One-Step Electrochemical Deposition of Hierarchical CuS

Aug 12, 2014 - A Player Often Neglected: Electrochemical Comprehensive Analysis of Counter Electrodes for Quantum Dot Solar Cells. Riccardo Milan , Me...
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One-Step Electrochemical Deposition of Hierarchical CuS Nanostructures on Conductive Substrates as Robust, HighPerformance Counter Electrodes for Quantum-Dot-Sensitized Solar Cells Feifan Wang,†,‡ Hui Dong,† Jinlong Pan,† Jingjian Li,† Qi Li,*,† and Dongsheng Xu*,†,‡ †

Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry and Molecular Engineering, and ‡Academy for Advanced Interdisciplinary Studies, Peking University, Beijing 100871, P. R. China S Supporting Information *

ABSTRACT: An ideal counter electrode, with high electrocatalytic activity, high performance stability, and applicable fabrication simplicity, is essential to give full play to the advantages of quantum-dot-sensitized solar cells (QDSSCs) such as high theoretical efficiency and simple synthetic procedure. Herein, we report a facile one-step electrochemical deposition approach for the growth of hierarchical covellite (CuS) nanostructures on conductive glass substrates. The as-synthesized copper sulfide can be employed directly as a robust, low-cost, and high-efficiency counter electrode without any post-treatments for QDSSCs filled with aqueous sulfide/polysulfide (S2−/Sn2−) electrolyte. The morphology and structure of the well-crystalline, strongly substrate-adhesive hierarchical CuS nanostructured film have been studied by X-ray and electron-based characterizations. QDSSC using this newly synthesized CuS as counter electrode achieves a higher power conversion efficiency of 4.32% than the one applying cuprous sulfide (Cu2S) on brass substrate (4.08%) or platinum counter electrode (2.85%). Furthermore, this CuS counter electrode shows a high and consistent electrocatalytic activity toward polysulfide reduction confirmed by the electrochemical measurements, destining the improved photovoltaic performance and superior stability of the corresponding QDSSC device. reaction (SILAR) methods,12,13 and postlinking of presynthesized colloidal QDs14−17 onto the microporous TiO2 matrix. Recently, the sulfide/polysulfide (S2−/Sn2−) redox electrolyte has gained its popularity in QDSSCs due to the effective scavenging of holes to regenerate the sensitizer and the resistivity against photoanodic corrosion.18,19 The choice of counter electrodes, to a great extent, depends on the established electrolyte for these solution-processed solar cells. Platinum counter electrode (henceforth denoted “Pt CE”), which has superior conductivity and electrocatalytic activity for iodide/triiodide redox couple in DSSCs, has been widely used as a high activity and perfect stability counter

1. INTRODUCTION Quantum-dot-sensitized solar cells (QDSSCs), as one of the promising liquid-junction photovoltaic devices, have attracted much attention due to their simple synthetic procedure and high theoretical thermodynamic efficiency (44%), which benefits from the multiple exciton generation, hot-carrier transfer, and tunable optical absorption of quantum dots (QDs).1−7 The photoelectrochemical mechanism of QDSSCs parallels that of dye-sensitized solar cells (DSSCs),8 with inorganic QDs replacing the dye molecules as incident light absorbers. Maximizing the charge separation and transfer at the various interfaces is crucial for improving the energy conversion efficiency in this solution-processed photovoltaics.9 The fabrications of the QD−TiO2 heterojunction interface include direct growth of nanoclusters, for example, chemical bath deposition (CBD),10,11 successive ionic layer adsorption and © XXXX American Chemical Society

Received: June 9, 2014 Revised: July 31, 2014

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Figure 1. Schematic depiction of one-step growth of hierarchical CuS nanostructures on FTO/glass substrate by a facile electrodeposition method (a) and a typical current density−time (J−t) curve during the potentiostatic (−1.05 V) deposition process (b).

simulated solar irradiation (100 mW cm−2). Notably, QDSSC devices assembled with this kind of CuS counter electrode exhibited excellent illumination stability and conservation stability in sulfide/polysulfide electrolyte.

