Light-emitting Electrochemical Cells Research Articles

Cationic Ir(III) Complex with 5-Phenyl-1H-1,2,4-triazole and 2-(1-Pyrazolyl)pyridine Derivatives for Blue Light-emitting Electrochemical Cells

Cationic iridium complex for blue light-emitting electrochemical cells (LECs) was synthesized from 1-(2,6-dimethylphenyl)-3-methyl-5-phenyl-1H-1,2,4-triazole (dphtz) cyclometalating ligand and 2-(1-phenyl-1H-pyrazol-3-yl)pyridine (phpzpy) ancillary ligand. The photophysical and electrochemical properties were investigated by UV-vis absorption, photoluminescence spectroscopy and cyclic voltammetry, and compared to those of the previously reported [Ir(ppy)2(phpzpy)]+. The LECs emitted blue light peaking around 488 nm and max luminance of 77.35 cd/m2 under constant current density of 10 mA/cm2.

From Textile Coloring to Light-emitting Electrochemical Devices: Upcycling of the Isoviolanthrone Vat Dye.

Produced at ton scale, vat dyes are major environmental pollutants generated by the textile industry. However, they represent ideal and accessible candidates for chemical upcycling since they are usually composed of large π-conjugated scaffolds. Based on the valorization of "old" products, waste or even contaminant into high-added value goods, this concept can be easily transposed to the laboratories. As a contribution to the current environmental and ecological transition, we demonstrate herein the valorization/upcycling of wastewaters generated during the dyeing procedure. To do so, the reduced (leuco) form of vat violet 10, also known as isoviolanthrone, was functionalized to afford a readily soluble derivative that was subsequently and successfully used as active material in operating solution processed light-emitting electrochemical cells, that is, from textile dyeing to high-tech application.

Electrical treeing and luminescence in a polymer p-n junction under extreme reverse bias

A polymer p-n junction was subjected to an extreme reverse bias, up to 1000 V, to investigate its reverse bias characteristics and possible breakdown. A full breakdown did not take place even after more than 2 h of continuous stress. Electrical treeing, a pre-breakdown phenomenon commonly encountered in insulating polymers, was instead observed. The electrical treeing was accompanied by intense light emission, and incurred significant material loss in the polymer film, the latter which was attributed to hot electron bombardment. The impact by hot electrons also caused light emission from the luminescent conjugated polymer. The polymer junction was realized via in situ redox doping reactions in a planar solid-state light-emitting electrochemical cell made from a polymer mixed conductor. The cell was activated with a forward bias and subsequently cooled to freeze the p-n junction. The reverse bias of a doped polymer junction brings interesting opportunities to study the electrical treeing phenomenon, the luminescent conjugated polymer and the still lacking reverse-bias characteristics of an organic p-n junction.

Tweaking the Optoelectronic Properties of S-Doped Polycyclic Aromatic Hydrocarbons by Chemical Oxidation.

Peri-thiaxanthenothiaxanthene, an S-doped analog of peri-xanthenoxanthene, is used as a polycyclic aromatic hydrocarbon (PAH) scaffold to tune the molecular semiconductor properties by editing the oxidation state of the S-atoms. Chemical oxidation of peri-thiaxanthenothiaxanthene with H2 O2 led to the relevant sulfoxide and sulfone congeners, whereas electrooxidation gave access to sulfonium-type derivatives forming crystalline mixed valence (MV) complexes. These complexes depicted peculiar molecular and solid-state arrangements with face-to-face π-π stacking organization. Photophysical studies showed a widening of the optical bandgap upon progressive oxidation of the S-atoms, with the bis-sulfone derivative displaying the largest value (E00 =2.99 eV). While peri-thiaxanthenothiaxanthene showed reversible oxidation properties, the sulfoxide and sulfone derivatives mainly showed reductive events, corroborating their n-type properties. Electric measurements of single crystals of the MV complexes exhibited a semiconducting behavior with a remarkably high conductivity at room temperature (10-1 -10-2 S cm-1 and 10-2 -10-3 S cm-1 for the O and S derivatives, respectively), one of the highest reported so far. Finally, the electroluminescence properties of the complexes were tested in light-emitting electrochemical cells (LECs), obtaining the first S-doped mid-emitting PAH-based LECs.

A metal-free and transparent light-emitting device by sequential spray-coating fabrication of all layers including PEDOT:PSS for both electrodes.

