Nickel substrate covered with a Sn-based protection bi-layer as a photoanode substrate for dye-sensitized solar cells

Ni is a metallic substrate commonly used in microelectronic and opto-electronic devices due to its high stability, cost-efficiency and flexibility. Up to now, Ni has not yet been used as the substrate for the photoanode in dye-sensitized solar cell (DSSC) because the formation of p-type Ni oxide hinders the electron transfer from n-type TiO2 mesoporous film to Ni substrate. In this paper, a Sn-based protection bi-layer coating on the Ni foil successfully turns Ni into a good photoanode substrate in DSSC. A Sn layer is initially electroplated onto the Ni surface to prevent its oxidation. The Sn layer is then transformed into a bi-layer of SnO2 and Ni–Sn intermetallic compound after high-temperature TiO2 sintering. Unlike the p-type Ni oxide layer, the top n-type SnO2 layer enables the electron transfer from the TiO2 mesoporous film to the conductive Ni substrate because its energy level of conduction band well matches that of TiO2. In addition, the interfacial Ni–Sn intermetallic compound is a good electrical conductor which facilitates the electron transfer. By carefully controlling the plating current density and time, a 1.2-μm-thick Sn layer with a smooth surface morphology is formed and is recognized as the best protection layer over the Ni substrate. The energy conversion efficiency of the resultant DSSC achieves 4.422% with a short-circuit current density of 10 mA/cm2, an open-circuit voltage of 0.643 V, and fill factor of 0.689 under AM 1.5 back-side illumination.