Designing a Stable Cathode with Multiple Layers to Improve the Operational Lifetime of Polymer Light‐Emitting Diodes

The short device lifetime of blue polymer light‐emitting diodes (PLEDs) is still a bottleneck for commercialization of self‐emissive full‐color displays. Since the cathode in the device has a dominant influence on the device lifetime, a systematic design of the cathode structure is necessary. The operational lifetime of blue PLEDs can be greatly improved by introducing a three‐layer (BaF₂/Ca/Al) cathode compared with conventional two‐layer cathodes (BaF₂/Al and Ba/Al). Therefore, the roles of the BaF₂ and Ca layers in terms of electron injection, luminous efficiency, and device lifetime are here investigated. For efficient electron injection, the BaF₂ layer should be deposited to the thickness of at least one monolayer (～3 nm). However, it is found that the device lifetime does not show a strong relation with the electron injection or luminous efficiency. In order to prolong the device lifetime, sufficient reaction between BaF₂ and the overlying Ca layer should take place during the deposition where the thickness of each layer is around that of a monolayer.     

Highly Efficient Hole Injection Using Polymeric Anode Materials for Small‐Molecule Organic Light‐Emitting Diodes

A novel, highly efficient hole injection material based on a conducting polymer polythienothiophene (PTT) doped with poly(perfluoroethylene‐perfluoroethersulfonic acid) (PFFSA) in organic light‐emitting diodes (OLEDs) is demonstrated. Both current–voltage and dark‐injection‐current transient data of hole‐only devices demonstrate high hole‐injection efficiency employing PTT:PFFSA polymers with different organic charge‐transporting materials used in fluorescent and phosphorescent organic light‐emitting diodes. It is further demonstrated that PTT:PFFSA polymer formulations applied as the hole injection layer (HIL) in OLEDs reduce operating voltages and increase brightness significantly. Hole injection from PTT:PFFSA is found to be much more efficient than from typical small molecule HILs such as copper phthalocyanine (CuPc) or polymer HILs such as polyethylene dioxythiophene: polystyrene sulfonate (PEDOT‐PSS). OLED devices employing PTT:PFFSA polymer also demonstrate significantly longer lifetime and more stable operating voltages compared to devices using CuPc.     

Self‐assembled Electroactive Phosphonic Acids on ITO: Maximizing Hole‐Injection in Polymer Light‐Emitting Diodes

In order to fulfill the promise of organic electronic devices, performance‐limiting factors, such as the energetic discontinuity of the material interfaces, must be overcome. Here, improved performance of polymer light‐emitting diodes (PLEDs) is demonstrated using self‐assembled monolayers (SAMs) of triarylamine‐based hole‐transporting molecules with phosphonic acid‐binding groups to modify the surface of the indium tin oxide (ITO) anode. The modified ITO surfaces are used in multilayer PLEDs, in which a green‐emitting polymer, poly[2,7‐(9,9‐dihexylfluorene)‐co‐4,7‐(2,1,3‐benzothiadiazole)] (PFBT5), is sandwiched between a thermally crosslinked hole‐transporting layer (HTL) and an electron‐transporting layer (ETL). All tetraphenyl‐diamine (TPD)‐based SAMs show significantly improved hole‐injection between ITO and the HTL compared to oxygen plasma‐treated ITO and simple aromatic SAMs on ITO. The device performance is consistent with the hole‐transporting properties of triarylamine groups (measured by electrochemical measurements) and improved surface energy matching with the HTL. The turn‐on voltage of the devices using SAM‐modified anodes can be lowered by up to 3 V compared to bare ITO, yielding up to 18‐fold increases in current density and up to 17‐fold increases in brightness at 10 V. Variations in hole‐injection and turn‐on voltage between the different TPD‐based molecules are attributed to the position of alkyl‐spacers within the molecules.     

Control of the Surface Composition of a Conducting‐Polymer Complex Film to Tune the Work Function

Control of surface composition of a hole‐injecting conducting polymer complex, poly(3,4‐ethylenedioxy thiophene) (PEDOT) doped with a polystyrene sulfonate (PSS) has been conducted in the spin‐cast films. We found that the work function of the polymeric complex films formed via single spin‐coating can be greatly increased up to 5.44 eV by increasing the surface concentration of the PSS dopant. As a result, we improved the device efficiency and the lifetime of green emitting polymer light‐emitting diodes (PLEDs). This implies that the PSS surface layer of the films spin‐cast from the conducting polymer complexes plays a key role in making high performance PLEDs.     

