Active Emission Layer Research Articles

Enabling Ultra‐Narrow‐Band Deep‐Blue Light‐Emitting Diode Via Trapping Injected Holes by Carbon Dots

AbstractNarrow‐band deep‐blue light‐emitting diodes (LEDs) (CIEy coordinate<0.08, full width at half maximum (FWHM)<20 nm) are important for the next generation of LED‐based displays. It remains a significant challenge for the design of narrow‐band deep‐blue emitters. Herein, a new strategy is proposed for constructing narrow‐band deep‐blue LEDs that are not dependent on the narrow‐band deep‐blue emitter, but only need to dope n‐type carbon dots (n‐CDs) in the active emission layer (EML) of the wide‐band blue LEDs. The n‐CDs‐LEDs show an ultra‐narrow deep‐blue emission at 430 nm with the color coordinates of (0.15, 0.04) and the FWHM of 19 nm. Wherein the EML is constructed by doping n‐CDs in Poly(9,9‐dioctylfluorene‐co‐N‐(4‐butylphenyl) diphenylamine) (TFB), which neither changes the band structure of TFB nor produces new energy levels. Transient photovoltage (TPV) and various photoelectrochemical characterizations jointly reveal that n‐CDs can trap holes to realize the carrier injection into a single energy level of TFB and achieve ultra‐narrow deep‐blue light emission. This work provides a brand‐new and simple way to realize the narrow‐band deep‐blue LEDs.

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.

Stable and Bright Electroluminescent Devices utilizing Emissive 0D Perovskite Nanocrystals Incorporated in a 3D CsPbBr3 Matrix.

The 0D cesium lead halide perovskite Cs4 PbBr6 has drawn remarkable interest due to its highly efficient robust green emission compared to its 3D CsPbBr3 counterpart. However, seizing the advantages of the superior photoluminescence properties for practical light-emitting devices remains elusive. To date, Cs4 PbBr6 has been employed only as a higher-bandgap nonluminescent matrix to passivate or provide quantum/dielectric confinement to CsPbBr3 in light-emitting devices and to enhance its photo-/thermal/environmental stability. To resolve this disparity, a novel solvent engineering method to incorporate highly luminescent 0D Cs4 PbBr6 nanocrystals (perovskite nanocrystals (PNCs)) into a 3D CsPbBr3 film, forming the active emissive layer in single-layer perovskite light-emitting electrochemical cells (PeLECs) is designed. A dramatic increase of the maximum external quantum efficiency and luminance from 2.7% and 6050cd m-2 for a 3D-only PeLEC to 8.3% and 11 200cd m-2 for a 3D-0D PNC device with only 7% by weight of 0D PNCs is observed. The majority of this increase is driven by the efficient inherent emission of the 0D PNCs, while the concomitant morphology improvement also contributes to reduced leakage current, reduced hysteresis, and enhanced operational lifetime (half-life of 129 h), making this one of the best-performing LECs reported to date.

Hybrid Polymeric Nanocomposites Based High Performance Oleds: A Review

This review reports the recent significant progress for achieving the synthesis of various types of polymer-based nanocomposites and understanding of the basic principle which determine their optical, electronic and magnetic properties. Some of the polymeric nanocomposite materials show remarkable electrical as well as optical properties regarding towards interest for applications in OLEDs. Nanoparticles which are basically inorganic, plays important role in the opto-electronic field. A nanoparticle with conjugate polymer matrix makes polymer nanocomposite and that nanocomposite used as an active emissive layer between the structures of optoelectronic device like OLEDs, OPVs etc. Application of such polymeric nanocomposite materials are discussed in this present communication with their success and failure, towards their optical as well as electrical properties in the fabrication of OLEDs.

Novel D-A-D Fluorescent Dyes Based on 9-(p-Tolyl)-2,3,4,4a,9,9a-hexahydro-1H-carbazole as a Donor Unit for Solution-Processed Organic Light-Emitting-Diodes.

