The latest report from the Intergovernmental Panel on Climate Change, published in spring 2022, makes for sober reading: 3.5 billion people are already highly vulnerable to climate change - regularly suffering water shortages, deadly heat or serious flooding [2]. The report warns that continued global warming, reaching 1.5°C in the near-term, would cause unavoidable increases in multiple climate hazards and present multiple risks to ecosystems and humans [2]. We urgently need to find sustainable ways to generate energy that allow us to move away from using fossil fuels.

The sun is the most abundant energy source in our solar system. Yet, only 2% of our worldwide electricity production currently comes from solar energy [3]. One of the challenges has been developing materials that can absorb light and convert this into electrical energy in a highly efficient way.

In 2021, together with Manager Magazin, we awarded one of our Curious Mind Researcher Awards to Prof Dr Michael Saliba from the University of Stuttgart and the Forschungszentrum Jülich for his work on perovskite solar cells, with which he recently set a new energy efficiency world record. Since its discovery in 2009, perovskite has been further developed with unprecedented speed, and this material now shows immense promise for transforming the sustainable energy and semi-conductor industries.

A solar cell is a device that converts the energy of light into electricity through a phenomenon known as the photovoltaic effect. Most solar cells are made of silicon and they are assembled together to form photovoltaic modules, or as we more commonly know them - solar panels.

In 2009, a new photovoltaic material was discovered – a so-called perovskite semiconductor that initially had modest power conversion efficiency (PCE) of just 3.8% [4] but after rapid development this material has reached a current world PCE record of 25.7% [5], an unprecedented leap in materials science.

This high performance has been attributed to exceptional properties of perovskite materials, such as their high absorption over the visible light spectrum. This has created great promise for highly sophisticated solar cells, lasers or miniaturized light sensors.

Unlike other semiconductor materials that need to be manufactured in a pure and clean environment, perovskites can be produced from simple inexpensive solution processing in a less pristine environment and still produce a semiconductor with remarkable properties. For example, they can be processed on a foil that is bent to produce flexible, lightweight and portable solar cells, enabling versatile new applications such as solar cells incorporated into clothing for charging a mobile phone.

Another advantage is that perovskite materials can be used in a semiconductor sandwich, allowing its use with an existing solar cell. “For example, if you have a silicon cell at the bottom of your semiconductor and a perovskite cell at the top, the perovskite uses the blue light in the sun’s spectrum. But it’s transparent for the red light, whereas the silicon cell at the bottom is very efficient at turning the red light into electricity. Using them together can allow solar cells to harness and transform more sunlight into sustainable electricity,” explains Saliba.

“The question is, can this be a game-changer that allows us to go beyond current solar energy electricity production - to deliver 40%, 50% or even 100% of the world’s electricity for our sustainable energy future? But to realize the potential of perovskite material we need to find a way to stabilize it, which has been a major challenge for the field,” says Saliba.

Perovskites are any material composed of a monovalent cation (A), a divalent metal (M) and an anion, usually a halide (X) – to a formula of AMX3. Their simplest form, with single cations, for example, struggle to reach top efficiencies consistently.

These materials are also sensitive and can degrade on contact with moisture, heat – and even light – which is far from ideal for a solar cell that needs to operate in exactly these conditions. Specifically, some formulations crystallize into a photo-inactive, non-perovskite ‘yellow-phase’ or a photo-active perovskite ‘black phase’ that is sensitive to solvents or humidity. Saliba’s team set out to overcome this challenge, to translate perovskite from a promising academic discovery to a material that could be realistically used in an industrial setting.

One of their key developments was a new perovskite composition that could sustain performance at high temperatures. In one study, they achieved this by using a triple cation mixture, where cesium is used in addition to formamidinium (FA) and methylammonium (MA). The addition of a small amount of cesium was enough to prevent the shift of the complex into the photo-inactive ‘yellow-phase’ and the use of all three cations yielded stabilized PCEs exceeding 21% after 250 hours in operational conditions - including fluctuations in temperature, solvent vapors and heating protocols [6].

In a second study, the addition of rubidium, improved performance even further [7]. Polymer coated perovskite solar cells containing rubidium as well as cesium maintained 95% of their initial performance at 85°C for 500 hours under full illumination.

“What we’ve found through careful screening is that alkali metals in the first periodic group such as cesium and rubidium are extremely helpful for replacing parts of the organic components and stabilizing the entire material at elevated temperatures,” says Saliba. “But a second thing we discovered is that solar cells are made of several layers which can negatively influence each other if not designed carefully.”