Alternating‐Current MXene Polymer Light‐Emitting Diodes

MXenes (Ti₃C₂) are 2D transition‐metal carbides and carbonitrides with high conductivity and optical transparency. However, transparent MXene electrodes suitable for polymer light‐emitting diodes (PLEDs) have rarely been demonstrated. With the discovery of the excellent electrical stability of MXene under an alternating current (AC), herein, PLEDs that employ MXene electrodes and exhibit high performance under AC operation (AC MXene PLEDs) are presented. The PLED exhibits a turn‐on voltage, current efficiency, and brightness of 2.1 V, 7 cd A^?1, and 12 547 cd m^?2, respectively, when operated under AC with a frequency of 1 kHz. The results indicate that the undesirable electric breakdown associated with heat arising from the poor interface of the MXene with a hole transport layer in the direct‐current mode is efficiently suppressed by the transient injection of carriers accompanied by the alternating change of the electric polarity under the AC, giving rise to reliable light emission with a high efficiency. The solution‐processable MXene electrode can be readily fabricated on a flexible polymer substrate, allowing for the development of a mechanically flexible AC MXene PLED with a higher performance than flexible PLEDs employing solution‐processed nanomaterial‐based electrodes such as carbon nanotubes, reduced graphene oxide, and Ag nanowires.     

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.     

Electron‐Rich Alcohol‐Soluble Neutral Conjugated Polymers as Highly Efficient Electron‐Injecting Materials for Polymer Light‐Emitting Diodes

We report the design and synthesis of three alcohol‐soluble neutral conjugated polymers, poly[9,9‐bis(2‐(2‐(2‐diethanolaminoethoxy) ethoxy)ethyl)fluorene] (PF‐OH), poly[9,9‐bis(2‐(2‐(2‐diethanol‐aminoethoxy)ethoxy)ethyl)fluorene‐alt‐4,4′‐phenylether] (PFPE‐OH) and poly[9,9‐bis(2‐(2‐(2‐diethanolaminoethoxy) ethoxy)ethyl)fluorene‐alt‐benzothiadizole] (PFBT‐OH) with different conjugation length and electron affinity as highly efficient electron injecting and transporting materials for polymer light‐emitting diodes (PLEDs). The unique solubility of these polymers in polar solvents renders them as good candidates for multilayer solution processed PLEDs. Both the fluorescent and phosphorescent PLEDs based on these polymers as electron injecting/transporting layer (ETL) were fabricated. It is interesting to find that electron‐deficient polymer (PFBT‐OH) shows very poor electron‐injecting ability compared to polymers with electron‐rich main chain (PF‐OH and PFPE‐OH). This phenomenon is quite different from that obtained from conventional electron‐injecting materials. Moreover, when these polymers were used in the phosphorescent PLEDs, the performance of the devices is highly dependent on the processing conditions of these polymers. The devices with ETL processed from water/methanol mixed solvent showed much better device performance than the devices processed with methanol as solvent. It was found that the erosion of the phosphorescent emission layer could be greatly suppressed by using water/methanol mixed solvent for processing the polymer ETL. The electronic properties of the ETL could also be influenced by the processing conditions. This offers a new avenue to improve the performance of phosphorescent PLEDs through manipulating the processing conditions of these conjugated polymer ETLs.     

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.     