The concept of a metal-free and all-organic electroluminescent device is appealing from both sustainability and cost perspectives. Herein, we report the design and fabrication of such a light-emitting electrochemical cell (LEC), comprising a blend of an emissive semiconducting polymer and an ionic liquid as the active material sandwiched between two poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) conducting-polymer electrodes. In the off-state, this all-organic LEC is highly transparent, and in the on-state, it delivers uniform and fast to turn-on bright surface emission. It is notable that all three device layers were fabricated by material- and cost-efficient spray-coating under ambient air. For the electrodes, we systematically investigated and developed a large number of PEDOT:PSS formulations. We call particular attention to one such p-type doped PEDOT:PSS formulation that was demonstrated to function as the negative cathode, as well as future attempts towards all-organic LECs to carefully consider the effects of electrochemical doping of the electrode in order to achieve optimum device performance.

Dinuclearization strategy of cationic iridium(iii) complexes for efficient and stable flexible light-emitting electrochemical cells

A simple and feasible di-nuclearization strategy of organometallic iridium(iii) complex used to achieve high-efficiency and long-lasting flexible light-emitting electrochemical cells concurrently is demonstrated.

Enhanced durability of cyclometalated Ir(III) photosensitizer through intramolecular π–π interaction for robust photocatalytic hydrogen evolution

Developing photosensitizers with high durability is desirable to boost the practical application of photocatalytic hydrogen evolution. Herein, referring to the successful strategy in the field of light-emitting electrochemical cells, the reported Ir(III) complex with intramolecular [Formula: see text] interaction, Ir2, is used as a photosensitizer to explore its durability. Photocatalytic hydrogen evolution experiment exhibits that the durability of Ir2 is significantly improved with the duration of 39 h, which is three times longer than that of the classical Ir(III) complex Ir1 (ca. 13 h) under the same condition. As revealed by theoretical calculation, the incorporation of intramolecular [Formula: see text] interaction inhibits the rupture of metal–ligand bond in the excited state, thereby reducing the possibility of complex degradation. This is a novel approach to achieve a durable Ir(III) photosensitizer, which stimulates new molecular engineering endeavors. The finding proves the applicability of molecular design strategy in the field of light-emitting electrochemical cells to the photocatalytic hydrogen evolution system, thus boosting the cross integration of different disciplines.

Cationic Iridium(III) Complexes with Benzothiophene-Quinoline Ligands for Deep-Red Light-Emitting Electrochemical Cells.

Three new cationic cyclometalated iridium(III) complexes equipped with differently substituted benzo[b]thiophen-2-ylquinoline cyclometalating ligands and with a sterically demanding tert-butyl-substituted 2,2'-bipyridine ancillary ligand were synthesized and structurally characterized by NMR and X-ray diffraction techniques. To tune the electronic properties of such complexes, the quinoline moiety of the cyclometalating ligands was kept pristine or equipped with electron-withdrawing phenyl and -CF3 substituents, leading to complexes 1, 2, and 3, respectively. A complete electrochemical and photophysical investigation, supported by density functional theory calculations, permits a deep understanding of their electronic properties. The emission of all complexes arises from ligand-centered triplet states in the spectral range between 625 and 950 nm, with excited-state lifetimes between 2.10 and 6.32 μs at 298 K. The unsubstituted complex (1) exhibits the most blue-shifted emission in polymeric matrix at 298 K (λmax = 667 nm, photoluminescence quantum yield (PLQY) = 0.25 and τ = 5.32 μs). The phenyl-substituted complex (2) displays the highest photoluminescent quantum yields (up to 0.30 in polymeric matrix), while the CF3-substituted counterpart (3) shows the most red-shifted emission, peaking at approx. 720 nm, but with lower quantum yields (e.g., 0.10 in polymeric matrix at 298 K). Complexes 1 and 2 were tested in single-layer nondoped light-emitting electrochemical cells (LEECs), using a nozzle-printing technique; both devices display deep-red electroluminescence with an external quantum efficiency close to 20%.

Versatile Electroluminescence Color-Tuning Strategy of an Efficient Light-Emitting Electrochemical Cell (LEC) by an Ionic Additive.

