Device Physics of White Polymer Light‐Emitting Diodes

The charge transport and recombination in white‐emitting polymer light‐ emitting diodes (PLEDs) are studied. The PLED investigated has a single emissive layer consisting of a copolymer in which a green and red dye are incorporated in a blue backbone. From single‐carrier devices the effect of the green‐ and red‐emitting dyes on the hole and electron transport is determined. The red dye acts as a deep electron trap thereby strongly reducing the electron transport. By incorporating trap‐assisted recombination for the red emission and bimolecular Langevin recombination for the blue emission, the current and light output of the white PLED can be consistently described. The color shift of single‐layer white‐emitting PLEDs can be explained by the different voltage dependencies of trap‐assisted and bimolecular recombination.     

Organic Light‐Emitting Diodes with Field‐Effect‐Assisted Electron Transport Based on α‐bi;,ω‐bi;‐Diperfluorohexyl‐quaterthiophene

Materials commonly used in the carrier transport layers of organic light‐emitting diodes, where transport occurs through the bulk, are in general very different from materials used in organic field‐effect transistors, where transport takes place in a very thin accumulation channel. In this paper, the use of a high‐performance electron‐conducting field‐effect transistor material, diperfluorohexyl‐substituted quaterthiophene (DFH‐4T), as the electron‐transporting material in an organic light‐emitting diode structure is investigated. The organic light‐emitting diode has an electron accumulation layer in DFH‐4T at the organic hetero‐interface with the host of the light‐emitting layer, tris(8‐hydroxyquinoline) aluminum (Alq₃). This electron accumulation layer is used to transport electrons and inject them into the active emissive host‐guest layer. By optimizing the growth conditions of DFH‐4T for electron transport at the organic hetero‐interface, high electron current densities of 750 A cm^?2 are achieved in this innovative light‐emitting structure.     

Cover Picture: An Organic Light‐Emitting Diode with Field‐Effect Electron Transport (Adv. Funct. Mater. 1/2008)

The cover shows an organic light‐emitting diode with remote metallic cathode, reported by Sarah Schols and co‐workers on p. 136. The metallic cathode is displaced from the light‐emission zone by one to several micrometers. The injected electrons accumulate at an organic heterojunction and are transported to the light‐emission zone by field‐effect. The achieved charge‐carrier mobility and in combination with reduced optical absorption losses because of the remoteness of the cathode may lead to applications as waveguide OLEDs and possibly a laser structure. (The result was obtained in the EU‐funded project “OLAS” IST‐ FP6‐015034.) We describe an organic light‐emitting diode (OLED) using field‐effect to transport electrons. The device is a hybrid between a diode and a field‐effect transistor. Compared to conventional OLEDs, the metallic cathode is displaced by one to several micrometers from the light‐emitting zone. This micrometer‐sized distance can be bridged by electrons with enhanced field‐effect mobility. The device is fabricated using poly(triarylamine) (PTAA) as the hole‐transport material, tris(8‐hydroxyquinoline) aluminum (Alq₃) doped with 4‐(dicyanomethylene)‐2‐methyl‐6‐(julolindin‐4‐yl‐vinyl)‐4H‐pyran (DCM₂) as the active light‐emitting layer, and N,N′‐ditridecylperylene‐3,4,9,10‐tetracarboxylic diimide (PTCDI‐C₁₃H₂₇), as the electron‐transport material. The obtained external quantum efficiencies are as high as for conventional OLEDs comprising the same materials. The quantum efficiencies of the new devices are remarkably independent of the current, up to current densities of more than 10 A cm^–2. In addition, the absence of a metallic cathode covering the light‐emission zone permits top‐emission and could reduce optical absorption losses in waveguide structures. These properties may be useful in the future for the fabrication of solid‐state high‐brightness organic light sources.     

