Observation of [V_Cu^1–In_i²⁺V_Cu^1–] Defect Triplets in Cu-Deficient CuInS₂

Abstract

Copper indium disulfide (CuInS 2 ) 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 CuInS 2 , 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 In In in the chalcopyrite structure to a tetrahedral interstitial site In i , while Cu remains in its normal Cu Cu position. Two In In rows of the bulk crystal merge into one row of In i , causing excess In i in the interphase. The concentrations of Cu Cu and In i reflect a ratio of Cu vacancies, V Cu , to an excess In i of ∼2. Their relative lattice positions, and the high electrical resistivity of the crystal, suggest that V Cu and excess In i "precipitate" as self-compensating, electrically neutral, [V Cu 1− In i 2+ V Cu 1− ] 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 cm 2 V −1 s −1 . The overall result is that making CuInS 2 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 CuInS 2 thin-film solar cells may be resolved by analyzing structure and composition at nanometer spatial resolution.

Defects go green: using defects in nanomaterials for renewable energy and environmental sustainability

“…The band gap for copper and silver doped QDs is still somewhat large, which limits spectral absorption coverage for LSCs and has driven research interest in Cu x In 2-x Se 2-y S y (CISeS) QDs. CISeS QDs have size tunable absorption all the way to the near-IR range, a large Stokes shift (300-500 meV), a near colorless emission wavelength that well-matches the peak EQE of the LSC PV cells (Figure 3B), and have recently achieved greater than 95% QY (Ueng and Hwang, 1989;Ueng and Hwang, 1990;Kim et al, 2012;Knowles et al, 2015;Jara et al, 2016;Fuhr et al, 2017;Xia et al, 2017;Bergren et al, 2018;Nelson and Gamelin, 2018;Yun et al, 2018;Houck et al, 2019;Hughes et al, 2019;Makarov et al, 2019;Fuhr et al, 2020a;Fuhr et al, 2020b;Du et al, 2020;Frick et al, 2020;Velarde et al, 2020;Hinterding et al, 2021;Xia et al, 2021;Harchol et al, 2022;Liang et al, 2023). The origin of CISeS Stokes shifted emission has commonly been ascribed to defects, but with several other proposed mechanisms depending on the type of defect that forms, and the band-edge transition itself (described later in the review).…”

“…Multinary nanomaterials-such as Cu x In 2-x Se y S 2-y (CISeS) nanocrystals-frequently have their defect distribution tuned by adjusting precursor ratios. In this case, Cudeficient nanocrystals tend to form V Cu / whereas Cu In // are more Frontiers in Nanotechnology frontiersin.org commonly observed in near-stoichiometric or Cu-rich nanocrystals (Ueng and Hwang, 1989;Ueng and Hwang, 1990;Kim et al, 2012;Jara et al, 2016;Fuhr et al, 2017;Yun et al, 2018;Houck et al, 2019;Fuhr et al, 2020a;Fuhr et al, 2020b;Du et al, 2020;Frick et al, 2020;Liang et al, 2023). Adjusting synthesis temperature and precursor ratios has been shown to enhance the density of vacancies during chemical vapor deposition (CVD) or chemical vapor transport (CVT) synthesis of transition metal dichalcogenides (TMDs, Figure 2) (Enyashin et al, 2013;Lin et al, 2016;Li et al, 2017b;Liang et al, 2021).…”

Identification of interfacial defects in the layered structure of a chalcopyrite compound

“…The cause of this low performance is not understood.Here, a single crystal grown from 1 mol % Cu-deficient melt was studied by using atomic resolution highangle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) and electron dispersive spectroscopy (EDS) (no beam damage issue). [3,4] Figure 1 shows that the CuInS2 crystal has a lamellar structure, taken in the ⟨11̅ 0⟩ direction. The lamellae are alternating pseudotwins that form a herringbone pattern (Fig.…”