Optical properties of Cu2ZnSnS4 and Cu2CdSnS4 quaternary compounds

Abstract

Nowadays, the efficiency of Cu2ZnSnS4 (CZTS) thin-film solar cells is still limited by various factors such as: electronic disorder, secondary phases and the presence of antisite defects. In order to avoid this limitations, the Zn substitution by heavier atoms like Cd was proposed, as it may inhibit the formation of antisite defects, thereby increasing the minority carrier lifetime and reducing electronic disorder in the system. Thus, the main goal of this work was to investigate the optical properties of Cu2ZnSnS4 (CZTS) and Cu2CdSnS4 (CCTS) quaternary compounds. Hence, the reflectance, transmittance and photoluminescence spectra were recorded over a wide temperature range (from 10 to 300 K). As a result, for the CZTS sample, the optical band gap energy at room temperature was found to be equal to 1.46 eV. Also, reflectance and photoluminescence spectra at 15 K revealed essential details about the excitonic behavior in the CCTS sample, in particular for the A type exciton, with ground and excited states (nA = 1 and nA = 2) observed. The binding energy for the A type exciton was found to be 64 meV, leading to an estimated band gap width (Eg) of about 1.39 eV. In addition, at higher energies, spectra revealed maxima associated with the ground and excited states (nB = 1 and nB = 2) of the B type exciton, with an estimated binding energy of 75 meV and a continuum energy of about 1.51 eV.