Sunrise for perovskite solar cells
Silicon cells dominate solar power, but a new family of materials are hot on their heels. Simon Frost finds out more.
Between 2009 and 2013, the World Intellectual Property Organisation published a total of 19 patents with the word ‘perovskite’ on the first page. In 2014 alone, there were 75 – about four times as many as in the previous five years put together. It reflects a remarkable rise in conversion efficiencies achieved by scientists developing perovskitic solar cells. Five years ago, researchers could barely achieve an efficiency of 4% – now, they’re exceeding 20%.
Perovskites are a family of organic-inorganic compounds with the same crystal structure as the mineral from which they take their name, a calcium titanium oxide with the chemical formula CaTiO3, which is found in the Earth’s mantle.
They boast several attractive properties – they are made from abundant and relatively cheap materials, such as ammonia, iodine, lead and tin, and can be manufactured using low-temperature solution processing. They offer high absorption, too – a micrometre-thick layer of perovskite can absorb as much sunlight as a 180 micrometre-thick silicon cell.
The field of perovskitic solar cells was born out of the dye-sensitised solar cell, borrowing the architecture whereby a charge-conducting mesoporous scaffold is coated with a light-absorbing dye, replacing the dye with a perovskite. In 2009, the Journal of the American Chemical Society published a study by a group of Japanese universities that had manufactured the first perovskitic solar cell, using organic-inorganic methylammonium lead tri-iodide to sensitise titanium dioxide for visible light conversion, achieving an efficiency of 3.8%.
Compared with the industry-dominating crystalline silicon cells, which can achieve around 25% efficiency, that was no big threat. But while the figure for silicon cells has essentially plateaued – advancing from a world-highest 25% to 25.6% in the past 15 years – rarely does a month pass without the publication of a new record-breaking efficiency for perovskites. At the time of writing, the record stands at 20.1%.
‘Since their discovery in 2009, there has been a constant stream of ever-increasing perovskite solar cell efficiencies reported’ says Dr Paul Coxon, who researches photovoltaic materials at the University of Cambridge Department of Materials Science and Metallurgy. Professor Saiful Islam, Materials Chemist at the University of Bath, describes this increase as, simply, ‘unprecedented’.
Not to scale
‘The emergence of a new PV technology rarely happens and has reinvigorated research in the field’, says Coxon. ‘This is our “graphene”, and not without good reason’. But it’s not all good news – like graphene, perovskites have a scalability problem. The highest efficiencies are found in very small, defect-free samples – it commonly falls to 10–15% in cells measuring more than 1cm2, although this is improving, too.
In June 2015, the National Institute for Materials Science (NIMS), Japan, achieved a record 15% energy conversion in perovskite solar cells larger than 1cm2. The NIMS team was the first to have perovskite efficiency results certified by an international public test centre.
Three months later, Brown University, USA, published findings of a 16.3% conversion efficiency in a 1.2cm2 planar perovskite cell in Advanced Materials. In the Brown team’s novel fabrication process, perovskite precursors are first dissolved in a solvent and coated onto a substrate. That substrate is then bathed in a second solvent, which selectively grabs the precursor-solvent and leaves behind an ultra-smooth film of perovskite crystals. Excess organic precursor is then added to ‘glue’ the small perovskite crystals together, allowing them to merge together into larger crystals during heat treatment, which also bakes away the excess precursor. What remains is a uniform film with fewer defects and, therefore, higher efficiency.
Researchers at SPECIFIC Innovation and Knowledge Centre at Swansea University, UK, have developed an alternative production method using a short burst of infrared radiation to stimulate the growth of perovskite crystals within seconds, reducing both time and cost – crystallisation in a conventional oven at 100°C can take 90 minutes and consumes much more power.
SPECIFIC has also developed the first fully solution-processed, flexible metal perovskite cells. The Swansea spinout is partnered with Tata Steel, which produces 100 million square metres of metal building cladding per year. ‘If we could print solar cells onto this material, we could put a sizeable dent in the UK’s carbon footprint, as well as creating jobs in the local economy,’ said lead researcher Joel Troughton.
Aside from scalability, perovskites have another serious obstacle to overcome, as Coxon explains, ‘They contain organic molecules which aren’t stable when exposed to moisture. Even worse, and somewhat cruelly, they break down in sunlight. Efforts are underway to improve the robustness of perovskites by encapsulating the active layers, but this will come at a cost of their ability to absorb light.’
At the European Photovoltaic Solar Energy Conference, held in Germany in September, Ricky Dunbar from CSIRO’s PV Performance Laboratory, Australia, said ‘It is difficult to measure the efficiency accurately. It’s degrading as you measure.’
In 2013, Professor Michael Grätzel, from the Swiss Federal Institute of Technology, Lausanne, invented a variant designed to negate this instability. Grätzel, it should be noted, is the co-inventor of the dye-sensitised solar cell.
In Grätzel’s perovskitic cell, the perovskite is present not as a separate layer to the TiO2 and/or ZrO2, which captures the electrons, but infused into the material. As well as improving stability, the cell architecture is designed to eliminate the use of expensive back contact conductors and also eliminates the use of a conventional organic hole-transport-material. In May 2015, Grätzel’s research team achieved 1,000 hours of stability under light soaking and more than 2,000 hours under temperatures of 80–85°C. Australian firm Dyesol is building a factory in Turkey to manufacture cells based on Gratzël’s infusion principle. It is expected to open in 2017.
But Coxon notes that the first commercially viable perovskitic cells may well be silicon-perovskite hybrids. Perovskites can be made in a variety of different formulations, each of which absorbs different areas of the visual light spectrum more or less effectively. The perovskitic layer of a