The challenge

Since photovoltaic (PV) solar panels emerged on the commercial market in the 1950s, the technology has evolved into a highly vibrant field of research worldwide, encompassing a large range of materials and technologies.

Most commercial technologies use silicon as a light absorber, a cheap and Earth-abundant semiconductor.

However, silicon has a limited capacity for absorbing light, which means the films that constitute the panels must be thick and free of elemental impurities (which could introduce energy conversion losses), leading to energy intensive manufacturing process with high CO2 footprint – known as energy payback time.

Furthermore, silicon panels are heavy and their performance is strongly affected by the angle of exposure to solar irradiation.

Current alternatives on the market rely on materials with significantly better solar light absorption (thin-film solar cells), such as cadmium telluride (CdTe) and Copper Indium Gallium Selenide (CIGS). However, these materials are made of elements which are rare, expensive and toxic, hence do not provide a sustainable solution in a fast-growing market (PV is the fastest growing energy market worldwide).

What we’re doing

Experiments conducted in the University of Bristol’s Electrochemistry Lab suggest that a more viable substitute for thin-film solar cells lies in using Earth-abundant, low toxicity, low cost materials incorporating elements such as copper, tin, sulphur and zinc.

Their studies, part of a four-year £2m EPSRC research project, reveal that solar PVs composed of these alternative materials would not only increase the scale of energy production, they also have the potential for being incorporated into flexible and portable products, representing an unprecedented step forward in solar technology.

In a parallel research project, a team of physicists, engineers and chemists are devising diamond-based solar thermionic energy converter devices, which use concentrated sunlight to heat surfaces to red heat temperatures so that they emit electrons directly into a vacuum.  If these electrons are collected at a cooled anode, electrical power can be generated with maximum efficiencies predicted to be much higher than is achievable using conventional silicon solar cells.

Semiconducting diamond produced by Plasma-assisted Chemical Vapour Deposition has been found to emit electrons therm

ionically at temperatures much lower than refractory metals, making terrestrial heat to electric power conversion feasible. This is made possible due to the low work function of such diamond surfaces and the chemical stability of the diamond surface at emitting temperatures.

How it helps

The project has revealed that the emission properties of semiconducting diamond material structures may be enhanced by the use of beta irradiation. This finding paves the way to devices capable of generating current densities of several A/cm2. Additionally, a multi-electrode thermionic converter has been developed that has the potential to mitigate the low output voltages (<1V) of thermionic converters and electron reflection losses, enabling higher power densities and efficiencies to be realised.

A further output of this project has been the world’s first tri-layer system comprising a gold-carbon-gold meta-surface, which has the potential to reach much higher temperatures than simple black surfaces while minimising the emission of thermal radiation.

Post-grant, this research is continuing in collaboration with Renewtec (Dubai) who are seeking to scale up the diamond thermionic converter technology to realise concentrated solar power stations in the Middle East, and also in collaboration with Prof Tappas Mallick, under the auspices of a UK/India Renewable Energy initiative on concentrated solar technologies.

Investigators

• Professor David Fermin, School of Chemistry
• Dr Neil Fox, School of Chemistry
• Professor Martin Cryan, Department of Electrical & Electronic Engineering