New fluorescent D-A-D dyes containing 9-(p-tolyl)-2,3,4,4a,9,9a-hexahydro-1H-carbazole as a donor unit and 2,1,3-benzochalcogenadiazoles as an electron-withdrawing group were synthesized. The photoluminescent and electroluminescent properties of novel dyes for fluorescent OLED application were investigated. It was demonstrated that the replacement of lightweight heteroatoms by heavier ones enables the fine tuning of the maximum emission without significantly reducing the luminescence quantum yield. The maximum quantum yield value of 62.6% for derivatives based on 2,1,3-benzoxadiazole (1a) in cyclohexane was achieved. Two devices with the architecture of glass/ITO/PEDOT-PSS/poly-TPD/EML/TPBi/LiF/Al (EML = emitting layer) were fabricated to check the suitability of the synthesized compounds as a single active emission layer in OLED. These OLEDs exhibited clear red electroluminescence of the dyes with the maximum current efficiency of 0.85 Cd/A.

Fluorescent Carbon Dots: Fantastic Electroluminescent Materials for Light‐Emitting Diodes

Fluorescent carbon dots (CDs) have emerged as fantastic luminescent nanomaterials with significant potentials on account of their unique photoluminescence properties, high stability, and low toxicity. The application of CDs in electroluminescent light‐emitting diodes (LEDs) have aroused much interest in recent years. Herein, the state‐of‐the‐art advances of CD‐based electroluminescent LEDs are summarized, in which CDs act as active emission layer and interface transport layer materials is discussed and highlighted. Besides, the device structure of CD‐based LEDs and preparation methods of CDs are also introduced. Furthermore, the opportunities and challenges for achieving high performance CD‐based electroluminescent LED devices are presented. This review article is expected to stimulate more unprecedented achievements derived from CDs and CD‐based electroluminescent LEDs, thus further promoting their practical applications in future solid‐state lighting and flat‐panel displays.

Ultra‐Bright and Stable Pure Blue Light‐Emitting Diode from O, N Co‐Doped Carbon Dots

AbstractThe pure blue light‐emitting diodes (LEDs) play a key role in the application of solid display and illumination. Most of pure blue LEDs reported so far, are based on semiconductor quantum dots and metal complex, involving heavy metal elements. Here, the fabrication of O, N co‐doped carbon dots (CDs) by a hydrothermal method from perylene‐3,4,9,10‐tetracarboxylic dianhydride and 2,3‐diaminophenazine is shown. These CDs show strong photoluminescence (PL) emission peaked at 447 nm (in ethanol) with absolute PL quantum yields of 88.9%. They are then doped into poly(vinyl carbazole) to form the active emission layer in the pure blue CDs LEDs. The electroluminescence emission of these CDs LEDs is centered at 452 nm with Commission Internationale d'Eclairage (CIE) coordinates of (0.14, 0.10). Moreover, the pure blue CDs LEDs show a high external quantum efficiency of 2.114%, high brightness of 648 cd m–2, long half‐lifetime (T50) of 200 h, and a favorable current efficiency of 2.2 cd A−1. This work opens a new way to develop cost‐effective, environmental‐friendly, and high‐efficient metal‐free pure blue LEDs.

Boosting the Efficiency and Stability of Perovskite Light-Emitting Devices by a 3-Amino-1-propanol-Tailored PEDOT:PSS Hole Transport Layer.

Properties of the underlying hole transport layer (HTL) in perovskite light-emitting devices (PeLEDs) play a critical role in determining the optoelectronic performance through influencing both the charge transport and the quality of the active perovskite emission layer (EML). This work focuses on manipulating the carrier transport behavior and obtaining a high-quality EML film by tailoring the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HTL with previously unused amino alcohol 3-amino-1-propanol (3AP). The modified PEDOT:PSS rendered a deeper work function that is more suitable for the hole injection from the HTL to EML. More importantly, the 3AP-modified PEDOT:PSS film can induce a low-dimensional perovskite phase that can passivate the defects in the EML, resulting in a significantly improved light emission. Such ameliorations consequently result in a dramatical enhancement in performance of PeLED with a low turn-on voltage of 2.54 V, a maximum luminance of 23033 cd/m2, a highest current efficiency of 29.38 cd/A, a corresponding maximum external quantum efficiency of 9.4%, and a prolonged lifetime of 6.1 h at a proper Cs/Pb ratio.

