EN19.04.28 : Combinatorial Investigation into the Phase Space of P-Type Transparent Cu-Zn-S

5:00 PM–7:00 PM Apr 3, 2018 (America - Denver)

PCC North, 300 Level, Exhibit Hall C-E

Rachel Woods-Robinson2 1 3 Yanbing Han4 3 Kristin Persson2 1 Andriy Zakutayev3

2, University of California, Berkeley, Berkeley, California, United States
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.