Stretchable light-emitting electrochemical cells fabricated by spray-coating

Interaction of current gain and avalanche multiplication in three-layer diodes is utilized to produce uniform avalanche effect, indicated by uniform light emission over the area of a junction. Proof that the effect is caused by the three-layer action is furnished by removing the emitter layer, which changes the light emission to the usually observed microplasma pattern.

Organic light emitting diodes (OLED) have been greatly investigated for application in flat panel display and light source. A different type of OLED is light emitting electrochemical cell (LEC), which consists of luminescent chromophore films with solid electrolyte such as ionic ruthenium complex film. Driving process of LEC is different from that of OLED. That is, upon applied voltage, the electric double layer is formed at the interface between the anode or cathode and ionic luminescent film and hole and electron are injected from both electrodes into the luminescent film and then light is emitted by charge recombination. Thus, LEC has attractive features which are single-layer device and no requirement to use low work function metal as a cathode. However, LEC has some drawback which is ill charge-injection balance between hole and electron caused by the difference in ion size between large ruthenium complex cation and small counter anion. To improve the charge-injection balance, incorporation of small cation into ionic luminescent layer will be expected. In this study, current efficiency for LEC was improved by introduction of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as additive electrolyte into the ruthenium complex film of LEC. LEC was prepared by spin-coating tris(2,2’-bipyridyl) ruthenium(II) complex TFSI2 (Ru(bpy)3(TFSI)2) and poly(methyl methacrylate) (PMMA) matrix in acetonitrile solution with LiTFSI solid electrolyte on ITO anodes. And then silver was vacuum-deposited onto the resulting ruthenium complex film with solid electrolyte on ITO anodes. The luminescent property of LEC was performed by applying constant current. The standard LEC without LiTFSI was not emitted and the lower voltage than 1 V was observed in the constant current of smaller than 1.5 mA whereas it was emitted and the voltage of around 2.5 V was observed in the constant current of larger than 1.5 mA. These results indicate that the only hole injection occurs and the electron injection does not occur until the applied current over 1.5 mA. It’s due to the thicker electric double layer of ruthenium complex at the cathode of LEC. As introduction of LiTFSI into the ionic luminescent layer, the emission starts at the smaller applied current with increasing amount of LiTFSI. The maximum current efficiency becomes 3.0 cd/A in the LEC containing 3 wt% LiTFSI. However, the addition of more than 3 wt% LiTFSI into LEC makes LEC performance worse because the distance between ruthenium complex become wide and the electron hopping rate becomes slow. Finally, addition of 10 wt% LiTFSI makes crystalize ionic luminescent layer and it does not function as LEC. Therefore, we found that the introduction of 3 wt% LiTFSI to LEC was the optimum condition.

We introduce ITO on graphene as a current-spreading layer for separated InGaN/GaN nanorod LEDs for the purpose of passivation-free and high light-extraction efficiency. Transferred graphene on InGaN/GaN nanorods effectively blocks the diffusion of ITO atoms to nanorods, facilitating the production of transparent ITO/graphene contact on parallel-nanorod LEDs, without filling the air gaps, like a bridge structure. The ITO/graphene layer sufficiently spreads current in a lateral direction, resulting in uniform and reliable light emission observed from the whole area of the top surface. Using KOH treatment, we reduce series resistance and reverse leakage current in nanorod LEDs by recovering the plasma-damaged region. We also control the size of the nanorods by varying the KOH treatment time and observe strain relaxation via blueshift in electroluminescence. As a result, bridge-structured LEDs with 8 min of KOH treatment show 15 times higher light-emitting efficiency than with 2 min of KOH treatment.

In the field of quantum dot (QD)‐based micro‐light‐emitting diode (µLED) full‐color display technology, achieving high color conversion efficiency (CCE) is one of the key performance indicators. In this work, a µLED architecture is presented that incorporates an optimized nanorod array, with QDs and nanogapped gold nanoparticles (AuNNPs) embedded in the inter‐rod gaps. By harnessing non‐radiative energy transfer (NRET) and localized surface plasmon resonance (LSPR), the absorption and utilization of quantum well (QW) energy by the QDs are significantly enhanced. To ensure efficient current spreading and uniform light emission, graphene is employed as a transparent conductive layer to interconnect the nanorods. As graphene can transfer photogenerated carriers to the QDs, enhancing their quantum yield, it is also introduced as an intermediate insertion layer and support layer, allowing the integration of a second layer of QDs and AuNNPs on the light‐emitting surface. This design maintains the electrical performance of the nanorod µLED while achieving ultra‐high CCE. Experimental results demonstrate that the proposed µLED with nanorod structures and AuNNPs achieves a maximum CCE of 94%, representing a 102% improvement compared to conventional planar µLEDs. These findings offer promising insights for advancing high‐performance, full‐color µLED display technologies through nanoscale engineering.

