Chemical Control of the Dimensionality of the Octahedral Network of Solar Absorbers from the CuI–AgI–BiI(3) Phase Space by Synthesis of 3D CuAgBiI(5)

[Image: see text] A newly reported compound, CuAgBiI(5), is synthesized as powder, crystals, and thin films. The structure consists of a 3D octahedral Ag(+)/Bi(3+) network as in spinel, but occupancy of the tetrahedral interstitials by Cu(+) differs from those in spinel. The 3D octahedral network of CuAgBiI(5) allows us to identify a relationship between octahedral site occupancy (composition) and octahedral motif (structure) across the whole CuI–AgI–BiI(3) phase field, giving the ability to chemically control structural dimensionality. To investigate composition–structure–property relationships, we compare the basic optoelectronic properties of CuAgBiI(5) with those of Cu(2)AgBiI(6) (which has a 2D octahedral network) and reveal a surprisingly low sensitivity to the dimensionality of the octahedral network. The absorption onset of CuAgBiI(5) (2.02 eV) barely changes compared with that of Cu(2)AgBiI(6) (2.06 eV) indicating no obvious signs of an increase in charge confinement. Such behavior contrasts with that for lead halide perovskites which show clear confinement effects upon lowering dimensionality of the octahedral network from 3D to 2D. Changes in photoluminescence spectra and lifetimes between the two compounds mostly derive from the difference in extrinsic defect densities rather than intrinsic effects. While both materials show good stability, bulk CuAgBiI(5) powder samples are found to be more sensitive to degradation under solar irradiation compared to Cu(2)AgBiI(6).