Bright Blue Solution Processed Triple‐Layer Polymer Light‐Emitting Diodes Realized by Thermal Layer Stabilization and Orthogonal Solvents

The realization of fully solution processed multilayer polymer light‐emitting diodes (PLEDs) constitutes the pivotal point to push PLED technology to its full potential. Herein, a fully solution processed triple‐layer PLED realized by combining two different deposition strategies is presented. The approach allows a successive deposition of more than two polymeric layers without extensively redissolving already present layers. For that purpose, a poly(9,9‐dioctyl‐fluorene‐co‐N‐(4‐butylphenyl)‐diphenylamine) (TFB) layer is stabilized by a hard‐bake process as hole transport layer on top of poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). As emitting layer, a deep blue emitting pyrene‐triphenylamine copolymer is deposited from toluene solution. To complete the device assembly 9,9‐bis(3‐(5′,6′‐bis(4‐(polyethylene glycol)phenyl)‐[1,1′:4′,1″‐terphenyl]‐2′‐yl)propyl)‐9′,9′‐dioctyl‐2,7‐polyfluorene (PEGPF), a novel polyfluorene‐type polymer with polar sidechains, which acts as the electron transport layer, is deposited from methanol in an orthogonal solvent approach. Atomic force microscopy verifies that all deposited layers stay perfectly intact with respect to morphology and layer thickness upon multiple solvent treatments. Photoelectron spectroscopy reveals that the offsets of the respective frontier energy levels at the individual polymer interfaces lead to a charge carrier confinement in the emitting layer, thus enhancing the exciton formation probability in the device stack. The solution processed PLED‐stack exhibits bright blue light emission with a maximum luminance of 16 540 cd m^?2 and a maximum device efficiency of 1.42 cd A^?1, which denotes a five‐fold increase compared to corresponding single‐layer devices and demonstrates the potential of the presented concept.     

Fluorescent,Carrier‐Trapping Dopants for Highly Efficient Single‐Layer Polyfluorene LEDs

Two fluorescent molecules with an alkynylanthracene core and pyrene end‐cappers have been synthesized and fully characterized. Carbazole moieties are introduced into one molecule at the C9 position of the fluorene linkages to enhance the hole‐transport ability of the molecule and to reduce intermolecular interactions. Both compounds exhibit high thermal stabilities and narrow energy bandgaps. Single‐layer polymer light‐emitting diodes (PLEDs) based on poly(9,9‐dioctylfluorene) (PFO) doped with the synthesized compounds exhibit excellent performance. A PLED with 0.2 % of dopant 7 had a high luminance efficiency of 10.7 ± 0.3 cd A^–1 as well as a brightness of 1400 cd m^–2 at a current density of 13 mA cm^–2, and a low turn on voltage (3.1 V) at a brightness of 10 cd m^–2. A maximum brightness of 20 500 ± 1400 cd m^–2 at 7 V was also measured. The high efficiency of the device's performance is attributed to the good electron and hole trapping ability of the dopants, which possess suitable energy levels as compared to those of PFO.     

Amino N‐Oxide Functionalized Conjugated Polymers and their Amino‐Functionalized Precursors: New Cathode Interlayers for High‐Performance Optoelectronic Devices

A series of amino N‐oxide functionalized polyfluorene homopolymers and copolymers (PNOs) are synthesized by oxidizing their amino functionalized precursor polymers (PNs) with hydrogen peroxide. Excellent solubility in polar solvents and good electron injection from high work‐function metals make PNOs good candidates for interfacial modification of solution processed multilayer polymer light‐emitting diodes (PLEDs) and polymer solar cells (PSCs). Both PNOs and PNs are used as cathode interlayers in PLEDs and PSCs. It is found that the resulting devices show much better performance than devices based on a bare Al cathode. The effect of side chain and main chain variations on the device performance is investigated. PNOs/Al cathode devices exhibit better performance than PNs/Al cathode devices. Moreover, devices incorporating polymers with para‐linkage of pyridinyl moieties exhibit better performance than those using polymers with meta‐linked counterparts. With a poly[(2,7‐(9,9‐bis(6‐(N,N‐diethylamino)‐hexyl N‐oxide)fluorene))‐alt‐(2,5‐pyridinyl)] (PF6NO25Py) cathode interlayer, the resulting device exhibits a luminance efficiency of 16.9 cd A^?1 and a power conversion efficiency of 6.9% for PLEDs and PSCs, respectively. These results indicate that PNOs are promising new cathode interlayers for modifying a range of optoelectronic devices.     

