Solar perovskite finds a way to overcome one of its major commercial hurdles: light-induced degradation

A team of researchers from the Hong Kong Polytechnic University (PolyU), in collaboration with scientists from Zhejiang University, has identified one of the mechanisms that accelerate the aging of perovskite solar cells when exposed to light and has developed a solution to slow down this process. This advance addresses one of the main obstacles to the large-scale commercialization of this photovoltaic technology, which is distinguished by its high efficiency, low manufacturing cost, lightness, and versatility.

The research has revealed that the degradation of these devices does not depend solely on the defects present in the material, but also on how the small crystals that form its structure interact. Specifically, the scientists have demonstrated that imbalances in charge losses between neighboring crystals accelerate the deterioration of certain areas of the cell during its operation under sunlight.

To address this issue, the team has developed a treatment capable of repairing the defects that occur at the interfaces between these crystals and balancing the behavior of electrical charges. The treated devices achieved a conversion efficiency of 26.55% and retained more than 90% of their initial performance after more than 2,200 hours of continuous operation under illumination equivalent to one sun.

One of the most promising alternatives

Perovskite solar cells are considered one of the most promising alternatives for the next generation of photovoltaic technology. However, their performance gradually declines after prolonged exposure to sunlight, a phenomenon known as photodegradation. This loss of performance constitutes a significant barrier to their market entry, as photovoltaic panels must maintain reliable operation for many years under real-world weathering conditions.

Until now, much of the scientific effort has focused on reducing material defects to improve stability. However, research by PolyU and Zhejiang University has shifted the focus to the interactions occurring between the small crystals that make up the layer responsible for generating electricity, identifying an additional mechanism that contributes to accelerating its aging.

This layer is composed of numerous tiny crystals, known as grains, whose surfaces exhibit different orientations. In mixed-facet perovskite cells, crystals with different orientations coexist side by side and come into contact through regions known as grain boundaries.

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When light strikes the cell, the material generates electrons and holes, two types of electrical charges, negative and positive respectively, which enable the production of electricity. However, for the energy to be efficiently converted into current, these charges must be properly transported and collected. The problem arises when they become trapped in defects present at the grain boundaries between crystals, where they can recombine and neutralize. This process reduces the amount of electricity the cell can produce and accelerates its progressive degradation.

Researchers have discovered that when two common crystallographic orientations, designated (100) and (111), come into contact, charge losses are not uniformly distributed on both sides of the grain boundary. The region corresponding to the (111) orientation exhibits a higher concentration of defects, primarily vacancies left in the material's structure when specific formamidinium (FA) ions are missing.

This accumulation of defects causes an anomalous concentration of positive charges, known as holes, in that region and accelerates its degradation. The finding is particularly significant because crystals with (111) orientation are, by nature, more resistant to aging caused by light. However, the charge imbalance causes them to deteriorate faster than expected.

Thus, the study demonstrates that the stability of perovskite solar cells does not depend solely on the individual properties of each crystal, but also on how they interact with one another. A conclusion that opens a new path to understand and combat the performance loss of this technology.

A treatment to repair defects between crystals

To address this issue, the team developed a strategy based on the incorporation of a compound known as 3-fluorophenethylammonium bromide (m-F-PEABr). The size of its molecules prevents them from penetrating the interior of the crystals, causing them to naturally concentrate in the regions where the crystals join.

Once in these locations, the positively charged portion of the molecules occupies the spaces left vacant by the absence of formamidinium ions at the junctions corresponding to the (111) orientation. Simultaneously, the fluorine-containing component helps keep the compound firmly anchored within the material.

Through this intervention, the researchers manage to repair the defects and balance the charge losses between the two crystal orientations. This prevents specific areas from accumulating greater degradation and improves both the performance and stability of the cells.

Experimental results show that the treated devices achieved an energy conversion efficiency of 26.55%. Moreover, they retained more than 90% of their initial efficiency after more than 2,200 hours of continuous operation under illumination equivalent to one sun.

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This final indicator is known as the T90 operational lifetime and measures the duration for which a cell maintains at least 90% of its initial efficiency. It is one of the parameters used to assess the durability of these devices and their ability to sustain performance over extended periods.

Yang Guang has pointed out that, although perovskite solar cells have already achieved high levels of efficiency, their stability remains the primary obstacle to commercialization. According to the researcher, previous studies had mainly focused on reducing the number of defects, whereas this study demonstrates that unequal charge losses between adjacent crystals constitute another fundamental cause of their aging during operation.

The team believes that balancing these losses allows for the simultaneous improvement of both the efficiency and stability of the cells, opening a new pathway to develop more durable and commercially viable perovskite devices. This advance contributes to addressing one of the major outstanding challenges of this photovoltaic technology, although further assessment of its performance and durability under real-world conditions and at a larger scale will still be required.