Charge Injection Layers Research Articles

Energy level alignment at the interfaces in a multilayer organic light-emitting diode structure

Received 2 March 2009; revised manuscript received 4 May 2009; published 11 June 2009We use photoelectron spectroscopy to study the electronic structure and energy level alignment throughoutan organic light-emitting diode. The structure under investigation is a state-of-the-art long-living red phospho-rescent device composed of doped charge-injection layers, charge-blocking layers, and an emission layer. Byconsecutively building up the whole device, the key parameters of every interface are measured. Our resultsshow that the doped layers have a significant influence on the device energetics, especially in controlling thebuilt-in potential, and that there are mostly only small dipoles present at the interfaces of the intrinsic organiclayers.DOI: 10.1103/PhysRevB.79.245308 PACS number s : 79.60.Fr, 85.60.Jb

30.2: Distinguished Paper: Charge Carriers and Charge Transfer Reactions in OLED Devices Studied by Electron Paramagnetic Resonance

AbstractWe use Electron Paramagnetic Resonance (EPR) as a quantitative method to measure charge carriers that are present in OLED devices under various bias conditions, and to investigate charge transfer reactions that occur at the cathode and anode interfaces through the interaction of charge injection layers and charge transport layers.

Improvement of subthreshold current transport by contact interface modification in p-type organic field-effect transistors

The charge injection efficiency of organic field-effect transistors (OFETs) is found to be a critical factor determining the subthreshold characteristics of these devices. OFETs fabricated using a wide band gap organic semiconductor and gold source/drain contacts display large threshold voltage and poor subthreshold characteristics. Insertion of a metal-oxide charge injection layer at the contact/semiconductor interface lower the injection barrier height, resulting in marked improvements in threshold voltage and subthreshold slope and strong suppression of the short-channel effect. The improved subthreshold characteristics are attributed to enhanced charge injection and the consequent promotion of charge accumulation.

The influence of subgap features in the electromodulation and built-in voltage measurements of polyfluorene blue light-emitting diodes with anodic charge injection layers

We report on electroabsorption (EA) and built-in voltage (VBI) measurements of polymer light-emitting diodes with the general structure ITO/PEDOT:PSS/emitting polymer/LiF∕Ca∕Al where ITO is indium tin oxide, PEDOT:PSS is poly(3,4-ethylene dioxythiophene) doped with poly(styrene sulfonate), and the emitting polymer is either poly(9,9-dioctylfluorene), poly(9, 9-dioctyl-fluorene-alt-bis-N,N′-(4-butyl-phenyl)-bis-N,N′-phenyl-1, 4-phenylenediamine), or poly(9,9-dioctylfluorene-alt-N-(4-butylphenyl)-diphenylamine). We find that the EA nulling voltage, i.e., the dc bias at which the EA signal vanishes, depends on the frequency of the ac voltage and on the incident photon wavelength. Such dependence poses a problem for accurate measurement of the built-in voltage (VBI), which is the voltage generated between the electrodes upon equilibration of the Fermi levels through the heterostructure. We find that the EA signal is mixed with a smaller intensity signal which can be ascribed to excited state absorption (ESA). We propose a method for separating the excited state absorption signal and producing accurate VBI measurements. We also demonstrate that in our devices the ESA contribution to the electromodulated (ΔT∕T) signal is negligible with respect to the accuracy with which VBI can be determined from the nulling voltage of ΔT∕T and can thus be safely ignored.

Stable, Highly Efficient and Temperature Resistant Organic Light-Emitting Devices

Highly efficient small molecule-based organic light-emitting devices (OLED) that use electrically doped charge injection layers are presented. These structures exhibit low power consumption, long lifetime and high stability when operated at ambient temperature. It is also shown that their performances do not change significantly even when devices are stored at 90 °C for 80 days or 110 °C for 24 h. Such OLED structures thus fulfill the most severe requirements for mobile display applications.

