SCIENCE CHINA Chemistry • ARTICLES •
July 2013 Vol.56 No.7: 977–981 doi: 10.1007/s11426-012-4810-8
Metal chalcogenide complex-mediated fabrication of Cu2S film as counter electrode in quantum dot sensitized solar cells YU XueChao, ZHU Jun*, LIU Feng, WEI JunFeng, HU LinHua & DAI SongYuan* Key Laboratory of Novel Thin Film Solar Cells, Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, China Received October 25, 2012; accepted November 29, 2012; published online January 21, 2013
Cu2S film onto FTO glass substrate was obtained to function as counter electrode for polysulfide redox reactions in CdS/CdSe co-sensitized solar cells by sintering after spraying a metal chalcogenide complex, N4H9Cu7S4 solution. Relative to Pt counter electrode, the Cu2S counter electrode provides greater electrocatalytic activity and lower charge transfer resistance. The prepared Cu2S counter electrode represented nanoflower-like porous film which was composed of Cu2S nanosheets on FTO and had a higher surface area and lower sheet resistance than that of sulfided brass Cu2S counter electrode. An energy conversion efficiency of 3.62% was achieved using the metal chalcogenide complex-mediated fabricated Cu2S counter electrode for CdS/CdSe co-sensitized solar cells under 1 sun, AM 1.5 illumination. metal chalcogenide complex, Cu2S counter electrode, catalytic activity, quantum dot sensitized solar cells
1 Introduction Quantum dot sensitized solar cells (QDSSCs) have attracted tremendous attention as an interesting energy device because of their impressive sunlight harvesting, multiple electron/hole generation, ease of fabrication and low cost [1, 2]. Quantum dot sensitized solar cells borrow the concepts of dye sensitized solar cells (DSCs) [3], which are based on a mesoporous metal oxide film, covered with a dye layer that is immersed into a redox electrolyte while the electric circuit is closed by a transparent conducting oxide front electrode and a counter electrode. As an alternative to dye molecules, semiconductor quantum dots (QDs) such as CdS [4], CdSe [5], PbS [6], InAs [7], In2S3 [8], InP [9], and others as well as extremely thin inorganic absorber layers have been used. QDs are very attractive because of their size dependent optical band gap [10], the ability to rapid charge separation for the large intrinsic dipole moment of quantum dots *Corresponding authors (email:
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[11], and their multiexciton generation that leads to external photocurrent quantum efficiency exceeding 100% [12]. Theoretically, the conversion efficiency of QDSSCs can reach 44% which is considerably higher than the ShockleyQueisser limit [13]. However, most QDs suffer from photodegradation when used in conjunction with the I/I3 redox couple. Consequently, quantum dot sensitized solar cells are often based on aqueous polysulfide electrolyte with minor use of the various Co2+/Co3+ couple [14], Fe(CN)63/Fe(CN)64 couple [15] and organic redox couples [16]. As a popular catalytic electrode for organic I/I3 redox electrolyte, Pt is also the most widespread counter electrode (CE) in DSCs. However, sulfur compounds are known to chemisorb on platinum surfaces and induce poisoning effects toward electrode performance, as a result, Pt and other novel metals like Au are not very catalytic and stable in conjunction with aqueous polysulfide electrolytes [17]. To address these issues, alternative metal chalcogenides counter-electrode materials for polysulfide solution have been investigated such as PbS [18], CoS [19], Cu2S [20], Au [21], and carbon [22], altchem.scichina.com
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hough each presents its own issues over long periods of time when used in conjunction with photoanodes (PEs) [23]. Among these materials, Cu2S is widely used in quantum dot sensitized solar cells because of its superior activity and stability. A common strategy to fabricate Cu2S counter electrodes is that Cu brass is exposed to sulfide solution after acid treatment to obtain an interfacial layer of metal sulfide. The problem is that such a preparative method suffers from continual corrosion and ultimately mechanical instability. What is more, screen-printed Cu2S combined with conductive carbon [24] and Cu2S reduced graphene oxide composite [25] is developed to improve the performance of the counter electrode. However, strategies to fabricate Cu2S films on TCO as counter electrodes are quite limited. Recently, metal chalcogenide complex (MCC) has attracted much attention as precursors for mesoporous metal chalcogenide films [26, 27]. In the present work, we developed a novel and simple fabrication method for Cu2S counter electrode that involves the use of a metal chalcogenide complex, N4H9Cu7S4 solution, as a precursor, which could easily be employed for large-scale production of counter electrodes.
