Quintenary Cu₂(Zn_1−xCo_x)SnS₄ is an analog of the promising solar absorber material Cu₂ZnSnS₄ (CZTS). The initial rapid progress in CZTS has stalled because the similar sizes of Cu and Zn cations lead to facile formation of antisite defects, which are thought to limit the solar cell performance. Cobalt substitution for Zn may reduce cation disorder. Herein, we report the synthesis of wurtzite Cu₂(Zn_1−xCo_x)SnS₄ across the entire composition range and a systematic study of the substitution of Co into the wurtzite CZTS lattice. The synthesis is based on microwave heating to only 160 °C and uses metal salts and thiourea as precursors and ethylene glycol as the solvent. The Cu₂(Zn_1−xCo_x)SnS₄ nanocrystals were phase pure wurtzite within the detection limits of X-ray diffraction and Raman scattering. The wurtzite lattice parameters, nanocrystal sizes, and A1 Raman mode peak positions depend on the Co concentration, x. The lattice parameters follow Vegard's law within the accuracy of our measurements, and the A1 Raman mode shifts nearly linearly with x. The nanocrystal size decreases from 8 nm to 4 nm as x increases from 0 to 1. The absorption band edge blue shifted from 1.1 eV for x = 0 to 1.35 eV for x = 1. These values are lower than those predicted by density functional theory calculations and previous attempts at determining the optical band gap for wurtzite Cu₂ZnSnS₄ (x = 0) and Cu₂CoSnS₄ (x = 1). Either the band gaps of wurtzite Cu₂ZnSnS₄ (x = 0) and Cu₂CoSnS₄ (x = 1) are lower or these materials have significant band tails due to defects. We also prepared polycrystalline Cu₂(Zn_1−xCo_x)SnS₄ thin films by thermal annealing, in sulfur, of coatings comprised of Cu₂(Zn_1−xCo_x)SnS₄ nanocrystals. Upon annealing in sulfur, the wurtzite Cu₂(Zn_1−xCo_x)SnS₄ nanocrystals transformed into larger grains (100 s of nm to microns) that have a kesterite structure. The films with x ≤ 0.4 were phase pure kesterite within the detection limits of XRD and Raman scattering, but, for x ≥ 0.6, secondary phases such as Cu_1.96S and Co_0.24Zn_0.76S were also detected.

中文翻译：

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Cu ₂（Zn _{1- x} Co _x）SnS ₄纳米晶体的合成和由其水分散体形成的多晶薄膜†

五元Cu ₂（Zn _{1- x} Co _x）SnS ₄是有前途的太阳能吸收材料Cu ₂ ZnSnS ₄（CZTS）的类似物。CZTS最初的快速进展已停止，因为类似大小的Cu和Zn阳离子会导致容易形成反位缺陷，这被认为限制了太阳能电池的性能。用钴代替锌可以减少阳离子紊乱。在这里，我们报道了纤锌矿型Cu ₂（Zn _{1− x} Co _x）SnS ₄的合成在整个组成范围内进行研究，并系统地研究了将Co替换为纤锌矿CZTS晶格的过程。该合成基于仅加热到160°C的微波，并使用金属盐和硫脲作为前体，乙二醇作为溶剂。Cu ₂（Zn _{1- x} Co _x）SnS ₄纳米晶体在X射线衍射和拉曼散射的检测极限内为相纯纤锌矿。纤锌矿晶格参数，纳米晶体尺寸和Al拉曼模峰位置取决于Co浓度x。在我们的测量精度范围内，晶格参数遵循Vegard定律，并且A1拉曼模式几乎随x线性移动。随着x从0增大到1 ，纳米晶体的尺寸从8 nm减小到4 nm 。吸收带边缘蓝色从x = 0的1.1 eV变为x = 1的1.35 eV 。这些值低于密度泛函理论预测的值计算和以前尝试确定纤锌矿型Cu ₂ ZnSnS ₄（x = 0）和Cu ₂ CoSnS ₄（x = 1）的光学带隙。纤锌矿型Cu ₂ ZnSnS ₄（x = 0）和Cu ₂ CoSnS ₄（x= 1）较低，或者由于缺陷而这些材料的尾带明显。我们还通过在硫中对包含Cu ₂（Zn _{1- x} Co _x）SnS ₄纳米晶体的涂层进行热退火，制备了多晶Cu ₂（Zn _{1- x} Co _x）SnS ₄薄膜。在硫中退火后，纤锌矿型Cu ₂（Zn _{1- x} Co _x）SnS ₄纳米晶体转变为具有钾镁石结构的较大晶粒（100 s纳米至微米）。带有x的电影≤0.4分别为X射线衍射和拉曼散射的检测限内的相纯锌黄锡矿，但是，对于X ≥0.6，如Cu二次相_1.96 S和CO _0.24锌_0.76还检测到S上。