The photovoltaic cell is poised for massive growth. According to the European Photovoltaic Technology Platform, it could cover up to 20% of the world’s electricity needs by the year 2040. Increasingly efficient and affordable, the photovoltaic cell is well placed in the race for green energies.

Although the photovoltaic effect was first discovered by French physicist, Becquerel, in 1839, it was not until semiconductors were invented in the 1950s that an application was found for it. A photovoltaic (PV) cell is a device that generates direct current electricity from the energy of photons alone, with no mechanical or thermal input.

Two layers of a semiconductor material (more often than not silicon) are sandwiched between two electrodes. The upper layer (N), which is doped with an impurity with higher valency than silicon, such as phosphorus, initially has spare electrons. The lower layer (P), which is doped with an impurity with lower valency, such as boron, has a deficit. At the PN junction, the electrons migrate from N to P until they achieve a balance, creating an electric field that prevents any subsequent transfer of charge. When light falls on the cell, the photons extract new electrons, so creating ‘holes’. As these negative and positive charges are unable to cross the P-N junction, they are forced to travel through the electrodes, generating an electric current.

The energy efficiency of PV cells has increased from 8% in the 1980s to between 11% and 17% at present, although this is still not enough to guarantee the sector’s profitability because of the high manufacturing cost. However, Germany has already succeeded in creating a market thanks to a highly proactive policy of incentives. In the space of eight years, the number of jobs in the photovoltaic sector has grown by a factor of 20, from 1 500 to 30 000. The sector’s future depends heavily on research, both to improve performance (with planned energy efficiencies of 25–45 % by the year 2030), and to reduce the costs of producing current modules.

This is the path that the European Commission has chosen to take with the CrystalClear project, which aims to make crystallinesilicon cells more affordable. The behaviour of crystalline silicon, which is used in around 85% of solar modules, has been studied extensively. CrystalClear researchers are exploring two main avenues of enquiry. The first is to use new types of silicon, such as solar-grade silicon, which is less purified and hence cheaper. The second is to maximise the usable portion of silicon, mainly by reducing scrap and improving cell architecture.

“We are aiming to halve the price of silicon PV cells by overhauling their design entirely. One idea is to place both electrodes behind the cell, rather than on the front and rear as at present”, explains Wim Sinke, CrystalClear project coordinator.

Reducing costs would also limit the environmental impact of production. Cutting down on the quantities of silicon used in PV cells also decreases the energy payback time (the amount of time a cell has to operate until it has produced the same amount of energy as was used to manufacture the cell), which is currently between one and two years. “So, reducing the costs goes hand in hand with reducing the environmental impact”, concludes Wim Sinke.

Marie-Françoise Lefèvre


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The flexible thin-film CIGS solar cell uses an innovative semiconductor made from copper, indium, gallium and selenium. With a junction unlike the P-N junction, it achieves a conversion efficiency of close to 20%. Various types of thin-film solar cell are the main rivals to the current silicon modules. The light and flexible plastic PV cell offers a conversion efficiency of 5% for a very low production cost. However, its sensitivity to oxygen and humidity makes it unsuitable for outdoor use, a drawback that researchers are endeavouring to resolve by encapsulating the cell. The Graetzel solar cell works on the photosynthesis principle and is made up of nanocrystals of titanium dioxide (TiO2) coated with organic dyes which sunlight causes to release electrons. The conversion efficiency of the Graetzel solar cell exceeds 10% in the laboratory and its inventor, Professor Graetzel, has announced that his cell will be five times cheaper to manufacture than a silicon cell.


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