Polylactide‐perylene derivative for blue biodegradable organic light‐emitting diodes

AbstractIn this work we demonstrate, for the first time, the use of polylactic acid (PLA) as a biodegradable host matrix for the construction of the active emissive layer of organic light‐emitting diode (OLED) devices for potential use in bioelectronics. In this preliminary study, we report a robust synthesis of two fluorescent PLA derivatives, pyrene‐PLA (AH10) and perylene‐PLA (AH11). These materials were prepared by the ring opening polymerisation of l‐lactide with hydroxyalkyl‐pyrene and hydroxyalkyl‐perylene derivatives using 1,8‐diazabicyclo[5.4.0]undec‐7‐ene as catalyst. OLEDs were fabricated from these materials using a simple device architecture involving a solution‐processed single‐emitting layer in the configuration ITO/PEDOT:PSS/PVK:OXD‐7 (35%):AH10 or AH11 (20%)/TPBi/LiF/Al (ITO, indium tin oxide; PEDOT:PSS, poly(3,4‐ethylenedioxythiophene) doped with poly(styrenesulfonic acid); PVK, poly(vinylcarbazole); OXD‐7, (1,3‐phenylene)‐bis‐[5‐(4‐tert‐butylphenyl)‐1,3,4‐oxadiazole]; TPBi, 2,2′,2″‐(1,3,5‐benzenetriyl)tris(1‐phenyl‐1H‐benzimidazole)). The turn‐on voltage for the perylene OLED at 10 cd m–2 was around 6 V with a maximum brightness of 1200 cd m–2 at 13 V. The corresponding external quantum efficiency and device current efficiency were 1.5% and 2.8 cd A–1 respectively. In summary, this study provides proof of principle that OLEDs can be constructed from PLA, a readily available and renewable bio‐source. © 2020 The Authors. Polymer International published by John Wiley & Sons Ltd on behalf of Society of Industrial Chemistry.

Green perovskite light-emitting diodes with simultaneous high luminance and quantum efficiency through charge injection engineering

Metal halide perovskite light emitting diodes (PeLEDs) have recently experienced rapid development due to the tunable emission wavelengths, narrow emission linewidth and low material cost. To achieve state-of-the-art performance, the high photoluminescence quantum yield (PLQY) of the active emission layer, the balanced charge injection, and the optimized optical extraction should be considered simultaneously. Multiple chemical passivation strategies have been provided as controllable and efficient methods to improve the PLQY of the perovskite layer. However, high luminance under large injection current and high external quantum efficiency (EQE) can hardly be achieved due to Auger recombination at high carrier density. Here, we decreased the electron injection barrier by tuning the Fermi-level of the perovskite, leading to a reduced turn on voltage. Through molecular doping of the hole injection material, a more balanced hole injection was achieved. At last, a device with modified charge injection realizes high luminance and quantum efficiency simultaneously. The best device exhibits luminance of 55,000 cd m−2, EQE of 8.02% at the working voltage of 2.65 V, current density of 115 mA cm−2, and shows EQE T50 stability around 160 min at 100 mA cm−2 injection current density.

New cyanopyridine based conjugated polymers carrying auxiliary electron donors: From molecular design to blue emissive PLEDs

Three new D-A (Donor-Acceptor) configured conjugated polymers, i.e.PPy1-3, centered on strong electron accepting cyanopyridine scaffold carrying varied auxiliary donors, viz. phenylene (PPy1), biphenyl (PPy2), and fluorene (PPy3) were designed and synthesized as blue emitters for PLEDs. The new polymers were subjected to spectral, thermal, photophysical and electrochemical characterization. Also, computational studies (DFT) were performed on the repeating units of polymer using Turbomole 7.2 V software package at the B3LYP/TZVP hybrid levels. Further, their weight average molecular masses were found to be 38.8 kDa, 38.9 kDa and 57.7 kDa, respectively as determined by GPC technique. Furthermore, the new polymers PPy1-3, were shown to be stable thermally up to 308–374 °C. Evidently, they exhibited good photophysical behavior with their optical energy band gaps of 2.53–2.64 eV. Finally, the polymers PPy1-3 were employed as an active emissive layer in standard ITO/PEDOT:PSS/Polymer/Al configured PLEDs. Interestingly, at 12 V all the newly fabricated devices exhibit a stable blue characteristic electroluminescence with low threshold voltages of 3.40–5.20 V, confirming an efficient injection of electrons in the diodes. From the results, it is clear that, the polymers PPy1-3, can be considered as prospective blue light emitters for PLED application.

