An international team of researchers led by the University of Cambridge found that a simple potassium solution could boost the efficiency of perovskite-based solar cells, by enabling them to convert more sunlight into electricity. The addition of potassium iodide seems to have a 'healing' effect on the defects and immobilized ion movement, which to date have limited the efficiency of perovskite solar cells.
Tiny defects in the crystalline structure of perovskites, called traps, can cause electrons to get 'stuck' before their energy can be harnessed. The easier it is for electrons to move around in a solar cell material, the more efficient that material will be at converting photons 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 color of light the material absorbs.
In the study, the researchers 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 means it is scalable and inexpensive. The potassium iodide formed a 'decorative' 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 immobilizing 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 ' so-called tandem solar cells. 'Potassium stabilizes 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 Program. '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 first author Mojtaba Abdi-Jalebi, a PhD candidate at the Cavendish Laboratory who is funded by Nava Technology Limited. '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, which is (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.
The research has also been supported in part by the Royal Society and the Engineering and Physical Sciences Research Council. The international team included researchers from Cambridge, Sheffield University, Uppsala University in Sweden and Delft University of Technology in the Netherlands.
