Aqueous-Solution-Processed Cu₂ZnSn(S,Se)₄ Thin-Film Solar Cells via an Improved Successive...

A facile improved successive ionic-layer adsorption and reaction (SILAR) sequence is described for the fabrication of Cu₂ZnSn(S,Se)₄ (CZTSSe) thin-film solar cells (TFSCs) via the selenization of a precursor film. The precursor films were fabricated using a modified SILAR sequence to overcome compositional inhomogeneity due to different adsorptivities of the cations (Cu⁺, Sn⁴⁺, and Zn²⁺) in a single cationic bath. Rapid thermal annealing of the precursor films under S and Se vapor atmospheres led to the formation of carbon-free Cu₂ZnSnS₄ (CZTS) and CZTSSe absorber layers, respectively, with single large-grained layers. The best devices based on CZTS and CZTSSe absorber layers showed total area (∼0.30 cm²) power conversion efficiencies (PCEs) of 1.96 and 3.74%, respectively, which are notably the first-demonstrated efficiencies using a modified SILAR sequence. Detailed diode analyses of these solar cells revealed that a high shunt conductance (G _sh), reverse saturation current density (J _o), and ideality factor (n _d) significantly affected the PCE, open-circuit voltage (V _oc), and fill factor (FF), whereas the short-circuit current density (J _sc) was dominated by the series resistance (R _s) and G _sh. However, the diode analyses combined with the compositional and interface microstructural analyses shed light on further improvements to the device efficiency. The facile layer-by-layer growth of the kesterite CZTS-based thin films in aqueous solution provides a great promise as an environmentally benign pathway to fabricate a variety of multielement-component compounds with high compositional homogeneities.