Electrodes In Organic Devices Research Articles

Mechanisms of methanol detection in graphene oxide and conductive polymer active layers for gas sensing devices

The admixture of PEDOT:PSS with Graphene Oxide (GO) in precise proportions achieves a substantial reduction in electrical resistivity, thereby augmenting its suitability as an electrode in organic devices. This study explores the electrical and morphological attributes of commercial PEDOT:PSS and chemically synthesized aqueous PEDOT ink when both are combined with GO. The investigation extends to the application of these conductive inks as active layers in flexible methanol sensing devices. Notably, a resistivity minimum is observed in the case of GO:PEDOT:PSS 78%, while the highest response to methanol is attained with GO:PEDOT:PSS 68%. To establish a theoretical underpinning for these findings, and to understand the interaction between gas/vapors with nanostructured materials, a model rooted in Kirchhoff’s Circuit approach is developed, with the aim of elucidating the factors behind the resistivity minimum and response maximum at distinct specific mass ratios between PEDOT and GO. Calculating the equivalent resistivity and response of the systems, the positions of minimum and maximum points are in agreement with the experimental data. Furthermore, the influence of PSS in the samples is examined, unveiling diverse interaction mechanisms between methanol molecules and the active layer, resulting in varying signals during the exposure to alcoholic vapor. The theoretical model is subsequently applied to these systems, demonstrating qualitative and quantitative agreement with the experimental results.

Stabilisation of Cu films in WO3/Ag/Cu:Al/WO3 structures through their doping by Al and Ag

Indium tin oxide (ITO) is the most common transparent conductive material used in industrial processes. It has many advantages, but also some disadvantages: Indium is scarce and ITO deposition techniques are aggressive for organic materials, making it difficult to use it as top electrode in organic devices. Moreover its ceramic structure limits its application in flexible devices. Among the possible new In free transparent conductive electrode, dielectric/metal/dielectric multilayer structures such as WO3/M/WO3 appear very promising. However, silver, which is the metal the more often used is expensive. Therefore it would be very profitable if copper, which is abundant on earth, could be substituted for silver. However the stability with time of the structure using Cu is questionable due to the high Cu diffusivity. In the present manuscript we improve significantly the lifetime of the structures using the alloy Cu:Al when a thin silver layer (2 nm) is introduced between the WO3 bottom layer and the Cu:Al. It is shown that the Cu atom mobility is significantly decreased by the presence of Al of the alloy and of Ag which appears to diffuse into the metal layer forming an eutectic with Cu.

Doping mechanism in organic devices: Effects of oxygen molecules in poly(3-hexylthiophene) thin films

Abstract The electrical properties of poly(3-hexylthiophene) (P3HT) are very sensitive to the presence of oxygen, as well as in contact with several metals and oxides that are used as electrodes in organic devices. Such atoms and molecules have deleterious impact on the devices performance, normally causing the diminishing of their efficiency and operation lifetime. Therefore, understanding how those molecules, especially the oxygen, interact with the active layer is mandatory to prevent device degradation. Here we performed CELIV and dark Time-of-Flight measurements in a simplified diode structure ITO/P3HT/Ag before and after exposing the device to ambient air. The results analysis give evidence of a slow intake of oxygen molecules by diffusion, a deliberate filling of the generated oxygen traps by electrons and, consequently, a shift of the P3HT Fermi level toward the HOMO of P3HT. Finally the dynamics of hole extraction during CELIV measurements and their recovery are also discussed.

Eumelanin–PEDOT:PSS Complementing En Route to Mammalian‐Pigment‐Based Electrodes: Design and Fabrication of an ITO‐Free Organic Light‐Emitting Device

The growing interest toward biocompatible and bioinspired materials is boosting the investigation and the engineering of natural products as active components in electronic devices. The human pigment melanin, and particularly its subgroup composed by the eumelanins, the black‐brown pigments derived from the oxidative polymerization of L‐3,4‐dihydroxyiphenylalanine (l‐DOPA) via 5,6‐dihydroxy‐indole intermediates are increasingly emerging as valuable candidates for organic (bio)electronics applications. Capitalizing on a recently developed protocol to prepare high quality eumelanin coatings, this paper reports herein the design and the integration of standard commercial poly(3,4‐ethylenedioxythiophene) with the poly(styrenesulfonate) (PEDOT:PSS) with eumelanin pigment. The new blend has made it possible to obtain a water stable quite transparent thin film, able to operate as an electrode for organic devices, complementing the PEDOT:PSS conductivity with the peculiar eumelanin properties, including adhesion, water stability, and ionic–electronic conductivity. As proof of concept, an unprecedented indium tin oxide‐free organic light emitting diode implementing an eumelanin–PEDOT layer as the anode is fabricated and characterized.

Investigations on Multilayered TCO as Transparent Electrode in Organic Devices

SummaryA sandwiched structure with a metal layer embedded between two metal oxide layers was fabricated on glass substrate to be used as transparent electrode in organic devices. The oxide layers of ZnO were dip coated with a thickness of about 60 nm each while the metal inter layer was sputter coated with a thickness of about 5 nm at room temperature. The deposited films were annealed at 200°C for 2 hrs to remove all additives. Such structures have been found to be suitable for applications in organic solar cell and OLED. The samples have been characterized using XRD, SEM and EDAX. Optical properties were measured by UV‐VIS and AFM was used to measure the roughness. The XRD pattern indicates the nearly amorphous structure of the thin film. The size of the aggregates as given by FESEM ranges from 30–50 nm and EDAX confirms the presence of elements of the three layers. The roughness as calculated using AFM is about 2.40 nm. UV‐VIS spectroscopy showed a high transparency of over 87% in the visible region for the sandwich structure making it suitable for TCO applications.

