Emissive Layer Research Articles

Infrared Emissivity–Resistivity Correlation of RU Thin Films for EUV Pellicle Applications

A high-infrared (IR) emissivity is required to dissipate heat effectively from high-temperature surfaces even in a near-vacuum environment. This radiative cooling capability is essential for the emissive layer of EUV pellicles operating under high vacuum conditions with increasing EUV source powers to ensure thermomechanical stability. We compute the IR emissivity of metal films by combining the classical oscillator model and DFT calculations. Based on the calculations, we predict a positive correlation between the electrical resistivity and IR emissivity of metal films, which is consistent with a recent experiment on Ru thin films. Our findings can provide a practical indicator for achieving high IR emissivity of metallic thin films based on electrical measurements. This emissivity–resistivity correlation can provide an effective way to search a large material space, and choose and synthesize a highly emissive layer of EUV pellicles.

Inverted device architecture for high efficiency single-layer organic light-emitting diodes with imbalanced charge transport

Many wide-gap organic semiconductors exhibit imbalanced electron and hole transport, therefore efficient organic light-emitting diodes require a multilayer architecture of electron- and hole-transport materials to confine charge recombination to the emissive layer. Here, we show that even for emitters with imbalanced charge transport, it is possible to obtain highly efficient single-layer organic light emitting diodes (OLEDs), without the need for additional charge-transport and blocking layers. For hole-dominated emitters, an inverted single-layer device architecture with ohmic bottom-electron and top-hole contacts moves the emission zone away from the metal top electrode, thereby more than doubling the optical outcoupling efficiency. Finally, a blue-emitting inverted single-layer OLED based on thermally activated delayed fluorescence is achieved, exhibiting a high external quantum efficiency of 19% with little roll-off at high brightness, demonstrating that balanced charge transport is not a prerequisite for highly efficient single-layer OLEDs.

Unraveling the Degradation Pathways in Deep Blue Phosphorescent OLEDs Depending on Charge Dynamics: Insights from Numerical Analysis and Magneto-Electroluminescence Characterization.

To analyze the lifetime difference based on the charge dynamics in the emitting layer (EML), we applied two electron transport layers (ETLs) with significantly different electron transporting characteristics to the same EML. Even with the same EML configuration, the device lifetime increased by approximately 4-fold, from 291 h to over 1000 h of LT50 (the time taken for the luminance to decrease to 50% of its initial value of 1000 cd/m2). Although trap/detrap of holes in the dopant molecules was observed through impedance spectroscopy, we found that the most significant difference in lifetime was caused by the quantity of electron current. Surprisingly, depending on the electron transporting layer, the primary bimolecular interaction in the EML (i.e., exciton-exciton, exciton-polaron interaction) dramatically changes even in the same EML configuration, which is theoretically analyzed by the numerical fitting of transient electroluminescence data and experimentally confirmed by magneto-electroluminescence (MEL) measurements. To the best of our knowledge, for the first time, the MEL measurements are demonstrated as a tool that can be utilized to intuitively discern the dominance of bimolecular interaction with respect to the operational stability of phosphorescent organic light-emitting diodes (PhOLEDs).

Highly enhanced Quantum dot light-emitting diode performance by controlling energy resonance in inorganic insertion layers

Quantum-dot light-emitting diodes (QLED) have become a research trend in the field of new displays due to their low cost, wide color gamut, narrow bandwidth, and characteristics that enable production through the solution-gel method. However, the electrical performance of QLED is consistently constrained by energy losses and imbalanced charge carrier injection. This motivates our focus on exciton recombination and energy losses within the quantum-dot layer to enhance the electrical efficiency of QLED. In this work, we introduce a method using a CdZnS quantum dot (B-QD) interlayer to modulate energy transfer and charge carrier transport in QLED devices employing CdSe quantum dot (G-QD) as the emissive layer. By strategically incorporating a B-QD layer between the G-QD and HTL/ETL, we facilitate energy transfer due to the overlap between the excitation wavelength of B-QD and the absorption wavelength of G-QDs. This leads to enhanced energy injection in QLED devices, resulting in a high current efficiency of 39.54 Cd/A and a peak brightness of 522,272 cd/m2 for efficient QLED. The corresponding external quantum efficiency (EQE) is greatly improved from 5.62 % to 9.4 %. Our work provides a straightforward and effective approach to modulate exciton recombination and energy injection and further can be applicable to other photo-electronics devices.