electrode in DSSCs.20 But in QDSSCs, it is incompatible with sulfide/polysulfide-based electrolyte caused by the strong chemisorption of sulfur species on the Pt surface (so-called poisoning effect).21 The low conductivity and poor electrocatalytic activity to polysulfide reduction results in low photocurrent density and fill factor (FF).18 Toward the optimization of the counter electrode−electrolyte interface in QDSSCs, researchers have paid great effort to develop new materials and novel structures of counter electrodes to satisfy the requirement of the established sulfide/polysulfide-based electrolytes, such as Cu2S,22 CoS,23 CuS-CoS,21 PbS,24 CoS2,25 Cu-based alloy chalcogenide,26,27 carbon-based counter electrodes,28−30 and so on. Among them, Cu2S counter electrodes show the highest electrocatalytic activity toward reduction of polysulfide species in QDSSCs.16 However, it has been found that Cu2S counter electrodes fabricated by sulfidization of brass foils (henceforth denoted “Cu2S/brass CE”) can be continually corroded by sulfide/polysulfide electrolyte, resulting in the poisoning of photoanode and detachment from substrates,31 which is also proved in our research. Furthermore, from the perspective of application, the high material costs restrict the use of platinum- and brass-based counter electrodes in QDSSCs. Recently, stable CuxS-based counter electrodes have been developed, including Cu2S reduced graphene oxide composite counter electrode manufactured by casting presynthesized Cu0-reduced graphene oxide and binder slurry on fluorine-doped tin oxide (FTO)32 and meso-texture CuxS cathode fabricated by exposing deposited Cu (on FTO) into polysulfide electrolyte.15,33 Nevertheless, these counter electrodes are not appropriate for large-scale industrialization, since they suffer from complex manufacture process, rigorous growth condition, or both. Based on the above considerations, an ideal counter electrode for QDSSCs should exhibit high electrocatalytic activity toward electrolyte, high photovoltaic performance stability, and applicable fabrication simplicity and economy. The electrochemical deposition method is a widely used synthetic strategy due to its low energy consumption and ease for growth control, but it is rarely reported to prepare counter electrodes for QDSSCs. In this report, we developed a facile one-step strategy to fabricate hierarchical CuS nanostructure thin films on FTO/glass substrates via electrochemical deposition method and applied them directly as counter electrodes into QDSSCs without any post-treatments such as annealing or coating with other auxiliary materials. The hierarchical CuS nanostructure thin film achieved a power conversion efficiency of 4.32% versus 4.08% and 2.85% for Cu2S/brass CE and Pt CE, respectively, under AM 1.5G

2. EXPERIMENTAL SECTION 2.1. Electrochemical Deposition of CuS Counter Electrode on FTO/Glass Substrate. In a typical synthesis (Figure 1), 3 mmol of Cu(NO3)2·3H2O as copper precursor and 3 mmol of NaNO3 as supporting electrolyte were dissolved in 60 mL of dimethyl sulfoxide (DMSO) to form a light blue solution. Afterward, 1.8 mmol of sulfur powder was added into this solution under vigorous stirring and dissolved gradually during heating to 80 °C. The electrodeposition process was performed potentiostatically at −1.05 V under continuously stirring, using a two-electrode system with a Pt plate (1.5 cm × 1.0 cm) as a counter electrode and FTO/glass (TEC-8, LOF, 9 Ω·sq−1) substrate as a working electrode. The electrolyte was heated up to 80 °C beforehand and maintained at this temperature throughout the deposition process. After deposition, the obtained CuS thin film well-adhered to the FTO/glass substrate was washed with hot DMSO and absolute ethanol several times each and dried in vacuum at 80 °C for 4 h for further characterization. 2.2. Materials Characterization. The morphology and composition of the in situ electrochemically deposited CuS film on FTO/glass were characterized by field-emission scanning electron microscopy (FESEM, Hitachi, S-4800) and X-ray photoelectron spectroscopy (XPS, Kratos, Axis Ultra imaging photoelectron spectrometer). The crystalline structure of the as-prepared hierarchical CuS nanostructures was demonstrated using X-ray powder diffraction (XRD, Rigaku, D/MAX-2500 diffractometer with Cu Kα radiation) and high-resolution transmission electron microscopy (HRTEM, Tecnai F20, FEI). The constituent and morphological changes and elemental mappings of different counter electrodes before and after solar cell performance test were analyzed by scanning electron microscopy (SEM, Nano430, FEI) and energy-dispersive X-ray spectroscopy (EDS). 2.3. CdS/CdSe QDs Co-Sensitized Mesoscopic TiO2 Photoanode Preparation and Sandwich-Style QDSSC Devices Assembly. The mesoscopic TiO2 photoanode was cosensitized by CdS/CdSe QDs via a published chemical bath deposition (CBD) method with slight modification,34 as detailed in the Supporting Information. Generally, the FTO/ glass was first covered with a dense nanosized TiO2 layer using a 40 mM TiCl4 aqueous solution. And the TiO2 paste was B

