Dr Paul Coxon, from the Materials Chemistry Group at the University of Cambridge, explores how the solar revolution began, how solar cells work, and how scientists are now harnessing natural structures and materials to make them more efficient.
Dr Coxon will be giving a presentation, Solar energy: past, present and future, at this year’s Cambridge Science Festival.
CSF: There are various renewable energy sources, so why have you concentrated on solar energy?
PC: I came into solar energy research by accident. My PhD was on silicon quantum dots – small clusters of silicon made up of less than 100 atoms.
We made these dots by etching ordinary silicon wafers in an electrochemical cell. The etching makes the top layer porous, full of holes with a dense nanostructure of spikes and wires. Unfortunately, the reaction wasn’t always consistent and instead of making the orange-brown porous silicon layer from which our clusters were derived, we occasionally made black silicon that was no use to me so it went in the bin! It turns out that while black silicon is terrible for making quantum dots, its special needle-like surface structure makes it excellent at absorbing light. Now I’m developing low cost ways to make black silicon using molten salts for antireflection surfaces for solar photovoltaic cells and we’ve had some encouraging results. There’s real commercial potential in our material so it always pays to check what you throw in your lab waste bin!
CSF: Are photovoltaics a better option that other renewable energy forms, eg wind power?
PC: There’s no one size fits all model for renewable energy. Solar PV is best suited in regions with a lot of sun; wind power is best where there is a lot of wind. The UK is ideally placed to make use of its large wind resources, so I’m not so sure largescale solar farms are the way forward for Britain – the land requirements for solar farms are a concern.
On the other hand, solar farms have low operational costs, with no or few moving parts and a typical lifetime of 20-25 years. Recent estimates place the lifetimes of wind turbines at only 12-15 years.
CSF: What are the downsides to photovoltaics?
PC: Solar energy isn’t perfect; there are some disadvantages – just like any energy source. Solar photovoltaics are a passive technology; they can only produce electricity as long as there is sunlight. Obviously, this means they don’t work at night or especially well on cloudy days – often the times when we our electricity demands are highest. Meeting our full energy demands over a 24-hour period is a challenge with solar panels alone and so we need ways to store this energy. One other critical issue is the energy costs in making solar panels – especially panels of silicon cells. Over 90% of the world’s installed solar panels are made from crystalline silicon, which takes a lot of energy to produce and refine. The cost of silicon has fallen by over 96% since 2007, but the energy needed to manufacture a silicon solar panel takes up to five years to pay back, although this is falling due to more efficient manufacturing methods.
CSF: What are the current technological and manufacturing obstacles or limitations in terms of photovoltaics?
PC: Silicon-based PV dominates the world market, with over 90% capacity share. Making silicon from silica (essentially sand) is a well-established process and the raw materials are plentiful and falling in cost. Unfortunately, turning silica into solar grade silicon is energy intensive, requiring considerable amounts of purification, high temperature processing, and generates a lot of expensive material waste.
This is always something we must bear in mind when shouting the ‘green’ credentials of solar PV. Also, silicon itself isn’t necessarily the best PV material. It’s a semiconductor with an indirect bandgap, which means it isn’t particularly efficient at absorbing light. The theoretical efficiency limit of a silicon solar cell is around 30% and practically, the best silicon solar cells today are around 24% efficient. There is extensive research around the world directed towards ways to make silicon PV more efficient, either by more efficient manufacturing processes, or by improving the ability of silicon to trap more light.
CSF: Do you see a time when solar energy might be able to meet most of our energy needs? And are we far off doing so?
PC: Solar energy has been growing at a rapid rate. Over the past two years, many largescale solar farms have been built around the world, especially in the US and China, and now the total worldwide solar capacity is estimated at over 200GW. India has recently announced an ambitious plan for 22GW solar capacity by 2022. Today, solar now accounts for 1% of all global electricity demand. It’s taken a long time to reach this share, and is largely focussed in countries such as Germany, which has aggressively pushed solar for many years, but the time taken to reach 2% is optimistically expected to happen sometime before 2020.
CSF: What do you think will be the developments over the next decade?
PC: It’s hard to predict the future, but we’ll undoubtedly see further uptake of solar photovoltaics around the world. The next decade will see a large expansion of massive-scale solar arrays, driven by further reductions in manufacturing and processing costs, especially in countries in the Middle East. By the end of this time, we may see the emergence of next-generation solar cells based upon perovskite materials. These abundant minerals are superior to silicon at absorbing light with electronic properties that could make them more efficient than silicon at generating electricity. They can also be produced relatively simply, using low cost solution processing techniques and are currently a hot topic of research. Small (1cm2) perovskite PV cells have achieved 20% efficiencies in the lab over (only) a few years and researchers are now looking at ways to combine perovskites with silicon to form tandem cells that could boost efficiencies even further.