Transparent InP Quantum Dot Light‐Emitting Diodes with ZrO2 Electron Transport Layer and Indium Zinc Oxide Top Electrode

Because of outstanding optical properties and non‐vacuum solution processability of colloidal quantum dot (QD) semiconductors, many researchers have developed various light emitting diodes (LEDs) using QD materials. Until now, the Cd‐based QD‐LEDs have shown excellent properties, but the eco‐friendly QD semiconductors have attracted many attentions due to the environmental regulation. And, since there are many issues about the reliability of conventional QD‐LEDs with organic charge transport layers, a stable charge transport layer in various conditions must be developed for this reason. This study proposes the organic/inorganic hybrid QD‐LEDs with Cd‐free InP QDs as light emitting layer and inorganic ZrO₂ nanoparticles as electron transport layer. The QD‐LED with bottom emission structure shows the luminescence of 530 cd m^?2 and the current efficiency of 1 cd/A. To realize the transparent QD‐LED display, the two‐step sputtering process of indium zinc oxide (IZO) top electrode is applied to the devices and this study could fabricate the transparent QD‐LED device with the transmittance of more than 74% for whole device array. And when the IZO top electrode with high work‐function is applied to top transparent anode, the device could maintain the current efficiency within the driving voltage range without well‐known roll‐off phenomenon in QD‐LED devices.     

Operating Modes of Sandwiched Light‐Emitting Electrochemical Cells

Light‐emitting electrochemical cells (LECs) are promising lighting devices in which the redistribution of ionic charges allows for double electronic carrier injection from air‐stable electrodes. Uncertainties about the mode of operation are limiting the progress of these devices. Using fast (with respect to the current growth time) but resolutive electrical measurement techniques, the electronic transport mechanism in state‐of‐the‐art sandwiched devices can be monitored as a function of the operation time. The results indicate the formation of doped transport layers adjacent to the electrodes that reduces the extent of the central neutral light‐emitting layer where electronic transport is limited by space‐charge. Prolonged growth of the doped regions beyond that required for efficient injection should be prevented, as this decreases the efficiency and leads to low luminance devices.     

The Role of Transition Metal Oxides in Charge‐Generation Layers for Stacked Organic Light‐Emitting Diodes

The mechanism of charge generation in transition metal oxide (TMO)‐based charge‐generation layers (CGL) used in stacked organic light‐emitting diodes (OLEDs) is reported upon. An interconnecting unit between two vertically stacked OLEDs, consisting of an abrupt heterointerface between a Cs₂CO₃‐doped 4,7‐diphenyl‐1,10‐phenanthroline layer and a WO₃ film is investigated. Minimum thicknesses are determined for these layers to allow for simultaneous operation of both sub‐OLEDs in the stacked device. Luminance–current density–voltage measurements, angular dependent spectral emission characteristics, and optical device simulations lead to minimum thicknesses of the n‐type doped layer and the TMO layer of 5 and 2.5 nm, respectively. Using data on interface energetic determined by ultraviolet photoelectron and inverse photoemission spectroscopy, it is shown that the actual charge generation occurs between the WO₃ layer and its neighboring hole‐transport material, 4,4',4”‐tris(N‐carbazolyl)‐triphenyl amine. The role of the adjacent n‐type doped electron transport layer is only to facilitate electron injection from the TMO into the adjacent sub‐OLED.     

Organic Tribotronic Transistor for Contact‐Electrification‐Gated Light‐Emitting Diode

Tribotronics is a new field about the devices fabricated using the electrostatic potential created by contact electrification as a “gate” voltage to tune/control charge carrier transport in semiconductors. In this paper, an organic tribotronic transistor is proposed by coupling an organic thin film transistor (OTFT) and a triboelectric nanogenerator (TENG) in vertical contact‐separation mode. Instead of using the traditional gate voltage for controlling, the charge carrier transportation in the OTFT can be modulated by the contact‐induced electrostatic potential of the TENG. By further coupling with an organic light‐emitting diode, a contact‐electrification‐gated light‐emitting diode (CG‐LED) is fabricated, in which the operating current and light‐emission intensity can be tuned/controlled by an external force–induced contact electrification. Two different modes of the CG‐LED have been demonstrated and the brightness can be decreased and increased by the applied physical contact, respectively. Different from the conventional organic light‐emitting transistor controlled by an electrical signal, the CG‐LED has realized the direct interaction between the external environment/stimuli and the electroluminescence device. By introducing optoelectronics into tribotronics, the CG‐LED has open up a new field of tribophototronics with many potential applications in interactive display, mechanical imaging, micro‐opto‐electro‐mechanical systems, and flexible/touch optoelectronics.     