Figure 2
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Results and discussion
Metal chalcogenide complex, N4H9Cu7S4 mediated prepared Cu2S powders by the heat treatment of the solution in N2 atmosphere was characterized by XRD (Figure 1). The diffraction patterns are in good agreement with that expected for chalcocite (PDF 33-0490). The SEM images of Cu2S counter electrode on FTO glass substrate are shown in Figure 2. Apparently, the elec-
Figure 1 der.
SEM, TEM, HRTEM and EDS of N4H9Cu7S4 mediated prepared Cu2S films.
X-ray diffraction of N4H9Cu7S4 mediated prepared Cu2S pow-
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trode had a relatively high coverage on the FTO. Meanwhile, the magnified image of the morphology revealed the microstructure of the Cu2S film. The noticeable feature of the microstructure was the nanoflower-like porous structure which was composed of Cu2S nanosheets. We used TEM and HRTEM to characterize the Cu2S nanosheets. The TEM image revealed that the aggregates of the Cu2S nanosheets prepared by the present method became apparent on the surface of FTO glass. The HRTEM and SAEM (inside) in Figure 2(c) clearly show hexagonally arranged spots that are 0.39 nm apart, and the angle between [010] and [100] is measured to be 120, corresponding to the parameters of high chalcocite Cu2S [28]. To further characterize the catalytic activities of the prepared counter electrode, cyclic voltammetry was performed and the results are shown in Figure 3. The rest potentials of all the electrodes when immersed in the electrolyte were found to be about 0.73 V which is in accordance with the values reported [23]. The value of Eredox is in good agreement with the predicated values using Nernst equation. For the Pt counter electrode, the current density at 1.2 V vs. SCE is 2.5 mA/cm2, similar to what Kamat et al. reported [25]. While for the N4H9Cu7S4 mediated prepared Cu2S electrode, the current density at 1.2 V vs. SCE is 14.4 mA/cm2, smaller than that of the graphene oxide(RGO)Cu2S composite, an outstanding counter electrode material developed by Kamat et al. [25]. However, N4H9Cu7S4 mediated prepared Cu2S electrodes, like the ones obtained by sulfided brass, showed a better response than Pt electrode in polysulfide electrolyte. Thus, the N4H9Cu7S4 mediated prepared Cu2S electrodes can be employed as a promising counter electrode for quantum dot sensitized solar cells. The fabrications of three different counter electrodes are shown in experimental details in Supporting Information. The effects of different counter electrode materials on the cell performances are illustrated in Figure 4(a) and Table S1, which indicate that the two different Cu2S electrodes were superior to Pt electrodes, for the same CdS/CdSe co-sensitized TiO2 electrodes after ZnS treatments. The substitution of the counter electrode strongly affects the fill
Figure 4 J-V (a), IPCE (b) and electrochemical impedance spectroscopy (c) of QDSSCs with different counter electrodes.
