductors is still meeting great challenges. For example, heavy atoms will lead to solubility and stability issues (tellurium substitution) and lack of versatile design strategy and effective synthetic methods to be incorporated into organic semiconductors, which limit their application in electronic devices. Therefore, this work timely summarizes the unique functionalities of heavy atom effects, and up-to-date progress in organic electronics including OFETs, OPVs, OLEDs, and OTEs, while the structure-performance relationships between molecular designs and electronic devices are clearly elucidated. Furthermore, this review systematically analyzes the remaining challenges in regulating heavy atoms within organic semiconductors, and design strategies toward efficient and stable organic semiconductors by the introduction of novel heavy atoms regulation are proposed.

Double-gate metal–oxide TFT pixel circuit for improved luminance uniformity of mobile OLED display

The small subthreshold swing (SS) of metal–oxide (MOx) thin-film transistors (TFTs) reduces the data voltage (VDAT) range of the organic light-emitting diode (OLED) display pixel circuit. This leads to a large OLED current error when a small change occurs in the gate-to-source voltage (VGS) of the driving TFT in the pixel. Therefore, we propose a new pixel circuit adopting a double-gate TFT structure for the driving TFT, which is mainly driven by the bottom gate with a thicker gate insulator to provide a wide VDAT range. The proposed pixel circuit further expands the VDAT range by employing threshold voltage (VTH) modulation effect depending on the gray level. It also allows for flexible adjustment of the VTH extraction time to reduce OLED current error at low gray levels. SPICE simulation and measured results verify that the proposed pixel circuit reduces the OLED current error rate to less than 10% even with a VTH variation of ±0.5 V or SS variation of ±5% for the current level from 0.1 nA to 30 nA.

Efficient and Stable Narrowband Pure-Red Light-Emitting Diodes with Electroluminescence Efficiencies Exceeding 43.

Advanced multiresonance-induced thermally activated delayed fluorescence (MR-TADF) materials exhibit exceptional promise for applications in state-of-the-art organic light-emitting diodes (OLEDs) owing to their unique narrowband emissions and high luminescent efficiencies. Despite substantial progress with blue and green MR-TADF materials, the development of pure-red MR-TADF emitters has lagged behind, thereby hindering the advancement toward high-performance ultrahigh-definition OLED displays. Here, we propose an effective approach for designing pure-red MR-TADF molecules based on the integration of secondary electron-donating units and π-skeleton extension into MR cores, which enables not only a redshift of narrowband emission but also an acceleration of reverse intersystem crossing (RISC) rate. The proof-of-the-concept emitter BNTPA showcases a bright and saturated red emission centered at 613 nm with a full-width at half-maximum of 0.14 eV, together with a more than twofold increase in the RISC rate. As a result, TADF OLEDs based on BNTPA achieved a record maximum external quantum efficiency (EQEmax) of up to 35.2% with Commission Internationale de l'Eclairage (CIE) coordinates of (0.657, 0.343), approaching the coordinates of the National Television Standards Committee (NTSC) red standard. Moreover, phosphor-assisted fluorescence devices exploiting BNTPA as a terminal emitter exhibit an exceptional EQEmax of 43.3%, accompanied by improved operational stability.

Purinylidene‐Based Iridium(III) Phosphors for Efficient Purple Phosphorescence and Organic Light‐Emitting Devices

It is very challenging to achieve highly efficient and low roll‐off purple to deep‐blue organic light‐emitting diodes (OLEDs) due to the limited selection of suitable emitters. Herein, a series of Ir(III) phosphors‐bearing purinylidene‐based N‐heterocyclic carbene (NHC) chelates are reported, namely, f‐ct2a–d, which have successfully turned the emission peak maxima to the purple region (427–432 nm) in degassed toluene, together with high quantum yields of 77%–81%. Notably, f‐ct2b is further employed as the phosphorescent dopant in the fabrication of OLEDs to afford deep‐blue emission paking at 448 nm and an EQE of 6.7% at 100 cd m−2. Furthermore, a true‐blue hyperphosphorescent OLED with 21 wt% of f‐ct2a as sensitizer and 1 wt% of ν‐DABNA as terminal emitter is also fabricated, giving excellent performance with max. EQE up to 22.2% and an EQE of 19.2% at a brightness of 100 cd m−2.

