Perovskite‐Derivative Valleytronics

Halide perovskites are revolutionizing the renewable energy sector owing to their high photovoltaic efficiency, low manufacturing cost, and flexibility. Their remarkable mobility and long carrier lifetime are also valuable for information technology, but fundamental challenges like poor stability under an electric field prevent realistic applications of halide perovskites in electronics. Here, it is discovered that valleytronics is a promising route to leverage the advantages of halide perovskites and derivatives for information storage and processing. The synthesized all‐inorganic lead‐free perovskite derivative, Cs3Bi2I9, exhibits strong light–matter interaction and parity‐dependent optically addressable valley degree of freedom. Robust optical helicity in all odd‐layer‐number crystals with inversion symmetry breaking is observed, indicating excitonic coherence extending well beyond 11 layers. The excellent optical and valley properties of Cs3Bi2I9 arise from the unique parallel bands, according to first principles calculations. This discovery points to new materials design principles for scalable valleytronic devices and demonstrates the promise of perovskite derivatives beyond energy applications. An atomically thin all‐inorganic lead‐free perovskite derivative, Cs3Bi2I9, which exhibits strong light–matter interaction, is synthesized by the chemical vapor deposition method. Robust optical helicity is observed in all odd‐layer‐number crystals with inversion symmetry breaking, indicating excitonic coherence extending well beyond 11 layers. The excellent optical and valley properties of Cs3Bi2I9 arise from the unique parallel bands, according to first‐principles calculations.