Performance Of Polymer Light-emitting Diodes Research Articles

Highly efficient transparent organic light-emitting diodes by ion beam assisted deposition-prepared indium tin oxide cathode

The authors have investigated the effects of indium tin oxide (ITO) deposited by ion beam assisted e-beam evaporation on the performance of polymer light-emitting diodes. ITO was evaporated as a cathode onto a thin Mg:Ag layer by an e-beam process, and its performance as a transparent cathode was subsequently compared to that of Mg:Ag and sputtering-prepared ITO. Polymer devices’ luminance and efficiency were improved by more than ten times by ion beam assisted deposition (IBAD)-prepared ITO deposition, with little observable damage to the organic layer. Implementation of the IBAD process resulted in the reduction of the interfacial energy barrier which induced band bending. Furthermore, outcoupling with ITO resulted in enhanced luminance.

Effect of oxygen plasma treatment on the surface properties of tin-doped indium oxide substrates for polymer LEDs

The effect of oxygen plasma treatment on the surface properties of tin-doped indium oxide (ITO) substrates and the changes in surface properties of treated ITO substrates with ageing time were investigated by X-ray photoelectron spectroscopy (XPS), contact angle and surface free energy measurements. Experimental results show that oxygen plasma treatment increases the oxygen concentration, decreases the carbon concentration, and enhances the surface free energy and polarity, and thereby improves the surface properties of ITO substrates. However, the improved ITO surface properties tended to decay and the surface free energy decreased, with ageing time. In addition, the ageing effect of treated ITO substrates on the performance of polymer light-emitting diodes (LEDs) was studied with respect to the driving voltage, electroluminescent luminance and efficiency. We observe that the ITO substrates aged for various times result in significant differences in optical and electrical characteristics which become worse as the ageing time increases. The optical and electrical performance of polymer LEDs is closely related to the surface properties of ITO substrate and the interface characteristics of ITO/polymer.

Modelling the effects of molecular arrangements in polymer light-emitting diodes

In order to understand how to enhance the performance of polymer light-emitting diodes(PLEDs), we used a mesoscopic hopping model, taking into account molecularproperties and polymer morphology, to investigate the impact of a number ofconjugated polymers and molecular arrangements on the functioning of single-layerdevices. The model is applied to devices with the active polymer consisting ofpoly(p-phenylene vinylene) (PPV) and PPV derivatives with stiff conjugated segments havingtheir long axis oriented parallel and perpendicular to the electrode surfaces as well asrandomly oriented, which are three of the molecular arrangements that can be obtainedexperimentally at microscopic scale in solution-processed conjugated polymer thin films.The model provides insight into current efficiency, charge distribution, internal electric fieldand consequently recombination throughout the polymer layer. We found that the detailsof molecular arrangement crucially affect the distribution of recombination events far fromthe electrodes and its field dependence, which has implications for the efficiency of PLEDs.In particular, we found a variation of recombination efficiency in the bulk of uniformordered polymer films depending on the molecular alignment relative to the electrodesurfaces. It turns out that molecular orientation perpendicular to the electrodes increasesrecombination in the centre of the polymer film as compared to the case of parallelorientation. We conclude that the orientational alignment perpendicular to theelectrode surfaces might be a viable strategy towards efficient polymer-based LEDs.

Influence of doping with titania nanotubes on performance of polymer light-emitting diodes

The performance of polymer light-emitting diodes was optimized by dispersing titania nanotubes into different functional layers. When titania nanotubes are doped into the hole-transport layer, the hole mobility of the device is greatly increased due to the good p-type conductibility of titania nanotubes. In the case of doping titania nanotubes into emitting layer, irradivative recombination was decreased since organic molecules wrapped on outer surface or entering into the nanotube make the chain conformation of polymer molecules more open. At last, the device efficiency of PLEDs was greatly improved by doping titania nanotubes into the hole buffer layer. This contributes to strong interaction between the titania nanotube and the polymer. This interaction reduces not only the hole conductibility but also the creation of defect states on interface region which can quench the luminescence.

