Professor Hazel Assender
Department of Materials
Tel: +44 1865 273781 (Room 179.30.06)
Tel: +44 1865 273700 (switchboard)
Fax: +44 1865 273789 (general fax)
Solar Energy Materials Initiative
Research on a range of polymer and polymer composite materials. Particular areas of interest include polymer surfaces and interfaces, nanocomposites, thin film deposition onto polymers including vacuum roll-to-roll deposition, thin film flexible electronics and photovoltaic materials.
Vacuum web processing
Dr. H.E. Assender, Dr. J. Topping
Development of coating techniques and materials using our unique roll-to-roll vacuum web processing capability. The coater can run a 35cm polymer web at speeds of up to 5m/s to allow the deposition of multiple layers from the following sources: i) evaporation, ii) dual magnetron sputter, iii) plasma iv) flash evaporation of organic materials with e-beam cure. Films can be produced for applications such as controlled optical properties and surface finish, high and low energy surfaces, barrier layers, biocompatibilisation or electronic devices.
Roll-to-roll deposition of flexible transparent barrier layers
V. Tobin, Dr. H.E. Assender, Dr S. Read*
Use of a roll-to-roll web coater to deposit, under vacuum, gas barrier layers onto polypropylene substrates combining vacuum deposited organic and inorganic materials. The study seeks to increase the flexibility of the barrier layer whil maintaining high barrier properties. *In collaboration with Innovia Films. Funded by EPSRC with support from Innovia Films.
Roll-to-roll processing of organic electronics
Dr G.A.W. Abbas, Z. Ding, Dr. H.E. Assender
Electronics components that can be manufactured using roll-to-roll processing offer the possibility of lower cost devices as well as those that might be mechanically flexible in use. Roll-to-roll (R2R) processing, using a flexible substrate (typically a polymer film) allows for cheap production of many components very rapidly, with low energy requirements. Key areas of exploitation of this technology include flexible displays, but there is also a wealth of lower-cost applications. Tagging and tracking of fast moving consumer goods is an example technology that truly exploit the very low-cost nature of the production and in which the manufacturing is closely linked to the manufacturing routes currently exploited for e.g. packaging technologies. This project seeks to exploit the existing industrialised technology of vacuum R2R processing, widely used for example in the packaging industry, to develop the manufacture of very low cost organic field-effect transistor (OFET)-based devices and circuits. This manufacturing route, like solvent based systems, is cheap and provides flexible product, and we can exploit high electrical mobility molecular semiconductors. Additional advantages of the solvent-free vacuum processes include: a) likely enhanced web-speed, b) integration with vacuum-based metal deposition for conducting channels, and metal or ceramic deposition for barrier layers and possible interfacial modification, and c) the ability to deposit multiple thin layers to build up device structures without solvent interactions with underlying layers. The project will exploit our existing R2R web processing facility to explore the principal manufacturing challenges to R2R vacuum production of OFET devices: 1) selection and adaptation of materials to vacuum deposition integrated with design of suitable circuitry, 2) patterning of the semiconductor and insulator layers to allow the formation of circuit connections between devices and 3) reliability of manufacture to be able to produce arrays of multiple transistors for circuits. It will allow us to explore and develop the deposition of molecular semiconductor and dielectric materials and then the subsequent reliability and thermo-mechanical resilience of the resulting product such that it might need to withstand, for example, during a lamination process.
Polymer insulator layers in organic transistor devices
Z. Ding, Dr. G.A.W. Abbas, Dr. H.E. Assender, Dr. G. Creech*
The project is investigating the use of novel acyrlic-based polymer materials for use as the gate-insulator layer in organic thin film transistor devices. The insulator is deposited using flash evaporation followed by curing of the polymer, for example using an electon beam. This process can be carried out in our roll-to-roll vacuum web coating facility. Tbe project will incestigate a range of monomer materials suitable for this process, as well as the proces conditions, and then investigate the impact of these parameters on the materials properties of the resulting layer, and in particular its performance in transistor devices. In collaboration with Scott Bader. Funded by IeMRC.