Chemical Composition of Additives That Spontaneously Form Cathode Interlayers in OPVs.

Interlayers between the active layer and the electrodes in organic devices are known to modify the electrode work function and enhance carrier extraction/injection, consequently improving device performance. It was recently demonstrated that chemical interactions between the evaporated electrode and interlayer additive can induce additive migration toward the metal/organic interface to spontaneously form the interlayer. In this work we used P3HT:PEG blends as a research platform to investigate the driving force for additive migration to the organic/metal interface and the source of the work function modification in OPVs. For this purpose PEG derivatives with different end groups were blended with P3HT or deposited on top of P3HT layer, topped with Al or Au evaporated electrodes. The correlation between the additive chemical structure, the Voc of corresponding devices, and the metal/organic interface composition determined by XPS revealed that the driving force for additive migration toward the blend/metal interface is the chemical interaction between the additives' end group and the deposited metal atoms. Replacing the PEG additives with alkyl additives bearing the same end groups has shown that the Al work function is actually modulated by the PEG backbone. Hence, in this work we have identified and separated between structural features controlling the migration of the interlayer additive to the organic/metal interface and those responsible for the modification of the metal work function.

Fabrication of high-performance composite electrodes composed of multiwalled carbon nanotubes and glycerol-doped poly(3,4-ethylenedioxythiophene):polystyrene sulfonate for use in organic devices

The organic doping facilitates the high performance of MWCNT/PEDOT:PSS composite electrode in organic devices.

Examining the transparency of gallium-doped zinc oxide for photovoltaic applications

In this work, we examine the optical properties of gallium-doped ZnO (GZO) thin films grown by pulsed laser deposition with varying oxygen pressures and substrate temperatures. In analyzing the optical properties of the thin films, we use a normalized transparency index that is directed at quantifying the transmission for usage as electrodes in photovoltaic devices. With this, we can clearly show the influence of oxygen on the transmission for different wavelengths. This allows us to clearly dissect the influence of band gap, oxygen pressure and carrier concentration on transparency. We achieved GZO films with a transparency index of 0.84 at room temperature and an improved index of 0.92 (92% of total solar spectrum transmitted) at a substrate temperature of 500 °C. The room temperature films, in particular, showed excellent transparency and conductivity, which are useful for use as electrodes in organic devices.

Insulator-metal Transition of an Alumina Cement Constituent, C12A7, Realized by Employing a Built-in Nanostructure: Fabrication of Highly Transparent Conductive Thin Films and Their Application to an Electron Injection Electrode in Organic Devices Utilizing the Low Work Function

12CaO·Al2O3 (C12A7) crystal is a constituent of alumina cements and an electrical insulator with a large band gap of ∼7 eV. It is possible to replace almost all the O2− ions in its crystallographic cages with electrons by reduction treatments. As a result, C12A7 is converted to a metallic conductor C12A7:e− with a low work function of ∼2.4 eV via an insulator-metal transition. We have developed a technique to prepare C12A7:e− thin films. The electrical and optical properties, and also an application to an electron injection layer for an organic light emitting device are reviewed.

Metal vs. Polymer Electrodes in Organic Devices: Energy Level Alignment, Hole Injection, and Structure

AbstractL We have investigated the electronic, structural, and charge injection properties of interfaces formed between three electroactive conjugated organic materials, i.e., N, N'-bis-(1-naphthyl)-N, N'-diphenyl1-1,1-biphenyl1-4,4'-diamine (á-NPD), pentacene, p-sexiphenyl, and two high work function electrode materials, i.e., gold and the conducting polymer poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS). Ultraviolet photoelectron spectroscopy shows that the hole injection barrier between the three organic materials and PEDOT:PSS is lower by 0.6-1.0 eV as compared to Au, despite a similar work function of the pristine electrode material surfaces (ca. 5 eV). This very large difference is due to an effective change of the metal work function due to the deposition of organic molecules, i.e., a decrease of the Au surface dipole due to adsorption. Accordingly, model device structures built from á-NPD and pentacene on the two different electrode materials show much higher current densities for hole injection from PEDOT:PSS than from Au. Hole injection from Au for á-NPD devices is independent of deposition sequence and substrate. Pentacene devices exhibit significant asymmetries in that respect, due to a strong dependence of the morphology and preferred molecular orientation of the crystalline material on the substrate, as shown by atomic force microscopy and X-ray diffraction. Consequently, great care must be taken when modeling current-voltage characteristics of devices comprised of crystalline organic solids, especially when the influence of film thickness or different substrate materials is to be studied.

A New Alternative for the Low-Workfunction Electrode in Organic Devices

ABSTRACTThe application of ele ctroactive organic materials in devices (light emitting diodes, photovoltaic cells) often requires electrodes with a low workfunction. Commonly, aluminum or alkaline earth metals are used, which usually exhibit strong interaction with the organic material, such as the formation of new covalent bonds or doping. This results in a strong modification of the electronic structure of the organic/metal interface, and in most cases does not yield the energy level alignment expected for the unreacted interface. As a n alternative to the above-mentioned metals we propose the use of samarium, with a workfunction of 2.7 eV, for the following reason: we have studied the interface formation between Sm and p -sexiphenyl (6P, which exhibits intense blue electroluminescence), with ultraviolet photoelectron spectroscopy. Sm was deposited stepwise onto thin films of 6P in ultrahigh vacuum, and the photoelectron spectra were recorded after each step. We did not find any indication for a strong interaction between the two materials. Metallic Sm is formed instantaneously and the valence electronic structure of 6P remains unchanged upon the metal deposition. The weak interaction at this interface allows one to determine the energy level alignment between a metal and an organic material in a direct manner from the photoelectron spectra, without the need for making any assumptions on the workfunction or ionization potential.