Solar power to the people
While many countries focus on the problems of reducing their emissions of carbon dioxide by switching to renewable energy sources, more than one billion people worldwide do not have access to electrical power of any description. According to Neil Robertson of the University of Edinburgh, solar energy has the potential to be a solution for all – and his department's work plays a critical role in making the technology not only more efficient but also more affordable...…
Article by Peter Barr
Photovoltaic cells were invented more than 50 years ago but even today, despite advances in technology and the economies of scale made possible by mass production, the cost of solar energy is still too high for many people all around the world.
During the 1950s and 60s, the space race boosted the development of solar cells for use in spacecraft, but it wasn't till the 1970s and 80s that the technology became cheap enough to come within the reach of consumers – and even then it was regarded as a luxury.
In recent years, especially in northern climates like Scotland which have about half the “usable” sunlight of countries like Spain, other sources like wind and marine have been grabbing the headlines, but there is little doubt that solar energy will play a major role in the renewables industry as time goes by, thanks to recent advances which make solar energy more cost-effective than ever before – even in Scotland.
According to Dr Neil Robertson of the Department of Chemistry in the University of Edinburgh, the sun provides 10,000 times more power than all human beings require, and a single hour of solar radiation is enough to run every electric device for a year. This may not be news, of course, but Robertson thinks that the tendency for many people in Scotland to dismiss solar power is a major mistake, because we'll soon have new technologies to help us exploit it.
Despite current prices, worldwide sales of PV cells are growing by about 25 per cent every year, and Robertson believes that we will soon reach a “tipping point” in solar power, when it becomes more cost-effective than fossil fuels, initially in warmer climates where conditions are better for photovoltaics. “And in Scotland,” he says, “it's only a matter of time before solar energy also takes off, as prices begin to decline.”
New technologies emerge
What makes Robertson so confident that we are on the verge of a breakthrough is the rapid development of new PV technologies that will make it much easier and cheaper to tap solar power. Robertson is working on two of these – a low-cost device called a “dye-sensitised solar cell” or DSSC, and a light-collecting device called a “luminescent solar concentrator” or LSC. Both these approaches are better at absorbing lower levels of sunlight, including the diffuse light which passes through clouds. DSSC units can also be made very thin – flexible enough to be attached to clothes or rucksacks.
Robertson's research group develops new dyes for PV devices – synthesising new functional molecules, understanding their properties and finding new applications:
1. Dye-sensitised solar cells (DSSC)
These cells use a cheap semiconductor – a network of titanium dioxide (TiO2) nanoparticles – which does not normally absorb visible light. When the dye is put onto the surface, it gives the cell the ability to absorb the visible light (photons) where the majority of solar radiation lies. When a photon “excites” the dye, it transfers an electron from the dye molecule to the electrode on the TiO2 semi-conductor and the positive charge (the “hole”) which is left on the dye is transferred to an electrolyte solution (liquid or gel). The interface between the dye and the titanium dioxide is “where it all happens,” says Robertson. The charge separation gives the electrical potential desired, and gets a current (the electrons) flowing through the circuit which can then be converted to power.
2. Luminescent solar concentrators
This is a light concentrating device – a flat perspex sheet with special luminescent dyes inside – which catches light over a large area and concentrates it onto the edge of the sheet, reducing the area and therefore the cost of the solar cell unit required. The dyes inside the perspex sheet absorb the incoming light and then re-emit it. The emitted light is trapped in the sheet in the same way light is trapped in fibre-optics and it is guided to the solar cells at the edge of the sheet, which convert the light into electrical energy. Robertson suggests that the sheets could be made into building components – like coloured bricks or windows that can be visually attractive as well as functional. (The University of Edinburgh is working on this project in collaboration with engineers in Heriot-Watt University.)
The rise of DSSC
Even though Robertson may sing the praises of DSSC, he believes there is room for a number of different approaches to PV design, including conventional crystalline silicon cells, in a market which is big enough for everyone and continues to grow. Different criteria make different options attractive. DSSC is much cheaper and can be used in much thinner, more portable units, while crystalline silicon cells are more costly, efficient and stable, which makes them much more suitable for use in solar power plants.
DSSC was first developed in the early 1990s, when it was shown to have a power-conversion efficiency of about 11 per cent – lower than crystalline Si cells but significantly cheaper, because it used a relatively low-cost semi-conductor and other cheap materials. To begin with, the new type of cell was less stable than crystalline silicon cells because it used solution-based electrolytes as well as more volatile solvents. This meant it was harder to manufacture and did not last as long as conventional cells, but Robertson explains that despite all these drawbacks, DSSC will have a huge impact worldwide simply because it is cheaper – and more practical for many applications. As advances are made to improve the stability, efficiency and manufacturability of the devices, and new applications emerge, sales could increase dramatically in coming years, in northern countries and in developing countries.
Robertson, who has studied and worked in the UK and Germany, specialising in synthetic chemistry and functional electronic materials, is particularly excited by the prospects of DSSC in developing countries, because it is a highly cost-effective alternative to other PV devices. “People in remote villages need very basic electric appliances like mobile phones and lights, and DSSC is a very cost-effective solution with the potential for production to be ramped up comparatively quickly,” he says.
As well as developing new types of dye, Robertson and his team also focus on the redox electrolyte – the transport medium which moves the “hole” from the dye to the positive electrode. When the electron separates from the dye and moves through the TiO2 to the negative electrode, it leaves behind on the dye a positive hole which is simply the absence of an electron.. The main objective is to move beyond liquids and gels to solid-state hole transporters, making the DSSC much more stable and easier to manufacture.
Robertson and his team are members of the SuperGen Excitonic Solar Cell Consortium, bringing together researchers at Imperial College and the Universities of Bath, Cambridge, Loughborough, Oxford, Warwick and Edinburgh. The main objectives of the project are to develop new materials and novel designs, as well as optimise the fabrication methods. There are also plans to work with other scientists in India to develop new transport media for DSSC.
Steady progress has been made with DSSC designs in recent years but this is measured more in terms of “understanding the mechanisms involved” rather than simply efficiency, as researchers seek to improve stability and manufacturability. This is already attracting investment. For example, a Cardiff-based company called G24 Innovations is now manufacturing DSSC products on a commercial scale, making flexible solar cells to charge mobile phones and power lighting – the first in the world to do so.
According to Robertson, the relatively low efficiency and shorter lifespan of DSSC is not necessarily a barrier since the whole point is to make low-cost devices which can be used for applications such as portable or wearable chargers. The major long-term benefits are less embedded energy (less manufacturing energy and materials) and lower capital costs – it is relatively easy to set up a production plant, compared to traditional silicon cells which require costly foundries.
The development of new dyes which harvest more photons and are also more stable (less likely to decompose), combined with solid-state designs, could lead to a “step-change” in DSSC, says Robertson – suggesting that when it comes to future technologies, the sky is the limit.