FA-Cs Perovskites: Cation Heterogeneity and Stability Solutions

Nature Communications China

Overview

Researchers have identified that Cs-enriched grain triple junctions act as the primary initiation points for ambient degradation in formamidinium-caesium mixed-cation perovskites. By introducing N-Octylsulfobetaine to homogenize cation distribution, the team achieved a power conversion efficiency of 26.37% and 96.47% performance retention after 2,200 h. This discovery provides a nanoscopic pathway to enhance the long-term durability of perovskite photovoltaics.

In Depth

Key Findings

The study identifies nanoscopic cation heterogeneity as the driving factor behind the ambient instability of formamidinium-caesium (FA-Cs) mixed-cation perovskites. Specifically, Cs-enriched grain triple junctions serve as intrinsic weak points where Cs⁺ segregation and phase separation begin upon exposure to ambient conditions. This localized instability leads to structural disordering and the crystallization of δ-CsPbI₃, which subsequently spreads into neighboring grains and degrades the perovskite matrix and interfacial optoelectronic properties.

Technical Details

To analyze these degradation pathways, the researchers utilized quasi-in situ scanning transmission electron microscopy. To mitigate the identified instability, they introduced N-Octylsulfobetaine into the fabrication process. This additive performs three primary functions:

• Regulates crystallization kinetics to suppress cation heterogeneity.
• Promotes a more homogeneous distribution of cations across the material.
• Passivates grain boundaries to increase resistance against the intrusion of water and oxygen.

Performance Results

The implementation of N-Octylsulfobetaine resulted in n-i-p perovskite solar cells with a champion power conversion efficiency of 26.37%. In terms of long-term stability, the cells retained 96.47% of their initial performance after 2,200 h of testing under the ISOS-L-1l protocol.

Significance and Outlook

The research clarifies the nanoscopic origins of degradation in FA-Cs perovskites, which has previously limited the efficiency and durability of solar cells. By establishing a strategy to control cation distribution and passivate grain boundaries, the findings offer a pathway toward robust perovskite photovoltaics that combine high power conversion efficiency with the long-term stability required for practical application.

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