Use of amorphous silicon tandem junction solar cells for hydrogen production in a photoelectrochemical cell

We report the use of amorphous silicon (a-Si) tandem junctions as part of an integral "hybrid" photoelectrochemical (PEC) cell to produce hydrogen directly from water using sunlight. The device configuration consists of stainless steel (SS)/ni₂pni₁p/ZnO/WO₃. When the device is immersed in an electrolyte and illuminated, O₂ is evolved at the WO₃/electrolyte interface and H₂ is produced at the counter electrode. A voltage >1.23V is required to split water; typically 1.6-1.8V are needed, taking account of losses in a practical water-splitting system. We use a-Si tandem cells, deposited by plasma-enhanced chemical vapor deposition, to supply this voltage. Current matching in the two a-Si subcells is achieved by altering the thicknesses of the two layers (i₁ and i₂) while keeping their band gaps at ~1.75eV, which results in a device with an open circuit voltage >1.6V, short circuit current density (J_sc) >6mA/cm² (on SS substrates), and a fill factor >0.6. Deposition on a textured SnO₂ coated glass has resulted in J_sc >9mA/cm². Photoactive WO₃ films, deposited using the RF sputtering technique, have achieved photocurrents >3mA/cm² at 1.6V vs. saturated calomel electrode (SCE). The PEC device operates at the point at which the WO₃ photocurrent IV curve and the a-Si (filtered by WO₃) light IV curve cross, leading to operating currents of 2.5mA/cm² and solar-to-hydrogen (STH) conversion efficiency of >3%.