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Investigation of metal sulfide composites as counter electrodes for improved performance of quantum dot sensitized solar cells
Materials Research Bulletin 100 (2018) 198–205
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Investigation of metal sulfide composites as counter electrodes for improved performance of quantum dot sensitized solar cells
T
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Beilei Yuana, Lianfeng Duana, , Qiqian Gaoa, Xueyu Zhanga, Xuesong Lia, Yue Yanga, Li Chenb, ⁎ Wei Lüa, a b
Key Laboratory of Advanced Structural Materials, Ministry of Education, Changchun University of Technology, Changchun 130012, PR China School of Basic Sciences & Advanced Institute of Materials Science, Changchun University of Technology, Changchun 130012, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Quantum dot-sensitized solar cell CuS/CoS Counter electrode Open-circuit voltage Power conversion efficiency
The electrochemical performance of counter electrodes (CEs) plays an important role for achieving high efficient quantum dot sensitized solar cells (QDSSCs). Considering the optical band gap energy of semiconductor materials and the Voc of the QDSSCs is determined by the energy difference between the quasi-Fermi levels (qEf) of the anode and cathode, so in this work, we synthesized the different metal sulfides and their composites including CuS/CoS, CuS/NiS, CuS, CoS and NiS by chemical bath deposition (CBD) method and used as CEs of QDSSCs. The effect of different CEs on performance of cells were investigated, and the best power conversion efficiency (η) of 5.22% was achieved by CuS/CoS CE, which is higher than that of CuS/NiS, CuS, CoS and NiS (η = 2.56%, 4.73%, 2.23% and 1.62%). The improved cell efficiency was attributed to the excellent electrical conductivity and catalytic activity of CuS and CoS, respectively. The result indicated that the combination of different metal sulfide composites could increase open-circuit voltage (Voc) value without sacrificing the short-circuit current (Jsc), and leaving good perspectives for significantly higher solar cell performances.
1. Introduction With the increasing problems of energy demands, global warming and emission of greenhouse gases, the development of renewable energy sources has been got much more attention [1–4]. Solar power is becoming a great choice to tackle the crisis and environmental problems. As one of the third generation solar cells, quantum dot-sensitized solar cells (QDSSCs) has attracted increasing attention, because of its relatively low cost, easy fabrication and high-efficiency energy conversion [5–7]. Basically, QDSSCs have similar structures with that of dye-sensitized solar cells (DSSCs). The semiconductor quantum dots (QDs) have presented the possibility of band gap tunability, high extinction coefficient, large intrinsic dipole moments, and potential processes of multiple exciton generation [8–13]. Furthermore, for the sensitizers of DSSCs, QDs also possess higher absorption coefficients [14], multiple exciton generation (MEG) [15], tunable bandgaps due to the quantum confinement effect [16], the possibility of hot electron injection [17], and easy fabrication processes [18]. Although the QDSSCs have been attracted great attention as the excellent properties for QDs, the highest power conversion efficiency of QDSSC is still lower than that of a DSSC in present study [19,20]. QDSSC is mainly composed of a transparent conductive glass, wide
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band gap oxide semiconductor thin films, quantum dots, electrolyte and counter electrode. The efficient charge transfers between sulfide sensitizers and the polysulfide redox couple generates a large short-circuit photocurrent (Jsc) in QDSSCs [21–27]. However, the large charge transfer resistance at the counter electrode results in a low fill factor (FF) and the small difference between the TiO2 Fermi level and the S2−/Sn2− redox level confines the open-circuit photovoltage (Voc). A promising strategy to increase the Voc value without sacrificing the high Jsc is to use a narrow band gap semiconductor as the counter electrode to provide an auxiliary tandem effect. Actually, different kinds of CEs have been found for QDSSCs, such as carbon, carbon derivatives, noble metals and conducting polymers [28–31]. The commonly used noble metal CEs, such as Pt and Au, exhibit poor catalytic property toward polysulfide electrolyte because they usually passivated by sulfide compound absorbed on their surface and the Pt produces a large charge transfer resistance in the polysulfide electrolyte lead to low fill factor [32]. Apart from noble metal, the carbon and carbon derivatives, including activated carbon, mesocellular carbon foam, have no better catalytic activity and there is not enough contact with the base [33]. Conducting polymers have poor power conversion efficiency. So the correlational studies have been reported rarely in CEs for QDSSCs [34]. Therefore, the optimization and
Corresponding authors. E-mail addresses: [email protected] (L. Duan), [email protected] (W. Lü).
https://doi.org/10.1016/j.materresbull.2017.12.021 Received 6 October 2017; Received in revised form 11 December 2017; Accepted 15 December 2017 Available online 20 December 2017 0025-5408/ © 2017 Elsevier Ltd. All rights reserved.
