Search

Electrodes with low work functions are required to efficiently inject electrons into semiconductor devices. However, when the work function drops below about 4 electronvolts, the electrode suffers oxidation in air, which prevents its fabrication in ambient conditions. Here we show that multivalent anions such as oxalate, carbonate and sulfite can act as powerful latent electron donors when dispersed as small ion clusters in a matrix, while retaining their ability to be processed in solution in ambient conditions. The anions in these clusters can even n-dope the semiconductor core of π-conjugated polyelectrolytes that have low electron affinities, through a ground-state doping mechanism that is further amplified by a hole-sensitized or photosensitized mechanism in the device. A theoretical analysis of donor levels of these anions reveals that they are favourably upshifted from ionic lattices by a decrease in the Coulomb stabilization of small ion clusters, and by irreversibility effects. We attain an ultralow effective work function of 2.4 electronvolts with the polyfluorene core. We realize high-performance, solution-processed, white-light-emitting diodes and organic solar cells using polymer electron injection layers with these universal anion donors, demonstrating a general approach to chemically designed and ambient-processed Ohmic electron contacts for semiconductor devices. Multivalent anions are found to be capable of electron-doping polymer semiconductors to realize conductive films with very low work functions, which enable efficient electron injection into materials with low electron affinity.

Micro-cavity architectures can be applied to printed P-OLED devices to improve the efficiency and colour that LEP materials can deliver in a display. As a result we have demonstrated not only high efficiency and required display colours, but also good T95 performance on a scalable platform.

We report on the development of hybrid P-OLEDs for display applications. Hybrid P-OLEDs combine polymeric hole side and emitter layers with small molecule layers as organic electron side. Experimental data for blue hybrid P-OLEDs is presented and compared to standard P-OLEDs with inorganic cathode side.

Solution processable materials are important for the fabrication of low cost displays and lighting. In this paper we present the development of state of the art white Polymer-LED materials with a long lifetime, high colour rendering index and good colour stability over operating lifetime.

Specular neutron reflectivity and nuclear reaction analysis (NRA) are used to measure the interfacial width between poly(9,9-dioctylfluorene) (F8) and poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) in a 500 nm bilayer. Annealing at temperatures up to 280 °C allows us to vary this width over a large range, from ∼1 nm to greater than 30 nm. Approximate calculations using the predictions of self-consistent field theory (SCFT) for both Gaussian and semiflexible chains suggest that in the liquid−liquid regime the Gaussian results of SCFT are valid for this system. We also demonstrate the ability to control the interfacial width in 100 nm bilayers, on both silicon and indium tin oxide (ITO). The procedure of “preannealing” the F8 and F8BT layers, before bringing them together to make a bilayer, allows us to independently control the interfacial width and the properties of the bulk of these 100 nm films.

Hexaazatriisothianaphthenes substituted with six alkylsulfanyl chains (propyl and dodecylsulfanyl) have been synthesised and their thermotropic, photophysical and oxidation–reduction properties characterised. Their synthesis has been motivated by the results of quantum-chemical calculations that point to efficient transport properties for these new electron-deficient mesogens since electron transport is predicted to be only slightly affected by rotational degrees of freedom in the discotic mesophase.

Although much progress has been made in improving polymer light-emitting diode performance, there has been little work to address device intrinsic degradation mechanisms due to the challenge of tracking minute chemical reactions in the 100-nm-thick buried active layers during operation. Here we have elucidated a hole-mediated electrical degradation of triarylamine-based blue polymer diodes using in situ Raman microspectroscopy. A slow irreversible hole-doping of polymer adjacent to the hole-injecting conducting-polymer leads to formation of oxidised triarylamine species counterbalanced by anions from the conducting-polymer. These charged species act as luminescence quenchers and hinder further hole injection across the interface leading to significant decreases in current density at low voltages.

We discuss some recent findings relating to the structure of interfaces in semiconducting polymer devices. The structure of three different types of interface is characterized via neutron reflectivity and scanning force microscopy. In the first example we find that enrichment of dopant occurs at the surface of a doped polymeric conductor, and that this enriched layer penetrates several nanometres into the material. Secondly, we find that the interface between a semiconducting polymer and an insulating polymer is not molecularly sharp, but is rather broad with a width typically of the order of 1 nm. Finally we present some initial neutron reflectivity measurements on the interface between two different semiconducting polymers (one of which is deuterated). Again we find that this interface is diffuse, with a typical width of the order of a few nanometres.

P-OLED materials development is usually reported using a spin-coated platform. This paper details the work done to realise considerable performance in spin-coated and printed devices. The challenges faced in achieving this result were: • Eliminating current leakage pathways that reduce device efficiency • Eliminating sources of contamination that reduce lifetime.

In order to analyze the alginate components of alginate dressings and the fractions which are released when the dressing is in contact with model biological fluids, the use of various analytical methods was considered. The first step was the conversion of a calcium alginate batch to pure sodium alginate. The recovery of the latter from either insoluble or soluble mixed sodium/calcium alginates was performed by complexation of calcium ions with sodium citrate followed by ultrafiltration. Comparisons were made between sugar analysis, 1H NMR and circular dichroism (CD) data to determinate the contents in guluronic and mannuronic acids of sodium alginate chains. It was shown that CD measurements afford a rapid and nondestructive method for determination of %G when one takes the ratio theta200/theta220 into account. Fractionation of crude alginate (generally ranging from 30 to 70% G) was achieved by the triangle dissolution/precipitation method in order to increase the range of alginate in sugar composition. The various validated procedures were applied to investigate the effects of irradiation sterilization on alginate dressings. It was shown that sugar composition is retained whereas molecular weight decreased dramatically due to chain scission.

