Piperidine-induced Switching of the direct band gaps of Ag(i)/Bi(iii) bimetallic iodide double perovskites

Abstract

The environmental toxicity of lead may eventually hamper the wide applications of hybrid perovskite material-based solar cells despite their excellent power conversion efficiency. Here, we have used silver(I) and bismuth(III) to replace lead(II) and obtain two two-dimensional (2D) iodide-based double perovskites, (AMP)₄[BiAgI₈]₂·H₂O (1) and (APP)₄[BiAgI₈]·H₂O (2), where AMP = 4-aminomethylpiperidine and APP = 4-aminopiperidine. Compared to our previously reported 1,4-cyclohexanediamine-based compound (C₆H₁₆N₂)₂[BiAgI₈]·H₂O with an indirect band gap, these two new 2D Ag–Bi double perovskites show direct band gaps (∼2.00 eV) due to the less distorted [AgI₆] coordination geometry. Moreover, the photo-current response experiments showed a difference of more than 50 nA between light and dark conditions for both the compounds, indicating the potential application for light harvesting. 1 and 2 were also stable under heat and moisture. They showed high proton conductivity (∼10⁻⁴ S cm⁻¹) at 95 °C and 90% RH. Thus, this work provides a new route for designing stable lead-free bimetallic iodide double perovskites with direct band gaps.