Development and synthesis of anthracene-based fluorescence material for non-doped OLEDs

Efficient organic blue materials are essential to develop highly efficient organic light-emitting diodes. However, there is a challenge to commercialize efficient blue compounds due to the fundamentally wide band gap and the lack of pure blue emitters. In this study, the new anthracene-based fluorescent emitter, (4-(10-(4-(9H-carbazol-9-yl)-2-methylphenyl)anthracen-9-yl)-3-methylphenyl)diphenylphosphine oxide (CZm-AN-mPO) was designed and synthesized. The thermal, photophysical and electrochemical properties of the CZm-AN-mPO were systematically investigated. The CZm-AN-mPO exhibits a deep-blue emission at 425 nm and AIE property with a relatively narrow full-width at half maximum of 34 nm. The CZm-AN-mPO based non-doped devices show pure blue emission. The optimized OLEDs indicated maximum external efficiencies of 4.3% and 4.2% at emission layer thickness of 20 nm and 40 nm, respectively.

Efficient Green Spin Light-Emitting Diodes Enabled by Ultrafast Energy- and Spin-Funneling in Chiral Perovskites.

Introducing molecular chirality into perovskite crystal structures has enabled the control of carrier spin states, giving rise to circularly polarized luminescence (CPL) in thin films and circularly polarized electroluminescence (CPEL) in LEDs. Spin-LEDs can be fabricated either through a spin-filtering layer enabled by chiral-induced spin selectivity or a chiral emissive layer. The former requires a high degree of spin polarization and a compatible spinterface for efficient spin injection, which might not be easily integrated into LEDs. Alternatively, a chiral emissive layer can also generate circularly polarized electroluminescence, but the efficiency remains low and the fundamental mechanism is elusive. In this work, we report an efficient green LED based on quasi-two-dimensional (quasi-2D) chiral perovskites as the emitting layer (EML), where CPEL is directly produced without separate carrier spin injection. The optimized chiral perovskite thin films exhibited strong CPL at 535 nm with a photoluminescence quantum yield (PLQY) of 91% and a photoluminescence dissymmetry factor (glum) of 8.6 × 10-2. Efficient green spin-LEDs were successfully demonstrated, with a large EL dissymmetry factor (gEL) of 7.8 × 10-2 and a maximum external quantum efficiency (EQE) of 13.5% at room temperature. Ultrafast transient absorption (TA) spectroscopic study shows that the CPEL is generated from a rapid energy transfer accompanied by spin transfer from 2D to 3D perovskites. Our study not only demonstrates a reliable approach to achieve high performance spin-LEDs but also reveals the fundamental mechanism of CPEL with an emissive layer of chiral perovskites.

Research Progress of Heavy-Metal-Free Quantum Dot Light-Emitting Diodes.

At present, heavy-metal-free quantum dot light-emitting diodes (QLEDs) have shown great potential as a research hotspot in the field of optoelectronic devices. This article reviews the research on heavy-metal-free quantum dot (QD) materials and light-emitting diode (LED) devices. In the first section, we discussed the hazards of heavy-metal-containing quantum dots (QDs), such as environmental pollution and human health risks. Next, the main representatives of heavy-metal-free QDs were introduced, such as InP, ZnE (E=S, Se and Te), CuInS2, Ag2S, and so on. In the next section, we discussed the synthesis methods of heavy-metal-free QDs, including the hot injection (HI) method, the heat up (HU) method, the cation exchange (CE) method, the successful ionic layer adsorption and reaction (SILAR) method, and so on. Finally, important progress in the development of heavy-metal-free QLEDs was summarized in three aspects (QD emitter layer, hole transport layer, and electron transport layer).

Dimensional and Doping Engineering of Chiral Perovskites with Enhanced Spin Selectivity for Green Emissive Spin Light-Emitting Diodes.