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Figure 2. Morphological and structural characterization of the as-synthesized CuS film on FTO/glass substrate. (a, b) SEM images describing the integral pattern and magnified morphology of hierarchical CuS nanostructures, respectively. (c) High-resolution TEM image of as-prepared CuS nanostructure acquired from the fragment of nanosheet denoised via a Bragg mask filtering process. The inset shows the corresponding inverse fast Fourier transfer (FFT) pattern. (d) Cross-sectional SEM image of the CuS counter electrode manifesting the intimate adhesion of CuS film on FTO/glass substrate. (e) XRD pattern of the as-synthesized hierarchical CuS nanostructures on FTO/glass substrate compared with the standard pattern for covellite (JCPDS 6-464) and the blue triangular symbols marked the peaks of FTO substrate. (f) XPS survey spectra of the surface of CuS film with inset magnified pattern of S 2p peaks.

in concentrated HCl solution at 70 °C for 5 min and subsequently dipped into sulfide/polysulfide electrolyte for 1 min, then rinsed with deionized water for several times. The resulted porous Cu2S/brass CE was dried in a tube furnace at 105 °C for 10 min in nitrogen flow. The commercial Pt CE (prepared by magnetron sputtering) was scanned in 1.0 M H2SO4 aqueous solution for at least 20 cycles via cyclic voltammetric method before electrochemical and photoelectrochemical measurements. 2.4. Electrochemical and Photovoltaic Characterization. Photocurrent−voltage (I−V) measurements were performed on a semiconductor characterization system (Keithley 4200) using simulated AM 1.5 sunlight with an intensity of 100 mW cm−2 produced by a Xe lamp solar simulator (Newport 91191), which was calibrated with a monocrystalline Si reference solar cell system (Newport 91150 V) before the photovoltaic characterization of QDSSC devices. The photocurrent density−voltage (J−V) curves were recorded between −0.1 and 0.7 V at a scan rate of 5 mV s−1, repeating the voltage scan one time per minute for sufficient time to achieve the best performance or to study the performance stability. The symmetrical dummy cells for electrocatalytic characterization were assembled with CuS, Cu2S/brass, and Pt counter

prepared by adding commercial P25 TiO2 nanoparticles (Degussa) into 10 wt % hydroxypropyl cellulose (Aldrich) in diethylene glycol solution with continuous stirring. The semitransparent mesoporous TiO2 layer was fabricated by the doctor blading method without casting a scattering layer of large TiO2 particles or any other TiO2 structural optimization. The thickness of the mesoscopic TiO2 layer was 10−11 μm as confirmed by using a step profiler (SP, KLA-Tencor, P6̅). Then, the mesoscopic TiO2 electrode was immersed into CdS QD deposition solution containing 20 mM CdCl2, 66 mM NH4Cl, 140 mM thiourea, and 0.23 M ammonia for 45 min and into CdSe QD deposition solution containing 26 mM CdSO4, 40 mM N(CH2COONa)3, and 26 mM Na2SeSO3 for 6 h, respectively. The whole process was carried out at 10 °C in the dark. The QDSSC device was assembled by sandwiching the mesoscopic TiO2 photoanode (active area about 0.16 cm2, measured by a vernier caliper three times averaged for the following calculation) and counter electrode with a Surlyn film (45 μm thickness) via a heat-sealed method, then filled with aqueous sulfide/polysulfide electrolyte (1.0 M Na2S and 1.0 M S). The Cu2S/brass CE was prepared by following a published procedure with minor modifications.22 The brass was immersed C

