Light-emitting Electrochemical Cells Research Articles

Polarized polymer light-emitting electrochemical cells using polyfluorene and polycaprolactone blend film by floating-film transfer method

Abstract Sandwich-type structured polarized Polymer Light-emitting Electrochemical Cells (PLECs) of poly(9,9-dioctylfluorene) (PFO) successfully fabricated by combination of floating-film transfer method (FTM) and Post-blending ionic liquid (IL) procedure. An ion-solvating polymer, polycaprolactone (PCL), was blended in PFO for improving IL penetration. The PFO:PCL film showed clear optical dichroism due to the PFO backbone orientation. The IL, 1-Ethyl-3-methylimidazolium (trifluoromethylsulfonyl)imide (EMIm-TFSI), easily post-blended into the PFO:PCL blend FTM film without disordering PFO backbone orientation. The fabricated thickness varied PLECs with ITO anode and aluminum cathode were demonstrated distinct anisotropy, whole device area emission and low turn-on voltage compared to PLEDs.

Scalable Fabrication of Neuromorphic Devices Using Inkjet Printing for the Deposition of Organic Mixed Ionic‐Electronic Conductor

AbstractRecent advancements in artificial intelligence (AI) have highlighted the critical need for energy‐efficient hardware solutions, especially in edge‐computing applications. However, traditional AI approaches are plagued by significant power consumption. In response, researchers have turned to biomimetic strategies, drawing inspiration from the ion‐mediated operating principle of biological synapses, to develop organic neuromorphic devices as promising alternatives. Organic mixed ionic‐electronic conductor (OMIEC) materials have emerged as particularly noteworthy in this field, due to their potential for enhancing neuromorphic computing capabilities. Together with device performance, it is crucial to select devices that allow fabrication via scalable techniques. This study investigates the fabrication of OMIEC‐based neuromorphic devices using inkjet printing, providing a scalable and material‐efficient approach. Employing a commercially available polymer mixed ionic‐electronic conductor (BTEM‐PPV) and a lithium salt, inkjet‐printed devices exhibit performance comparable to those fabricated via traditional spin‐coating methods. These two‐terminal neuromorphic devices demonstrate functionality analogous to literature‐known devices and demonstrate promising frequency‐dependent short‐term plasticity. Furthermore, comparative studies with previous light‐emitting electrochemical cells (LECs) and neuromorphic OMIEC devices validate the efficacy of inkjet printing as a potential fabrication technique. The findings suggest that inkjet printing is suitable for large‐scale production, offering reproducible and stable fabrication processes. By adopting the OMIEC material system, inkjet printing holds the potential for further enhancing device performance and functionality. Overall, this study underscores the viability of inkjet printing as a scalable fabrication method for OMIEC‐based neuromorphic devices, paving the way for advancements in AI hardware.

Electrochemical self-doping in intrinsically ionic, thermally activated delayed fluorescence materials enables high-performance light-emitting electrochemical cells

Intrinsically ionic, thermally activated delayed fluorescence (TADF) materials are promising active materials for solid-state light-emitting electrochemical cells (LECs), but the origin for their superior device performances has remained elusive and how to further boost their device performances toward real world applications has remained an open question. Here, three TADF materials containing the same TADF unit have been prepared, which include a neutral one (R), an ionic one with an imidazolium cation being anchored to the acceptor through an alkyl chain (E1), and an intrinsically ionic one with an imidazolium cation being directly anchored to the acceptor (E2). By comparison between R, E1 and E2, it is revealed that n-type electrochemical self-doping occurs in E2 upon electron-injection onto its acceptor in operating LECs, due to in-situ formed, closely bound [electron-imidazolium cation] pair, which significantly enhances the device performance. By enhancing the chemical stability of the material and centering the carrier recombination zone in the active layer, the green LECs afford peak brightness up to 1461cd/m−2, external quantum efficiencies (EQEs) up to 9.1 %, and half-lifetimes up to 371h at constant current of 50 A/m−2. Peak brightness/EQE/half-lifetime at 322 cd/m−2/10.1 %/∼1800 h has further been achieved at constant current of 10 A/m−2. These LECs represent the most efficient, bright, and stable TADF LECs reported so far. The work clarifies the great advantage and huge potential of intrinsically ionic TADF materials, with unique electrochemical self-doping capacity, for fabricating efficient, bright and stable light-emitting devices.

