Potassium Could Boost Efficiency of Next-Generation Solar Cells

An international research team led by the University of Cambridge discovered that the incorporation of potassium iodide ‘healed’ the defects and immobilized ion movement, which, thus far, have restricted the efficiency of economical perovskite solar cells. These next-generation solar cells could be employed as an efficiency-boosting layer on top of present silicon-based solar cells, or be made into standalone solar cells or colored LEDs. The results are reported in the Nature journal.

The solar cells used in the research are based on metal halide perovskites – a potential group of ionic semiconductor materials that in only a few short years of development current rival commercial thin film photovoltaic technologies in relation to their efficiency in changing sunlight into electricity. Perovskites are inexpensive and easy to manufacture at low temperatures, which makes them appealing for next-generation solar cells and lighting.

Regardless of the potential of perovskites, certain limitations have hindered their consistency and efficiency. Minuscule defects in the crystalline structure of perovskites, known as traps, can result in electrons getting ‘stuck’ before their energy can be harnessed. The easier the electrons can travel about in a solar cell material, the more efficient that material will be at changing photons, particles of light, into electricity. Another problem is that ions can travel around in the solar cell when illuminated, which can result in a change in the bandgap – the color 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. This would enable perovskites to be used as versatile solar cells or as coloured LEDs, which are essentially solar cells run in reverse.

Dr Sam Stranks, Cavendish Laboratory, University of Cambridge

In the research, the team modified the chemical composition of the perovskite layers by incorporating potassium iodide to perovskite inks, which then self-assemble into thin films. The method matches with roll-to-roll processes, which means it is scalable and economical. The potassium iodide developed a ‘decorative’ layer on top of the perovskite which had the effect of ‘healing’ the traps so that the electrons could travel more freely, as well as stopping the ion movement, which makes the material more stable at the preferred bandgap.

The team proved favorable performance with the perovskite bandgaps suitable for layering on top of a silicon solar cell or with another perovskite layer – so-called tandem solar cells. Silicon tandem solar cells are the most probable first common application of perovskites. By integrating a perovskite layer, light can be more efficiently collected from a broader 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 sponsored 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. 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.

Mojtaba Abdi-Jalebi, PhD Candidate, Cavendish Laboratory, University of Cambridge

The perovskite and potassium devices displayed good stability in tests, and were 21.5% efficient at changing light into electricity, which is analogous to the best perovskite-based solar cells and not much below the practical efficiency limit of silicon-based solar cells, which is 29%. Tandem cells composed of two perovskite layers with suitable bandgaps have a theoretical efficiency limit of 45% and a practical limit of 35% - both of which are greater than the present practical efficiency boundaries for silicon. “You get more power for your money,” said Stranks.

The study has also been supported partly by the Royal Society and the Engineering and Physical Sciences Research Council. The international team included scientists from Cambridge, Sheffield University, Uppsala University in Sweden, and Delft University of Technology in the Netherlands.

Reference:

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