Researchers at Zhengzhou University in China have reviewed the potential applications of zwitterions in perovskite solar cells, concluding that the compounds could improve device efficiency and stability through mechanisms extending beyond conventional defect passivation.

Zwitterions are electrically neutral molecules containing both positively and negatively charged functional groups. “This dual-charge structure gives zwitterions a broader interaction scope than conventional Lewis acid/base passivators, single ionic additives, or some polymer-based modifiers,” the scientists explained. “Depending on their molecular structure and location in the device, zwitterions can participate in defect passivation, crystallization modulation, and interfacial energy-level tuning within one integrated molecular unit.”

Previous research has primarily focused on using zwitterions to passivate ionic defects in perovskite absorbers. More recent studies have explored their incorporation into precursor solutions and application at device interfaces to reduce non-radiative recombination, suppress ion migration, and improve charge transport.

According to the researchers, the effectiveness of these mechanisms depends on the molecular structure, incorporation method, and location within the device, making zwitterions promising multifunctional additives rather than single-purpose passivation agents.

Bulk engineering

The review identified three main functions of zwitterions within the perovskite absorber: defect passivation, crystallization regulation, and phase stabilization.

The researchers explained that the oppositely charged groups can interact with defects such as undercoordinated lead ions, halide vacancies, and defects associated with organic cations, reducing charge-carrier trapping and non-radiative recombination.

Betaine-type zwitterions have been the most extensively investigated, although amino-acid-based and other functionalized compounds have also shown promising results. In tin-based perovskites, for example, the addition of L-asparagine increased cell efficiency from 10.54% to 13.31%, while formamidine sulfinic acid enabled an efficiency of 22.06% in a wide-bandgap device.

Zwitterions can also influence precursor coordination, nucleation, and crystal growth, producing more uniform films with fewer pinholes and improved crystallinity. Depending on their chemical structure and processing conditions, they can accelerate or delay crystallization and modify grain size and orientation.

The review also highlighted their ability to stabilize the photoactive black phase, particularly in cesium- and formamidinium-based perovskites. Other reported benefits include reduced ion migration, improved lattice integrity, suppressed tin oxidation, and reduced residual mechanical stress.

Interface engineering

The researchers also examined the use of zwitterions at interfaces between the perovskite absorber and the electron transport layer (ETL) or hole transport layer (HTL), where they can simultaneously passivate defects and improve energy-level alignment.

At ETL/perovskite interfaces involving tin oxide (SnO2), titanium dioxide (TiO2), and zinc oxide (ZnO), zwitterions can interact with defects on both sides of the interface. Their molecular dipoles can also modify work functions and reduce electron extraction barriers, while influencing perovskite nucleation and crystal growth.

The review identified liquid-type zwitterions as a potential solution to the poor solubility and processing limitations of conventional solid-state compounds.

At HTL/perovskite interfaces, zwitterions can facilitate hole extraction, suppress surface recombination, and improve chemical stability. They can be applied as surface modifiers or incorporated directly into transport materials such as PEDOT and nickel oxide (NiOx).

In one study, a zwitterionic additive increased the average efficiency of PEDOT devices from 10.29% to 14.29%. Another approach using L-tryptophan at the NiOx/perovskite interface achieved efficiencies of 23.79% and 20.41% for devices with bandgaps of 1.56 eV and 1.68 eV, respectively. The latter devices also maintained stable performance for more than 1,600 hours.

Electrode engineering

The review also explored zwitterionic interlayers at electrode contacts, where molecular dipoles can adjust metal work functions, improve energy-level alignment, and facilitate charge extraction.

In one study, a zwitterionic polymer interlayer increased cell efficiency from 17.31% to 19.16%, while a carboxylate-based compound improved efficiency from 15.08% to 17.10%.

Beyond contact optimization, zwitterions can limit ion migration and electrode corrosion, particularly at silver contacts. The researchers highlighted a naphthalene diimide-based zwitterion capable of capturing both silver and iodide ions, thereby suppressing interfacial degradation.

Looking forward, the academics said future research on zwitterions should develop clearer design and evaluation frameworks. “By constructing a comprehensive database encompassing molecular descriptors, processing parameters, and device performance metrics, supervised learning models can be trained to predict optimal zwitterion candidates for specific functional layers. Furthermore, generative models could be explored to propose novel zwitterionic structures beyond the existing chemical space,” they stated.

“Zwitterions may evolve from important auxiliary modifiers in current high-efficiency perovskite solar cells into a key molecular platform for advancing perovskite photovoltaics toward long-term stability, scalability, and practical application,” the scientists concluded.

Their work was presented in the paper “Research progress of zwitterions in perovskite solar cells,” which was recently published in Sustainable Chemistry for Energy Materials.