Light-emitting Electrochemical Cells Research Articles

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 (>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.

Efficiency Roll-Off in Light-Emitting Electrochemical Cells.

Understanding "efficiency roll-off" (i.e., the drop in emission efficiency with increasing current) is critical if efficient and bright emissive technologies are to be rationally designed. Emerging light-emitting electrochemical cells (LECs) can be cost- and energy-efficiently fabricated by ambient-air printing by virtue of the in situ formation of a p-n junction doping structure. However, this in situ doping transformation renders a meaningful efficiency analysis challenging. Herein, a method for separation and quantification of major LEC loss factors, notably the outcoupling efficiency and exciton quenching, is presented. Specifically, the position of the emissive p-n junction in common singlet-exciton emitting LECs is measured to shift markedly with increasing current, and the influence of this shift on the outcoupling efficiency is quantified. It is further verified that the LEC-characteristic high electrochemical-doping concentration renders singlet-polaron quenching (SPQ) significant already at low drive current density, but also that SPQ increases super-linearly with increasing current, because of increasing polaron density in the p-n junction region. This results in that SPQ dominates singlet-singlet quenching for relevant current densities, and significantly contributes to the efficiency roll-off. This method for deciphering the LEC efficiency roll-off can contribute to a rational realization of all-printed LEC devices that are efficient at highluminance.

Achieving High-efficiency, High-color-purity Light-emitting Electrochemical Cells through TADF-sensitized Fluorescence Strategy

Achieving High-efficiency, High-color-purity Light-emitting Electrochemical Cells through TADF-sensitized Fluorescence Strategy

Controlling aggregation-induced emission by supramolecular interactions and colloidal stability in ionic emitters for light-emitting electrochemical cells.

Chromophores face applicability limitations due to their natural tendency to aggregate, with a subsequent deactivation of their emission features. Hence, there has been a fast development of aggregation induced emission (AIE) emitters, in which non-radiative motional deactivation is inhibited. However, a fine control of their colloidal properties governing the emitting performance is fundamental for their application in thin film optoelectronics. In addition, ion-based lighting devices, such as light emitting electrochemical cells (LECs), requires the design of ionic AIE emitters, whose structure allows (i) an easy ion polarizability to assist charge injection and (ii) a reversible electrochemical behavior. To date, these fundamental questions have not been addressed. Herein, the hydrophilic/hydrophobic balance of a family of cationic tetraphenyl ethene (TPE) derivatives is finely tuned by chemical design. The hydrophilic yet repulsive effect of pyridinium-based cationic moieties is balanced with hydrophobic variables (long alkyl chains or counterion chemistry), leading to (i) a control between monomeric/aggregate state ruling photoluminescence, (ii) redox behavior, and (iii) enhanced ion conductivity in thin films. This resulted in a LEC enhancement with the first ionic AIE emitters, reaching values of 0.19 lm W-1 at ca. 50 cd m-2. Overall, this design rule will be key to advance ionic active species for optoelectronics.

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

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

Synthesis and Characterization of Iridium(III) Complexes with Substituted Phenylimidazo(4,5-f)1,10-phenanthroline Ancillary Ligands and Their Application in LEC Devices.

In this work, we report on the synthesis and characterization of six new iridium(III) complexes of the type [Ir(C^N)2(N^N)]+ using 2-phenylpyridine (C1-3) and its fluorinated derivative (C4-6) as cyclometalating ligands (C^N) and R-phenylimidazo(4,5-f)1,10-phenanthroline (R = H, CH3, F) as the ancillary ligand (N^N). These luminescent complexes have been fully characterized through optical and electrochemical studies. In solution, the C4-6 series exhibits quantum yields (Ф) twice as high as the C1-3 series, exceeding 60% in dichloromethane and where 3MLCT/3LLCT and 3LC emissions participate in the phenomenon. These complexes were employed in the active layer of light-emitting electrochemical cells (LECs). Device performance of maximum luminance values of up to 21.7 Lx at 14.7 V were observed for the C2 complex and long lifetimes for the C1-3 series. These values are counterintuitive to the quantum yields observed in solution. Thus, we established that the rigidity of the system and the structure of the solid matrix dramatically affect the electronic properties of the complex. This research contributes to understanding the effects of the modifications in the ancillary and cyclometalating ligands, the photophysics of the complexes, and their performance in LEC devices.

