Potential energy: dye-sensitised solar cells

A low-cost solar cell that imitates photosynthesis opens up new applications for photovoltaics.

Solar power works; if it didn’t, we’d all be dead. The sun provides the power for every plant on Earth and therefore underpins the food chain. But man’s attempts to copy photosynthesis, the process by which plants harness solar energy, haven’t always been so successful.

Successive generations of solar cells have been developed since the 1950s, with the promise of solar power always seemingly just about to make a breakthrough. Now, silicon-based solar cells compete with a newer generation of plastic photovoltaics, and with interest in renewable power at its highest level for years, solar power may yet make it into the big time.

One of the most interesting entrants into the photovoltaic field, however, is a cell that copies very closely the mechanisms of photosynthesis. Based on cheap materials and manufacturable through simple processes, it is set to make a big impact into the most neglected of global energy markets, the developing world.

The dye-sensitised solar cell is also known as the Grätzel cell, after its inventor, Prof Michael Grätzel of the Ecole Polytechnique of Lausanne in Switzerland. The invention, which won Grätzel this year’s Millennium Technology Prize, has recently hit the market for the first time, courtesy of Cardiff-based company G24 Innovations.

In a conventional silicon-based photovoltaic cell, sunlight strikes the crystalline silicon surface and displaces an electron, which can then diffuse through the structure of the semiconductor, creating a current. The silicon is ’doped’ with elements to increase its conductivity, easing the path of the electron. However, the efficiency is limited, and this is partly because the silicon meets the role of both energy harvesting and electron transport.

This isn’t the case in photosynthesis. Chlorophyll, the pigment that makes leaves green, acts as the energy harvester; it absorbs the light and generates a free electron. Other mechanisms within the plant’s cells then handle the movement of the electron, carrying it into the processes where it reacts with carbon dioxide and water to make glucose and oxygen.

In the early 1970s, concerns about the price of oil triggered a wave of research into alternative energy sources, and Grätzel became interested in photosynthesis. ’At the time I was educated, the detail on how photosynthesis worked was not well established,’ he said. ’I was interested by the way that plants use their molecules to generate charges and separate those charges.’

Grätzel cells copy this process, using three essential ingredients: a conductive electrolyte solution, in which is dispersed nanocrystals of the white pigment titanium dioxide (TiO2) coated with an organic dye. The dye takes the place of the chlorophyll – in early versions, chlorophyll itself was used, but electrons couldn’t diffuse through it – and absorbs sunlight, which knocks electrons free from the organic molecular structure of the highly coloured material. These are knocked into the TiO2, which has semiconducting properties, and carries the charge through the electrolyte to an external circuit. Electrons flowing back into the cell through the other side of the circuit replace those that are displaced from the dye molecules.

Sunlight entering the cell displaces an electron from the dye molecule into the TiO2 particle, which diffuses to the anode. The dye strips an electron from iodide ions in the electrolyte, and the charged iodide diffuses to the cathode where it recovers an electron. The released electrons flow around the external circuit as electric current.

Grätzel’s first cells were perfected in the 1980s, as a result of work with new nanoscale forms of TiO2 created at Lausanne. ’This was a fundamental study, driven by our curiosity – no one had done it before,’ he said. The research came to fruition in 1988, using a porous film coated with TiO2 crystals, creating a very high surface roughness to absorb sunlight.

’I asked my PhD student Hans Desilvestro how the experiment had gone,’ Grätzel remembered. ’He did not seem too enthusiastic initially, adding that he had only measured a few milliampères current. I knew it was a lot; other researchers had only measured micro or nano-amps.’
The initial cells achieved efficiencies of around seven per cent – a thousand times better than original versions from the 1970s – and currently reach around 12 per cent.

Although not as efficient as silicon cells, whose efficiencies are generally around 15 per cent, Grätzel cells are cheaper. TiO2 is a readily available material, used widely in white paint and sunscreens; organic dyes are also inexpensive. Grätzel is fond of demonstrating the system in schools, asking pupils to make their own cells using blackberry juice as the organic dye.

Moreover, the cells can be made using a simple roll-to-roll process, using machinery similar to that used in printing, coating and packaging – a system that is being used by G24 Innovations in Wales.

The sheets of dye-sensitised cells rolling off their production line are cut up to be stitched into the company’s first commercial product, a range of backpacks with integral solar panels to charge mobile phones and MP3 players.

This technology and application have a wider significance, however. ’This really comes into its own in Africa,’ Grätzel explained. ’More than 90 per cent of internet access there is via mobile phone, but the electricity grid is poor so people can’t easily charge their phones.’ Much of this mobile phone use is to check prices for agricultural products, so that smallholders can locate the best market to maximise their profits, Grätzel said.

G24 Innovations is planning a portable phone charger that would be given out free, with the cost spread across call and data download charges, to allow users to charge easily and regularly.

Further applications could be extended to solar power for water treatment, making it possible for households to operate their own small purification units without the need for an electricity connection or a diesel generator; similarly, the units could power fridges, both for domestic use and to keep pharmaceuticals and blood supplies cool in medical clinics. The low cost, light weight and flexibility of the printed panels makes them ideal for such applications, Grätzel said.

Other applications include coatings for windows, as the dye component of the cell could be made transparent, absorbing only the near-IR and ultraviolet component of the solar radiation. This is a particularly interesting application, Grätzel believes, as the cells can produce electricity in lower light conditions, unlike conventional silicon cells that require bright sunlight to work effectively. Pilkington’s North American operation is working with a US-based Grätzel cell provider, Dyesol, to produce electricity-generating glass panels for building-integrated photovoltaics.
Another potential spin-off is currently in Grätzel’s laboratories at Lausanne, using nanocrystalline iron-oxide films working in conjunction with Grätzel cells to generate hydrogen from water.

Meanwhile, work continues to further refine the Grätzel cell. Solid versions are being developed, using a solid inorganic matrix to replace the electrolyte solution; other work is focusing on improving the efficiency of the dye or its sensitivity to specific portions of the solar spectrum.