Vacuum deposition of polymer photovoltaic devices
N. Klein, Dr. A.A.R. Watt, Dr. H.E. Assender
Conjugated polymers have demonstrated enhanced properties in terms of light absorption and hole-transport, and in combination with fullerene electron-acceptors the highest power conversion efficiency organic solar cells. However, the use of solvents substantially limits the complexity of the devices as the coating solutions interfere with already deposited layers. Vacuum deposition is a solvent-free process, advantageous for its simplicity and ability to evaporate unlimited number of layers with well controlled thickness and composition. Although some polymer materials have been deposited by physical vapour deposition techniques, there have not been any attempts to deposit conjugated polymers in the same way. This project will involves the comparison of evaporated polymer-based photovoltaic devices with those deposited by solution casting, and development of the vacuum deposition processesElectroabsoprtion of nanocomposite photovoltaic materials
Crystallization and phase separation in thin film polymers
L. Jiang, Dr. H.E. Assender
The properties of polymer materials in thin film and close to surface and interfaces can in difffer from those in the bulk. We have previously shown, along with others, that some polymers (e.g. PET, PEN and PVA) can form thin film or surface crystals with characteristic morphology at a lower temperature than the bulk crystallisation temperature, as a result of a lower glass transistion temperature in the near-surface region. We are investigating the range of polymer materials, and their characteristics, in which this phenomenon can be observed. Phase separation processes can also be modified in the thin-film/near surface region for the same reason, and we will investigate a number of technologically important phase separating systems.
Enhancing the efficiency of thin film solar cells using optical confinement
M. Wincott, A. Powell, Dr. H.E. Assender, Dr. A.A.R. Watt, Dr. J.M. Smith
Thin film solar cells offer an inexpensive means to generate clean energy, but current efficiencies are limited to around five percent, about three times lower than commercial polycrystalline silicon cells. One of the main reasons behind the low efficiency is that a tension exists between the desire to absorb as much as possible of the incident light, in which case the optical path length should be thick (at least several hundred nanometres), and the desire to extract the photogenerated charge carriers efficiently from the cell, in which case the exciton transport path length should be short (no more than a few tens of nanometers). Most attempts to solve this problem involve using a thick cell, and focusing the advanced aspects of cell design on building in some means for ensuring a short transport path length. Here we take the opposite viewpoint; that the optical path can be elongated for a given cell geometry by the use of wave guiding and cavitation, thereby reducing the burden placed on the transport related features of the device. This new project involves the design, fabrication, and testing of devices that explore this theme by employing inexpensive approaches to encourage light to propagate in the plane of the film.
Melanin-based thin films
I. Yeom, Dr A.A.R. Watt, Dr H.E.Assender
Morphological, optical and electronic characterisation of thin films based on a dispersion of melanin. This material shows intriguing optical and electronic properties that may be of interest. e.g. for optoelectronic devices, however to dat, it has not been investigated extesnsisvely in thin-film form. We are developing techniques to create thin films containing well-dispered melanin and their subsequent characterisation.
Novel high energy density high reliability capacitors
Dr. A. Mahadevegowda, Dr. C. Johnston, Dr. H.E. Assender, Professor P.S. Grant
Current capacitor technology significantly limits the temperature capability and electrical performance of power electronics relative to the "More Electric Airframe" systems requirements, which are emerging rapidly as a key priority for both aeroengine and airframe manufacturers. Novel capacitor materials combining high dielectric ceramics and high performance polymers are being developed for aero-engine applications, particularly within the more electric aircraft concept. Investigations include characterisation of the fundamental material properties using advanced analytical instruments, clean room characterisation of the electrical properties, development of fabrication routes, and modelling of behaviour for lifetime prediction. (Funded by Technology Strategy Board, Labinal Power, ICW Ltd)
9 public active projects
Abbas, G., Ding, Z., Mallik, K., Assender, H., Taylor, M. (2013) IEEE ELECTRON DEVICE LETTERS, 34(2), 268-270. doi: 11.1109/LED.2012.2234434.