Herein, sandwich-structured hybrid flexible transparent conductive electrodes (FTCEs), implemented by sandwiching silver nanowire (AgNWs) networks between 2D Ti3C2Tx MXene layers and applying pressure treatment is proposed. The synergistic effect of MXene flakes and mechanical pressure promotes efficient welding of wire-to-wire junctions, resulting in a flexible electrode with a low sheet resistance of 20.5 Ω sq-1 and a high transmittance of 92.3%. Thanks to strong hydrogen bonding interactions between MXene layers and the PET substrate, combined with effective embedding of AgNWs into the substrate, the pressure-treated MXene/AgNWs/MXene FTCE exhibits outstanding thermal stability, remarkable mechanical durability, and chemical stability (Sheet resistance only slightly changes after annealed at 160 °C for 3.5h or 1000 bending tests). As a consequence, the resulting sandwich-structured FTCE-based green phosphorescent organic light-emitting diode (OLED) achieves state-of-the-art electroluminescence performance with a maximum external quantum efficiency (EQE) of 22.9% and a current efficiency of 81.4cd A-1, which is significantly higher than those of ITO-based reference device. Most interestingly, the flexible red OLED demonstrates an EQE reaching 24.6%, extremely small efficiency roll-off (23.1% at 2144cd m-2), and a large-area 25 × 25 mm2 panel with uniform light emission, which represents the best performance reported to date for flexible red-wavelength OLEDs.

Two blue‐emitting cationic iridium complexes with 2‐(1H‐pyrazol‐1‐yl)pyridine (pzpy) as the ancillary ligands, namely, [Ir(ppy)2(pzpy)]PF6 and [Ir(dfppy)2(pzpy)]PF6 (ppy is 2‐phenylpyridine, dfppy is 2‐(2,4‐difluorophenyl) pyridine, and PF6− is hexafluorophosphate), have been prepared, and their photophysical and electrochemical properties have been investigated. In CH3CN solutions, [Ir(ppy)2(pzpy)]PF6 emits blue‐green light (475 nm), which is blue‐shifted by more than 100 nm with respect to the typical cationic iridium complex [Ir(ppy)2(dtb‐bpy)]PF6 (dtb‐bpy is 4,4′‐di‐tert‐butyl‐2,2′‐bipyridine); [Ir(dfppy)2(pzpy)]PF6 with fluorine‐substituted cyclometalated ligands shows further blue‐shifted light emission (451 nm). Quantum chemical calculations reveal that the emissions are mainly from the ligand‐centered 3π–π* states of the cyclometalated ligands (ppy or dfppy). Light‐emitting electrochemical cells (LECs) based on [Ir(ppy)2(pzpy)]PF6 gave green‐blue electroluminescence (486 nm) and had a relatively high efficiency of 4.3 cd A−1 when an ionic liquid 1‐butyl‐3‐methylimidazolium hexafluorophosphate was added into the light‐emitting layer. LECs based on [Ir(dfppy)2(pzpy)]PF6 gave blue electroluminescence (460 nm) with CIE (Commission Internationale de L'Eclairage) coordinates of (0.20, 0.28), which is the bluest light emission for iTMCs‐based LECs reported so far. Our work suggests that using diimine ancillary ligands involving electron‐donating nitrogen atoms (like pzpy) is an efficient strategy to turn the light emission of cationic iridium complexes to the blue region.

