Optical properties of Cu2ZnSn(SxSe1-x)4 solar absorbers: Spectroscopic ellipsometry and ab initio calculations

Abstract

Dielectric functions of Cu2ZnSn(SxSe1-x)4 thin film absorbers with varied x were determined by spectroscopic ellipsometry and ab initio calculations. From the combination of experimental and theoretical studies, the fundamental interband transition energy E0 (∼1–1.5 eV) and the next following transition energy E1 (∼2–3 eV) were identified and found to blue-shift with increasing sulfur anion content, while keeping the energy separation E1−E0 almost constant, ∼1.4 eV from experiments, and 1 eV from theory. In addition, the average dielectric responses were found to decrease with sulfur anion content from both theoretical and experimental results. The Tauc optical bandgap value Eg determined on samples prepared on Mo and soda lime glass substrate showed a positive linear relationship between x and bandgap Eg. The bandgap bowing factor determined from the theoretical data is 0.09 eV.

High absorption coefficients of the CuSb(Se,Te)2 and CuBi(S,Se)2 alloys enable high-efficient 100 nm thin-film photovoltaics

“…Also, the lighter group-VI anion Se atom in CuSbSe 2 yields a 0.34 eV larger gap compared with CuSbTe 2 with E g = 0.87 eV. Similarly, the lighter S atom in CuBiS 2 (E g = 1.30 eV) yields a 0.23 eV larger gap compared with CuBiSe 2 ; this energy difference is somewhat smaller than corresponding value of ∼0.5 eV for both Cu 2 ZnSn(S/Se) 4 and CuIn(S/Se) 2 when comparing their group-VI anion atoms [28,29]; heavier group-V atoms yields smaller energy difference. Thus, the heavier Bi atom yields the smallest variation of E g with respect to anion alloying.…”

Influence of Cu 2 S, SnS and Cu 2 ZnSnSe 4 on optical properties of Cu 2 ZnSnS 4

“…Cu-Zn related intrinsic defect plays critical role in band gap fluctuations and small open-circuit voltage of the CZTS-based solar cells [10][11]. Possibility of tuning the band gap between 1.0 and 1.5 eV is reported [12] by increasing the S/Se ratio in CZTS. Since CZTS is a quaternary compound, during thermal processing and synthesis secondary phases might be formed [13] that play important role in its electrical and optical properties.…”

Spectroscopic ellipsometry study of Cu2ZnSnS4 bulk poly-crystals

“…10 Despite of the considerable efforts aimed at the development of kesterite thin film technology and the photovoltaic conversion improvements, there is still a limited number of works centered on their fundamental material properties. Most reports deal with the structural investigation, 11,12 the theoretical calculation of electronic band structure and point defects, 7,[13][14][15] as well as the analysis of electrical transport, 16,17 vibrational 18,19 and optical properties. [1][2][3]7,13 However, for the optoelectronic applications it is mandatory to have a deeper and more accurate knowledge on the optical properties in a wide photon energy range.…”

“…An examination of the spectroscopic ellipsometry (SE) literature shows that the dielectric function data, ε, have been reported for CZTS thin films. 7,20,21 However the optical response of the thin film has been often treated with the multi-layer stack model composed from three to six sub layers, 7,21 which has a limited accuracy. In this context the data obtained from the bulk poly-crystals in principle should be more reliable and have a greater value for the actual polycrystalline devices design over the measurements on singlecrystalline material that show strong polarization dependence of the dielectric function.…”

“…The obtained values of ∞ are in the range 7.0 -7.2 for the studied samples, while the recent ab-initio calculations give close but lower values of 6.5 -6.8 depending on the employed functional. 7 Let us now consider the obtained energies of the interband transitions. For the E0 transition we get a value from 1.53 to 1.67 eV, which can be assigned to the transition between the valence band maximum (VBM) to the conduction band minimum (CBM) at the Γ(0,0,0) point in the BZ.…”