The daily input of solar energy to the earth's surface is enough to fulfil our energy needs many times over, but cheap and efficient ways of converting it, especially to electricity, have remained elusive. Yes, there is a lot of photovoltaic material installed around the world today – more than 100 gigawatts – but the efficiency of conversion to kilowatt hours is relatively poor, usually 15% or less. More than 85% of the photovoltaics (PV) used today are made from crystalline silicon, but scientific research continues into new materials that could do the job better. The criteria are greater efficiency, with cheap materials that are readily available, solid, durable under prolonged exposure to sunlight and weather, and, if possible, fairly transparent. There have been many false dawns. A number of these have scored high on efficiency, but have used materials so exotic that any scale-up would be limited by availability and cost.
Now there is excitement, and it is over a class of materials known as perovskites. Perovskite is the term for a particular mineral crystal structure, most commonly a calcium titanium trioxide mineral, and is applied to anything that adopts this same structure. Perovskite materials for solar cells were first reported in 2009 (organolead halides, in case you are interested), but they were very low efficiency and had to remain liquid in use. Not promising. But it caught the attention of several research groups, in particular that of Henry Snaith at Oxford, and Andrew Rappe at the University of Pennsylvania. By doing some clever polymer chemistry, they were able to make solid perovskite solar cells, and then, in only 18 months, engineered these up to efficiencies of 16%. It took decades of research on silicon to get such improvements. But the polymer was expensive and complex. Still, Science magazine classed perovskite solar cells as one of the breakthroughs of 2013.
The great solar cell scientist and inventor Michael Graetzel in Lausanne was intrigued, and figured out just how these cells were transporting the electrons, and how that polymer worked. With that information, it has recently been possible for scientists at Notre Dame University in the US to discover that the complex polymer could be replaced by something as simple as copper iodide. OK, their first cell is not yet up to the 16% efficiency, but there was a benefit: while the polymer based cells declined in current over time, the copper iodide ones did not.
So perovskite PV is at the point of, perhaps, maximum optimism. There are predictions that the efficiencies could reach 50%, the costs could fall to well below where silicon might get to in 10 years, and the science points to it being desirable to make these cells thicker, rather than as thin films, meaning they can be suitable as window or roofing materials. And the prototype cells produced have partial transparency.
There are already companies working on commercialising perovskite. Snaith has formed Oxford PV, and has already attracted £2m of start-up funding. Perovskites are the clean tech material development most worth watching right now.
Bernie Bulkin is a director of Ludgate Investments Ltd and of HMN Colmworth Ltd. He was chair of the Office of Renewable Energy for the UK Government from 2010-2013, and a member of the UK Sustainable Development Commission. He was formerly chief scientist of BP.