Point defects in Cu₂ZnSnSe₄ (CZTSe): Resonant X‐ray diffraction study of the low‐temperature order/disorder transition

Abstract

The interest in Cu 2 ZnSn(S,Se) 4 (CZTS) for photovoltaic applications is motivated by similarities to Cu(In,Ga)Se 2 while being comprised of non-toxic and earth abundant elements. However, CZTS suffers from a V oc deficit, where the V oc is much lower than expected based on the band gap, which may be the result of a high concentration of pointdefects in the CZTS lattice. Recently, reports have observed a low-temperature order/disorder transition by Raman and optical spectroscopies in CZTS films and is reported to describe the ordering of Cu and Zn atoms in the CZTS crystal structure. To directly determine the level of Cu/Zn ordering, we have used resonant-XRD, a site, and element specific probe of long range order. We used CZTSe films annealed just below and quenched from just above the transition temperature; based on previous work, the Cu and Zn should be ordered and highly disordered, respectively. Our data show that there is some Cu/Zn ordering near the low temperature transition but significantly less than high chemical order expected from Raman. To understand both our resonant-XRD results and the Raman results, we present a structural model that involves antiphase domain boundaries and accommodates the excess Zn within the CZTS lattice.

Temperature dependence of Raman scattering in the Cu2ZnSnSe4 thin films on a Ta foil substrate

“…A similar asymmetrically broadening dominant Raman peak of the CZTSe was observed in few works (Khare et al, 2012;Djemour et al, 2013;Juskenas et al, 2016;Rey et al, 2014;Schelhas et al, 2017). For…”

“…Nevertheless, according to Khare et al (2012), Fontane et al (2012), Djemour et al (2013), andDimitrievska et al (2014) this second line is associated to a different crystal modification (like, stannite and Cu-Au ordering) or the phonon confinement effects because of the presence of lattice defects in the scattering volume (for example, disordered kesterite). In addition, a similar broadening and shift towards greater Raman shift of the CZTSe dominant Raman peak was observed at studying the structural transition in the cation sublattice of CZTSe (i.e., transitioning from disordered to ordered kesterite) by Rey et al (2014) and Schelhas et al (2017). Nevertheless, the existence of this second line at dominant Raman peak of the CZTSe is not discussed in these works, but it has been established that the disordered keserite leads to asymmetric broadening and shift of peak towards greater Raman shift.…”

Resolving the effects of compositional change on structures in Cu2ZnSnS4 nanocrystals by X-ray absorption fine structure

“…In both cases, deviations from the 1:1 ratio of Zn:Sn contribute to bond shortening, and a contraction of the observed crystal lattice. This leaves Sample 3 with the least vibrational rigidity, but the most consistent kesterite crystal lattice 5,[8][9][10]. Unlike the Cu and Zn centers, Sn has been consistent throughout all of the samples, and throughout all of the analyses discussed herein.…”

“…6 Previous work has shown maximal photoresponse within small compositional ranges, though the exact range appears to be dependent on the method of synthesis. 5,[7][8][9] Using a facile, low-temperature solvothermal method, the maximal photocurrent range appears to be relatively large. This occurs due to the increased number of antisite substitutions and disordered sites associated with low-temperature fabrication.…”

“…16 Previous work has identified many different defect structures that form in CZTS films, and how they contribute to the overall photoresponse of the layer. 7,9 These defects, including antisite substitutions and atomic vacancies, are associated with stoichiometric regions that correspond with different photoresponses. Using X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS), the changes in antisite structure within the narrow stoichiometric band that produces high photoresponse can be isolated for many different stoichiometries.…”