Shinotsuka, K., Bliznyuk, V.N., Assender, H.E. (2012) Near-surface crystallization of PET. POLYMER, 53, 5554-5559. doi: 10.1016/j.polymer.2012.09.048
Willis, S.M., Cheng, C., Assender, H.E., Watt, A.A.R., (2012). Defect mediated extraction in InAs/GaAs quantum dot solar cells. SOLAR ENERGY MATERIALS AND SOLAR CELLS, 102, 142-147. doi: 10.1016/j.solmat.2012.03.010
Moghal, J., Suttle, H., Cook, A. G., Grovenor, C. R. M., & Assender, H. E. (2012, March 15). Investigation of the mechanical properties of aluminium oxide thin films on polymer substrates by a combination of fragmentation and scratch testing. SURFACE & COATINGS TECHNOLOGY, 206, 3309. doi:10.1016/j.surfcoat.2012.01.040
Kovacik, P., Willis, S. M., Matichak, J. D., Assender, H. E., & Watt, A. A. R. (2012). Effect of side groups on the vacuum thermal evaporation of polythiophenes for organic electronics. ORGANIC ELECTRONICS: PHYSICS, MATERIALS, APPLICATIONS, 13(4), 687-696. doi: 10.1016/j.orgel.2012.01.005
Droessler, L. M., Assender, H. E., & Watt, A. A. R. (2012). Thermally deposited lead oxides for thin film photovoltaics. MATERIALS LETTERS, 71, 51-53.
Willis, S. M., Cheng, C., Assender, H. E., & Watt, A. A. (2012, March 14). The transitional heterojunction behavior of PbS/ZnO colloidal quantum dot solar cells. NANO LETT, 12(3), 1522-1526. doi:10.1021/nl204323j
Abbas, G.A.W., Assender, H., Ibrahim, M. Taylor, D.M. (2011) Organic thin-film transistors with e-beam cured and flash vacuum deposited polymeric gate dielectric. J. VAC. SCI & TECH B, 29(5) 52401. doi: 10.1116/1.3628635
Kovacik, P., Sforazzini, G., Cook, A. G., Willis, S. M., Grant, P. S., Assender, H. E., Watt, A. A. (2011, January). Vacuum-deposited planar heterojunction polymer solar cells.. ACS Appl Mater Interfaces, 3(1), 11-15. doi:10.1021/am1008985
Barkhouse, D. A. R., Bishop, H. E., Henry, B. M., Webster, G. R., Burn, P. L., & Assender, H. E. (2010, April). Improving efficiency of MEH-PPV/TiO2 solar cells by lithium salt modification. ORG ELECTRON, 11(4), 649-657. doi:10.1016/j.orgel.2010.01.005
Beal, R. M., Stavrinadis, A., Warner, J. H., Smith, J. M., Assender, H. E., & Watt, A. A. R. (2010, March 9). The Molecular Structure of Polymer-Fullerene Composite Solar Cells and Its Influence on Device Performance. MACROMOLECULES, 43(5), 2343-2348. doi:10.1021/ma902211u
Cattley, C., Stavrinadis, A., Beal, R., Moghal, J., Cook, A., Grant, P., Smith, J., Assender, H., Watt, A. (2010). Colloidal synthesis of lead oxide nanocrystals for photovoltaics. CHEM COMM, 46, 2802-2804. doi: 10.1039/b926176
Lancaster, T., Pratt, F. L., Blundell, S. J., McKenzie, I., & Assender, H. E. (2009, August 26). Muon-fluorine entanglement in fluoropolymers. J PHYS-CONDENS MAT, 21(34), . doi:10.1088/0953-8984/21/34/346004
Stavrinadis, A., Beal, R., Smith, J. M., Assender, H. E., & Watt, A. A. R. (2008, August 18). Direct formation of PbS nanorods in a conjugated polymer. ADV MATER, 20(16), 3105-3109. doi:10.1002/adma.200702115
Howells, D. G., Henry, B. M., Leterrier, Y., Manson, J. A. E., Madocks, J., & Assender, H. E. (2008, April 25). Mechanical properties of SiOx gas barrier coatings on polyester films. SURF COAT TECH, 202(15), 3529-3537. doi:10.1016/j.surfcoat.2007.12.030