Evaluation of degradation phenomena in the electric potential distribution inside organic light-emitting diodes by electron holography

Understanding the intrinsic degradation processes of organic light-emitting diodes is necessary to improve their lifetimes. This intrinsic degradation is typically caused by carrier injection at the interface between the hole transport layer (HTL) and the emissive layer (EML). However, revealing the charge behavior in this local region with a high spatial resolution remains challenging. Thus, this study employed electron holography, a transmission electron microscopy (TEM) technique, to measure the nanometer scale potential distribution inside an OLED composed of N,N′-di-[(1-naphthyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4,4′-diamine (α-NPD) and tris-(8-hydroxyquinoline)aluminum (Alq3) that was degraded via continuous voltage application. The α-NPD and Alq3 functioned as the HTL and EML, respectively. The degraded OLED was found to exhibit several potential distributions, depending on the local positions from which the TEM samples were lifted out of the same bulk sample. The distributions included (i) formation of a potential valley at the α-NPD/Alq3 interface, (ii) disappearance of electric fields within the organic layers, and (iii) similar distribution to original before degradation. We suggest that the degradation was caused by charge accumulation, cationization of Alq3, and local failures. Thus, this study revealed the influence of electric degradation at the nanometer scale because of charge injection to the α-NPD/Alq3 interface. Electron holographic degradation analysis near the HTL/EML interface is expected to aid in the development of design guidelines for preventing device degradation and thus extend device lifetime.

High‐Efficiency and Low Efficiency Roll‐Off Blue‐Emission Crystalline Organic Light‐Emitting Diodes by embedding Triplet‐Triplet Annihilation Nanoaggregates

AbstractCrystalline host matrix (CHM)‐nanoaggregates (NA) structure combines the advantages of a crystalline host matrix and high‐performance luminescent materials. This structure emerges as a highly effective strategy for achieving efficient blue organic light‐emitting diodes (OLEDs). This approach can tackle the current challenge of achieving low voltage and high brightness in amorphous OLEDs. In this work, triplet‐triplet annihilation (TTA) material NA are embedded into CHM and sensitize the fluorescent dopant (D) BD1. As a result, a CHM‐TTANA‐D structure device is created, demonstrating an outstanding external quantum efficiency (EQE) of 8.46%. Notably, the efficiency roll‐off is mitigated. The device maintains an EQE of ≈7.50% at 1000 cd m−2, which is the highest values among blue crystalline OLEDs. The device leverages the benefits of CHM and exhibits swift turn‐on and a rapid increase in brightness and current density. This leads to a significant improvement in blue photon output and a reduced series‐resistance Joule heat loss ratio. The findings underscore the promising nature of the CHM‐TTANA‐D structure as a favorable design paradigm for high‐performance crystalline thin‐film OLEDs (C‐OLEDs).

Simple‐Structure and Anti‐Quenching Deep‐Blue Multi‐Resonance TADF Emitters Enable High‐Efficiency OLEDs with BT.2020 Blue Gamut

AbstractDeveloping highly efficient pure‐blue organic light‐emitting diodes (OLEDs) that meet the stringent BT.2020 standard by using multi‐resonance thermally activated delayed fluorescence (MR‐TADF) materials has long been a formidable challenge. In this study, a strategy is demonstrated for high‐performance blue MR‐TADF emitters by gradually decorating the MR framework with alkyl groups, and subsequently introducing a diarylamino group at the para‐position of boron atom. The proof‐of‐concept molecule, IPrBN‐mCP, exhibits a narrowband deep‐blue emission peaking at 452 nm, with a very narrow full‐width at half maximum (FWHM) of 19 nm in solution, and a remarkably high photoluminescence quantum yield (PLQY) approaching unity in doped films. As a result, OLEDs based on IPrBN‐mCP achieve not only a high maximum external quantum efficiency (EQEmax) of 33.4% but also ultrapure blue emission with a Commission Internationale de L'Eclairage (CIE) y value of 0.046, fully satisfying the BT.2020 blue standard. This represents the first OLED example fully meeting the rigorous color requirements of the BT.2020 blue standard while simultaneously achieving an EQE exceeding 30%.

Boron-Nitrogen Compounds in Luminescent Materials: A Review of Applications and Synthesis Methods

Boron-nitrogen compounds (B-N) have emerged as a promising material due to their unique electronic structure and excellent luminescent properties, showing great potential in organic light-emitting diodes (OLED) and electroluminescent devices. This paper systematically reviews the molecular structure, optical, and electronic properties of B-N compounds, discusses their applications in luminescent materials, and explores synthesis methods, including Friedel-Crafts-type reactions, borohydride reduction reactions, and olefin metathesis. Additionally, the paper analyzes key factors in controlling the performance of luminescent materials, investigating the impact of synthesis conditions, doping, and the preparation of composite materials on luminescent efficiency and stability. Despite significant progress in B-N compound research, challenges remain in synthesis precision, luminescent efficiency, and industrial production. Future research should focus on improving targeted synthesis capabilities, optimizing electronic structure and luminescent performance, and exploring more efficient, cost-effective production processes. With continued innovation, B-N compounds are poised to become a foundation for the next generation of high-efficiency luminescent materials, driving advancements in optoelectronic technology.

