Scalable Triple Cation Mixed Halide Perovskite–BiVO4 Tandems for Bias‐Free Water Splitting

Abstract Strong interest exists in the development of organic–inorganic lead halide perovskite photovoltaics and of photoelectrochemical (PEC) tandem absorber systems for solar fuel production. However, their scalability and durability have long been limiting factors. In this work, it is revealed how both fields can be seamlessly merged together, to obtain scalable, bias‐free solar water splitting tandem devices. For this purpose, state‐of‐the‐art cesium formamidinium methylammonium (CsFAMA) triple cation mixed halide perovskite photovoltaic cells with a nickel oxide (NiO x ) hole transport layer are employed to produce Field's metal‐epoxy encapsulated photocathodes. Their stability (up to 7 h), photocurrent density (–12.1 ± 0.3 mA cm −2 at 0 V versus reversible hydrogen electrode, RHE), and reproducibility enable a matching combination with robust BiVO 4 photoanodes, resulting in 0.25 cm 2 PEC tandems with an excellent stability of up to 20 h and a bias‐free solar‐to‐hydrogen efficiency of 0.35 ± 0.14%. The high reliability of the fabrication procedures allows scaling of the devices up to 10 cm 2 , with a slight decrease in bias‐free photocurrent density from 0.39 ± 0.15 to 0.23 ± 0.10 mA cm −2 due to an increasing series resistance. To characterize these devices, a versatile 3D‐printed PEC cell is also developed.