Research Centre UK
Photovoltaics is a large activity within the group, our overall aim is to improve the efficiency (or decrease the cost) of photovoltaics. By the application of new nanotechnology concepts. we are particularly interested in anti-reflection and light-trapping technique based around "optical biomimetic surfaces" (such as moth-eyes) the use of "plasmonic nanostructures" to enhance absorption or to couple light into thin film solar cells. We are also exploring semi-conductor growth, including thin film silicon depostion (in conjunction with Plasmaquest Ltd) and the fabrication of self-organised nanodevcies.
Antireflection
The surface of the eyes of specific kinds of night-flying moths are covered in sub-wavelength pillars which act as an antireflective coating. By mimicking these "moth-eye" structures, high quality antireflective surfaces can be made for solar cells .
Most of the moth-eye structures that have been fabricated so far are expensive, and based on electron beam lithography. Nevertheless, these structures have allowed a detailed optical investigation of a large number of patterns and symetries. We are now trying to find the very best nanostructured black for all wavelengths and all angles of incidence. We are also exploring cheaper depostion techniques particulary Nanoimprint lithography , and nanosphere lithography (where tiny spheres are deposited on a surface then etched to form pillars).
These "moth-eye" surfaces, are important for many applications, the same techniques applied to transparent materials to reduce reflection, (we have been exploring the transparent wings of some species of moth). Reduced reflection from windows, will also be useful for the window used in solar cell modules, as well as displays. The technique can also be form the basis of anti-glare and stealth technologies.
Plasmonics
Another idea which could improve the efficiency of solar cells is to exploit the phenomenon of plasmonics, where the free electrons at the surface of a metal are displaced when an incident electric field is present, i.e. light is incident on the metal. The most common use of plasmonics is in the form of localised surface plasmon resonances which occur in particles a few nanometers in diameter. At a certain frequency, a peak in the reflection spectrum exists, which is dependent on the size and shape of the nanoparticle. A local enhancement in electric field is also observed, which allows light to be transferred into waveguiding modes into the substrate.
Nanoparticles of gold or silver can be fabricated either by e-beam lithography and a lift-off technique, evaporation of a deposited thin film, or deposition of a colloidal solution. All three techniques are being researched in the group.
Thin Film Silicon
Thin film silicon is of interest in the field of photovoltaics to form thin film solar cells due to their lower fabrication cost and larger area. However, thin film silicon solar cells are less efficient than a standard silicon solar cell, and research is being undertaken in this area to improve the efficiency.
Recently, in conjunction with Plasma Quest Ltd., the ability to fabricate thin film amorphous silicon solar cells has arisen, allowing students to undertake projects in this field.
