Stable pure-iodide wide-band-gap perovskites for efficient Si tandem cells via kinetically controlled phase evolution

Abstract

Halide perovskites, promising top-cell materials for efficient Si tan-dem solar cells, suffer from halide segregation, which results from the halide mixing necessary for achieving band-gap widening. We report pure-iodide wide-band-gap perovskite top cells that are fundamentally free of halide segregation. Cs and dimethylammo-nium 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 re-sulted in the simultaneous formation of orthorhombic and hexago-nal secondary phases rather than forming the pure perovskite phase, owing to the different precipitation kinetics between cat-ions. We 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).