Wavy Graphene Nanoribbons Containing Periodic Eight-Membered Rings for Light-Emitting Electrochemical Cells.

Precision graphene nanoribbons (GNRs) offer distinctive physicochemical properties that are highly dependent on their geometric topologies, thereby holding great potential for applications in carbon-based optoelectronics and spintronics. While the edge structure and width control has been a popular strategy for engineering the optoelectronic properties of GNRs, non-hexagonal-ring-containing GNRs remain underexplored due to synthetic challenges, despite offering an equally high potential for tailored properties. Herein, we report the synthesis of a wavy GNR (wGNR) embedding periodic eight-membered rings into its carbon skeleton, which is achieved by the A2B2-type Diels-Alder polymerization between dibenzocyclooctadiyne (6) and dicyclopenta[e,l]pyrene-5,11-dione derivative (8), followed by a selective Scholl reaction of the obtained ladder-type polymer (LTP) precursor. The obtained wGNR, with a length of up to 30 nm, is thoroughly characterized by solid-state NMR, FT-IR, Raman, and UV-Vis spectroscopy with the support of DFT calculations. The non-planar geometry of wGNR efficiently prevents the inter-ribbon π-π aggregation, leading to photoluminescence in solution. Consequently, the wGNR can function as an emissive layer for organic light-emitting electrochemical cells (OLECs), offering a proof-of-concept exploration in implementing luminescent GNRs into optoelectronic devices. The fast-responding OLECs employing wGNR will pave the way for advancements in OLEC technology and other optoelectronic devices.

Enhanced performance of ambient-air processed CsPbBr3 perovskite light-emitting electrochemical cells via synergistic incorporation of dual additives

Metal halide perovskite light-emitting electrochemical cells (MHP-LECs) are promising due to their facile solution processability and exceptional optoelectrical properties. However, commercialization is hindered by poor thin-film coverage and the need for glovebox processing. This study addresses these issues by incorporating poly(ethylene oxide) (PEO) and potassium hexafluorophosphate (KPF6) into cesium lead bromide perovskite (CsPbBr3) emitting layers (EML), processed under ambient-air conditions. These perovskite composite thin films were spin-coated on a preheated substrate, and the device structure was ITO/PEDOT:PSS/EML/Al. Notably, the MHP-LEC based on CsPbBr3:PEO:KPF6 (100:65:0.2, weight ratio) exhibited pure-green emission with a peak at 524 nm and a narrow full width at half-maximum of 27 nm. Compared to a reference device (CsPbBr3:PEO (100:65, weight ratio)), the new device showed 1.5-, 4-, and 5-fold improvements in current density, electroluminescence (EL) intensity, and lifetime, respectively. These enhancements are attributed to reduced non-radiative current leakage, improved film surface coverage and passivation, and balanced charge carrier injection and transportation. This study offers a straightforward approach for processing CsPbBr3 thin films under ambient-air conditions, enhancing the EL performance of MHP-LECs.

Unveiling the key role of metal coordination mode and ligand's side groups on the performance of deep-red light-emitting electrochemical cell

Three novel deep-red to near-infrared (DR to NIR) emitters based on mononuclear and dinuclear ruthenium(II) complexes with bulky structures were presented herein. For the first time, the unusual effects of metal coordination mode on the electroluminescence properties of a binuclear emitter were investigated. Unexpectedly, the mononuclear complexes showed superior performance in deep-red light-emitting electrochemical cells (DR-LEC) compared to the dinuclear complex. Likewise, substituting various ancillary ligands improved the radiance and lifetime of devices by 2.5 and 1.5 times, respectively. To the best of our knowledge, the obtained efficiency is among the best reported to date for DR-LECs based on ruthenium polypyridyl complexes.

Thermally Activated Delayed Fluorescence (TADF) Materials Based on Earth-Abundant Transition Metal Complexes: Synthesis, Design and Applications.

