Hidden cause of perovskite solar cell aging identified

Researchers from the Hong Kong Polytechnic University (PolyU) have found a cause of aging in perovskite solar cells exposed to light. The team also developed a solution that extends the cells’ lifetime, an important step towards bringing the technology to market.

The study was conducted by Professor Yang Guang, Assistant Professor of the PolyU Department of Electrical and Electronic Engineering, and is published in the journal Watt.

Perovskite solar cells are efficient, lightweight and versatile, making them 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 become a 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.

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 aging under light. However, the build-up of holes unexpectedly makes the (111) grains degrade faster, turning them into a source of aging in the cell. The findings show that a cell’s lifetime depends not only on how stable each grain is, but also on how neighbouring grains interact.

To address this, the researchers 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 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 strategy reportedly reached a power conversion efficiency of 26.55%. They also kept over 90% of their initial efficiency after more than 2200 hours of continuous operation under bright conditions. This measure, known as the T90 operational lifetime, is an indicator of long-term durability.

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