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sodium sulfite (Na2SO3) were supplied by Sigma-Aldrich. Acetic acid (C2H4O2), cadmium acetate (Cd(CH3COO)2·2H2O), L-cysteine (C3H7NO2S), zinc acetate (CH3(COOH)2Zn) and potassium chloride (KCl) were bought from Aladdin. Sodium sulfide (Na2S·9H2O), copper sulfate pentahydrate (CuSO4·5H2O), selenium (Se), sodium hyposulfite (Na2S2O3) and urea (CH4N2O) were purchased from Tianjin Guangfu Technology Development Co., Ltd. All chemicals were used without further purification.
fabrication of CEs with high electrocatalytic activity and good stability are still very important for improving the performances of QDSSCs. Recently, for improving electrocatalytic activity and low fill factor of the CE, metal sulfides have been got much attention due to its low cost, low resistance, good electrocatalytic activity and facile fabrication [5,35]. Furthermore, the metal sulfides would be good choice as CEs for covering related problems discussed above. Several works have reported the preparation and application of metal sulfides as CEs in QDSSCs. Lin et al. applied PbS thin films as CEs for QDSSCs by SILAR method and these exhibited the highest η of 4.7% [36]. Savariraj et al. prepared CuS thin films using a facile CBD method and got the highest η of 4.53% [37]. Yuan et al. reported that CoS thin films be synthesized and applied to the CEs of QDSSCs and achieved the highest η of 5.20% [38]. However, single metal sulfides as CE generally suffers poor electrical conductivity or catalytic activity. Therefore, we use metal sulfide composites as CEs of QDSSCs and these different metal sulfide composites may change the value of Voc2 (the difference between the potential of redox/reduction of electrolyte and the Fermi level of CE), resulting in a noticeable change in the Voc of QDSSCs. In this paper, inspired by above discussion, CuS/CoS, CuS/NiS, CuS, CoS and NiS thin films are deposited on fluorine doped tin oxide (FTO) glass substrates by chemical bath deposition (CBD), which are further used as CEs of QDSSCs, and the cell performances based on these CEs are systematically investigated. In Fig. 1, it has been described that the transport pathways of electrons and photoelectrical conversion configuration of a TiO2/CdS/CdSe/ZnS for anode with the counter electrode of CuS/CoS for QDSSCs. The cell using CuS/CoS CE show a 5.22% power conversion efficiency (PCE) under one sun illumination, which is higher than that of CuS/NiS, CuS, CoS and NiS (η = 2.56%, 4.73%, 2.23% and 1.62%, respectively). The enhanced PCE of CuS/CoS CE is attributed to the combination of different metal sulfides, for that CuS shows the exceptional electrical conductivity and CoS shows the excellent catalytic activity. The result indicates that the combination of different metal sulfide composites could increase open-circuit voltage (Voc) value without sacrificing the short-circuit current (Jsc), and leaving good perspectives for significantly higher solar cell performances.
2.2. Preparation of counter electrodes The CBD method was used to deposit metal sulfide (CuS/CoS, CuS/ NiS, CuS, CoS and NiS) to serve as CEs. For CuS CE, 1 M Na2S2O3 aqueous solution and 1 M CuSO4 aqueous solution were mixed with the volume ratio of 4:1, and the pH value was adjusted to 2 by Acetic acid. Then, the FTO glasses were immersed into as-prepared mixed solution and heated to 70 ℃ for 3 h. After cooling down to the room temperature, the substrates were washed with DI and then heated to 130 ℃ and kept for 30 min. For NiS CE, a solution included 8 g NiSO4·6H2O and 2 g C3H7NO2S in 40 ml deionized water was prepared. The solution was stirred vigorously for 10 min to get a bottle-green solution. Under constant stirring, 4 g CH3CSNH2 and 12 g C2H4O2 were added to the above solution and stirred ultrasonically for another 15 min. Then, the FTO substrates were put into the solution and kept at 90 ℃ for 120 min. For CoS CE, a solution included 5 g CoCl2 and 5 g CH3CSNH2 in 30 ml deionized water was prepared. The solution was stirred vigorously for 10 min to acquire a amaranth solution. Next, 12 g of C2H4O2 was added to the above amaranth solution, and the reaction mixture was stirred vigorously for a further 15 min. To prepare CuS/CoS and CuS/NiS CEs, 0.1 M CuSO4·5H2O, 0.5 M CH4N2O and 0.5 M CH3CSNH2 were dissolved in 30 ml deionized water, and the FTO glasses were immersed into as-prepared mixed solution and heated to 70 ℃ for 2 h. Then the as-prepared CuS substrates were put into a solution of containing 0.1 M CoCl2·6H2O, 1 M CH4N2O, 0.5 M CH3CSNH2 and 0.6 M C2H4O2 for further deposition of CoS. This film substrate named CuS/CoS. After cooling down to the room temperature, the substrates were washed with DI water and heated to 60 ℃ for 30 min. To deposit the CuS/NiS, the as-prepared CuS substrates were put into a solution that consisted of 0.1 M NiSO4·6H2O, 1 M CH4N2O, 0.5 M CH3CSNH2 and 0.6 M C2H4O2 and kept at 90℃ for 2 h.
