Electronic Structure and Optical Properties ofCu2ZnGeSe4: First-Principles Calculations and Vacuum-Ultraviolet Spectroscopic Ellipsometric Studies

Abstract

Cu 2 ZnGeSe 4 is of interest for the development of next-generation thin-film photovoltaic technologies. To understand its electronic structure and related fundamental optical properties, we performed first-principles calculations for three structural variations: kesterite, stannite, and primitive-mixed CuAu phases. The calculated data are compared with the room-temperature dielectric function ε = ε 1 + iε 2 spectrum of polycrystalline Cu 2 ZnGeSe 4 determined by vacuumultraviolet spectroscopic ellipsometry in the photon-energy range of 0.7 to 9.0 eV. Ellipsometric data are modeled with the sum of eight Tauc-Lorentz oscillators, and the best-fit model yields the bandgap and Tauc-gap energies of 1.25 and 1.19 eV, respectively. A comparison of overall peak shapes and relative intensities between experimental spectra and the calculated ε data for three structural variations suggests that the sample may not have a pure (ordered) kesterite phase. The complex refractive index N = n + ik, normal-incidence reflectivity R, and absorption coefficients α are calculated from the modeled ε spectrum, which are also compared with those of Cu 2 ZnSnSe 4. The spectral features for Cu 2 ZnGeSe 4 appear to be weaker and broader than those for Cu 2 ZnSnSe 4 , which is possibly due to more structural imperfections presented in Cu 2 ZnGeSe 4 than Cu 2 ZnSnSe 4 .

Point defects, compositional fluctuations, and secondary phases in non-stoichiometric kesterites

“…Higher levels in the conduction band can be interpreted from the contribution of (Ge,Sn)-s, Zn-s, and (Ge,Sn)-p states [165]. It is also worth mentioning that Choi et al [154] have observed in Cu 2 ZnGeSe 4 calculations the possibility of multiple transitions at energies around 2.5 eV in the kesterite structure compared to the stannite and PMCA ones.…”

“…Most of the band structure calculations in the Cu 2 Zn(Ge,Sn)(S,Se) 4 compounds, along the Brillouin zone, have been performed based on the density-functional theory formalism in combination either with the local density or generalized gradient approximations [147][148][149][150][151][152][153], while others are based on a restricted selfconsistent GW scheme [154]. It is a common result that the maximum of the valence band (MVB) is mainly formed by the antibonding orbitals from the hybridization of the Cu-d and (S,Se)-p states, while the minimum of the conduction band (MCB) is mainly formed by the antibonding orbitals from the hybridization of the [151], and Li et al [152].…”

“…Several features can be observed after these calculations. Some authors [147,148,153,154] [164]. Their calculations show that kesterite and stannite structures exhibit a direct band gap while an indirect transition should appear in the PMCA structure [154].…”

“…Some authors [147,148,153,154] [164]. Their calculations show that kesterite and stannite structures exhibit a direct band gap while an indirect transition should appear in the PMCA structure [154]. In addition, apart from the fact that the kesterite structure possesses the lowest formation energy although not significantly smaller than the stannite and PMCA structures, and the PD-KS one results in the highest among the four [147], the band gap value is higher for the kesterite structure [148].…”

Stability and electronic properties of planar defects in quaternary I2-II-IV-VI4 semiconductors

“…Many efforts have been devoted to replacing each component of the host material with other chemical elements in the same column in the periodic table. [9][10][11][12][13][14][15][16][17][18][19] Change in the fundamental material properties like the lattice constants and the band edge positions affect the defect properties, and thus the effect of chemical element substitution on the defects should be thoroughly investigated.…”