Exploration, Prediction and Experimental Verification of Structure and Optoelectronic Properties in I₂-Eu-IV-X₄ (I = Li, Cu, Ag; IV = Si, Ge, Sn; X = S, Se) Chalcogenide Semiconductors

Recent extensive research into the defect-resistant I2-II-IV-X4 (I = Li, Cu, Ag; II = Ba, Sr, Eu, Pb; IV = Si, Ge, Sn; X = S, Se) family of quaternary chalcogenide semiconductors suggests excellent potential for applications in photovoltaics, thermoelectrics, and nonlinear optics. Among these compounds, Eu containing members are understudied, with only five previously synthesized members. Herein, we undertake a comprehensive study of the possible structures and electronic properties of all eighteen of the Eu-based combinations within I2-Eu-IV-X4 (I = Li, Cu, Ag; IV = Si, Ge, Sn; X = S, Se). To further understand the broader I2-II-IV-X4 family and test the geometric tolerance factor (reported in our previous work) as a tool for predicting potential stable structures, we first use hybrid density functional theory to systematically study these rare-earth-including I2-Eu-IV-X4 semiconductors. Lowest-energy quaternary structure candidates, energy band structures, and densities of states are computationally predicted for all eighteen compounds. Following this screening process, the previously unknown compound Cu2EuSnSe4 was selected and synthesized due to its predicted photovoltaics-relevant direct band gap. Optimal synthesis conditions were determined, and the experimentally derived structure, lattice parameters, and bandgap of Cu2EuSnSe4 were found to be consistent with the predictions from both geometric tolerance factors and hybrid DFT, validating our predictive approaches and confirming a 1.55 eV band gap. Along with strong optical absorption in the visible range, this band gap suggests potential for Cu2EuSnSe4 in photovoltaic and other optoelectronic applications.