Novel Approach for Alternating Current (AC)‐Driven Organic Light‐Emitting Devices

A novel approach for alternating current (AC)‐driven organic light‐emitting devices is reported, which uses the concept of molecular doping in organic semiconductors. Doped organic charge‐transport layers are used to generate charge carriers within the device, hence eliminating the need for injecting charge carriers from external electrodes. Bright luminance of up to 1000 cd m^?2 is observed when the device is driven with an AC bias. The luminance observed is attributed to charge‐carrier generation and recombination, leading to the formation of excitons within the device, without injection of charge carriers through external electrodes. A mechanism for internal charge‐carrier generation and the device operation is proposed.     

Synthesis and Characterization of n‐Type Materials for Non‐Doped Organic Red‐Light‐Emitting Diodes

Two compounds, 2,3‐dicyano‐5,6‐di(4′‐diphenylamino‐biphenyl‐4‐yl)pyrazine (CAPP) and 6,7‐dicyano‐2,3‐di(4′‐diphenylamino‐biphenyl‐4‐yl)quinoxaline (CAPQ), capable of intramolecular charge transfer, have been designed and synthesized in high yield by a convenient procedure. The compounds have been fully characterized spectroscopically. They have a high thermal stability and show bright light emission both in non‐polar solvents and in the solid state. Moreover, they exhibit excellent reversible oxidation and reduction waves. The higher energy level of the highest occupied molecular orbital (–5.3 eV) and the triphenylamine group are advantageous for hole‐injection/transport. In addition, the high electron affinities of 3.4 eV and the observed reversible reductive process suggest that these compounds enhance electron injection and have potential for use in electron transport. Three types of non‐doped red‐light‐emitting diodes have been studied using CAPP and CAPQ as the electron‐transporting and host‐light‐emitting layers, respectively. The devices exhibit red electroluminescence (EL), and constant Commission Internationale de l'Eclairage coordinates have been observed on increasing the current density. Pure red EL of CAPP, with a maximum brightness of 536 cd m^–2 and an external quantum efficiency of 0.7 % in ambient air, was achieved.     

Efficient Charge Carrier Injection and Balance Achieved by Low Electrochemical Doping in Solution‐Processed Polymer Light‐Emitting Diodes

Charge carrier injection and transport in polymer light‐emitting diodes (PLEDs) is strongly limited by the energy level offset at organic/(in)organic interfaces and the mismatch in electron and hole mobilities. Herein, these limitations are overcome via electrochemical doping of a light‐emitting polymer. Less than 1 wt% of doping agent is enough to effectively tune charge injection and balance and hence significantly improve PLED performance. For thick single‐layer (1.2 µm) PLEDs, dramatic reductions in current and luminance turn‐on voltages (V_J = 11.6 V from 20.0 V and V_L = 12.7 V from 19.8 V with/without doping) accompanied by reduced efficiency roll‐off are observed. For thinner (<100 nm) PLEDs, electrochemical doping removes a thickness dependence on V_J and V_L, enabling homogeneous electroluminescence emission in large‐area doped devices. Such efficient charge injection and balance properties achieved in doped PLEDs are attributed to a strong electrochemical interaction between the polymer and the doping agents, which is probed by in situ electric‐field‐dependent Raman spectroscopy combined with further electrical and energetic analysis. This approach to control charge injection and balance in solution‐processed PLEDs by low electrochemical doping provides a simple yet feasible strategy for developing high‐quality and efficient lighting applications that are fully compatible with printing technologies.     

