Frank Nüesch

Water Vapor and Oxygen Degradation Mechanisms in Organic Light Emitting Diodes

The degradation of organic light emitting diodes (OLEDs) due to the growth of dark spots can be attributed to the synergy of three external causes: dust particles deposited during the fabrication process, pollution by water vapor, and pollution by oxygen. On the basis of a set of new experiments performed on benchmark devices, we demonstrate that, for a given distribution of dust particles and a given concentration of the polluting agent, water is a thousand times more destructive than oxygen at room temperature. While the thermal diffusion of oxygen causes the oxidation of both the metal at the interface and the dye in the bulk of the device, water acts by an electrochemical process causing the delamination of the electrode.

Chemical potential shifts at organic device electrodes induced by grafted monolayers

Abstract Charge injection from an electrode into an organic semiconductor is strongly dependent on the height of the energy barrier between the electrode workfunction and the energy level of the HOMO or LUMO molecular orbitals. In the case of the well-known emitter tris-(8-hydroxyquinoline) aluminium (Alq) on clean indium tin oxide (ITO), an energy barrier of about 1 eV hinders both negative and positive charge carriers to be injected from the oxide electrode. By grafting molecules with appropriate dipole moments on ITO we have constructed light-emitting diodes based on Alq to demonstrate that the ITO workfunction can be modified in such a way that holes and even electrons can be efficiently injected.

The role of copper phthalocyanine for charge injection into organic light emitting devices

Abstract We investigate the charge injection efficiency of plasma treated indium tin oxide (ITO) anodes into copper phthalocyanine (CuPc) in single-layer diodes fabricated under inert conditions. Using electroabsorption and Kelvin p`robe surface potential measurements, we demonstrate that the effective ITO work function is pinned at the energy level of the highest occupied molecular orbital of CuPc. We ascribe this effect to oxygen doping from the ITO electrode. Such doping results in high-efficiency hole injection from ITO as inferred from the current–voltage characteristics. We find evidence for a time-dependent modification of the device characteristics particularly in reverse bias that we attribute to oxygen diffusion from ITO into bulk CuPc. Oxygen plasma treatment of ITO produces an oxide surface that is stable with respect to oxygen diffusion.

Cyanine dye acting both as donor and acceptor in heterojunction photovoltaic devices

The use of cyanine dyes in thin-film heterojunction photovoltaic devices is investigated. It is demonstrated that a carbocyanine dye acts as donor in conjunction with buckminsterfullerene (C60). Due to its high electron affinity, the cyanine can also act as acceptor when using copoly(dicyano-phenylvinylene-triphenylamine) as donor. H and J aggregates of the cyanine dye play a determining role both in the photocurrent spectrum and in the open circuit voltage. Open-circuit voltages ranging from 0.25 to 1.28 V are obtained for devices using the cyanine as donor and acceptor, respectively. When the cyanine layer is sandwiched between the polymer donor and the C60 acceptor, incident photon to current conversion efficiencies greater than 10% are observed. The possibility of using cyanine dyes at the same time as donors and acceptors is a promising strategy to improve conversion efficiency.

Protonated metal-oxide electrodes for organic light emitting diodes

Abstract Protonation of the indium tin oxide (ITO) electrode surface by nitric acid vapor gives rise to an ionic double layer producing an important increase in the effective electrode workfunction. This shift of ∼0.8 eV can be determined from the photoresponse of single layer diodes consisting of tris (8-hydroxyquinoline) aluminium (Alq) sandwiched between protonated ITO and aluminium electrodes. Due to the chemical treatment, the contact between the protonated hole injecting oxide electrode and Alq becomes ohmic. Under an applied electric field, the efficient injection of holes produces a positive space charge in the organic layer which enhances the emission of electrons from the cathode.

H-aggregation and correlated absorption and emission of a merocyanine dye in solution, at the surface and in the solid state. A link between crystal structure and photophysical properties

Abstract The formation of H-aggregates as a function of solution, substrate and ambient variables is considered for the merocyanine dye 3-acetyl-5-12-(3-ethyl-2-benzothiazolydene) rhodanine. Colloidal semiconductor particles are shown to be a powerful tool to control the size of the aggregates. In water the blue shifted absorption band has been assigned to a dimer. Its spectrum has been isolated and the thermodynamical variables derived for the dissociation reaction are: Δ r G 0 =21.16 kJ/mol, Δ r H 0 =32.36 kJ/mol and Δ r S 0 =37.24 J/mol K. Exciton band absorption maxima for aggregates in solution and at the water-TiO 2 and water-Al 2 O 3 interface, respectively, have been correlated to the aggregation geometry using the extended dipole model in conjunction with crystallographic data. Microcrystals showing a hypsochromical shift in the absorption band have been produced within the pores of nanocrystalline semiconductor films. The calculation shows that these aggregates are needle shaped and are composed of about 2250 monomer units. A broad emission band appears when the organic molecules assemble in a head to tail stacking geometry which could be attributed to excimer fluorescence. It is not quenched by charge injection into TiO 2 and indicates the existence of dislocations within the merocyanine stacks.

