Cation/Anion Substitution in Cu₂ZnSnS₄ for Improved Photovoltaic Performance

Cations and anions are replaced with Fe, Mn, and Se in CZTS in order to control the formations of the secondary phase, the band gap, and the micro structure of Cu₂ZnSnS₄. We demonstrate a simplified synthesis strategy for a range of quaternary chalcogenide nanoparticles such as Cu₂ZnSnS₄ (CZTS), Cu₂FeSnS₄ (CFTS), Cu₂MnSnS₄ (CMTS), Cu₂ZnSnSe₄ (CZTSe), and Cu₂ZnSn(S_0.5Se_0.5)₄ (CZTSSe) by thermolysis of metal chloride precursors using long chain amine molecules. It is observed that the crystal structure, band gap and micro structure of the CZTS thin films are affected by the substitution of anion/cations. Moreover, secondary phases are not observed and grain sizes are enhanced significantly with selenium doping (grain size ~1 μm). The earth-abundant Cu₂MSnS₄/Se₄ (M = Zn, Mn and Fe) nanoparticles have band gaps in the range of 1.04-1.51 eV with high optical-absorption coefficients (~10⁴ cm^-1) in the visible region. The power conversion efficiency of a CZTS solar cell is enhanced significantly, from 0.4% to 7.4% with selenium doping, within an active area of 1.1 ± 0.1 cm². The observed changes in the device performance parameters might be ascribed to the variation of optical band gap and microstructure of the thin films. The performance of the device is at par with sputtered fabricated films, at similar scales.