MgxZn1−xO 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. MgxZn1−xO (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.