No abstract

As the world’s demand for energy grows, the search for cost competitive and earth abundant thin film photovoltaic absorbers is becoming increasingly important. A promising approach to tackle this challenge is through thin film photovoltaics made of elements that are abundant in the Earth’s crust. In this work, we focus on Cu2SnS3, a promising earth abundant absorber material. Recent publications have presented 3% and 6% device efficiencies using Cu2SnS3-based absorber materials and alloys, respectively. However, little is understood about the fundamental defect and doping physics of this material, which is needed for further improvements in device performance. Here, we identify the origins of the changes in doping in sputtered cubic Cu2SnS3 thin films using combinatorial experiments and first-principles theory. Experimentally, we find that the cubic Cu2SnS3 has a large phase width and that the electrical conductivity increases with increasing Cu and S content in the films, which cannot be fully explained by the theoretical point defect model. Instead, theoretical calcuations suggest that under Cu-rich conditions alloying with an isostructural metallic Cu3SnS4 phase occurs, causing high levels of p-type doping; this theory is consistent with experimental Raman and NEXAFS spectroscopy data. These experimental and theoretical works lead to the conclusion that Cu2SnS3 films must be grown both S-poor and Cu-poor in order to achieve moderate hole concentrations. These new insights enable the design of growth processes that target the desired carrier concentrations for solar cell fabrication. Using the strategies described above, we have been able to tune the carrier concentration over >3 orders of magnitude and achieve films with p-type doping of ≤1018 cm–3, facilitating future device integration of these films.

Next-generation thin film solar cell technologies require earth abundant photovoltaic absorber materials. Here we demonstrate an alternative approach to design of such materials, evaluating candidates grouped by constituent elements rather than underlying crystal structures. As an example, we evaluate thermodynamic stability, electrical transport, electronic structure, optical and defect properties of Cu-Sn-S candidates using complementary theory and experiment. We conclude that Cu2SnS3 avoids many issues associated with the properties of Cu4SnS4, Cu4Sn7S16, and other Cu-Sn-S materials. This example demonstrates how this element-specific approach quickly identifies potential problems with less promising candidates and helps focusing on the more promising solar cell absorbers.

Polycrystalline thin film copper chalcogenide solar cells show remarkable efficiencies, and analogous but less-explored semiconducting materials may hold similar promise. With consideration of elemental abundance and process scalability, we explore the potential of the Cu-Sb-S material system for photovoltaic applications. Using a high-throughput combinatorial approach, Cu-Sb-S libraries were synthesized by magnetron co-sputtering of Cu 2 S and Sb 2 S 3 targets and evaluated by a suite of spatially resolved characterization techniques. The resulting compounds include Cu 1.8 S (digenite), Cu 12 Sb 4 S 13 (tetrahedrite), CuSbS 2 (chalcostibite), and Sb 2 S 3 (stibnite). Of the two ternary phases synthesized, CuSbS 2 was found to have the most potential, however, when deposited at low temperatures its electrical conductivity varied by several orders of magnitude due to the presence of impurities. To address this issue, we developed a self-regulated approach to synthesize stoichiometric CuSbS 2 films using excess Sb 2 S 3 vapor at elevated substrate temperatures. Theoretical calculations explain that phase-pure CuSbS 2 is expected to be formed over a relatively wide range of temperatures and pressures, bound by the sublimation of Sb 2 S 3 and decomposition of CuSbS 2. The carrier concentration of CuSbS 2 films produced within this regime was tunable from 10 16 − 10 18 cm −3 through appropriate control of Sb 2 S 3 flux rate and substrate temperature. CuSbS 2 displayed a sharp optical absorption onset indicative of a direct transition at 1.5 eV and an absorption coefficient of 10 5 cm −1 within 0.3 eV of the onset. The results of this study suggest that CuSbS 2 holds promise for solar energy conversion due to its tolerant processing window, tunable carrier concentration, solar-matched band gap, and high absorption coefficient.

Compositional inhomogeneities in multi--elemental materials typically form due to lowering of the energy relative to the homogeneous phase. Here, we demonstrate an entropy--driven mechanism in the zinc--blende derived cation-substituted multinary compounds Cu2SnS3 (CTS) and Cu2ZnSnS4 (CZTS). Using a motif--based model Hamiltonian and Monte--Carlo simulations, we find that disorder leads to a redistribution of the structural motifs in such a way to create cation-clustering. The associated formation of (sub) nanometer--scale compositional inhomogeneities can cause potential fluctuations with detrimental consequences for photovoltaic applications.