Monolithically Integrated BiVO₄/Si Tandem Devices Enabling Unbiased Photoelectrochemical Water Splitting

Abstract

Abstract A photoelectrochemical (PEC) water splitting device based on a dual‐junction monolithic tandem cell that utilizes NiOOH/FeOOH/BiVO 4 /SnO 2 /Ta:SnO 2 (TTO)/tunnel oxide passivated contact (TOPCon) Si is reported. The PEC device achieves a maximum photocurrent density of 1.4 mA cm −2 (equal to a solar‐to‐hydrogen conversion efficiency of 1.72%) in 1.0 m potassium borate solution (pH 9) when illuminated with air mass 1.5 G simulated solar irradiation, which is the highest value among dual‐junction monolithic photoelectrochemical cells except for III–V materials. The TOPCon Si not only works as an appropriate bottom photoelectrode for subsequent high‐temperature BiVO 4 processing but also offers a high photovoltage of 590 mV. Transparent and conductive TTO grown by pulsed laser deposition serves as a recombination layer to achieve effective integration. In addition, the TTO provides chemical and physical protection, allowing the surface of the TOPCon Si to exhibit 24 h of tandem cell stability under weak base electrolyte conditions. The SnO 2 hole‐blocking layer inserted between TTO and BiVO 4 enhances the charge separation of BiVO 4 , allowing the device to achieve high efficiency. Artificial leaf‐type monolithic tandem cells consisting of NiFe/BiVO 4 /SnO 2 /TTO/TOPCon Si/Ag/Ti/Pt with a solar‐to‐hydrogen efficiency of 0.44% are also demonstrated.