Research Background
Organic-inorganic hybrid halide perovskites are regarded as the core material for next-generation optoelectronic devices due to their outstanding optoelectronic properties. Among them, perovskite solar cells (PSCs) have made rapid progress, with single-junction cell efficiencies already exceeding 26%. However, constrained by the Shockley-Queisser (S-Q) limit, efficiency improvements for single-junction cells have encountered a bottleneck. To overcome this limitation, constructing multi-junction tandem solar cells has become a feasible pathway toward higher efficiencies. Both perovskite/silicon and all-perovskite tandem cells require a high-performance wide-bandgap (WBG) perovskite top cell (typically with a bandgap range of 1.68 eV to 1.80 eV) to match the narrow-bandgap bottom cell. However, the commonly used 1.80 eV WBG perovskite (such as Cs_0.2FA_0.8Pb(I_0.6Br_0.4)₃) faces severe challenges: complex composition leads to uncontrollable crystallization, and defect-mediated halogen migration triggers severe "phase separation". These issues directly result in poor film quality and significant voltage loss, seriously hindering the performance improvement of tandem devices.

Research Progress
To address these challenges, a research team from the Frontiers Science Center for Flexible Electronics at Northwestern Polytechnical University, jointly with Xi'an Shiyou University and Xidian University, developed a novel crystallization regulation strategy. The team introduced all-inorganic 2D CsPb₂Br₅ nano-flakes as a "heteronucleation agent" for the first time, successfully achieving precise control over the crystallization process of wide-bandgap perovskites. The research team discovered that CsPb₂Br₅ can serve as heterogeneous nucleation sites in the precursor solution, significantly reducing the nucleation barrier and promoting homogeneous nucleation. More importantly, this heterointeraction guides the Vertical Growth of perovskite crystals. Unlike the randomly stacked grains in traditional films, the films prepared by the new strategy are composed of vertically thorough and laterally tightly arranged crystals. The "top-down" growth mode induced by CsPb₂Br₅ not only forms vertical grain boundaries beneficial for charge transport but also effectively releases residual tensile strain within the film. Theoretical calculations and experimental results indicate that the introduction of CsPb₂Br₅ increases the formation energy of vacancy defects, thereby reducing defect density. This fundamentally cuts off the defect-mediated ion migration path, significantly suppressing the phase separation phenomenon of wide-bandgap perovskites under illumination. Based on this strategy, the optimized 1.80 eV WBG PSCs achieved a champion Power Conversion Efficiency (PCE) of 20.14% and a Fill Factor (FF) as high as 85.39%, which is one of the highest fill factors reported for this bandgap to date.

Future Prospects
This research not only achieved a breakthrough in single-junction wide-bandgap cells but also demonstrated immense application potential in tandem devices. The team constructed 4-terminal (4T) tandem solar cell systems: the perovskite/silicon tandem cell achieved an efficiency of 31.13%, and the all-perovskite tandem cell reached 28.39%. Furthermore, unencapsulated devices exhibited excellent stability under both illumination and storage conditions. This achievement proves that rational crystallization control strategies can effectively resolve the intrinsic instability issues of wide-bandgap perovskites. In the future, this growth regulation technology based on heteronucleation is expected to be extended to the fabrication of large-area modules, laying a solid theoretical and technical foundation for the commercial application of efficient and stable tandem photovoltaic modules.

About the Authors
Chenxin Ran: Associate Professor and Doctoral Supervisor at the Frontiers Science Center for Flexible Electronics, Northwestern Polytechnical University. He is mainly engaged in the research of novel halide perovskite optoelectronic materials and devices.

Wei Huang: Academician of the Chinese Academy of Sciences, Professor at Northwestern Polytechnical University, and one of the pioneers in the field of flexible electronics, dedicated to the research and application of organic optoelectronics and flexible electronics

The complete study is accessible via DOI:10.34133/research.0892