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on FTO/glass substrate (Figure 2d) showed the thickness of CuS film was about 400−500 nm. Furthermore, there was no detachment between CuS film and FTO substrate proving the intimate adhesion of CuS nanostructure on FTO, which was beneficial for electron transport. The XRD pattern (Figure 2e) was used to clarify the crystalline structure of the CuS film, which can be indexed as hexagonal CuS (JCPDS 6-464; space group 194, P63/mmc), as shown in the inset of Figure 2e. Comparing with the standard card, it can be found that the intensities of (100), (101), and (110) peaks are enhanced and the ones of (006) and (008) peaks are reduced. This result indicates the growth of CuS nanosheets along the [001] direction is retarded and the nanosheets preferentially expose the (001) plane. The selective growth of the hierarchical CuS nanostructures on FTO induces the disappearance of the high-index facet peaks. The pure phase and sharp peaks in the XRD pattern implied the good crystallinity of the CuS film synthesized by this electrochemical deposition method, which was also confirmed by the wellresolved 2D lattice fringes in Figure 2c. XPS was used to characterize the surface chemistry state of the hierarchical CuS nanostructure (Figure 2f). The peaks detected at 932.4 and 952.2 eV were assigned to Cu 2p binding energy, and the peaks measured at 162.4 and 163.6 eV (magnified in the inset) were attributed to the S 2p3/2 and 2p1/2, respectively. The peak areas of Cu 2p and S 2p regions were quantified to determine the atomic ratio of Cu/S to be 57:43, suggesting that the surface of this CuS film was Cu rich. Overall, this electrochemical deposition strategy was a facile method for fabricating wellcrystalline, intimately substrate-adhesive hierarchical CuS nanostructures directly on FTO/glass. The electrodeposition method facilitated the study of growth mechanism by shutting off the chemical reaction at any time using an electric switch. It can be seen from the morphology evolution of a single CuS nanoparticle shown in Figure 3a−d that the hierarchical nanostructure underwent the following four growth stages: (i) a single plate grown on FTO substrate as the main plate; (ii) small irregular pieces of thin CuS sheets emerged on both sides of the main plate; (iii) the sheets became larger and more regular geometrically; and (iv) a welldefined concaved cuboctahedron-like nanostructure formed with degradation of the as-emerged main plate. The proposed growth mechanism can be classified as the crystallizationdissolution processes for the transition from irregular nanoparticles to regular nanostructures.35 The stacking density of the well-defined nanostructures increased with the extension of deposition time, and the CuS nanostructure lost its symmetric cuboctahedral geometry gradually instead of the closely stacked CuS nanosheets (Figure 3e−h). 3.2. Electrochemical and Photoelectrochemical Performance of Different Counter Electrodes and the Assembled QDSSC Devices. To evaluate the electrochemical performance of the as-synthesized CuS counter electrode in sulfide/polysulfide electrolyte, we assembled sandwich-like thin-layer liquid-junction QDSSCs (Figure 4a) using the hierarchical CuS nanostructures on FTO/glass as counter electrode (henceforth denoted “h-CuS/FTO CE”). The matrix of photoanode was prepared simply by the commercial TiO2 nanoparticles (Degussa P25) without casting a scattering layer of large TiO2 particles or any other TiO2 structural optimization. For direct comparison of counter electrode performance, the current density−voltage (J−V) curves of QDSSCs assembled using the same batch of photoanodes and

electrodes to construct a sandwich-like structure similar to the QDSSC devices, using punched Surlyn films as spacers. The electrode area formed by the punched rectangular spacer was measured three times averaged by using a vernier caliper. Tafel polarization measurements were carried out in the symmetrical dummy cells using a CHI 660A electrochemical analyzer with a scan rate of 10 mV s−1. Electrochemical impedance spectroscopy (EIS) was conducted at the open circuit potential with an amplitude of 10 mV using a potentiostat (EG&G, M283) equipped with a frequency response detector (EG&G, FRD100). The spectra were obtained at a frequency ranging from 100 mHz to 120 kHz at room temperature in the dark. To characterize the stability of electrocatalytic activity, the EIS spectra were measured before, after 60 cycles, and after 120 cycles of cyclic voltammetry scanning from 0 to 0.65 V at a scan rate of 50 mV s−1.