Chiral perovskites play a pivotal role in spintronics and optoelectronic systems attributed to their chiral-induced spin selectivity (CISS) effect. Specifically, they allow for spin-polarized charge transport in spin light-emitting diodes (LEDs), yielding circularly polarized electroluminescence at room temperature without external magnetic fields. However, chiral lead bromide-based perovskites have yet to achieve high-performance green emissive spin-LEDs, owing to limited CISS effects and charge transport. Herein, we employ dimensional regulation and Sn2+-doping to optimize chiral bromide-based perovskite architecture for green emissive spin-LEDs. The optimized (PEA)x(S/R-PRDA)2-xSn0.1Pb0.9Br4 chiral perovskite film exhibits an enhanced CISS effect, higher hole mobility, and better energy level alignment with the emissive layer. These improvements allow us to fabricate green emissive spin-LEDs with an external quantum efficiency (EQE) of 5.7% and an asymmetry factor |gCP-EL| of 1.1 × 10-3. This work highlights the importance of tailored perovskite architectures and doping strategies in advancing spintronics for optoelectronic applications.

An n-type host with a high triplet energy level and wide bandgap for RGB phosphorescent OLEDs

We applied 4,6-Bis(3,5-di(pyridin-2-yl)phenyl)-2-methylpyrimidine (B2PyMPM), a material with a wide bandgap and excellent electron mobility, as the host for the emissive layer in red, green, and blue organic phosphorescence-emitting devices. In the case of red emission, it exhibited a stable and outstanding external quantum efficiency (EQE) of 10.2% along with color coordinates (0.68, 0.31), indicating its potential as a candidate material for smartphone displays. For green and blue emissions, relatively low electroluminescence characteristics were observed due to insufficient energy transfer. With a high triplet energy level (T1) of 2.89 eV, B2PyMPM holds promise for use with other phosphorescent dopants in the future.

Progress in Research on White Organic Light-Emitting Diodes Based on Ultrathin Emitting Layers.

White organic light-emitting diodes (WOLEDs) hold vast prospects in the fields of next-generation displays and solid-state lighting. Ultrathin emitting layers (UEMLs) have become a research hotspot because of their unique advantage. On the basis of simplifying the device structure and preparation process, they can achieve electroluminescent performance comparable to that of doped devices. In this review, we first discuss the working principles and advantages of WOLEDs based on UEML architecture, which can achieve low cost and more flexibility by simplifying the device structure and preparation process. Subsequently, the successful applications of doping and non-doping technologies in fluorescent, phosphorescent, and hybrid WOLEDs combined with UEMLs are discussed, and the operation mechanisms of these WOLEDs are emphasized briefly. We firmly believe that this article will bring new hope for the development of UEML-based WOLEDs in the future.

Synthesis of 5-(Aryl)amino-1,2,3-triazole-containing 2,1,3-Benzothiadiazoles via Azide-Nitrile Cycloaddition Followed by Buchwald-Hartwig Reaction.

An efficient access to the novel 5-(aryl)amino-1,2,3-triazole-containing 2,1,3-benzothiadiazole derivatives has been developed. The method is based on 1,3-dipolar azide-nitrile cycloaddition followed by Buchwald-Hartwig cross-coupling to afford the corresponding N-aryl and N,N-diaryl substituted 5-amino-1,2,3-triazolyl 2,1,3-benzothiadiazoles under NHC-Pd catalysis. The one-pot diarylative Pd-catalyzed heterocyclization opens the straightforward route to triazole-linked carbazole-benzothiadiazole D-A systems. The optical and electrochemical properties of the compound obtained were investigated to estimate their potential application as emissive layers in OLED devises. The quantum yield of photoluminescence (PLQY) of the synthesized D-A derivatives depends to a large extent on electron-donating strengths of donor (D) component, reaching in some cases the values closed to 100%. Based on the most photoactive derivative and wide bandgap host material mCP, a light-emitting layer of OLED was made. The device showed a maximum brightness of 8000 cd/m2 at an applied voltage of 18 V. The maximum current efficiency of the device reaches a value of 3.29 cd/A.