The color-tuning strategies of solid-state light-emitting devices (ss-LEDs) are mainly focused on engineering molecular structures. In this paper, for the first time, we developed a facile strategy for tuning the electroluminescence (EL) color from orange to green through the addition of the ionic additive TBAP (tetrabutylammonium perchlorate). To achieve the active ionic emissive compound for use in a light-emitting electrochemical cell (LEC), the neutral biscyclometalated bromo tetrazole iridium(III) [Ir(ppy)2(BrTz)] was exchanged to its cationic complex, [Ir(ppy)2(BrTz-Me)]ClO4 (ppy = 2-phenyl pyridine, BrTz = 4-bromo-2-pyridine tetrazole, BrTz-Me = 4-bromo-2-pyridine methyl tetrazole) with a new synthetic strategy. This method allows employing neutral Ir-cyclometalated complexes, which are ruled out for use in LECs because of their non-ionic behaviors. In the following, an LEC based on the new cationic [Ir(ppy)2(BrTz-Me)]ClO4 as the emissive layer was fabricated between the FTO (fluorine-doped tin oxide) anode and Ga:In alloy cathode without using any additive or polymers, which makes this configuration the simplest ss-LED so far. By adding the ionic additives, the electroluminescence characteristics of [Ir(ppy)2(BrTz-Me)]ClO4 were dramatically increased, including luminance (L) from 162.8 cd/m2 for the device with an additive to 212.9 and 355.9 cd/m2 for devices containing LiTFSI (bis(trifluoromethane)sulfonamide lithium salt) and TBAP, respectively. In particular, when TBAP was added to the [Ir(ppy)2(BrTz-Me)]ClO4 complex, the irradiance was significantly increased from 166.4 to 220.8 μW/cm2 with an efficacy of 1.78 cd/A and external quantum efficiency (EQE) value of 2.14%. The obtained EL results clearly showed that adding TBAP and LiTFSI significantly improved the electroluminescence characteristics and tuned the electroluminescence color.

Long-Wavelength Light-Emitting Electrochemical Cells: Materials and Device Engineering.

Long-wavelength light-emitting electrochemical cells (LECs) are potential deep-red and near infrared light sources with solution-processable simple device architecture, low-voltage operation, and compatibility with inert metal electrodes. Many scientific efforts have been made to material design and device engineering of the long-wavelength LECs over the past two decades. The materials designed the for long-wavelength LECs cover ionic transition metal complexes, small molecules, conjugated polymers, and perovskites. On the other hand, device engineering techniques, including spectral modification by adjusting microcavity effect, light outcoupling enhancement, energy down-conversion from color conversion layers, and adjusting intermolecular interactions, are also helpful in improving the device performance of long-wavelength LECs. In this review, recent advances in the long-wavelength LECs are reviewed from the viewpoints of materials and device engineering. Finally, discussions on conclusion and outlook indicate possible directions for future developments of the long-wavelength LECs. This review would like to pave the way for the researchers to design materials and device engineering techniques for the long-wavelength LECs in the applications of displays, bio-imaging, telecommunication, and night-vision displays.

Ion transfer into solution-processed electrodes can significantly shift the p–n junction and emission efficiency of light-emitting electrochemical cells

A light-emitting electrochemical cell (LEC) comprises mobile ions in its active material, which enable for in situ formation of a p–n junction by electrochemical doping. The position of this emissive p–n junction in the interelectrode gap is important, because it determines whether the emission is affected by constructive or destructive interference. An appealing LEC feature is that the entire device can be fabricated by low-cost solution-based printing and coating. Here, we show, somewhat unexpectedly, that the replacement of conventional vacuum-deposited indium-tin-oxide (ITO) for the positive anode with solution-processed poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) can result in an increase in the peak light-emission output by 75%. We demonstrate that this emission increase is due to that the p–n junction shifts from a position of destructive interference in the center of the interelectrode gap with ITO to a position of constructive interference closer to the anode with PEDOT:PSS. We rationalize the anodic p–n junction shift by significant anion transfer into the soft and porous PEDOT:PSS electrode during LEC operation, which is prohibited for the ITO electrode because of its compact and hard nature. Our study, thus, contributes with important design criteria for the attainment of efficient light emission from solution-processed LEC devices.

Ionic multiresonant thermally activated delayed fluorescence emitters for light emitting electrochemical cells.

We designed and synthesized two new ionic thermally activated delayed fluorescent (TADF) emitters that are charged analogues of a known multiresonant TADF (MR-TADF) compound, DiKTa. The emission of the charged derivatives is red-shifted compared to the parent compound. For instance, DiKTa-OBuIm emits in the green (λPL = 499 nm, 1 wt % in mCP) while DiKTa-DPA-OBuIm emits in the red (λPL = 577 nm, 1 wt % in mCP). In 1 wt % mCP films, both emitters showed good photoluminescence quantum yields of 71% and 61%, and delayed lifetimes of 316.6 μs and 241.7 μs, respectively, for DiKTa-OBuIm and DiKTa-DPA-OBuIm, leading to reverse intersystem crossing rates of 2.85 × 103 s−1 and 3.04 × 103 s−1. Light-emitting electrochemical cells were prepared using both DiKTa-OBuIm and DiKTa-DPA-OBuIm as active emitters showing green (λmax = 534 nm) and red (λmax = 656 nm) emission, respectively.

