Flower-like SnS2/rGO composites with efficient iodide-triiodide redox performance for counter electrodes in Pt-free dye-sensitized solar cells

The work function (WF) of the counter electrode (CE) predominantly determines the device performance of dye sensitized solar cells (DSSCs) as it controls the electron transfer rate for dye regeneration. Hence in this work, tin disulphide/reduced graphene oxide (SnS₂/rGO) composites were synthesized with different wt.% of rGO (3, 5, 7, 10 %) and investigated their performance in DSSCs applications. At the outset, the formation of pristine SnS₂ and SnS₂/rGO composites were confirmed through X-ray diffraction and Raman spectral analysis. The morphological imaging revealed the micro flower-like structure of SnS₂ with cross-sectional width of 5 $\pm$ 0.2 μm, and petal thickness ranges from 30.7 nm to 46.4 nm. The binding energy spectra confirms the 4+ oxidation state of Sn in both SnS₂ and 7 wt% SnS₂/rGO, suggesting that the rGO does not alter the chemical states of SnS₂. Further, the peak-to-peak separation values obtained from Cylic-Voltammetry analysis were found to be 0.37 and 0.57 V for S7 and Pt CE respectively, indicating the rapid electrolyte reduction of 7 wt% SnS₂/rGO. Finally, a photo conversion efficiency (PCE) of 7.3 % has been achieved with 7 wt% SnS₂/rGO CE which is greater than that of the PCE with Pt CE (6.5 %) and also higher than that of pristine SnS₂ with 4.7 %. The improved PCE is attributed to the reduced WF of the 7 wt% SnS₂/rGO composite as measured from the Kelvin probe force microscopy analysis, enabling the rapid redox reaction towards triiodide reduction.