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 (n
A = 1 and n
A = 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 (n
B = 1 and n
B = 2) of the B type exciton, with an estimated
binding energy of 75 meV and a continuum energy of about 1.51 eV.