Transparent Organic Photodetector using a Near-Infrared Absorbing Cyanine Dye

Organic photodetectors are interesting for low cost, large area optical sensing applications. Combining organic semiconductors with discrete absorption bands outside the visible wavelength range with transparent and conductive electrodes allows for the fabrication of visibly transparent photodetectors. Visibly transparent photodetectors can have far reaching impact in a number of areas including smart displays, window-integrated electronic circuits and sensors. Here, we demonstrate a near-infrared sensitive, visibly transparent organic photodetector with a very high average visible transmittance of 68.9%. The transmitted light of the photodetector under solar irradiation exhibits excellent transparency colour perception and rendering capabilities. At a wavelength of 850u2005nm and at −1u2005V bias, the photoconversion efficiency is 17% and the specific detectivity is 1012 Jones. Large area photodetectors with an area of 1.6u2005cm2 are demonstrated.

Highly stretchable dielectric elastomer composites containing high volume fractions of silver nanoparticles

The dielectric permittivity (e′) of a polymeric material can be significantly increased when blended with conductive fillers at concentrations approaching the percolation threshold. However, reproducible synthesis of such composites is after decades of research still a major challenge and a bottleneck for their application. Difficulties arise in controlling the size and shape of the filler as well as in its homogenous distribution within the composite. These parameters strongly affect the dielectric as well as mechanical properties of the composite. While a substantial amount of literature deals with the influence of conductive fillers on the dielectric properties of composites, little is known about their mechanical properties. It is therefore still an important goal to synthesize materials with simultaneously high e′ and good mechanical properties. Here, we report the synthesis of dielectric elastomers that combine key properties such as high flexibility and stretchability, high thermal stability, increased e′, low dielectric loss and conductivity. Such materials were prepared by solution processing using quasi-spherical silver nanoparticles (AgNPs) of a defined size in a polydimethylsiloxane matrix (Mw = 692 kDa). To prevent percolation, the AgNPs were coated with a thin silica shell (<4 nm). To increase their compatibility with the silicone matrix, these core–shell nanoparticles were passivated with a silane reagent. The insulating silica shell around the particles precisely defines the minimum approach distance between the cores as twice the shell thickness. The dielectric properties of the passivated particles (filler) were measured in pellets and found to have an almost frequency independent value of e′ = 90 and a very low loss factor tanu2006δ = 0.023 at high frequencies. When such particles were used as fillers in a polydimethylsiloxane matrix, composites with low dielectric losses were obtained. A composite containing a 31 vol% filler with e′ = 21 and tanu2006δ = 0.03 at ∼1 kHz was achieved. At a AgNP volume fraction of 20%, the composite has e′ = 5.9 at ∼1 kHz, a dielectric strength of 13.4 V μm−1, an elastic modulus as low as 350 kPa at 100% strain, and a strain at break of 800%. Due to the high specific energy density per volume at low electric fields, these composites are attractive materials in applications involving low electric fields.

Dissociation of Charge Transfer States and Carrier Separation in Bilayer Organic Solar Cells: A Time-Resolved Electroabsorption Spectroscopy Study

Ultrafast optical probing of the electric field by means of Stark effect in planar heterojunction cyanine dye/fullerene organic solar cells enables one to directly monitor the dynamics of free electron formation during the dissociation of interfacial charge transfer (CT) states. Motions of electrons and holes is scrutinized separately by selectively probing the Stark shift dynamics at selected wavelengths. It is shown that only charge pairs with an effective electron-hole separation distance of less than 4 nm are created during the dissociation of Frenkel excitons. Dissociation of the coulombically bound charge pairs is identified as the major rate-limiting step for charge carriers generation. Interfacial CT states split into free charges on the time-scale of tens to hundreds of picoseconds, mainly by electron escape from the Coulomb potential over a barrier that is lowered by the electric field. The motion of holes in the small molecule donor material during the charge separation time is found to be insignificant.

1,3-Diphenyl-5-(9-phenanthryl)-4,5-dihydro-1H-pyrazole (DPPhP): structure, properties, and application in organic light-emitting diodes

A novel hole-transport material, 1,3-diphenyl-5-(9-phenanthryl)-4,5-dihydro-1H-pyrazole (DPPhP), was synthesized and fully characterized. The crystal structure of DPPhP was determined by X-ray diffraction analyses. DSC and AFM analysis demonstrate that DPPhP has a high Tg of 96 °C and good film forming ability. The hole-transport performance of DPPhP was examined by fabricating a multilayer device with structure of ITO/DPPhP (60 nm)/AlQ (60 nm)/LiF (0.8 nm)/Al, using DPPhP as the hole-transport layer along with an emitting-material, tris-(8-hydroxyquinolino)aluminium (AlQ). Both the brightness and efficiency of the device are about 30% higher than those of the device using N,N′-di-1-naphthenyl-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (α-NPD) as the hole-transport layer.