2. Experimental details 2.1. Materials FTO glasses were purchased from Zhuhai Kaivo Optoelec-tronic Technology Co., Ltd., cobalt chloride hexahydrate (CoCl2·6H2O), nickel sulfite hexahydrate (NiSO4·6H2O), thioacetamide (CH3CSNH2) and
2.3. Preparation of photoanode (TiO2 /CdS/CdSe/ZnS) TiO2 films were made using the doctor-blade method by coating Fig. 1. Schematic structure of QDSSCs based CuS/ CoS CE.
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Fig. 2. SEM images of the (a) CuS; (b) CoS and (c) NiS thin films deposited on FTO substrates.
Fig. 3. SEM images of the (a) CuS/NiS; (c) CuS/CoS thin films deposited on FTO substrates; (b) and (d) are the magnified images of (a) and (c), respectively.
2.4. Fabrication of qdsscs The as-prepared CuS/CoS, CuS/NiS, CuS, CoS and NiS counter electrodes and TiO2/CdS/CdSe/ZnS photoanode were assembled to a sandwich-type cell and penetrated with a polysulfide electrolyte of containing 2 M Na2S and 2 M S in methanol and H2O solution (v/v= 7:3). 2.5. Characterization The morphology and structure of counter electrodes were observed by scanning electron microscopy (SEM) (S4800, Hitachi). X-ray diffraction (XRD) was performed on D-MAX II A X-ray diffractometer. The electrochemical impedance spectroscopy (EIS), Cyclic voltammetry (CV) and Tafel polarization curve were obtained with Solartron Impedance Analyzer. A VG ESCALAB MKII spectrometer can be used to measure the XPS spectra. The incident photon-to-current conversion efficiency (IPCE) was measured by a solar cell scan 100 (Zolix, Beijing). AM 1.5 solar simulator (Zolix, Beijing) was used to measure the performances of the cell.
Fig. 4. XRD spectra ofCuS/CoS, CuS/NiS, CuS, CoS and NiS thin film on FTO substrates.
TiO2 paste on FTO glass substrates, followed by sintering at 450 ℃ for 30 min, and obtain a film of ∼7.5 μm [39]. Next, CdS QDs were deposited on the TiO2 films by the successive ionic layer adsorption and reaction (SILAR) method [40,41]. For deposition of CdSe QDs on TiO2 films, chemical bath deposition (CBD) method was used to deposit CdSe QDs [1,42,43]. Finally, ZnS passivation layer was deposited by SILAR method and it can avoid corrosion caused by the polysulfide electrolyte and prevent the recombination of electrons in the electrolyte [14,26,28].
3. Results and discussion 3.1. Synthesis and characterization of the counter electrodes Fig. 2 shows SEM images of the CuS, CoS and NiS thin films prepared separately on FTO substrate, respectively. It is obvious that the three CEs have different morphologies. The CuS thin film consists of many nanoparticles with spheres-like structures as shown in Fig. 2(a). The NiS thin film in Fig. 2(c) is more like nano-flowers, and the CoS film in Fig. 2(b) shows the intermediate structures. For CuS/NiS film as shown in Fig. 3(a), many cracks between 200
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Fig. 5. XPS spectra of CuS/CoS thin film on FTO surface: (a) survey scan, high resolution spectrum of the (b) Co 2p; (c) Cu 2p; and (d) S 2p.
Fig. 6. (a) CV curves for the CuS/CoS, CuS/NiS, CuS, CoS and NiSCEs (b) Nyquist plots of EIS for the symmetric cells. (c) Tafel polarization curves of the symmetric cells. (d) UV–vis results of the symmetric cells.
