The opportunities presented by perovskite solar cell technology are exciting, but hurdles remain.
There is great excitement worldwide about a ‘magic’ material, perovskite crystals, that have shown great promise for use in solar cells.
The material has exceptional properties, including a very high magnetoresistance, ferroelectricity, charge ordering, spin dependent transport (spintronics), high thermopower, and catalytic properties. It was in a perovskite – an yttrium barium copper oxide – that high temperature superconductivity was first observed.
Perovskite minerals were first discovered in the Ural mountains of Russia and named after Russian mineralogist Count Lev Aleksevich von Perovski who was the founder of the Russian Geographical Society.
Perovskites are based on ingredients that are abundant, and combining them in various ways is easy and inexpensive. They can be made into thin films with a crystalline structure similar to that of silicon wafers, at low temperatures, while the production of silicon wafers requires costly, high temperature processing. Silicon wafers are thick and rigid, whereas perovskites can be manufactured in thin and flexible films that could be used as lightweight, bendable solar sheets.
The perovskite mineral is made of calcium, titanium and oxygen in the form CaTiO3. A perovskite structure, on the other hand, is any material with the form ABX3 with the same crystal structure as the mineral. A perovskite can be seen as large atomic or molecular cation (positively charged) of type A in the centre of a cube. The corners of the cube are then occupied by atoms B (also positively charged) and the faces of the cube are occupied by a smaller atom X with negative charge (anion).
In the case of perovskite solar cells, the most efficient devices produced so far are – in the usual perovskite form ABX3 composed of:
• an organic cation (A) – methylammonium (CH3NH3+ )
• a big inorganic cation (B) – usually lead (II) (Pb2+)
• an X3 – a smaller halogen anion – usually chlorine (Cl-) or iodine (I-).
The typical compound is methylammonium lead triiodide (CH3NH3PbI3).
One of the main reasons why there is so much excitement about perovskite solar cells is in only a few years since they were first reported, huge progress has been made. The efficiencies of perovskite solar cells in converting sunlight to electricity went from 3.8 per cent in 2009 to 20.1 per cent in 2015, making them almost competitive with silicon solar cells with which efficiencies of about 25 per cent were achieved after decades of research and development. Even higher efficiencies could be achieved with perovskite solar cells and, combined with their very low production cost, they could soon become commercially attractive. A few start-up companies are already promising to introduce perovskites solar modules into the market by 2017.
The first incorporation of perovskite materials into a solar cell was reported in 2009 by Tsutomu Miyasaka et al from Toin University of Yokohama, Japan, in the Journal of the American Chemical Society. That solar cell had a power conversion efficiency of only 3.8 per cent. A major breakthrough came in 2012, when Professor Henry Snaith and Mike Lee at Clarendon Laboratory at the University of Oxford, UK, and Michael Grätzel at the École Polytechnique Fédérale de Lausanne, Switzerland, achieved efficiencies of almost 10 per cent using the “sensitised” TiO2 architecture with a solid-state hole transporter. Higher efficiencies were attained by replacing the transporter with an inert scaffold.
In November 2014 researchers from the Korea Research Institute of Chemical Technology achieved an efficiency of
20.1 per cent and, in December 2015, a new record of 21 per cent was achieved
The efficiency of a solar cell is closely related to the band gap – roughly the minimum energy needed to liberate electrons. Sunlight includes all wavelengths, but only certain wavelengths have enough energy to exceed the energy band gap. There is a maximum theoretical efficiency of a solar cell, called the Shockley-Queisser limit. The limit places maximum solar conversion efficiency at about 33.7 per cent assuming a single p-n junction with a band gap of 1.34eV. That means that of all power in the sunlight falling on a solar cell (about 1000W/sq m), only 33.7 per cent could ever be turned into electricity (337W/sq m). Silicon has a less favourable band gap of 1.1eV, resulting in a maximum theoretical efficiency of about 32 per cent.
The Shockley-Queisser limit only applies to cells with a single p-n junction. Cells with multiple layers can outperform that limit. Multijunction solar cells ,“tandem cells”, are one path to overcoming the Shockley-Queisser limit. They use multiple p-n junctions, each tuned to a particular frequency of the spectrum.
Several problems will have to be overcome before perovskite solar cells can compete with silicon cells, however. Next to the advantages of perovskites over silicon cells, there are also drawbacks. One of the negative aspects of perovskites is the fact that lead has been a major constituent of all highly performing perovskite cells, raising toxicity issues during device fabrication, deployment and disposal. Another major drawback of perovskite cells is they undergo degradation on exposure to moisture and ultraviolet radiation. There is also a need to scale up perovskite solar cells from laboratory size small-area cells.
Researchers around the world are trying to overcome those drawbacks and to take advantage of the exceptional properties of perovskites.
A pioneer in the field of photovoltaics is Professor Martin Green, of the University of New South Wales’ Australian Centre for Advanced Photovoltaics. Called “the father of photovoltaics”, he has led the world in silicon cell efficiency since 1983. “The rapid emergence of perovskites has been astonishing, although problems with stability are both unresolved and serious and are likely to frustrate any near-term commercialisation efforts,” he said.
Rather than competing commercially, silicon and perovskite solar cells could operate together, with greater efficiency than when operating alone. A silicon cell has only one band gap, but when fabricating a perovskite cell, varying band gaps are possible. Each band gap may absorb a different part of the sun’s spectrum. Combining a silicon cell with a perovskite cell, or even combining two perovskite cells, may result in higher efficiencies. Several researchers have tried to obtain higher efficiencies that way.
Professor Green and his team have been investigating tandem cells consisting of a perovskite cell sitting on top of a silicon cell.
“This would be a winner if the perovskite cells were stable”, he said.
“A remarkable energy conversion efficiency of 23.4 per cent is achieved when a CH3NH3PbBr3 solar cell is coupled with a 22.7 per cent efficient silicon passivated emitter rear locally diffused solar cell.
“Relative enhancements of more than 10 per cent are demonstrated by CH3NH3PbBr3/CH3NH3PbI3 and Ch3NH3PbBr3/multicrystalline-screen-printed-Si spectral splitting systems with tandem efficiencies of 13.4 per cent and 18.8 per cent respectively. The former is the first demonstration of an all perovskite split spectrum system.”
There have been few studies attempting to fabricate large-area perovskite solar cells. The reported efficiencies are usually based on devices of about 0.1sq cm or even smaller. This has led people to question the reported advances, partly because the measurement errors tend to increase as the active cell area becomes smaller. Therefore, the development of perovskite solar cells with active areas on the square-centimetre scale is strongly recommended.
A group from the National Renewable Energy Laboratory in Golden, Colorado, US, and Brown University in Providence, Rhode Island, US – led by the university’s Professor Nitin P Padture – has tackled the question of producing higher efficiency square-centimetre CH3NH3PbI3 perovskite solar cells. The group has demonstrated a solution-based synthetic route for the formation of uniform, large-grain and high-crystallinity planar perovskite films. They achieved efficiencies exceeding 15 per cent.
It will be difficult for perovskite solar cells to displace silicon cells. These represent a very mature, well-known and successful technology. However, the next few years, will see much R&D on organic-inorganic halide perovskite solar cells, with ongoing improvements in efficiency.
It is a fair prediction there will be many efforts toward the commercialisation of perovskite photovoltaics in the coming years. Watch this space.
By Paul Grad, engineering writer