Silver bismuth iodides in various compositions as potential Pb-free light absorbers for hybrid solar cells

Abstract

Polycrystalline silver bismuth iodide (SBI) powders of various compositions (Ag : Bi = 2 : 1–1 : 1 in atomic ratio) were synthesized via a solid-state reaction in an evacuated Pyrex tube. Regardless of the composition of Ag and Bi, Ag₂BiI₅ in the hexagonal phase was preferentially formed and BiI₃ impurity in the rhombohedral phase was formed with the increase of the Bi component. The synthesized SBI powders of various compositions were applied as light absorbers for hybrid solar cells (HSCs), which employ mesoporous TiO₂ as the electron transporting layer and pristine spiro-OMeTAD as the hole transporting layer. To deposit light absorber layers, the coating solutions were prepared by dissolving the synthesized SBI powders in DMSO/DMF/HI and spin-coated over the mesoporous TiO₂ layer. For the solar cell employing pure Ag₂BiI₅, a photovoltaic conversion efficiency (PCE) of 1.74% was achieved, whereas the inclusion of the BiI₃ impurities in the Ag₂BiI₅ phase significantly increased the device performance. The highest PCE of 2.31% was achieved with the SBI of Ag : Bi = 55 : 45. Furthermore, the SBI solar cells show no hysteresis in the J–V curve measurements and are highly stable under ambient conditions, exhibiting excellent long-term stability at a relative humidity of 50%.