Improving the balance of carrier mobilities of host-guest solid-state light-emitting electrochemical cells

We report efficient host-guest solid-state light-emitting electrochemical cells (LECs) utilizing a cationic terfluorene derivative as the host and a red-emitting cationic transition metal complex as the guest. Carrier trapping induced by the energy offset in the lowest unoccupied molecular orbital (LUMO) levels between the host and the guest impedes electron transport in the host-guest films and thus improves the balance of carrier mobilities of the host films intrinsically exhibiting electron preferred transporting characteristics. Photoluminescence measurements show efficient energy transfer in this host-guest system and thus ensure predominant guest emission at low guest concentrations, rendering significantly reduced self-quenching of guest molecules. EL measurements show that the peak EQE (power efficiency) of the host-guest LECs reaches 3.62% (7.36 lm W(-1)), which approaches the upper limit that one would expect from the photoluminescence quantum yield of the emissive layer (～0.2) and an optical out-coupling efficiency of ～20% and consequently indicates superior balance of carrier mobilities in such a host-guest emissive layer. These results are among the highest reported for red-emitting LECs and thus confirm that in addition to reducing self-quenching of guest molecules, the strategy of utilizing a carrier transporting host doped with a proper carrier trapping guest would improve balance of carrier mobilities in the host-guest emissive layer, offering an effective approach for optimizing device efficiencies of LECs.     

Tailoring carrier injection efficiency to improve the carrier balance of solid-state light-emitting electrochemical cells

We study the influence of the carrier injection efficiency on the performance of light-emitting electrochemical cells (LECs) based on a hole-preferred transporting cationic transition metal complex (CTMC) [Ir(dfppz)(2)(dtb-bpy)](+)(PF(6)(-)) (complex 1) and an electron-preferred transporting CTMC [Ir(ppy)(2)(dasb)](+)(PF(6)(-)) (complex 2) (where dfppz is 1-(2,4-difluorophenyl) pyrazole, dtb-bpy is 4,4'-di(tert-butyl)-2,2'-bipyridine, ppy is 2-phenylpyridine and dasb is 4,5-diaza-9,9'-spirobifluorene). Experimental results show that even with electrochemically doped layers, the ohmic contacts for carrier injection could be formed only when the carrier injection barriers were relatively low. Thus, adding carrier injection layers in LECs with relatively high carrier injection barriers would affect carrier balance and thus would result in altered device efficiency. Comparison of the device characteristics of LECs based on complex 1 and 2 in various device structures suggests that the carrier injection efficiency of CTMC-based LECs should be modified according to the carrier transporting characteristics of CTMCs to optimize device efficiency. Hole-preferred transporting CTMCs should be combined with an LEC structure with a relatively high electron injection efficiency, while a relatively high hole injection efficiency would be required for LECs based on electron-preferred transporting CTMCs. Since the tailored carrier injection efficiency compensates for the unbalanced carrier transporting properties of the emissive layer, the carrier recombination zone would be located near the center of the emissive layer and exciton quenching near the electrodes would be significantly mitigated, rendering an improved device efficiency approaching the upper limit expected from the photoluminescence quantum yield of the emissive layer and the optical outcoupling efficiency from a typical layered light-emitting device structure.     

The efficiency of light-emitting electrochemical cells

We report on the efficiency behavior of light-emitting electrochemical cells (LECs) fabricated from a methyl-substituted ladder-type poly(p-phenylene) (mLPPP) that was blended with a crown ether based solid state electrolyte. Unlike organic light-emitting diodes (oLEDs) utilizing mLPPP as an active layer, the LECs suffer from a loss of efficiency at elevated current densities. From scan rate dependent studies we deduce that this efficiency drop is not only due to device decomposition upon high voltage operation and we also reveal the intrinsic mode of LEC operation. The decreasing width of the intrinsic region between the p- and n-type doped zones upon ongoing pin-junction formation causes distinct (either field or electrode induced) luminance quenching effects.     

