Light-emitting Electrochemical Cells Research Articles

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.

Supramolecular Chalcogen‐Bonded Semiconducting Nanoribbons at Work in Lighting Devices

AbstractThis work describes the design and synthesis of a π‐conjugated telluro[3,2‐β][1]‐tellurophene‐based synthon that, embodying pyridyl and haloaryl chalcogen‐bonding acceptors, self‐assembles into nanoribbons through chalcogen bonds. The ribbons π‐stack in a multi‐layered architecture both in single crystals and thin films. Theoretical studies of the electronic states of chalcogen‐bonded material showed the presence of a local charge density between Te and N atoms. OTFT‐based charge transport measurements showed hole‐transport properties for this material. Its integration as a p‐type semiconductor in multi‐layered CuI‐based light‐emitting electrochemical cells (LECs) led to a 10‐fold increase in stability (38 h vs. 3 h) compared to single‐layered devices. Finally, using the reference tellurotellurophene congener bearing a C−H group instead of the pyridyl N atom, a herringbone solid‐state assembly is formed without charge transport features, resulting in LECs with poor stabilities (<1 h).

Supramolecular Chalcogen-Bonded Semiconducting Nanoribbons at Work in Lighting Devices.

This work describes the design and synthesis of a π‐conjugated telluro[3,2‐β][1]‐tellurophene‐based synthon that, embodying pyridyl and haloaryl chalcogen‐bonding acceptors, self‐assembles into nanoribbons through chalcogen bonds. The ribbons π‐stack in a multi‐layered architecture both in single crystals and thin films. Theoretical studies of the electronic states of chalcogen‐bonded material showed the presence of a local charge density between Te and N atoms. OTFT‐based charge transport measurements showed hole‐transport properties for this material. Its integration as a p‐type semiconductor in multi‐layered CuI‐based light‐emitting electrochemical cells (LECs) led to a 10‐fold increase in stability (38 h vs. 3 h) compared to single‐layered devices. Finally, using the reference tellurotellurophene congener bearing a C−H group instead of the pyridyl N atom, a herringbone solid‐state assembly is formed without charge transport features, resulting in LECs with poor stabilities (<1 h).

Blue light-emitting electrochemical cells incorporating aggregation-induced emission small molecules

The light-emitting electrochemical cells (LECs) have received extensive attention from researchers in recent years due to their simple single-layer structure, air-stable metal electrodes, and low manufacturing cost. So far, reports on the use of LEC active layer emitter materials have mainly focused on ionic transition metal complexes (iTMCs) and conjugated polymers (CPs). The application of the above materials in blue LECs is restricted due to high cost, complicated synthesis process, and poor stability, which results in the slow development of blue LECs. A novel blue LECs device based on aggregation-induced emission (AIE) small molecule (2,3-diphenylbenzo[b]thiophene S, S-sulfur dioxide, DP-BTO) with a peak luminance of 36.58 cd/m2, a CEmax of 1.14 cd/A, and an EQEmax of 0.46% was reported in this work. The blue light emission centered at ~460 nm was exhibited at a voltage above 6 V. This work successfully constructed blue light-emitting LECs devices based on AIE materials, opening up a new research avenue for the application of AIE materials in the field of thin-film device development.

Purely organic pyridium-based materials with thermally activated delayed fluorescence for orange-red light-emitting electrochemical cells

Light-emitting electrochemical cells (LECs) based on ionic transition metal complexes (iTMCs) have become a trend in the field of electroluminescence research due to their simple device structure. However, high cost has become a major obstacle to the development of these precious metal-containing materials. Addressing this challenge, two pyridium-based thermally activated delayed fluorescence (TADF) emitters were designed and synthesized to obtain high-efficiency orange-red-emitting light-emitting electrochemical cells (LECs). Pyridine moiety is used as the precursor of electron acceptor, as well as the embedded ionic part, connecting with different donors, carbazole (CZ) and tert-butyl carbazole (tBuCZ), named Pym-CZ and Pym-tBuCZ. The lifetime measurements showed delayed fluorescence at 2887 and 1746 ns for Pym-CZ and Pym-tBuCZ, respectively, confirming their TADF characteristics. The orange-red emission of Pym-CZ is considered to be due to the strong intermolecular interaction in high concentration, while green emission occurs when the molecules are in monomer form. Notably, compounds Pym-CZ and Pym-tBuCZ are the first pure ionic organic TADF materials for orange-red LECs, and the maximal EQE of LEC based on Pym-CZ reaches approximately 1.2%, demonstrating the potential of a new method for manufacturing low-cost orange-red LECs.

