Perovskite materials are cheap and easy to produce at low temperatures, which makes them attractive for next-generation solar cells and lighting. However, defects in the crystalline structure of perovskites, called traps, can cause electrons to get ‘stuck’ before their energy can be harnessed. The easier that electrons can move around in a solar cell material, the more efficient that material will be at converting photons, particles of light, into electricity. Another issue is that ions can move around in the solar cell when illuminated, which can cause a change in the bandgap – the colour of light the material absorbs.
“So far, we haven’t been able to make these materials stable with the bandgap we need, so we’ve been trying to immobilise the ion movement by tweaking the chemical composition of the perovskite layers,” said Dr Sam Stranks from Cambridge’s Cavendish Laboratory, who led the research. “This would enable perovskites to be used as versatile solar cells or as coloured LEDs, which are essentially solar cells run in reverse.”
The international team included researchers from Cambridge, Sheffield University, Uppsala University in Sweden and Delft University of Technology in the Netherlands. They altered the chemical composition of the perovskite layers by adding potassium iodide to perovskite inks, which then self-assemble into thin films. The technique is compatible with roll-to-roll processes, which is key to a scalable and inexpensive process. The potassium iodide formed a layer on top of the perovskite which had the effect of ‘healing’ the traps so that the electrons could move more freely, as well as immobilising the ion movement, which makes the material more stable at the desired bandgap.
The researchers demonstrated promising performance with the perovskite bandgaps ideal for layering on top of a silicon solar cell or with another perovskite layer in tandem solar cells that are expected to be the first widespread application of perovskites. By