Identifying and alleviating electrochemical side-reactions in light-emitting electrochemical cells

We demonstrate that electrochemical side-reactions involving the electrolyte can be a significant and undesired feature in light-emitting electrochemical cells (LECs). By direct optical probing of planar LECs, comprising Au electrodes and an active material mixture of {poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) + poly(ethylene oxide) (PEO) + KCF3SO3}, we show that two direct consequences of such a side-reaction are the appearance of a "degradation layer" at the negative cathode and the formation of the light-emitting p-n junction in close proximity to the cathode. We further demonstrate that a high initial drive voltage and a high ionic conductivity of the active material strongly alleviate the extent of the side reaction, as evidenced by the formation of a relatively centered p-n junction, and also rationalize our findings in the framework of a general electrochemical model. Finally, we show that the doping concentrations in the doped regions at the time of the p-n junction formation are independent of the applied voltage and relatively balanced at approximately 0.11 dopants/MEH-PPV repeat unit in the p-type region and approximately 0.15 dopants/MEH-PPV repeat unit in the n-type region.     

A supramolecularly-caged ionic iridium(III) complex yielding bright and very stable solid-state light-emitting electrochemical cells

A new iridium(III) complex showing intramolecular interligand pi-stacking has been synthesized and used to improve the stability of single-component, solid-state light-emitting electrochemical cell (LEC) devices. The pi-stacking results in the formation of a very stable supramolecularly caged complex. LECs using this complex show extraordinary stabilities (estimated lifetime of 600 h) and luminance values (average luminance of 230 cd m-2) indicating the path toward stable ionic complexes for use in LECs reaching stabilities required for practical applications.     

Phosphorescent dyes for organic light-emitting diodes

This article presents general concepts that have guided important developments in our recent research progress regarding room-temperature phosphorescent dyes and their potential applications. We first elaborate the theoretical background for emissive metal complexes and the strategic design of the chelating C-linked 2-pyridylazolate ligands, followed by their feasibility in functionalization and modification in an aim to fine-tune the chemical and photophysical properties. Subsequently, incorporation of 2-pyridylazolate chromophores is illustrated in the synthesis of the highly emissive, charge-neutral Os, Ru, Ir, and Pt complexes. Insights into their photophysical properties are gained from spectroscopy, relaxation dynamics, and theoretical approaches, from which the lowest-lying excited states, competitive radiative decay, and radiationless processes are then analyzed in detail. In view of applications, their potentials for OLEDs have been evaluated. The results, in combination with the fundamental basis, give a conceptual design contributed to the future advances in the field of OLEDs.     

Dynamic doping and degradation in sandwich-type light-emitting electrochemical cells

Photoluminescence spectroscopy has been performed in situ during device operation and after switch-off on ionic transition metal complex (iTMC)-based sandwich-type light-emitting electrochemical cells (LECs). It is demonstrated that the photoluminescence of the LECs decreases with increasing operating time. For operating times up to three hours the decline in photoluminescence is fully recoverable after switching off the bias. These results imply that doping of the iTMC layer is responsible, not only, for the turn-on of LECs but also for their lifetimes.     

Efficient green-blue-light-emitting cationic iridium complex for light-emitting electrochemical cells

A highly luminescent novel cationic iridium complex [iridium bis(2-phenylpyridine)(4,4'-(dimethylamino)-2,2'-bipyridine)]PF6 was synthesized and characterized using NMR, UV-visible absorption, and emission spectroscopy and electrochemical methods. This complex displays intense photoluminescence maxima in the green-blue region of the visible spectrum and exhibits unprecedented phosphorescence quantum yields, 80 +/- 10% with an excited-state lifetime of 2.2 mus in a dichloromethane solution at 298 K. Single-layer light-emitting electrochemical cells with the charged complex as conducting and electroluminescent material sandwiched between indium-tin oxide and Ag electrodes were fabricated, which emit green-blue light with an onset voltage as low as 2.5 V. Density functional theory calculations were performed to provide insight into the electronic structure of the [iridium bis(2-phenylpyridine)(4,4'-(dimethylamino)-2,2'-bipyridine)]PF6 complex, comparing these results with those obtained for [iridium bis(2-phenylpyridine)(4,4'-tert-butyl-2,2'-bipyridine)]PF6.     

A near-infrared fluorescent probe for lipid hydroperoxides in living cells

An NIR (near-infrared) fluorescent probe TCP (tricarbocyanine diphenylphosphine) including a non-conjugated 'pre-tricarbocyanine' was designed and synthesized for visualizing lipid hydroperoxides (ROOH) in living cells. The excitation and emission spectra of tricarbocyanine in the NIR region can effectively avoid background fluorescence interference in biological systems. The probe exhibited a rapid fluorescence response to ROOH and high selectivity for ROOH over other ROS (reactive oxygen species) and some biological compounds, and the limit of detection was 38 pM. In addition, the probe was stable, and less cytotoxic, which indicated that it has potential application in detecting lipid hydroperoxides in living biological systems.     