Application of carbazole derivatives as a multifunctional material for organic light-emitting devices

The object of research is newly synthesized carbazole-derived compounds and organic light-emitting structures based on them. The problem lies in the complex solution of scientific and technical problems of improving the characteristics and stability of organic light-emitting diodes (OLEDs), namely improving the brightness and energy-efficient parameters. Organic light-emitting structures of blue, blue, and green radiation with color coordinates were formed by the thermovacuum sputtering method and the solution deposition method. The turn-on voltage of the white OLED is 6 V, the maximum brightness of the light-emitting structures was 10,000 cd/m2. The devices demonstrated a sufficiently high external quantum efficiency of 5 % to 7 %. This paper reports the multifunctional application of a simple donor-acceptor organic compound, as active and host material in the emission layer of organic light emitting devices. Em1 has been used as active components in OLEDs, where Em1 is the guest emitter (Device A), the acceptor part of the excited emitter (Device B) and the host matrix of the CdSeS/ZnS alloy quantum dot. At least four different OLEDs have been designed and characterized where Em1 plays the role of the guest emitter (Device C). The external quantum efficiencies of devices A–C are characterized by values common to pure fluorescent OLEDs (up to 5 % of the theoretical limit), but these devices sustain low-efficiency roll-off of electroluminescence over a wide range of current densities. Organic light-emitting diodes based on carbazole-derived compounds, due to their color characteristics, are promising candidates for use in the latest lighting systems. A separate advantage of the data light-emitting structures is a multifunctional application of one compound for different types of light-emitting structures. In addition, organic LEDs on based on carbazole-derived compounds have low energy consumption and are environmentally friendly due to the absence of toxic substances in their architecture, which creates prerequisites for saving energy resources and reducing the industrial burden on the environment.

Perfluorooctanoic acid interface modification enabling efficient true-blue perovskite light-emitting diodes and air-processing compatibility

The pure-bromide quasi-2D perovskite has shown advantages in fabrication of blue perovskite light-emitting diodes (PeLEDs) due to its relatively good color stability and capability of moisture resistance. However, the performances of efficiency and luminance of pure-bromide quasi-2D PeLEDs with true-blue (470 nm−480 nm) emission are still inferior. Here, we show a interfacial modification strategy of involving perfluorooctanoic acid (PFOA) to a hole transport layer of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), to improve the performances of the true-blue PeLEDs. The PFOA regulates the molecular conformation of PEDOT:PSS, forming ordered PEDOT domains that improves the carrier transport and a more favorable band alignment with the perovskite emissive layers. The achieved true-blue PeLEDs show peak EQE of 6.86 % and maximum luminance of 854.10 cd/m2 at wavelength of 478 nm, which are the record values for the pure-bromide quasi-2D PeLEDs in true-blue region. Moreover, owing to the hydrophobic fluorine-carbon (C-F) chains of PFOA, we successfully fabricate the blue PeLEDs in air condition without inert gas protection. This work paves the way for commercial applications in next-generation display technologies.

Regioisomerism vs Conformation: Impact of Molecular Design on the Emission Pathway in Organic Light-Emitting Device Emitters.

Despite the design and proposal of several new structural motifs as thermally activated delayed fluorescent (TADF) emitters for organic light-emitting device (OLED) applications, the nature of their interaction with the host matrix in the emissive layer of the device and their influence on observed photophysical outputs remain unclear. To address this issue, we present, for the first time, the use of up to four regioisomers bearing a donor-acceptor-donor electronic structure based on the desymmetrized naphthalene benzimidazole scaffold, equipped with various electron-donating units and possessing distinguished conformational lability. Quantum chemical calculations allow us to identify the most favorable conformations adopted by the electron-rich groups across the entire pool of regioisomers. These conformations were then compared with conformational changes caused by the interaction of the emitter with the Zeonex and 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP) matrices, and the correlation with observed photophysics was monitored by UV-vis absorption and steady-state photoluminescence spectra, combined with time-resolved spectroscopic techniques. Importantly, a CBP matrix was found to have a significant impact on the conformational change of regioisomers, leading to unique TADF emission mechanisms that encompass dual emission and inversion of the singlet-triplet excited-state energies and result in the enhancement of TADF efficiency. As a proof of concept, regioisomers with optimal donor positions were utilized to fabricate an OLED, revealing, with the best-performing dye, an external quantum emission of 11.6%, accompanied by remarkable luminance (28,000 cd/m2). These observations lay the groundwork for a better understanding of the role of the host matrix. In the long term, this new knowledge can lead to predicting the influence of the host matrix and adopting the structure of the emitter in a way that allows the development of highly efficient and efficient OLEDs.

