Combinatorial Sputtering of Ga-Doped (Zn,Mg)O for Contact Applications in Solar Cells

Development of tunable contact materials based on environmentally friendly chemical elements using scalable deposition approaches is necessary for existing and emerging solar energy conversion technologies. In this paper, the properties of ZnO alloyed with magnesium (Mg), and doped with gallium (Ga) are studied using combinatorial thin film experiments. As a result of these studies, the optical band gap of the sputtered Zn_1−xMg_xO thin films was determined to vary from 3.3 to 3.6 eV for a compositional spread of Mg content in the 0.04<x<0.17 range. Depending on whether or not Ga dopants were added, the electron concentrations were on the order of 10¹⁷ cm⁻³ or 10²⁰ cm⁻³, respectively. Based on these results and on the Kelvin Probe work function measurements, a band diagram was derived using basic semiconductor physics equations. The quantitative determination of how the energy levels of Ga-doped (Zn, Mg)O thin films change as a function of Mg composition presented here, will facilitate their use as optimized contact layers for both Cu₂ZnSnS₄ (CZTS), Cu(In, Ga)Se₂ (CIGS) and other solar cell absorbers.