Bandgap Optimization by Cu Doping of Ag-In Halide Double Perovskites for Photovoltaic Applications

Lead iodide perovskites (APbI₃, where A represents organic cations) have been used as photoactive layers in perovskite photovoltaics due to their excellent optical and electronic properties. However, the toxicity of Pb and the ambient instability caused by the degradation of organic cations and iodide anions have hindered their widespread use. Metal halide double perovskites (A₂B^IB^IIIX₆, where B^I, B^III, and X represent monovalent and trivalent metal cations, and halide anions, respectively) have been theoretically proposed as Pb-free halide perovskites. Theoretical calculations predict that Rb₂CuInCl₆ double perovskites have a direct and optimal bandgap of 1.36 eV, whereas they cannot be synthesized directly due to their low thermodynamic stability. Instead, we focused on Cs₂AgInCl₆ double perovskites, which can be synthesized directly and exhibit superior ambient stability; however, they have a large bandgap (&gt;3.0 eV), making them unsuitable for photoactive layers. Recent studies have reported that Cu²⁺ doping reduces the bandgap of Cs₂AgInCl₆ from 3.0 to 2.2 eV. Furthermore, it was demonstrated that Cu⁺-Cu²⁺-In³⁺-based layered perovskites exhibited better optical properties than perovskites composed of Cu⁺ or Cu²⁺ alone. In this study, we present the synthesis of Ag-In double perovskites with an optimal bandgap as photoactive layers through Cu^+/2+ doping.<br/>All of the following processes were performed at 75°C in ambient air. Ag-In precursor aqueous solutions (denoted as S1) were prepared by dissolving AgCl and InCl₃･4H₂O in concentrated HCl. Precursor aqueous solutions containing only Cu²⁺ (denoted as S2) were prepared by dissolving CuCl₂･2H₂O in concentrated HCl. Precursor aqueous solutions containing only Cu⁺ (denoted as S3) were prepared by dissolving CuCl with reductants in concentrated HCl. Precursor aqueous solutions containing Cu⁺ and Cu²⁺ simultaneously (denoted as S4) were prepared by dissolving CuCl and/or CuCl₂･2H₂O with reductants in concentrated HCl. Ultraviolet-visible (UV-Vis) absorption spectra of the S2-S4 solutions were measured to evaluate the valence states and complex structures of Cu species in these solutions. The solid samples precipitated by adding CsCl into the S1 solutions, the S1/S2 mixtures, the S1/S3 mixtures, or the S1/S4 mixtures were collected and washed. The composition, structure, and bandgap of the resulting samples were evaluated by inductively coupled plasma-mass spectrometry (ICP-MS), X-ray diffraction (XRD), and optical absorption measurements.<br/>The S2, S3, and S4 solutions turned yellow, colorless, and dark, respectively. The UV-Vis spectra of these solutions indicated that the S2 and S3 contained only [Cu^IICl₄]^2- and [Cu^ICl₄]^3-complexes, respectively, whereas the S4 contained mixed-valence Cu^+/2+ species. The resulting samples from the S1 solutions were completely white in color. XRD analysis confirmed the presence of the Cs₂AgInCl₆ phase in these samples. In contrast, the resulting samples from the S1/S2 and the S1/S3 mixtures were yellow and pale yellow, respectively. These samples had a smaller bandgap compared to Cs₂AgInCl₆. Trace amounts of Cu were detected via ICP-MS, indicating that Cu doping of the Cs₂AgInCl₆can reduce the bandgap. Interestingly, the resulting samples from the S1/S4 mixtures exhibited darker colors than the samples doped with Cu⁺ or Cu²⁺ alone, suggesting that simultaneous Cu^+/2+ co-doping leads to emergent optical properties in these samples.