Renewable Energy by Nick Jelley

Renewable Energy by Nick Jelley

Author:Nick Jelley
Language: eng
Format: epub
ISBN: 9780192559319
Publisher: OUP Oxford
Published: 2020-01-24T16:00:00+00:00


Developments in solar cell technology

One route to improved efficiency is by using both surfaces of a silicon solar cell. These bifacial cells convert light reflected onto the back surface by replacing the aluminium contact there by an aluminium grid, as well as the illumination on the top surface of the cell. By mounting these cells about a metre above a reflective surface such as light-coloured stones or white-painted concrete, the output can be increased by up to 30 per cent, and potentially produce cheaper electricity.

Solar cells can also be made from very thin films of certain photovoltaic materials, and it was thought that these could be much cheaper than silicon cells, as they used much less material. Most types have lost out in the rapid fall in cost of silicon cells, but a few may prove very effective. One is the perovskite solar cell, whose efficiency improved dramatically from less than 5 per cent in 2009 to over 24 per cent by 2019. Perovskites are crystals with the same structure as the mineral calcium titanium oxide, and excellent cells have been made with the perovskite methylammonium lead trihalide. It absorbs photons with energies greater than 1.6 electron-volts (eV) and needs to be only a third of a micron thick—very much thinner than a silicon cell. The absorption energy can be changed by altering the composition of the perovskite. The thin film of perovskite can be simply applied, which should enable production costs to be low, either on solid or flexible surfaces, and there are already plans to make the cells commercially available.

They can also be used to improve the efficiency of a silicon solar cell, and this may be a particularly cost-effective application. This can be done by applying a layer of perovskite on top of the silicon cell. In such a double p–n junction (tandem) cell, photons with energies between 1.1 eV and 1.6 eV are absorbed in the silicon, while those with energies greater than 1.6 eV are absorbed in the perovskite. The tandem cell acts like two batteries in series, with the upper one delivering current at a voltage of 1.2 volts, the lower at 0.7 volts. The current is about half that from a single silicon cell but the voltage at 1.9 volts is more than twice this, so the efficiency could increase from around 22 per cent to 30 per cent. In December 2018, a perovskite–silicon tandem solar cell, manufactured by the company Oxford PV, achieved an efficiency of 28 per cent—higher than the record single junction silicon efficiency of 26.7 per cent. As the gain in efficiency is expected to be much higher than the increase in cost of adding the perovskite layer, the cost of electricity from these tandem cells could be significantly lower than from silicon cells. The company, together with the University of Oxford, is also exploring the possibility of making a low-cost triple junction solar cell, with three different perovskite layers that absorb photons of differing energies, which could potentially have 37 per cent efficiency.



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