Cross-Linked Polyfluorene Polymer Precursors: Electrodeposition, PLED Device Characterization, and Two-Site Co-deposition with Poly(vinylcarbazole)

Electropolymerization of π-conjugated polymers is considered as an alternative method for depositing and patterning charge injection, transport, and luminescent layers in organic electronic devices. In this study, a series of novel poly(fluoren-9,9-diyl-alt-alkan-α,ω-diyl) polymers with increasing alkyl spacer lengths were synthesized and used as precursors for the deposition of conjugated polymers formed by the oxidative coupling of the fluorene units. The precursor polymers and their conjugated polymer network were characterized in solution and in film form, and their charge-transport characteristics were investigated by fabricating electroluminescent polymer light emitting diode (PLED) devices. Interesting charge carrier transport properties based on thickness and co-deposition with another precursor polymer gives the potential for optimized solid-state device performance and film processing conditions. Finally, a “two-pixel” prototype PLED was demonstrated using a sequential electrochemical co-deposit...

High-performance organic thin-film transistors with metal oxide/metal bilayer electrode

We demonstrate bilayer source-drain (S-D) electrodes for organic thin film transistors (OTFT). The bilayer consists of a transition metal oxide (MoO3,WO3, or V2O5) layer and a metal layer. The metal oxide layer, directly contacting the organic semiconducting layer, serves as the charge-injection layer. The overcoated metal layer is responsible for the conduction of charge carriers. We found that the metal oxide layer coupled between pentacene and metal layers played an important role in improving the field-effect transistor characteristics of OTFTs. Devices with the bilayer S-D electrodes showed enhanced hole-injection compared to those with only metal electrode. High field-effect mobility of 0.4cm2∕Vs and on/off current ratios of 104 were obtained in the pentacene based TFTs using the bilayer S-D electrodes at a gate bias of −40V. The improvement is attributed to the reduction in the contact barrier and the prevention of metal diffusion into the organic layer and/or unfavorable chemical reaction between the organic layer and the metal electrode.

Multicolor Light-Emitting Diodes Based on Semiconductor Nanocrystals Encapsulated in GaN Charge Injection Layers

Numerous technologies including solid-state lighting, displays, and traffic signals can benefit from efficient, color-selectable light sources that are driven electrically. Semiconductor nanocrystals are attractive types of chromophores that combine size-controlled emission colors and high emission efficiencies with excellent photostability and chemical flexibility. Applications of nanocrystals in light-emitting technologies, however, have been significantly hindered by difficulties in achieving direct electrical injection of carriers. Here we report the first successful demonstration of electroluminescence from an all-inorganic, nanocrystal-based architecture in which semiconductor nanocrystals are incorporated into a p-n junction formed from GaN injection layers. The critical step in the fabrication of these nanocrystal/GaN hybrid structures is the use of a novel deposition technique, energetic neutral atom beam lithography/epitaxy, that allows for the encapsulation of nanocrystals within a GaN matrix without adversely affecting either the nanocrystal integrity or its luminescence properties. We demonstrate electroluminescence (injection efficiencies of at least 1%) in both single- and two-color regimes using structures comprising either a single monolayer or a bilayer of nanocrystals.

Current-induced chain migration in semiconductor polymer blends

We have used the technique of neutron reflectometry in order to evaluate the changes that occur within polymer light-emitting devices as they are operated. The devices examined consisted of an indium-tin-oxide, anode, a deuterated-poly(styrene sulfonate)/poly(3,4 ethylenedioxythiophene) (d-PSS/PEDT), charge injection layer, poly(fluorene) (F8), and a low work-function cathode. We found clear evidence for segregation of d-PSS to the d-PSS:PEDT/F8 interface in a device driven until its brightness had fallen to 10% of its initial value. We suggest that this effect is due to Joule heating of the device during operation.

10.4: High Performance and Innovative OLED Structures and Display

High performance small molecules OLED structures based on doped charge injection layers are presented. These structures exhibiting a low operation voltage are implemented in 3.25″ QVGA active matrix display made of laser annealed poly-silicon. The display exhibits a luminance of 120 cd/m2 at an overall voltage of less than 7 V and a power consumption of 280 mW.

