Thin films of absorber material Cu₂ZnSnS₄ for solar cells

Pulsed Laser Deposition technique is applied to the production of thin films of CZTS (Cu₂ZnSnS₄).This vacuum technique has proven to be particularly successful in the production of films with acomplex stoichiometry, as in the case of high temperature superconductors. The material ablated bythe laser pulse is transferred to the substrate at very high kinetic energy (~ keV), thus resulting inhigh mobility of the adsorpted atoms yet at low substrate temperatures. Since the reaction ofdecomposition of CZTS via S and SnS evaporation is the main problem all vacuum techniques haveto deal with, it is of interest to see here how the crystallinity develops out of such high energetic,stoichiometric transfer. We investigate the optical and structural properties of thin films produced in high vacuum (p < 10^-6mbar) with a single target made with sintered powder with stoichiometry: Cu2ZnSnS4. The films are deposited on Mo coated SLG in the temperature range from 25C to 500C. X-ray diffraction patterns show an increase in the intensity of main peak associated to kesterite CZTS up to asubstrate temperature of 300 C, then secondary phases start to show up and the main peak associated to kesterite drops down in intensity. Optical measurements (direct and diffusereflectance) and ellipsometry analysis are used to investigate the optical constants of the films produced and to estimate the bandgap, while AFM images are used to investigate the roughness.The same measurements are carried out on the same samples after annealing at 500 C for 20 mins inN2 + S atmosphere and the results are compared.The films produced are in the thickness range 600 – 1000 nm, the excimer laser used is a LambdaPhysik filled with KrF working at 248 nm, with pulse length of 20 ns. Pulse repetition rate was setat 10 Hz, the deposition process was lasting 1 hour.