Crystal growth, electronic and optical properties of Tl2CdSnSe4, a recently discovered prospective semiconductor for application in thin film solar cells and optoelectronics.

A single crystal of the non-centrosymmetric low-temperature modification of Tl 2 CdSnSe 4 (space group I 4 ‾ 2m) was grown, for the first time, and its electronic and optical properties were studied from both experimental and theoretical viewpoints. The Tl 2 CdSnSe 4 crystal was grown from the solution-melt by vertical Bridgman-Stockbarger method. Centimeter dimensions of the crystal allow its application in practice. The bandgap energy, E g = 1.32 eV, estimated from the optical absorption coefficient agrees well with the value of E g = 1.29 eV calculated from the photosensitivity measurements. X-ray photoelectron spectroscopy (XPS) was used to investigate the electronic structure and charge states of atoms composing the crystal under consideration. To detect the best agreement of the curve of total densities of states (DOS) with the experimental XPS spectrum of valence electrons of Tl 2 CdSnSe 4 , we performed theoretical calculations within a density functional theory (DFT) framework treating different models for exchange correlation (XC) potential. Our findings yield that the best agreement of the experimental and theoretical distributions of the valence electronic states is derived when using in the computation procedure for XC potential the modified Becke-Johnson potential in the form of Tran-Blaha (TB-mBJ), which involves also the Hubbard parameter U for strongly correlated d electrons and spin-orbit coupling (SOC) effect (TB-mBJ + U + SOC model). Based on this approach, we calculated partial densities of states, energy band dispersion, and main optical constants of Tl 2 CdSnSe 4. Importantly, the TB-mBJ + U + SOC model reveals the E g value which is close to those determined experimentally; therefore, scissors correction adjustment is not required when performing DFT calculations of the optical constants in such a case. The present experimental XPS measurements of the treated with middle-energy Ar+-ions Tl 2 CdSnSe 4 crystal surface and the theoretical DFT calculations data indicate, in spite of its rather hazardous chemical elements, thallium and cadmium, the crystal surface is rather stable and, coupled with suitable energy band gap, makes quaternary selenide under discussion a very promising material for using in thin film solar cells and optoelectronics as well as in highly efficient photocatalytic devices. Image 1 • For the first time, a crystal of non-centrosymmetric Tl 2 CdSnSe 4 (space group I 4 ‾ 2m) was grown. • Electronic structure of Tl 2 CdSnSe 4 crystal was studied by XPS. • Total and partial densities of states are calculated withinDFT framework. • Calculations within TB-mBJ + SOC + U model are in agreement with XPS measurements. • Principal optical constants of Tl 2 CdSnSe 4 are explored in detail. [ABSTRACT FROM AUTHOR]