Red solid-state fluorescent aminoperfluorophenazines

The solid-state fluorescence of 2-amino-, 2-ethylamino, 2-diethylamino-, and 2,7-bis(diethylamino)perfluorophenazines was examined. They showed their fluorescence maxima in the range of 584-637 nm. The solid-state fluorescence quantum yield of 2-diethylamino derivative was highest among these derivatives, there being 0.22. X-ray crystallographic analysis suggests that the 2-diethylamino derivative has no strong intermolecular interactions among adjacent molecules to show intense fluorescence, whereas the other derivatives have network NH/F hydrogen, π-π, and CH/F interactions to reduce solid-state fluorescence intensity.     

Electrical properties of electrochemically doped organic semiconductors using light-emitting electrochemical cells

We present a study of the electrical properties of electrochemically doped conjugated polymers using polymeric light-emitting electrochemical cells (PLECs) and interpreting the results according to a phenomenological model (PM) which assumes that, above the device turn-on voltage, the bulk transport properties of the doped organic semiconductor are responsible for the main contribution to the whole device conductivity. To confirm the predictions of this model, the dependence of the conductivity of PLECs with different parameters is evaluated and compared with the behavior expected for a doped semiconducting polymeric material. The organic semiconductor doping level, the blend concentration of organic semiconducting molecules, the device thickness, the charge carrier mobility, and the temperature are the parameters varied to perform this analysis. We observed that the device conductivity is independent of the active layer thickness, weakly dependent on the temperature, but strongly dependent on the semiconductor doping level, on the semiconductor fraction in the blend, and on the intrinsic charge carrier mobility. These results were well described by the variable range hopping (VRH) model, which has been widely employed to describe the charge transport in doped semiconducting polymeric materials, confirming the prediction of the phenomenological model. The current analysis demonstrates that PLECs are a suitable system for studying, in situ, the electrochemical doping of semiconducting polymers, permitting the evaluation of material properties as, for instance, the density of electronic charge carriers (and, consequently, the ionic charge carrier concentration) necessary to achieve the maximum electrochemical doping level of the organic semiconductor.     

Solid-state white light-emitting electrochemical cells using iridium-based cationic transition metal complexes

White electroluminescent (EL) emission from single-layered solid-state light-emitting electrochemical cells (LECs) based on host-guest cationic iridium complexes has been successfully demonstrated. The devices show white EL spectra (Commission Internationale de l'Eclairage coordinates ranging from (x, y) = (0.45, 0.40) to (0.35, 0.39) at 2.9-3.3 V with high color rendering indices up to 80. Peak external quantum efficiency and peak power efficiency of the white LEC reach 4% and 7.8 lm/W, respectively. These results suggest that white LECs based on host-guest cationic transition metal complexes may be a promising alternative for solid-state lighting technologies.     

Microcapillary electrochemical droplet cells: applications in solid-state surface analysis

Capillary–based microcells, also known as microcapillary electrochemical droplet cells, have proved their capabilities in various electrochemical surface investigations in recent decades. Due to the large measured current density and the high limiting current, this technique provides high–resolution electrochemical responses. Current densities in the range from a few femto to pico Acm^?2 to hundreds of Acm^?2 can be measured using this technique. Various applications for microcapillary cells have been reported. Technical limitations, such as the Ohmic drop and changes in the composition of the measurement area near the tip of the microcapillary have also been considered by some researchers. The rapid increase in the application of microcells and the increase in the number of related reports published in the literature have paralleled recent attempts to develop and improve microcell setups, showing that this technique is already well established for electrochemical surface studies.     

A selective near-infrared fluorescent probe for singlet oxygen in living cells

A selective near-infrared fluorescent probe (His-Cy), which features a fast response to (1)O(2) with high sensitivity and selectivity, was designed, synthesized and applied to bioimaging.     