Quantitative estimation of exciton quenching strength at interface of charge injection layers and organic semiconductor

The performance of polymer light emitting diodes (PLEDs) degrades due to exciton quenching at the interface with charge injection layers and electrodes. We investigate the photo-physics of singlet excitons in Poly (9, 9-dioctylfluorene-alt-benzothiadiazole) (F8BT) conjugated polymer interfaced with various commonly used hole and electron injection layers. Absolute, steady-state and transient photoluminescence (PL) studies are carried out on pristine F8BT film and films with injection layer/F8BT to understand the role of injection layers on exciton quenching. Exciton quenching by the charge injection layers is treated by accounting for both exciton diffusion and the non-radiative transfer of energy to the charge injection layer. The non-radiative transfer of energy is modelled using dipole-dipole interaction theory coupled with diffusion of excitons, from which we obtain the exciton capture radius (x₀) in the range of 1–7 nm. We also correlate x₀ with PL decay time (τ) using the relation τ α 1/x₀³. The steady-state PL yield for each case also shows correlation with the PL decay lifetime. This study provides interesting insight on the selection criterion for injection layer to be used in PLEDs for minimizing optical losses while preserving the electronic injection properties.     

Composite film of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) and MoO3 as an efficient hole injection layer for polymer light-emitting diodes

We demonstrate improved performances in polymer light-emitting diodes (PLEDs) using a composite film of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and MoO₃ powder as a hole injection layer. The PLED with the composite film exhibits the current efficiency of 13.5 cd/A, driving voltage of 3.4 V, and half lifetime of 108.1 h, while those values of the PLED with a pristine PEDOT:PSS was 11.3 cd/A, 3.8 V, and 41.5 h, respectively. We also analyze the morphological, optical and electrical properties of the composite films by atomic force microscopy (AFM), UV–Vis-IR absorption, and ultraviolet photoemission spectroscopy (UPS). This work suggests that mixing MoO₃ into PEDOT:PSS is a simple and promising technique for use solution-based devices as an hole injection layer.     

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.     

Degradation Effects Related to the Hole Transport Layer in Organic Solar Cells

The influence of the hole transport layer on device stability in polymer:fullerene bulk‐heterojunction solar cells is reported. Three different hole transport layers varying in composition, dispersion solvent, electrical conductivity, and work function were used in these studies. Two water‐based hole transport layers, poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) and polyaniline:poly(styrene sulfonate), and one isopropyl alcohol‐based polyaniline:poly(styrene sulfonate) transport layer were investigated. Solar cells with the different hole transport layers were fabricated and degraded under illumination. Current–voltage, capacitance–voltage, and capacitance–frequency data were collected at light intensities of 16, 30, 48, 80, and 100 mW cm^?2 over a period of 7 h. Device performance and stability were compared between nonencapsulated and encapsulated samples to gain understanding about degradation effects related to oxygen and water as well as degradation mechanisms related to the intrinsic instability of the solar cell materials and interfaces. It is demonstrated that the properties of the hole transport layer can have a significant impact on the stability of organic solar cells.     

Interface Modification to Improve Hole‐Injection Properties in Organic Electronic Devices

The performance of organic electronic devices is often limited by injection. In this paper, improvement of hole injection in organic electronic devices by conditioning of the interface between the hole‐conducting layer (buffer layer) and the active organic semiconductor layer is demonstrated. The conditioning is performed by spin‐coating poly(9,9‐dioctyl‐fluorene‐co‐N‐ (4‐butylphenyl)‐diphenylamine) (TFB) on top of the poly(3,4‐ethylene dioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) buffer layer, followed by an organic solvent wash, which results in a TFB residue on the surface of the PEDOT:PSS. Changes in the hole‐injection energy barriers, bulk charge‐transport properties, and current–voltage characteristics observed in a representative PFO‐based (PFO: poly(9,9‐dioctylfluorene)) diode suggest that conditioning of PEDOT:PSS surface with TFB creates a stepped electronic profile that dramatically improves the hole‐injection properties of organic electronic devices.     

