Influence of alkali iodide fluxes on Cu2ZnSnS4 monograin powder properties and performance of solar cells

Abstract

One promising and cost-effective method to produce flexible solar panels in the future is monograin layer (MGL) solar cell technology based on monograin powder (MGP) crystals. The results of the present study demonstrate the influence of different alkali salts (LiI, NaI, KI, RbI and CsI) on the properties of Cu₂ZnSnS₄ (CZTS) MGPs and their effect on the characteristics of MGL solar cells. SEM and EDX studies revealed that the morphology and composition of the formed crystals are influenced by the nature of the flux materials. Structural studies by XRD showed good crystallinities for all MGPs. However, CZTS crystals grown in LiI exhibited a shift of all diffraction peaks towards lower angles and larger lattice parameter values. In addition, powder grown in LiI exhibited the broadest main Raman peak (FWHM = 7.06 cm⁻¹). When CsI was used, the Raman peaks were sharper and narrower with FWHM of 4.5 cm⁻¹, showing a higher level of crystallinity compared to other produced powders. The estimated band gap energy values obtained from EQE measurements were ∼1.57 eV for NaI, KI, RbI, CsI and 1.65 eV for LiI grown CZTS MGPs. The analysis of temperature-dependent current–voltage characteristics indicated that tunneling enhanced interface recombination is the prevailing process in all materials. At low temperatures, the powder crystals grown in LiI, NaI, and RbI revealed the presence of recombination channels that were not observed at room temperature. The MGL solar cell based on CZTS powder grown in CsI resulted in the highest power conversion efficiency of 10.9%.