By Sam Stranks
A new material has entered the emerging low-cost photovoltaics arena and is threatening to blow much of the existing competition away. Power conversion efficiencies (how efficiently incident sunlight is converted to electrical power) in perovskite-based solar cells have increased from a starting point of 3.8% in 2009 to a staggering 19.3% by May 2014. Such rapid improvement is unprecedented, and signs are promising for perovskite solar cells to very shortly exceed the efficiencies of established thin film technologies such as cadmium telluride (record certified efficiency 20.4%), CIGS (20.8%), and, more pertinently, to approach those of the market-dominating crystalline silicon solar cells (25%), all at a fraction of the cost. This breakthrough is a useful spark for the emerging PV field, and the excitement is widespread – so much so that the editors of the journal Science selected perovskite-based solar cells as runner-up for Breakthrough of the Year 2013, and the journal Nature highlighted these materials in their summary of what’s in store for science in 2014.
The perovskite family of materials is itself not new. Perovskite, named after Russian mineralogist Lev Perovski, refers to any material sharing the crystal structure of calcium titanate (CaTiO3), based on the general formula ABX3. When used in solar cells, A is typically a small carbon-based (organic) molecular cation, B is a metal ion such as lead, and X is a halide such as iodide, bromide or chloride. These “organo-metal halide” perovskites were studied extensively throughout the 1990s but were overlooked for solar cells until 2009, when researchers at the Toin University of Yokohama used these materials in liquid electrolyte dye-sensitised solar cells. However, the liquid electrolyte dissolved the perovskite, rendering the solar cells highly unstable. In 2012, our group in Oxford, at the same time as researchers at École polytechnique fédérale de Lausanne (EPFL) in Switzerland and Sungkyunkwan University in Korea, replaced the problematic liquid component with a stable solid-state version, paving the way for dramatic improvements in efficiency.
Organo-metal halide perovskites have several key advantages over traditional solar cell materials such as crystalline silicon, which generally require intensive, high-temperature processing. Firstly, these perovskites can be processed using very simple, low-cost methods – the perovskite precursor solution, containing a mixture of inexpensive salts, is simply cast onto the bottom electrode of the solar cell, heated gently to form the crystalline perovskite material, and sandwiched with a top electrode. This allows ‘printing’ of these solar cells using a large inkjet-style printer. We can also process them on flexible substrates, such as plastic or fabric, opening up a number of portable electronics applications. Using some tricks, we can make the solar cells semi-transparent enough to be used on window panes. Secondly, the constituent elements in the ABX3 crystal structure can be widely tuned to give a range of desired optical and electrical properties. Tweaking the halide composition, for example, allows the solar cell color to be tuned to any color of the rainbow. This gives them the huge advantage of being able to be fabricated in aesthetically-pleasing ways. This means consumers may be more willing to put them on their roofs, and building-integrated PV applications become attractive. They can even be processed as additional layers on top of established technologies such as silicon, where we can use their color tunability to harvest more of the solar spectrum and improve the current state-of-the-art panels.
While the applications are promising, there are a number of challenges these materials need to overcome before we see widespread deployment. We need to prove that these solar cells, assembled as modules, can last for several years under illumination and in the elements – the silicon industry standard is currently 20-30 years. These perovskites are particularly sensitive to moisture, so they need to be very well sealed from the atmosphere to prevent premature degradation. Presently there is insufficient stability data to indicate how long they will last, but ongoing laboratory tests on well-sealed devices under simulated sunlight over 1000s of hours are very encouraging. Another issue is the presence of trace amounts of lead in these materials. While it is perfectly possible to contain the lead throughout the entire life cycle of the panel, this low toxicological risk could still be problematic for the technology, particularly if policy stipulates against it. However, just last month both our group and researchers at Northwestern University reported the first lead-free (tin-based) perovskite solar cells, albeit with much lower stability and efficiency than their lead-based counterparts. These results are particularly promising for the technology, and with optimisation to improve stability and performance, we could see the tin analogues surpassing the lead-based materials.
With such an unprecedented increase in solar cell efficiency after only a few years of academic research, the future is certainly looking bright for these materials. The sky really does seem to be the limit – recent reports have shown that these perovskites can emit light very efficiently, also opening up light-emitting diodes (LEDs) and lasers as potential applications. By further exploiting their remarkable properties and improving their stability, we could see perovskites playing a major role in an electrified future world.
Dr Sam Stranks is a Junior Research Fellow at Worcester College, Oxford, and a Lecturer in Physics at Corpus Christi College, Oxford. He is currently working with Prof. Henry Snaith in the Department of Physics at the University of Oxford, and will commence a Marie Curie Fellowship at MIT in October 2014.
Image Credit: Oxford PV