Spectroscopic ellipsometry study of Cu2ZnSnS4 bulk poly-crystals

Abstract

The linear optical properties of Cu2ZnSnS4 bulk poly-crystals have been investigated using spectroscopic ellipsometry in the range 1.2-4.6 eV at room temperature. The characteristic features identified in the optical spectra are explained by using the Adachi analytical model for the interband transitions at the corresponding critical points in the Brillouin zone (BZ). The experimental data have been modeled over the entire spectral range taking into account the lowest E0 transition near the fundamental absorption edge, and E1A and E1B higher energy interband transitions. In addition, the spectral dependences of the refractive index, extinction coefficient, absorption coefficient, and normal-incidence reflectivity values have been determined and are provided since they are essential data for the design of Cu2ZnSnS4 based optoelectronic devices. In the span of the last decade, much progress has been made in the kesterite based Cu2ZnSnS4 (CZTS) thin film solar cells. This is because kesterites show excellent properties as absorber materials such as an optimal band gap of ~ 1.5 eV 1-3 which can be tuned by adjusting anion and cation compositions 4,5 and a high optical absorption coefficient 6,7 over ~10 4 cm-1. Moreover, the kesterites are constituted by low-cost elements with low toxicity and abundant in the Earth's crust, which allows avoiding the use of critical raw materials.

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

“…In many of these works slight variations in the band gap value have been observed depending on the deviations from stoichiometry: in Cu 2 ZnSnS 4 and Cu 2 ZnSnSe 4 compounds in thin film form, an increase in the band gap with the Cu/(Zn+Sn) ratio was observed [155], including the intermediate Cu 2 ZnSn(S, Se) 4 compounds [163], probably due to a decrease in the MVB level because of a lower degree of hybridization between the Cu-d and (S, Se)-p states, consistent with the theoretical predictions. However, the opposite effect was pointed out by Levcenko et al [100] in Cu 2 ZnSnS 4 polycrystals. The same authors [159] observed a decrease in the E g value (and E 1A and E 1B ones) when the Ge/Zn ratio is increased in Cu 2 ZnGeS 4 , because of a lowering of the MCB, which is also consistent with the band structure calculations.…”

“…Recently, Raman spectroscopy has become a widely used technique for the confirmation of quaternary phase formation, as in the case of its application in solar cell devices [84][85][86][87][88][89][90][91][92], as well as for further fundamental material analysis [93][94][95][96][97][98][99][100]. The increased popularity of Raman spectroscopy for the assessment of kesterite-type quaternary compounds, as well as of other inorganic materials, is mostly due to the strong increase in the usability and versatility of Raman systems and low qualification requirements for the general spectra analysis.…”

“…The agreement between the theoretical predictions and the experimental values is rather satisfactory in most of the cases, given that the energy transitions consistently identified with the interband ones after the theoretical calculations for E g , E 1A , and E 1B . Only Levcenko et al [100] have indicated the possibility of E 1A also corresponding to a second conduction band transition at the Γ(G):(0 0 0) point.…”

“…Theoretically calculated and experimentally obtained by SE energy transition values for Cu 2 Zn(Ge,Sn)(S,Se) 4 compounds (superscripts correspond to cited references). Tetragonal structure corresponding to stannite after the authors.Complementarily, the energy interband transitions in Cu 2 ZnSnS 4[100,155,158,[166][167][168], Cu 2 ZnSnSe 4[100,155], Cu 2 ZnGeS 4[159] and Cu 2 ZnGeSe 4[160] compounds, as well as their intermediate pseudoquaternary ones, Cu 2 Zn(Ge, Sn)S 4…”

Revisiting the Cu-Zn Disorder in Kesterite Type Cu2ZnSnSe4 Employing a Novel Approach to Hybrid Functional Calculations

“…. 1.67 eV (CZTS) [3], their solid solution Cu 2 ZnSn(S x Se 1−x ) 4 (CZTSSe) includes the optimal band gap region of about 1.34 eV for single junction solar cell devices according to the Shockley-Queisser limit [4]. Ultimately, actual devices based on CZTSSe already demonstrated power conversion efficiencies of about 13% [5].…”