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 $PE{A}_{2}\mathrm{Cu}{\mathrm{Cl}}_{4}\phantom{\rule{4pt}{0ex}}(PE{A}^{+}={\mathrm{C}}_{6}{\mathrm{H}}_{5}{\mathrm{C}}_{2}{\mathrm{H}}_{4}\mathrm{N}{\mathrm{H}}_{3})$ nanocrystals (NCs). The direct band gap of $PE{A}_{2}\mathrm{Cu}{\mathrm{Cl}}_{4}$ 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 $\mathrm{Cu}\text{\ensuremath{-}}3d$ and $\mathrm{Cl}\text{\ensuremath{-}}3p$ interaction, which raises the valence-band maximum toward the conduction-band minimum. The study contributes to a better understanding of 2D layered OIHP $PE{A}_{2}\mathrm{Cu}{\mathrm{Cl}}_{4}$ NCs and enables pressure processing as a reliable technique for improving materials by design in applications.