Performance Of Light-emitting Electrochemical Cells Research Articles

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.

Recent advances in light-emitting electrochemical cells

Light-emitting electrochemical cells (LECs) are solution-processable thin-film electroluminescent devices consisting of a luminescent material in an ionic environment. The simplest type of LEC is based on only one material, ionic transition-metal complexes (iTMCs). These materials are of interest for different scientific fields such as chemistry, physics, and technology as selected chemical modifications of iTMCs resulted in crucial breakthroughs for the performance of LECs. This short review highlights the different strategies used to design these compounds with the aim to enhance the performances of LECs.

Separating Ion and Electron Transport: The Bilayer Light-Emitting Electrochemical Cell

The current generation of polymer light-emitting electrochemical cells (LECs) suffers from insufficient stability during operation. One identified culprit is the active material, which comprises an intimate blend between an ion-conducting electrolyte and an electron-transporting conjugated polymer, as it tends to undergo phase separation during long-term operation and the intimate contact between the ion- and electron-transporting components provokes side reactions. To address these stability issues, we present here a bilayer LEC structure in which the electrolyte is spatially separated from the conjugated polymer. We demonstrate that employing this novel device structure, with its clearly separated ion- and electron-transport paths, leads to distinctly improved LEC performance in the form of decreased turn-on time and improved light emission. We also point out that it will allow for the utilization of combinations of active materials having mutually incompatible solubilities.

Luminescent and electrical performance of polymer light-emitting electrochemical cells with treated indium-tin-oxide electrodes

The surfaces of indium-tin-oxide (ITO) electrodes for polymer light-emitting electrochemical cells (LECs) were modified by O2 plasma (OP) treatment. The properties of the ITO surfaces were evaluated by chemical composition, contact angle and surface energy. The influence of surface properties of the ITO electrodes on the performance of LECs was investigated in terms of the driving voltage, turn-on voltage, injection current and luminance. When the OP treatment was employed to modify the ITO electrodes, the surface properties of ITO are optimised due to the improvement of surface stoichiometry and the enhancement of wettability. And the improved surface properties benefited from the OP treatment is observed to decay with the time after the OP treatment. In addition, the luminescent and electrical performance of the devices is found to become worse with the increasing time after the OP treatment on ITO electrodes. The results demonstrate that the device performance strongly depends on the ITO surface properties and the ITO/organic interface characteristics.

Variable-force tapping atomic force microscopy as a tool in the characterization of organic devices

A common method for characterizing the phase separation of materials in mixtures is tapping mode atomic force microscopy (AFM). However, AFM results are influenced by surface-energy effects and the employed tapping force, and it might therefore be difficult to attain correct information regarding the bulk with such a surface-imaging technique. In this work, we present a way of imaging material phase separation in an improved manner by recording a series of AFM images at different tapping force. More specifically, we have employed the variable-force AFM method on organic mixtures, comprising a conjugated polymer (MEH-PPV) and an ion-conducting polymer electrolyte (PEO-XCF 3SO 3, X=Li, K, Rb), and we demonstrate that it is capable of reversibly sampling such materials not only on the surface, but also (indirectly) in the topmost part of the bulk. The analysis of the evolution of AFM phase images allows us to (indirectly) gain information about the bulk-phase separation of materials. We find that the variable-force AFM results correlate well with the device performance of light-emitting electrochemical cells employing such organic mixtures as the active material.

The influence of electrodes on the performance of light-emitting electrochemical cells

We demonstrate that the electrochemical properties of the electrode material can have a dramatic impact on the performance of light-emitting electrochemical cells (LECs). Specifically, we report results from planar wide-gap LECs containing a blend of poly(2-methoxy,5-(2′-ethylhexyloxy)- p-phenylene vinylene) (MEH-PPV), poly(ethylene oxide) and LiCF 3SO 3 as the active material. We find that Au electrodes are preferable over Al electrodes, since Au-electrode devices exhibit fast turn-on (i.e., p–n junction formation time) and clearly visible light emission during operation at 5 V and 360 K, while Al-electrode devices exhibit slow turn-on (due to a delayed onset of p-doping progression) and no visible light emission. These results are rationalized with a cyclic voltammetry study, which demonstrates that Al is oxidized at a lower potential than the p-doping (oxidation) potential of MEH-PPV, while Au is electrochemically inert over the entire voltage range spanned by the p- and n-doping potentials of MEH-PPV. Consequently, the oxidation charge injected into Al-electrode devices results in a combination of p-doping of MEH-PPV and formation of Al ions. The latter process is undesired since it results in a slow turn-on time and quenched light emission. Finally, we find that planar LECs in a bottom-electrode configuration exhibit a faster turn-on time than identical devices with the electrodes on top of the active material.