The First Spatially Resolved Detection of 13CN in a Protoplanetary Disk and Evidence for Complex Carbon Isotope Fractionation

Recent measurements of carbon isotope ratios in both protoplanetary disks and exoplanet atmospheres have suggested a possible transfer of significant carbon isotope fractionation from disks to planets. For a clearer understanding of the isotopic link between disks and planets, it is important to measure the carbon isotope ratios in various species. In this paper, we present a detection of the 13CN N = 2 − 1 hyperfine lines in the TW Hya disk with the Atacama Large Millimeter/submillimeter Array. This is the first spatially resolved detection of 13CN in disks, which enables us to measure the spatially resolved 12CN/13CN ratio for the first time. We conducted nonlocal thermal equilibrium modeling of the 13CN lines in conjunction with previously observed 12CN lines to derive the kinetic temperature, H2 volume density, and column densities of 12CN and 13CN. The H2 volume density is found to range between (4 − 10) × 107 cm−3, suggesting that CN molecules mainly reside in the disk's upper layer. The 12CN/13CN ratio is measured to be 70−6+9 at 30 < r < 80 au from the central star, which is similar to the 12C/13C ratio in the interstellar medium. However, this value differs from the previously reported values found for other carbon-bearing molecules (CO and HCN) in the TW Hya disk. This could be self-consistently explained by different emission layer heights for different molecules combined with preferential sequestration of 12C into the solid phase toward the disk midplane. This study reveals the complexity of the carbon isotope fractionation operating in disks.

New Phenanthro[9,10-d]oxazole-Based Fluorophores with Hybridized Local and Charge-Transfer Characteristics for Highly Efficient Blue Non-Doped OLEDs with Negligible Efficiency Roll-Off.

It is vital to develop highly efficient non-doped blue organic light-emitting diodes (OLEDs) with high color purity and low-efficiency roll-off for applications in display and lighting. Herein, two blue D-A fluorophores TPA-PO and TPA-DPO are designed and synthesized, in which phenanthro[9,10-d]oxazole (PO) acts as the acceptor and triphenylamine as the donor. TPA-PO and TPA-DPO display good thermal stability and efficient luminescence efficiency in neat film. Results based on photophysical property and theoretical calculation demonstrate that TPA-PO and TPA-DPO possess the hybridized local and charge-transfer (HLCT) feature, which can utilize the triplet exciton to achieve highly efficient electroluminance (EL). The non-doped OLEDs with TPA-PO/TPA-DPO as pure emissive layer show the uniform EL emission peak at 468 nm, corresponding to CIE coordinates of (0.168, 0.187) and (0.167, 0.167), respectively. The TPA-DPO-based non-doped OLEDs provide the maximum external quantum efficiency (EQE) of 7.99 % and high exciton utility efficiency of 48.4 %~72.6 %. Moreover, the TPA-DPO-based device exhibits low-efficiency roll-off, still maintaining the EQE of 6.03 % at the high luminance of 5000 cd m-2. Those findings state clearly that PO is a promising building block of blue fluorophore with a potential HLCT feature to be applied in non-doped OLEDs.

Abnormal temperature-dependent transient electroluminescence spikes induced by the leakage of hole carriers in organic light-emitting diodes

The amplitude of the emission spike at the transient electroluminescence (TEL) falling edge is an important benchmark for evaluating the quantities of trapped charges existed in organic light-emitting diodes and often shows a normal temperature dependence which increases with the decreasing temperature. Surprisingly, an unreported abnormal temperature-dependent TEL spike was observed in this work. A series of experimental results relevant to the electroluminescence spectrum and TEL measurements demonstrate that this abnormal temperature-dependent behavior is induced by the leakage of hole carriers from the emission layer (EML) to an electron transport layer (ETL). After the voltage pulse is turned off, these holes already leaked into the ETL drift back toward the EML, subsequently engaging in radiative recombination with trapped electrons on guest molecules to generate a spike at the TEL falling edge. However, the drift process is hindered by the reduced carrier mobility of the ETL material with the decrease in temperature. As a result, the spike intensity weakens as the temperature decreases, which contradicts the conclusions reported in previous literatures. Therefore, this study not only leads to the reconsideration for the judgment criteria of the number of trapped charges but also provides valuable insight into the TEL research field of organic optoelectronic devices.

