TY - JOUR
T1 - Observation of [VCu1-In i2+VCu1-] Defect Triplets in Cu-Deficient CuInS2
AU - Frick, Jessica J.
AU - Cheng, Guangming
AU - Kushwaha, Satya
AU - Yao, Nan
AU - Wagner, Sigurd
AU - Bocarsly, Andrew B.
AU - Cava, Robert J.
N1 - Publisher Copyright:
© 2020 American Chemical Society. All rights reserved.
PY - 2020/12/3
Y1 - 2020/12/3
N2 - Copper indium disulfide (CuInS2) is a semiconductor with a direct energy band gap of 1.53 eV - an optimal value for highly efficient thin-film solar cells. But it has reached only ∼11% power conversion efficiency, far less than the theoretically achievable value of ∼30%. The cause of this low performance is not understood. A single crystal grown from 1 mol % Cu-deficient melt was studied by using atomic resolution high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) and electron dispersive spectroscopy (EDS). While the bulk crystal is exactly stoichiometric CuInS2, it contains nanometer thick, structurally coherent, Cu-deficient interphases that form along rotational twin boundaries in the {112} plane. Transition zones from the bulk crystal to the interphase are observed, where In is seen to move from its normal site InIn in the chalcopyrite structure to a tetrahedral interstitial site Ini, while Cu remains in its normal CuCu position. Two InIn rows of the bulk crystal merge into one row of Ini, causing excess Ini in the interphase. The concentrations of CuCu and Ini reflect a ratio of Cu vacancies, VCu, to an excess Ini of ∼2. Their relative lattice positions, and the high electrical resistivity of the crystal, suggest that VCu and excess Ini "precipitate"as self-compensating, electrically neutral, [VCu1-Ini2+VCu1-] defect triplets. This is the first atomic-level observation of the ordered defect that has been invoked as the basic structural modifier in chalcopyrite compound homologues. The interphases introduce an optical gap of 1.47 eV. Electron trapping in band tail states, evident from a photoconductivity exponent of 0.54, is the likely cause of an unusually low electron mobility of 0.1 cm2 V-1 s-1. The overall result is that making CuInS2 slightly copper-poor inserts nanometer thick layers of the interphase into the bulk crystal. This study shows that apparently conflicting results of the effect of Cu deficiency on CuInS2 thin-film solar cells may be resolved by analyzing structure and composition at nanometer spatial resolution.
AB - Copper indium disulfide (CuInS2) is a semiconductor with a direct energy band gap of 1.53 eV - an optimal value for highly efficient thin-film solar cells. But it has reached only ∼11% power conversion efficiency, far less than the theoretically achievable value of ∼30%. The cause of this low performance is not understood. A single crystal grown from 1 mol % Cu-deficient melt was studied by using atomic resolution high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) and electron dispersive spectroscopy (EDS). While the bulk crystal is exactly stoichiometric CuInS2, it contains nanometer thick, structurally coherent, Cu-deficient interphases that form along rotational twin boundaries in the {112} plane. Transition zones from the bulk crystal to the interphase are observed, where In is seen to move from its normal site InIn in the chalcopyrite structure to a tetrahedral interstitial site Ini, while Cu remains in its normal CuCu position. Two InIn rows of the bulk crystal merge into one row of Ini, causing excess Ini in the interphase. The concentrations of CuCu and Ini reflect a ratio of Cu vacancies, VCu, to an excess Ini of ∼2. Their relative lattice positions, and the high electrical resistivity of the crystal, suggest that VCu and excess Ini "precipitate"as self-compensating, electrically neutral, [VCu1-Ini2+VCu1-] defect triplets. This is the first atomic-level observation of the ordered defect that has been invoked as the basic structural modifier in chalcopyrite compound homologues. The interphases introduce an optical gap of 1.47 eV. Electron trapping in band tail states, evident from a photoconductivity exponent of 0.54, is the likely cause of an unusually low electron mobility of 0.1 cm2 V-1 s-1. The overall result is that making CuInS2 slightly copper-poor inserts nanometer thick layers of the interphase into the bulk crystal. This study shows that apparently conflicting results of the effect of Cu deficiency on CuInS2 thin-film solar cells may be resolved by analyzing structure and composition at nanometer spatial resolution.
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U2 - 10.1021/acs.jpcc.0c08872
DO - 10.1021/acs.jpcc.0c08872
M3 - Article
AN - SCOPUS:85097895259
SN - 1932-7447
VL - 124
SP - 26415
EP - 26427
JO - Journal of Physical Chemistry C
JF - Journal of Physical Chemistry C
IS - 48
ER -