This Achilles heel in the perovskite solar cell is due to a ‘hole transporter’ material, one of the mentioned layers. It extracts the positive electrical charge from the active light absorber (the perovskite) and transmits it to an electrode. The most commonly used material for the hole transporter becomes permeable at high temperatures. It allows the metal electrode (needed to transport the electricity) to diffuse into and destroy the perovskite layer. To prevent this, Saliba’s team molecularly engineered a new hole transporter material and used this as a buffer layer to prevent metal leaching. Not only does this protect the perovskite at long-term high temperatures, but it also dissolves in more environmentally friendly solvents and costs about a fifth of the price of the original material to produce [8,9].

“Now we have found a way to stabilize the perovskite cells, the potential impact on industry is tremendous,” says Saliba. “For silicon power plants, for example, we just add one more item to the assembly line. This can increase the efficiency of the overall solar cell at a small increase in cost. We are making an economic difference for industry while doing something good for the planet.”

Other applications of perovskite solar cells also show promise. The ability to manufacture a thin solar cell that is lightweight, for example, means that these cells could be exported to sun-rich developing countries where regions can benefit from autonomous energy production. As semiconductors, perovskite cells could be used in several other areas – as hard-drives for storage or as photodetectors – and they are already being explored as materials for LEDs.

“One thing to understand about solar cell research is that we have recently surpassed the threshold of 400 ppm CO2 concentration in the atmosphere, which means the clock is ticking, the planet is heating up. We need to stop burning fossil fuels and transition towards regenerative energy sources,” says Saliba. “No other materials have advanced as rapidly as perovskites. It took silicon over 40 years to obtain similar efficiencies. We hope this unprecedented innovation can be rapidly translated into other areas too. This kind of disruptive technology is what we need if we want to be able to act within the next decade.”

For his work in advancing perovskite solar cells, Saliba was awarded the Curious Mind award in the "Materials and Active Substances" category. The Curious Mind Researcher Award recognizes young scientists under the age of 40 whose work not only demonstrates the highest level of academic excellence, but also indicates that it will provide impetus for the future of the global economy.

On receiving his award, Saliba told us: ”The recognition is a huge encouragement and shows that research on renewable energy is highly relevant. The award event also provided us with valuable feedback and contacts for future collaborations. Most importantly, the great effort and continuous hard work of the team at the Institute for Photovoltaics is rewarded. It is my privilege to work with such highly motivated researchers from so many backgrounds.”

^{[1] Mertens, K. Photovoltaics Fundamentals, Technology and Practice. (2018; second edition) Wiley. ISBN: 9781119401049
[2] Intergovernmental Panel on Climate Change, 2022. Climate Change 2022: Impacts, Adaptation and Vulnerability. [online] Available: https://www.ipcc.ch/report/ar6/wg2/downloads/report/IPCC_AR6_WGII_SummaryForPolicymakers.pdf
[3] https://ourworldindata.org/renewable-energy
[4] Kojima A, Teshima K, Shirai Y, Miyasaka T. Organometal halide perovskites as visible-light sensitizers for photovoltaic cells. J Am Chem Soc. 2009;131(17):6050-6051. doi:10.1021/ja809598r
[5] National Renewable Energy Laboratory, Best Research-Cell Efficiencies chart; https://www.nrel.gov/pv/cell-efficiency.html
[6] Saliba M, Matsui T, Seo JY, et al. Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency. Energy Environ. Sci., 2016,9, 1989-1997. doi: 10.1039/C5EE03874J
[7] Saliba M, Matsui T, Domanski K, et al. Incorporation of rubidium cations into perovskite solar cells improves photovoltaic performance. Science. 2016;354(6309):206-209. doi:10.1126/science.aah5557
[8] Saliba, M., Orlandi, S., Matsui, T. et al. A molecularly engineered hole-transporting material for efficient perovskite solar cells. Nat Energy 1, 15017 (2016). doi.org/10.1038/nenergy.2015.17
[9] Matsui T, Petrikyte I, Malinauskas T, Domanski K, Daskeviciene M, Steponaitis M, Gratia P, Tress W, Correa-Baena JP, Abate A, Hagfeldt A, Grätzel M, Nazeeruddin MK, Getautis V, Saliba M. Additive-Free Transparent Triarylamine-Based Polymeric Hole-Transport Materials for Stable Perovskite Solar Cells. ChemSusChem. 2016 Sep 22;9(18):2567-2571. doi: 10.1002/cssc.201600762.}