Discrepancy between integral and local composition in off-stoichiometric Cu2ZnSnSe4 kesterites: A pitfall for classification

Abstract

High-efficiency kesterite-based thin film solar cells typically feature Cu-poor, Zn-rich absorbers although secondary phases occur easily in non-stoichiometric Cu2ZnSnSe4. We therefore applied high-resolution X-ray fluorescence analysis using a synchrotron nanobeam to study the local composition of a CZTSe cross section lamella cut from a sample with an integral composition of Zn/Sn = 1.37 and Cu/(Zn+Sn) = 0.55. We find submicrometer-sized ZnSe-, SnSe/SnSe2-, and even CuSe/Cu2Se-like secondary phases, while the local compositions of the kesterite are highly Zn-rich yet barely Cu-poor with 1.5 ≤ Zn/Sn ≤ 2.2 and Cu/(Zn+Sn) ∼ 1.0. Consequently, great care must be taken when relating the integral composition to other material properties including the device performance.

Point defects, compositional fluctuations, and secondary phases in non-stoichiometric kesterites

“…The integral layer composition may thus not be a reliable measure for anticipating the electronic properties of the actual kesterite material. Furthermore, figure 14(b) demonstrates a significant spread of the local CZTSe compositions on a submicrometer scale [119]. Variations of the cation ratios on a nano-to micrometer scale were also observed by TEM-EDX for solution-grown CZTS single crystals [124] and by APT for CZTSe thin films formed by selenization of co-evaporated Cu-Zn-Sn-Se precursors [121].…”

“…To investigate an extreme case of non-stoichiometry, a CZTSe thin film with an integral layer composition of Zn/Sn=1. 37 and Cu/ (Zn+Sn)=0.55 was grown by two-stage selenization of a sputtered metallic Cu-Zn-Sn precursor [119]. A thin cross-section lamella of approximately 270 nm thickness was prepared from the sample using a focused ion Figure 13.…”

“…Plotting the intensity of the different K lines as a function of the beam position yields the elemental distribution maps of Cu, Zn, Sn, and Se shown in figure 14(a) [119]. The Se exhibits a homogeneous intensity distribution except for some rather large voids within the layer.…”

“…The co-localization with Se indicates the formation of binary compounds, namely ZnSe, SnSe x , and even Cu x Se as marked by the black arrows in figure 14(a). Consequently, all three binary phases are present in the layer and coexist within an area of only a few micrometers in size [119]. Nano-to micrometer-sized secondary phases, in particular ZnSe segregations, were also observed by TEM-EDX and atom probe tomography (APT) for CZTSe thin films with an overall Zn-rich composition formed by selenization of co-evaporated Cu-Zn-Sn-Se precursors [121,122] and for CZTSSe thin films prepared in a sequential solution-based process [123].…”

“…As shown in figure 14(b), the integral layer composition corresponds to the A-type material with Cu vacancies V Cu and Zn Cu anti-sites. The local composition of the CZTSe domains was determined by fitting the individual XRF spectra and calculating the cation ratios from the atomic concentrations thus obtained [119]. The results for two line scans, one horizontally (h) and one vertically (v) as indicated in figure 14(a), are plotted in figure 14(b).…”