Materials exhibiting thermally activated delayed fluorescence (TADF) based on transition metal complexes are currently gathering significant attention due to their technological potential. Their application extends beyond optoelectronics, in particular organic light-emitting diodes (OLEDs) and light-emitting electrochemical cells (LECs), and include also photocatalysis, sensing, and X-ray scintillators. From the perspective of sustainability, earth-abundant metal centers are preferred to rarer second- and third-transition series elements, thus determining a reduction in costs and toxicity but without compromising the overall performances. This review offers an overview of earth-abundant transition metal complexes exhibiting TADF and their application as photoconversion materials. Particular attention is devoted to the types of ligands employed, helping in the design of novel systems with enhanced TADF properties.

Efficient white light-emitting electrochemical cells based on quantum-dot color conversion layers with high EQE >20 %

Solid-state light-emitting electrochemical cells (LECs) offer promising benefits, including simple device architecture, low operating voltage, and insensitivity to electrode work functions. These advantages make them highly suitable for low-cost display and lighting applications, with significant potential for widespread adoption. However, great challenges remain in realizing stable and efficient white LECs. In this work, the blue LECs integrated with red or yellow/red quantum-dot (QD) color conversion layers (CCLs) are shown to achieve efficient and stable white emissions. When the blue LECs are combined with red QD CCLs, they achieve an external quantum efficiency (EQE) exceeding 20 % at a low current density of 0.003 mA cm−2. Alternatively, the blue LECs integrated with yellow/red QD CCLs deliver white emissions with a low CCT <2500 K and a high CRI up to 93 while the peak EQE at a low current density of 0.005 mA cm−2 still remains high (>16 %). The data effectively showcase that efficient and stable white emission can be achieved through partial energy down-conversion from the blue LECs to the QD CCLs, and the proposed white light sources are potential candidates for lighting applications.

Enhancing the Performance of Light-Emitting Electrochemical Cells by Incorporating Quantum Dots

Enhancing the Performance of Light-Emitting Electrochemical Cells by Incorporating Quantum Dots

In Situ Surface-Enhanced Raman Spectroscopy on Organic Mixed Ionic-Electronic Conductors: Tracking Dynamic Doping in Light-Emitting Electrochemical Cells.

In the domain of organic mixed ionic-electronic conductors (OMIECs), simultaneous transport and coupling of ionic and electronic charges are crucial for the function of electrochemical devices in organic electronics. Understanding conduction mechanisms and chemical reactions in operational devices is pivotal for performance enhancement and is necessary for the informed and systematic development of more promising materials. Surface-enhanced Raman spectroscopy (SERS) is a potent tool for monitoring electrochemical evolution and dynamic doping in operational devices, offering enhanced sensitivity to subtle spectral changes. We demonstrate the utility of SERS for in situ tracking of doping in OMIECs in an organic light-emitting electrochemical cell (LEC) containing a conjugated polymer (poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]; MEH-PPV), a molecular anion (lithium triflate), and an electrolyte network (poly(ethylene oxide); PEO). SERS enhancement is achieved via an interleaved layer of gold particles formed by spontaneous breakup of a deposited thin gold film. The results successfully highlight the ability of SERS to unveil time-resolved MEH-PPV doping and polaron formation, elucidating the effects of triflate ion transfer in the operating device and validating the electrochemical doping model in LECs.

Synthesis and characterization of UV organic light-emitting electrochemical cells (OLECs) using phenanthrene fluorene derivatives for flexible applications

This paper details how two new small molecules, based on phenanthrene, were developed, and tailored for light-emitting device applications. An account is provided of both the compound synthesis and the methodologies employed in device fabrication. The ink formulation was improved by the use of triflate counterions. Standard bottom emitting devices were constructed on ITO glass along with top emitting devices on a sputter coated silver on glass substrate. Both structures exhibit UV emissions from the synthesized molecules. Successful EL emission within the UV spectrum range has been achieved by spray coating these active molecules onto glass slides. The optimized solution-processed devices produce UV emission using a semi-transparent silver nanowire top electrode. This results in electroluminescence (EL) peaking at 398 nm, with a maximum EL emission intensity of 20.5 μW/cm2.