Howells, D. G., Henry, B. M., Madocks, J., & Assender, H. E. (2008, March 31). High quality plasma enhanced chemical vapour deposited silicon oxide gas barrier coatings on polyester films. THIN SOLID FILMS, 516(10), 3081-3088. doi:10.1016/j.tsf.2007.11.017
Lochab, B., Burn, P. L., Barkhouse, A., Kirov, K. R., Assender, H. E., Keeble, D. J., . . . Samuel, I. D. W. (2007, December). Electronically asymmetric poly(1,4-phenylenevinylene)s for photovoltaic cells. ORG ELECTRON, 8(6), 801-812. doi:10.1016/j.orgel.2007.07.003
Martin, C. M., Burlakov, V. M., Assender, H. E., & Barkhouse, D. A. R. (2007, November 15). A numerical model for explaining the role of the interface morphology in composite solar cells. J APPL PHYS, 102(10), . doi:10.1063/1.2817603
Xie, Z. B., Henry, B. M., Kirov, K. R., Barkhouse, D. A. R., Burlakov, V. M., Smith, H. E., . . . Tsukahara, Y. (2007, April 11). Correlation between photoconductivity in nanocrystalline titania and short circuit current transients in MEH-PPV/titania solar cells. NANOTECHNOLOGY, 18(14), . doi:10.1088/0957-4484/18/14/145708
Xie, Z., Henry, B. M., Kirov, K. R., Smith, H. E., Barkhouse, A., Grovenor, C. R. M., . . . Tsukahara, Y. (2006, July 26). Study of the effect of changing the microstructure of titania layers on composite solar cell performance. In THIN SOLID FILMS Vol. 511 (pp. 523-528).
Martin, C. M., Burlakov, V. M., & Assender, H. E. (2006, May 5). Modeling charge transport in composite solar cells. SOL ENERG MAT SOL C, 90(7-8), 900-915. doi:10.1016/j.solmat.2005.05.009
Xie, Z., Burlakov, V. M., Henry, B. M., Kirov, K. R., Smith, H. E., Grovenor, C. R. M., . . . Tsukahara, Y. (2006, March). Intensity-dependent relaxation of photoconductivity in nanocrystalline titania thin films. PHYS REV B, 73(11), . doi:10.1103/PhysRevB.73.113317
Barkhouse, D. A. R., Carey, M. J., Xie, Z. B., Kirov, K. R., Henry, B. M., Assender, H. E., . . . Burn, P. L. (2006). Twofold efficiency increase in nanocrystalline-TiO2/polymer photovoltaic devices by interfacial modification with a lithium salt. In Z. H. Kafafi, & P. A. Lane (Eds.), Organic Photovoltaics VII Vol. 6334 (pp. U90-U99). doi:10.1117/12.679424
Kirov, K. R., & Assender, H. E. (2005, November 1). Quantitative ATR-IR analysis of anisotropic polymer films: Surface structure of commercial PET. MACROMOLECULES, 38(22), 9258-9265. doi:10.1021/ma050495i
Pratt, F. L., Lancaster, T., Brooks, M. L., Blundell, S. J., Prokscha, T., Morenzoni, E., . . . Assender, H. E. (2005, September). Surface dynamics of a thin polystyrene film probed by low-energy muons. PHYS REV B, 72(12), . doi:10.1103/PhysRevB.72.121401
Burlakov, V. M., Kawata, K., Assender, H. E., Briggs, G. A. D., Ruseckas, A., & Samuel, I. D. W. (2005, August). Discrete hopping model of exciton transport in disordered media. PHYS REV B, 72(7), . doi:10.1103/PhysRevB.72.075206
Kawata, K., Burlakov, V. M., Carey, M. J., Assender, H. E., Briggs, G. A. D., Ruseckas, A., . . . Samuel, I. D. W. (2005, May). Description of exciton transport in a TiO2/MEH-PPV heterojunction photovoltaic material. In SOL ENERG MAT SOL C Vol. 87 (pp. 715-724).
Xie, Z. B., Burlakov, V. M., Henry, B. M., Kirov, K. R., Grovenor, C. R. M., Assender, H. E., . . . Tsukahara, Y. (2005). Time evolution of photoconductivity in TiO2 electrodes fabricated by a sol gel method. In M. Durstock, D. Friedman, R. Gaudiana, & A. Rockett (Eds.), Materials for Photovoltaics Vol. 836 (pp. 43-48).