Insight into luminescent iridium complexes: Their potential in light-emitting electrochemical cells

Cyclometallated iridium complexes possess fascinating electrochemical and photophysical properties that make them excellent candidates for a variety of photonic and optoelectronic applications. In particular, light-emitting electrochemical cells (LEECs) based on iridium-containing ionic transition-metal complexes (Ir-iTMCs) are a promising alternative to conventional organic light-emitting diodes with several advantages, including a simpler device structure, solution processability, and reduced manufacturing costs. This review aims to provide a comprehensive and systematic overview of the current status of Ir-iTMC-based LEECs using the archetypal complex [Ir(ppy)₂(bpy)]PF₆ as a reference emitter. After a discussion of the device fundamentals and important photophysical and device parameters, key strategies for tuning the emission characteristics and device stability through LUMO and HOMO stabilization/destabilization are presented using numerous examples from the literature, with a particular focus on ligand modification with hydrophobic, electron-withdrawing, and electron-donating substituents, π-stacking interactions, and alternative ancillary and cyclometalated ligand skeletons. Comprehensive data tables summarizing the photophysical and LEEC properties of the various classes of iridium complexes reported to date are also provided. Finally, in an effort to highlight promising directions for future research, the current champion iridium complexes for fabricating state-of-the-art LEECs are identified, and the merits and limitations of existing approaches are discussed.     

A near-infrared fluorescent probe for monitoring ozone and imaging in living cells

A near-infrared fluorescent probe (Trp-Cy) for endogenous ozone is presented, which exhibited a large stokes shift about 140 nm and a rapid fluorescence response to ozone with high selectivity and sensitivity.     

Tetraphenylethylene-BODIPY aggregation-induced emission luminogens for near-infrared polymer light-emitting diodes

The aggregation-induced emission(AIE) phenomenon provides a new direction for the development of organic light-emitting devices. Here, we present a new class of emitters based on 4,4-difluoro-4-bora-3 a,4 a-diaza-s-indacene(BODIPY), functionalized at different positions with tetraphenylethylene(TPE), which is one of the most famous AIE luminogens. Thanks to this modification, we were able to tune the photoluminescence of the BODIPY moiety from the green to the near-infrared(NIR)spectral range and achieve PL efficiencies of ~50% in the solid state. Remarkably, we observed an enhancement of the AIE and up to ~100% photoluminescence efficiencies by blending the TPE-substituted BODIPY fluorophores with a poly[(9,9-di-noctylfluorene-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,7-diyl)](F8 BT) matrix. By incorporating these blends in organic lightemitting diodes(OLEDs), we obtained electroluminescence peaked in the range 650–700 nm with up to 1.8% external quantum efficiency and ~2 m W/cm2 radiance, a remarkable result for red/NIR emitting and solution-processed OLEDs.     

Highly fluorescent solid-state asymmetric spirosilabifluorene derivatives

A series of four asymmetrically aryl-substituted 9,9'-spiro-9-silabifluorene (SSF) derivatives, 2,2'-di-tert-butyl-7,7'-diphenyl-9,9'-spiro-9-silabifluorene (PhSSF), 2,2'-di-tert-butyl-7,7'-dipyridin-2-yl-9,9'-spiro-9-silabifluorene (PySSF), 2,2'-di-tert-butyl-7,7'-dibiphenyl-4-yl-9,9'-spiro-9-silabifluorene (BPhSSF), and 2,2'-di-tert-butyl-7,7'-bis(2',2' '-bipyridin-6-yl)-9,9'-spiro-9-silabifluorene (BPySSF) are prepared through the cyclization of the corresponding 2,2'-dilithiobiphenyls with silicon tetrachloride. These novel spiro-linked silacyclopentadienes (siloles) form transparent and stable amorphous films with relatively high glass transition temperatures (T(g) = 203-228 degrees C). The absorbance spectrum of each compound shows a significant bathochromic shift relative to that of the corresponding carbon analogue as a result of the effective sigma-pi conjugation between the sigma orbital of the exocyclic Si-C bond and the pi orbital of the oligoarylene fragment. Solid-state films exhibit intense violet-blue emission (lambda(PL) = 398-415 nm) with high absolute photoluminescence quantum yields (phi(PL) = 30-55%).     

Efficient and stable solid-state light-emitting electrochemical cell using tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) hexafluorophosphate

The complex tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II), prepared via a simple microwave-assisted synthesis, was used to prepare a single-layer light-emitting electrochemical cell. This device reaches a high power efficiency of 1.9 Lum/W at a brightness of 390 cd/m2. Moreover, its lifetime is an order of magnitude longer than that of a similar cell making use of tris(bipyridine)ruthenium(II) as the emitting complex.     

Modular solid-state unit for electrochemical studies

A relatively inexpensive unit based upon solid-state operational amplifiers is described; its modular design makes it an extremely versatile instrument for many electrochemical techniques, e.g., normal direct current polarography and linear sweep voltammetry, cyclic voltammetry, alternating current polarography, and coulometry and electrolysis at controlled electrode potential. It can be readily adapted to many other functions.