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

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)center dot H2O (1) and (APP)(4)[BiAgI8]center dot H2O (2), where AMP = 4-aminomethylpiperidine and APP = 4-aminopiperidine. Compared to our previously reported 1,4-cyclohexanediamine-based compound (C6H16N2)(2)[BiAgI8]center dot H2O with an indirect band gap, these two new 2D Ag-Bi double perovskites show direct band gaps (similar to 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.1and2were also stable under heat and moisture. They showed high proton conductivity (similar to 10(-4)S cm(-1)) at 95 degrees C and 90% RH. Thus, this work provides a new route for designing stable lead-free bimetallic iodide double perovskites with direct band gaps.