Simon J. Martin

Electrical transport modelling in organic electroluminescent devices

This paper was originally accepted as a Topical Review but published, in error, as a regular article.

Optical limiting properties of a zinc porphyrin polymer and its dimer and monomer model compounds

We report on the strong decrease in laser light transmission with increasing excitation intensity observed in thin films of a zinc porphyrin polymer and its dimer and monomer model compounds. Nonlinear transmission measurements performed at 532 nm using 500 ps laser pulses show decreases of 40%, 60% and 65% at input fluences of 0.01 J/cm2 for the monomer, dimer and polymer thin films, respectively. Its application as an optical limiting device for the protection of the eyes and optical sensors from high-fluence laser pulses is assessed. Additional pump–probe measurements performed at 595 nm using 0.8 ps excitation allow us to discuss the strong response in terms of a six-level model of the electronic structure.

Hydrothermal carbonisation of sewage sludge: effect of process conditions on product characteristics and methane production.

Hydrothermal carbonisation of primary sewage sludge was carried out using a batch reactor. The effect of temperature and reaction time on the characteristics of solid (hydrochar), liquid and gas products, and the conditions leading to optimal hydrochar characteristics were investigated. The amount of carbon retained in hydrochars decreased as temperature and time increased with carbon retentions of 64-77% at 140 and 160°C, and 50-62% at 180 and 200°C. Increasing temperature and treatment time increased the energy content of the hydrochar from 17 to 19 MJ/kg but reduced its energy yield from 88% to 68%. Maillard reaction products were identified in the liquid fractions following carbonisations at 180 and 200°C. Theoretical estimates of the methane yields resulting from the anaerobic digestion of the liquid by-products are also presented and optimal reaction conditions to maximise these identified.

Surface segregation and self-stratification in blends of spin-cast polyfluorene derivatives

We have used helium-3 nuclear reaction analysis to study the morphology of spin-cast blends of poly(9,-dioctylfluorene) (F8) and poly(9,-dioctylfluorene-alt-benzothiadiazole) as a function of composition, casting solvent, and initial solvent concentration. In blends cast from toluene, a surface segregated layer is observed for a broad range of F8 compositions. The surface excess has a maximum of ?nm at an F8 volume fraction = 0.5, dropping down to ?2?nm for = 0.2 and?0.8. The existence of such surface segregated layers could play an important role in charge transport in optoelectronic devices made from blends of conjugated polymers. Films cast from chloroform show negligible surface segregation. Scanning probe microscopy experiments show that films cast from toluene exhibit significant in-plane structure, in contrast to those cast from chloroform, which show minimal lateral structure.

The internal electric field distribution in bilayer organic light emitting diodes

Abstract The internal electric field distribution in a bilayer 4,4′-bis[N-(1-napthyl)-N-phenylamino]-biphenyl/tris-(8-hydroxyquinoline) aluminium organic light emitting diode has been investigated experimentally using electroabsorption spectroscopy. The experimental results have been compared to those obtained from a drift–diffusion device simulation, further validating the model and highlighting the potential worth of such modelling. With the aid of the simulation, the electric field distribution can be explained in terms of charge carrier accumulation at the interface between the two organic layers, due to the HOMO and LUMO band offsets, and charge injection into the device, demonstrating the influence of contact materials on device behaviour.

Electroabsorption studies of phthalocyanine/perylene solar cells

We report electroabsorption (EA) studies of electric fields in bilayer molecular organic solar cells made from zinc phthalocyanine (ZnPc) and a methyl substituted perylene pigment (MPP). We have detected an electric field at the metal/organic interface which is sensitive to the external DC bias. The interface field has a different spectral signature from that of the bulk of the two layers, which we attribute to interface species such as charge transfer-induced dipoles. The electric field is proportional to the applied bias in devices containing only ZnPc or MPP, but rectifying behavior is observed in the bilayer solar cell.

Enhanced performance of pulse driven small area polyfluorene light emitting diodes

We have investigated the performance of conjugated polymer light emitting diodes (LEDs) as their active areas are reduced to dimensions typical for pixellated displays. We find that by reducing the active diameter of the device from our standard size (1.5×3 mm) to 50 μm, the current density that can be sustained before LED failure increases by a factor of 9. This increase in current density is mirrored by similar increases in maximum LED brightness. We propose that such effects occur due to improved thermal management in small area devices. Using pulsed driving schemes, devices with a diameter of 50 μm sustained current densities of 5.1 MA m−2, and have emitted electroluminescence with a brightness of 6.5 Mcd m−2.

In situ imaging and height reconstruction of phase separation processes in polymer blends during spin coating

Spin coating polymer blend thin films provides a method to produce multiphase functional layers of high uniformity covering large surface areas. Applications for such layers include photovoltaics and light-emitting diodes where performance relies upon the nanoscale phase separation morphology of the spun film. Furthermore, at micrometer scales, phase separation provides a route to produce self-organized structures for templating applications. Understanding the factors that determine the final phase-separated morphology in these systems is consequently an important goal. However, it has to date proved problematic to fully test theoretical models for phase separation during spin coating, due to the high spin speeds, which has limited the spatial resolution of experimental data obtained during the coating process. Without this fundamental understanding, production of optimized micro- and nanoscale structures is hampered. Here, we have employed synchronized stroboscopic illumination together with the high light gathering sensitivity of an electron-multiplying charge-coupled device camera to optically observe structure evolution in such blends during spin coating. Furthermore the use of monochromatic illumination has allowed interference reconstruction of three-dimensional topographies of the spin-coated film as it dries and phase separates with nanometer precision. We have used this new method to directly observe the phase separation process during spinning for a polymer blend (PS-PI) for the first time, providing new insights into the spin-coating process and opening up a route to understand and control phase separation structures.

Direct visualization of the real time swelling and collapse of a poly(methacrylic acid) brush using atomic force microscopy

The reversible height change dynamics of a poly(methacrylic acid) brush, a weak polyelectrolyte in response to pH changes, were observed using atomic force microscopy. The brush thickness could be repeatably and reversibly switched between 40 nm at pH 3 and 120 nm at pH 10.5. The swelling and collapse transitions took 6 s in the AFM measurements, but high resolution force spectroscopy measurements showed that the collapse can be made to happen in less than a second. We conclude that the response time of these switches is limited in practise by the physical time taken to exchange solvents and by the intrinsic slow dynamics in the brush.

THEORETICAL INVESTIGATION OF THE LOW-LYING ELECTRONIC STRUCTURE OF POLY(P-PHENYLENE VINYLENE)

The two-state molecular orbital model of the one-dimensional phenyl-based semiconductors is applied to poly(para-phenylene vinylene). The energies of the low-lying excited states are calculated using the density matrix renormalization group method. Calculations of both the exciton size and the charge gap show that there are both