The past decade has witnessed a rapid evolution of research on metal halide perovskite-based optoelectronic and energy devices. In light of this remarkable progress, photoelectrochemical (PEC) cells based on halide perovskite photoabsorbers have recently emerged as a promising solar fuel technology. Notably, the bandgap tunability and low-temperature processing make monolithic all-perovskite tandem cells ideal candidates for achieving efficient, cost-effective, unassisted solar-driven water electrolysis devices.

Here, we report our progress on fabricating monolithic all-perovskite tandem cells consisting of two solution-processed perovskite subcells for unassisted water-splitting applications. The all-perovskite tandem devices are achieved by monolithically integrating a wide-bandgap (1.7 – 2.1 eV) Pb-based mixed-halide (Br-I) perovskite top subcell and a narrower-bandgap (1.25 - 1.55 eV) bottom subcell based on Pb-based or mixed Pb-Sn iodide perovskites. Varying the halide perovskite composition for each subcell enables us to tailor the photovoltaic performance of the tandem devices. We demonstrate that all-perovskite tandem devices with various bandgap compositions can deliver open-circuit voltages of more than 2 V. The high photovoltage provides a sufficient overpotential to drive unassisted PEC water splitting with a solar-to-hydrogen conversion efficiency of more than 10%. Additionally, we show that proper water-impermeable encapsulants are needed to prevent degradation of the halide perovskite absorbers in an aqueous environment and enable a long operational lifetime.