Material and Interface Engineering for Highly Efficient Polymer Light Emitting Diodes

Polymer light‐emitting diodes (PLEDs) have great potential to compete with LCD displays that are currently used for computer and television screens. The efficiency and stability of PLEDs still need to be improved in order to fully realize the advantages of low cost and ease of fabrication provided by organic materials. Our effort in improving the PLED's performance have been focused on two parallel approaches: 1) Modify the interface between polymer and the charge‐injection electrodes for more efficient device structures; 2) Enhance the efficiency of PLEDs through the development of new conjugated materials with balanced charge‐transporting properties. In this paper, we review our recent progress on the interface engineering between polymer and electrodes to optimize charge‐injection, ‐transport, and ‐recombination in PLEDs, as well as on the material engineering to tune the emission color, electron affinity, and charge mobility.

Efficient electron injection from bilayer cathode with aluminum as cathode

A new bilayer cathode has been developed for varieties of RGB emitting polymers to form an effective electron injector. An effective electron injection from high work function metal, such as Al cathode can be realized by incorporation of a thin layer of amino-ended alkyl-substituted polyfluorene copolymer and corresponding ammonium salt-cationic polyelectrolyte between metal cathode and emitting layer. The performance of polymer light emitting-diodes (PLEDs) fabricated with such bilayer cathodes can be enhanced to the levels comparable or even higher than that obtained by using Ca and Ba as the cathode (for red QE = 2–3%, for green 5–8%, for blue 1–2% respectively). The new electron-injection layer can be processed from environment-friendly solvents such as alcohol or water in which most of light-emitting polymers are insoluble. The mechanism of lowering the barrier height for electron injection from Al is discussed based on I-V characteristics, build-in potential, and X-ray diffraction data.

Dramatically improving polymer light-emitting diode performance by doping with inorganic salt

A simple method is described that dramatically lowers the turn-on voltage of polymer light-emitting diodes (PLEDs) by doping the electroluminescent polymer with an inorganic salt. Single-layer PLEDs were fabricated by spin-coating a solution of poly(9,9′-dioctylfluorene), doped with controlled amounts of LiBr, onto an indium tin oxide-coated glass (anode) followed by physical vapor deposition of an Al cathode. The turn-on voltage dropped from the undoped value of 7.4V as low as to 3.0V. These results are discussed in terms of alterations of the cathode-polymer and anode-polymer interfaces by Li+ and Br−.

Plasma treatment and its prospective application to polymer light-emitting diodes fabricated by ink-jet printing method

The influence of CF4 plasma treatment of indium-tin-oxide (ITO) and polyimide (PI) on the patterning of ink-jet printed polymer is presented. Not much difference between the as-received ITO and PI surface energies was found, but a significant difference in surface energies between ITO and PI after CF4 plasma treatment was noted. It is expected that precise patterning can be achieved by using the difference in surface energies between the inside of the pixel and its surroundings. Also the effects of CF4 plasma treatment of ITO have been studied on the performance of polymer light-emitting diodes (PLEDs). X-ray photoelectron spectroscopy revealed that CF4 plasma treatment led to a decrease in the surface content of carbon contaminants and an increase in the surface content of fluorine, which in turn enhance the performance of PLEDs.

Correlation between the Indium Tin Oxide morphology and the performances of polymer light-emitting diodes

This paper reports on performance enhancement of polymer light-emitting diodes (PLEDs) based on poly(2,5-bis(3′,7′-dimethyl-octyloxy)1,4-phenylene-vinylene) (BDMO-PPV) after Indium Tin Oxide (ITO) pre-treatment. Diluted aqua regia surface treatments of ITO substrates were investigated before PLEDs manufacturing. The correlation between ITO surface morphology and the properties of devices were discussed. The residual roughness, the global ITO thickness and developed anodic active area were found extremely dependent on treatment time. The performances of the ITO/BDMO-PPV/Ca/Al diodes were improved. With long treatments, the roughness was dramatically increased and poor PLEDs performances were achieved. The incorporation of a conducting polymer layer, poly(3,4-ethylene dioxythiophene) doped with polystyrene sulfonate, between the ITO layer and the emissive polymer, was also studied. With this buffer layer, no effect of the acidic treatment was observed on luminance–voltage characteristics and low turn-on voltage devices were made.

Improved Performance of 2,3-Dibutoxy-1,4-Phenylene Vinylene Based Polymer Light-Emitting Diodes by Thermal Annealing

2,3-dibutoxy-1,4-phenylene vinylene (DB-PPV) thin films were used to fabricate polymer light emitting diodes (PLEDs). Before the PLEDs were fabrication, DB-PPV thin films were annealed at various temperatures. The surface morphologies of the films were measured by atomic force microscopy (AFM). From the measured J–V and L–I curves, the performance of PLED was found to have been improved. The best operational properties were observed for the DB-PPV based PLED annealed at 150°C.

