A special form of man-made crystals known collectively as perovskites have stunned scientists with their capacity to convert sunlight into electricity, and further development could reduce the cost of renewable energy.
The ability of perovskites, a class of synthesised materials with a specific crystalline structure, to produce electricity from sunlight was largely unknown until 2009.
When Professor Henry Snaith, at Oxford University, UK, first tinkered with the material for applications in solar cells, the cells' performance flew off the charts. ‘It was an absolute shock,’ he said. ‘Perovskites revealed wonderful properties and we are still trying to understand how they work today.’
For many years, scientists have been trying to develop a high-performing yet cost-effective solar cell. The problem has been that the choice of materials to build the cells has so far been limited to high-purity crystals of silicon or gallium arsenide, which are efficient but expensive, or so-called thin-film alternatives, which are cheaper but less reliable.
‘The question for material scientists is not how good perovskites can be, but what else might be possible.’
Professor Henry Snaith, Oxford University, UK
Perovskites could provide the best of both worlds. The main source of optimism is the speed with which scientists have been able to improve how efficiently perovskite-based solar cells convert sunshine into electricity – from 3.8 % to 19.3 % in just four years.
‘In terms of efficiency, this has brought them almost neck-and-neck with the leading thin-film technologies … that have been under development for over 40 years,’ said Prof. Snaith. He leads the HYPER project, funded by the European Research Council, which is investigating the potential of perovskites.
‘One target in my proposal was to achieve 10 % energy conversion efficiency in a solid electrolyte solar cell,’ he said. ‘That deadline is still two years away and we have already technically surpassed it.’
The advantage of developing perovskite-based solar cells is not only that they would reduce the cost of renewable electricity, but also that the cells could be manufactured relatively easily.
‘Perovskites are unusual in that they do not require cost-intensive deposition techniques to produce a quality material,’ explained Dr Mohammad Nazeeruddin from the École Polytechnique Fédérale in Lausanne, Switzerland. His laboratory has pioneered methods for manufacturing the materials in these cells in clever new ways. He said that by simply mixing salts of lead iodide and methylammonium iodide in a solution, the material naturally forms highly symmetric crystals, whose structure helps free electric charges once they are excited by sunlight.
‘What we have here is a technology that could transform the electricity sector,’ said Dr Nazeeruddin. ‘A handful of technicians in even the most remote locations could combine these ingredients to produce solar panels for their community.’
Dr Nazeeruddin is participating in the EU-funded GLOBASOL project which is investigating how to combine perovskites with other energy conversion devices in order to harvest all the wavelengths of light that come from the sun and achieve unprecedented power conversion efficiency.
Dr Nazeeruddin and Prof. Snaith are also working together on the EU-funded MESO project to bring perovskite solar cells to the market. However, they are up against daunting challenges. The material is chemically similar to a salt and changes colour when it absorbs humidity. Temperatures reached in the sun can loosen and rearrange its chemical bonds. Also, lead contained in perovskite cells impose additional safety measures.
‘Every material in these devices must be improved,’ said Prof. Snaith. He stressed that perovskites still face technical hurdles, notably in terms of their stability, but is confident that overcoming these challenges will be worth the effort.
His own start-up, Oxford Photovoltaics, is field testing a new generation of components that can withstand showers, operate at 85 °C and be manufactured on 30 by 20 centimetre areas. He hopes to see the first products coming out in 2017. ‘We are aiming to start with perovskite modules stacked on top of crystalline silicon ones to boost energy conversion efficiencies above 23 %,’ he said.
Prof. Snaith also believes the discovery of perovskites’ electricity-generating properties will have effects beyond the field of solar power. ‘This semiconductor was just sitting there waiting to be found,’ he muses. ‘Now that we know that it exists, the question for material scientists is not how good perovskites can be, but what else might be possible.’
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