Characterization of ITO/CuPc/AI and ITO/ZnPc/Al structures using optical and capacitance spectroscopy

Metal-substituted phthalocyanine (MPc) thin films as zinc- or copper-phthalocyanine are often used as charge injection layers for organic electroluminescent or photovoltaic devices. It is then important to characterize their electronic defect density and band structure near their gap. In this work the monolayer structures were prepared by vacuum sublimation of the organic thin film sandwiched between indium tin oxide (ITO) and aluminum electrodes. Electrically active defects were investigated with space-charge capacitance spectroscopy, as a function of temperature and frequency, in the range 80–330 K and 40 Hz to 10 MHz, respectively. Organic materials are best described on the basis of individual molecular orbital (HOMO and LUMO) energies instead of valence and conduction band. Such energies were derived from cyclic voltammetry and optical absorption spectroscopy measurements. Experimental results were correlated to the electrical J– V characteristics of these MPc based devices to gain more insight on the charge injection processes and their limitations.

Effects of additives in polymer thick film-organic light emitting diodes (PTF-OLED)

The optimum composition and film thickness determined for a single-layer organic light emitting diodes (120–150 cd/m 2 at 30 mA/cm 2) has been determined in one of our previous publication [Display 21 (2001) 199]. In this following study, a series of additives was included in the original composition for the purposes of color tuning, enhancement of the device efficiency as well as the stability of the polymer thick film-organic light emitting diodes (PTF-OLED). The additives attempted including three dyes (rubrene, perylene and DCM), three sensitizers (naphthalene, anthracene and p-terphenyl), several stabilizers (antioxidant and photo-stabilizer) and a number of organic soluble salts known as phase transfer catalyst. Rubrene was found being able to improve the OLED efficiency by 3-fold and its durability by at least 60 times at a concentration of 5–10 pph (in part per hundred of Alq 3 replaced). The color of the OLED can also be tuned according to the additional dye used. The sensitizer naphtahlene can also improved the efficiency of the OLED by 2-fold while the effect of the other additives was less significant. In addition, the lowering of the turn-on voltage and thus a higher current efficiency resulted by the addition of a charge injection layer (lithium fluoride or calcium fluoride) in between the cathode and the organic layer. The performance of the doped PTF-OLED is within an order of magnitude compared to a heterojunction small molecule-based OLED.

Organic polymer thick film light emitting diodes (PTF-OLED)

A guest–host approach was used to fabricate a one-layer organic light emitting diode (OLED). The thick film ink approach allows the two-dimensional OLED to be processed using traditional methods such as silk-screen printing. The I– V– L characteristics of the polymer thick film (PTF)-OLED were studied as a function of the device chemical compositions and physical configurations. Different polymers, hole and electron transporters, and emitters at different weight ratios were studied for its composition dependence. Device configuration also plays a significant role on its overall performance. Dependence on film thickness, electrode type, and the usage of additional charge injection layers were also investigated. The simplified one-layer device allows a straightforward interpretation for the charge-transport and recombination phenomena which shed light for its future improvement.

Light-Emitting Diodes from Fluorene-Based π-Conjugated Polymers

We report the synthesis of novel fluorene-based π-conjugated polymers and the investigation of their electroluminescent properties in organic light-emitting devices. We also report on the photo- and electroluminescence of materials of the same class whose synthesis was recently published. The alternated incorporation of phenylene or thiophene moieties in fluorene-based π-conjugated polymers leads to the tunability of the electroluminescent properties. The spectral emission varies from blue to green or yellow, depending on the composition of the copolymers. To enhance the luminous efficiency of the devices, the hole injection and hole transport into the polymer were improved by insertion of an insulating buffer layer and the incorporation of efficient hole transport material in the polymer. The insertion of a charge injection layer such as LiF and a hole transport layer such as N,N‘-diphenyl-N,N‘-bis(3-methylphenyl)-1,1-biphenyl-4,4‘-diamine significantly improves the electroluminescence efficiency of the ...