Achieving Record Efficiency and Luminance for TADF Light-Emitting Electrochemical Cells by Dopant Engineering.

Thermally activated delayed fluorescence (TADF) light-emitting electrochemical cells (TADF-LECs) are appealing due to their simple sandwich structure and potential applications in wearable displays and sensors. However, achieving high performance remains challenging. In this paper, we demonstrate that the use of TADF emitters with a low aggregated-caused quenching (ACQ) tendency is crucial to address this challenge. To verify it, two types of TADF-LECs are compared in parallel using different kinds of TADF emitters. The control device uses 2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN) as the dopant, which suffers from a serious ACQ issue and thus dramatically limits the doping concentrations of 4CzIPN in these TADF-LECs. At the best doping condition (0.5 wt %), insufficient host-to-dopant energy transfer (ET) does exist, thereby displaying very limited efficiency and luminance, i.e., 2.43% and 1483 cd m-2. By contrast, the TADF-LECs using 3,6-di(tert-butyl)-1,8-di(4-(bis(4-(tert-butyl)phenyl)amino)phenyl)-9-(4-(4,6-diphenyl-1,3,5-triazin-2-yl) phenyl) carbazole (BPAPTC) can tolerate a much higher doping concentration because BPAPTC is a satisfactory TADF emitter featuring a low ACQ tendency. At the optimized doping condition of 18 wt %, the BPAPTC-based emissive layer possesses the best TADF property, including the longest τDF (2646 ns), the largest rDF (69%), and the highest kRISC of 7.50 × 105 s-1. Moreover, the corresponding TADF-LEC simultaneously displays the most efficient host-to-dopant ET. It thus achieves unprecedented performance, e.g., the highest external quantum efficiency (EQEmax.) of 7.6%, the highest luminance (Lmax.) of 3696 cd m-2, and an EQE of 7.01% at a practical high luminance of 1000 cd m-2.

Carbon nanodots: A metal-free, easy-to-synthesize, and benign emitter for light-emitting electrochemical cells

Carbon nanodots: A metal-free, easy-to-synthesize, and benign emitter for light-emitting electrochemical cells

The Effect of Interface Modification by PEDOT: PSS on the Hole Mobility of the LEC Device

The purpose of the work is to understand how to effect the interface PEDOT: PSS on the hole mobility of the LEC device by Space Charge Limited Current (SCLC) approaches technique. PEDOT: PSS plays a significant role in organic electronics device as interface modification, particularly on Light-emitting electrochemical cells (LEC) due to fundamental structure of hole only device. This study analyses the hole mobility of the device based on current-voltage characteristic approach at room temperature. It has been observed that the PEDOT: PSS interface increases the hole mobility of the LEC device by a factor of 108 .

Multivariate Analysis Identifying [Cu(N^N)(P^P)]+ Design and Device Architecture Enables First-Class Blue and White Light-Emitting Electrochemical Cells.

White light-emitting electrochemical cells (LECs) comprising only [Cu(N^N)(P^P)]+ have not been reported yet, as all the attempts toward blue-emitting complexes failed. Multivariate analysis, based on prior-art [Cu(N^N)(P^P)]+ -based thin-film lighting (>90papers) and refined with computational calculations, identifies the best blue-emitting [Cu(N^N)(P^P)]+ design for LECs, that is, N^N: 2-(4-(tert-butyl)phenyl)-6-(3,5-dimethyl-1H-pyrazol-1-yl)pyridine and P^P: 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene, to achieve predicted thin-film emission at 490nm and device performance of 3.8cdA-1 @170cdm-2 . Validation comes from synthesis, X-ray structure, thin-film spectroscopic/microscopy/electrochemical characterization, and device optimization, realizing the first [Cu(N^N)(P^P)]+ -based blue-LEC with 3.6cdA-1 @180cdm-2 . This represents a record performance compared to the state-of-the-art tricoordinate Cu(I)-complexes blue-LECs (0.17cdA-1 @20cdm-2 ). Versatility is confirmed with the synthesis of the analogous complex with 2-(4-(tert-butyl)phenyl)-6-(3,5-dimethyl-1H-pyrazol-1-yl)pyrazine (N^N), showing a close prediction/experiment match: λ=590/580nm; efficiency=0.55/0.60cdA-1 @30cdm-2 . Finally, experimental design is applied to fabricate the best white multicomponent host:guest LEC, reducing the number of trial-error attempts toward the first white all-[Cu(N^N)(P^P)]+ -LECs with 0.6cdA-1 @30cdm-2 . This corresponds to approximately ten-fold enhancement compared to previous LECs (<0.05cdA-1 @<12cdm-2 ). Hence, this work sets in the first multivariate approach to design emitters/active layers, accomplishing first-class [Cu(N^N)(P^P)]+ -based blue/white LECs that werepreviously elusive.

