Researchers from the University of Fribourg and École Polytechnique Fédérale de Lausanne in Switzerland, Pandit Deendayal Petroleum University in India and Benemérita Universidad Autónoma de Puebla in Mexico have revealed new clues about the stability of perovskite thin films and solar cells.

“Our chief aim is to stabilize perovskite solar cells for many years and decades,” explains Michael Saliba, principal investigator at the Adolphe Merkle Institute, University of Fribourg. “Without long-term stability, any commercialization efforts will fail.”

The researchers are especially focused on an organic component, the methylammonium (MA) molecule, which is found in nearly all high-performance perovskite solar cells. MA is a highly volatile molecule that is very sensitive to heat, making it a concern for the long-term stability of perovskite solar cells. The molecule tends to degas at high temperatures, so commercial devices are being developed that avoid its use.

Most previous studies of MA stability, however, have used thin films, which can behave quite differently from actual, multi-layered devices under real-life conditions. Saliba and his colleagues investigated what are considered to be ‘weakest link’ devices based on MA (MAPbI3), which are at an increased risk of degradation. Nevertheless, these devices show remarkably good stability, retaining 100% of their initial efficiency over 1000 hours of aging under continuous illumination at 20°C. The researchers wanted to find out what effect MA has on the long-term resilience of perovskite materials at both higher (50°C, 65°C, and 95°C) and lower (-10°C) temperatures.

“We conducted a systematic study using accelerated aging conditions,” says Saliba. “We found that MA is indeed highly unstable in some cases and, therefore, a risk factor for industry. On the other hand, we also found that MA stability is improved in a multilayer device stack and when solar cells are operated in forward bias, the so-called maximum power point mode.”

The researchers’ findings indicate that MA degrades on much longer timescales than previously thought. While this does not rule out MA as a risk factor, it implies that there are other degradation mechanisms at work, such as metal migration from electrodes or the decay of materials other than perovskite in the device stack, which are under-appreciated at the moment.

“We need more research on the long-term stability of perovskite materials,” says Saliba. “We have to establish standards for analyzing degradation mechanisms within this novel class of materials. This is the only way to enable stable, low-cost, high-efficiency perovskite solar cells for a sustainable energy future.”