Expectations to the commercialization of the perovskite/Si tandem solar cell are higher than ever due to its skyrocketing efficiency, but the stability issues still pose a big hurdle. Halide perovskites are promising top-cell materials for high-efficiency silicon tandem solar cells, but it suffers from halide segregation problem. This is a consequence of the halide mixing necessary to achieve bandgap widening. One of the key stability issues is the segregation of multiple halide anions that are critical for the wide frequency gap of peroxide top cells. The fundamental solution to the problem of halide segregation is the use of monohalogen elements, but no one has demonstrated monohalogen peroxide top cells so far due to the inevitable formation of secondary phases originating from precipitation kinetics.
        In a report published in JOULE in October 2022, the research team composed of Jin Young Kim and Jungwon Park of Seoul National University and Dong Hoe Kim of Korea University demonstrated a pure iodide wide-gap peroxide top-layer cell with monohalogen elements by adding a new kinetically controlled phase evolution path of Cl anions. The formation of a new 2D mesophase is crucial for kinetic control. The resulting peroxide crystal top-layer cells containing pure iodine exhibit significantly enhanced photostability and high tandem efficiency exceeding 29%. High efficiency 29.4%!
     Cs and dimethylammonium cations were incorporated simultaneously into the A-site of perovskite structure to increase the band gap while maintaining the tolerance factor. However, the incorporation of dual cations resulted in the simultaneous formation of orthorhombic and hexagonal secondary phases rather than forming the pure perovskite phase, owing to the different precipitation kinetics between cations.
       The research team demonstrated that this strategy can only be implemented by the phase-controlled nucleation of the Cs-rich composition that governs the desired phase evolution. The pure-iodide perovskite top cell exhibited excellent photo-stability (1% degradation after 1,000 h of continuous operation; ISOS-L-1I, white LED), and its Si tandem exhibited a high conversion efficiency of 29.4% (28.37% certified).