Researchers at Zhengzhou University in China have reviewed the use of zwitterions in perovskite solar cells, concluding that these electrically neutral molecules could contribute to device efficiency and stability through several mechanisms beyond conventional defect passivation.
Zwitterions contain both positively and negatively charged functional groups. According to the researchers, this dual-charge structure allows them to interact with defects, influence crystallization, and tune energy levels at interfaces within a single molecular unit. Their effectiveness depends on the molecular structure, the incorporation method, and the position within the device.
In the perovskite absorber, zwitterions can interact with undercoordinated lead ions, halide vacancies, and defects associated with organic cations. These interactions may reduce charge-carrier trapping and non-radiative recombination. The review notes that betaine-type compounds have been widely studied, while amino-acid-based and other functionalized zwitterions have also produced promising results.
The reported performance improvements include an increase in efficiency from 10.54% to 13.31% in a tin-based perovskite cell using L-asparagine. Formamidine sulfinic acid enabled an efficiency of 22.06% in a wide-bandgap device. Zwitterions can also affect precursor coordination, nucleation, and crystal growth, helping produce more uniform films with fewer pinholes and improved crystallinity.
The compounds may also stabilize the photoactive black phase in cesium- and formamidinium-based perovskites. Other reported effects include reduced ion migration, improved lattice integrity, suppressed tin oxidation, and lower residual mechanical stress.
At the interfaces between the perovskite layer and electron or hole transport layers, zwitterions can passivate defects while improving energy-level alignment. At interfaces involving tin oxide, titanium dioxide, or zinc oxide, their molecular dipoles can modify work functions and reduce electron extraction barriers. The review also points to liquid zwitterions as a possible way to address the poor solubility and processing limitations of some solid compounds.
At hole-transport interfaces, zwitterions can facilitate hole extraction, suppress surface recombination, and improve chemical stability. One reported additive increased the average efficiency of devices using PEDOT from 10.29% to 14.29%. Another approach based on L-tryptophan at a nickel oxide/perovskite interface achieved efficiencies of 23.79% and 20.41% for devices with bandgaps of 1.56 eV and 1.68 eV, respectively. Those devices maintained stable performance for more than 1,600 hours.
Zwitterionic interlayers can also be used at electrode contacts. There, molecular dipoles may adjust metal work functions, improve energy-level alignment, and support charge extraction. In the studies reviewed, one zwitterionic polymer raised efficiency from 17.31% to 19.16%, while a carboxylate-based compound increased it from 15.08% to 17.10%.
The researchers further highlighted the potential of zwitterions to limit ion migration and electrode corrosion, particularly at silver contacts. A naphthalene diimide-based zwitterion was reported to capture both silver and iodide ions, helping suppress degradation at the interface.
The review calls for clearer design and evaluation frameworks, including databases combining molecular descriptors, processing parameters, and device-performance data. The researchers suggest that supervised learning could help identify suitable zwitterions for specific functional layers, while generative models could propose new molecular structures.
The work, titled “Research progress of zwitterions in perovskite solar cells,” was published in Sustainable Chemistry for Energy Materials.
Sources: pv magazine.