Preparation of the Sb2Se3 thin films: Preparation method for the Sb2Se3 thin films is identical to our previous report. Metallic Sb was electrodeposited onto Au-coated FTO substrates from a solution containing 15 mM K2Sb2(C4H2O6)2 and 50 mM tartaric acid. The pH of the solution was adjusted using concentrated sulfuric acid to 1.3 and a potential of -0.3 V vs.