Phosphonium-Based Ionic Thermally Activated Delayed Fluorescence Emitters for High-Performance Partially Solution-Processed Organic Light-Emitting Diodes

Phosphonium-Based Ionic Thermally Activated Delayed Fluorescence Emitters for High-Performance Partially Solution-Processed Organic Light-Emitting Diodes

Non-invasive probing of dynamic ion migration in light-emitting electrochemical cells by an advanced nanoscale confocal microscope

In this study, we firstly propose an optical approach to investigate the ion profile of organic films in light-emitting electrochemical cells (LECs) without any invasive sputtering processes. In contrast to previous literatures, this pure optical strategy allows us to record clear and non-destructive ion profile images in the (Ru(dtb-bpy)3(PF6)2) consisted organic layer without interferences of complex collisions from the bombardment of secondary sputter induced ions in a conventional time-of-flight secondary ion mass spectrometry. By using the advanced position sensitive detector (PSD)-based Nanoscale Confocal Microscope, ion distribution profiles were successfully acquired based on the observation of nanoscale optical path length difference by measuring the refractive-index variation while the thickness of the LEC layer was fixed. Dynamic time-dependent ion profile displayed clear ion migration process under a 100 V applied bias at two ends of the LEC. This technique opens up a new avenue towards the future investigations of ion distributions inside organic/inorganic materials, Li-ion batteries, or micro-fluid channels without damaging the materials or disturbing the device operation.

Muscle fibers inspired electrospun nanostructures reinforced conductive fibers for smart wearable optoelectronics and energy generators

Bioinspired muscle fiber–based wearable electronics are a breakthrough in the production of lightweight, mechanically robust next-generation smart textiles. To meet the urgent demand for next generation wearable electronics, the use of interpenetrating reinforced conductive fibers produced through electrospinning is proposed herein. Strain-insensitive and mechanically robust electrospun reinforced conductive fiber (ERCF) electrodes (elongation of 711 % and toughness of 10.05 MJ m−3) contain compliant mechanical reinforcements and deeply adhered conductive silver nanoparticles led to durable wearable optoelectronics. An ambient condition processable and fully solution processable ERCFs exhibits strain-insensitive conductive electrical endurance presents robust hysteresis-free smart gloves and light-emitting electrochemical cells performances, establishing the wearable cognitive human–computer interfaces. A designed ERCF-based nanogenerator can yield outstanding piezoelectric voltage (29.5 V), current (0.39 μA), and power output (11.57 μW) values, surpassing the performance of expensive, toxic, non-biocompatible dopants and technologies requiring highly energy-intensive poling processes.

Intrinsically-ionic donor-acceptor molecules featuring thermally-activated delayed fluorescence for high-performance host-guest blue light-emitting electrochemical cells

High-performance blue light-emitting electrochemical cells (LECs) have remained a formidable challenge. Ionic donor–acceptor molecules, especially those featuring thermally-activated delayed fluorescence (TADF), are promising hosts for developing high-performance blue LECs, but such hosts have remained undeveloped. Here, we report two intrinsically-ionic donor–acceptor molecules, 1-(3-(4-(4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)-6-phenylpyrimidin-2-yl)phenyl)-3-ethyl-1H-imidazol-3-ium hexafluorophosphate (H1) and 1-(3-(4-(2-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)-6-phenylpyrimidin-2-yl)phenyl)-3-ethyl-1H-imidazol-3-ium hexafluorophosphate (H2), and their use as hosts for high-performance blue LECs. 3,6-di-tert-butylcabazole and imidazolium-substituted 2,4-diphenylpyrimidine are used as the donor and acceptor, respectively, which are located para (for H1) or ortho (for H2) to each other on the central phenyl linker. While H1 shows a low triplet energy (<2.6 eV), a large singlet–triplet energy gap (ΔEST) (>0.4 eV) and no TADF, H2 exhibits a high triplet energy (>2.8 eV), a small ΔEST (∼0.2 eV) and distinct TADF, due to its twisted donor–acceptor conformation. Both H1 and H2 show excellent electrochemical stability. Host-guest blue LECs using H2 as the host and a phosphorescent cationic Ir(III) complex as the guest afford sky-blue electroluminescence with high blue-color stability and peak brightness/peak external quantum efficiency/half-lifetime at 1078 cd m−2/8.6%/66 min under a constant-current driving. Upon adding LiPF6 into the active layer, a half-lifetime of ∼ 2 h is achieved at 918 cd m−2. These performances are the highest for host–guest blue LECs reported so far. The efficiencies and operational stability achieved at the high brightness are also the highest for blue LECs with inert-metal cathodes reported so far.

