5 Things to Know about Perovskite Solar Cells

1. What makes perovskite solar cells so special?

Never before has progress been so rapid with solar cell technology as with perovskite solar cells. It was in Japan in 2009 that researchers first used perovskite material for solar cells. The solar cell itself did not yet offer much due to its low conversion efficiency (3.8%), a small surface area (0.24 cm²) and a stability of only a few minutes. But thanks to a worldwide effort, we are now able to achieve more than 25% conversion efficiency with lab cells.

The production process is also relatively simple, the cells can be used at lower light intensities, and the material is relatively cheap and available in large quantities. In addition, the solar cells can be made translucent, in different colors, and also on flexible substrates.

The material comprising perovskites is quite fascinating. The first perovskites used in research were minerals that were mined in nature. The first member of the perovskite family was calcium titanate (CaTiO₃), mined in 1839 by the German chemist Gustav Rose during an expedition in the Ural Mountains. He named the mineral "perovskite" after the Russian mineralogist Lev Perovski. All materials with the same crystal structure as calcium titanate— ^XIIA2+VIB4+X^2-₃—which were subsequently discovered or synthesized, were given the name perovskite. This family of materials is huge, precisely because of the enormous flexibility in ions that can be used in the formula. More than 90% of the metals in the Mendeljev periodic table can be used. In this way you can perfectly "tune" perovskites for certain wavelengths or applications.

Comparative graph showing the evolution of different solar cell technologies. The strong growth of perovskite-based cells is striking. (Source: EPKI White Paper)

2. Is it just a hype, or will they soon actually appear in real-life applications?

The stability of perovskite solar cells has long been a problem, and the PV industry was very skeptical about this new kid on the block. But here, too, a lot of progress has been made, and the first perovskite solar cells are expected to roll off the belt within 2 to 3 years. The EPKI White Paper states that the production capacity (worldwide) will increase from 0.4 to 1.3 GWp in that period. Dozens of companies around the world are preparing their production processes and expanding their production capacity, and both Europe and China are expected to play an important role in this new technology.

There is a standard test for (silicon) solar cells to guarantee a service life of 20 years. By 2020, perovskite cells will be able to pass this test. The question is whether this test can offer the same guaranteed 20-year lifespan for perovskite cells, since it was specifically developed for silicon cells. Therefore, in parallel, the research community needs to gain more insights into degradation mechanisms in perovskite cells in order to develop new, more suitable tests in the long run.

There are two types of perovskite-based solar cells that are worth mentioning. One is a silicon cell with a layer of perovskite on top of it, known as a tandem solar cell. Perovskite silicon solar cells are now almost at their limit in terms of conversion efficiency, and that extra perovskite layer on top of it can add a huge boost to efficiency, without drastically changing the production process. This type of cell will mainly be used in solar cell parks. For solar cell parks in less sunny areas—where the light intensity and clouds are more variable-bifacial silicon solar cells can also be used with a layer of perovskite on top. This allows you to also capture light that is reflected from the ground.

The other option—pure thin-film perovskite cells—will be used in different integrated applications, such as in cars, building materials, windows, and clothing. As this requires a completely new approach to the integration process and business model, this type of perovskite-based cells is likely to appear on the market a little later.

3. Where will these cells be produced?

Perovskite solar cells for integrated applications like cars and buildings are best produced locally because each market has its own regulations and organization that is strongly country-specific. Europe—which has a lot of expertise in the field of perovskite material and cells—has the opportunity to become an important player in this story.

For the silicon-perovskite tandem solar cells, it is possible for nations with high manufacturing costs to purchase cheaper silicon cells in other countries and then apply the perovskite finish on it locally.

4. Are perovskite solar cells really going to be cheaper than the already cheap silicon cells?

The silicon-perovskite tandem cells will be more expensive than the current silicon cells because extra process steps and material are needed compared to the standard cells. However, the gain in efficiency will compensate considerably for this extra cost, resulting in a lower cost/watt peak. Tandem solar cell modules will certainly win the race with current cheap silicon modules.

Thin-film perovskite cells have the potential to be very cheap. Of course, the final price will depend on the material used, the design of the stack and the type of process used, as well as the application and market size of this application. But as a guide, the EPKI White Paper mentions about 20 eurocents/Wp for the next 5 to 10 years with a reduction to 10 and maybe even 4 eurocents/Wp as further progress is made in the development and efficiency of the cells.

5. Does the lead in perovskite solar cells make them environmentally unfriendly?

Perovskite technology is considered the most environmentally friendly PV technology available, due to, among other things, the use of synthetic-manufactured material (which does not require mining or complex purification processes), the very small amount of material required, and the low process temperature.

The best efficiencies are achieved today with lead-containing perovskite cells. Alternatives are being considered, but they do not score very well for the time being. The amount of lead in the cells is very low: the lead-containing layers in a perovskite cell are typically about 0.3 µm thick, which translates into 1 g of lead iodide/m². This is in accordance with the RoHS directive. In addition, solutions are being sought to further minimize the very small chance that lead would end up in the environment as a result of damage to the PV cell. For example, materials can be integrated into the cell that bind with the lead in case of exposure and form water-insoluble components.

Conclusion

Perovskite-based solar cells—tandem cells with silicon or thin-film cells for integrated applications—will hit the market very soon. Both the R&D community and the PV industry will play a major role in making this a success by further improving efficiency and stability, and finding new business models to enter new markets with solar-based cars, building elements, clothing, and more. The future will be an exciting time for the PV industry.

Tom Aernouts, R&D Manager of Thin-Film PV at imec, initiated the research on thin film solar cells at imec in 1999. Up until 2006 he was a researcher working on fundamental understanding of organic solar cells and developing bulk heterojunction solar cells and fully flexible modules. Since 2006, he has been the group leader of imec's Thin Film PV group, steering imec's thin film PV research activities. An initial infrastructure upgrade in 2009 enabled processing devices up to 15 × 15 cm², and a subsequent upgrade completed in 2018 extended the device size to 35 × 35 cm². He received his MSc in semiconductor physics from the KU Leuven, Belgium, in 1998, and a PhD in Science in 2006 from the same university.