Sol–gel prepared CZTS (Cu₂ZnSnS₄) films doped and co-doped with alkali metals (Li, Na and K)

In this study, we present the synthesis and characterisation of Cu₂ZnSnS₄ (CZTS) thin films—undoped, singly doped, and co-doped with alkali metals (Li, Na, and K), where 1.5% of the Cu atoms were substituted. The investigation focused on analysing and comparing the stoichiometry, morphology, crystalline structure, and optical and electrical properties as influenced by individual dopants (Li, Na, K) and their binary combinations (LiNa, LiK, and NaK). To date, no study has systematically explored the effects of both single and co-doping of CZTS films with alkali metals using a single deposition technique. This work addresses that gap by providing a comparative analysis of the impact of all three alkali metals—both individually and in binary combinations—using a consistent spin-coating method. SEM/EDX analysis revealed uniform surfaces across all doped samples, with notable variations in surface morphology and porosity depending on the specific dopant. The elemental content of the films exhibited slight Cu enrichment and Sn and S deficiency. XRD patterns and the calculated lattice parameters (a, c) remained within the theoretical and experimental ranges characteristic of tetragonal kesterite/stannite CZTS structures, regardless of the dopant. Doping led to increased micro-strain and a reduction in crystallite size, from approximately 78 nm in pristine CZTS to around 50 nm in the NaK co-doped sample. Raman spectroscopy confirmed the CZTS phase, showing the characteristic A vibrational modes of the most common kesterite structure around 284 cm⁻¹ and 330 cm⁻¹. Optical analysis demonstrated a significant effect of doping on the bandgap, which decreased from 1.45 eV for the undoped film to a range of 1.43–1.28 eV for the doped samples. Changes in carrier type (n- or p-type) were primarily observed in Cu-rich and Sn-poor films, particularly in the undoped and LiNa co-doped samples. Overall, the results show that alkali metal doping has a pronounced impact on the structural, optical, and electrical properties of CZTS thin films.