Manganese-Containing Quaternary Chalcogenides: New Earth-Abundant Semiconductors for Solar Energy Conversion

The need of eco-friendly compound semiconductors in thin film photovoltaics has constantly pushed the research towards more environmental friendly materials, such as the Kesterite-type compounds Cu₂ZnSnS₄, Cu₂ZnSnSe₄ 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 14.9 % [1] highlights the great potential of this abundant chalcogenides to support a sustainable energy transition.<br/>Recently, other quaternary semiconductors have raised attention: Cu₂MnSnS₄/Se₄. Photovoltaic devices based on their solid solution reached an efficiency of 1.8% [2]. An efficiency of 20.3% was calculated for a Cu₂MnSnS₄ based solar cell [3]. Including Cu₂MnGe(S/Se)₄, a band gap energy range of 1.2 to 2.1 eV can be covered by the respective anion as well cation mutations. Moreover Mn is more abundant than Zn (1100 ppm against 79 ppm) [4], leading to a potentially cheaper final device. Thus Mn-based quaternary compound semiconductors have a strong potential in solar energy conversion technologies.<br/>In this work, we study four cation and anion mutation series: Cu₂Mn(Ge,Sn)S₄ and Cu₂Mn(Ge,Sn)Se₄ as well as Cu₂MnSn(S,Se)₄ and Cu₂MnGe(S,Se)₄ aiming for insights into the crystal structure, structural disorder and their band gap energies. In contrast to the kesterite-type materials discussed above, these Mn-containing quaternaries crystallize in the tetragonal stannite-type (when Sn is the four-valent cation) or orthorhombic wurtz-stannite-type (in case Ge is the four-valent cation) structure. This change in the crystal structure, with respect to the kesterites, can block Cu/B^II disorder (Cu/Zn disorder in kesterites), as shown before [5].<br/>We studied the crystal structure, cation distribution and intrinsic point defect scenario in the mixed crystals of these four series (powder samples) by neutron diffraction. This method enables us to differentiate the isoelectronic cations Cu⁺ and Ge⁴⁺ as well as electronic similar Mn²⁺ in the crystal structure analysis giving the possibility to conclude on anti site defects including these cations. From our experimental findings it became evident, that the off-stoichiometry type model developed for kesterites [6] also applies in Mn-containing quaternaries despite their different crystal structure.<br/>We observed Cu-Mn swapping (very small fractional amount of Cu_Mn and Mn_Cu anti sites) in all compounds and mixed crystals with the exception of Cu₂MnSnSe₄ [7]. This type of cation swapping was also observed in Cu₂MnSnS₄ thin films [8].<br/>We found that in the Cu₂Mn(Ge,Sn)S₄/Se₄ series Sn-rich mixed crystals adopt the stannite structure, whereas Ge-rich mixed crystals adopt the wurtz-stannite structure. Within the intermediate range, two chemically identical but structurally different quaternary phases coexist, adopting the tetragonal and the orthorhombic structure respectively. The structural transition is driven by an increasing structural distortion with increasing Ge-content. Both end members of the Cu₂MnSn(S,Se)₄ and Cu₂MnGe(S,Se)₄series crystallize in the same structure type, avoiding a structural transition. But a change in the structural disorder can be noticed within these series, showing an increase around the center of the solid solutions.<br/>Finally, the band gap energy variations within these four cation and anion mutation series will be presented, showing the correlation between crystal structure and this important optoelectronic material property.<br/>[1] www.nrel.gov/pv/cell-efficiency.html<br/>[2] Sun et al., Sol. En. Mater. Sol. Cell. 219 (2021) 110788<br/>[3] Pansuriya et al., Opt. Mater. 126 (2022) 112150<br/>[4] Le Donne et al., Front. Chem. 7 (2019) 297<br/>[5] Gurieva et al., Phys. Rev. Mat. 4 (2020) 054602<br/>[6] Schorr et al., J. Phys.: Energy 2 (2020) 012002<br/>[7] Gurieva et al., Faraday Discuss. 239 (2022) 51<br/>[8] Rudisch et al., Phys. Stat. Solidi B 256 (2019) 1800743