Electrophosphorescent Devices Based on Cationic Complexes: Control of Switch‐on Voltage and Efficiency Through Modification of Charge Injection and Charge Transport

This paper reports an analysis of the properties of polymer light‐emitting devices (PLEDs) doped with iridium complexes. Devices based on charged and neutral complexes doped into poly(vinylcarbazole) (PVK) are presented, and the role of the ions and the charge‐transport properties of the complexes are discussed. In devices with the charged complexes, the concentration of the complex is found to have a profound effect on both the switch‐on voltage and the efficiency. At higher doping concentrations the efficiency is increased and the switch‐on voltage decreased. The increase in efficiency and decrease in switch‐on voltage at higher dopant concentration are found to be due to an alternative charge transport path via the iridium dopant [Ir(bpy)]⁺ (bis(2‐phenylpyridine‐C²,N′)(2,2′‐bipyridine)iridium hexafluorophosphate). However, at lower concentrations the complex becomes an electron trap and the efficiency is reduced. The devices are found to be significantly less efficient than those with neutral complexes. This difference is attributed to the ionic content and the charge trapping properties of the charged complexes. The low efficiency of the charged‐complex‐based devices could be overcome by utilizing a hole‐blocking layer; devices with efficiencies as high as 23 cd A^–1 were obtained.     

Highly efficient electrophosphorescent polymer light-emitting devices

We report a high performance polymer electroluminescent device based on a bi-layer structure consisting of a hole transporting layer (poly(vinylcarbazole)) and an electron transporting layer poly(9,9-bis(octyl)-fluorene-2,7-diyl) (BOc-PF) doped with platinum(II)-2,8,12,17-tetraethyl-3,7,13,18-tetramethylporphyrin (PtOX). The devices show red electrophosphorescence with a peak emission at 656 nm and a full width at half maximum of 18 nm, consistent with exclusive emission from the PtOX dopants. BOc-PF emission is not observed at any bias. The required doping levels for these phosphorescence-based polymer light-emitting diodes (PLEDs) are significantly lower than for other reported phosphorescence-based PLEDs or organic light-emitting diodes (OLEDs). A doping level of 1% or more give an LED with exclusive PtOX emission, whereas related PLEDs or OLEDs doped with phosphorescent dopants require doping levels of >5% to achieve exclusive dye dopant emission. The device external efficiency was enhanced from 1% to 2.3% when doped with PtOX. The lower doping level in BOc-PF/PtOX based PLEDs decreases triplet–triplet annihilation in these devices, leading to quantum efficiency that is only weakly dependent on current density. The luminescence transient decay time for this device is 500 μs.     

Work function modification of solution-processed tungsten oxide for a hole-injection layer of polymer light-emitting diodes

We report a solution-processed tungsten oxide hole injection layer for polymer light-emitting diodes (PLEDs). Unlike vacuum evaporated tungsten oxide, solution-processed tungsten oxide has a reduced work function due to contamination of the ambient atmosphere. To increase the work function of tungsten oxide layer, an organic ionic solution containing poly(ethylene oxide) and tetra-n-methylammonium tetrafluoroborate was coated on the tungsten oxide layer. Organic ions infiltrated into the tungsten oxide layer and formed an interfacial dipole, increasing the layer’s work function by 0.4 eV. As a result, a PLED, which had a hole-injection layer of the tungsten oxide and organic ionic solution, showed a performance equivalent to that of the PEDOT:PSS-based device with even higher maximum luminance of 27,560 cd/m² at 10 V.     

Reduced Efficiency Roll‐Off and Improved Stability of Mixed 2D/3D Perovskite Light Emitting Diodes by Balancing Charge Injection

Perovskite light emitting diodes (PeLEDs) have reached external quantum efficiencies (EQEs) over 21%. Their EQE, however, drops at increasing current densities (J) and their lifetime is still limited to just a few hours. The mechanisms leading to EQE roll‐off and device instability require thorough investigation. Here, improvement in EQE, EQE roll‐off, and lifetime of PeLEDs is demonstrated by tuning the balance of electron/hole transport into a mixed 2D/3D perovskite emissive layer. The mixed 2D/3D perovskite layer induces exciton confinement and beneficially influences the electron/hole distribution inside the perovskite layer. By tuning the electron injection to match the hole injection in such active layer, a nearly flat EQE for J = 0.1–200 mA cm^?2, a reduced EQE roll‐off until J = 250 mA cm^?2, and a half‐lifetime of ≈47 h at J = 10 mA cm^?2 is reached. A model is also proposed to explain these improvements that account for the spatial electron/hole distributions.