3. RESULTS AND DISCUSSION 3.1. Structural Characterization and Growth Mechanism of Hierarchical CuS Nanostructures on FTO/glass Substrates. The direct growth of hierarchical CuS nanostructures on FTO/glass substrates was achieved via a facile one-step electrochemical deposition strategy in a two-electrode system, as illustrated schematically in Figure 1a. Typically, the electrodeposition was performed potentiostatically at −1.05 V under continuous stirring at 80 °C for 5 min. The current density decreased gradually during the deposition process (Figure 1b), which was a common behavior exhibited by the potentiostatic deposition method under the control of electrolyte diffusion. The deposition voltage applied between FTO/glass working electrode and Pt foil counter electrode was determined according to the cyclic voltammetry spectrum characterized in a three-electrode system. The reduction peak at negative potential (around −1.0 V vs Ag/AgCl) can be identified as the formation of CuS, based on the electrochemical reaction Cu 2 + + S (DMSO) + 2e− → CuS↓

This reaction is driven by both the applied electric field and the equilibrium shifting caused by the precipitation of CuS from electrolyte solution. Therefore, a moderate electrodeposition current is necessary for the formation of well-built hierarchical CuS nanostructures. We characterized the morphology of the CuS film on FTO/ glass substrate by SEM. The CuS film was constituted by close packed hierarchical nanostructures about 550−700 nm in diameter (Figure 2a). For a single particle shown in Figure 2b, the overall geometry was similar to the concaved cuboctahedra possessing 14 cavities (six square ones and eight triangular ones) demonstrated by Yu’s group,35 but with curved instead of straight edges. More interestingly, the plates of the well-defined concaved cuboctahedron were layered by several sheets (about 20 nm in thickness) each in our work, from which came the socalled hierarchical nanostructure. This two-dimensional nanosheet structure provided higher surface area and much more active sites for electrocatalytic reduction toward polysulfide electrolyte. The denoised HRTEM image (Figure 2c) confirmed the single-crystalline structure of each nanosheet of the hierarchical CuS nanostructure. The well-resolved 2-D lattice fringes showed a hexagonal structure with lattice spacing of 3.2 Å, indicating that the preferential exposure of (0001) plane of covellite.36 The cross-sectional SEM image of CuS film D

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Table 1. Performance Parameters of Quantum DotSensitized Solar Cell Devices Assembled with Different Counter Electrodes counter electrode a

h-CuS/FTO Cu2S/brassa Pta h-CuS/FTOb Cu2S/brassb Ptb

JSC (mA cm−2)

VOC (V)

η (%)

FF (%)

16.05 17.18 12.81 15.45 15.08 13.35

0.550 0.483 0.540 0.565 0.550 0.525

4.32 4.08 2.85 3.91 3.95 2.43

48.9 49.2 41.2 44.8 47.6 34.6

a

The highest recorded champion devices performances measured in different batches of photoanodes with h-CuS/FTO, Cu2S/brass, and Pt counter electrodes. bQDSSC characteristics with different counter electrodes measured using the same batch of photoanodes.

compared to those of Pt CE demonstrated that h-CuS/FTO CE had high electrocatalytic activity toward sulfide/polysulfide electrolyte similarly with that of the Cu2S/brass CE. While the activities of the h-CuS/FTO CE and Cu2S/brass CE were both much higher than that of Pt CE. As a result, the QDSSC device fabricated with h-CuS/FTO CE displayed an overall efficiency (η) of 3.91%, similar to that of 3.95% with Cu2S/brass CE, which was much higher than that of Pt CE, 2.43%. The incident photon-to-electron conversion efficiency (IPCE) spectra of these QDSSC devices showed the spectral response uniformity of the same batch of photoanodes (Figure S1, Supporting Information), indicating that the difference of power conversion efficiency was attributed to the different electrocatalytic activity of h-CuS/FTO, Cu2S/brass, or Pt CE. To demonstrate the high reproducibility of the h-CuS/FTO CE, photovoltaic characteristics of QDSSC devices assembled by different batches of photoanodes and h-CuS/FTO CEs were measured and are shown in Table S1 in the Supporting Information, with performance comparison of QDSSCs paired with Cu2S/brass and/or Pt CE using the same batch of photoanodes. The highest recorded performances measured with different batches of photoanodes and different CEs are shown in Figure 4b by lines with open symbols. The highest power conversion efficiency, 4.32%, was achieved with h-CuS/ FTO CE, which was higher than 4.08% with Cu2S/brass CE