Using Self‐Assembling Dipole Molecules to Improve Hole Injection in Conjugated Polymers

AbstractSurface modification of indium‐tin‐oxide (ITO)‐coated substrates through the use of self‐assembled monolayers (SAMs) of molecules with permanent dipole moments has been used to control the ITO work function and device performance in polymer light‐emitting diodes based on a polyfluorene hole transporting copolymer. Measured current–voltage characteristics of the devices reveal greatly increased hole injection currents from the SAM‐altered electrodes with higher work function, in agreement with an expected reduction in the barrier for hole injection. In particular, it is shown that the SAM‐modified electrode with the highest work function provides an ohmic contact for hole injection into the studied polymer. Injection from the widely used poly(2,3‐ethylenedioxythiophene)/polystyrenesulphonic acid (PEDOT:PSS)‐coated ITO anode system, is less efficient compared with some of the studied SAM‐coated ITO anodes despite the significantly higher work function measured by a Kelvin probe. This apparently anomalous situation is attributed to the inhomogenities in the injection processes that occur over the area of the device when the PEDOT:PSS‐coated ITO electrode is used.

Effect of molecular properties on the performance of polymer light-emitting diodes

The performance of a single layer polymer light-emitting diode depends on several interdependent factors, although recombination between electrons and holes within the polymer layer is believed to play an important role. Our aim is to carry out computer experiments in which bipolar charge carriers are injected in polymer networks made of poly(p-phenylene vinylene) chains randomly oriented. In these simulations, we follow the charge evolution in time from some initial state to the steady state. The intra-molecular properties of the polymer molecules obtained from self-consistent quantum molecular dynamics calculations are used in the mesoscopic model. The purpose of the present work is to clarify the effects of intra-molecular charge mobility and energy disorder on recombination efficiency. In particular, we find that charge mobility along the polymer chains has a serious influence on recombination within the polymer layer. Our results also show that energy disorder due to differences in ionization potential and electron affinity of neighbouring molecules affects mainly recombinations that occur near the electrodes at polymer chains parallel to them.

Electrical degradation of triarylamine-based light-emitting polymer diodes monitored by micro-Raman spectroscopy

Although much progress has been made in improving polymer light-emitting diode performance, there has been little work to address device intrinsic degradation mechanisms due to the challenge of tracking minute chemical reactions in the 100-nm-thick buried active layers during operation. Here we have elucidated a hole-mediated electrical degradation of triarylamine-based blue polymer diodes using in situ Raman microspectroscopy. A slow irreversible hole-doping of polymer adjacent to the hole-injecting conducting-polymer leads to formation of oxidised triarylamine species counterbalanced by anions from the conducting-polymer. These charged species act as luminescence quenchers and hinder further hole injection across the interface leading to significant decreases in current density at low voltages.

Indium tin oxide anode modification for enhanced polymer light‐emitting devices

Abstract— Effort has been focused on exploring indium tin oxide (ITO) anode modification for enhanced performance of polymer light‐emitting diodes (PLEDs). It was found that oxidative treatment, e.g., a commonly used oxygen plasma, modifies ITO surface effectively to produce a low‐conductivity oxygen‐rich region. As a consequence, oxygen plasma‐treated ITO behaves somewhat similarly to specimens where there is an ultra‐thin insulating layer on its surface. It shows that the presence of such an ultra‐thin insulating interlayer between the ITO and the polymer layer favors the efficient operation of the PLEDs. The result of this effort provides an insight to better understand optimal anode contact for enhanced PLED performance.

Improving the performance of polymer light-emitting diodes using polymer solid solutions

Optical and electronic properties of conjugated polymers can be modified significantly by arranging the polymer chains differently. For example, it is well known that polymer solutions of lower concentrations have different photoluminescence spectra from solutions of higher concentrations. In this manuscript, we demonstrated that such effects can also be achieved in solid states, i.e., in polymer solid solutions consisting of a narrower band-gap material—poly(2-methoxy-5(2′-ethyl-hexyloxy)-1,4-phenylene vinylene (MEH-PPV) with a wider band-gap polymer–poly(9,9-dioctylfluorene). At very low MEH-PPV concentrations, emission spectra of these solid solutions are similar to the photoluminescence spectrum of a diluted MEH-PPV solution in an ordinary organic solvent, suggesting the absence of a significant amount of interchain species. In addition, the electroluminescence efficiency is significant enhanced. Furthermore, the full width at half maximum (FWHM) is dramatically reduced at lower MEH-PPV concentrations, as a result of suppressed interchain interactions. This approach is particularly important for the application to display technology, in which well-defined narrow FWHM emission spectra of red, green, and blue colors are required.