Efficient Near‐Infrared Fluorescence in Deuterated Host–Guest System for Near‐Infrared Organic Light‐Emitting Diodes

AbstractNear‐infrared organic light‐emitting diodes (NIR‐OLEDs) possess substantial potential for future valuable applications, such as a light source for sensing applications. However, the low photoluminescence quantum yield (PLQY) of NIR‐emitting molecules represents a significant impediment to these applications. In this study, the impact of the deuteration of both of host (mCP‐d20: 1,3‐Dicarbazole‐benzene‐d20) and guest (BBT‐TPA‐d28: 4,8‐bis[4‐(N,N‐diphenylamino)phenyl]benzo[1,2‐c:4,5‐c']bis[1,2,5]thiadiazole‐d28) on NIR PL properties in the host–guest codeposited film is reported. The 1 wt%‐BBT‐TPA‐d28:mCP‐d20 codeposited film exhibited PLQY of 15 ± 2% with an emission peak wavelength at ≈900 nm, which is about three times higher than that of the film composed of undeuterated molecules. Importantly, the deuteration of only the host or guest does not yield the PLQY up to 15%, underlining the importance of the deuteration of both host and guest molecules in suppressing nonradiative decay processes. The NIR‐OLED with the deuterated codeposited film as an emissive layer demonstrates a maximum external electroluminescence quantum efficiency of 2.3 ± 0.2%.

High‐Permittivity Polysiloxanes for Bright, Stretchable Electroluminescent Devices

AbstractStretchable alternating current electroluminescent (ACEL) devices have a bright future in wearable electronics and soft robotics. Still, their market application is hindered by high operating voltages. The voltage can be reduced by increasing the relative permittivity of the dielectric elastomer in the emissive layer. Here, a fluorine‐free high‐permittivity silicone elastomer functionalized with cyanopropyl side groups, specially designed for application in stretchable ACEL devices, is introduced. The polar silicone elastomer exhibits excellent mechanical properties and a dielectric permittivity four times higher than commercial PDMS. Light‐emitting devices based on the polar elastomer reach 7.5 times higher maximum luminance at the same electric field than PDMS‐based devices and turn on at a 50% lower electric field. Besides, the polar elastomer‐based devices perform better than all materials tested in literature in achieving high luminance at low electric fields. Stretchable ACEL devices are built from the polar elastomer which shows bright and uniform light emission and can be operated up to 50% strain. The high‐permittivity silicones are promising materials for stretchable ACEL devices and can help their breakthrough to market application by overcoming the drawback of high operating voltages.

Over 30% external quantum efficiency for doping-free D-A-type thermally activated delayed fluorescence organic light-emitting diodes

The application of doping-free emitting layers in organic light-emitting diodes (OLEDs) device fabrication has attracted extensive attention due to the simplified process and the low cost. Meanwhile, the application of interfacially doping layer (or ultrathin emissive layer, UEML) with a thickness below 1 nm showed great potential in achieving high device efficiency by reducing the aggregation-caused quenching (ACQ) effect, which was usually observed in neat film and highly doped dopant-host system. This work developed a simple molecular design strategy via peripheral decoration of TADF emissive core with multiple shielding groups (t-butyl and aryl units) to effectively alleviate the intermolecular interaction and the ACQ effect. The resulting OLEDs based on BIPH-TPA exhibited a record-setting external quantum efficiency (EQE) of 15.8 % at 640 nm with a conventional doping-free configuration (25 nm). Furthermore, the ultrathin doping-free (0.2 nm) OLEDs based on NA-TPA achieved unprecedented EQE of 33.1 % at 620 nm. The outstanding EL performance clearly demonstrated promising potential of this molecular design strategy for the exploration of doping-free OLEDs.