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

Abstract

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.