Donal Bradley

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Donal Bradley

Professor of Physics and Engineering & Head of MPLS

I joined the University of Oxford in September 2015 as Head of the Mathematical, Physical and Life Sciences Division and Professor of Physics and Engineering Science; I am also a Professorial Fellow at Jesus College.

I was elected a Fellow of the Royal Society (FRS) in 2004 and was appointed Commander of the Order of the British Empire (CBE) for services to science in 2010. I am also a Fellow of the Institute of Physics (FInstP), a Fellow of the Institution of Engineering and Technology (FIET) and a Chartered Engineer (CEng).

I studied undergraduate Physics (BSc, ARCS) at Imperial College in London and received the Royal Society for the Encouragement of Arts, Manufactures and Commerce Silver Medal (1983) as an outstanding graduate of the Royal College of Science.

My postgraduate research was undertaken at Churchill College and the Cavendish Laboratory in Cambridge where I was a member of the Physics and Chemistry of Solids Group. I received my PhD in 1987 for a thesis entitled "Spectroscopic studies of the solution processable conjugated polymers poly(p-phenylenevinylene) and poly(4,4'-diphenylenediphenylvinylene)" and was subsequently appointed Unilever Research Fellow in Chemical Physics at Corpus Christi College and, simultaneously, Toshiba Research Fellow at the Chemical Laboratory, Toshiba R&D Center, Kawasaki, Japan. I spent one year in Japan studying the nonlinear optical properties of precursor-route poly(p-phenylenevinylene), producing a patent on patterning methods for polymer optical waveguides in addition to several journal papers.

Shortly after my return to the UK I was a co-inventor, in 1989, of conjugated polymer electroluminescence with Jeremy Burroughes and Richard Friend. I was subsequently appointed to a University Assistant Lectureship in Physics with a Fellowship at Churchill College, where I also then became Director of Studies and College Tutor. I worked extensively on the development of conjugated polymer LEDs and co-founded Cambridge Display Technology (www.cdtltd.co.uk) in 1992 (with Jeremy Burroughes, Richard Friend, Andrew Holmes, Paul Burn and Arno Kraft). I also contributed significantly to the then contentious debate on the nature of the excited states in conjugated polymers, strongly supporting the molecular exciton picture that is now widely accepted. In addition, I helped to develop the use of nonlinear spectroscopy to probe conjugated polymer excited states and device structures and investigated the role of molecular conformation and film microstructure in determining electronic (and optical) properties. Industrial research collaborators included Toshiba Corporation.

In 1993 I was appointed to a Readership in Physics at the University of Sheffield and founded the Molecular Electronic Materials and Devices Group which continues as the Electronic and Photonic Molecular Materials Group under the leadership of Professor David Lidzey. I was promoted Professor of Physics in 1995 and became Co-Director then Director of the Sheffield Centre for Molecular Materials; I also served as Warden of Tapton Hall. My MEMD group was one of the first to study in detail the polyfluorene family of polymers for light emitting diode and laser applications, working closely with the Dow Chemical Company (Midland, Michigan, USA). We identified and named the β-phase of poly(9,9-dioctylfluorene (PFO), first showed the thermotropic liquid crystalline ordering of these materials and established their potential for highly linearly-polarised light emitting diodes. We also demonstrated strong coupling in organic semiconductor micro-cavities for the first time and showed giant Rabi splitting at room temperature. Studies were additionally undertaken on cavities containing thermotropically aligned PFO films whose birefringence enabled exceptional polarisation of light emission. More fundamental studies addressed the nature of charge carrier injection and transport in polymer films and diode structures. We further undertook early studies of optical gain and lasing and probed some of the limitations to electrical pumping. More general exciton physics, including resonant energy transfer, exciton dissociation and defect-related quenching was also studied. Additional industrial research collaborators included Sharp, Merck, Avecia and Zeneca.

In 2000, after seven years in Sheffield, I was asked to return to Imperial College to lead a strategic initiative in Solid State Physics to establish a programme on organic semiconductors. This culminated in the 2009 founding of the Centre for Plastic Electronics that within five years, involved some 25 academic research groups in the Departments of Physics, Chemistry, Materials and Chemical Engineering, comprising in total some 100 PhD students and 50 PDRAs and visitors and a research portfolio in excess of £35M. It is also home to the EPSRC Centre for Doctoral Training in Plastic Electronic Materials and Devices (Directed by Dr Paul Stavrinou), an activity that is jointly run with the University of Oxford and Queen Mary, University of London. I was the Founding Director (2009-15) and supported the growth of a major programme across the breadth of materials design and synthesis, materials processing and characterisation, device fabrication and optimisation and theoretical modelling; five members of the CPE have become ISI Highly Cited Scientists. I also served as Head of the Experimental Solid State Physics Group (2003-05), as Head of the Department of Physics (2005-08), as Deputy Principal of the Faculty of Natural Sciences (2008-2011) and as Vice-Rector then Vice-Provost for Research (2011-2015).

