•The addition of PMMA enhances the stability of P3HT:PCBM photovoltaic devices.•PMMA changes the film morphology and forms pillars.•PMMA absorbs water in the active layer.•It proposed that the PMMA slows the rate of deep trap formation.We report on the lifetime of unencapsulated organic photovoltaic diodes (OPVs) based on a ternary blend of poly(3-hexylthiophene) (P3HT), phenyl-C61-butyric acid methyl ester (PCBM) and a soft insulating polymer, poly(methyl methacrylate) (PMMA) as compared to reference binary P3HT:PCBM OPVs. The performance of ternary devices was shown to decay more slowly than that of their binary counterparts to an extent that depends on the relative humidity (RH). The power conversion efficiency of ternary OPVs when stored in a low humidity environment (1% RH) decayed to 80% of their initial value after 200 h, almost double that of the reference binary OPVs. AFM measurements suggest that the PMMA forms pillars within the P3HT:PCBM matrix. It is proposed that the PMMA absorbs water in the active layer, and in doing so, slows the rate of deep trap formation that would otherwise lead to enhanced Shockley-Read-Hall recombination.

We report on the optimization of the plasma treatment conditions for a solution-processed silicon dioxide gate insulator for application in zinc oxide thin film transistors (TFTs). The SiO2 layer was formed by spin coating a perhydropolysilazane (PHPS) precursor. This thin film was subsequently thermally annealed, followed by exposure to an oxygen plasma, to form an insulating (leakage current density of ∼10−7 A/cm2) SiO2 layer. Optimized ZnO TFTs (40 W plasma treatment of the gate insulator for 10 s) possessed a carrier mobility of 3.2 cm2/(V s), an on/off ratio of ∼107, a threshold voltage of −1.3 V, and a subthreshold swing of 0.2 V/decade. In addition, long-term exposure (150 min) of the pre-annealed PHPS to the oxygen plasma enabled the maximum processing temperature to be reduced from 180 to 150 °C. The resulting ZnO TFT exhibited a carrier mobility of 1.3 cm2/(V s) and on/off ratio of ∼107.Keywords: low temperature; oxygen plasma; solution process; solution-processed silicon dioxide; zinc oxide; zinc oxide field-effect transistor

We report on the low-temperature formation (180 °C) of a SiO2 dielectric layer from solution-processed perhydropolysilazane. A bottom-gate zinc oxide thin-film transistor has subsequently been fabricated that possesses a carrier mobility of 3 cm2 V s−1, an on/off ratio of 107 and minimal hysteresis in its transfer and output characteristics.

A synthetically versatile strategy has been employed for luminescence colour tuning in a new series of bipolar carbazole–2,5-diaryl-1,3,4-oxadiazole hybrid molecules 1–7. Their syntheses, solution absorption and emission properties and cyclic voltammetric data are reported. Calculations using DFT (density functional theory) establish that they possess molecular orbitals which favour bipolar charge-transport. Single-active-layer organic light emitting devices (OLEDs) have been fabricated by thermal evaporation using the bipolar compounds as the emitters in the architecture ITO:PEDOT-PSS:X:Ca/Al (X = 1–7). The structure–property relationships within the series of compounds are assessed with emphasis on the OLED performance and emission colour. The HOMO–LUMO gap has been varied by systematic modifications of the molecular subunits of 1–7, allowing the colour of the electroluminescence to be tuned from deep blue (CIE x,y 0.157, 0.079) through to green (CIE x,y 0.151, 0.096). These materials are very attractive for further development due to the combination of good processability of the molecules, their bipolar structure, colour tunability and efficient performance of OLEDs using a simple device architecture.

An ion-sensitive organic field-effect transistor (ISOFET) has been fabricated by using poly(3-hexylthiophene) as the semiconductor and polymethylmethacrylate as the gate insulator. With the gate metallization replaced by an Ag/AgCl reference electrode, and following suitable encapsulation, the ISOFETs exhibit transistor behavior in an aqueous environment. Moreover, the devices are shown to respond to changes in pH of the solution. A significant increase in the sensitivity of the ISOFET to H+ ions was found by depositing a Langmuir–Blodgett film of arachidic acid on top of the gate insulator.

Thin films containing single-wall carbon nanotubes (SWCNTs) have been prepared using the Langmuir–Blodgett (LB) technique. Atomic force microscopy has been used to investigate the morphology of these films. Films of pure SWCNTs were in the form of a network of interconnected bundles of tubes. The in-plane electrical characteristics of the films were measured at room temperature. Approximately ohmic electrical conductivity was observed at low applied voltages for LB films of pure SWCNTs. High power dissipation was found to lead to a reorganisation of the nanotubes in these ultra-thin films. In contrast, multilayer architectures built up from mixtures of SWCNTs and cadmium arachidate exhibited a current I versus voltage V dependence of the form ln(I/V) ∝ V1/2, indicative of Poole–Frenkel conductivity.

