With oil reserves running out, silicon solar cells offer an alternative source of energy. How do they work and how can we exploit their full potential?
Indirectly, the Sun is the source of most of the energy we use on Earth: not only of fossil fuels and biomass, but also wind and tidal energy, to mention just a few. Increasingly, there is interest in capturing the energy from the Sun more directly, using photovoltaic cells.
A relatively old, medium-sized star made of hot plasma, the Sun radiates energy as electromagnetic radiation over a wide spectrum. At a distance of 150 million kilometres, our planet receives an irradiance of around 1366 W/m2 (1 W= 1 J·s) from the Sun, but not all of this actually reaches us because Earth’s atmosphere reflects and absorbs about 30 % of this energy. Nonetheless, every square metre of Earth’s surface receives an average of nearly 1000 Joules per second from the Sun.
To put this into perspective, the total energy consumed globally in 2010 was around 5 x 1020 J. If we assume that our planet is a perfect sphere with a radius of 6370 km, Earth receives 1.8 x 1017 J/s, of which about 1.3 x 1017 J/s reaches Earth’s surface. Thus in one hour, the Sun provides Earth with all the energy we need for a whole year.
It isn’t quite that simple, however. Due to meteorological factors, the Sun’s declination and Earth’s rotation, the irradiance is actually closer to 230 W/m2. If we repeat the last calculation using that figure, the time needed to power Earth with energy from the Sun for a year is about five and a half hours – still an impressively short time.
Solar radiation is therefore a promising energy reservoir, but how can we collect it and use it?
The foundations for modern solar energy collection were laid in 1839, when the French physicist Edmond Becquerel observed an increase in the electrical conductivity of some materials when they were exposed to light; this became known as the photovoltaic effect. It was not until quantum mechanics was developed, though, that the phenomenon was explained. Electromagnetic radiation can be described as a stream of quantum objects called photons. When these photons are absorbed by some materials, they can promote electrons in the material into a higher energy state (the conduction band), potentially enhancing the material’s conductivity.
Semiconductors, such as silicon, are photovoltaic because a photon’s energy matches that required to move one of the semiconductor’s electrons up into the conduction band. However, semiconductors themselves have few free electrons and, therefore, low conductivity. To increase their electrical conductivity, tiny amounts of other materials (impurities) can be added, a process called doping.
Doped silicon is the most frequently used material in electronics. Pure silicon has four valence electrons that it shares with four neighbouring atoms. Adding impurities with more or fewer valence electrons (such as phosphorus or boron) modifies the conductivity properties of the material. Phosphorus has five valence electrons and, when a phosphorus atom is surrounded by silicon atoms, the fifth electron is only loosely bound. This means it can easily reach the conduction band, helping to increase the material’s conductivity. Phosphorus-doped silicon is called n-type (negative type) since doping increases the number of negative free charges (electrons). In contrast, boron has only three valence electrons, and the lack of one electron in the silicon lattice creates a ‘hole’. As free electrons move through the lattice, from one hole to another, the positively charged holes appear to move through the material. Boron-doped silicon is known as p-type (positive-type) silicon.
These phenomena can be exploited in solar cells to collect energy from the Sun and transform it into electrical energy. The simplest solar cell is formed by the junction of two semiconductors, one p-doped and one n-doped, called a p-n junction. At this junction, the electrons in the n-type silicon ‘see’ the holes in the p-type silicon and travel to fill them – creating electron-hole pairs. When a photon strikes one of these pairs, however, it breaks apart and the flow through the material of these newly freed charge carriers, both positive and negative, generates an electric current.
Not all the freed charge carriers generated by this process will contribute to the current, however. Instead, a significant proportion of the electrons and holes will pair up again, generating heat. This reduces the energy conversion efficiency of a photovoltaic material: the percentage of the incoming solar energy that is converted into electrical energy. This is one of the most important parameters of a solar cell’s quality. Currently, commercially available silicon solar cells are approximately 20 % efficient, but extensive efforts are being made to improve this value.
We now know what is happening inside a solar cell, but what are the practicalities of using solar cells to capture energy from the Sun? A standard solar module is approximately 1.3 m2 and consists of an array of around 50 single solar cells. Depending on the technology, one module will deliver about 200 W, so an assembly of five modules can supply the energy needs of an average household – about 1 kW. In theory, the total energy demand of Europe could be satisfied by covering just 1 % of the continent with solar cells. Realistically, however, solar power is only going to be part of the solution to our energy needs.
In Europe in 2010, about 1 % of energy was obtained using photovoltaic technology, but optimistic estimates of what proportion of Europe’s energy needs could, realistically, be met by solar power range from 30 to 50 %. More precise figures are not yet possible because the necessary technological innovations are still being developed.
One of the limitations of solar energy is that the amount of electricity generated by solar cells is strongly dependent on environmental factors including cloudy weather; the angle at which the sunlight strikes the panel; snow, rain, leaves and other debris on the surface; and, of course, night time. One way to address these issues is by incorporating solar energy into a smart grid, a new concept of a power grid that co-ordinates electricity production from several sources – including solar cells, thermal generators and nuclear plants – to meet consumer demand. In this kind of power distribution, solar cells are playing an increasingly important role.
They are also becoming more popular on a smaller scale where the electricity produced may be used on-site – in people’s houses, for roadside telephones, at industrial plants, and on boats, cars and even the International Space Stationw1.
So while we are still unreachably far from meeting our annual global energy needs from five hours of sunshine, photovoltaic technology is increasingly a feasible source of energy. Next time you switch on the kettle or the television, think of the sunlight that helped to power it.
Photovoltaic panels based on crystalline semiconductors – as described in this article – are relatively expensive to produce and process. An alternative is offered by organic photovoltaic materials, which allow large solar panels on flexible substrates to be produced using low-cost processes such as inkjet printing. However, much more research is necessary to improve their efficiency.
Most organic photovoltaic devices are based on thin films comprising an electron-accepting component (such as a fullerene derivative) and an electron-donating component (usually a conjugated polymer) between two electrodes. An important requirement is to mix these two components to obtain a continuous network of donor and acceptor paths for the carriers (electrons or holes) to reach the appropriate electrode (see image). Analysis with synchrotron X-rays at the European Synchrotron Radiation Facility (ESRF) is a good way to examine these materials closely, enabling their characteristics to be improved.
To learn more, see the ESRF websitew2.
ESRF is a member of EIROforumw3, the publisher of Science in School.
Find out more about organic photovoltaics at ESRF.
Shallcross D, Harrison T, Henshaw S, Sellou L (2009) Looking to the heavens: climate change experiments. Science in School 12: 34-39. www.scienceinschool.org/2009/issue12/climate