With predictions of up to 50% efficiency and named one of the breakthroughs of 2013, perovskites are the clean tech material development to watch right now. 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. . . .