Variable chemical decoration of extended defects in Cu-poor $\mathrm{C}{\mathrm{u}}_2\mathrm{ZnSnS}{\mathrm{e}}_4$ thin films

We report on atom probe tomography studies of variable chemical decorations at extended defects in Cu-poor and Zn-rich $\mathrm{C}{\mathrm{u}}_2\mathrm{ZnSnS}{\mathrm{e}}_4$ thin films. For a precursor film, which was co-evaporated at $320{}^{\circ }\mathrm{C}$, grain boundaries and dislocations are found enriched with Cu. Furthermore, Na out-diffusion from the soda-lime glass substrate occurs even at such a low temperature, resulting in Na segregation at defects. In contrast, stacking faults in the precursor film show clear Zn enrichment as well as Cu and Sn depletion. After an annealing step at $500{}^{\circ }\mathrm{C}$, we detect changes in the chemical composition of grain boundaries as compared to the precursor. Moreover, we measure an increase in the grain boundary excess of Na by one order of magnitude. We show that grain boundaries and dislocations in the annealed $\mathrm{C}{\mathrm{u}}_2\mathrm{ZnSnS}{\mathrm{e}}_4$ film exhibit no or only slight variations in composition of the matrix elements. Thus, the effect of annealing is a homogenization of the chemical composition.

Figure 1

Schematic presentation of the investigated samples. APT measurements were carried out on samples without having CdS/i-ZnO/ZnO:Al layers. The Ni-Al grid is not shown here.

Figure 2

(a)–(d) Three-dimensional elemental maps of Na (green) atoms from a dataset of the Cu-poor precursor. (c) and (d) are rotated by ${110}^{\circ }$ with respect to (a) and (b). The blue isocomposition surfaces in (b) and (d) mark 24.8 at.% Cu and, hence, Cu-enriched CZTSe GBs. (e), (f) Composition profiles across the GBs I and II, i.e., along the red arrows given in (b) and (d), respectively. (g) and (h) show the calculated relative composition fluctuations of the composition profiles given in (e) and (f), respectively. To view the rotating elemental maps, see the Supplemental Material [48].

Figure 3

(a)–(c) Three-dimensional elemental maps of Na (green) atoms from a dataset of the Cu-poor absorber. (c) is rotated by ${70}^{\circ }$ with respect to (a) and (b). The gray isocomposition surfaces in (a) mark 32.0 at.% Zn and, hence, ZnSe precipitates. (d) The composition profiles across the GBs VII, i.e., along the red arrow given in (b). (e)–(g) The calculated relative composition fluctuations across the GBs VII–IX, i.e., along the red arrows given in (b) and (c). (b) Shows additionally by a circle marked area, where Na is enriched at dislocations that are located along a ZnSe/CZTSe interface (see text for more details). For rotating elemental maps, see the Supplemental Material [48].

Figure 4

(a), (b) APT dataset from the Cu-poor precursor. (b) is rotated by ${90}^{\circ }$ with respect to (a). The gray and blue isocomposition surfaces in (a) and (b) mark 20.0 at.% Zn and 26 at.% Cu, respectively Two stacking faults (I and II) and a dislocation (III) are observed. (c), (d) Relative composition fluctuation profiles across the stacking fault I and the dislocation III, i.e., along the red arrows given in (b). The black arrow in (d) marks the position of the dislocation.

Figure 5

(a), (b) Three-dimensional elemental maps of Na (green) atoms from an APT dataset of the Cu-poor absorber. The gray isocomposition surfaces in (a) mark 32.0 at.% Zn and, hence, ZnSe precipitates. (c), (d) The relative composition fluctuation profiles across the dislocations I and II, i.e., along the red arrows given in (b). The black arrows in (c) and (d) mark the position of the dislocations. For elemental maps viewed from different perspectives, see the Supplemental Material [48].