Although several wide-bandgap (1.6-2.0 eV) chalcopyrites (e.g. CuGaSe2, Cu(In,Ga)S2, Cu(In,Al)Se2, (Ag,Cu)GaSe2) have been thus far studied as top cell absorbers for photoelectrochemical (PEC) water splitting tandem devices, CuGa(S,Se)2 is a wide-Eg chalcopyrite that has not yet received any attention in this regard. In this communication, we report on the performance of wide-bandgap chalcopyrite CuGa(S,Se)2 photocathodes as a top cell for PEC tandem water splitting. To be able to assess the PEC performance of CuGa(S,Se)2 as well as its optical transmittance, transparent conductive fluorinated tin oxide (FTO) substrates were used as back contacts. Through our research, we found that synthesizing CuGa(S,Se)2 films through sulfurization of a CuGaSe2 precursor would degrade the optoelectronic properties of the FTO substrate. As such, a specific synthesis approach was developed to circumvent this problem and successfully fabricate functioning CuGa(S,Se)2 PEC photocathodes on FTO. We present the PEC performance of our CuGa(S,Se)2 photocathodes (JSAT≈10 mA/cm2) as well as measurements relevant to its performance as a top cell, such as sub-bandgap transmittance. Finally, we present quantum efficiency of a low-bandgap Cu(In,Ga)Se2 shaded by the CuGa(S,Se)2 PEC photocathode, and discuss the potential of Cu(In,Ga)Se2/CuGa(S,Se)2 tandem structures for non-assisted PEC water splitting.
