Elastic and optical properties of Cu₂ZnSn(Se_xS_{1 −x})₄alloys: density functional calculations

Abstract

Cu 2 ZnSn(S 1−x Se x ) 4 (CZT(S, Se)) is emerging as a very credible alternative to CuIn 1−x Ga x Se 2 (CIGS) as the absorber layer for thin film solar cells. The former compound has the important advantage of using abundant Zn and Sn instead of the expensive In and Ga. A better understanding of the properties of CZT(S, Se) is being sought through experimental and theoretical means. Thus far, however, very little is known about the fundamental properties of the CZT(S, Se) alloys. In this work, theoretical studies on the structural, elastic, electronic and optical properties of CZT(S, Se) alloys through first-principles calculations are reported. We use a density functional code (AIMPRO), along with the Padé parametrization for the local density approximation to the exchange correlation potential. For the alloying calculations we employed 64 atom supercells (approximately cubic) with a 2 × 2 × 2 k-point sampling set. These supercells possess a total of 32 chalcogen species and the CZTSe x S 1−x alloys are described by using the ordered alloy approximation. Accordingly, to create a perfectly diluted alloying host, the species type of the 32 chalcogen sites is selected randomly with uniform probability x and 1 − x for Se and S, respectively. Properties of alloys (structural, elastic, electronic and optical) are obtained by averaging the results of ten supercell configurations generated for each composition. For each configuration, lattice vectors and atomic positions were allowed to relax (although enforcing the tetragonal lattice type) and the Murnaghan equation of state was fitted to the total energy data. The results presented here permit a better understanding of the properties of the CZT(S, Se) alloys which in turn result in the design of more efficient solar cells.

Optical Properties of Cu2ZnSn(SxSe1‐x)4 by First‐Principles Calculations

“…The optical properties of CZTS and CZTSe compounds have been studying extensively . There has also been a number of computational and experimental studies addressing structural, electronic, and optical properties of their mixtures, that is, CZTSSe. The highest efficiency attained so far for CZTSSe is 12.6%, which was achieved by partially substituting S anions with Se forming a band‐gap grading through the depth of the film with optical gaps ranging from 1.1 to 1.5 eV.…”

“…In assessing strategies to enhance photovoltaic efficiency, first‐principles calculations of optical properties can be used to optimize the atomic substitution and in the analysis of optical measurements. For instance, Camps et al calculated using density functional theory (DFT) the absorption coefficient of kesterite and stannite Cu 2 ZnSn(Se x S 1‐ x ) 4 in the local density approximation with 64 atom supercells. Recently, Li et al calculated the dielectric function of the kesterite phase using the modified meta‐GGA of Becke–Johnson (mBJ) functional for an eight atom supercells and a 20 × 20 × 20 k ‐mesh.…”

Optical Properties

“…The static and high-frequency dielectric constants for some Cu 2 -II-IV-VI 4 quaternaries have been theoretically calculated by several authors (CZTS [8][9][10][11], CZTSe [9][10][11], and Cu 2 HgGeS 4 [12]). Experimentally, Choi et al [13] obtained ε ∞ = 9.2 for CZTSe from their measured SE spectrum.…”

Electronic Energy‐Band Structure

“…This difference in the crystal-field strength is related to the different crystalline structures rather than to the lattice constant ratio c/a (as in the Cu(Ga,In) Se 2 alloy [14]) or to the atomic displacements from the ideal crystal (which can strongly affect the crystal field [15]). The calculation was made by a DFT code, along with the local density approximation [16]. The total DOS curve is also shown in Figure 5.3b.…”

“…(b) total DOS curves for stannite-type CZTSe calculated using a DFT code, along with the local density approximation. Reproduced with permission from Camps et al [16]. © Institute of Physics Publishing Ltd Figure 5.4 shows the electronic energy-band structures of (a) kesterite-type CZTSe, (b) stannite-type Cu 2 CdSnSe 4 , and (c) stannite-type Cu 2 HgSnSe 4 quaternary semiconductors [13].…”