10.1021/acs.chemmater.7b04010.s004 Christopher J. Bosson Christopher J. Bosson Max T. Birch Max T. Birch Douglas P. Halliday Douglas P. Halliday Chiu C. Tang Chiu C. Tang Annette K. Kleppe Annette K. Kleppe Peter D. Hatton Peter D. Hatton Polymorphism in Cu<sub>2</sub>ZnSnS<sub>4</sub> and New Off-Stoichiometric Crystal Structure Types American Chemical Society 2017 crystal structure disorder time CZTS crystal structures lattice parameter ratios Cu 2 ZnSnS 4 decrease open-circuit voltage charge-neutral defect complexes CZTS phases New Off-Stoichiometric Crystal Structure Types Cu 2 ZnSnS 4 2017-10-30 00:00:00 Dataset https://acs.figshare.com/articles/dataset/Polymorphism_in_Cu_sub_2_sub_ZnSnS_sub_4_sub_and_New_Off-Stoichiometric_Crystal_Structure_Types/5596654 Cu<sub>2</sub>ZnSnS<sub>4</sub> (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 <i>c</i>/<i>a</i> 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<sub>Sn</sub><sup>3–</sup> + Cu<sub>Zn</sub><sup>–</sup> + Cu<sub>i</sub><sup>+</sup> + 3V<sub>S</sub><sup>2+</sup>] defects are prevalent, and H-type, in which [3S<sub>i</sub><sup>2–</sup> + V<sub>Cu</sub><sup>–</sup> + Zn<sub>Cu</sub><sup>+</sup> + 2Sn<sub>Cu</sub><sup>3+</sup>] 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.