Elsevier

Journal of Power Sources

Volume 275, 1 February 2015, Pages 80-89
Journal of Power Sources

Investigation of electrodeposited cobalt sulphide counter electrodes and their application in next-generation dye sensitized solar cells featuring organic dyes and cobalt-based redox electrolytes

https://doi.org/10.1016/j.jpowsour.2014.11.003Get rights and content

Highlights

  • C-AFM shows that the current density of CoS film increases with deposition cycles.

  • CoS–III shows the highest catalytic activity with RCT of 0.75 Ω cm2.

  • Use of high absorption coefficient organic dye C218 as sensitizer in DSSC.

  • CoS–I, CoS–III, CoS–V & Pt used as CEs in DSSC with cobalt-redox electrolyte.

  • CoS–III shows the highest JSC of 12.84 mA cm−2, VOC of 805 mV and PCE of 6.72%.

Abstract

Cobalt sulphide (CoS) films are potentiodynamically deposited on fluorine-doped tin oxide (FTO) coated glass substrates employing one, three and five sweep cycles (CoS–I, CoS–III and CoS–V respectively). Analysis of the CoS–III film by impedance spectroscopy reveals a lower charge transfer resistance (RCT) than that measured for Pt CE (0.75 Ω cm−2 and 0.85 Ω cm−2, respectively). The CoS films are used as counter electrodes (CE) in dye-sensitized solar cells (DSSCs) featuring the combination of a high absorption coefficient organic dye (C218) and the cobalt-based redox electrolyte [Co(bpy)3]2/3+. DSSCs fabricated with the CoS–III CE yield the highest short-circuit current density (JSC) of 12.84 mA cm−2, open circuit voltage (VOC) of 805 mV and overall power conversion efficiency (PCE) of 6.72% under AM 1.5G illumination (100 mW cm−2). These values are comparable to the performance of an analogous cell fabricated with the Pt CE (PCE = 6.94%). Owing to relative lower cost (due to the inherit earth abundance of Co) and non-toxicity, CoS can be considered as a promising alternative to the more expensive Pt as a CE material for next-generation DSSCs that utilize organic dyes and cobalt-based redox electrolytes.

Graphical abstract

Cobalt sulphide counter electrodes, fabricated by electrodeposition, are a viable, low-cost, earth abundant and non-toxic alternative to traditionally used platinum in next-generation dye-sensitized solar cells using the high absorption coefficient organic dye C218 and cobalt-based electrolytes, giving a highest JSC of 12.84 mA cm−2, VOC of 805 mV and PCE of 6.72% (platinum = 6.94%).

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Introduction

A combination of low component costs, coupled with simple and environment friendly fabrication processes makes dye sensitized solar cells (DSSCs) an attractive solar energy conversion technology [1]. The device architecture consists of a photoanode, a counter electrode (CE) and an electrolyte. A dye [2], [3], [4] is employed to sensitize the mesoporous photoanode, enabling injection of an excited electron from the dye into the TiO2 photoanode [5], [6], [7] after absorption of light. The photogenerated holes, residing on the oxidized dye, are transported to the CE via the redox electrolyte [8], [9], [10], completing the circuit and restoring the ground state of the sensitizer. The difference between the quasi Fermi level of the TiO2 photoanode and redox potential of the electrolyte determines the upper limit of the open circuit voltage (VOC) in the device. Improvements in VOC is achievable by replacing the commonly used iodide/triiodide redox electrolyte with a cobalt-based redox electrolyte [11], [12], which possesses a higher redox potential, allowing access to a greater VOC and therefore enhanced power conversion efficiency (PCE). Employing this strategy enables the fabrication of DSSCs with a high VOC leading to PCE beyond 12% [13], [14], [15]. The CE plays an important role in DSSCs as it is the conduit between the redox electrolyte and the photoanode. Pt thin films deposited on FTO coated glass are commonly used as CE because Pt exhibits a high catalytic behavior with the traditionally used iodide electrolyte. Platinum (Pt) also demonstrates reasonable compatibility with cobalt-based redox electrolytes, however high cost and low availability of Pt have prompted research towards identifying alternative materials with comparable catalytic activity. This includes materials like conducting polymers [16], [17], [18], [19], carbon based materials [20], [21], [22], [23], [24], inorganic materials [25], [26] and electrodeposited cobalt sulphide (CoS) [27], [28], [29], [30]. CoS in particular is a non-toxic, low cost and earth abundant compound that may be extremely useful as CE material, particularly for the cobalt-based redox electrolytes employed in next-generation DSSCs.

