Continuous direct band-gap narrowing and piezochromism of lead-free two-dimensional perovskite nanocrystals under high pressure

Lead halide perovskites are widely applied in photovoltaic devices and solar cells. However, the economic potential is restricted by the toxicity of lead. Two-dimensional (2D) layered organic-inorganic hybrid perovskite (OIHP) copper-based perovskites with good stability and hypotoxicity greatly fulfill environmentally favorable standards. Herein, we systematically report a synthesis and band-gap modulation of 2D layered OIHP PEA(2)CuCl(4) (PEA(+) = C6H5C2H4NH3) nanocrystals (NCs). The direct band gap of PEA(2)CuCl(4) NCs is effectively decreased from 2.93 to 1.77 eV by using high-pressure technique, accompanied by the piezochromism of products from yellow to dark red and then to opaque black, which enhances the photovoltaic application to a greater extent. Combining experiments and theoretical analysis indicates that direct band-gap narrowing is caused by the shorted average Cl-Cu-Cl bond length, resulting in increased Cu-3d and Cl-3p interaction, which raises the valence-band maximum toward the conduction-band minimum. The study contributes to a better understanding of 2D layered OIHP PEA(2)CuCl(4) NCs and enables pressure processing as a reliable technique for improving materials by design in applications.