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.
Highlights
The quaternary semiconductor material Cu2ZnSn(S,Se)4 (CZTSSe), one of the socalled kesterites, has received great attention for thin-film solar cells owing to its direct, tunable bandgap energy between 1.0 and 1.5 eV,[1] its high absorption coefficient,[1,2] and its earth abundant, nontoxic constituents
The layered geometry of the sample is clearly visible by scanning electron microscopy (SEM) and energydispersive X-ray spectroscopy (EDS), and contrast variations within the absorber, small voids, and a thin Pt layer that was deposited to protect the absorber during the lamella preparation.[22]
Segregations of ZnS(Se) were found for CZTSSe thin films prepared by a non-toxic solvent-based process using transmission electron microscope (TEM)-EDS.[27]
Summary
The quaternary semiconductor material Cu2ZnSn(S,Se) (CZTSSe), one of the socalled kesterites, has received great attention for thin-film solar cells owing to its direct, tunable bandgap energy between 1.0 and 1.5 eV,[1] its high absorption coefficient,[1,2] and its earth abundant, nontoxic constituents Despite these ideal properties, the conversion efficiency is still limited to 12.6%,[3] which is mainly attributed to losses in the open circuit voltage in relation to the optical bandgap. In this work, high-efficiency solar cell absorbers with different nominal compositions are examined by energydispersive X-ray spectroscopy (EDS) and high-spatial-resolution X-ray fluorescence analysis (nano-XRF) These techniques enable the detection, identification, and localization of secondary phases and off-stoichiometry types[13,19,20,21] caused by local elemental fluctuations in CZTSSe solar cells. Compositional modifications at selected grain boundaries are found, which could influence the related solar cell performance
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