FeSe Q 1 Q 2 2 3 films with controllable morphologies as efficient counter electrodes for dye-sensitized solar cells †

Dye-sensitized solar cells (DSSCs) are promising alternatives to conventional solid-state photovoltaic devices based on materials such as Si, CdTe and CuIn1 xGaxSe2, owing to their low cost, ease of production and high efficiency. In general, a typical DSSC usually comprises three main components: a dye-sensitized porous nanocrystalline titanium dioxide (TiO2) film as the photoanode, an electrolyte traditionally containing an iodide/triiodide (I /I3 ) redox couple and a counter electrode (CE). As a crucial component, CE plays an important role in the performance of a DSSC in that it collects electrons from external circuit and Q5 fulfills electron transfer from the CE interface to the electrolyte by catalyzing the reduction of I3 to I . Usually, fluorine doped tin oxide (FTO) glass loaded with noble metal platinum (Pt) is the preferred material for the CE due to its superior conductivity, electrocatalytic activity, and stability. However, in view of the low abundance and high cost of platinum, much incentive exists in developing other cost-effective alternatives for Pt. For instance, carbon materials, conducting polymers and inorganic compounds have been utilized as CEs in DSSCs. In particular, some metal chalcogenides among these Pt-free electrocatalysts have been shown to be quite effective and have exceeded the performance of Pt. As Q6 another compound in the metal chalcogenide series, iron diselenide (FeSe2) is an important VIII–VI transition metal selenide semiconductor and has drawn enormous attention owing to its unusual structure and electronic properties, as well as the opportunity it offers to engineer special shapes for application in the development of advanced functional devices. In recent years, much effort has been devoted to the synthesis of FeSe2 films, lowdimensional FeSe2 micro/nanostructures and anisotropic threedimensional (3D) FeSe2 nanostructures. However, to the best of our knowledge, no research on the use of FeSe2 as CEs in DSSCs has been reported so far. Herein, as the electrocatalytic activity may depend on the morphology, we have prepared the FeSe2 films with controllable morphologies, including 3D flower-like FeSe2 assembled with nano/ microrods (Fr) and sphere-shaped FeSe2 assembled with sphereshaped particles (Fs), via a facile hydrothermal synthetic approach, and used them as Pt-free electrocatalysts for DSSCs, respectively. Electrochemical results of the Fr CE showed Pt-like catalytic activity for triiodide reduction, surpassing that of the Fs CE. As a result, the power conversion efficiency (Z) of the DSSC with the Fr CE was 8.00%, which was comparable to that of the Pt CE (7.87%), while the Z of the Fs CE was 7.38%. Details of the fabrication method are shown in the experimental section of the ESI.† The crystallographic structures of the as-synthesized FeSe2 products were characterized by X-ray diffraction (XRD). From the XRD patterns (Fig. 1), all diffraction peaks of Fr and Fs can be readily indexed to an orthorhombic phase of FeSe2 (JCPDS No. 65-2570). The morphologies of the FeSe2 samples (Fr and Fs) were observed using typical scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Fig. 2a, the as-obtained Fr 1