Tuning Electrical, Optical, and Thermal Properties through Cation Disorder in Cu₂ZnSnS₄

Abstract

Chemical disorder in semiconductors is important to characterize reliably because it affects materials performance, for instance by introducing potential fluctuations and recombination sites. It also represents a means to control material properties, to far-exceed the limits of equilibrium thermodynamics. We present a study of highly-disordered Cu-Zn-Sn-S (d-CZTS) films along the Cu2SnS3-Cu2ZnSnS4-ZnS binary line, deposited by physical vapor deposition. Deposition at low temperature kinetically stabilizes compositions that are well outside of the narrow, equilibrium solid solution of kesterite (Cu2ZnSnS4). Here we study d-CZTS and its thermal treatment using complementary characterization techniques: X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), and transmission electron microscopy (TEM). We find that cations in d-CZTS are highly-disordered while the sulfur anions remain in a well-defined, cubic close-packed lattice. On the atomic scale, composition fluctuations are accommodated preferentially by stacking faults. Kinetically-stabilized cation disorder can produce nonequilibrium semiconductor alloys with a wide range of band gap, electronic conductivity, and thermal conductivity. d-CZTS therefore represents a processing route to optimizing materials for optoelectronic device elements such as light absorbers, window layers, and thermal barriers.

Raman and X-ray Photoemission Identification of Colloidal Metal Sulfides as Potential Secondary Phases in Nanocrystalline Cu2ZnSnS4 Photovoltaic Absorbers

“…Nevertheless, one should not exclude the possibility of the formation of CuS during the growth of CZTS NCs under conditions slightly deviating from the protocol optimized in our work. It is known that electrical and optical properties of CZTS are strongly dependent on its composition . Therefore, depending on the targeted application (photovoltaics, catalysis, charge storage, etc.…”

Probing Ultrafast Charge Separation in CZTS/CdS Heterojunctions through Femtosecond Transient Absorption Spectroscopy

“…Currently, copper indium gallium selenide (CIGS) is dominating the thin-film solar cell market, but its prospect is limited due to the low abundance of indium in the Earth’s crust. In this scenario, copper zinc tin sulfide (CZTS) emerges as a potential photovoltaic material due to its excellent earth abundance, nontoxicity, direct band gap (1.5 eV), high absorption coefficient (10 4 cm –1 ), , and high carrier mobility. Among the different heterostructures, CZTS and cadmium sulfide (CdS) is the most studied heterojunction where narrow band gap CZTS forms a staggered type II band alignment with wide band gap CdS .…”

Room-Temperature Electron Paramagnetic Resonance Study of a Copper-Related Defect in Cu2ZnSnS4 Colloidal Nanocrystals

“…The performance improvement of kesterite-based devices is obviously limited by the still poor understanding of the fundamental physical properties of these materials and the lack of control over the material parameters during production. This is largely due to an intrinsic tolerance of CZTS and alike compounds to disorder in the cationic sublattice, nonstoichiometry, point defects, and segregation of secondary phases. − The situation is complicated even more by the fact that the best device performance is achieved not for the ideal stoichiometry of the kesterite Cu 2 ZnSnS 4 . One prerequisite for a high PV efficiency is, for instance, a remarkable degree of Cu deficiency, , while Sn deficiency is claimed in some of the studies to suppress the band tails .…”

“…This is largely due to an intrinsic tolerance of CZTS and alike compounds to disorder in the cationic sublattice, nonstoichiometry, point defects, and segregation of secondary phases. − The situation is complicated even more by the fact that the best device performance is achieved not for the ideal stoichiometry of the kesterite Cu 2 ZnSnS 4 . One prerequisite for a high PV efficiency is, for instance, a remarkable degree of Cu deficiency, , while Sn deficiency is claimed in some of the studies to suppress the band tails . Despite the fact that the determinant role of defects and related electronic states in the properties of materials is generally accepted in the field, ,, its systematic investigation has been pursued so far mainly theoretically; see − and references therein.…”

“…One prerequisite for a high PV efficiency is, for instance, a remarkable degree of Cu deficiency, , while Sn deficiency is claimed in some of the studies to suppress the band tails . Despite the fact that the determinant role of defects and related electronic states in the properties of materials is generally accepted in the field, ,, its systematic investigation has been pursued so far mainly theoretically; see − and references therein. Works gaining physical insights into the nature of lattice defects by direct experimental methods remain relatively rare, focusing mainly on one of the defects, Cu–Zn antisite. − Some established experimental physical methods for the investigation of lattice defects have not been applied so far to these materials.…”