Recently, organic light-emitting transistors (OLETs) which combined organic light-emitting diodes (OLEDs) with organic thin film transistors (OTFTs) have being intensively investigated for application to display. They can control light emission by the gate voltage. So far, we have investigated light-emitting electrochemical cells (LECs) and electrochemical transistors (ECTs) independently. LECs have some advantages about a simple device structure and usage of air-stable metal cathode compared to OLEDs. ECTs also have unique features for lower driving gate voltage than that of OTFTs because the electrochemical doping becomes easier for the formation of the electric double layer in the ECTs. Therefore, a novel concept for the light-emitting electrochemical transistor (LECT) which consists of the light-emitting electrochemical cell (LEC) and the electrochemical transistor (ECT) was introduced. To aim at constructing LECT, four-electrode electrochemical system using interdigitated array (IDA) electrodes was employed to demonstrate the concept of LECT in solution. Cyclic voltammetric (CV) measurement was carried out in acetonitrile solution containing Ru(bpy)3(PF6)2 as redox species and a chemiluminescent material. We confirmed that the light emission was controlled at the electrode potential difference between both fingers of IDA electrodes as source and drain electrodes of the transistor and reference/counter electrodes as gate electrode. The light emission was observed at the electrogenerated chemiluminescent condition where the excited state Ru(II) was produced on the gap distance by the electron exchange reaction of Ru(I) and Ru(III) produced at both fingers of IDA electrodes, respectively. If the electrode potential would be shifted to anodic or cathodic directions, whether Ru(I) or Ru(III) would be not produced, the light emission disappear even at the same bias voltage between the source and drain electrodes (VDS) where the light emission is possible. If VDS is not enough to produce Ru(I) and Ru(III), the light emission does not occur. If VDS is enough to produce Ru(I) and Ru(III) in the diffusion limited condition, the light emission becomes steady state intensity. Therefore, we demonstrated that the light emission can be controlled by VDS and the gate electrode potential in the LECT. We also confirmed that current behavior flowed between source and drain electrodes shows bipolar transistor characteristics. We are now trying to solidify the LECT system in solution to realize LECT device.

The synthesis, photophysical/electrochemical characterization, and implementation in light‐emitting electrochemical cells (LECs) of three novel red‐emitting heteroleptic [Cu(N^N)(P^P)]PF6 complexes are reported. The complex design consists in combining i) the bridged di(pyrazin‐2‐yl)sulfane N^N ligand with expanded π‐conjugation and electro‐withdrawing 4‐(trifluoromethyl)pyrimidine moieties and ii) the [(diphenylphosphino)phenyl] ether (DPEphos) P^P ligand. The effect of the N^N ligand substitution on the photophysical, electrochemical, ion conductivity, and morphological features in their respective powders and thin films is thoughtfully rationalized. This is rounded by the trends noted in single‐layered red‐emitting LECs (λmax = 630–660 nm) featuring moderate performances with irradiances of around 60–90 µW cm−2 and total emitted energies in the range of 4–12 mJ. A further optimization using a multilayered architecture, in which hole injection/transport and exciton formation processes are decoupled, leads to a significant enhancement of the irradiance up to ≈150 µW cm−2 and total emitted energies of 91 mJ without affecting device chromaticity. Finally, the best red‐emitting complex in LECs is used to fabricate host:guest white devices, achieving luminances of 12 cd m−2 in concert with an excellent white color quality (x/y CIE color coordinates of 0.31/0.32; color rendering index of 90) stable over lifespan.

It has recently been demonstrated that light‐emitting electrochemical cells (LECs) can be designed to deliver strong emission with high efficiency when the charge transport is effectuated by a majority host and the emission is executed by a minority guest. A relevant question is then: should the guest be physically blended with or chemically incorporated into the host? A systematic study is presented that establishes that for near‐infrared‐(NIR‐) emitting LECs based on poly(indacenodithieno[3,2‐b]thiophene) (PIDTT) as the host and 4,7‐bis(4,4‐bis(2‐ethylhexyl)‐4H‐silolo[3,2‐b:4,5‐b′]dithiophen‐2‐yl)benzo[c][1,2,5]‐thiadiazole (SBS) as the guest the chemical‐incorporation approach is preferable. The host‐to‐guest energy transfer in LEC devices is highly efficient at a low guest concentration of 0.5%, whereas guest aggregation and ion redistribution during device operation severly inhibits this transfer in the physical‐blend devices. The chemical‐incorporation approach also results in a redshifted emission with a somewhat lowered photoluminescence quantum yield, but the LEC performance is nevertheless very good. Specifically, an NIR‐LEC device comprising a guest‐dilute (0.5 molar%) PIDTT‐SBS copolymer delivers highly stabile operation at a high radiance of 263 µW cm−2 (peak wavelength = 725 nm) and with an external quantum efficiency of 0.214%, which is close to the theoretical limit for this particular emitter and device geometry.

