A simple potassium solution could boost the efficiency of next-generation solar cells, by enabling them to convert more sunlight into electricity. Together with an international research team, led by the University of Cambridge, researchers from Delft University of Technology found that the addition of potassium iodide 'healed' the defects and immobilised ion movement, which to date have limited the efficiency of cheap perovskite solar cells. These next-generation solar cells could be used as an efficiency-boosting layer on top of existing silicon-based solar cells, or be made into stand-alone solar cells or coloured LEDs. The results are reported in the journal Nature.

Metal halide perovskites form a promising group of ionic semiconductor materials that have recently been used for solar cells. Interestingly, after just a few years of development these materials now rival traditional photovoltaic technologies, such as multicrystalline silicon solar cells, in terms of their efficiency in converting sunlight into electricity. Furthermore, these perovskites are cheap and easy to produce at low temperatures, which makes them attractive for commercial applications. Perovskite-based solar cells could be used as stand-alone solar cells, coloured LEDs or as an efficiency-boosting top cell in combination with traditional silicon-based solar cells.

Electron collection

Despite the potential of perovskites, some limitations have hampered their efficiency and stability. For efficient solar cells, light absorption has to result in free electrons which have to be collected before these decay back to the ground-state. More efficient electron collection means a higher efficiency of converting sunlight into electricity. Defects in the crystalline structure of perovskites can cause electrons to get ‘stuck’ before they can be collected. Another issue is that some of the constituents, negatively charged ions, can move around in the solar cell when illuminated. This ion movement can cause a local change in the bandgap: the colour of light that the perovskite absorbs.

Together with an international research team, led by the University of Cambridge, researchers from Delft University of Technology found that the addition of potassium iodide to perovskites ‘healed’ the defects and prevented ion movement, thereby boosting the efficiency of next-generation perovskite solar cells. The results are reported in the journal Nature. “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.”

Scalable and inexpensive

In the study, the researchers altered the chemical composition of the perovskite layers by adding potassium iodide to inks containing the perovskite precursors. The technique is scalable and inexpensive. The researchers demonstrated promising performance with the perovskite bandgaps ideal for layering on top of a silicon solar cell or another perovskite layer – so-called tandem solar cells. Tandem solar cells are the most likely first widespread application of perovskites. By adding a perovskite layer, sunlight can be more efficiently harvested from a wider range of the solar spectrum.

“Interestingly, we measured that the transport of free electrons in the perovskite layer improved on adding small amounts of potassium” said Eline Hutter of TU Delft. “This results in better electron collection and thus, explains the higher efficiencies of the solar cells.” The perovskite devices containing the potassium 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 maximum obtained efficiency of silicon-based solar cells (27%). Tandem cells comprised 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.

Support

Eline Hutter is an NWO-funded PhD candidate at Chemical Engineering, TU Delft. This research has also been supported in part by the Royal Society and the Engineering and Physical Sciences Research Council. The international team was led by the University of Cambridge, and included next to Delft University of Technology, researchers from Sheffield University in the United Kingdom and Uppsala University in Sweden.

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Reference:

Mojtaba Abdi-Jalebi et al. ‘Maximising and Stabilising Luminescence from Halide Perovskites with Potassium Passivation.’ Nature (2018). DOI: 10.1038/nature25989

For more information, contact:

Eline Hutter
Tel: +31 (0)15 2783460
E.M.Hutter@tudelft.nl

Tom Savenije
+31 (0)15 2786537
T.J.Savenije@tudelft.nl

Sarah Collins
Office of Communications, University of Cambridge
Tel: +44 (0)1223 765542    Mob: +44 (0)7525 337458
sarah.collins@admin.cam.ac.uk