A team of researchers, led by the Chinese Academy of Sciences (CAS), has developed a generalizable strategy to control crystallization kinetics in all-perovskite tandem solar cells, enabling certified power conversion efficiencies of 30.3% in rigid devices and 28.0% in flexible configurations.
Synchronized crystallization drives efficient rigid and flexible perovskite tandems. Image credit: NIMTE
All-perovskite tandems can enable high efficiency and compatibility with low-temperature solution processing. However, their performance has been consistently limited by asynchronous crystallization in multicomponent perovskite systems. This effect arises from mismatched coordination chemistry and crystallization rates among mixed halide systems and Pb²⁺/Sn²⁺ cations, leading to vertical compositional gradients, structural inhomogeneity, and elevated non-radiative recombination losses.
To address this, the CAS-led team introduced an additive engineering approach guided by hard-soft acid-base (HSAB) theory. By selecting additives with tailored chemical hardness, the researchers were able to selectively coordinate different perovskite precursor systems and synchronize nucleation and crystal growth processes across both wide- and narrow-bandgap layers.
Specifically, the team employed difluoro(oxalato)borate (DFOB⁻), a borderline base, to coordinate with wide-bandgap perovskite precursors, and tetrafluoroborate (BF₄⁻), a hard base, for narrow-bandgap systems. This dual-additive strategy effectively balanced the crystallization kinetics of PbI₂/PbBr₂ and PbI₂/SnI₂ systems, suppressing phase segregation and eliminating vertical compositional non-uniformities.
In situ optical and structural characterization confirmed that the approach promotes homogeneous nucleation and direct crystal growth, avoiding intermediate halide redistribution - a key source of defect formation and internal stress. As a result, the films exhibited reduced defect densities and suppressed ion migration, both critical for improving device efficiency and stability.
At the device level, the improvements were reflected in enhanced single-junction subcell performance. Wide-bandgap cells improved from 18.5% to 20.1% efficiency, while narrow-bandgap devices increased from 21.6% to 23.3%.
When integrated into monolithic two-terminal tandem architectures, the optimized devices achieved a certified efficiency of 30.3%, with an open-circuit voltage of 2.16 V and a fill factor of 85.2%. Flexible tandem devices also demonstrated strong performance, reaching 28.2% efficiency (28.0% certified).
Operational stability was significantly improved as well. The rigid tandem devices retained 92% of their initial efficiency after 1,000 hours of maximum power point tracking. Flexible devices maintained 95.2% of their original efficiency after 10,000 bending cycles, highlighting their suitability for lightweight and flexible photovoltaic applications.
“The findings provide a pathway to simultaneously improve efficiency and durability in both rigid and flexible devices, thereby advancing the development of lightweight, scalable photovoltaic technologies,” the scientists pointed out.
The work establishes chemical hardness matching as a universal principle for regulating crystallization in compositionally complex perovskite systems, offering a pathway toward scalable, high-efficiency tandem solar technologies.