Optimum charge-carrier mobility for a polymer light-emitting diode

The performance of polymer light-emitting diodes (PLEDs) consisting of various poly-p(phenylene vinylene) (PPV)-based derivatives has been investigated. It is demonstrated that an increased charge carrier mobility gives rise to an enhancement of the quenching of the electroluminescence at the metallic cathode. As a result the power efficiency of PPV-based LEDs under operating conditions exhibits a distinct optimum at a mobility of about 10−11 m2/V s.

Device performance and polymer morphology in polymer light emitting diodes: morphology dependent emission spectra

Polymer thin films for the light-emitting devices (LEDs) are usually obtained by spin-casting from slightly viscous solutions. The performance of polymer light-emitting diodes is strongly influenced by the thin film morphology. It is known that the polymer thin film morphology (or aggregation) is determined by the solvents used for obtaining polymer solutions. In this manuscript, we demonstrate that the polymer aggregation can also be controlled by the concentration of the polymer solution and the rotational speed used during spin-casting of the films. The correlation between the film morphology and emission spectra for polymer light-emitting diodes fabricated using poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV) has been studied. Our findings indicate that there is a strong correlation between the “vibronic structure” observed in the emission spectra of the MEH-PPV devices and the aggregation of the polymer chains.

Polymer/metal interfaces and the performance of polymer light-emitting diodes

Conjugated polymers are often treated as semiconductors with low doping concentrations. Unlike the traditional semiconductors which have a high density of surface states (mainly due to the dangling bonds), the nature of the metal/polymer interface, including barrier height and charge injection efficiency, is quite sensitive to the work function of the contact metal. In this article, we present evidence to show that the pinning of the surface Fermi level effect commonly observed at the silicon/metal interface can also be observed at the metal/polymer interface. It is achieved by controlling the doping level at the metal/polymer [poly(2-methoxy-5(2′-ethyl-hexyloxy)-1,4-phenylene vinylene) or MEH-PPV] interface. ITO/MEH-PPV/Al devices doped with 2 Å of calcium on the cathode side of the interfacial layer have the same device performance as the ITO/MEH-PPV/Ca devices. The heavily n-doped region pins the surface energy level, hence the polymer interface at the cathode side is no longer sensitive to the work function of the overcoated metal. It is believed that either the midgap bipolaron energy states created by the dopants or the sharp band bending at the interface is responsible for facilitating the electron injection. On the other hand, a p-doped region at the anode side, obtained by using a thin layer of an acid at the interface, pins the surface energy level and makes the contact insensitive to the work function of the anode. Therefore, an efficient polymer light-emitting diode with the p-i-n structure has been demonstrated without the matching of the work function of the metal electrodes.

An iodine-doped polymer light-emitting diode

The performance of polymer light-emitting diodes (LEDs) having the configuration Al/MEH–PPV/ITO, where MEH–PPV=poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene-vinylene) has been improved by light iodine doping of the emissive MEH–PPV polymer layer whereby the turn-on voltage is reduced and the external quantum efficiency is increased by an order of magnitude. It differs from nondoped MEH–PPV LEDs in that light emission is observed in both forward and reverse bias modes. It is believed that the mobility of the I3− anions in the (MEH–PPV)x+y(I3)xy− species formed on doping the MEH–PPV compensate for injected charges at the electrodes, thus reducing the electron and hole injection barriers.

Polymeric anodes for improved polymer light-emitting diode performance

We have studied polyaniline and polyethylenedioxythiophene transparent electrodes for use as hole-injecting anodes in polymer light emitting diodes. The anodes were doped with a variety of polymer and monomer-based acids and cast from either water or organic solvents to determine the effect of the dopant and solvent on the hole-injection properties. We find that the anodes with polymeric dopants have improved device quantum efficiency and brightness relative to those with small molecule dopants, independent of conductivity, solvent, or type of conducting polymer. For the most conducting polymer anodes [σ>2(Ωcm)−1], diodes could be made without an indium tin oxide underlayer. These diodes show substantially slower degradation.