Lead-free halide double perovskites with a generic formula of A2B'(III)B"(I)X6 (A and B are cations and X is a halide anion) are being explored as a less toxic, higher thermal-and moisturestable alternatives to well-studied lead halide perovskite (APbX3) solar energy absorbers. However, the absorption profiles of most of the 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 Cu 2+-doped Cs2SbAgCl6 double perovskites that exhibit dramatic shifts in their bandgaps from ~ 2.6 eV (Cs2SbAgCl6, parent) to ~1 eV (Cu 2+-doped Cs2SbAgCl6). Powder X-ray diffraction patterns of the Cu 2+-doped polycrystalline materials indicate long-range crystallinity with non-uniform 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), unravel the complex short-and medium-range structure of these novel double perovskite materials. Variable temperature 133 Cs NMR spectroscopy reveals that paramagnetic Cu 2+ ions are incorporated within the double perovskite material impacting the 133 Cs 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 indicate excellent resistance to environmental exposure.