Triple-Cation Perovskite Photoanodes for Solar Water Splitting: From Photovoltaic-Assisted to Immersed Photoelectrochemical Operation

Mixed-halide perovskite solar cells with the composition Cs0.1(MA0.17FA0.83)0.9Pb(I0.83Br0.17)3 were fabricated obtaining solar cells as glass/ITO/SnO2/triple-cation perovskite/HTL/Au, and subsequently used as photoanodes for efficient solar-driven water splitting by attaching commercial catalytic nickel foils to the Au back-contact pads of solar cells. To enable operation in alkaline media, the devices were encapsulated using commercial PET–EVA multilayer films, providing an effective barrier while leaving the Ni foils exposed as the electrochemically active interface. Two operating configurations were investigated and compared: (i) an outside configuration, where the perovskite device powered the external electrochemical cell, and (ii) an immersed configuration, in which the encapsulated perovskite solar cell was directly integrated, together with the Ni catalyst, into the electrolyte. In both configurations, the onset potential for the oxygen evolution reaction shifted from ~1.32 V vs. RHE, when the Ni electrode was not powered by the perovskite solar cell, to ~0.34 V vs. RHE, when the perovskite device powered the Ni foil for both immersed and outside configurations. The immersed configuration delivered the highest performance, achieving a maximum Applied Bias Photon-to-Current Efficiency of ~20% under AM 1.5 G illumination (100 mW cm−2), among the highest values reported for perovskite-based photoanodes. Importantly, the enhanced performance does not arise from changes in catalyst composition or direct semiconductor–electrolyte interaction, but from improved photovoltage delivery and reduced resistive losses enabled by the integrated device architecture. These results demonstrate that device architecture is a key factor in controlling photovoltage utilization and charge-transfer kinetics, providing a viable strategy for efficient and scalable perovskite-based photoelectrochemical systems.