Figure 3. Morphology evolution of CuS nanostructures. SEM images of the CuS nanostructures electrodeposited on FTO/glass substrates at different times: (a) 0 s, (b) 10 s, (c) 20 s, (d) 30 s, (e) 2.5 min, (f) 5 min, (g) 20 min, and (h) 60 min. Scale bar represents (a−d) 200 nm and (e−h) 2 μm.

h-CuS/FTO, Cu2S/brass, and Pt CEs were recorded (shown in Figure 4b), and the solar cell performance parameters are summarized in Table 1. The higher short-circuit current density (JSC) and fill factor (FF) of h-CuS/FTO and Cu2S/brass CEs

Figure 4. (a) Schematic depiction of the mechanism of CdS/CdSe quantum-dots cosensitized thin-layer liquid-junction solar cell assembled with asprepared CuS counter electrode using P25-TiO2 nanoparticles as a matrix and filled with a 1.0 M S2−/1.0 M S aqueous electrolyte. (b) Current density−voltage (J−V) curves of QDSSCs assembled with h-CuS/FTO, Cu2S/brass, and Pt counter electrodes. The lines with hollow circles represented the highest recorded power conversion efficiency devices measured in different batches of photoanodes with different CEs. The lines without hollow circles were the QDSSC performances of different CEs using the same batch of photoanodes. E

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Figure 5. Electrochemical characterization of h-CuS/FTO, Cu2S/brass and Pt electrodes in symmetrical dummy cells filled with aqueous 1.0 M S2−/ 1.0 M S electrolyte. (a) Tafel polarization obtained by different electrodes with the ordinates of the crossing point of two tangent lines showing the logarithmic exchange current density. (b) Nyquist plots of symmetrical dummy cells with inset magnified these of h-CuS/FTO and Cu2S/brass electrodes. The equivalent circuit used to fit the nyquist plots was shown in the inset.

Figure 6. Illumination stability of QDSSC devices assembled with h-CuS/FTO, Cu2S/brass and Pt CEs using the same batch of photoanodes. Stability of (a) power conversion efficiency, (b) short-circuit current density, (c) open-circuit voltage, and (d) fill factor of each QDSSC device under continuous illumination for 2 h.

current density across the range of applied overpotential: Cu2S/ brass > h-CuS/FTO > Pt. The exchange current density (J0) obtained by extrapolating the anodic and cathodic branches of each curve to the zero overpotential indicated the highly electrocatalytic activity of Cu2S/brass and h-CuS/FTO CEs toward the polysulfide reduction. The EIS spectra of different symmetrical dummy cells (Figure 5b) were fitted by the equivalent circuit shown in the inset, which consisted of series resistance (RS), constant phase element (CQE), Warburg impedance (W), and charge transfer resistance (RCT). The values of RCT fitted from the semicircle at the middle frequency regions are 47.2, 5.8, and 870.8 Ω cm2 for h-CuS/FTO, Cu2S/ brass, and Pt CEs, respectively. According to the Tafel equation

and much higher than 2.85% with Pt CE (Table 1), using the same procedures of photoanode preparation and device assembly under the identical testing conditions. Therefore, it is clear that the performance of QDSSCs can be improved by replacing Cu2S/brass or Pt CE with h-CuS/FTO CE due to the high electrocatalytic activity of hierarchical CuS nanostructures on FTO/glass toward polysulfide reduction. To further investigate the interfacial charge-transfer properties of h-CuS/FTO electrode in aqueous S2−/Sn2− redox couple solution, we carried out Tafel polarization measurement and EIS experiments in symmetrical dummy cells prepared with two identical CEs. The comparison of Tafel polarization curves of different cells (Figure 5a) showed the magnitude of logarithmic F

