Cation ratio fluctuations in Cu<sub>2</sub>ZnSnS<sub>4</sub> at the 20 nm length scale investigated by analytical electron microscopy

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  • Jeffery A. Aguiar
    National Renewable Energy Laboratory Golden CO 80401 USA
  • Mehmet E. Erkan
    Department of Electrical and Computer Engineering University of Utah UT 84112 USA
  • Dennis S. Pruzan
    Department of Materials Science and Engineering University of Utah UT 84112 USA
  • Akira Nagaoka
    Department of Materials Science and Engineering University of Utah UT 84112 USA
  • Kenji Yoshino
    Department of Applied Physics and Electronic Engineering University of Miyazaki Miyazaki 889 2192 Japan
  • Helio Moutinho
    National Renewable Energy Laboratory Golden CO 80401 USA
  • Mowafak Al‐Jassim
    National Renewable Energy Laboratory Golden CO 80401 USA
  • Michael A. Scarpulla
    Department of Electrical and Computer Engineering University of Utah UT 84112 USA

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<jats:sec><jats:label /><jats:p>Kesterite Cu<jats:sub>2</jats:sub>ZnSn(S,Se)<jats:sub>4</jats:sub> (CZTSSe) is a sustainable material for thin‐film photovoltaics with device efficiencies greater than 12% have been demonstrated. Despite similar crystal structure and polycrystalline film microstructures, there is widespread evidence for larger‐amplitude potential and bandgap fluctuations in CZTS than in the analogous Cu(In,Ga)Se<jats:sub>2</jats:sub> (CIGSe) chalcopyrite material. This disorder is believed to account for a sizable part of the larger open‐circuit voltage (<jats:italic>V</jats:italic><jats:sub>OC</jats:sub>) deficit in CZTS devices, yet the detailed origins and length scales of these fluctuations have not been fully elucidated. Herein, we present a transmission electron microscopy study focusing on composition variation within bulk multicrystals of CZTS grown by the travelling heater method (THM). In these slow‐cooled, solution grown crystals we find direct evidence for spatial composition fluctuations of amplitude <1 at.% (∼5 × 10<jats:sup>20</jats:sup> cm<jats:sup>−3</jats:sup>) and thus, explainable by point defects. However, rather than being homogeneously‐distributed we find a characteristic 20 nm length scale for these fluctuations, which sets a definite length scale for band gap and potential fluctuations. At Σ3 grain boundaries, we find no evidence of composition variation compared to the bulk. The finding highlights such variations reported at grain boundaries in polycrystalline thin‐films are direct consequences of processing methods and not intrinsic properties of CZTS itself.</jats:p></jats:sec>

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