Film thickness effect on 2D lead-free hybrid double perovskite properties: Band gap, photocurrent and stability

Lead-free hybrid double perovskites (LFHDPs) have received a lot of attention due to their environmental friendliness and promising attributes. However, studying the effect of film thickness on LFHDPs optoelectronic properties has not yet been investigated. Herein, we synthesized two new Ruddlesden–Popper LFHDPs, namely (C₅H₁₂N)₄AgBiI₈ (CAB-1) and (C₆H₁₄N)₄AgBiI₈ (CAB-2) using cyclopentylamine and cyclohexylamine as monoamine ligands. Indeed, these two Ag(I)-Bi(III) LFHDPs form smooth and uniform films ranging in thickness from 250 nm to 1 μm, with preferred orientations. Notably, the studies on the optical properties showed that the direct band gap value decreased from 2.17 eV to 1.91 eV for CAB-1 and from 2.05 eV to 1.86 eV for CAB-2 with increasing thickness. Accordingly, photo-current response using a xenon lamp revealed a significant difference of over 1000 nA between light and dark conditions for 1 μm-thickness films, suggesting potential for light harvesting. Other than that, thicker films of CAB-1 and CAB-2 exhibit high stability for 90 days in a relatively humid environment (RH of 55 %), paving the way for promising optoelectronic applications.