Martin R. Willis

Organic electroluminescent devices : enhanced carrier injection using an organosilane self assembled monolayer (SAM) derivatized ITO electrode

A protocol for the reproducible silylation of indium–tin oxide coated glass (ITO) using small molecule chlorosilanes is reported, and shown to be a convenient means of dramatically improving the performance of the ITO anode invariable used in organic electroluminescent devices. Using the model system: ITO|TPD|Alq3|Al (where TPD is N,N′-bis(3-methylphenyl)-N,N′diphenyl-1,1′biphenyl-4,4′-diamine and Alq3 is tris(quinolin-8-olato)aluminium) luminance-voltage, current–voltage, quantum efficiency and luminance efficiency data illustrate the superior performance of the silane modified device over a reference. Static contact angle measurements confirming the successful silylation of the ITO electrode surface, are supported by direct measurement of the effect the dipolar monolayer has on the work function of the underlying ITO, using a scanning Kelvin probe. This research builds on our earlier work with dipolar phosphonic acids. However, unlike phosphonic acids, chlorosilanes are known to adhere to oxide surfaces via a covalent bond, and so the silylated ITO electrodes are expected to exhibit improved durability. Such electrode modification provides a method of tuning the work function of the ITO electrode to the HOMO of the hole-transporting layer and thus, improving device performance.

Electroluminescence: enhanced injection using ITO electrodes coated with a self assembled monolayer

Abstract In a typical organic bilayer electroluminescent device the hole injecting electrode is almost invariably ITO glass, but a number of electron injecting metal electrodes are possible. Unfortunately the low work function materials used readily oxidise and restrict the lifetime of the device. It is known that appropriate monolayers can change the work function of a solid and also that phosphonic acids can form self assembled monolayers on ITO glass. Using an ITO glass electrode coated with a self assembled monolayer of an electron accepting phosphonic acid (2-chloroethanephosphonic acid) and aluminium as the electron injecting electrode, it was found that the threshold voltage was significantly reduced to the same value as achieved with the less stable Mg:Ag electrode. The use of such modified ITO electrodes would obviate the use of highly reactive metal electrodes and help to overcome one of the factors which limit device lifetime.

NO2 detection at room temperature with copper phthalocyanine thin film devices

In this work, we report the effect of post-deposition film treatment on the NO2 sensing properties of CuPc thin films for room temperature operation. The gas-sensitive response of the electrical conductivity to doping with NO2, doping with oxygen (in air) and cooling to 77 K in liquid nitrogen are reported. The pretreatment with NO2 is shown to improve the gas sensing properties by providing both an increase in the magnitude of the conductivity change for a given NO2 concentration and a significant improvement in the recovery time. Data is analysed using an Elovich model, which suggests that the cooled devices have the best fit to this model; the data for the NO2 doped devices suggest a Langmuir behaviour. For all devices, a simple time derivative of the change in current provides a measure of concentration for real time gas sensing applications.

The use of charge transfer interlayers to control hole injection in molecular organic light emitting diodes

In order to improve the performance of the indium–tin oxide (ITO) electrode frequently used as the anode in electroluminescent devices, we report its modification using ultrathin films of C60 and 11,11,12,12-tetracyanonaphtho-2,6-quinodimethane (TNAP). In both cases the interaction between the film and the ITO substrate is found to shift the work function of the electrode, thereby modifying the barrier to hole injection in the model system ITO∣TPD∣Alq3∣Al (where TPD is N,N′-bis(3-methylphenyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine and Alq3 is tris(quinolin-8-olato) aluminium). Scanning Kelvin probe measurements show that the ITO work function is increased by as much as 0.25 eV with a TNAP overlayer, whilst C60 overlayers are found to reduce the work function by a comparable amount. The former has been attributed to a charge transfer effect, however, for C60 overlayers the variation in the electrostatic potential across the interface cannot be attributed to charge transfer alone. The performance of devices incorporating these modified ITO electrodes are rationalised in terms of the work function modification, film thicknesses and the hole transport properties of the two films.

A robust ultrathin, transparent gold electrode tailored for hole injection into organic light-emitting diodes

A viable alternative to the problematic indium tin oxide (ITO) glass substrates invariably employed as the anode in organic light-emitting diodes is reported; namely ultrathin gold electrodes supported on glass. The key element to the successful fabrication of these electrodes is the pre-treatment of the glass substrate with the adhesion promoter 3-mercaptopropyl(methyl)dimethoxysilane prior to gold thermal deposition. The resulting films are exceptionally robust and exhibit high transparency over the visible spectrum combined with very low sheet resistance. Unlike ITO glass, these electrodes are chemically well defined and are fabricated at room temperature with no post-deposition annealing.