Porfyrakis, K., & Assender, H. E. (2004, May). Mesoscale modelling of processing rubber-toughened acrylic polymers. PLAST RUBBER COMPOS, 33(5), 223-232. doi:10.1179/146580104225021027
Swaminathan, P., Disley, P. F., & Assender, H. E. (2004, May 1). Surface modification of ion exchange membrane using amines. J MEMBRANE SCI, 234(1-2), 131-137. doi:10.1016/j.memsci.2004.01.022
Kirov, K. R., & Assender, H. E. (2004, February). Quantitative ATR-IR analysis of anisotropic polymer films: extraction of optical constants. Macromolecules, 37(3), 894-904. doi:10.1021/ma030369j
Kirov, K. R., Burlakov, V. M., Carey, M. J., Henry, B. M., Xie, Z. B., Grovenor, C. R. M., . . . Briggs, G. A. D. (2004). Non-steady state operation of polymer/TiO2 photovoltaic devices. In MATER RES SOC SYMP P Vol. 822 (pp. 71-76).
Kirov, K. R., Burlakov, V. M., Xie, Z. B., Henry, B. M., Carey, M. J., Grovenor, C. R. M., . . . Briggs, G. A. D. (2004). Non-steady state operation of polymer/TiO2 photovoltaic devices. In P SOC PHOTO-OPT INS Vol. 5520 (pp. 68-75).
Assender, H. E. (2004). The joy of parenthood. Physics World, 3(17), 60.
Carey, M. J., Burlakov, V. M., Henry, B. M., Kirov, K. R., Webster, G. R., Assender, H. E., . . . Grovenor, C. R. M. (2004). Nanocomposite titanium dioxide/polymer photovoltaic cells: effects of TiO2 microstructure, time and illumination power.. In P SOC PHOTO-OPT INS Vol. 5215 (pp. 32-40).
Assender, H. (2003). Modelling the heirarchical structure of synthetic polymers. In R. A. Pethrick, & C. Viney (Eds.), Techniques for polymer organisation and morphology characterisation (1st ed.). Chicester: Wiley.
Bliznyuk, V. N., Assender, H. E., & Briggs, G A D. (2002, August). Surface glass transition temperature of amorphous polymers: a new insight with SFM. Macromolecules, 35(17), 6613-6622. doi:10.1021/ma011326a
Porfyrakis, K., Assender, H. E., & Robinson, I. M. (2002, August). The interrelationship between processing conditions, microstructure and mechanical properties for injection moulded rubber-toughened poly(methyl methacrylate) (RTPMMA) samples. POLYMER, 43(17), 4769-4781.
Assender, H. E., Bliznyuk, V. N., & Porfyrakis, K. (2002). How surface topography relates to materials properties. Science, 973-976.
Goldbeck-Wood, G., Bliznyuk, V N., Burlakov, V., Assender, H. E., Briggs, G. A. D., Tsukahara, Y., . . . Windle, A. H. (2002, June). Surface structure of amorphous polystyrene: comparison of AFM imaging and lattice chain simulations. Macromolecules, 35(13), 5283-5289. doi:10.1021/ma0119777
Porfyrakis, K., Kolosov, O. V., & Assender, H. E. (2001, December 9). AFM and UFM surface characterization of rubber-toughened poly(methyl methacrylate) samples. J APPL POLYM SCI, 82(11), 2790-2798.
Bliznyuk, V. N., Burlakov, V. M., Assender, H. E., Briggs, G. A. D., & Tsukahara, Y. (2001, March). Surface structure of amorphous PMMA from SPM: Auto-correlation function and fractal analysis. MACROMOL SYMP, 167, 89-100.
Assender, H. E. (2001). Aerospace Materials (1st ed.). B. Cantor, H. E. Assender, & P. S. Grant (Eds.), Bristol: IoPP.