A blue organic light-emitting device, based on an iridium phosphorescent dopant in a polyvinylcarbazole host, has been modified by the addition of an external CaS:Eu inorganic phosphor layer. By incorporating a surfactant in the phosphor mixture, a uniform coating could be achieved by drop-casting. The resulting hybrid device exhibited white light emission, with Commission Internationale de l’Eclairage, CIE (x, y) coordinates of x = 0.32, y = 0.35. No significant change in these coordinates was observed for current densities in the range 25–510 A m−2. The maximum power efficiencies of the white device was 2.3 lm W−1 at a brightness of 254 cd m−2.

This work describes bipolar 2,5-diaryl-1,3,4-oxadiazole–fluorene hybrids which incorporate triphenylamine or carbazole units within the π-electron system, viz. compounds 7, 8, 14 and 16. A related bipolar bis(oxadiazolyl)pyridine system 20 is reported. The syntheses of these five new materials are discussed, along with their optoelectronic absorption and emission properties, and their solution electrochemical redox properties. Anodic electropolymerisation of 20 was observed. Calculations using DFT (density functional theory) establish that they all possess a significantly higher HOMO energy level (by 0.60–1.02 eV) than 1,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazol-5-yl]benzene (OXD-7) due to the presence of electron-rich amine moieties and increased conjugation lengths, thereby leading to more balanced charge-transport characteristics. Devices were fabricated by spin-coating techniques using the bipolar compounds as the emitters in the simple device architecture ITO:PEDOT-PSS:X:Ca/Al (X = 7, 8, 14, 16 or 20). The turn-on voltages were 2.9, 5.5, 3.6, 4.5 and 3.4 V for the devices incorporating 7, 8, 14, 16 and 20, respectively. The highest external quantum efficiency (EQE) was observed for compound 7: viz. EQE 0.36%; current efficiency 1.00 cd A−1; power efficiency 0.56 lm W−1 at 5.7 V. The EQE of the device fabricated from 8 was considerably lower than for devices using other materials due to low light emission. The EL emission peaked at λmax 430, 487, 487 and 521 nm for 8, 14 and 16, and 7, respectively. For the 20 device λmax = 521 nm and 564 nm. Thus the HOMO–LUMO gap has been modified, allowing the colour of the emitted light to vary from light blue through to green by the systematic chemical modification of the molecular subunits. The high chemical and thermal durability of these materials combined with the simplicity of the device structure and low turn-on voltages offers considerable potential for OLED applications.

This work focuses on the first 2,5-diaryl-1,3,4-oxadiazole–fluorene hybrids which incorporate pyridine units within the π-electron system, viz. 2,7-bis{5-[2-(4-dodecyloxyphenyl)-1,3,4-oxadiazol-5-yl]-2-pyridyl}-9,9-dihexylfluorene (6) and 2,7-bis{5-[2-(4-dodecyloxyphenyl)-1,3,4-oxadiazol-5-yl]-2-pyridyl}spirobifluorene (7). The thiophene analogue 2,7-bis{5-[5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]-thien-2-yl}-9,9-dihexylfluorene 11 was also synthesised and its X-ray crystal structure was obtained. There is a progressive red shift in the UV–Vis absorption and photoluminescence spectra on replacing benzene (8) with pyridine (6) and thiophene (11) consistent with increased planarity of the π-system and reduced HOMO–LUMO gap along the series. Calculations at the DFT (density functional theory) level establish that inclusion of the pyridyl rings in 6 and 7 considerably enhances the electron affinity of the system, compared to phenyl analogues. Single-layer organic light-emitting diodes (OLEDs) have been fabricated by spin-coating blends of poly[2-(2-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene] (MEH–PPV) as the emissive material with added electron transport compounds 6 or 7 to enhance electron injection. The external quantum efficiencies of the devices were greatly enhanced compared to pure MEH–PPV reference devices. ITO/PEDOT ∶ PSS/MEH–PPV : 7 (30 ∶ 70% by weight)/Al devices exhibited an external quantum efficiency (EQE) of 0.5% and a luminous efficiency of 0.93 cd A−1 at 9.5 V and a luminance of 100 cd m−2. The modest increase in efficiency for the same device when Al was replaced by a Ca/Al cathode (EQE 0.6% and 1.2 cd A−1 at 10.5 V) suggests that the two methods of enhancing electron injection into the MEH–PPV emitter are mutually exclusive. Utilising blended layers is an attractive alternative to using Ca electrodes, which are highly reactive and are unstable in the atmosphere.

