Cation Mutation in Cu₂Zn(Ge_xSi_1-x)Se₄ Solid Solution as a Path Towards CRM-Free Top Absorber Layer for Tandem Solar Cells

The search for sustainable, efficient, and cost-effective photovoltaic materials continues to be a challenge in the field of solar energy production. In particular, the development of critical raw material free top absorber layers for tandem solar cells is crucial in the effort to transition away from fossil fuels and towards a greener energy future. The emergence of compound semiconductors has provided an opening into low-cost fabrication of thin-film solar cells, made possible a reduction in absorber layer thickness, and therefore lead to a decrease in production costs.<br/>In this work, we explore the potential of tetravalent cation mutation in Cu-based quaternary chalcogenide semiconductors (Cu₂ZnSi_xGe_1-xSe₄ [1]) with the aim of finding a material with increased bandgap (ideally around 1.7eV) and reduced structural disorder, which is considered to be the main culprit of the low V_OC in the only CRM-free material used in PV technology at the moment (Cu₂ZnSn(S,Se)₄). One of the ways, previously shown to completely block structural disorder, especially the Cu/Zn disorder, is a change of the atomic crystal structure, e.g. [2]. The substitution of the tetravalent cation in the compound semiconductor brings a drastic structural change from the tetragonal kesterite type structure in Cu₂ZnGeSe₄ to the monoclinic wurtz-kesterite type structure in Cu₂ZnSiSe₄, with a significant increase in the bandgap as well [3].<br/>We studied crystal structure, cation distribution and intrinsic point defect scenario in Cu₂ZnSi_xGe_1-xSe₄ mixed crystals (powder samples) by neutron diffraction. This method enables us to differentiate the isoelectronic cations Cu⁺, Zn²⁺and Ge ⁴⁺ in the crystal structure analysis. These investigations enabled us to deduce the structural transition scenario of the Cu₂Zn(Ge,Si)Se₄, series which is going via a region where two phases with different crystal structures but with the same chemical composition coexist. Interestingly both quaternary phases show the same cation distribution within the element specific cation sites. In the Cu₂Zn(Ge,Si)Se₄, series the distortion of the coordination tetrahedra is increasing with increasing Si content thus indicating that the structural transition from the kesterite type to the wurtz-kesterite type structure is distortion driven.<br/>The results of this study highlight the importance of considering alternative materials beyond the known kesterites in the search for CRM-free top absorber layers and demonstrate that cation mutation in quaternary chalcogenides is a promising path towards the development of highly efficient tandem solar cells.<br/><br/>[1] Niedenzu, S. et al. In: IEEE, 2018. - ISBN 978-1-5386-8529-7, p. 3290-3293<br/>[2] Gurieva, G. et al. Phys. Rev. Mat. 4 (2020), p. 054602/1-8<br/>[3] Schorr, S. Sol. En. Mat. and Sol. Cells 249 (2022), p. 112044<br/><i>This work has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement no.952982. A portion of this research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the ORNL.</i>