Polymorphism in Cu2ZnSnS4 and New Off-Stoichiometric Crystal Structure Types
2017
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- + Zn...
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