Highly efficient organic light‐emitting diodes and light‐emitting electrochemical cells employing multiresonant thermally activated delayed fluorescent emitters with bulky donor or acceptor peripheral groups

AbstractMultiresonant thermally activated delayed fluorescence (MR‐TADF) emitters have been the focus of extensive design efforts as they are recognized to show bright, narrowband emission, which makes them very appealing for display applications. However, the planar geometry and relatively large singlet–triplet energy gap lead to, respectively, severe aggregation‐caused quenching (ACQ) and slow reverse intersystem crossing (RISC). Here, a design strategy is proposed to address both issues. Two MR‐TADF emitters triphenylphosphine oxide (TPPO)‐tBu‐DiKTa and triphenylamine (TPA)‐tBu‐DiKTa have been synthesized. Twisted ortho‐substituted groups help increase the intermolecular distance and largely suppress the ACQ. In addition, the contributions from intermolecular charge transfer states in the case of TPA‐tBu‐DiKTa help to accelerate RISC. The organic light‐emitting diodes (OLEDs) with TPPO‐tBu‐DiKTa and TPA‐tBu‐DiKTa exhibit high maximum external quantum efficiencies (EQEmax) of 24.4% and 31.0%, respectively. Notably, the device with 25 wt% TPA‐tBu‐DiKTa showed both high EQEmax of 28.0% and reduced efficiency roll‐off (19.9% EQE at 1000 cd m−2) compared to the device with 5 wt% emitter (31.0% EQEmax and 11.0% EQE at 1000 cd m−2). The new emitters were also introduced into single‐layer light‐emitting electrochemical cells (LECs), equipped with air‐stable electrodes. The LEC containing TPA‐tBu‐DiKTa dispersed at 0.5 wt% in a matrix comprising a mobility‐balanced blend‐host and an ionic liquid electrolyte delivered blue luminance with an EQEmax of 2.6% at 425 cd m−2. The high efficiencies of the OLEDs and LECs with TPA‐tBu‐DiKTa illustrate the potential for improving device performance when the DiKTa core is decorated with twisted bulky donors.

Polarizable H‐Bond Concept in Aromatic Poly(thiourea)s: Unprecedented High Refractive Index, Transmittance, and Degradability at Force to Enhance Lighting Efficiency

AbstractDeveloping transparent and highly refractive environmentally friendly polymers has not been realized yet toward sustainable optoelectronics. This work describes poly(thiourea)s (PTUs) design following a new “polarizable group synergy” concept, combining highly polarizable hydrogen bonding groups and aromatic‐based spacers to form densely packed and high‐refractive‐index polymer networks. Specifically, PTUs containing m‐ and p‐phenylene spacers exhibit an easy synthesis, high thermostability (Tg = 159 °C), visible transparency (&gt;92%T at 1 µm‐film), ultra‐high refractive index (nD = 1.81) based on the random H‐bonding arrays with a high packing constant (Kp = 0.738), and straightforward preparation of flexible films via solvent‐based techniques. Capitalizing on these assets, PTU‐films are integrated into benchmark graphene‐based lighting device architectures based on the light‐emitting electrochemical cells (LECs) concept. A joint optical modeling and experimental validation confirm the increase in external quantum efficiency expected by the enhanced light out‐coupling of PTU‐films. Finally, PTUs are efficiently depolymerized to low molecular weight compounds by simply adding diamines under heating, following the dynamic covalent bond exchange between thiourea moieties. Overall, this work highlights the PTU family as new promising materials with a unique polarizable H‐bond design to meet efficient and sustainable thin‐film lighting devices.

High‐Performance Narrowband Light‐Emitting Electrochemical Cells Enabled by Intrinsically Ionic Multi‐Resonance Thermally Activated Delayed Fluorescence Emitters