Enhanced hole injection in organic light-emitting diodes using a SAM-derivatised ultra-thin gold anode supported on ITO glass

In order to increase the performance of organic light-emitting diodes (OLEDs) we report the modification of the indium-tin oxide (ITO) anode invariably used in OLEDs, with a dipolar self-assembled monolayer-derivatised (4-nitrophenylthiolate) ultra-thin gold overlayer. This composite approach allows the work function of the anode to be tuned to the hole-transporting band of the adjacent semiconductor, while facilitating good mechanical adhesion at this interface. When this modification is incorporated into the model OLED system ITO/TPD/Alq3/C6H5CO2Li/Al [where TPD is N,N′-bis(3-methylphenyl)-N,N-diphenyl-1,1-biphenyl-4,4′-diamine, Alq3 is tris(quinolin-8-olato)aluminium and C6H5CO2Li is lithium benzoate], the power efficiency is dramatically enhanced. Furthermore, these devices exhibit a maximum external quantum efficiency of more than 5 cd A−1 and a peak luminance of ∼36,000 cd m−2. In combination with the current–voltage–luminance (LIV) characterisation of these devices, scanning Kelvin probe, polarisation modulation reflection absorption infrared spectroscopy and time of flight secondary ion mass spectroscopic techniques have been employed to probe the ITO–Au–SAM interface. This research builds on our earlier work with dipolar organosilanes, phosphonic acids and charge-transfer films at the ITO–organic interface.

Deposition of ordered phthalocyanine films by spin coating

Thin films of octaalkyl-substituted phthalocyanines were prepared by spin coating. The films were remarkably smooth and showed optical absorption spectra with Davydov splitting, characteristic of crystalline order, similar to those of structurally related compounds prepared by the Langmuir–Blodgett technique. The crystallinity was confirmed by X-ray diffraction. The technique may have advantages for device fabrication.

Fabrication and performances of a double-layer organic electroluminescent device using a novel starburst molecule, 1,3,5-tris[N-(4-diphenylaminophenyl)phenylamino]benzene, as a hole-transport material and tris(8-quinolinolato)aluminum as an emitting material

A double-layer organic electroluminescent (EL) device was fabricated using a novel starburst molecule, 1,3,5-tris[N-(4-diphenylaminophenyl)phenylamino]benzene (p-DPA-TDAB), as a hole transport material and tris(8-quinolinolato) aluminum (Alq/sub 3/) as an emitting material, and its performance characteristics were investigated. It was found that p-DPA-TDAB, which forms a stable amorphous glass with a glass-transition temperature of 108/spl deg/C, functions as a good hole-transport material and that the EL device is thermally stable, operating at a temperature of 120/spl deg/C.

Nickel phthalocyanine photovoltaic devices

Metal substituted phthalocyanines are a class of organic dye material that are weakly semiconducting. Many of these materials incorporating different metals have been investigated for molecular switching, gas sensing and photovoltaic properties. In this work we report the use of nickel phthalocyanine (NiPc) as the semiconductor in Schottky barrier photovoltaic devices. Sandwich structures of aluminium-NiPc-silver have been fabricated by vacuum deposition and investigated for both dark conductivity and photovoltaic response using a solar simulator. Previous workers have shown that this type of device is highly sensitive to different gasses, consequently a deposition process was used that did not break the vacuum from phthalocyanine deposition to characterization. This process clearly shows that the presence of oxygen is essential to observe a photovoltaic response (i.e. short circuit current and open circuit voltage) in this type of device. The effect of incorporating tetracyanoquinodimethane (TCNQ), a strong electron acceptor, in the nickel phthalocyanine structure to enhance the conductivity is also reported.

The effect of NO2 doping on the gas sensing properties of copper phthalocyanine thin film devices

In this work we report the effect on the NO2 gas sensing properties of initially doping CuPc thin films with oxygen (in air) and NO2 for room temperature operation. The pre-treatment with NO2 is shown to improve the gas sensing properties by providing both an increase in the magnitude of the conductivity change for a given NO2 concentration and a significant improvement in the recovery time. Data presented suggest that a simple time derivative of the change in current may provide a measure of concentration for real time gas sensing applications.