CuS nano flakes and nano platelets as counter electrode for quantum dots sensitized solar cells
Introduction
Ever since the discovery of dye sensitized solar cells (DSSC) by Michael Gratzel to meet the energy need, there have been several evolutions and developments to make them cost effective as well as to find an alternative material owing to the photo degradation observed in DSSC [1], [2], [3], [4], [5], [6], [7]. Quantum dots sensitized solar cells (QDSSC) has been found to be an effective alternative candidate due to its easy synthesis, size dependant tunability of bandgap [8], [9] with selective light absorption over a range of wavelength, highest extinction coefficient [10] and large intrinsic dipole moment [11], [12] becasue of quantum confinement. Mostly, chalcogenide based quantum dots (QDs) are used as photon harvesters [13], whose properties can be tuned by altering the reaction parameters such as concentration, reaction type and deposition time [14], either by direct adsorption on to the photoanode by chemical bath deposition or by succesive ionic layer adsorption and reaction (SILAR) [15], [16] with and without the assistance of bifunctional linker- assisted attachment of presynthesized QDs [17], [18]. The best photovoltaic performance for QDSSC is reported using polysulfide redox system in water methanol mixture [19]. Though, platinum (Pt) is very stable and performs high electrocatalytic activity with I−/I3− redox electrolyte, it is found to have unstable photovoltaic performance with S2−/Sn2− due to easy adsorption of polysulfide redox species on to the Pt surface leading to increased resistance [20], [21] and thus reducing its stability [22]. It also leads to low efficiency (η) and fill factor (FF) due to very high over potential in regenerating the charge carriers [23]. Carbon based materials such as carbon black, graphene and carbon nanotubes also have disadvantages; carbon black lacks thickness for effective charge conduction although it possess large surface area, while graphene has poor catalytic activity even though it has high charge conduction [24]. In recent times, nanostructured metal chalcogenides (MCs) are found to be a cost efffective alternative material to Pt CE [20], [25]. Among the MCs, copper sulfide has been found to be a suitable CE material which belongs to IB-VIA group and the composition varies from sulfur rich (CuS2) to copper rich (Cu2S) [26] exhibiting variety of morphologies like nano-flakes, nanorods, nanowire and nanodiscs etc [27]. Copper sulfide has enhanced redox activity towards the reduction of polysulfide electrolyte couple and increases the short circuit current (Jsc) [28], FF and η as well [7]. The covalently bonded MCs with sulfur enrichment have been reported to have better electrocatalytic activity due to weak inter-layer van der Waals forces, exhibiting gaphite like structures [29], [30], [31].
Herein, we report a facile seed layer assisted chemical bath deposition route in prepararing CuS nanoflakes and nano platelets CE by CBD without any surfactant’s assistance. By varying the diping time of the seed layer, we have synthesized CuS nanoflakes and nano platelets. The surface morphology is found to affect the electrocatalytic properties and eventually influences the cell performance. We have achieved the highest η of 4.02% using the CuS CE prepared at 60 °C.
Section snippets
Experimental Section
CuS nano flakes and nano platlets were prepared using simple CBD as reported elsewhere using copper sulfate pentahydrate (CuSO4·5H2O) and thioacetamide (CH3CSNH2) as copper and sulfur sources respectively [32] with little modification such as seed layer and the position of the substrate inside the growth solution. All the precursors used for the synthesis were analytical grade and purchased from Sigma–Aldrich and the synthesis was carried out without further purification. 0.1 M of CuSO4·5H2O and
Characterizations
The crystalline phase and the phase purity of the CuS thin films were analysed using X-ray diffraction (XRD; Bruker D8-Advance) with Cu Kα radiation(λ = 1.54056) source operated at 40 kV and 30 mA in the range of 10–80°. The surface morphology of the samples was analysed using FE-SEM (Hitachi, Model S-4200). UV–vis spectroscopic analysis was carried out using optizen 3220UV. The current-voltage charecteristics of the QDSSCs were studied under 1 sun illumination (AM 1.5 G 100 mW cm−2) using ABET
Results and discussion
Fig. 1 represents the X-ray diffracted pattern of the CuS thin film coated on the FTO. The diffracted peaks of the as synthesized thin film samples corresponds to a CuS phase. The diffracted peaks at (1 0 0), (0 0 6), (1 0 5), (1 1 4), (2 0 4) and (2 1 2) are corresponding to hexagonal CuS of standard ICDD file [No. 79-2321]. Fig. 2 shows the surface morphology of CuS thin film samples deposited at 60 °C by CBD. The SEM image of CE0V shows uniformly coated nano flake like structure while CE0H displays a
Conclusion
In summary we have successfully synthesized CuS thin films deposited at 60 °C by simple chemical bath deposition (CBD) without any surfactant and TiCl4 treatment, could be better counter electrodes towards achieving high photoelectrical conversion efficiency. They are found to possess better electrocatalytic performance than Pt and economical too. An enhanced electrocatalytic activity and lower charge transfer was exhibited by CE0H CE gives the highest efficiency of 4.02%, Jsc of 12.506 mA/cm−2, V
Acknowledgement
This work was supported for two years by Pusan National University Research Grant.
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