AbstractThe development of efficient, bright, and stable narrowband light‐emitting electrochemical cells (LECs) has remained a challenge. Here, intrinsically ionic multi‐resonance thermally activated delayed fluorescence (MR‐TADF) emitters are reported as guest emitters for narrowband LECs, which are developed by attaching an imidazolium cation onto a typical MR‐TADF emitter. In solution, the emitters show green–blue emission peaked at 486−497 nm with small full widths at half‐maximum (FWHMs) at 24−26 nm. In doped films, they show narrowband green–blue emission with high luminescent efficiencies at ≈90%. LECs using an ionic exciplex host and the ionic MR‐TADF guest emitters show green–blue emission peaked at 494−503 nm with small FWHMs at 31−34 nm, and afford high external quantum efficiencies (EQEs) up to 10% under constant‐voltage driving. With ionic TADF small‐molecule hosts, the narrowband LECs show high EQEs up to 13.0% under constant‐voltage driving, which is the highest among all reported narrowband LECs, and afford peak brightness/EQE/half lifetime at 780 cd m−2/5.6%/62.2 h under constant‐current driving. A long half‐lifetime of ≈630 h has further been achieved at 136 cd m−2. The work demonstrates the great potential for the use of intrinsically ionic MR‐TADF guest emitters and ionic TADF hosts to develop efficient, bright, and stable narrowband LECs.

Oxygen reduction reaction induced electrode effects in polymer light-emitting electrochemical cells

The perceived diminished sensitivity of Polymer Light-Emitting Electrochemical Cells (LECs) to electrode material and active layer thickness compared to Organic Light-Emitting Diodes (OLEDs) due to electrochemical doping, positions them as a focal point in emerging electronic applications. However, empirical evidence reveals the electrode still influences device performance. Simultaneously, electrochemical doping involves side reactions at the cathode, predominantly with oxygen. Nevertheless, the impact of oxygen reduction reaction on the device performance concerning electrode effects remains underexplored. This study centers on the pivotal influence of oxygen reduction reaction on the electrode effect in LECs. The investigation of the doping process for various electrodes and active layer thicknesses was conducted using photoluminescence imaging. Through controlling temperatures and vacuum levels, obtained time-current curves undergo fitting procedures, which enables quantitative analysis of oxygen reduction reaction effects on both electrode and film thickness. The results underscore the impact of oxygen reduction reaction on the performance of the device induced by electrodes, emphasizing the pronounced effect on the activation energy of the reduction reaction dictated by both the electrode work function and oxygen concentration. In addition, this study elucidates that the utilization of a low-work-function bottom contact in conjunction with a thicker active layer exerts a discernible influence on the device's current magnitude.

Cationic Ir(III) Complexes with 4-Fluoro-4'-pyrazolyl-(1,1'-biphenyl)-2-carbonitrile as the Cyclometalating Ligand: Synthesis, Characterizations, and Application to Ultrahigh-Efficiency Light-Emitting Electrochemical Cells.

Light-emitting electrochemical cells (LECs) promise low-cost, large-area luminescence applications with air-stabilized electrodes and a versatile fabrication that enables the use of solution processes. Nevertheless, the commercialization of LECs is still encountering many obstacles, such as low electroluminescence (EL) efficiencies of the ionic materials. In this paper, we propose five blue to yellow ionic Ir complexes possessing 4-fluoro-4'-pyrazolyl-(1,1'-biphenyl)-2-carbonitrile (ppfn) as a novel cyclometalating ligand and use them in LECs. In particular, the device within di[4-fluoro-4'-pyrazolyl-(1,1'-biphenyl)-2-carbonitrile]-4,4'-di-tert-butyl-2,2'-bipyridyl iridium(III) hexafluorophosphate (DTBP) shows a remarkable photoluminescence quantum yield (PLQY) of 70%, and by adjusting the emissive-layer thickness, the maximal external quantum efficiency (EQE) reaches 22.15% at 532 nm under the thickness of 0.51 μm, showing the state-of-the-art value for the reported blue-green LECs.

The first proof-of-concept of straightforward and ambient-processed CsPbBr3 perovskite light-emitting electrochemical cell

All-inorganic cesium lead-halide perovskite thin films show great promise for optoelectronic applications. This study introduces a novel synthesis strategy for CsPbBr3 thin films, conducted on a preheated substrate. The synthesis process involves precise control and optimization of parameters under ambient conditions, eliminating the need for a glovebox. Therefore, a facile and low-cost way to fabricate CsPbBr3 perovskite thin films for potential optoelectronic applications has been provided by this method. The effects of various parameters, such as precursor concentration, annealing temperature, and polyethylene oxide (PEO) addition, on the structure, morphology, and optical properties of the perovskite films were systematically studied. The optimal conditions for the highest photoluminance and maximum surface coverage were found to be 0.26 M precursor concentration, 100 °C annealing temperature, and 100:65 CsPbBr3:PEO weight ratio. Uniform and bright perovskite films with over 99% surface coverage, reduced grain size, and suppressed trap density were obtained as a result of these conditions. The PL intensity of the optimized composite thin film exhibited a minor decrease of 6% from its initial value over six months in an ambient environment. A proof-of-concept perovskite light-emitting electrochemical cell (PeLEC) was constructed utilizing the optimized single-layer perovskite thin film. Consequently, green electroluminescence at 523 nm, featuring a full width at half maximum (FWHM) of 20 nm and a chromaticity coordinate of (0.121, 0.800), was successfully generated from the PeLEC. This showcases the viability of the mentioned methodology for the future fabrication of light-emitting devices.