Energy level alignment in polyfluorene-based polymer light-emitting diodes (PLEDs) is studied with electroabsorption spectroscopy and cyclic voltammetry (CV). Using an active layer containing a blend of three polyfluorenes, it is found that the built-in potential values are determined by the interplay of the contacts work function and the lowest and highest lying energy levels of the blend components.

When a blue polymer OLED device is driven under DC constant current the electroluminescence decays over time. Over the last two years there have been large advances in the time taken for the luminescence to fall to 50% of its original value. This is in part due to an improved recognition and understanding of the degradation mechanisms involved in the luminescence decay. Measurements made from devices during lifetest have identified a number of characteristic degradation-related changes to the device and to the light-emitting polymer. In this paper we will compare these degradation signatures together with the material properties between light-emitting polymers which have been designed to give a long DC-driven lifetime and a class of polymers which have been specifically designed to give a long lifetime under pulsed driving schemes.

In an operating LED, electrons are injected into the polymer's LUMO level from the cathode and holes from the anode into the polymer's HOMO level. The value of the electron affinity (EA) is usually inferred (often incorrectly) from the experimental HOMO level energy and the value of the optical band gap. Using carefully controlled experimental conditions we can now directly access the EA position. Reduction events have been observed on poly(9,9-dioctylfluorene) (F8) and blue emitting polymers based on 9,9-dioctylfluorene. This event is consistent and reversible. As predicted by theory, the LUMO level is entirely delocalised over F8 blocks. An EA of approx. –2.3eV, equal to that of F8, is therefore a common feature of blue polymers containing sequences of 9,9-dioctylfluorene. Blue emitting 9,9-dioctylfluorene -triarylamine AB copolymers containing either 4-sec-butylphenyl diphenyl amine (TFB) or N,N'-bis(4-butylphenyl)-N,N'-diphenyl phenylenediamine (PFB) are characterized by an electron affinity of –2.1eV. This result shows that the LUMO wave function in these systems is localized on the Ph-F8-Ph units. In the case of PFB homopolymer the LUMO level is again localized, this time on the biphenyl unit at the junction of each 2 repeating units. The EA for this material is therefore lower than in the previous cases: -1.84eV, as a result of the increased localization of the LUMO wave function Electrochemical characterisation provides a direct probe for the dynamic of the charge injection in a polymer system, as the HOMO and LUMO are the states into which the holes and electrons are injected. Electrochemical measurement together with theoretical modelling gives a more complete understanding of molecular properties and behaviour. However, electrochemical measurements are not necessarily relevant to the optical energy gap, as the optical excitation does not necessarily involve molecular orbitals probed by electrochemical methods.

Blends of conjugated polymers are frequently used as the active semiconducting layer in light-emitting diodes and photovoltaic devices. Here we report the use of scanning near-field optical microscopy, scanning force microscopy and nuclear-reaction analysis to study the structure of a thin film of a phase-separated blend of two conjugated polymers prepared by spin-casting. We show that in addition to the well-known micrometre-scale phase-separated morphology of the blend, one of the polymers preferentially wets the surface and forms a 10-nm-thick, partially crystallized wetting layer. Using near-field microscopy we identify unexpected changes in the fluorescence emission from the blend that occurs in a 300-nm-wide band located at the interface between the different phase-separated domains. Our measurements provide an insight into the complex structure of phase-separated conjugated-polymer thin films. Characterizing and controlling the properties of the interfaces in such films will be critical in the further development of efficient optoelectronic devices.

Pseudomonas oleovorans has been previously shown to produce a polyester containing a phenyl pendant group when grown with 5-phenylpentanoic acid under nutrient-limiting conditions. The same polyester was produced when 5-phenyl-2,4-pentadienoic acid was the only carbon source, and a mixture of two different polymers was produced when this bacterium was grown on a mixture of 5-phenyl-2,4-pentadienoic acid and nonanoic acid. The polymer blend obtained was separated by fractional crystallization to yield poly(3-hydroxy-5-phenylpentanoate) and the copolymer which is normally produced with nonanoic acid alone.

We report that adding a thin (∼10nm) semiconducting polymer interlayer between apoly(styrenesulphonate)-doped poly(3,4-ethylenedioxythiophene) (PEDT:PSS) hole transporter and an emissive semiconductor significantly improves the device efficiency of polymer light-emitting diodes (LEDs). With the interlayer, the external quantum efficiency (EQE) increases from 0.7%(0.4cd∕A at 3.7V) to 1.9% (1.0cd∕A at 3.3V) at 100cd∕m2 for red LEDs and from 1.9%(6.2cd∕A at 3.4V) to 3.0% (10.1cd∕A at 3.0V) at 1000cd∕m2 for green LEDs. An EQE of 4.0% is also observed in blue LEDs (35% increase). The interlayer is spin-coated directly on top of the PEDT:PSS layer from a poly(2,7-(9,9-di-n-octylfluorene)-alt-(1,4-phenylene-((4-sec-butylphenyl)imino)-1,4-phenylene)) (TFB) solution. This interlayer prevents significant quenching of radiative excitons at the PEDT:PSS interface by acting as an efficient exciton blocking layer.