1. Polymorphism in Cu2ZnSnS4 and New Off-Stoichiometric Crystal Structure Types
- Author
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Annette K. Kleppe, Chiu C. Tang, Peter D. Hatton, M. T. Birch, Christopher J. Bosson, and Douglas P. Halliday
- Subjects
010302 applied physics ,Diffraction ,Materials science ,General Chemical Engineering ,02 engineering and technology ,General Chemistry ,Crystal structure ,021001 nanoscience & nanotechnology ,01 natural sciences ,Crystallography ,chemistry.chemical_compound ,Lattice constant ,chemistry ,Polymorphism (materials science) ,0103 physical sciences ,Materials Chemistry ,CZTS ,0210 nano-technology ,Stoichiometry - Abstract
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 [2CuSn3- + CuZn- + Cui+ + 3VS2+] defects are prevalent, and H-type, in which [3Si2- + VCu- + ZnCu+ + 2SnCu3+] 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.
- Published
- 2017
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