In-Situ and Green Method To Prepare Pt-Free Cu₂ZnSnS₄ (CZTS) Counter Electrodes for Efficient and Low Cost Dye-Sensitized Solar Cells

A novel in-situ and green method with the combination of TiCl₄ pretreatment and solvothermal growth was proposed to prepare the Pt-free Cu₂ZnSnS₄ (CZTS) counter electrodes (CEs) on FTO substrate. It was found that TiCl₄ pretreatment toward the substrates was crucial to a high quality and high efficiency CZTS film, which consists of nanosheet and nanoparticle structure. The highly crystallized nanosheets were assembled from kesterite structure CZTS nanocrystals, with an average diameter of 10–30 nm. The detailed morphology, crystal structure, and composition of the CZTS nanostructures were characterized by SEM, TEM, SAED, XRD, Raman, and EDS analysis, respectively. Electrocatalytic abilities of the films toward I^–/I₃^– were verified through cyclic voltammograms (CV), electrochemical impedance spectroscopy (EIS), and Tafel polarization measurements. A maximum power conversion efficiency of 5.65% was achieved for a cell with CZTS 3, under 100 mW/cm², which was higher than that of a cell with a commercial Pt counter electrode (4.96%). This high efficiency was mainly attributed to the good bonding strength between CZTS films and FTO substrate (R_S = 4.41 Ω cm²), and higher electron-transfer process at the electrolyte/CE interface (high J_SC), along with superior electrochemical catalytic ability (R_Ct = 2.40 Ω cm²). The in-situ prepared CZTS CEs are proven to be suitable for high efficiency Pt-free dye-sensitized solar cells (DSSCs), leading to significantly decrease of the cell cost and simplification of preparation process.