Recent advances of organic emitters in deep-red light-emitting electrochemical cells.

Light-emitting electrochemical cells (LECs) are kind of easily fabricated and low-cost light-emitting devices that can efficiently convert electric power to light energy. Compared with blue and green LECs, the performance of deep-red LECs is limited by the high non-radiative rate of emitters in long-wavelength region. While various organic emitters with deep-red emission have been developed to construct high-performance LECs, including polymers, metal complexes, and organic luminous molecules (OLMs), but this is seldom summarized. Therefore, we overview the recent advances of organic emitters with emission at the deep-red region for LECs, and specifically highlight the molecular design approach and electrochemiluminescence performance. We hope that this review can act as a reference for further research in designing high-performance deep-red LECs.

Hydrophilic Conjugated Polymers for Sustainable Fabrication of Deep‐Red Light‐Emitting Electrochemical Cells

AbstractIt is crucial to develop functional electronic materials that can be processed from green solvents to achieve environmentally sustainable and cost‐efficient printing fabrication of organic electronic devices. Here, the design and cost‐efficient synthesis of two hydrophilic and emissive conjugated polymers, TQ‐OEG and TQ2F‐OEG, are presented, which are rendered hydrophilic through the grafting of oligo(ethylene glycol) (OEG) solubilizing groups onto the thiophene‐quinoxaline conjugated backbone and thereby can be processed from a water:ethanol solvent mixture. The results show that the introduction of the OEG groups enables for a direct dissolution of salts by the neat polymer for the attainment of solid‐state ion mobility. These properties are utilized for the design and development of light‐emitting electrochemical cells (LECs), the active materials of which can be solution cast from a water:ethanol‐based ink. It is specifically shown that such an LEC device, comprising an optimized blend of the TQ2F‐OEG emitter and a Li salt as the active material positioned between two air‐stabile electrodes, delivers deep‐red emission (peak wavelength = 670 nm) with a radiance of 185 µW m−2 at a low drive voltage of 2.3 V. This study contributes relevant information as to how polymers and LEC devices can be designed and fabricated to combine functionality with sustainability.

Optical Probing of Operation Processes in Light-Emitting Electrochemical Cells

Optical Probing of Operation Processes in Light-Emitting Electrochemical Cells

Trifluoromethyl substituted cationic cyclometallated iridium(III) complexes: Photophysical properties and theoretical studies

Three new trifluoromethyl substituted (–CF3) cationic iridium(III) complexes 1–3, with 3,5-dimethyl-1-(3-(trifluoromethyl)phenyl)-1H-pyrazole (DTPP) as the cyclometallated ligand, namely, [Ir(DTPP)2bpy](PF6) (1, bpy = 2, 2′–bipyridine), [Ir(DTPP)2phen](PF6) (2, phen = 1,10-phenanthroline) and [Ir(DTPP)2pphen](PF6) (3, pphen = pyrazino[2,3-f][1,10]phenanthroline) have been rationally designed and synthesized in good yields (60–77%). The photophysical, electrochemical, and electroluminescent properties were fully characterized. Complexes 1–3 show bright blue to yellow emission in the both solution and neat film states with maximum emission wavelength in the range of 484–560 nm. Photoluminescence quantum yields (PLQYs) are very high ranging from 62% to 72% in the solution state with microsecond excited-state lifetimes (0.39–0.77 μs). More detailed photophysical and theortical calculation studies reveal that introduction of –CF3 substituent onto the cyclometallated ligand can induce a blue-shift emission compared with conventional fluorinated phenylpyridine or phenylpyrazole complexes, which can be contributed to the decline in HOMO energy level of complexes. Single-crystal structure analysis of complex 3 shows a distorted octahedral geometry with strong intermolecular π–π stacking (face-to-face) interactions between ancillary ligands in adjacent molecules, which might be responsible for the obviously red-shifted emission compared with complexes 1 and 2 in the aggregated state. In addition, electroluminescent properties of all three new complexes have been studied in the application of light-emitting electrochemical cells (LECs), which show green to yellow electroluminescence (λmax, EL = 516–540 nm).