Elaboration et caractérisation des couches minces de matériau CZTS (Cu2ZnSnS4) obtenues par voie sol gel

Abstract

Recently, considerable work has been done on the Cu2ZnSnS4 (CZTS) quaternary compound semiconductor, due to its simplicity, low cost and above all its flexibility in the combination of several chemical compounds, to make it a good absorbent layer for thin film solar cells and thermoelectric energy generator. CZTS thin films are a kind of p-type direct band gap semiconductor with a band gap value of Eg ≈ 1.5 eV for photoelectric energy conversion and are characterized by a large absorption coefficient (> 104 cm - 1). In this way, the aim of the present work is to study the parameters of the annealing temperature and optimal annealing time for the formation of pure CZTS thin layer. For this, we have developed a simple and non-toxic sulfur-free process using the sol gel process and the vacuum channels to prepare the thin layers of pure CZTS phase deposited on glass substrates by dip-coating technique and annealed at different temperatures. and different annealing times. We studied the effect of annealing temperature on the optical properties of CZTS thin films. The last ones were deposited by dip-coating technique using the sol-gel method. They found that increasing the annealing temperature from 200 to 500 °C for one hour (1h) reduced the energy gap from 1.72 to 1.50 eV. So, they judged that these thin layers are good for solar cells. Consideration of these results prompted us to study the effect of the annealing time from 5 to 60 min on the properties of CZTS thin films. The samples obtained were studied by several techniques such as XRD, Raman spectroscopy, SEM, Atomic force microscopy (AFM), UV-vis spectroscopy and photoluminescence. Confirmed by Raman spectroscopy, XRD analysis reveals the formation of a single-phase Cu2ZnSnS4 compound, of quasistoichiometric composition, in the tetragonal structure of kesterite with a preferential orientation along the direction [112]. The grain size tends to increase as the duration of the annealing increases and the temperature of the annealing also increases, a result confirmed by SEM. The study of the morphology by SEM and AFM confirms that the thin films thus produced are homogeneous, without cracking, of nanometric structure and having a very low roughness (of the order of 1 nm). Optical measurements analysis reveals that the layers have a relatively high absorption coefficient in the visible spectrum with a range gap of 1.62−1.50 eV, which is somewhat close to the optimal value of the solar cell. The Luminous Spectra (PL) shows broad bands with a limited intensity of a maximum between 1.62 and 1.50 eV, corresponding to the optical range gap of CZTS and increasing with the annealing time. The study of electrical properties using I(V) also showed that increasing annealing time improves the electrical properties of the structure. In addition, we examined the problem of secondary phases and suggested a way to avoid its formation.