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

Molten salt synthesis-growth method is one possibility to produce very high-quality monocrystalline absorber materials in powder form for flexible solar cells. Results of the current study show the influence of different alkali salts on the CZTS monograin powder (MGP) properties and impact on the performance of monograin layer (MGL) solar cells. Cu1.84Zn1.09Sn0.99S4 powders were synthesized from CuS, ZnS and SnS by isothermal molten salt synthesis-growth method in the presence of molten LiI, NaI, KI, RbI and CsI salts as flux materials in sealed vacuum quartz ampoules at 740 oC. SEM and EDX studies showed that the morphology and composition of the formed crystals are influenced by the nature of the flux materials. Structural studies by XRD revealed a shift of all diffraction peaks towards lower angles for CZTS crystals grown in LiI and a larger lattice parameter values in comparison with powder crystals formed in CsI, RbI, NaI and KI. CZTS MGPs grown in LiI also showed the widest main Raman peak (FWHM=7.06 cm-1). In case of CsI, Raman peaks were sharper and narrower (FWHM=4.5 cm-1) compared to the other produced powders, showing a higher level of crystallinity. The estimated effective bandgap energy values from EQE measurements were similar to 1.57 eV and 1.66 eV for CZTS MGPs grown in NaI, KI, RbI, CsI and in LiI, respectively. In addition, detailed analysis of photoluminescence spectroscopy and temperature dependent solar cell parameters will be discussed. The highest efficiency of 10.88% was achieved with MGL solar cell based on CZTS grown in CsI.