High efficient Light-Emitting Electrochemical Cells based on ionic liquids 1,2,3-triazolium

Abstract The Light-Emitting Electrochemical Cell (LEEC) is one of the simplest kind of electroluminescent solid-state devices. Generally, they are fabricated with only one emitting material layer (EML), so-called active emission layer, that consist of an emitter material mixed with an ionic electrolyte. Apart from the electrolyte, their structure is similar to that of a single layer organic light-emitting diode (OLED). Not only LEECs assemble most of OLEDs applications and their technological advantages, but they also bring additional ones such as (i) low dependence of the electrodes work function, (ii) low dependence to the EML thickness, (iii) low operation voltage and high brightness and finally (iv) LEECs can be printed or sprayed in large areas. To investigate the performance of ionic liquid (IL) based devices we synthesized four organic salts that were compared with devices using four inorganic salts commonly used as electrolyte for LEECs. All devices were fabricated with a commercial poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) as emitter polymer. As a result, we showed that the best device performance was achieved with the Ionic Liquid 1-dodecyl-4-(hydroxymethyl)-3-methyl-1H-1,2,3-triazol-3-ium iodide (dohmtI) presenting a current efficiency of 3.6 cd/A, an external quantum efficiency (EQE) of 1.4% as well as a luminance of 3.2 × 104 cd/m2. Despite the use of MEH-PPV, a polymer not so efficient when compared to the state-of-art of the novel polymers employed nowadays, our realization showed these new ILs have been able to provide the specific requirements of the LEEC devices.

Hydrochromic full-color MXene quantum dots through hydrogen bonding toward ultrahigh-efficiency white light-emitting diodes

Multiple-color emissive MXene quantum dots (MQDs) exhibit vast application prospects in various fields, including optoelectronics, bioimaging, and catalysis. However, the majority of the recent MQDs display limited maximum emission in the blue-light region. Herein, we employ the hydrogen bonds as an adjustment method to prepare novel full-color MQDs using Ti3C2 MXene as the starting material. By doping with sodium thiosulfate and ammonia water (sulfur-doped, nitrogen-doped), the maximum emission of the obtained MQDs demonstrates entire light spectrum covering from blue to orange light. Interestingly, the as-synthesized Ti3C2 MQDs aqueous solutions unveil multiple-color, excitation-independent emission wavelength and fluorescence (lifetime and quantum yield) enhancement compared with in the dry state. The fluorescence shift and enhancement are confirmed by comprehensive spectroscopic techniques (e.g. grazing incidence X-ray diffraction (GIXRD)) and complementary density functional theory (DFT) calculations. The results indicate that the construction of stalwart bridge-like hydrogen-bonded networks between the MQDs by highly ordered bound water on the doped MQDs surface can give rise to the immobilization of the CO and C–O bonds of the MQDs, thus strengthening the rigidity of the entire system. As a benefit of the bandgap emission, white light-emitting diodes (WLEDs) with stable emission color by directly utilizing MQDs as an active emission layer have been realized for the first time. As such, the as-prepared full-color MQDs developed herein can remarkably broaden the prospect of MXene 2D quantum dots (2D-QDs) in a myriad of technological applications such as electronics, batteries, bioimaging, and cancer therapy.

