Preparation of iron diselenide/reduced graphene oxide composite and its application in dyesensitized solar cells

In recent years, dye-sensitized solar cells (DSSCs) have attracted much attention because of their easy fabrication, good flexibility low cost and relatively high efficiency. As a crucial component, the function of counter electrode (CE) is to collect the electrons from external circuits and transfer them to electrolyte by catalyzing the reduction of I3- into I-. Platinum (Pt) is a conventional material of CE in DSSCs due to its high conductivity and outstanding catalytic activity towards the reduction of triiodide (I3-). However, the high cost and low abundance of Pt restrict the commercial application of DSSCs. Moreover, Pt could be dissolved slowly in the I-/I3- redox electrolyte, which will deteriorate the long term stability of DSSCs. Therefore, it is necessary to explore novel material with high conductivity, catalytic activity and stability to replace Pt. In this paper, with Fe(NO3)39H2O and graphene oxide (GO) serving as raw materials and deionized water as the solvent, we synthesize iron diselenide (FeSe2) nanorods (with diameters in a range of about 100-200 nm)/reduced graphene oxide (rGO) composite through a facile hydrothermal method and use the composite as CE material of DSSCs for the first time. The structure and morphology of FeSe2/rGO are characterized by using X-ray diffraction (XRD), Raman spectrum, field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). The XRD pattern shows that the FeSe2 is typically orthorhombic phase. The SEM images show that the FeSe2 has a structure of nanonods and can be attached to the surface of rGO closely The surface of FeSe2/rGO composite is rough and exhibits a porous structure. The TEM image shows that the FeSe2 has a high degree of crystallinity and orientation. To evaluate the catalytic activity and conductivity of FeSe2/rGO, we perform cyclic voltammetry (CV) measurements, electrochemical impedance spectroscopy and obtain Tafel polarization curves for FeSe2/rGO electrode and also for Pt, FeSe2 and rGO electrodes for comparison. The results indicate that the CE based on FeSe2/rGO composites has the lowest peak-to-peak voltage separation (E_{pp}) charge transfer resistance (Rct) and series resistance (Rs) in the four different CEs, suggesting that the FeSe2/rGO CE has an excellent electrocatalytic performance for the reduction I3-. The current density-voltage (J-V) curves of DSSCs with different CEs under the illumination of 1 sun (100 mW cm-2) show that the cell with FeSe2/rGO CE has an open-circuit voltage (Voc) of 0.727 V, a short-circuit current (Jsc) of 18.94 mA cm-2, a fill factor (FF) of 0.65 and an excellent power conversion efficiency (PCE) of 8.90%, which is a notable improvement compared with the PCE of the cell with an FeSe2 CE (7.91%) and an rGO CE (5.24%). It can be attributed to the synergetic effects between the FeSe2 nanorods and rGO which eventually improve the PCE of DSSC We also conducte the experiments on the electrochemical stability of FeSe2/rGO CE by sequential CV measurements the result indicates that the FeSe2/rGO composite has a better stability than Pt in I-/I3- electrolyte In summary, we synthesize a novel FeSe2/rGO conductive catalyst. This hybrid material possesses the features of FeSe2 and rGO, exhibiting both highly catalytic activity and high conductivity Therefore, the low-cost and high-performance FeSe2/rGO composite can be a promising CE material to replace Pt in the large-scale industrial production of DSSCs.