Organic Light‐Emitting Transistors Containing a Laterally Arranged Heterojunction

An approach to produce organic light‐emitting transistors (OLETs) containing a laterally arranged heterojunction structure, which minimizes exciton quenching at the metal electrodes, is described. This device configuration provides an organic light‐emitting diode (OLED) structure where the anode (source) electrode, hole‐transport material (field‐effect material), light‐emitting material, and cathode (drain) electrode are laterally arranged, thus offering a chance to control the electroluminescent intensity by changing the gate bias. Pentacene and tris(8‐quinolinolato)aluminum (Alq₃) are employed as the field‐effect and light‐emitting materials, respectively. The laterally arranged heterojunction structures are achieved by successively inclined deposition of the field‐effect and light‐emitting materials. After deposition of pentacene, a narrow gap of about 10–20 nm between the drain electrode and pentacene was obtained, thereby creating an opportunity to fabricate a laterally arranged heterojunction. In the OLETs, unsymmetrical source and drain electrodes, that is, Au and LiF/Al ones, are used to ensure efficient injection of holes and electrons. Visible‐light emission from OLETs is observed under ambient atmosphere. This result is ascribed to efficient carrier injection and transport, formation of a heterojunction, as well as good luminescence from the organic emissive layer. The device structure serves as an excellent model system for OLETs and demonstrates a general concept of adjusting the charge‐carrier injection and transport, as well as the electroluminescent properties, by forming laterally arranged heterojunctions.     

Luminescent Poly(p‐phenylenevinylene) Hole‐Transport Layers with Adjustable Solubility

The active part of present polymer light‐emitting diodes (PLEDs) consists of only a single layer. Multilayer devices have the advantage that the electron and hole transport can be balanced and that the recombination can be removed from the metallic cathode, leading to higher efficiencies. A major problem for polymer‐based multilayer devices is the solubility of the materials used; a multilayer can not be fabricated when a spin‐cast layer dissolves in the solvent of the subsequent layer. We demonstrate the development of high‐mobility poly(p‐phenylenevinylene) (PPV)‐based hole‐transport layers with tunable solubility by chemical modification. Enhanced charge‐transport properties are achieved by using symmetrically substituted PPVs; copolymers of long and short side chains enable us to tune the solubility without loss of the enhanced charge transport. Dual‐layer PLEDs, in which the holes are efficiently transported via this copolymer towards the luminescent layer, exhibit an enhanced efficiency at high voltages (> 10 V) and a strongly improved robustness against electrical breakdown.     

Organic Light‐Emitting Diodes Based on Poly(9,9‐dioctylfluorene‐co‐bithiophene) (F8T2)

A study of the optical properties of poly(9,9‐dioctylfluorene‐co‐bithiophene) (F8T2) is reported, identifying this polymer as one that possesses a desirable combination of charge transport and light emission properties. The optical and morphological properties of a series of polymer blends with F8T2 dispersed in poly(9,9‐dioctylfluorene) (PFO) are described and almost pure‐green emission from light emitting diodes (LEDs) based thereon is demonstrated. High luminance green electroluminescence from LEDs using only a thin film of F8T2 for emission is also reported. The latter demonstration for a polymer previously primarily of interest for effective charge transport constitutes an important step in the development of emissive materials for applications where a union of efficient light emission and effective charge transport is required.     

Inverted Solution Processable OLEDs Using a Metal Oxide as an Electron Injection Contact.

A new type of bottom‐emission electroluminescent device is described in which a metal oxide is used as the electron‐injecting contact. The preparation of such a device is simple. It consists of the deposition of a thin layer of a metal oxide on top of an indium tin oxide covered glass substrate, followed by the solution processing of the light‐emitting layer and subsequently the deposition of a high‐workfunction (air‐stable) metal anode. This architecture allows for a low‐cost electroluminescent device because no rigorous encapsulation is required. Electroluminescence with a high brightness reaching 5700 cd m^–2 is observed at voltages as low as 8 V, demonstrating the potential of this new approach to organic light‐emitting diode (OLED) devices. Unfortunately the device efficiency is rather low because of the high current density flowing through the device. We show that the device only operates after the insertion of an additional hole‐injection layer in between the light‐emitting polymer (LEP) and the metal anode. A simple model that explains the experimental results and provides avenues for further optimization of these devices is described. It is based on the idea that the barrier for electron injection is lowered by the formation of a space–charge field over the metal‐oxide–LEP interface due to the build up of holes in the LEP layer close to this interface.     