Deep blue organic light-emitting diodes of 1,8-diaryl anthracene

We report on the optimization of organic light emitting diode (OLED) devices using 1,8-di-(4-trifluromethylphenyl)-anthracene (CF3-DPA) as the active emissive layer. CF3-DPA emits in the deep blue region with an emission peak at 432 nm in solution which showed a slight red shift in thin films. CF3-DPA has high reported fluorescence quantum efficiency, $${\sim } 67\%$$ , as compared to 9,10-diphenyl anthracene (9,10-DPA). We optimized the OLED devices with different hole transporting layers (HTLs). Bilayer devices formed with $$N,N^{\prime }$$ -di(1-naphthyl)- $$N,{N}^{\prime }$$ -diphenyl-( $$1,1^{\prime }$$ -biphenyl)- $$4,4^{\prime }$$ -diamine (NPD) as the HTL gave a reasonable light output. We observed that trilayer or multilayer devices with the inclusion of poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS) and/or copper phthalocyanine as an additional HTL reduced the turn on voltage by $$\sim $$ 5 to 9 V, though the brightness of the light emission also decreased. Including suitable carrier (electron or hole) transporting layers like $$2,2^{\prime }$$ , $$2^{\prime \prime }$$ -(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) and $$4,4^{\prime }$$ -Bis(N-carbazolyl)- $$1,1^{\prime }$$ -biphenyl (CBP) increases the efficiency of the devices. From our studies, we conclude that though NPD/CF3-DPA interface is crucial for light emission, the performance of the devices is limited by the mismatch of the hole and electron mobilities and the low internal quantum efficiency of CF3-DPA in the solid state. Devices having ITO/NPD/CF3-DPA/TPBi/LiF-Al geometry were observed to be the most efficient. Synopsis: This paper reports the application of 1,8-diaryl anthracene derivative (CF3-DPA) in blue OLEDs which is observed to emit at 439 nm in thin films. OLED devices with different geometries and combination of ETLs and HTLs were fabricated. Device with ITO/NPD/CF3-DPA/TPBi/LiF-Al was found to be the most efficient.

Enhanced device efficiency in organic light-emitting diodes by dual oxide buffer layer

Abstract We have demonstrated an improvement in device performance of fluorescent organic light-emitting diodes (OLEDs) by inserting a dual anode buffer layer composed of tungsten oxide (WO3) and molybdenum oxide (MoO3). The advantage of adding dual anode buffer layers with different deposition sequences over individual and composite oxide buffer layers has been systematically analyzed based on their electronic and optical properties. The incorporation of single and composite buffer layers has been revealed to induce a very low injection barrier for holes in tri-layer fluorescent OLEDs which results in a charge imbalance in the emission layer. In contrast, a proper sequence of buffer layers (WO3/MoO3) exhibiting higher contact angle (lower surface energy) and higher surface roughness, together with a step-wise increment of potential barrier leads to a better overall charge balance in the active emission layer. Therefore, an enhanced current efficiency and power efficiency of ∼5.8 cd/A and ∼5.2 lm/W respectively were recorded for the WO3/MoO3 buffer unit, which was better than the insertion of individual and composite layers.

Synthesis and optical characterization of novel carbazole Schiff bases

In this study, newly substituted carbazole derivatives of S1; (Z)-4-((9-isobutyl-9H-carbazol-3-ylimino)methyl)phenol, S2; (Z)-9-butyl- N-(2,3,4-trimethoxybenzylidine)-9H-carbazol-3-amine, S3; (Z)-4-((9-octyl-9H-carbazol-3-ylimino)methyl)benzene-1,2-diol and S4; (Z)-3-((9-octyl-9H-carbazol-3-ylimino)methyl)benzene-1,2-diol compounds are synthesized by using condensation reaction between carbazole amines and aromatic aldehydes. All synthesized carbazole Schiff bases are purified by crystallizing from chloroform. The structural and optical characterizations of synthesized compounds are investigated by FT-IR (Fourier Transform-Infrared Spectroscopy), 1H NMR (Proton Nuclear Magnetic Resonance), 13C NMR (Carbon Nuclear Magnetic Resonance), LC-MS (Liquid Chromatography-Mass Spectrometry) and temperature dependent PL (Photoluminescence) measurements. The formations of synthesized Schiff bases were confirmed by FT-IR, NMR and microanalysis. Due to stronger π-conjugation and efficient charge transfer from host material, the broad and complex bands centered at about ∼2.16 and ∼1.76 eV are observed in PL spectra for all samples. Their relative intensities depend on functional groups associated with the carbazole. These newly synthesized Schiff bases could be considered as an active emissive layer for organic light emitting diodes.

