Cation substitution induced structural transition, band gap engineering and grain growth of Cu₂Cd_xZn_1−xSnS₄thin films

Cu₂Cd_xZn_1−xSnS₄(CCZTS) thin films with varying the degree of cation substitution were fabricated employing a facile procedure by spin-coating the precursor solutions followed by post-sulfurized treatment. Combining the results from X-ray diffraction and Raman spectroscopy, the substitution of Zn with Cd was confirmed as well as a phase transition from kesterite to stannite with the increase of Cd content in CCZTS. The progressive cation substitution was demonstrated by the red shift in XRD patterns along with the Raman peaks move towards lower wavenumber, and the increasing calculated unit cell, which indicates the lattice expansion. It is observed that the grain size was dramatically improved with the incorporation of Cd into Cu₂ZnSnS₄(CZTS) films. In particular, the band gaps of the CCZTS films determined by optical transmittance spectra were presented to be controlled linearly by adjusting the Cd/(Cd + Zn) ratios of the synthesis precursors from 1.35 (x = 0.0) to 1.15 eV (x = 1.0) with a small bowing constant of the bowing model (0.10 eV). The knowledge gained about the kesterite-stannite transition in CCZTS allowed fundamental understanding of alloying Cd into CZTS of pre-annealed and post-sulfurized consequently for high crystallinity. This further leads to awareness about practical possibility of cation substitution induced grain growth, phase transition in CCZTS, and suggests that such treatments are potential to develop efficient band gap engineering of these compounds towards the fabrication of Cu-based solar cells.