Coordination Chemistry Enables Stable Wide-Bandgap Perovskites for Efficient Tandem Solar Cells

A team from Beijing Institute of Technology and partner institutes has introduced a coordination-regulated strategy to stabilize wide-bandgap perovskites used in high-efficiency tandem solar cells. Their approach centers on bis(2-pyridylmethyl) sulfide (2PyS), a molecule designed to reshape the local coordination environment around Pb²⁺ during perovskite formation.

Wide-bandgap perovskites are attractive top absorbers in tandem architectures because their bandgaps can be tuned to better match the solar spectrum. Yet mixed-halide compositions inside these materials often suffer from illumination- and heat-driven halide ion migration. That migration can produce iodine-rich and bromine-rich regions, creating bandgap inhomogeneity, increasing non-radiative recombination, and accelerating efficiency loss.

The researchers traced a key instability origin to undercoordinated Pb²⁺ defects and the associated halide vacancies. These defects not only act as electronic recombination centers, but also provide structural disorder that facilitates ionic transport. Traditional post-treatment passivation can reduce some defects, but it typically offers limited control over defect formation during crystal growth—leaving photoinduced segregation hard to suppress over long operating periods.

Their solution uses 2PyS as a coordination “regulator.” Strong binding between 2PyS and Pb²⁺ suppresses the formation of undercoordinated lead sites and reduces halide vacancy density. With fewer defect-assisted pathways, halide migration is constrained, and photoinduced phase segregation is mitigated.

Computational analysis supported this mechanism by showing that 2PyS binds PbI₂ more strongly than common solvents such as DMF and DMSO, suggesting it plays a dominant role rather than merely influencing solubility. In situ photoluminescence measurements further indicated that 2PyS modulates crystallization kinetics—dampening rapid nucleation and promoting more uniform film growth.

As a result, the optimized wide-bandgap perovskite solar cells reached a power conversion efficiency of 22.21% and an open-circuit voltage of 1.20 V. Operational durability improved substantially, with the devices retaining more than 91% of their starting performance after 2000 hours of continuous operation.

To demonstrate practical tandem relevance, the semi-transparent wide-bandgap perovskite top cell was integrated with a CIGS bottom cell in a four-terminal configuration. The tandem delivered an overall efficiency of 29.71%, highlighting the strategy’s potential for stable, high-output solar conversion.

Looking ahead, the team plans to design more coordination-active molecules with tailored binding geometries and multifunctional passivation behavior. They also emphasize the need for advanced operando characterization to capture how defects and halides evolve dynamically under real illumination and thermal stress.

Keywords

Perovskite solar cells; wide-bandgap perovskites; coordination chemistry; defect passivation; halide migration; tandem photovoltaics; CIGS; molecular design