Iontronic Electroluminescence Devices: Comparing Halide Perovskites and Conjugated Polymers

Halide perovskites and polymer light-emitting electrochemical cells exhibit similar iontronic properties, wherein ion migration under an external bias can be coupled with electronic doping of the perovskites and conjugated polymers, leading to efficient electroluminescence devices with an ultrasimplified device architecture. In this work, it is discovered that a major difference exists between halide perovskites and polymer light-emitting electrochemical cells through low-temperature measurements of photovoltaic properties, photoluminescence spectra, and current–voltage characteristics using precharged devices. In perovskites, ion migration is simultaneously accompanied by doping, and both n-type and p-type doping can occur at a bias much lower than the bandgap/e of the perovskite. On the contrary, ion migration in polymer light-emitting electrochemical cells leads to doping only when the bias exceeds the bandgap/e of the conjugated polymer.

Controlling the Emission Zone by Additives for Improved Light-Emitting Electrochemical Cells.

The position of the emission zone (EZ) in the active material of a light-emitting electrochemical cell (LEC) has a profound influence on its performance because of microcavity effects and doping- and electrode-induced quenching. Previous attempts of EZ control have focused on the two principal constituents in the active material-the organic semiconductor (OSC) and the mobile ions-but this study demonstrates that it is possible to effectively control the EZ position through the inclusion of an appropriate additive into the active material. More specifically, it is shown that a mere modification of the end group on an added neutral compound, which also functions as an ion transporter, results in a shifted EZ from close to the anode to the center of the active material, which translates into a 60% improvement of the power efficiency. This particular finding is rationalized by a lowering of the effective electron mobility of the OSC through specific additive: OSC interactions, but the more important generic conclusion is that it is possible to control the EZ position, and thereby the LEC performance, by the straightforward inclusion of an easily tuned additive in the active material.

Self-Assembly of Double-Helical Metallopolymers.

Metal-containing polymers, or metallopolymers, have diverse applications in the fields of sensors, catalysis, information storage, optoelectronics, and neuromorphic computing, among other areas. The approach of metal-templated subcomponent self-assembly using dynamic covalent linkages allows complex architectures to be formed with relative synthetic ease. The dynamic nature of the linkages between subunits in these systems facilitates error checking during the assembly process and also provides a route to disassemble the structure, rendering these materials recyclable. This Account summarizes a class of double-helical metallopolymers. These metallopolymers are formed via subcomponent self-assembly and consist of two conjugated helical strands wrapping a linear array of CuI centers. Starting from discrete model helicates, we discuss how, through the judicious design of subcomponents, long helical metallopolymers can be obtained and detail their subsequent assembly into nanometer-scale aggregates. Two approaches to generate these helical metallopolymers are compared. We describe methods to govern (i) the length of the metallopolymers, (ii) the relative orientations (head-to-head vs head-to-tail) of the two organic strands, and (iii) the screw-sense of the double helix. Achieving structural control allowed the growth behavior of these systems to be probed. The structure influenced properties in ways that are relevant to specific applications; for example, the length of the metallopolymer determines the color of the light it emits in solution. In the solid state, the ionic nature of these helices renders them useful as both emitters and ionic additives in light-emitting electrochemical cells. Moreover, recent experimental work has clarified the role of the linear array of Cu ions in the transport of charge through these materials. The conductivity displayed by a film of metallopolymer depends upon its history of applied voltage and current, behavior characteristic of a memristor. In addition to the prospective applications already identified, others may be on the horizon, potentially combing stimuli-responsive electronic behavior with the chirality of the helical twist.

TADF: Enabling luminescent copper(i) coordination compounds for light-emitting electrochemical cells.

