What more can you say about solar photovoltaics (PV)? They basically tick all the boxes – completely clean, cheap, limitless, there’s enough to power the world and, most importantly, they’re bendy – and we are now so close to seeing it do its thing in a big way. In some ways, you could compare it to a promising young athlete (Gareth Bale, perhaps, for the football-minded) – you’re not sure just how good they can become, but they’re already exciting to watch.

Despite this, some feel that the technology is still not getting the support it needs from business to reach its potential. ‘For some reason, [solar energy] is never a big mix in the predicted 2020 or 2050 calculations,’ says Henry Snaith at the University of Oxford, UK. ‘I don’t think people who do the calculations really figure in the potential for technological evolution and development advancement.’

The best is yet to come

Snaith’s recent work certainly demonstrates this kind of evolution. Whilst working on a class of dye-sensitised solar cells (DSSCs) modified with perovskites, he made a crucial discovery. He found that some perovskites, which were being used as the sensitiser component, could themselves transport charge, making one of the key components of DSSCs redundant, greatly reducing energy loss.

‘People just thought [the perovskites] were light absorbing pigments and not really semiconductors,’ he explains. ‘[Then] we got our shock result, literally on the first set of devices.’This fortuitous discovery resulted in a huge jump in the efficiency of perovskite-sensitised solar cells from about 7% to 10.9%.

‘From there, it all becomes possible,’ Snaith enthuses, ‘because when you realise that this is a semiconductor, we can make low cost solution processed thin film cells and there’s no loss due to electron transfer into the TiO2 which is a major loss in the dye cell.’

Taking this exciting development another step, Snaith showed in his latest paper in Energy & Environmental Science that using colloidal chemistry avoided the need for the 500°C sintering step typically necessary for DSSCs.1 In fact, the highest temperature his new process needs is a 150°C drying step. This will help make devices cheaper and provide more flexibility in terms of materials that can be used. Perhaps more importantly, though, it could help to make the cells even more efficient.

Multi-junction cells work on the principle that having separate absorbers to specialise in specific parts of the solar spectrum will result in more efficient devices. Using Snaith’s new low-temperature process, these different cells can be deposited on top of each other in one device. This would not previously be possible, as the sintering step would damage the cell underneath.

His new device already has a 12.3% efficiency, which rivals the best DSSCs available. However, Snaith is confident that it can reach the 25% efficiency of current silicon solar cells, something that traditional DSSCs have failed to do.

He is also already looking forward to getting this technology onto the market. He expects to commercialise these cells in the next two years, through his spin out company Oxford Photovoltaics. Not only that, he is predicting that his devices will cost a fraction of current silicon ones. ‘We’ve costed it out and we think, at our first level of manufacturing scale, it’ll be something like 30 cents (20p) per Watt peak. Current silicon is about $1 per Watt peak.’

Although Snaith is confident about his new material, he is adamant that the search for better solar cells is by no means over. ‘What this [work] really shows is that there’s stuff out there that we haven’t noticed before. I’m sure there’s other stuff that will turn up over the next few years.’

Snaith’s new cells are still in their infancy, so we can expect great things. I’d say the future for PV is very bright indeed.

Yuandi Li

1 J M Ball et alEnergy Environ. Sci., 2013, DOI: 10.1039/c3ee40810h

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