New passivation strategy improves scalability and efficiency of perovskite solar cells

Solar cells, devices that can convert sunlight into electrical energy, are becoming increasingly widespread, with many households and industries worldwide now relying on them as a source of electricity. While crystalline silicon-based photovoltaics and other widely available solar cells perform relatively well, manufacturing them can be expensive, and they do not perform well in low-light or other unfavorable conditions.

Renewable energy engineers have thus been trying to develop alternative solar cells that are easier to manufacture, while also efficiently converting sunlight into electrical energy. These include perovskite solar cells, photovoltaic devices that rely on perovskites, light-absorbing materials with a characteristic crystal structure resembling that of the mineral calcium titanate (CaTiO₃).

Despite their good power-conversion efficiencies (PCEs), most perovskite solar cells developed so far have been found to rapidly degrade over time, due to defects on the surface of perovskite layers that can trap charge carriers. Passivation strategies, which aim to neutralize these defects, could help to overcome the limitations of existing perovskite solar cells, ultimately improving their long-term performance and facilitating their large-scale adoption.

Researchers at Westlake University, Zhejiang University and other institutes in China have recently introduced a new passivation strategy that could neutralize electronic irregularities in a wide range of perovskites and is also easier to integrate with existing solar cell manufacturing processes than other passivation methods introduced in the past. This strategy, outlined in a paper published in Nature Energy, relies on the use of a modified alcohol solvent called fluorinated isopropanol, which limits undesirable chemical reactions between a passivation agent and perovskite layers.

“Surface defect passivation is crucial for improving the efficiency and stability of perovskite solar cells,” Sisi Wang, Weizhong Tian and their colleagues wrote in their paper. “However, its reproducibility and universal applicability have not been fully explored, limiting large-scale production. We introduce a passivation strategy based on fluorinated isopropanol for full passivation of surface defects with only a thin layer of low-dimensional perovskite, which does not interfere with charge transport.”

Many previously proposed strategies to neutralize defects on perovskite layers are not universally applicable to different materials and types of solar cells. Moreover, they often do not attain consistent results on batches of the same material, thus they would be difficult to reliably implement on a large scale.

In contrast, the passivation strategy introduced by Wang, Tian and their colleagues was found to be effective for perovskite solar cells with different underlying designs and material compositions. The strategy entails rinsing perovskites with a mixture of fluorinated and standard isopropanol, solvents that collectively ensure the removal of excess chemicals, thereby enhancing the uniformity of perovskite layers.

“Fluorinated isopropanol reduces the reactivity of passivator molecules with the perovskite and allows the use of high passivator concentrations, ensuring complete defect passivation,” wrote Wang, Tian and their colleagues.

“A subsequent rinse with a solvent mixture of fluorinated isopropanol and isopropanol removes the excess passivator molecule. We demonstrate that the strategy has a broad processing window with high tolerance for deviations to the passivator concentration and is applicable to various device architectures, perovskite compositions and device areas.”

The researchers used their passivation strategy to prepare uniform perovskite films, which they then used to fabricate solar cells. When they tested the performance of these solar cells, they found that they consistently exhibited high power conversion efficiencies.

As the strategy developed by Wang, Tian and their colleagues could be easier to integrate with existing manufacturing processes and appears to attain more consistent results, it could ultimately contribute to the large-scale and cost-effective industrial production of perovskite solar cells. In the future, other research teams could apply the same strategy or adaptations of it to their own solar cells to improve their uniformity and performance.

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