Ag/Bi Iodide Films Fabricated by Single-Source Thermal Ablation for Lead-Free Perovskite-Inspired Solar Cells

Abstract

For the first time, we investigate the preparation of Ag/Bi halide double salts with the formula Ag_aBi_bI_a+3b by single source thermal ablation (SSTA), using different precursor stoichiometries (2.0 ≥ Ag/Bi ≥ 0.5). Silver bismuth iodides are interesting because they combine outstanding stability under ambient conditions with low toxicity and large light absorption coefficients. Thanks to their optical band gaps in the 1.4–2.0 eV range, they have been proposed for different applications, such as indoor photovoltaics and top cell in Si- and CIGS-based tandem solar cells. Independently of the precursor composition, films obtained by SSTA are all characterized by a dominant phase that is closely related to that of Ag₂BiI₅. We demonstrate that the Ag₂BiI₅ and BiI₃ rhombohedral phases coexist in the films prepared from precursors with Ag/Bi ≤ 1 even though no evidence of BiI₃ is achieved by XRD powder diffractometry in the θ–2θ Bragg–Brentano geometry or by UV–vis absorption measurements. The presence of the BiI₃ phase worsens the performance of planar solar cells, so the Ag/Bi = 2.0 precursor provides the best solar cells. Remarkably, these devices show a power conversion efficiency of 1.02%, an open-circuit voltage (V_OC) of 0.71 V, and a short-circuit current density (J_SC) of 3.09 mA/cm², which are comparable to those reported in the literature for planar solar cells, despite the lack of any device optimization. The reported results confirm that SSTA can be successfully used for the exploitation of silver bismuth iodides in photovoltaic applications.