Physical,photoelectrochemical properties of CuIn3Se5 and relevance for hydrogen production

CuIn₃Se₅, prepared by the fusion technique crystallizes in the P-chalcopyrite structure and exhibits n-type conduction ascribed to indium excess. The electrical conductivity follows an Arrhenius-type law with activation energy of 0.35 eV and an electron mobility of 10⁻⁴ cm² V⁻¹ s⁻¹ in conformity with small polaron hopping. The optical gap (1.19 eV), determined from the diffuse reflectance spectrum, is properly matched to the sun spectrum. CuIn₃Se₅ is chemically stable and a corrosion rate of only 1.2 μmol year⁻¹ is found at neutral pH. The slope and the intercept to C⁻² = 0 of the Mott Schottky plot gives respectively an electron density of 3.75 × 10¹⁶ cm⁻³ and a flat band potential of −0.22 V_SCE. The conduction band (−0.74 V_SCE) therefore lies below the potential of H₂O/H₂ couple and as application, H₂ photo-production is successfully achieved over CuIn₃Se₅. The best performance is obtained in S₂O₃²⁻ solution (10⁻² M, pH ∼ 7) with an evolution rate of 0.54 mL g⁻¹ min⁻¹. The conversion efficiency (0.13%) is due to the formation of small depletion width (230 nm) and a large diffusion length compared to a very large penetration depth (∼1 μm). Attempts have been made to improve the photoactivity and the hetero-system CuIn₃Se₅/WO₃ is compared favorably with respect to CuIn₃Se₅. The photoactivity is ascribed to electrons transfer from the sensitizer CuIn₃Se₅-conduction band (CB), acting as electrons pump, to WO₃-CB (−0.4 V_SCE) resulting in the enhanced water reduction.