Previously, electrochemically deposited CoS layers have been employed in DSSCs utilizing ruthenium dyes (Z-907 and N719), iodide electrolytes and flexible as well as rigid substrates resulting in PCEs of about 6% [27], [28], [29], [30]. The majority of DSSCs employing ruthenium dyes (i.e. N719) and iodide electrolytes utilize composite films with a thicker (12 μm) high surface area, mesoporous, transparent layer (TiO2 particle size of 19 nm) responsible for the majority of dye adsorption. To ensure the maximum utilization of incident light, a scattering layer (5 μm, TiO2 particle size of 400 nm) is also employed in the classic device structure [31], [32]. By taking advantage of the higher molar absorptivity of organic dyes, drastic reductions to the thickness of the mesoporous transparent TiO2 layer to a 5 μm thickness can be realized, thereby improving the material cost of the overall device. Furthermore, employing organic dyes in the DSSC allows for an improvement in the overall fabrication time, as organic dyes routinely require shorter (0.5–6 h [14], [15], [16], [33]) dye adsorption times compared to ruthenium dyes (i.e. N719 requires 20–24 h [32]). Also there are some major drawbacks associated with the iodine-based redox couple including immoderate light absorption of triiodide and corrosiveness of the electrolyte towards most metal current collectors [34]. Therefore there is a unique possibility of CoS to provide an avenue toward alternative CE materials in the new generation of DSSC that employ high absorption coefficient organic dye [35] and cobalt redox based electrolytes.

In this paper we report a study on the potentiodynamic deposition of CoS films on FTO glass substrates by varying the number of deposition cycles. The performance of these CEs is studied in DSSCs where organic dye C218 is used as sensitizer and [Co(bpy)3]2/3+as redox system for the electrolyte [33], [36]. The DSSC fabricated with CoS CE deposited at three sweep cycles is reported with the maximum efficiency ∼6.72% and VOC of 805 mV.

Section snippets

Experimental

FTO glass substrates (7 Ω/□, Pilkington) were cleaned consecutively using soap solution, deionized (DI) water, acetone and propanol in an ultrasonic bath, each for 15 min. The CoS solution employed in electrodeposition was composed of 0.5 M thiourea and 0.005 M CoCl2.6H2O in DI water. The pH of the solution was maintained at 7.0 by adding the few drops of liquor ammonia. Potentiodynamic deposition of CoS films on FTO substrates was performed with a computer controlled potentiostat (PAR 273A-2,

Morphological properties

Fig. 1 shows the SEM images of the CoS–I, CoS–III and CoS–V films deposited on FTO substrates. There is a persistent honeycomb-like structure present in CoS–III film, while these structures are just starting to grow in CoS–I. The CoS–V film reveals increased aggregation in the honeycomb-like structure, consistent with the previous report [29].

Surface roughness and morphology of the films are analyzed using 10 × 10 μm2 AFM images (Fig. 2). The root mean square (RMS) surface roughness of the CoS

Conclusions

CoS films are successfully deposited on the FTO glass substrate employing an electro-deposition technique using different sweep cycles. The CoS films were utilized as CE materials in DSSCs that employ the high absorption coefficient organic dye (C218) and a cobalt-based redox electrolyte. CoS–III CE representing three sweep cycle exhibits a lower RCT due to a good catalytic behavior and honeycomb like surface structure with good interconnectivity. C-AFM result shows that the interconnectivity

Acknowledgments

The work presented in this paper was performed under the Department of Science and Technology (DST) project “Efficient solar cells based on organic and hybrid technology (ESCORT)” and the European Union Seventh Framework Programme (FP7/2007–2013) under grant agreement ‘ENERGY-261920, ESCORT’. S.K.S acknowledges the financial support from Ministry of New and Renewable Energy (MNRE), New Delhi & IITD-EPFL student exchange programme. We wish to thank to Mr. Pascal Comte for providing the titania

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