Electrical current modeling for polymer light-emitting electrochemical cells: Contributions from electrons, ions, and oxygen

A very promising approach to achieving stable polymer P-N junctions is polymer light-emitting electrochemical cells (LECs). In LECs, under a specific voltage bias, the injection of carriers into the polymer occurs through a redox reaction and subsequently gets compensated by opposite ions, resulting in the creation of electrochemical doping. Unlike organic light-emitting diodes, which have numerous mature electrical current models serving as invaluable tools for understanding the underlying mechanism and predicting device performance, LECs lack such modeling. This lack of modeling stems from the greater complexity of LECs, as the electrical current in LECs is composed of not only electronic components but also ionic contributions, along with a side-reaction portion arising from the electrochemical reaction. This work demonstrates an electrical current model for LECs, which is simple and accurate enough for practical applications. The model achieves a quantitative separation of electronic and ionic charge contributions to the electrical currents, as well as provides insights into the distribution of oxygen through operation schemes. Additionally, this paper incorporates the relationships between oxygen level, voltage, temperature, and current into the current model, thereby discerningly formulating expressions for ionic and electronic currents within the model. This demonstrates a precise equation for LEC electric current.

Morphological and structural defect optimization in CsPbBr3 nanoparticle films for light-emitting electrochemical cells

Crystalline and morphological defects in the perovskite film affect the operation of light-emitting devices. Thus, advanced and scalable fabrication techniques can improve device properties. In this work, we use slot-die coating at ambient conditions, followed by hot air drying, to produce CsPbBr3 light-emitting electrochemical cells. We compare this method to spin-coating and analyze film morphology and optical properties. We reveal that annealing the film on a hot plate increases PLQY and Shockley-Read-Hole lifetime, but worsens film morphology. In contrast, hot air drying during deposition improves morphology but reduces photoluminescence. The slot-die coating shows better results for device fabrication. With InGa and Al top electrodes, we achieve luminance 8100 cd m−2 and 2900 cd m−2 at a 5 V bias, respectively.

Mild and Efficient Extraction of Fluorescent Chlorophyll a from Spinach Leaves for Application as the Sustainable Emitter in Light‐Emitting Electrochemical Cells

AbstractNatural pigments are sustainable compounds that can be employed as emitters, sensors and sensitisers in optoelectronics. The most abundant pigment, chlorophyll, offers advantages of easily available and plentiful feedstock, biodegradability and non‐toxicity. However, strenuous extraction and separation limit its application on larger scale. In this work, a practically mild and scalable extraction and separation method for rapid isolation of chlorophyll a from spinach is presented. Three different stationary phases for column chromatography were evaluated, and a new solvent system was developed for the elution of chlorophyll a on a neutral alumina chromatography column. The purified product was obtained with a yield of 0.98 mg ⋅ g−1 with respect to the dry leaves. A first light‐emitting electrochemical cell (LEC) based on chlorophyll a as the emitter is reported, using the extracted chlorophyll a as the guest compound dispersed in a blend‐host matrix in a concentration of 2.5 or 5 mass %. The higher‐chlorophyll‐concentration LEC exhibits emission solely from the chlorophyll emitter, with the main emission peak located at 675 nm. The lower‐chlorophyll‐concentration LEC features two distinct emission bands, one in the red region that is originating from the chlorophyll guest and one in the blue region (main peak at 430 nm) that stems from the blend host. This combined red:blue emission can be attractive for, e. g., greenhouse applications, since it matches the action spectrum of plant photosynthesis.