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Figure 7. Electrochemical and chemical stability characterization of h-CuS/FTO, Cu2S/brass, Pt electrodes and QDSSC devices assembled with different CEs. (a-c) showed the changes in electrochemical impedance spectra obtained before, after 60 cycles and 120 cycles of I−V scanning (from 0 to 0.65 V) using symmetrical dummy cells filled with aqueous 1.0 M S2−/1.0 M S electrolyte. (d) The EDS spectra of bare photoanode and photoanodes paired with different CEs in QDSSCs after the 2-h illumination stability measurements, with magnified Cu Kα peaks around 8.0 keV shown in the inset. (e) SEM image of photoanode after 2-h continuous J-V photovoltaic test paired with Cu2S/brass counter electrode (left) and the in situ EDS mapping of Cu element (right), indicating the uniform and dense distribution of copper sulfide (CuxS) particles on the TiO2 mesoscopic matrix of photoanode.

J0 =

RT FR CT

continuous AM 1.5G simulated solar irradiation for 2 h. The QDSSC performance parameters are depicted in Figure 6 with a time-dependent behavior. The power conversion efficiencies of QDSSCs paired with h-CuS/FTO, Cu2S/brass and Pt CEs (shown in Figure 6a) all experienced a sharp increase at the initial several minutes. Following the sharp increase, the photovoltaic device with h-CuS/FTO CE showed a steady increase and reached its peak of 4.22% at 45 min and plateaued at about 4%. The steady increase was mainly attributed to the increment of fill factor (Figure 6d) during the continuous J−V measurement. However, for Cu2S/brass CE, the efficiency decreased rapidly after reaching its highest value of 3.96% at 5 min, along with the attenuation of JSC, VOC, and FF. The rebound of these parameters around 20−30 min probably resulted from the further corrosion of brass plate in sulfide/

where R is the gas constant and F is the Faraday constant, the exchange current density was calculated to be 544, 4420, and 29.5 μA cm−2 for h-CuS/FTO, Cu2S/brass, and Pt electrode materials, respectively, and they agreed with the values of J0 obtained from the Tafel polarization curves. Thus, the electrocatalytic activity of h-CuS/FTO and Cu2S/brass CEs are much higher than that of Pt CE for the reduction of polysulfide species. 3.3. Stability Tests of the Resulting Cell Devices. To characterize the illumination stability of different CEs assembled QDSSC devices, we measured the photocurrent density−voltage (J−V) curves once per minute under G

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Figure 8. Conservation stability measurement of QDSSC devices incorporated with h-CuS/FTO CEs. (a) Variation of photovoltaic performance characteristics (η, JSC, FF, and VOC) versus conservation time. (b) J−V curves obtained at different conservation time with the inset table showing the photovoltaic parameters.

of conversion efficiency in QDSSC using Cu2S/brass as CE can be mainly attributed to two reasons: (i) the competitive lightabsorption of deposited CuxS particles against CdS/CdSe QDs, which cannot inject photoelectrons into CdS/CdSe QDs or mesoscopic TiO2 due to the mismatch of energy band; (ii) local reduction of polysulfide species with negative charged copper complexes and the consequent impediment of electrolyte diffusion. These conditions reduced the short-circuit current density, open-circuit voltage, and fill factor of QDSSC device when using Cu2S/brass as CE. In contrast, QDSSCs showed a greater and more stable JSC under chopped irradiation when the Cu2S/brass or Pt CE was replaced with an h-CuS/FTO CE (Figure S3, Supporting Information), suggesting the excellent electrocatalytic activity and illumination stability of the h-CuS/ FTO CE. Moreover, we also characterized the conservation stability measurement of QDSSC device incorporated with an h-CuS/FTO CE (shown in Figure 8). This device reached its highest power conversion efficiency (η) of 4.06% in 6 days because of the increment of fill factor. The device maintained the peak performance for 10 days, implying the high conservation stability of QDSSCs assembled with h-CuS/ FTO CEs.

