Cu(II)-Doped Cs2SbAgCl6 Double Perovskite: A Lead-Free, Low-Bandgap Material

Lead-free halide double perovskites with a generic formula of A(2)B'(III)B ''(I)X-6 (A and B are cations and X is a halide anion) are being explored as a less toxic, higher thermal- and moisture-stable alternative to well-studied lead halide perovskite (APbX(3)) solar energy absorbers. However, the absorption profiles of most double perovskites reported to date have larger bandgaps (>2 eV) that are poorly aligned with the solar spectrum, reducing their photoconversion efficiency. Here, we present new heterovalent paramagnetic Cu2+-doped Cs2SbAgCl6 double perovskites that exhibit dramatic shifts in their bandgaps from similar to 2.6 eV (Cs2SbAgCl6, parent) to similar to 1 eV (Cu2+-doped Cs2SbAgCl6). Powder X-ray diffraction patterns of the Cu2+-doped polycrystalline materials indicate long-range crystallinity with nonuniform microstrain in the crystal lattice. To decode the dopant, complementary magnetic resonance spectroscopy techniques, solid-state nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR), are used to unravel the complex short- and medium-range structure of these novel double perovskite materials. Variable temperature Cs-133 NMR spectroscopy reveals that paramagnetic Cu2+ ions are incorporated within the double perovskite material impacting the Cs-133 NMR through a Fermi contact interaction. Finally, a comprehensive stress test of the material's long-term (up to 365 days) thermal and moisture stability indicates excellent resistance to environmental exposure.