Structure-property relations in Ag-Bi-I compounds: potential Pb-free absorbers in solar cells

Recently, Ag-Bi-I system has attracted significant attention as an emergent material for absorbers in solar cells. We have investigated the single-phase compositions, analyzed the detailed crystal structures, measured optical and electronic properties, and determined the structure-property relations of Ag 2−3 x Bi x I 2 . Single-phase samples with CdCl 2 -type rhombohedral structures were obtained with Ag-rich compositions ( x = 0.45-0.48), while defect-spinel-type cubic structures were stabilized with Bi-rich compositions ( x = 0.52-0.57). Both the rhombohedral and cubic structures consisted of the cubic close-packed I-ion sublattices, and the small difference in the Ag/Bi composition resulted in distinct crystal structures. Both the CdCl 2 -type rhombohedral and defect-spinel-type cubic compounds exhibited semiconducting behavior with proper band gap energies, which indicates that both materials are potentially useful as absorbers in solar cells. The rhombohedral structure compounds have shallower valence band energies, larger indirect band gap energies, and higher electrical conductivity with lower activation energy than the cubic structure compounds. The distinct properties result from the difference in the defect structures, additional Ag occupation in the rhombohedral phase and deficiencies of the octahedral site in the cubic phase. Structure-property relations of Ag 2-3 x Bi x I 2 have been revealed.