My research at Imperial College encompassed projects on (i) materials design and characterisation for light emission, charge carrier transport and photocharge generation functions using fluorene, thiophene, phenylenevinylene, benzothiadiazole, arylamine, acene and other conjugated functional units; (ii) photophysical studies of optical and optoelectronic behaviours; (iii) Development of electrode and interlayer materials, including indium tin oxide replacements especially for flexible substrates and more stable systems for cathodes (replacing reactive metals). These include carbon (fullerene, nanotube, graphene, graphene oxide), metal-oxide (zirconia, zinc oxide, molybdenum oxide, etc), pseudohalide (CuSCN), conducting polymer (PEDOT:PSS based and VPP-PEDOT systems), nanowire (Ag and Cu), polyelectrolytes, self-assembled dipolar monolayers, etc; (iv) High efficiency solution processed OLEDs including polymer/small molecule and fluorescent/phosphorescent/thermally activated delayed fluorescence systems; (v) Detailed charge injection and transport studies using time of flight photocurrent, dark injection transient, SCLC diode and transistor measurements; (vi) Optical probes of local environment (structural and electrical) using Raman, ellipsometry, electroabsorption and photoluminescence measurements; (vii) Photonics spanning optical gain, amplification, switching, wavelength conversion and lasing, using 1-D, 2-D and mixed 1-D/2-D distributed feedback structures, waveguides and vertical microcavities with Bragg and/or metal reflectors. Gain media studied have been primarily polymers and/or molecular glasses both single component and blend. Also involving the development of novel conformational molecular metamaterials (using β-phase formation in dialkylfluorene based polymers), plasmonic processes, strong/ultra-strong coupling and associated polariton physics; (viii) Bulk heterojunction (fullerene/organic or organic/organic) and hybrid (metal oxide/organic) solar cells (including inverted structures) and the role of microstructure in generating efficient charge generation and collection. Solar cell stability provided a less-generally-studied component of these studies; (ix) microfluidic and lateral flow lab-on-a-chip diagnostic structures featuring organic semiconductor photodetector and/or light source instrumentation. This work led to the founding (with John and Andrew de Mello) of Molecular Vision Ltd, now part of the Abingdon Health Group (www.abingdonhealth.com). Photodetector structures were also studied as X-ray detection elements for imaging (digital X-ray) applications; (x) Hybrid nitride/organic structures using organic emission materials for down conversion and colour control. Both radiative and non-radiative coupling between inorganic Mott-Wannier and organic Frenkel excitons was studied; (xi) Development of novel deposition and patterning methods for high-throughput device fabrication including: breath figure formation, interlayer lithography, gravure printing, stamp transfer printing, spray coating, zone casting and dip pen nanolithography; (xii) Electronics including organic and metal-oxide devices and circuits, also photo- and light-emitting-transistors and high frequency (GHz) diodes. Industrial research collaborators involved in this work included Philips, Merck, Sharp, Dow, Dupont, Matsushita, LG Philips, Toshiba, Sumitomo Chemical Company, BP Solar, Cambridge Display Technology, Molecular Vision, Solar Press and Unilever.

Finally, I am a keen supporter of Ireland Rugby and of Everton FC.

Graduate level courses on: Display Technologies, Molecular Photophysics, Nonlinear Optics

I am a Trustee of the Rank Prize Funds (http://www.rankprize.org/) and Chair of the Optoelectronics Fund Committee. The Rank Prize Funds organisation seeks to recognise excellence in specific fields of research and reward innovators for their dedication and outstanding contribution. The Funds have as their objectives the advancement and promotion for the public benefit of knowledge, education and learning in nutrition and optoelectronics recognising the dual interests (flour milling and cinema) of the Rank family businesses at the time of the Funds' establishment. This is done through the award of prizes, the organisation of symposia and the provision of student bursaries.

My research concerns the science and application of novel semiconductor materials and devices and contributes to the associated development of a new technology platform, widely known as Plastic Electronics. This platform embodies a paradigm shift towards low temperature, solution-based device fabrication, with great potential for use in, for example, energy efficient displays and lighting, photovoltaic energy generation, large-area electronics, medical diagnostics and polymer waveguide/plastic fibre based datacomms. The discovery of conjugated polymer electroluminescence for poly(p-phenylenevinylene) in 1989 proved to be a key event in the development of the Plastic Electronics field, with the 1990 Nature paper on this topic acting as the trigger point for an explosion of interest in solution processed semiconductors. This paper now has more than 11,300 Google Scholar citations and is featured in both the Nature Physics and Chemistry looking-back collections: www.nature.com/physics/looking-back/burroughes/index.html and http://www.nature.com/chemistry/looking_back/index.html.

