Researchers from the Ningbo Institute of Materials Technology and Engineering (NIMTE) of the Chinese Academy of Sciences and Fudan University have developed a unified colloidal-chemistry strategy that pushes all-perovskite tandem solar cells (TSCs) close to 30% efficiency, while directly targeting the crystallization mismatch between their subcells.
Tandem solar cells stack two photoactive layers so that each subcell absorbs a different portion of the solar spectrum, enabling higher power conversion efficiency (PCE) than single-junction devices. All-perovskite TSCs, which use perovskite absorbers in both the wide-bandgap (WBG) and narrow-bandgap (NBG) subcells, are particularly attractive and have a theoretical efficiency potential above 40%. In practice, however, their performance has been limited by mismatched crystallization kinetics: the WBG and NBG perovskites form and grow via different pathways, which can lead to phase segregation, non-uniform grain growth, and a high density of defects at interfaces and grain boundaries.
To overcome this issue, the team designed a unified carboxylate-based Lewis modulator system that engineers the colloidal chemistry of both subcells within the same tandem stack. Guided by hard–soft acid–base (HSAB) theory, they introduced two graded carboxylate anions - tartrate (Ta−) and citrate (Cit−) - that exhibit distinct basicity and coordination preferences. In the WBG perovskite colloids, the borderline-basic Ta− preferentially coordinates with Pb2+, stabilizing the Pb2+ network, suppressing phase segregation, and promoting more uniform nucleation and crystal growth. In parallel, the harder-base Cit− modulates the Sn–I bonding environment in the NBG colloids, effectively passivating Sn2+ defects and improving charge transport.
Beyond these dual carboxylate anions, the researchers further incorporated choline cations, which synergize with Ta− and Cit− at the crystal–colloid interfaces. These choline species help passivate undercoordinated metal ions, contributing to a robust stabilization matrix across both WBG and NBG films. Density functional theory calculations and detailed morphological analyses support this mechanistic picture, confirming that the carboxylate modulators operate at the colloidal and interfacial levels to harmonize crystallization in both subcells.
Device-level metrics highlight the impact of this approach. The optimized WBG subcell achieves an open-circuit voltage of 1.32 V with an 84.35% fill factor, indicating suppressed non-radiative recombination and high-quality film formation. When integrated into a monolithic all-perovskite tandem configuration, the co-engineered device reaches a PCE of 29.76%, with an independently certified efficiency of 29.22%. A large-area 1 cm² tandem cell maintains a PCE of 28.87%, demonstrating that the colloidal modulation strategy is compatible with scale-up beyond small-area lab devices.
Stability is another notable strength of the reported tandems. Under continuous operation at the maximum power point, the devices retain more than 90.2% of their initial efficiency after 700 hours, underscoring that the defect passivation and crystallization control translate not only into higher initial performance but also into durable operation.
Taken together, the unified carboxylate-based modulator system provides what the authors describe as a universal route to harmonize multijunction crystallization, positioning carboxylate coordination chemistry as a versatile platform for advancing high-efficiency, scalable all-perovskite tandem photovoltaics.