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.

Printable biflourene based ultra-violet (UV) organic light-emitting electrochemical cells (OLECs) with improved device performance

A series of printable UV emitting ionic bifluorene derivatives have been prepared incorporating pendent alkylimidazolium groups. Herein, we detail the synthesis of compounds and the methods used in device fabrication. We show how ink formulation is improved by increasing the solubility of the active bifluorene through extension of the alkyl chain length and switching the counter ion from PF 6 − to CF 3 SO 3 − . We also show how organic light emitting electrochemical cells (OLECs) can be fabricated by spray coating to achieve an active layer with a thickness of ∼150–200 nm, leading to working devices with a turn on voltage of around 6.5 V. This gives electroluminescent (EL) that peaks between 385 nm and 390 nm with a maximum EL emission intensity of 1.29 μW/cm 2 . Thus, EL emission within the UV range has been demonstrated successfully with the synthesised molecules via spray coating onto glass slides. UV light emitting ionic bifluorene small molecules have been synthesised and used to fabricate the OLECs on ITO glass slides. Spray coating technique has been utilized to deposit the functional layers in the OLECs. While the OLEC light emission layer is optimized to the thickness of ∼250 nm, the encapsulated devices demonstrated the turn on voltage of 6.5 V. This fabrication process can be adopted to achieve the light emitting textiles.

Visualizing the effects of salt concentration in planar polymer light-emitting electrochemical cells

The effects of salt concentration have been visualized in bilayer, planar polymer light-emitting electrochemical cells (PLECs) with a gap size of 2 mm. Electrochemical doping and junction electroluminescence (EL) have been observed in a luminescent conjugated polymer (CP) top layer with a different solid polymer electrolyte (SPE) underlayer. By varying the salt concentration and thickness of the SPE layer, the initial cell current, peak cell current, doping propagation speed and junction formation time can all be altered/controlled. This is explained by a limiting ionic resistance in the SPE layer which is both salt concentration and thickness dependent. This study establishes the role of electrolyte ions to support doping current in PLECs. The unexpected observation of a systematic junction shift towards the cathode suggests the electrolyte ions, when functioning as counter ions for doping, are also affected by the salt concentration. At high salt concentrations, the light-emitting junction overlaps with the cathode edges which is undesirable. Thus, the bilayer PLECs allowed for the study and visualization of functions of mobile ions in PLEC operation. Moreover, the bilayer PLECs can potentially be used to study the SPE layer itself using highly sensitive imaging techniques.

Photophysical characteristics of Phenylimidazo(4,5-f)1,10-phenanthroline Rhenium(I) complexes - A theoretical approach to their potential applications in lighting devices

In the search for new luminescent semiconductors to be applied in energy conversion devices, we report via DFT/TD-DFT calculations the structure, UV–Vis absorption, and phosphorescent properties of Re(I) complexes denoted as [ReCl(CO)3(Ln)] and [ReTMP(CO)3(Ln)]+, where TMP is 2,4,6-trimethylpyridine and Ln is a bidentate ligand that corresponds to phenylimidazo(4,5-f)1,10-phenanthroline. This last ligand was analyzed for their electron-donating and -withdrawing characters with different substituents on the peripheral phenyl ring (Ln = -H, –CH3, o –F). Regarding the absorptions, at lower energy in the [ReCl(CO)3(Ln)], it was possible to identify a mixture of metal-to-ligand and ligand-to-ligand charge-transfer excitations, while the [ReTMP(CO)3(Ln)]+ series showed ligand centered transitions as responsible for absorption maximum. The phosphorescence of [ReCl(CO)3(Ln)] complexes exhibits an emission wavelength of 609–623 nm, whose deactivation path comes from π* orbital located at Ln ligands, towards the metal and ligands orbitals. By contrast, the emission process of the [ReTMP(CO)3(Ln)]+ complexes are mainly due to ligand-centered electron promotion (70%) in line with absorption features, showing emission wavelengths in the range of 521–527 nm. The change of the substituent Cl for TMP significantly influences the absorption and emission energies, unlike substitution on the Ln ligand. However, this last substitution pattern becomes relevant when considering injection/transport of holes-electrons where the reorganization energy (λ) for [ReCl(CO)3(L3)] neutral complexes improves due to the more balance charge transfer ability required for organic light-emitting diodes (OLED), while [ReTMP(CO)3(L2)]+ is the best choice for a light-emitting electrochemical cell (LEEC). Then, rhenium complexes can be considered as promising luminophores for device applications.