Perovskite solar cells are considered one of the most promising areas in solar energy: they are efficient, cheap, lightweight and can be flexible. However, under light their performance gradually declines - a process called photodegradation. For mass use, panels must work reliably outdoors for years, so photodegradation remains the main obstacle to commercialization.
A research group led by Yang Guang, an associate professor in the Department of Electrical and Electronic Engineering at Hong Kong Polytechnic University (PolyU), together with colleagues from Zhejiang University, discovered a previously overlooked cause of aging. The work was published in the journal Watt, and its results were commented on by Professor Nam-Gyu Park of Sungkyunkwan University (South Korea), a pioneer in the field of perovskite photovoltaics.
The electricity-generating layer consists of many tiny crystals - grains. The surface of each grain has a certain orientation, called a crystal facet. In cells with mixed facets, grains with different facets are adjacent, and the places where they meet are called grain boundaries. When sunlight is absorbed, negatively charged electrons and positively charged holes are formed. Ideally they create current, but at defects at grain boundaries they can be trapped and annihilate each other, losing energy and gradually damaging the cell.
The team found that when grains with two common facets - (100) and (111) - are nearby, charge losses on opposite sides of the boundary turn out to be uneven. The (111) side has more defects, mainly vacancies - empty spots where formamidinium (FA) ions should be. As a result, an anomalous accumulation of holes occurs there. The (111) facet by itself is more resistant to light, but because of the hole accumulation it unexpectedly begins to degrade faster, becoming a source of aging for the entire cell. This shows that service life depends not only on the stability of each grain but also on the interaction of neighboring grains.
To eliminate this cause of degradation, the researchers added the compound 3-fluorophenylethylammonium bromide (m-F-PEABr). Its molecules are too large to penetrate inside the grains, so they accumulate at the boundaries. The positively charged end of the molecule takes the place of the missing FA ions and fills vacancies at the (111) boundaries, while the fluorine-containing part firmly holds the molecule. After this treatment, charge losses on both sides are equalized, and no spot becomes a focus of destruction.
For the treated cells, the power conversion efficiency reached 26.55%. In addition, after more than 2,200 hours of continuous operation under illumination equivalent to one sun, they retained over 90% of their initial efficiency. This indicator, called the T90 operating lifetime, is considered a key measure of durability.
"Perovskite solar cells are already very efficient, but stability remains the main barrier to commercialization. Previous studies mainly focused on reducing the number of defects. We have shown for the first time that uneven charge losses between neighboring grains are a key cause of aging during operation," Yang Guang noted. "By balancing charge losses between grains, we can improve both efficiency and stability. This provides a new understanding of cell aging and opens the way to creating more durable, commercially viable perovskite solar cells."
It should be emphasized: this concerns laboratory samples, not finished panels available for purchase. It is not yet clear how the technology will behave in real conditions and in mass production.