Photoelectrochemical Water Reduction over Wide Gap (Ag,Cu)(In,Ga)S2 Thin Film Photocathodes
The effects of partial replacement of Cu in a Cu(In,Ga)S2 (CIGS) compound thin film with Ag on its structural, optical, electrostructural, and photoelectrochemical (PEC) properties were investigated in order to improve its performance for PEC water reduction under sunlight illumination. Results of X-ray diffraction (XRD) analyses revealed successful partial replacement of Cu with Ag to form solid-solutions with different Ag/(Ag+Cu) ratios (A(x)CIGS, x = Ag/(Ag+Cu) = 0.1, 0.2, 0.3, and 0.4), as confirmed by gradual increments of (112) reflections to smaller 2θ angles with increase in the Ag/(Ag+Cu) ratio. Analyses of their photoabsorption properties using photoacoustic spectroscopy indicated increments of band gap energies in association with increase in the Ag/(Ag+Cu) ratio. In addition, valence band maximum potentials of A(x)CIGS were deepened gradually with increase in the Ag/(Ag+Cu) ratio. After modifications of these A(x)CIGS films with a CdS ultrathin (ca. 70 nm) layer and a Pt catalyst, PEC water reduction properties were evaluated in an electrolyte solution with pH adjusted to 6.5 under simulated sunlight (AM 1.5G) radiation. Compared to the CdS- and Pt-modified Ag-free A(x)CIGS (A(0)CIGS) film, appreciable increments of PEC properties were observed on electrodes based on A(x)CIGS (x>0) films: the best PEC performance was obtained on the electrode based on the A(0.2)CIGS film. However, the electrode derived from the A(x)CIGS film with Ag/(Ag+Cu) ratios higher than 0.3 showed drops of PEC properties due to the partial conversion of its semiconducting property form p-type to n-type.