Speaker
Description
The need of handling eco-friendly and cheap compound semiconductors in thin film photovoltaics (PV) has constantly pushed the research towards more environmentally friendly materials, such as Kesterite-type compound semiconductors Cu2ZnSnS4 (CZTS), Cu2ZnSnSe4 (CZTSe) and Cu2ZnSn(S,Se)4 (CZTSSe). They present the only critical raw material (CRM) free thin film PV technology with tunable band gap energy and excellent long-term stability. The current record efficiency for CZTSSe solar cells of 16.5 %[1] highlights the great potential of these abundant chalcogenides to support a sustainable energy transition. Additionally, finding alternative semiconductor materials with suitable properties remains a task of utmost importance for the widespread use of renewable energies.
In record devices these chalcogenide absorber materials generally show a non-stoichiometric composition which is possible due to a remarkable flexibility of their crystal structure. This ability to accept deviations from stoichiometry is correlated to the formation of intrinsic point defects [2]. The formation of such defects, like vacancies, antisites and interstitials, is driven thermodynamically by minimizing the Gibbs free energy of the crystal. The key issue for kesterite solar cells is the large open-circuit voltage deficit which is caused by three main factors: (i) a non-homogeneous absorber composition resulting in bandgap fluctuations [3] and formation of secondary phases, (ii) structural disorder like Cu/Zn disorder [4] or intrinsic point defects[2] causing strong band tailing [3] and (iii) the multi-element composition and the variable valence of Sn resulting in complicated defect properties[5].
Ternary nitrides ZnGeN2 and ZnSnN2 are being considered as promising candidates for photovoltaic absorber materials, containing uniquely elements of low toxicity and low resource criticality. Further to bandgap tuning by alloying, we have shown that these compounds possess a second mechanism for bandgap tuning through cation disorder [6].
The presentation will discuss the potential of point defect engineering to optimize the key factors limiting the open circuit voltage of the potential solar cell. Based on the off-stoichiometry type model for quaternary chalcogenides [2] correlating chemical composition and occurring intrinsic point defects in these compound semiconductors, a compositional region for highly efficient devices can be revealed. Changing the degree of Cu/Zn disorder varies the band gap energy as well as the value of PLmax[7]. The Cu/Zn disorder can be influenced by a thermal annealing process [8] nowadays technologically applied as post-deposition treatment. Cation disorder is also important for the bandgap engineering in ZnGeN2, showing an interplay with the unavoidable oxygen content in the material[6].
The presentation is based on our systematic experimental studies of quaternary compound semiconductors applying advanced analytical methods like neutron and synchrotron X-ray diffraction, photoluminescence and UV-Vis spectroscopy.
[1] https://www.nlr.gov/pv/cell-efficiency
[2] S.Schorr et al., J. Phys.: Energy 2 (2020) 012002
[3] T.Gokmen et al., Appl. Phys. Lett. 103 (2013) 103506
[4] S.Schorr et al., Europ. J. Mineral. 19 (2007) 65
[5] R.Fonoll-Rubio et al., Energy Environ. Sci. 14 (2021) 507
[6] Z.Wang et al., J. Mat. Chem. C 12 (2024) 1124
[7] G.Gurieva et al., Sol. En. Mat. Sol. Cells 248 (2022) 112009
[8] D.Toebbens et al., Phys. Stat. Solidi B 253 (2016) 1890
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