Silver bismuth sulfoiodide solar cells: tuning optoelectronic properties by sulfide modification for enhanced photovoltaic performance

Silver bismuth iodides (Ag_aBi_bI_a+3b) are nontoxic and comparatively cheap photovoltaic materials, but their wide bandgaps and downshifted valence band edges limit their performance as light absorbers in solar cells. Herein, a strategy is introduced to tune the optoelectronic properties of Ag_aBi_bI_a+3b by partial anionic substitution with the sulfide dianion. A consistent narrowing of the bandgap by 0.1 eV and an upshift of the valence band edge by 0.1–0.3 eV upon modification with sulfide are demonstrated for AgBiI₄, Ag₂BiI₅, Ag₃BiI₆, and AgBi₂I₇ compositions. Solar cells based on silver bismuth sulfoiodides embedded into a mesoporous TiO₂ electron-transporting scaffold, and a poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] hole-transporting layer significantly outperform devices based on sulfide-free materials, mainly due to enhancements in the photocurrent by up to 48%. A power conversion efficiency of 5.44 ± 0.07% (J_sc = 14.6 ± 0.1 mA cm⁻²; V_oc = 569 ± 3 mV; fill factor = 65.7 ± 0.3%) under 1 sun irradiation and stability under ambient conditions for over a month are demonstrated. The results reported herein indicate that further improvements should be possible with this new class of photovoltaic materials upon advances in the synthetic procedures and an increase in the level of sulfide anionic substitution.