Research is carried out on the growth, processing and characterisation of thin film materials for photovoltaic devices (solar cells) and functional coatings.
Photovoltaics (PV) offers tremendous potential as an environmentally friendly and reliable form of renewable electricity generation. It is currently dominated by solar cells based on bulk crystalline Si wafers but the high specific cost of this technology (cost per watt of electricity produced) is a barrier to its widespread dissemination. Our research is aimed at investigating novel thin film routes, processes and device structures to reduce the cost of Si solar cells and improve their performance. We have strong links with industry as well as UK and overseas Universities and research institutes. We are part of the UK's Engineering and Physical Sciences Research Council (EPSRC) PV-21 SUPERGEN consortium of 9 universities and several industrial partners. This is aimed at competitive PV solar energy through novel inorganic thin film approaches.
Currently, our research is focused in the following areas:
A number of routes for the growth of TFPS solar cells on glass are being investigated. They include solid phase crystallisation (SPC) and low temperature epitaxial thickening of thin, polycrystalline seed layers formed by laser crystallisation or aluminium induced crystallisation (AIC). An example of a TFPS layer on an AIC seed layer on glass, with grains in excess of 10 mm, is shown in the figure to the right. Electron cyclotron resonance chemical vapour deposition (ECRCVD) was used to epitaxially thicken the seed layer to 1.7 µm at temperatures <600 °C.
The figure is an overlay of an electron backscatter diffraction (EBSD) orientation map in the sample normal direction and a scanning electron microscope (SEM) image. The grain structure and orientation can be seen clearly. Red, blue and green in the EBSD map correspond to (100), (111) and (101) grain orientations, respectively. Grains oriented towards (001) predominate.
We are investigating a number of ideas to increase the absorption of light in thin film silicon solar cells to increase efficiency. They include the use of plasmonic effects in metal nanoparticles to enhance light scattering into silicon films and device geometries based on silicon nanowires. Nanosphere lithography (NSL) and annealing of very thin (< 20 nm) metal layers are being used to form nanoparticle arrays on silicon films and solar cell surfaces. The impact on optical absorption and cell efficiency is being studied. The experimental work is being supported by finite difference time domain (FDTD) simulations.
The work on silicon nanowire growth is illustrated in the figure above which shows vertical nanowires on (111) silicon. The nanowires are being grown by the vapour liquid solid (VLS) mechanism using microwave plasma assisted CVD. Solar cells based on these structures are being studied.
Research is being carried out on developing novel, lower temperature plasma processes for single and multi crystalline silicon cells. The techniques include epitaxial emitter formation and defect/surface passivation (by direct ECR discharges and using ECR-deposited SiNx/SiOx layers). Efficiencies approaching 15% have been demonstrated in non-optimised device structures using processing temperatures below 500 °C.
Expertise exists in the growth, characterisation and processing of a wide range of nano- and micro-scale thin films and coatings for a variety of applications. Our comprehensive growth facilities include ECRCVD, microwave plasma CVD and RF plasma CVD, RF magnetron sputtering, e-beam evaporation and thermal evaporation. In addition to silicon, thin films and coatings studied include silicon carbide for solar cells and sensors, SiNx passivating and anti-reflecting coatings, transparent conducting oxides and microcrystalline and nanocrystalline diamond coatings for anti-wear applications.
The thin film and device fabrication facilities are housed in a class 10000 cleanroom facility with an adjoining yellow room for photolithographic processing. The facilities include those outlined below.
A wide range of state-of-the-art techniques is available for studying the structural, chemical, electrical and optical properties of thin films, materials and devices. The facilities include:
Professor Hari Reehal
Tel: 020 7815 7513
Funded with £1,420,000 by the European Commission we’re interested in developing a new type of glass-based protective coating for engineering components. We have 9 industrial partners and over the last year we have developed a sol-gel process for synthesizing precursor materials for nanostructured coatings with enhanced properties.
We’re developing polymers and a new process for thermoset coatings. The project is managed by Akzo Nobel plc and includes Philips Electronics and Network Rail. During the past year, we have developed a new formulation for thermoset coatings and carried out successful trials on bridge sections supplied by Network Rail.
Looking at flexible automated processes for PVD coatings this project is funded by the European Commission and looks at developing new materials and equipment for physical vapour deposited coatings. Many common electroplating processes damage the environment and are harmful to workers’ health. The project is aimed at developing physical vapour deposition as a replacement for electroplating.
Prof David Gawne
Tel: 020 7815 7641
Dr Yuqing Bao
Tel: 020 7815 7680