Perovskite – the future of the solar cell

Dark-blue panels on red tiled roofs and green meadows – this is the image usually generated upon entering the term “solar energy” in an online search engine. The familiar blue solar cells made from silicon are indeed the most widespread technology for photovoltaics. For some time now, however, it has by no means been the only one. One new technology is about to get out of the starting blocks: the solar cell made from perovskite. This is the collective term for innovative materials whose crystal structure resembles that of the natural mineral also known as perovskite.

Perovskite solar cells have developed quickly. Over the past ten years, their level of efficiency has literally exploded and increased from a modest 3 % to more than 20 %. Perovskites have outstanding properties: they absorb light especially efficiently and are adept at routing the generated electricity. They are cost-effective as well as simple with respect to both production and processing. However, solar cells based on perovskites also have a significant downside: they do not yet work consistently enough and are not sufficiently durable for large-scale use. Researchers from the Swiss Federal Institute of Technology Lausanne (EPFL) and the University of Fribourg are working to improve the stability and efficiency of this promising technology.

Doping for solar cells

So-called doping is often applied here – an absolutely legitimate means of improving performance in the area of semiconductor technology. This process sees semiconducting materials enriched with a small amount of a foreign element. The progress achieved in this way is striking. For example, by doping a titanium oxide electrode with the element lithium, the researchers increased the performance of a perovskite solar cell by two percentage points to more than 19 %

Upon using neodymium instead of lithium for the doping procedure, the level of efficiency at just over 18 % was not quite as impressive. However – and this is no less important – this solar cell worked for much longer with a high output. The researchers found out that the neodymium atoms worked like a putty and filled in the cracks and fissures in the crystal lattice. This allows for so-called electron traps to be mitigated. These pitfalls disrupt the desired movement of the charge carriers and have a negative impact on the performance off a solar cell. These emerge due to, among other reasons, ultraviolet radiation that attacks the titanium oxide.

In a further study, the researchers replaced the UV-sensitive titanium oxide with another compound: tin oxide. While this is less susceptible to UV light, it has disappointed so far in terms of electrical output. “Doping” was once again the solution here. This time with the element gallium. The result impressed with a good efficiency level of 16.5 % and, in particular, an outstanding resistance to UV radiation.

Stability thanks to “butterfly molecule”

Doping is, however, not always the answer to the challenges faced by perovskite technology. For example, a further development of the EPFL does not use doping at all. It involves an organic compound with molecules that are reminiscent of a butterfly due to their complex form.

The rather esoteric-sounding task of this material is to transport “holes” – positive charges that form when electrons are knocked out of their environment via the absorption of light. The organic “butterfly molecules” allow for a similar efficiency level to that achieved by conventional materials. However, they are significantly more stable.

In the cases mentioned so far, the researchers focussed their efforts on improving electrodes and other electricity-transporting layers. Yet among the greatest successes are their developments at the heart of the solar cell – the light-absorbing perovskite. They discovered that the addition of a small amount of caesium to the organic components of the perovskite led to much purer crystals – and a peak level of efficiency of 21.1 %.

The mixture with caesium also responds more favourably to small inaccuracies in the manufacturing process. This makes the production of perovskite solar cells with efficiency levels of more than 20 % routine – a breakthrough in terms of reproducibility and a big step towards the industrialisation of this new technology.

On a direct path to becoming an industrial product

A further important stage on the way towards becoming a marketable product is the step from laboratory-scale solutions to the production of extensive solar modules. The electrical interconnecting of these modules takes place at an industrial level with the help of lasers which engrave insulating dividing lines or channels for pathways into the materials. Whether this industrial process also works for perovskites was unknown until now. With the first laser-produced solar module made from perovskite, the EPFL researchers were able to provide this evidence.

With each innovation, the objective of turning economically viable and highly efficient perovskite solar cells into a reality moves a step closer. And the new cells will not only provide technical support for the energy turnaround, but also make it more aesthetically attractive. This is because the developed materials and processes allow for discreet alternatives to the standardised blue solar cells. Michael Grätzel from the EPFL believes that the chances of a quick introduction of the perovskite solar cell are good. In particular, this is because the new developments can be integrated in existing industrial production chains. Grätzel is convinced that only in this way will Swiss companies and research groups be able to maintain their advantage in this contested international environment.