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Andrew Watt

Professor Andrew Watt
Associate Professor of Materials

Department of Materials
University of Oxford
16 Parks Road
Oxford OX1 3PH
UK

Tel: +44 1865 613456 (Room 352.10.45)
Tel: +44 1865 273700 (switchboard)
Fax: +44 1865 273789 (general fax)

Solar Energy Materials Initiative

Summary of Interests

Andrew leads a group of 14 scientists on energy materials research from the synthesis of new nanomaterials through to the fabrication, characterization and application of devices. The group has considerable experience in the synthesis of nanomaterials, thin film device fabrication, optoelectronic materials characterisation and advanced transmission electron microscopy techniques. Recent highlights include, the demonstration of vacuum thermal evaporation of conducting polymers for solar cells, high resolution TEM imaging of polymer lamellae in bulk heterojunction photovoltaic devices,  and the initial demonstration of a SnS nanocrystal heterojunction thin film photovoltaic devices.

Current Research Projects

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

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.

2 public active projects

Research Publications

Visit http://scholar.google.co.uk/citations?user=X3qNsJkAAAAJ&hl=en for full publication list.

Key publications:

1. Low temperature phase selective synthesis of Cu 2 ZnSnS 4 quantum dots CA Cattley et al. Chemical Communications 49, p3745 (2013).

2. Morphology control in co-evaporated bulk heterojunction solar cells P Kovacik et al. Solar Energy Materials and Solar Cells 117, p22 (2013).

3. The transitional heterojunction behavior of PbS/ZnO colloidal quantum dot solar cells SM Willis, Nano letters 12, p1522-1526 (2012).

4. The Molecular Structure of Polymer− Fullerene Composite Solar Cells and Its Influence on Device Performance, RM Beal et al. Macromolecules 43, p2343 (2010).

5. Vacuum-deposited planar heterojunction polymer solar cells, P Kovacik et al. ACS applied materials & interfaces 3, p11 (2010).

6. SnS/PbS nanocrystal heterojunction photovoltaics A Stavrinadis. Nanotechnology 21, p185202 (2010).

7. The supramolecular structure of melanin. AAR Watt et al. Soft Matter 5, p3754 (2009).

8. Lead sulfide nanocrystal: conducting polymer solar cells, AAR Watt et al. Journal of Physics D: Applied Physics 38 , p2006 (2005).

Projects Available

Colloidal Quantum Dot Photovoltaics
Andrew Watt

The ability to tune the band gap across the solar spectrum through quantum confinement allows the possibility to create broadband multi-junction and multi-band gap solar cells. If this can be harnessed alongside multiple exciton generationand emerging energy transfer mechanisms there is a very real opportunity to break the Shockley Queisser limit. This project will address the challenge of improving device efficiency by using a number of strategies including improving energy level matching and limiting recombination losses by passivation.

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Metal Nanowires for Optoelectronics
Andrew Watt

Transparent conductors are part of life from iPod touch screens to solar cells. Current technologies utilise doped metal oxides, however the constituent materials and processing method are expensive and not applicable to a wide range of substrates (eg plastics). This project will involve the synthesis of metal nanowire alloys, thin film processing on a variety of substrates, conductivity and mobility measurements along with physicochemical characterization eg XRD, SEM, XPS, TEM.  The last stage of the project will be to apply the new materials in photovoltaic and LED devices.

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Colloidal Quantum Dot Displays and Lighting
Andrew Watt

Optoelectronic devices made from Colloidal Quantum Dots (CQD) have started to transition from the lab to consumer products, the Sony Triluminous display being a prime example. CQD have great promise in a number of applications, but there are still challenges to be met, primarily methods for continuous production in bulk and lowering the toxicity of the constituent material, and for the integration process of devices and systems. This project focuses on manufacturing process of CQD materials and next-generation smart display/lighting devices based on LEDs.

The latest CQD display technology uses back lit LCD units and Cd based CQD as the phosphor. A second generation of LED driven by electroluminescent (EL) CQD devices (CQD LED) is envisaged to replace current organic LED (OLED) for use in both displays and smart lighting. The main advantage of the CQD LED over OLED are improved reliability/stability, lower production costs, lower power consumption and improved colour purity/gamut. This project is focused on the manufacturing technologies we believe are needed to augment current, and develop additional markets for CQD based companies and also create new businesses.

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Vacuum Deposited Organic Photovoltaics
Andrew Watt

In the last 3 years there has been a surge in the power conversion efficiency of organic photovoltaic devices to over 10% . This has been brought about by the development of new materials with improved electronic structure and molecular co-doping to create graded p-i-n structures. This project aims to build on this work and start from a first device operation principles to determine what the key metrics are for a good organic photovoltaic device. From here we will assess the conjugated molecules available and build devices using a new deposition tool that will be built as part of the project. Particular attention will be paid to using materials which are stable, low cost, environmentally friendly and amenable to large volume productio

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Graphene electrodes for nanocrystal solar cells
Jamie Warner and Andrew Watt

Graphene is an ideal 2D material for utilization as a transparent conducting electrode in photovoltaics (solar cells). High efficiency photovoltaic devices will require the effective integration of other nanomaterials with graphene to produce hybrid nanosystems. Inorganic nanocrystals such as PbS, ZnSe, TiO2 and Si, have unique semiconducting properties with band gaps that span from the near-IR to UV. This project will focus on synthesizing inorganic nanocrystals using solution-phase chemistry. Control over the shape to tailor spherical, rod and branched structures will be investigated. Variation of surface state morphology will be conducted through various chemical approachs to control the inter-nanocrystal interactions. Synthetic graphene will be produced using chemical vapour deposition. Composite hybrid devices will be fabricated that use synthetic graphene as a working transparent conducting electrode and the inorganic nanocrystal as the active functional nanomaterial.

Also see homepages: Jamie Warner Andrew Watt

Also see a full listing of New projects available within the Department of Materials.