It is referred to as the field of emerging photovoltaics, and 2013 was certainly one of its most glowing years. Three parameters matter in a solar cell: its cost, efficiency of conversion (of sunlight into electricity), and stability of performance over time. With crystalline silicon cells one can tick successfully the latter two, but cost remains an issue. Alternative technologies – from thin films to organic semiconductors – have produced a variety of solutions with mixed outcomes. Perovskite, a material used in a range of applications including light-emitting diodes and non-volatile ferromagnetic memory (FeRAM), entered the field in 2009 and, in the words of a leading researcher in photovoltaics, brought in the magic.
What is special about perovskite? The term identifies any material with a specific crystal structure, that is, a prescribed way for its atomic building blocks to arrange themselves on a lattice. Each type of building block is “one knob you can adjust, so there is room for individual tunability,” observes Sam Stranks, a postdoctoral researcher in Henry Snaith’s group at the University of Oxford among the leading teams on perovskite-based solar cells.
At the core of the photovoltaic technology is the p-n junction, a boundary between two different semiconductors. The electric field at the junction guides positive and negative charges – holes and excited electrons, formed when the sunlight hits the cell – to the electrodes where they are collected to generate an electric current.
Snaith and collaborators use perovskites as light absorbers. Indeed, these are extremely efficient in harnessing sunlight and allowing for holes and electrons to travel through the cell. The figures reported by Snaith’s group in recent publications are impressive: their perovskite-based cells show efficiencies of 15%, and they are characterised by a large diffusion length – a measure of how far the charges can reach inside the material. The latter parameter is crucial, as it ensures high efficiencies without the need for increased thickness. Further to this success, the group simplified the geometry of the cells by adopting a planar configuration. Flat, thin solar cells are easier to fabricate, and this in turn reduces their cost.
Perovskite-based solar cells are efficient and inexpensive – they thus meet two of the three “golden criteria”. What about their performance over time? Snaith and coworkers published their first results on this aspect in December 2013. As the authors point out, longevity of the cells is partly an industrial task, for example in realising effective encapsulation – perovskites dislike moisture – and design of the modules. Academic research should investigate the stability of the “basic chemistry” in the cells. The findings presented in Nature Communications are promising: the photocurrent produced by the perovskite-based cells under full sunlight (that is, without ultraviolet filters, whose use is widespread but controversial) remains stable for at least 1000 hours – about 42 days. As for the conversion efficiency, it drops to around 6% within the first week but it stays constant afterwards, and this value is still higher than the figures obtained with different cells.
In a review article on perovskite-based solar cells, Henry Snaith discusses potential future developments for this technology. For example, he describes a solution where an all-perovskite cell could be used in conjunction with an ordinary silicon-based one to form a tandem structure. This “enhanced silicon technology” would further improve efficiencies (while limiting costs), and it might be more attractive to the market of photovoltaics than an entirely new product.
With a combined effort from different disciplines within academia as well as from industry, solar research based on the recent perovskite breakthrough has the potential to become, quite literally, the sunny side of scientific progress.
Thank you to Dr Stranks for the helpful discussions about solar cells and the role of perovskites. Visit the web page of Professor Snaith’s group at http://www2.physics.ox.ac.uk/research/photovoltaic-and-optoelectronic-device-group