Piperidine Switches on Direct Band Gaps of Ag(I)/Bi(III) Bimetallic Iodide Double Perovskites
The environmental toxity of lead may eventually hamper the wide application of hybrid perovsite material based solar cells despite their excellent power conversion efficiency. Here we use 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. Comparing to our previously reported 1,4-cyclohexanediamine based compound (C6H16N2)2[BiAgI8].H2O with 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 show more than 50 nA difference between Ilight and Idark for both compounds, indicating the potential application for light harvesting. 1 and 2 are also stable under heat and moisture. They show high proton conductivity (~10-4 S cm-1) at 95 oC and 90% RH. Thus, this work provides a new road for design stable lead-free bimetallic iodide double perovskites with direct band gaps.