Small-molecule ionic liquids are frequently used as efficient bulk phase modifiers for perovskite materials. However, their inherent characteristics, such as high volatility and ion migration, pose challenges in addressing the stability issues associated with perovskite solar cells (PSCs). Recently, researchers at China's Northwestern Polytechnical University and CNPC Tubular Goods Research Institute designed improved poly ionic liquids (ILs) with multiple active sites as efficient additives for perovskite materials.
The team's recent work shows how additive engineering with a polymerized ionic liquid to the metal halide perovskite material can improve the solar cell's function, helping to pave the way for the adoption of perovskite solar cells.
The scientists explained that common solution processing methods for fabricating perovskite layers introduce many defects in both the bulk and surface of the perovskite layer, that pose a significant constraint on the overall performance of the devices. To improve the ionic liquids added to perovskite solar cells, researchers can create polymerized or poly ionic liquids. Polymers are added to ionic liquids, enhancing the properties of the ionic liquid. In this study, the researchers synthesized a poly ionic liquid called poly4-styrenesulfonyl(trifluoremethylsulfonyl)imidepyridine, or PSTSIPPyri for short.
The addition of PSTSIPPyri to the metal halide perovskite solar cell reportedy has many benefits. It can prevent halide ion migration, which helps maintain the crystal structure, and facilitate the fixation of organic and halide ions, which improves the solar cell's stability.
"To date, researchers have devoted considerable attention to the meticulous selection of additives that enhance the performance of perovskite solar cells. Among these, ionic liquids have received widespread attention. Ionic bonds in ionic liquids tend to be stronger and more stable, and they offer various tunable properties, including viscosity, polarity, and conductivity," said Xuanhua Li, a researcher at Northwestern Polytechnical University. "This tunability makes it possible to fine-tune the ionic liquid properties to meet the specific requirements of the perovskite film, thereby optimizing device performance."
To test the success of the addition of PSTSIPPyri, researchers conducted comprehensive studies of the perovskite solar cells with the poly ionic liquid, especially for factors that are essential for the performance of the solar cells. The perovskite films were aged for 300 hours at 85°C and 60% relative humidity. The enhanced perovskite film had a slower rate of change than the perovskite film with the poly ionic liquid. In a high humidity, high heat environment, it also maintained 84.5% of its efficiency after 1000 hours, while the control perovskite solar cell only maintained 43.6% of its initial efficiency.
Since perovskite solar cells are often less durable than other alternatives, it was important to test the long-term durability of the perovskite solar cell with the addition of PSTSIPPyri. With the poly ionic liquid, the perovskite solar cell maintained 87.6% of its power conversion efficiency after 1,500 hours of continuous light, compared to the control which only maintained 61.1% of its power conversion efficiency.
"Incorporating PSTSIPPyri as an additive leads to a significant enhancement in the power conversion efficiency of inverted perovskite solar cells from 22.06% to 24.62%. They also demonstrate excellent long-term operational stability," said the team. "This strategy illustrates the potential of poly ionic liquids as a promising additive for perovskite solar cells, offering both high performance and stability."
