Cu₂ZnSnS₄ (CZTS) is a very promising material for the absorber layer in sustainable thin-film solar cells. Its photovoltaic performance is currently limited by crystal structure disorder, which causes fluctuations in electrostatic potential that decrease open-circuit voltage. The origin of this disorder is still not fully understood. This work investigates five samples of CZTS over a range of compositions, fabricated by solid-state reaction. Their crystal structures are conclusively identified using high-resolution anomalous X-ray diffraction. Three of the samples display two distinct CZTS phases, evident in minute splitting of some diffraction peaks (by ∼0.02°) due to different c/a lattice parameter ratios. These are attributed to different composition types of CZTS, defined by different charge-neutral defect complexes. In addition to such types previously reported, two new types are proposed: G-type, in which [2Cu_Sn^3– + Cu_Zn^– + Cu_i⁺ + 3V_S²⁺] defects are prevalent, and H-type, in which [3S_i^2– + V_Cu^– + Zn_Cu⁺ + 2Sn_Cu³⁺] defects are prevalent. In both cases, the defects probably do not form a single complex due to their large number. Above the order–disorder phase transition the two CZTS phases generally converge to a single phase. A mechanism of phase formation in CZTS is thus proposed. This is the first time CZTS crystal structures have been investigated with sufficiently high resolution to distinguish these different CZTS phases, thereby establishing polymorphic behavior in CZTS.

中文翻译：

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Cu ₂ ZnSnS _4中的多态性和新的非化学计量晶体结构类型

Cu ₂ ZnSnS ₄（CZTS）是可持续薄膜太阳能电池吸收层的一种非常有前途的材料。目前，其光伏性能受到晶体结构紊乱的限制，晶体紊乱会引起静电势的波动，从而降低开路电压。这种疾病的根源仍不完全清楚。这项工作研究了通过固态反应制备的一系列组成范围内的五个CZTS样品。使用高分辨率异常X射线衍射可以最终确定其晶体结构。样品中的三个显示出两个不同的CZTS相，由于c / a的不同，一些衍射峰的微小分裂（约0.02°）非常明显。晶格参数比率。这些归因于由不同电荷中性缺陷复合物定义的CZTS的不同组成类型。除了先前报道的这种类型，还提出了两种新类型：G型和H型，其中[2Cu _Sn ^3– + Cu _Zn ^– + Cu _i ⁺ + 3V _S ²⁺ ]缺陷普遍存在。 [3S _i ^2– + V _Cu ^– + Zn _Cu ⁺ + 2Sn _Cu ³⁺缺陷是普遍存在的。在这两种情况下，由于缺陷数量众多，缺陷可能不会形成一个单一的复合体。在有序-无序相变之上，两个CZTS相通常会聚为一个相。因此提出了CZTS中相形成的机理。这是首次以足够高的分辨率研究CZTS晶体结构以区分这些不同的CZTS相，从而在CZTS中建立多态行为。