The simple device architecture as well as the solution-based processing makes light-emitting electrochemical cells (LECs) a promising device concept for large-area flexible lighting solutions. The lack of deep-blue emitters, which are, at the same time, efficient, bright, and long-term stable, complementary to the wide variety of yellow-orange-emitting LECs, hampers the creation of white LECs. We present a hybrid device concept for the realization of white light emission by combining blue colloidal quantum dots (QDs) and an Ir-based ionic transition-metal complex (iTMC) LEC in a new type of white QD-LEC hybrid device (QLEC). By careful arrangement of the active layers, we yield light emission from both the blue QDs and the yellow iTMC emitter already at voltages below 3 V. The QLEC devices show homogeneous white light emission with high color rendering index (up to 80), luminance levels above 850 cd m-2, and a maximum external quantum efficiency greater than 0.2%.

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.

Light emitting devices based on a bifunctional block copolymer containing crown ether spacers

Self‐stability in light‐emitting electrochemical cells (LECs) based on Ir(III) ionic transition metal complexes (Ir‐iTMC) has been long overlooked. Herein, it is demonstrated that the nature of the active layer blending an archetype Ir‐iTMC as emitter and ionic electrolytes—ionic liquid (IL) or ionic polyelectrolyte (IP)—is paramount for the storage and mechanical stability of rigid/flexible LECs. Strikingly, devices with ionic polyelectrolytes (IPs) stand out compared to those with traditional configurations with or without ILs. They exhibit i) superior brightness and efficiencies in rigid/flexible devices due to the higher photoluminescence quantum yield, ii) the best performance at pulsed current driving mode under inert/ambient operation conditions due to a slower growth of the doped regions, iii) enhanced device stabilities upon ambient/inert storage, resulting in <10% performance loss after 1 month of aging, and iv) the smallest performance loss (<10%) upon bending stress, since IPs prevent mechanically induced damage, preserving morphological and spectroscopic features. These findings are supported by steady‐state and time‐resolved emission spectroscopy, electrochemical impedance spectroscopy, microscopic and mechanical assays, along with the analysis of fresh and aged devices driven at different modes under inert/ambient conditions. Overall, this work highlights the need of revisiting new emitter:electrolyte combinations toward realizing highly self‐stable LECs.

The characteristic doping process in polymer light‐emitting electrochemical cells (LECs) causes a tradeoff between luminescence intensity and efficiency. Experiments and numerical modeling on thin film polymer LECs show that, on the one hand, carrier injection and transport benefit from electrochemical doping, leading to increased electron‐hole recombination. On the other hand, the radiative recombination efficiency is reduced by exciton quenching by polarons involved in the doping. Consequently, the quasi‐steady‐state luminescent efficiency decreases with increasing ion concentration. The transient of the luminescent efficiency shows a characteristic roll‐off while the current continuously increases, attributed to ongoing electrochemical doping and the associated exciton quenching. Both effects can be modeled by exciton polaron‐quenching via diffusion‐assisted Förster resonance energy transfer. These results indicate that the tradeoff between efficiency and intensity is fundamental, suggesting that the application realm of future LECs should be sought in high‐brightness, low‐production cost devices, rather than in high‐efficiency devices.

Solid‐state light‐emitting electrochemical cells (LECs) using sustainable and eco‐friendly materials and affording high brightness, efficiency, and stability are highly desired. Here, intrinsically ionic, thermally activated delayed fluorescence (TADF) materials 1–3 for efficient, bright, and stable LECs are reported. 1–3 feature carbazole‐type donors and cationic triazine‐type acceptors, which are located ortho to each other on the phenyl linkers. Through‐space charge‐transfer (CT) dominates the CT transitions in 1–3. In doped and neat films, 1–3 show blue and green TADF emission, respectively, with reverse intersystem crossing rates at around 7.0 × 105 s−1. 1–3 possess excellent electrochemical stability (except for the oxidation of 1) and film‐forming abilities. LECs using neat films of 1–3 as the single active layers afford green electroluminescence with peak brightness/peak external quantum efficiency (EQE) of up to 572 cd m−2/6.8% under 4.0 V and peak brightness/peak EQE/half‐lifetime of up to 860 cd m−2/5.4%/48 h under 50 A m−2. A longer half‐lifetime of 218 h has further been achieved at 162 cd m−2 under 10 A m−2. The work reveals the bright prospect for the development of efficient, bright, and stable LECs with intrinsically‐ionic TADF materials.