Atomic Scale Structure of (Ag,Cu)2ZnSnSe4 and Cu2Zn(Sn,Ge)Se4 Kesterite Thin Films

Abstract

Kesterite based materials are being researched and developed as affordable, efficient, and mechanically flexible absorber materials for thin film photovoltaics. Both (Ag,Cu)2ZnSnSe4 and Cu2Zn(Sn,Ge)Se4 based devices have shown great potential in overcoming some of the remaining challenges for further increasing the conversion efficiency of kesterite based solar cells. This study therefore investigates the long range crystallographic structure and the local atomic scale structure of technologically relevant thin films by means of grazing incidence X-ray diffraction and low temperature X-ray absorption spectroscopy. As expected, the unit cell dimensions change about an order of magnitude more than the element specific average bond lengths. In case of Cu2Zn(Sn,Ge)Se4, the thin film absorbers show a very similar behavior as Cu2Zn(Sn,Ge)Se4 powder samples previously studied. Small amounts of residual S in the thin films were taken into account in the analysis and the results imply a preferential formation of Sn-S bonds instead of Ge-S bonds. In (Ag,Cu)2ZnSnSe4, the dependence of the Ag-Se and Cu-Se bond lengths on Ag/(Ag+Cu) might indicate an energetic advantage in the formation of certain local configurations.

Short- and Long-Range Cation Disorder in (AgxCu1–x)2ZnSnSe4 Kesterites

“…Figure f shows the Cu EXAFS spectra (see Table S2) for a detailed list of coordination numbers ( CN s), bond distances ( R ), mean-squared relative displacements (σ 2 ), and energy shifts (Δ E 0 ). The Cu–Se bond length was ∼2.40 ± 0.01 Å for all n x -CZTSe samples, in agreement with other reports . The CN Cu–Se of n 0.1 -ACZTSe was 3.8 ± 0.3, close to the ideal value (i.e., 4.0 ± 0.2) calculated for pristine n 0 -CZTSe.…”

“…Figure h and Table S5 show the Sn EXAFS and the corresponding fitting data, respectively. From Sn EXAFS, a bond length of ∼2.55 ± 0.1 Å observed for the three samples could be assigned to the Sn–Se bond, according to other literature reports . In addition, the CN value (4.0 ± 0.1) remained unchanged for n 0 -CZTSe and n 0.1 -ACZTSe but decreased to 3.7 ± 0.1 for n 0.2 -ACZTSe.…”

“…The Cu−Se bond length was ∼2.40 ± 0.01 Å for all n x -CZTSe samples, in agreement with other reports. 62 The CN Cu−Se of n 0.1 -ACZTSe was 3.8 ± 0.3, close to the ideal value (i.e., 4.0 ± 0.2) calculated for pristine n 0 -CZTSe. However, CN Cu−Se decreased to 3.6 ± 0.6 for n 0.2 -ACZTSe.…”

“…From Sn EXAFS, a bond length of ∼2.55 ± 0.1 Å observed for the three samples could be assigned to the Sn−Se bond, according to other literature reports. 62 In addition, the CN value (4.0 ± 0.1) remained unchanged for n 0 -CZTSe and n 0.1 -ACZTSe but decreased to 3.7 ± 0.1 for n 0.2 -ACZTSe. Finally, the CN Ag-Se value was estimated to be 4.0 ± 0.6 for n 0.1 -ACZTSe and decreased to 3.3 ± 0.3 for n 0.2 -ACZTSe, while the corresponding bond length changed to 2.61 ± 0.01 Å (Table S6).…”

Phonon Raman spectroscopy of nanocrystalline multinary chalcogenides as a probe of complex lattice structures

“…Therefore, determining the intrinsic vibrational spectra of nanosized ternary sulphides is crucial for their identification as impurity inclusions in quaternary materials and establishing Raman spectroscopy as an affordable quantitative diagnostic tool for the vast population of I-III-VI and I-II-IV-VI families [62]. On the other hand, reliable differentiation between segregation of the elements, non-stoichiometry, and formation of secondary (impurity) phases usually requires time-consuming and costly investigations involving several complementary methods, including, for instance, resonant XRD, energy-dispersive x-ray spectroscopy and high-spatial-resolution x-ray fluorescence analysis (nano-XRF) [61,63,64].…”