Cu(In,Ga)(S,Se)₂ Photocathodes with a Grown-In Cu_xS Catalyst for Solar Water Splitting

As CuInGa-based chalcopyrite photocathodes suffer from poor hydrogen evolution activity, n-type overlayers and hydrogen evolution catalysts (HECs) need to be deposited on the film surface to drive surface band bending and reduce the overpotential for the hydrogen evolution reaction (HER). Here, we present a Cu(In,Ga)(S,Se)2 (CIGSSe) photocathode with grown-in CuxS HECs enabling solar water splitting without the deposition of additional n-type overlayers and HECs. The controlled two-step chalcogenization using a Cu-rich CuInGa precursor film resulted in the natural formation of the CuxS phase at the CIGSSe film surface and an increase in S content by substituting Se. Electrochemical water reduction tests elucidated that the naturally formed CuxS alters the surface state of CIGSSe and reduces the overpotential for HER. Also, the S incorporation allows fine-tuning to make the CIGSSe band gap favorable for solar water splitting. Consequently, the CIGSSe photocathode showed −25.7 mA·cm–2 photocurrent density and 3 h photostability for photoelectrochemical hydrogen evolution.