polysulfide electrolyte and the surface reconstruction of Cu2S/ brass CE. For Pt CE, the conversion efficiency increased slowly and stabilized around 2.85%, which was much smaller than that of h-CuS/FTO CE assembled device. Overall, the comparison explicitly demonstrates that the QDSSCs employing h-CuS/ FTO CEs show much better illumination stability than these using Cu2S/brass and Pt as CEs. To further examine the stability of electrocatalytic activity of different CEs, we characterized the changes of electrochemical impedance spectra during the cyclic voltammetry scanning in the symmetrical dummy cells (Figure 7a−c). After 120 cycles of I−V scanning (from 0 to 0.65 V), the charge-transfer resistance (RCT) increased from 800 to 866 then to 906 Ω cm2 for Pt electrode and from 5.81 to 7.42 then to 9.26 Ω cm2 for Cu2S/ brass electrode, respectively. However, the value of RCT for hCuS/FTO electrode decreased from 45.6 to 42.8 then finally to 40.3 Ω cm2, which meant the cyclic voltammetry scanning in aqueous 1.0 M S2−/1.0 M S electrolyte facilitates the chargetransfer process at the interface of h-CuS/FTO electrode. These changes in EIS indicated better stability of electrocatalytic activity for h-CuS/FTO CE toward polysulfide reduction than Cu2S/brass or Pt CE. Although the h-CuS/ FTO CE showed better stability than Cu2S/brass CE, the value of RCT for the latter was still lower than that for h-CuS/FTO CE, which meant the exchange current density of Cu2S/brass electrode in sulfide/polysulfide electrolyte was still higher than that of h-CuS/FTO CE. Therefore, there were other reasons for the poor stability of QDSSC photovoltaic performance assembled with Cu 2S/brass CE using aqueous S2−/Sn2− electrolyte. After the 2 h illumination stability measurements, we noticed that the active layer of photoanode paired with Cu2S/brass CE had been changed from green color to dark blackish green color. So, we carried out EDS analysis on all the photoanodes paired with the three types of CEs. The EDS result of the photoanode tested with Cu2S/brass CE showed apparent Cu Kα peaks around 8.0 keV in the inset of Figure 7d and in Supporting Information Figure S2, while the bare photoanode without photocurrent−voltage test and the photoanodes assembled with h-CuS/FTO and Pt CEs after the 2 h illumination stability measurements did not present the Cu Kα peaks. Furthermore, the elemental mapping of Cu based on EDS (Figure 7e) illustrated the uniform and dense deposition of copper sulfide (CuxS) particles on the photoanode via an electric field-assistant migration.31 The rapid decay

4. CONCLUSIONS In summary, we have demonstrated that the hierarchical CuS nanostructures on FTO/glass synthesized by a facile electrochemical deposition method can serve as a robust, highperformance counter electrode in QDSSCs without any posttreatments. When applying an h-CuS/FTO counter electrode, QDSSC device achieved a power conversion efficiency as high as 4.32% without further optimization for photoanode structures. In comparison to Cu2S/brass and Pt CE, the improved QDSSC performance obtained by the h-CuS/FTO CE is attributed to the high and consistent electrocatalytic activity of this h-CuS material toward the reduction of polysulfide species. Furthermore, the h-CuS/FTO CE exhibits an excellent chemical and electrochemical stability in the sulfide/polysulfide electrolyte and the corresponding QDSSC device shows high illumination and conservation stability. This strategy paves the way for fabricating cheap, stable, and highly efficient counter electrodes for QDSSCs, which can act as a substitute for Cu2S/brass and Pt counter electrodes. H

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ASSOCIATED CONTENT

S Supporting Information *

Details for the preparation of CdS/CdSe QDs cosensitized mesoscopic TiO2 photoanode and sandwich-style QDSSC devices assembly, IPCE spectra of QDSSCs, photovoltaic parameters of different batches of cells, element content analysis of photoanode paired with Cu2S/brass CE after 2 h test, JSC response and stability of cells with different CEs. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*(D.X.) Tel./Fax: 86-10-62760360. E-mail: [email protected]. *(Q.L.) Tel./Fax:86-10-62766235. E-mail: [email protected]. cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 21133001 and 51121091) and the National Key Basic Research Program of China (Grant Nos. 2011CB808702, 2013CB932601, and 2014CB239303).

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