Mg _x Zn_1−x O contact to CuGa₃Se₅ absorber for photovoltaic and photoelectrochemical devices

Abstract CuGa₃Se₅ is a promising candidate material with wide band gap for top cells in tandem photovoltaic and photoelectrochemical (PEC) devices. However, traditional CdS contact layers used with other chalcopyrite absorbers are not suitable for CuGa₃Se₅ due to the higher position of its conduction band (CB) minimum. Mg _x Zn_{1− x} O (MZO) is a transparent oxide with adjustable band gap and CB position as a function of magnesium composition, but its direct application is hindered by CuGa₃Se₅ surface oxidation. Here, MZO is investigated as a contact (n-type ‘buffer’ or ‘window’) material to CuGa₃Se₅ absorbers pretreated in Cd²⁺ solution, and an onset potential close to 1 V vs reversible hydrogen electrode in 10 mM hexaammineruthenium (III) chloride electrolyte is demonstrated. The Cd²⁺ surface treatment changes the chemical composition and electronic structure of the CuGa₃Se₅ surface, as demonstrated by photoelectron spectroscopy measurements. The performance of CuGa₃Se₅ absorber with Cd²⁺ treated surface in the solid-state test structure depends on the Zn/Mg ratio in the MZO layer. The measured open circuit voltage of 925 mV is promising for tandem PEC water splitting with CuGa₃Se₅/MZO top cells.