Light-emitting electrochemical cells using a molten delocalized salt

The device performances of light-emitting electrochemical cells are improved by adding a room-temperature molten salt (tetrahexylammonium-bis-trifluoro-methyl-sulfonyl imide) directly into the light-emitting layer. For poly(9,9-dihexyl-fluorene-2,7-diyl) with an indium-tin-oxide anode and an aluminum cathode, the power efficiency can be increased by more than one order of magnitude. An even more pronounced effect is observed for poly [2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene]. Equally important for both luminescent polymers, the operating voltage is drastically reduced.

Local Ions Distribution Inhomogeneities in Polymer Based Light Emitting Cells

In the last years the development of light emitting electrochemical cells on a polymer base has made a great progress. They are extremely interesting due to their low cost, but still, problems concerning the stability and lifetime of these elements remain. The performance of light-emitting electrochemical cells (LEC) is strongly influenced by the inhomogeneous distribution of the three main components (light emitting polymer, ion conductive polymer, ionic molecules) in the device. The local ions distribution of lithium and triflate (short for the CF3SO3 − species) ions in a blend with the ionic conducting polymer poly(ethylene oxide) (PEO) and the conjugated polymer m-LPPP (methyl substituted poly(para phenylene)) throughout the polymer layer has been investigated by secondary ion mass spectroscopy (SIMS). Different combinations of top- and base-electrodes have been investigated and have been found to influence the distribution of the ions strongly.

Charge distribution in light emitting electrochemical cells

The performance of light-emitting electrochemical cells (LEC) is strongly influenced by the inhomogeneous distribution of the three main components (light emitting polymer, ion conductive polymer and ionic molecules) in the device. We report on the local ion distribution of lithium and triflate ions in a blend of the ionic conducting polymer poly (ethylene oxide) (PEO) and the conjugated polymer m-LPPP (methyl substituted poly (para phenylene)) by secondary ion mass spectroscopy(SIMS). In the system ITO/active layer/Al the ions are found to be located close to their counter electrodes. The behavior of these devices is investigated by impedance spectroscopy.

Polymer Light-Emitting Electrochemical Cells: A Theoretical Study of Junction Formation under Steady-State Conditions

The performance of light-emitting electrochemical cells is studied under steady-state conditions. The theoretical model presented is based on the approach to electrodiffusion−reaction problems familiar with electrochemical cells. However, some links with the traditional semicondutor physics approach are also presented. The reversible redox electrode reactions are considered to always be in thermodynamic equilibrium, and the distribution of the applied potential between the two electrode/polymer interfaces and the active layer is carefully analyzed by the numerical solution of the transport equations. The theory is able to explain most of the experimental observations. In particular, the junction formation is proved to be a bulk process related to the electrodiffusion and recombination reaction of polarons, thus explaining its macroscopic nature. Current−voltage and light−voltage characteristics in qualitative agreement with experiment are found. The observed changes in the junction width and the electric ...

Light‐Emitting Electrochemical Cells with Crown Ether as Solid Electrolyte

Light‐emitting electrochemical cells (LECs) are reported with lithium salt/crown ether complex as the solid electrolyte. By careful choice of the combination of electroluminescent polymer, crown ether, and lithium salt, LECs are demonstrated with good quantum efficiency, fast response, and long stress life. The results demonstrate the importance of phase compatibility to LEC performance.

Efficient, fast response light-emitting electrochemical cells: Electroluminescent and solid electrolyte polymers with interpenetrating network morphology

The device performance of light-emitting electrochemical cells is improved by adding a bifunctional liquid additive into the light-emitting layer. Because of the surfactant-like character of the additive, the light-emitting layer exhibits a high surface area bicontinuous three-dimensional network morphology. The semiconducting polymer forms a continuous network phase enabling electronic transport of injected electron and holes: the electrolyte forms a continuous network phase enabling fast ion transport; the nm length scale of the phase separated network enables rapid, effective transport of the ions into the conducting polymer during electrochemical doping.