Interplay of Performance‐Limiting Nanoscale Features in Cu₂ZnSn(S,Se)₄ Solar Cells

Abstract

Highly performing kesterite‐based Cu2ZnSn(S,Se)4 (CZTSSe) thin‐film solar cells are typically produced under Cu‐poor and Zn‐rich synthesis conditions. However, these processing routes also facilitate the formation of secondary phases as well as deviations from stoichiometry, causing intrinsic point defects. Herein, the local composition of CZTSSe absorbers prepared with different nominal cation concentrations is investigated by applying energy dispersive X‐ray spectroscopy and synchrotron X‐ray fluorescence spectroscopy at the nanoscale to cross‐sectional lamellae. The findings confirm the formation of ZnS(Se) secondary phases, whose presence, number, and dimension strongly increase with the reduction of the nominal Cu and increment of the nominal Zn content. Furthermore, the local compositions of the CZTSSe phase within the absorber reveal strong variations, leading to collateral and multiple off‐stoichiometry types of the kesterite phase in the absorber, which cause different intrinsic point defects. Therefore, the off‐stoichiometry type determined from the integral composition does not represent the complete true picture of this complex material system. Accordingly, the correlation of integral composition with electrical properties or conversion efficiency may be misleading. Overall, the approach provides new experimental insights into the nanoscale relationship among local compositional fluctuations, off‐stoichiometry types, and secondary phases in these promising photovoltaic materials.

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

“…The investigation of Raman/IR phonon spectra of ternary sulphides is important from the viewpoint of acquiring new physical knowledge and characterization of ternary NC materials for applications. Both binary and ternary sulphides occur as secondary (impurity) phases (inclusions) in the nano-or microcrystallites of quaternary chalcogenides, usually deteriorating the properties of the latter [61]. The spectra of NCs can be qualitatively different from their bulk counterpart, as will be exemplified in this review.…”

“…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].…”

“…For Cu-rich composition, correspondingly, CuS is often reported as a major secondary phase [41,46,49]. Often local compositions of the CZTSS(e) phase within the absorber layer exhibit strong variations [61], leading to the formation of other structural phases and intrinsic point defects. Accordingly, the correlation of an average composition with electrical properties or conversion efficiency may be misleading.…”

Atomic spectrometry update – a review of advances in X-ray fluorescence spectrometry and its special applications

“…The nanoscale characterisation of photovoltaic materials represents an emerging class of applications employing nanoXRF spectrometry combined with complementary spectroscopic/imaging techniques. Ritzer et al 44 used X-ray and electron microscopies to study the composition and microstructure of Cu 2 ZnSn(S,Se) 4 (CZTSSe) solar cell absorbers with different nominal compositions and performance at the nanoscale. The experiments showed the coexistence of the CZTSSe absorber material and ZnS(Se) secondary phase segregations, the size and number of which increased when the synthesis conditions become depleted in Cu and richer in Zn.…”