A research team at The Hong Kong Polytechnic University (PolyU) has found a previously overlooked cause of ageing in perovskite solar cells exposed to light. The team has also developed a solution that effectively extends the cells’ lifetime, an important step towards bringing the technology to market. The study was conducted by Prof. YANG Guang, Assistant Professor of the PolyU Department of Electrical and Electronic Engineering, together with a team from Zhejiang University, and has been published in the international journal Watt. Prof. Nam-Gyu PARK of Sungkyunkwan University, Korea, a pioneer in perovskite photovoltaics, commended the work in a research highlight in the same journal.
Perovskite solar cells are efficient, low-cost, lightweight and versatile, and are widely seen as one of the most promising next-generation solar technologies. However, their performance gradually declines after long exposure to sunlight, a process known as photodegradation. Solar panels must work reliably outdoors for many years, so photodegradation has long been a major barrier to large-scale perovskite solar cell commercialisation.
The layer of a perovskite solar cell that generates electricity comprises countless tiny crystals called grains. The surface of each grain has a particular orientation, known as its crystal facet. In mixed-facet perovskite solar cells, grains with different facets sit side by side. Each facet has its own properties, which can affect how charges move, how defects form and how the material ages under light. The places where grains meet are called grain boundaries.
When the cell absorbs sunlight, it produces negatively charged electrons and positively charged holes. Ideally, the two collect to form an electric current, but if they meet defects at grain boundaries, they can become trapped and cancel each other out. This wastes solar energy and gradually damages the cell.
The team found that in mixed-facet structures, when grains with two common facets, (100) and (111), sit next to each other, charge losses on the two sides of the boundary between them are uneven. The (111) side of the boundary has more defects. Most of these are vacancies: gaps left where formamidinium (FA) ions should be in the material. As a result, an abnormal build-up of holes develops on that side.
The (111) facet is naturally more resistant to ageing under light. However, the build-up of holes unexpectedly makes the (111) grains degrade faster, turning them into a source of ageing in the cell. The finding shows that a cell’s lifetime depends not only on how stable each grain is, but also on how neighbouring grains interact.
To address this source of degradation, the team added a compound called 3-fluorophenylethylammonium bromide (m-F-PEABr). Its molecules are too bulky to enter the grains, so they naturally collect at the grain boundaries. The positively charged end of each molecule takes the place of the missing FA ions and fills the vacancies at the (111) boundaries. Meanwhile, the fluorine-containing part holds the molecule firmly in place. Once the boundaries are repaired, charge losses on both sides become balanced, so no single spot becomes a focal point for degradation.
Cells treated with this targeted strategy reached a power conversion efficiency of 26.55%. They also kept over 90% of their initial efficiency after more than 2,200 hours of continuous operation under light equivalent to one sun. This measure, known as the T90 operational lifetime, is a key indicator of long-term durability.
Prof. Yang said, “Perovskite solar cells are already highly efficient, but stability is still the biggest barrier to commercialisation. Previous research has mostly focused on reducing the number of defects. We have shown for the first time that uneven charge losses between neighbouring grains are a key cause of ageing while the cells are working. By balancing the charge losses between grains, we can improve both the efficiency and the stability of the cells. This offers a new way of understanding cell ageing and opens a new path for developing more durable, commercially viable perovskite solar cells.＂
