1, Lawrence Berkeley National Lab, Berkeley, California, United States
3, National Renewable Energy Laboratory, Golden, Colorado, United States
4, Fudan University, Shanghai, , China
Recently, the ternary material system of Cu-Zn-S has shown significant promise as a p-type transparent conductor (TC) for photovoltaic and optoelectronic applications. Previous studies have found evidence of both Cu doping onto the Zn antisite (CuxZn1-xS) and a solid solution of CuyS and ZnS (CuyS:ZnS), depending on the thermodynamics and kinetics of the growth process. Here, we investigate this material system using combinatorial sputtering, high-throughput characterization and percolation theory to explore the phase transitions (both in chemical potential and temperature space) and resulting structure-property relations. Samples are grown with copper concentrations across the entire chemical space, with Cu/(Cu+Zn) ranging from 0 to 1, and films are found to crystallize at room temperature with optimized p-type conductivity and transparency within the range of 0.2 < x < 0.4, in agreement with the “TC regime” of previous studies. We find conductivity to increase monotonically as a function of Cu concentration, which is evidence for a solid solution of amorphous CuyS and ZnS, yet it increases with distinct jumps by an order of magnitude at two different Cu concentrations. Using high spatial resolution synchrotron x-ray diffraction, we find these jumps in conductivity to correlate with structural changes between the wurtzite and zinc-blende crystal structures. This could indicate either (1) greater Cu incorporation into the wurtzite phase than zinc-blende or (2) higher transport in wurtzite due to lower computed effective mass. Additionally, we find conductivity within the “TC regime” to increase to up to 250 S/cm at growth temperatures of 185 - 200 deg. C with no decrease in transparency. At elevated temperature films are found to be solid solutions of Cu2S5 and zinc-blende ZnS, so conductivity is likely due to larger crystal grains and a connected network of Cu2S5 regions within a transparent ZnS matrix. We also present initial results from heterojunction solar cells with combinatorially sputtered Cu-Zn-S and discuss this material’s excellent TC figure of merit in the context of both state-of-the-art p-type TCs and the authors’ recent computational screenings.