Perovskite: the next big thing in solar cells?
When perovskite first appeared on the solar power stage seven years ago, few could have foreseen how far it would go, or how quickly. Dr Gareth Evans reports.
Back in 2009, when perovskite made its debut, it had a photon-to-electricity conversion efficiency of less than 4%; today that has risen to 22%. Not only does it represent the fastest jump in photo-voltaic (PV) history to date, but it also puts perovskite in the same general league as conventional silicon solar cells – and some researchers believe it could go even further.
However, it is not this alone that has fuelled the material’s meteoric rise to become one of PV’s hottest topics. It also seems that perovskite could potentially resolve the long-standing trade-off between cost and performance of current solar cells, and enable future solar power installations to reach the efficiency of crystalline silicon for less than the price of thin-film copper indium gallium selenide (CIGS).
This is a popular and highly competitive field with serious reputations to be gained and, arguably, serious money too, by whoever can achieve the critical breakthrough of turning this promising technology into a commercially viable PV solution. Unsurprisingly, a number of teams worldwide and several start-ups are trying to make that happen.
Stability problems in solar technology
One of the most significant challenges of incorporating peroskite into solar technology is addressing the humidity problem. The nature of the material used as the active light-harvesting layer – typically an organic-inorganic lead or tin-containing hybrid compound – which contributes to its low cost of manufacture, also makes it susceptible to moist environments.
In humid conditions, the water solubility of the organic component means that both stability and performance rapidly suffer. According to Keith Emery, manager of the US National Renewable Energy Laboratory’s (NREL) photovoltaic cell and module performance characterisation group, the stability of most perovskites is marginal, and this prevents prolonged exposure to light or elevated temperatures.
“Silicon, which this technology hopes to displace, has set a high bar for stability with modules expected to degrade much less than 1% in power per year of exposure in the field for 25 years or more,” Emery explains.
He says that although, in general, perovskite cells from some of the research groups are more stable now than they were a year ago, there is still a long way to go, and the issue remains a big obstacle to commercialisation. However, in July, a team led by Taiho Park, professor of chemical engineering at Pohang University of Science and Technology (POSTECH), Korea, unveiled a new method that may eventually help point the way to overcoming this problem.
Additive-free panel materials
Professor Park and his team created a novel hydrophobic conducting polymer that has the high electron hole (hole) mobility within its crystal lattice required for the solar cell to work efficiently, but without the additives often necessary to achieve it. This is particularly problematic as the hole transport materials conventionally used in many perovskite solar cells are themselves partially soluble, while the chemicals commonly added to improve hole transport also draw in water from the air.
The idea is not a new one; a team from the University of California, Los Angeles (UCLA) demonstrated an efficient planar perovskite solar cell using an additive-free donor acceptor conjugated small molecule as a hole transport material a year earlier, but what marks out Park’s cell is its efficiency. Whereas the UCLA cell achieved a power conversion efficiency (PCE) of 14.9%, the Korean researchers recorded a PCE of 17.3%. Moreover, the POSTECH press release also claims that it “dramatically improved stability too, the cells retaining the high-efficiency for over 1,400 hours, almost two months, under 75% humidity”.
Streamlining energy research efforts
While it is undoubtedly a step forward, it also highlights one of the problems with perovskite research; the approach to testing is not always the same, and not all results are independently verified. As Emery has notably said, many of the efficiency figures “should be taken with a grain of salt”.
He points out that the standard module qualification test calls for 1,000 hours at 85°C and 85% humidity, not the 75% at room temperature used by the Korean group – something Professor Park acknowledges, but says couldn’t be done in his lab. Nevertheless, Park believes that once problems such as light and thermal stability, and large-scale reproducibility have been overcome “this technology will be much up-graded and [we] will see this type of solar cells in markets”.
It is clearly a view echoed by Oxford Photovoltaics, a spin-out from the University of Oxford founded in 2010 that is working to commercialise a solid-state, thin-film perovskite solar cell which, it is claimed, will change the way glass is used in buildings.
The idea is to print thin-film cells, either onto silicon or CIGS solar cells, creating a ‘tandem’ cell architecture and so boost their efficiency. Alternatively, it could be applied directly onto glass as a semi-transparent coating, said to be ideal for building integrated photovoltaic (BIPV) systems. According to Oxford Photovoltaics the technology should enable a building to generate a significant proportion of its electrical demand directly from sunlight.
The company says its perovskite solar cells are already performing at 17% PCE under laboratory conditions and it expects to reach 25% “within a few years”, with hopes of tandem perovskite / perovskite solar cells achieving 30% in the same timeframe.
It may be too soon to consider equipping all new buildings with integrated PV systems or perovskite cells to generate commercially useful power. Before that, Emery says, there must first be a prototype module built that can pass the module qualification test.
“Until this hurdle is crossed the product and cell design is a lab curiosity and not ready for the PV power market,” he observes.
If perovskite really is going to be the next big thing in solar energy it will obviously have to overcome the existing limits on humidity and stability, but even then Emery sounds a note of caution on the road to commercialisation.
“Gaining market share with a new product with unproven reliability will be a daunting challenge once they have crossed the module qualification hurdle because they will be competing with a silicon PV industry that has giga-watts and millions of modules of manufacturing experience while they have no manufacturing experience,” he says.Nonetheless, with scientists from the US Department of Energy’s Lawrence Berkeley National Laboratory having recently discovered a method that could, potentially, bring efficiency closer to the material’s theoretical energy conversion limit of 31% – at least at the nanoscale – interest in perovskite is unlikely to wane any time soon.