We describe the synthesis of 2,5-diaryl-1,3,4-oxadiazole–fluorene hybrid molecules, e.g. 2,7-bis[2-(4-tert-butylphenyl-1,3,4-oxadiazol-5-yl]-9,9-dihexylfluorene 6, 2,7-bis{4-[2-(4-tert-butylphenyl)-1,3,4-oxadiazol-5-yl]phenyl}-9,9-dihexylfluorene 10, 2,7-bis{4-[2-(4-dodecyloxyphenyl)-1,3,4-oxadiazol-5-yl]phenyl}-9,9-dihexylfluorene 11, 2,7-bis{4-[2-(4-dodecyloxyphenyl)-1,3,4-oxadiazol-5-yl]phenyl}-spirobifluorene 13 and analogue 16, comprising the 9,9-dihexylfluorene or spirobifluorene core units to which are attached aryl- or diaryl-oxadiazole units to provide linearly extended π-conjugated systems. The X-ray crystal structure is reported for compound 11. We have fabricated single-layer organic light-emitting diodes (OLEDs) using blends of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) as the emissive material with the electron transport (ET) compounds 6, 10, 11, 13 and 16 added to enhance electron injection. For all the devices studied electroluminescence originates exclusively from the MEH-PPV material. The external quantum efficiencies of the devices increased with increasing concentration of the ET compound up to 95% by weight, and are greatly enhanced (>two orders of magnitude) compared to pure MEH-PPV reference devices. Further improvements have been achieved by adding a layer of PEDOT : PSS and efficiencies reach ca. 0.4% at 30 mA cm−2 for devices in the configuration ITO/PEDOT : PSS/MEH-PPV–13 (5 : 95% by weight)/Al.

A novel method to detect added water to full fat milk has been developed using single frequency electrical conductance measurements. The characteristics at 100 kHz and 8 °C for all skimmed milk samples revealed a linear decrease in conductance with increasing water content over the entire range of water concentrations. In contrast, the conductance of full fat milk showed a decrease only at added water concentrations higher than 10%. At lower added water concentrations, the full fat milk exhibited an anomalous conductivity maximum at 2–3% added water.

We report on the use of polyelectrolyte molecular films, assembled on a gold substrate by the layer-by-layer (LbL) electrostatic deposition technique, to monitor small quantities of positively and negatively charged ions in solution. The detection system is based on surface plasmon resonance (SPR), the optical configuration consisting of a light emitting diode as a photon source and a CCD camera as the detector. Reflectivity changes in real time have been used to follow the adsorption steps during the deposition of the multilayer film. The sensing behaviour of the organic film is shown to depend of the architecture of molecular assembly. Concentrations of copper ions of less than one part per million could be detected by the compact SPR equipment.

The method of electrical admittance spectroscopy has been used to study the water and fat content of milk. Over the frequency range 5 Hz to 1 MHz, the electrical circuit was dominated by a single time constant. To eliminate the effect of electrode polarisation, the conductance of the milk was measured at high frequencies where it showed a saturation value. The characteristics at 100 kHz and 8 °C for all milk samples revealed a linear decrease in conductance with increasing water content. Admittance data for full fat, semi-skimmed and skimmed milk showed an increase in milk conductance with decreasing fat content.

The operating lifetimes of light-emitting diode structures incorporating poly(2-methoxy-5-(2′-ethylhexyloxy)-p-phenylenevinylene) (MEH-PPV) Langmuir–Blodgett films are reported. To remove the moisture from the organic layer, a number of post-deposition treatments have been investigated prior to the deposition of the metal top contact. The best external quantum efficiency was found for an indium–tin oxide/MEH-PPV/aluminium structure dried in high vacuum. However, this device possessed a relatively short lifetime. Experiments at constant current and constant voltage revealed that annealing at an elevated temperature could enhance the lifetime. Further improvements were found for devices in which a lithium fluoride layer (≈2 nm) was sandwiched between the aluminium electrode and the polymer layer, and by encapsulating the device with adhesive tape.

The application of electrical impedance spectroscopy to gas sensors and light emitting devices (LEDs) based on Langmuir–Blodgett (LB) films is reviewed. The sensing material was a co-ordination polymer formed by reaction of the bifunctional amphiphilic ligand 5,5′-methylenebis (N-hexadecylsalicylideneamine)) (MBSH) and copper ions in an interfacial reaction at the water surface. Changes of the device capacitance and conductance during exposure to ethanol, acetonitrile and benzene were related to the polarity of the organic vapour. Impedance measurements on LEDs incorporating a substituted polypyridine derivative, poly(6-hexyl-2,5-pyridinediyl), provided an understanding of the equivalent circuit of the device structure.