Emission and evaporation properties of 75 at.% Re-25 at.% W mixed matrix impregnated cathode

We present a comprehensive study on the phase, emission performance, surface composition, chemical states and evaporation properties of a 75at.% Re-25at.% W (75Re) mixed matrix impregnated cathode by several modern analyzers, including XRD, electron emission test device, in situ AES, XPS and Quartz Crystal Oscillation Instrument (QCOI). On the basis of experimental results, the adsorption energy and charge transfer of the Ba-O dipole adsorbed on cathode surface was investigated by the first-principles density functional theory calculations. The in situ AES analyses indicate that the atomic ratio of Ba:O of the active emission layer on the cathode surface converged to 3:2 for a conventional Ba-W cathode and to about 3:1 for the 75Re cathode. Due to the larger adsorption energy of Ba and Ba-O on 75Re cathode surface, the total evaporation rate of Ba and BaO in the 75Re cathode is much lower than that for the Ba-W cathode, which is agreed favorably with the experimental evaporation data. Our characterizations and calculations suggest that rhenium in the matrix of impregnated cathodes improves the stability of Ba-O dipole on the cathode surface and enhances the emission capability substantially.

Bright Multicolor Bandgap Fluorescent Carbon Quantum Dots for Electroluminescent Light-Emitting Diodes

Themonochrome light-emitting diodes (LEDs) from blue to red that directly use multicolor bandgap fluorescent carbon quantum dots (MCBF-CQDs) as the active emission layer will be reported. The MCBF-CQDs were synthesized by a facile solvothermal method and exhibited emission color from blue to red with a quantum yield (QY) up to 75%. Even without using hole transport layer, the maximum luminance (Lmax) of blue LEDs reached about 136 cd/m2, a record for CQDs-based monochrome electroluminescent LEDs. We further demonstrated the distinctive advantage of such LEDs, the remarkably stable and voltage-independent emission color, which enabled the BF-CQDs great opportunities to achieve high-performance LEDs. Also presented are white LEDs (WLEDs) fabricated by using BF-CQDs blended poly(N-vinyl carbazole) as the emissive layer, which showed a high-performance even comparable to semiconductor QDs-based LEDs.

Bright Multicolor Bandgap Fluorescent Carbon Quantum Dots for Electroluminescent Light-Emitting Diodes.

Multicolor bandgap fluorescent carbon quantum dots (MCBF-CQDs) from blue to red with quantum yield up to 75% are synthesized using a solvothermal method. For the first time, monochrome electroluminescent light-emitting diodes (LEDs) with MCBF-CQDs directly as an active emission layer are fabricated. The maximum luminance of blue LEDs reaches 136 cd m-2 , which is the best performance for CQD-based monochrome electroluminescent LEDs.

Enhanced polymer light-emitting diode property using fluorescent conducting polymer-reduced graphene oxide nanocomposite as active emissive layer

The present article reports the polymer light-emitting diode property of the nanocomposite comprising poly 9,9-dioctyl fluorene-alt-bithiophene and reduced graphene oxide used as an emissive layer. Two times repetition of Hummers oxidation and hydrazine hydrate reduction method produce reduced graphene oxide (term as rGO2) with more uniform distribution in size and thickness. In addition, this uniquely synthesized rGO2 induces favorable shift in balance of electron and hole recombination zone toward the center of emissive layer owing to increase in in-plane crystallite size and high localize aromatic confinement. Five times increase in maximum device efficiency (Cd/A) and three times increase in maximum brightness (Cd/m2) are achieved with the LED device using nanocomposite as emissive layer compared to neat polymer. Also, the fabricated device requires relatively low turn-on voltage (4 V) because of low energy barrier between PEDOT work function (−5.0 eV) and HOMO levels of bi-thiophene copolymer −5.67 eV) and nanocomposite (−5.66 eV).