Figure 3 Cyclic voltammetry of (a) Cu2S electrode (N4H9Cu7S4 mediated prepared), (b) Cu2S electrode (sulfided brass) and (c) Pt electrode.
factor, as has been previously suggested [20]: Cu2S (MCC) (FF = 53.9%), sulfided brass (FF = 50.8%), Pt (FF = 42.7%). Meanwhile, the short circuit current and the open circuit voltage represent a slight increase for cells employed both Cu2S counter electrodes compared with Pt electrodes. As Zaban et al. have suggested [18], the morphology of counter electrodes has a significant influence on their performance. Therefore, the better performance of N4H9Cu7S4 mediated prepared Cu2S electrodes than that of the brass sulfided Cu2S electrode can also be attributed to their nanoflower-like porous structure. On the other hand, it can be observed from Figure 4(b) that the cell with Pt counter electrode showed comparable IPCE (Incident Photon-to-Electron Conversion Efficiency)
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with other two Cu2S counter electrodes at the whole spectrum region, even though it exhibited a lower Jsc than that in J–V results. We attribute it to the difference of the light intensity power used in the IPCE measurement system and the 1 sun J–V measurement. The IPCE measurement is carried out under low-intensity monochromatic illumination, where a relatively small amount of electron–hole pairs is generated at the sensitizer, and under this condition, regeneration of holes at photoanodes is not much affected by the catalytic activities of CEs. However, under high-intensity illumination like 1 sun, the electron–hole pair generation rate is significantly enhanced and the catalytic role of the counter electrode becomes important. The low Jsc of the cell with Pt counter electrode implies that hole regeneration is not sufficient, which may be due to the poor electrocatalytic activity of the Pt for reducing the oxidized species of the redox system. The lowest FF also suggests the poor electrocatalytic activity of Pt which consequently leads to more recombination and lower reduction. To further reveal the electrochemical characteristics of the counter electrodes, electrochemical impedance spectroscopy (EIS) was carried out with QDSSCs under opencircuit conditions as shown in Figure 4(c). The Nyquist plot of the cells with Pt counter electrode has the largest arc at high frequency, indicating lower charge transfer rate at the Pt/electrolyte interface. In addition, the EIS shown in Figure 4(c) also indicated that the N4H9Cu7S4 solution-mediated Cu2S counter electrode had a better electrochemical activity thus faster charge transfer than that of sulfided brass counter electrode. The fast charge transfer at the counter electrode/electrolyte interface causes a change in the ions’ concentration in the solution and by that influences the recombination rates at the working electrode/electrolyte interface. In other words, the oxidized species concentration at the counter electrode will decrease due to the higher reductive reaction speed and thus lead to the decease of oxidized species concentration at the working electrode by mass transport. Then the recombination reaction between the TiO2 electron and the oxidized species in the electrolyte is suppressed. The relative increase of Voc seems to be the result of less recombination of the catalytic Cu2S. Consequently, the measured improvement in all cell parameters upon introduction of the N4H9Cu7S4 solution-mediated Cu2S counter electrode to the cell is a strong indication for an enhanced catalytic activity of the N4H9Cu7S4 solutionmediated Cu2S counter electrode in conjunction with the polysulfide electrolyte compared with sulfided brass counter electrode. The enhancement may originate from the nanoflower-like porous morphology with a high surface area. Additionally, the magnified view in Figure 4(c) indicated that sulfided brass counter electrode had the greatest sheet resistance due to the low conductivity of brass substrate. However, the N4H9Cu7S4 solution-mediated strategy avoided this defect by depositing Cu2S film on FTO.
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Conclusion
In this paper, we have shown that a N4H9Cu7S4 solutionmediated Cu2S counter electrode is able to reach 3.62% efficiency at AM 1.5 intensity by optimizing the catalytic activity and conductivity. The Cu2S counter electrode prepared by this strategy has made a considerable leap in overcoming the limitations of the low catalytic activity of Pt counter electrode and low conductivity and surface area of sulfided brass counter electrode. The method to fabricate Cu2S counter electrode is appealing for large-area, roll-toroll counter electrodes for QDSSCs. This work was supported by the National Basic Research Program of China (2011CBA00700), the National High Technology Research and Development Program of China (2011AA050527), the External Cooperation Program of the Chinese Academy of Sciences (GJHZ1220), and the National Natural Science Foundation of China (21003130, 21173228). We acknowledge Dr. Qing Shen for providing the brass material. 1 2
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