I have published more than 600 research articles that have been cited more than 60,000 times in total and I have an h-index of 105 (Google Scholar). I am an ISI Highly Cited researcher, placing in the 1% most cited authors globally for publications in materials science over the past decade. My research has been further recognized by Fellowship of the Royal Society (FRS, 2004), Institute of Physics (FInstP, 2005) and Institution of Engineering and Technology (FIET, 2013) and by award of the European Union Descartes Prize (2003), the Society for Information Display Jan Rachman Prize (2005), the European Science Foundation European Latsis Prize (2005), the IOP Faraday Medal (2009), the IET Faraday Medal (2010), the Royal Society Bakerian Medal (2010) and the IOP Polymer Physics Group Founder’s Prize (2013). I was appointed a Commander of the Order of the British Empire (CBE) in 2010 for services to science and received an honorary DSc from the University of Sheffield in 2014. I am a chartered engineer (CEng) and also a Fellow of the Royal Society for the Encouragement of Arts, Manufactures and Commerce (FRSA).

My long-standing research interests include the critical control of conjugated polymer electronic/optoelectronic/photonic properties via chemical and physical structure and consequent approaches to device optimization. Understanding the similarities and differences between molecular and more traditional inorganic semiconductors has been a major theme of my work. Within this theme, the influence of localized wavefunctions, of the anisotropy in intra- and inter-molecular interactions and of conformational and other disorder has been a particular focus. This focus is very much motivated by the soft solids nature of molecular electronic materials (strong intra-molecular covalent bonding versus weaker, typically van der Waals, inter-molecular bonding) that makes their properties very sensitive to environment and state of order. The resulting understanding has made a substantial contribution to describing the semiconductor physics of conjugated polymers and to developing the foundations of materials design and processing that have allowed the field to emerge as an important branch of solid-state science.

Nanometer dimensions provide a natural length scale on which molecular properties can be engineered: Kuhn segment lengths for conjugated polymers are of order ≈ 10-30 nm, exciton diffusion lengths ≈ 5-50 nm, Förster transfer radii ≈ 1-10 nm, 1/e absorption depths ≈ 20-200 nm, and so on. These values, together with limitations on carrier transport (hopping within/between nanometric domains), then lead to typical layer thicknesses in LED and photovoltaic device structures of ≈ 10-100 nm. In turn this leads to important optical interference and confinement effects in the near UV, through visible, to near IR spectral range of interest: The nano-scale structure provides the necessary modulation of properties on an optical path length scale to tune light propagation and produce resonance conditions that can adjust the light matter interaction. For instance, Raman scattering, stimulated emission and strong coupling processes are exquisitely sensitive to both the electronic/spectral features of the molecule(s) (controlled by state of order, environment and external stimulus) and the nature of the resonance structure (shape, dimensions, Q-value). In this context my work also constitutes a commercially promising component of the “bottom-up” approach to nano-science/-technology.

Another important aspect of my work has been to learn how best to exert control over the required electronic properties through chemical and physical structure, especially via molecular orientation, a process that exposes the intrinsic anisotropy of many polymer properties, and chain conformation, as exemplified by the β-phase in poly(9,9-dioctylfluorene). State of order (glass/crystal/liquid crystal; mono-/poly-domain) has also been used to good effect in establishing and understanding trade-offs between processing protocols and ultimate properties, especially for blends. Technique development for in-situ probing of optical, morphological and electronic properties in device structures is another strong interest. Finally, hybrid structures that combine organic and inorganic semiconductors as layers or blends and solution-processed metal-oxides are both of increasing current interest.

Beyond academic research, I have been closely involved in the establishment of IPR related to the outcomes of my research, being a co-inventor for > 25 patent families. The 1990 fundamental patent on conjugated polymer electroluminescence (Friend, Burroughes and Bradley US5247190, referenced in > 2000 subsequent patents) led to the founding of Cambridge Display Technology Ltd (Founded 1992, NASDAQ IPO 1999, acquired by Sumitomo Chemical Company 2007: www.cdtltd.co.uk), a company that currently employs > 170 people. A more recent patent on polymer detection systems for microanalysis led to the founding of Molecular Vision Ltd, a start up company that is now part of the Abingdon Health Group (www.abingdonhealth.com). I am a member of the Advisory Board of Abingdon Health and also a non-executive director of the Solar Press (UK) Ltd a start up company funded by the Carbon Trust to accelerate the development of organic photovoltaic technology. I have also acted as a technical consultant for a wide range of organisations in the UK, USA, Europe, Japan, Korea and China.