Solution‐Processible Multi‐component Cyclometalated Iridium Phosphors for High‐Efficiency Orange‐Emitting OLEDs and Their Potential Use as White Light Sources

The synthesis and photophysical studies of several multifunctional phosphorescent iridium(III) cyclometalated complexes consisting of the hole‐transporting carbazole and fluorene‐based 2‐phenylpyridine moieties are reported. All of them are isolated as thermally and morphological stable amorphous solids. Extension of the π‐conjugation through incorporation of electron‐pushing carbazole units to the fluorene fragment leads to bathochromic shifts in the emission profile, increases the highest occupied molecular orbital levels and improves the charge balance in the resulting complexes because of the propensity of the carbazole unit to facilitate hole transport. These iridium‐based triplet emitters give a strong orange phosphorescence light at room temperature with relatively short lifetimes in the solution phase. The photo‐ and electroluminescence properties of these phosphorescent carbazolylfluorene‐functionalized metalated complexes have been studied in terms of the coordinating position of carbazole to the fluorene unit. Organic light‐emitting diodes (OLEDs) using these complexes as the solution‐processed emissive layers have been fabricated which show very high efficiencies even without the need for the typical hole‐transporting layer. These orange‐emitting devices can produce a maximum current efficiency of ～ 30 cd A^–1 corresponding to an external quantum efficiency of ～ 10 % ph/el (photons per electron) and a power efficiency of ～ 14 lm W^–1. The homoleptic iridium phosphors generally outperform the heteroleptic counterparts in device performance. The potential of exploiting these orange phosphor dyes in the realization of white OLEDs is also discussed.     

All-solution-processed multilayer polymer/dendrimer light emitting diodes

A fully solution-processed deep-blue emitting organic light emitting diode (OLED) based on a highly efficient fluorescent dendritic material with a pyrene core, a phenylene shell and triphenylamine surface groups coupled with polymeric hole (HTL) and electron (ETL) transport layers is demonstrated. Each layer formed smooth and pinhole-free films as demonstrated by Atomic Force Microscopy (AFM) as well as by X-ray Photoelectron Spectroscopy (XPS). Furthermore, detailed Ultraviolet Photoelectron Spectroscopy (UPS) surveys revealed a beneficial energy level alignment and hence improved charge carrier confinement. The resulting triple-layer device saw a 7.7-fold increase in current efficiency compared to a single-layer device while maintaining a deep-blue emission color characterized by the CIE1931 coordinates of x = 0.153 and y = 0.155.     

Achieving Highly Efficient Fluorescent Blue Organic Light‐Emitting Diodes Through Optimizing Molecular Structures and Device Configuration

Based on the results of first‐principles calculations of the electronic properties of blue light‐emitting materials, the molecular structures of oligofluorenes are optimized by incorporating electron‐withdrawing groups into the molecules to balance hole and electron injection and transport for organic light‐emitting diodes (OLEDs). The result is a remarkable improvement in the maximum external quantum efficiency (EQE) of the undoped device from 2.0% to 4.99%. Further optimization of the device configurations and processing procedures, e.g., by changing the thickness of the emitting layer and through thermal annealing treatments, leads to a very high maximum EQE of 7.40% for the undoped sky‐blue device. Finally, by doping the emitter in a suitable host material, 4,4’‐bis(carbazol‐9‐yl)biphenyl (CBP), at the optimal concentration of 6%, pure blue emission with extremely high maximum EQE of 9.40% and Commission Internationale de l’Eclairage (CIE) coordinates of (0.147, 0.139) is achieved.     