Deng, C. S., Assender, H. E., Dinelli, F., Kolosov, O. V., Briggs, G. A. D., Miyamoto, T., . . . Tsukahara, Y. (2000, December 1). Nucleation and growth of gas barrier aluminium oxide on surfaces of poly(ethylene terephthalate) and polypropylene: Effects of the polymer surface properties. J POLYM SCI POL PHYS, 38(23), 3151-3162.
Assender, H. E., Bowditch, M. R., Grey, N. F. C., Harris, A. E., O'Gara, P. M., & Shaw, S. J. (2000, December). A novel system for self-validating adhesive joints. INT J ADHES ADHES, 20(6), 477-488.
Cuberes, M. T., Assender, H. E., Briggs, G. A. D., & Kolosov, O. V. (2000, October 7). Heterodyne force microscopy of PMMA/rubber nanocomposites: nanomapping of viscoelastic response at ultrasonic frequencies. J PHYS D APPL PHYS, 33(19), 2347-2355.
Bliznyuk, V. N., Burlakov, V. M., Assender, H. E., Briggs, G. A. D., & Tsukahara, Y. (2000, August 20). Auto-correlation function analysis of the surface structure of amorphous PMMA.. ABSTR PAP AM CHEM S, 220, U311.
Bliznyuk, V. N., Kirov, K., Assender, H. E., Briggs, G. A. D., & Tsukahara, Y. (2000, August 20). In situ crystalization study in PET films by elevated temperature AFM/UFM.. ABSTR PAP AM CHEM S, 220, U311.
Dinelli, F., Assender, H. E., Kirov, K., & Kolosov, O. V. (2000, May). Surface morphology and crystallinity of biaxially stretched PET films on the nanoscale. POLYMER, 41(11), 4285-4289.
Dinelli, F., Assender, H. E., Takeda, N., Briggs, G. A. D., & Kolosov, O. V. (1999, May). Elastic mapping of heterogeneous nanostructures with ultrasonic force microscopy (UFM). In SURF INTERFACE ANAL Vol. 27 (pp. 562-567).
Assender, H. E., & Windle, A. H. (1998, August). Crystallinity in poly(vinyl alcohol). 1. An X-ray diffraction study of atactic PVOH. POLYMER, 39(18), 4295-4302.
Assender, H. E., & Windle, A. H. (1998, August). Crystallinity in poly(vinyl alcohol) 2. Computer modelling of crystal structure over a range of tacticities. POLYMER, 39(18), 4303-4312.
Assender, H. E., & Windle, A. H. (1997, February). The solvation of poly(vinyl alcohol). In MACROMOLECULAR SYMPOSIA Vol. 114 (pp. 199-204).
Assender, H. E., & Windle, A. H. (1997, February). Domain structures in magnetically orientated liquid crystalline polymers. POLYMER, 38(3), 677-688.
Assender, H. E., & Windle, A. H. (1996, January). The relaxation of a magnetically orientated liquid crystalline polymer. POLYMER, 37(2), 371-375.
Windle, A. H., Assender, H. E., & Lavine, M. S. (1994, July 15). Modelling of form in thermotropic polymers. PHILOS T ROY SOC A, 348(1686), 73-96.
Assender, H. E., & Windle, A. H. (1994, June 6). 2-Dimensional Lattice Model of Disclinations in Liquid-Crystals - Choice of Energy Function. MACROMOLECULES, 27(12), 3439-3441.
Deposition of organic an inorganic layers on polymer substrates by roll-to-roll coating in vacuum
Prof H E Assender
The project will make use of our state-of-the art roll-to-roll polymer web coater to deposit under vacuum acrylate or other organic layers on polymer substrates, followed by evaporation or magnetron sputtering deposition of thin film inorganic layers such as metals or oxides. The resulting materials will then be characterized using a suite of methods. Possible applications include optical coatings, gas barrier films (often for electronics applications), or polymer electronics.
Also see homepages: Hazel Assender
Phase separation and self-ordering in thin film polymers
Prof Hazel Assender
The project will examine phase separation and self-ordering processes and morphological changes in thin film polymers, comparing the processes and kinetics in thin film systems with those in the bulk. The work will consider the effect of substrate interactions as well as processing characteristics on the resulting structures.
Also see homepages: Hazel Assender
Also see a full listing of New projects available within the Department of Materials.