The last decade has seen a surge of interest in the emissive behaviour of copper(i) coordination compounds, both neutral compounds that may have applications in organic light-emitting doides (OLEDs) and copper-based ionic transition metal complexes (Cu-iTMCs) with potential use in light-emitting electrochemical cells (LECs). One of the most exciting features of copper(i) coordination compounds is their possibility to exhibit thermally activated delayed fluorescence (TADF) in which the energy separation of the excited singlet (S1) and excited triplet (T1) states is very small, permitting intersystem crossing (ISC) and reverse intersystem crossing (RISC) to occur at room temperature without the requirement for the large spin–orbit coupling inferred by the presence of a heavy metal such as iridium. In this review, we focus mainly in Cu-iTMCs, and illustrate how the field of luminescent compounds and those exhibiting TADF has developed. Copper(i) coordination compounds that class as Cu-iTMCs include those containing four-coordinate [Cu(P^P)(N^N)]+ (P^P = large-bite angle bisphosphane, and N^N is typically a diimine), [Cu(P)2(N^N)]+ (P = monodentate phosphane ligand), [Cu(P)(tripodal-N3)]+, [Cu(P)(N^N)(N)]+ (N = monodentate N-donor ligand), [Cu(P^P)(N^S)]+ (N^S = chelating N,S-donor ligand), [Cu(P^P)(P^S)]+ (P^S = chelating P,S-donor ligand), [Cu(P^P)(NHC)]+ (NHC = N-heterocyclic carbene) coordination domains, dinuclear complexes with P^P and N^N ligands, three-coordinate [Cu(N^N)(NHC)]+ and two-coordinate [Cu(N)(NHC)]+ complexes. We pay particular attention to solid-state structural features, e.g. π-stacking interactions and other inter-ligand interactions, which may impact on photoluminescence quantum yields. Where emissive Cu-iTMCs have been tested in LECs, we detail the device architectures, and this emphasizes differences which make it difficult to compare LEC performances from different investigations.

Materials and design strategies for stretchable electroluminescent devices.

Stretchable displays have recently received increasing attention as input and/or output interfaces for next-generation human-friendly electronic systems. Stretchable electroluminescent (EL) devices are a core component of stretchable displays, and they can be classified into two types, structurally stretchable EL devices and intrinsically stretchable EL devices, according to the mechanism for achieving their stretchability. We herein present recent advances in materials and design strategies for stretchable EL devices. First, stretchable devices based on ultrathin EL devices are introduced. Ultrathin EL devices are mechanically flexible like thin paper, and they can become stretchable through various structural engineering methods, such as inducing a buckled structure, employing interconnects with stretchable geometries, and applying origami/kirigami techniques. Secondly, intrinsically stretchable EL devices can be fabricated by using inherently stretchable electronic materials. For example, light-emitting electrochemical cells and EL devices with a simpler structure using alternating current have been developed. Furthermore, novel stretchable semiconductor materials have been presented for the development of intrinsically stretchable light-emitting diodes. After discussing these two types of stretchable EL devices, we briefly discuss applications of deformable EL devices and conclude the review.

Deep-red light-emitting electrochemical cells based on phosphor-sensitized thermally activated delayed fluorescence

Efficient deep-red electroluminescence can be obtained from light-emitting electrochemical cells (LECs) based on phosphor-sensitized thermally activated delayed fluorescence. The maximum external quantum efficiency of these LECs reaches &gt;5%.

New molecularly engineered binuclear ruthenium(II) complexes for highly efficient near-infrared light-emitting electrochemical cells (NIR-LECs).

From the practical point of view, the stability, response time and efficiency of near-infrared light-emitting electrochemical cells (NIR-LECs) are key factors. By using the high potential of chemical modification potential of the phenanthroimidazole ligand, three new binuclear ruthenium(II) complexes with an alkyl spacer as the NIR-emitter were designed and synthesized. NIR-LECs based on these complexes exhibit near-infrared emission at the maximum wavelength of up to 705 nm and with an EQE of up to 0.72% at 4.0 V, which are among the highest values for NIR-LECs based on cationic binuclear ruthenium(II) complexes reported so far. The lifetimes of NIR-LECs based on binuclear complexes were increased about 1.5-to-4-fold with respect to the ones based on mononuclear complexes. Furthermore, a significant decrease in the turn-on time of NIR-LECs by chemical tethering of a new ionic methylpyridinium moiety from 6.3 to 1.4 minutes was observed. It seems that this combinational modification approach can open a new avenue for practical applications.

Facile generation of thenil and furil based blue emitters for the fabrication of non-doped and solution-processed light-emitting electrochemical cells

Environmentally sustainable, energy-efficient and economical devices have drawn great attention and are considered to be the future of artificial lighting device market. LECs provide promising benefits such as their compatible nature with inert cathode materials, solution-processability, and low voltage drive.