Image quality enhancement in variable refresh rate LTPO-based AMOLED displays using a gate in panel voltage compensation scheme

This study investigated novel driving scheme for low-temperature polycrystalline silicon and oxide (LTPO) active-matrix organic light-emitting diode (AMOLED) displays under variable refresh rate (VRR) driving conditions with aim of reducing the brightness instability (i.e., flicker and luminance variation) for seamless performance. The mechanisms underlying brightness fluctuations in displays were examined, focusing on frequency-dependent differences in gate-inpanel (GIP) operations. Notably, our study revealed additional factors within the GIP operation that cause LTPO flicker and VRR performance, in addition to previously identified factors of thin-film transistor hysteresis and OLED charging. Consequently, variable GIP voltage driving method that reduced these issues and enhanced display performance was proposed. The proposed method could minimize brightness fluctuations to levels imperceptible to users and facilitate performance improvement through driving options in peripheral circuits without modifying panel design and is applicable across various applications utilizing LTPO OLED technology. Its driving scheme was validated through simulation and measurement of luminance variation during frequency transition using 6.0-inch LTPO-applied AMOLED displays. The experimental results indicated a decrease in luminance difference from 14.2% to 1.5%, particularly at extremely low brightness level of 0.2 nit. The luminance distortion decreased from 7 to less than 1, which is a just noticeable color difference (JNCD).

Electron transport and quantum phase transitions in cumulene-connected C80H20 fulleryne: DFT and tight-binding studies

Molecular bridges are opening up exciting new applications in diverse fields, improving the efficiency of conductive inks, enhancing the performance of devices such as organic light-emitting diodes and low-cost solar cells, and advancing the development of highly sensitive sensors, chemical reactions, drug delivery systems, and more. In this paper, we study the electron transport properties of a C80H20 fulleryne (dodecahedryne) connected to two cumulene electrodes. Using density functional theory (DFT), we determine the optimal molecular bridge structure. Based on the IR vibration spectra, different stable phases of the molecular bridge are obtained. The corresponding tight-binding (TB) parameters of the cage are obtained by assigning appropriate values of the length and type of bonds for the fulleryne cage through matching the HOMO-LUMO gap between the DFT calculations and the TB parameters. The electron transport for the desired structures is investigated using the obtained tight-binding parameters and the non-equilibrium Green's function (NEGF) method. Finally, it is concluded that among the five possible configurations for the cumulene-dodecahedryne -cumulene molecular bridge, only one specific configuration—where the electrodes are one edge apart—exhibits metallic behavior, while other positions act as insulators. In addition, the system exhibits quantum phase transitions from metal to semiconductor and from insulator to metal in the presence of critical electric fields. The ability to control quantum phase transitions in these molecular systems can be leveraged to develop qubits for quantum computing. The unique properties can be utilized to design advanced molecular electronic devices.

Device physics of perovskite light-emitting diodes

Perovskite light-emitting diodes (LEDs) have emerged as a potential solution-processible technology that can offer efficient light emission with high color purity. Here, we explore the device physics of perovskite LEDs using simple analytical and drift-diffusion modeling, aiming to understand how the distribution of electric field, carrier densities, and recombination in these devices differs from those assumed in other technologies such as organic LEDs. High barriers to electron and hole extraction are responsible for the efficient recombination and lead to sharp build-up of electrons and holes close to the electron- and hole-blocking barriers, respectively. Despite the strongly varying carrier distributions, bimolecular recombination is surprisingly uniform throughout the device thickness, consistent with the assumption typically made in optical models. The current density is largely determined by injection from the metal electrodes, with a balance of electron and hole injection maintained by redistribution of electric field within the device by build-up of space charge.

Synergistic π-Extension and Peripheral-Locking of B/N-Based Multi-Resonance Framework Enables High-Performance Pure-Green Organic Light-Emitting Diodes.

Multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters offer natural advantages for creating power-efficient, wide-color-gamut OLEDs. However, current green MR-TADF emitters face challenges in simultaneously achieving high color purity and efficient reverse inter-system crossing (RISC), leading to suboptimal device performance. In this study, we propose a synergistic molecular design approach that combines π-extension and peripheral locking to address these challenges. This approach allows for the construction of quadruple borylated MR-TADF emitters that not only deliver precisely tuned pure-green emission with a narrow full width at half maximum (FWHM) of 15 nm, but also exhibit close-to-unity quantum yield, rapid RISC, and optimal horizontal dipole orientation. The resulting sensitizer-free OLED approaches the BT.2020 standard with CIE coordinates of (0.18, 0.74) and demonstrates impressive external quantum efficiency (EQE) of 36.6% at maximum and 31.8% at 1000 cd m-2. Additionally, the device shows good operational stability, with a lifetime (LT80) of 485 hours at an initial luminance of 1000 cd m-2. This study hence offers a promising molecular design strategy that effectively enhances the comprehensive OLED performance.