Spray-coated ligand-free Cu2ZnSnS4 nanoparticle thin films

We have fabricated Cu2ZnSnS4 (CZTS) thin films from spray-coating ligand-free nanoparticle inks. The as-synthesized CZTS nanoparticles were inherently ligand-free [1], which allows the use of polar solvents, such as water and ethanol. Another advantage of these particles is that user- and environmentally-friendly alkali metal chloride salts can be directly dissolved in controllable amounts. The homogeneous distribution of alkali metals in the ink allows uniform grain growth within the deposited absorber layer as a result of liquid phase assisted sintering. We find that particularly beneficial effects are seen by including potassium in the film. This produced micron-sized grains, an increased photoluminescence signal, and a bandgap of approx. 1.43 eV. The composition was Cu-poor and Zn-rich, and phase-pure kesterite CZTS was detected by Raman spectroscopy and X-ray diffraction (XRD) as well as an unquantifiable amount of ZnS. A Sono-tek spray-coating system is used which utilizes ultrasonic atomization. We investigate the effect of different binders, ink concentration, and spray-coating conditions, i.e. spray power, flow rate from syringe pump, distance between spray nozzle and the substrate, and time between consecutive spray layers for multiple layers. The spray-coated films are characterized by scanning electron microscopy (SEM), optical microscopy, and a Dektak profilometer.