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 adding a perovskite layer, light can be more efficiently harvested from a wider range of the solar spectrum.
“Potassium stabilises the perovskite bandgaps we want for tandem solar cells and makes them more luminescent, which means more efficient solar cells,” said Stranks, whose research is funded by the European Union and the European Research Council’s Horizon 2020 Programme. “It almost entirely manages the ions and defects in perovskites.”
“We’ve found that perovskites are very tolerant to additives – you can add new components and they’ll perform better,” said researcher Mojtaba Abdi-Jalebi. “Unlike other photovoltaic technologies, we don’t need to add an additional layer to improve performance, the additive is simply mixed in with the perovskite ink.”
The perovskite and potassium devices showed good stability in tests, and were 21.5% efficient at converting light into electricity, which is similar to the best perovskite-based solar cells and not far below the practical efficiency limit of silicon-based solar cells at 29%. Tandem cells made of two perovskite layers with ideal bandgaps have a theoretical efficiency limit of 45% and a practical limit of 35% – both of which are higher than the current practical efficiency limits for silicon. “You get more power for your money,” said Stranks.
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