Pyridine‐Containing Electron‐Transport Materials for Highly Efficient Blue Phosphorescent OLEDs with Ultralow Operating Voltage and Reduced Efficiency Roll‐Off

A series of pyridine‐containing electron‐transport materials are developed as an electron‐transport layer for the FIrpic‐based blue phosphorescent organic light‐emitting diodes. Their energy levels can be tuned by the introduction of pyridine rings in the framework and on the periphery of the molecules. Significantly reduced operating voltage is achieved without compromising external quantum efficiency by solely tuning the nitrogen atom orientations of those pyidine rings. Unprecedented low operating voltages of 2.61 and 3.03 V are realized at 1 and 100 cd m^?2, giving ever highest power efficiency values of 65.8 and 59.7 lm W^?1, respectively. In addition, the operating voltages at 100 cd m^?2 can be further reduced to 2.70 V by using a host material with a small singlet‐triplet exchange energy, and the threshold voltage for electroluminescence can even be 0.2–0.3 V lower than the theoretical minimum value of the photon energy divided by electron charge. Aside from the reduced operating voltage, a further reduced roll‐off in efficiency is also achieved by the combination of an appropriate host material.     

Reduction of Tungsten Oxide: A Path Towards Dual Functionality Utilization for Efficient Anode and Cathode Interfacial Layers in Organic Light‐Emitting Diodes

Here, we report on the dual functionality of tungsten oxide for application as an efficient electron and hole injection/transport layer in organic light‐emitting diodes (OLEDs). We demonstrate hybrid polymer light‐emitting diodes (Hy‐PLEDs), based on a polyfluorene copolymer, by inserting a very thin layer of a partially reduced tungsten oxide, WO_2.5, at the polymer/Al cathode interface to serve as an electron injection and transport layer. Significantly improved current densities, luminances, and luminous efficiencies were achieved, primarily as a result of improved electron injection at the interface with Al and transport to the lowest unoccupied molecular orbital (LUMO) of the polymer, with a corresponding lowering of the device driving voltage. Using a combination of optical absorption, ultraviolet spectoscopy, X‐ray photoelectron spectroscopy, and photovoltaic open circuit voltage measurements, we demonstrate that partial reduction of the WO₃ to WO_2.5 results in the appearance of new gap states just below the conduction band edge in the previously forbidden gap. The new gap states are proposed to act as a reservoir of donor electrons for enhanced injection and transport to the polymer LUMO and decrease the effective cathode workfunction. Moreover, when a thin tungsten oxide film in its fully oxidized state (WO₃) is inserted at the ITO anode/polymer interface, further improvement in device characteristics was achieved. Since both fully oxidized and partially reduced tungsten oxide layers can be deposited in the same chamber with well controlled morphology, this work paves the way for the facile fabrication of efficient and stable Hy‐OLEDs with excellent reproducibility.     

Integrating Efficient Optical Gain in High‐Mobility Organic Semiconductors for Multifunctional Optoelectronic Applications

Simultaneously integrating efficient optical gain and high charge carrier mobility in organic semiconductors for multifunctional optoelectronic applications is challenging. Here, a new thiophene/phenylene derivative, 5,5′‐bis(2,2‐diphenylvinyl)‐bithiophene (BDPV2T), containing an appropriate butterfly molecular configuration in a π‐conjugated structure, is designed to achieve both solid‐state emission and charge transport properties. The prepared BDPV2T crystals exhibit excellent light‐emitting characteristics with a photoluminescence quantum yield of 30%, low light‐amplification threshold of 8 kW cm^?2, high optical net gain up to 70 cm^?1, and high charge carrier mobility up to 1 cm² V^?1 s^?1 in their J‐aggregate single crystals. These BDPV2T single crystal characteristics ensure their application potential for photodetectors, field‐effect transistors, and light‐emitting transistors. High optoelectronic performances are achieved with photoresponsivity of 2.0 × 10³ A W^?1 and light on/off ratio of 5.4 × 10⁵ in photodetectors, and efficient ambipolar charge transport (µ_h: 0.14 cm² V^?1 s^?1, µ_e: 0.02 cm² V^?1 s^?1) and electroluminescence characteristics in light‐emitting transistors. The remarkably integrated optoelectronic properties of BDPV2T suggest it is a promising candidate for organic multifunctional and electrically pumped laser applications.