adjacent NiS structures could be observed, which are more obvious compared with that of pure NiS deposited on FTO substrate (as seen in Fig. 2(c)). The enlarged SEM image in Fig. 3(b) could clearly observe the morphology, which may be due to the different substrates for deposition. For CuS/NiS film, the NiS layer is deposited onto the CuS layer, and the CuS film is composed of nanoparticles as shown in Fig. 3(a), the uneven surface of substrate induces the large space between adjacent NiS crystal particles. For CuS/CoS film, the top CoS layer also exhibits different morphology with that of CoS film on FTO as shown in Fig. 2(b). The CoS on CuS layer shows porous structure composed of many thin nanosheets. As it could be expected to yield electrodes, the highly porous structure of the CuS/CoS CE would provide more catalytically active sites for the redox process, which is with
Table 1 Photovoltaic parameters of QDSSCs based on various counter electrodes. Counter electrodes
Rs(Ω)
Rct(Ω)
Zw(Ω)
CPE(μF)
CuS/NiS CuS/CoS CuS CoS NiS
2.491 2.203 2.284 3.017 3.021
55.61 22.17 37.75 55.72 78.11
1.9 0.9 1.4 2.1 2.5
37.27 41.88 38.87 37.01 32.99
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peak in the diffraction pattern of NiS [40,45]. The sharp and strong diffraction peaks of CuS at 27.70°, 29.57°, 32.05°, 48.61° and 59.30° can be observed, corresponding to the (101), (102), (006), (110) and (116) lattice planes (JCPDS NO. 06-0464). Similarly, the XRD patterns of as prepared CuS/CoS film can be attributed to the diffraction on (101), (102), (006), (110), (116) lattice planes of CuS, (101) and (112) lattice planes of CoS, which can be well indexed to CuS/CoS phases (JCPDS PDF NO. 75-0605 and JCPDS NO. 06-0464). The XRD patterns of the CuS/NiS film are resulted from (101), (102), (006), (110) and (116) lattice planes of CuS, (100), (101), and (110), (116) lattice planes of NiS, which can be well indexed to CuS/NiS phases (JCPDS NO. 06-0464 and JCPDS NO. 02-1280). For CuS/CoS composites, there are some peaks at around 20 degrees and 78 degrees can be observed, this peaks can be ascribed to FTO substrate. For CuS/ NiS composites, there are some peaks at around 20-25 degrees and 62–65° can be observed, this peaks can be attributed to the FTO substrate [40,41]. These results confirm the successful deposition of CuS/CoS and CuS/NiS compounds on the FTO substrates. Fig. 5 is the XPS spectrum of the CuS/CoS thin film on FTO surface for examining the chemical valence state and elemental compositions of the as prepared CEs. Fig. 5(a) is overall scan of XPS and all elements involved in synthesis could be observed. In the high-resolution XPS spectrum of Co 2p (Fig. 5(b)), there are two peaks of Co 2p3/2 at 781 eV and Co 2p1/2 at 797 eV. For Co 2p3/2 peak at 781 eV, it is assigned to CoS formation as well as a broad peak at higher binding energies (about 781 eV) [46]. Moreover, there is a little shake-up satellite peak of the metallic Co at 778.5 eV, which is contributed to the presence of Co0 species are being formed [47,48]. This could be attributed to the cobalt ion has a very strong affinity to oxygen and it is difficult to exclude it impurities from the resultant materials [31,47]. As shown in Fig. 5(c), the XPS spectrum exhibited two strong peaks at binding energies of 931.3 eV and 951.4 eV, which can be attributed to Cu 2p3/2 and Cu 2p1/2, respectively. Except for them, there are several weak satellite peaks around 943 eV due to the presence of Cu2+ [48,49]. In addition, there are S 2p3/2 and S 2p1/2 peaks at 162.9 eV and 168.36 eV in Fig. 5(d), indicating that the S species exist as S2− in the deposits [48]. From the XPS analyses, it is indicated that CuS/CoS composites had been synthesized successfully onto the FTO surface by chemical bath deposition method.
Fig. 7. (a) J–V and (b) IPCE curves characteristics of the QDSSCs with CuS/CoS, CuS/ NiS,CuS, CoS and NiSCEs.
Table 2 Photovoltaic parameters of CuS/CoS, CuS/NiS, CuS, CoS and NiS CEs based on QDSSCs under one Sun. Samples
Jsc(mA cm−2)
Voc
FF
η (%)
NiS CoS CuS CuS/NiS CuS/CoS
9.52 12.39 17.82 13.09 19.96
0.42 0.42 0.58 0.45 0.56
0.40 0.43 0.45 0.44 0.47
1.62 2.23 4.73 2.56 5.22
3.2. Electrochemical performance of the counter electrodes The electrochemical catalytic activity of as-prepared CuS/CoS, CuS/ NiS, CuS, CoS and NiS CEs in polysulfide electrolyte are investigated. The cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), Tafel polarization and UV–vis measurements are performed, as shown in Fig. 6. In the CV curves, the high current peak indicates active electrocatalytic reaction of the electrode for the S2−/Sn2− redox couples in the polysulfide electrolyte [49]. The negative currents represent the reduction of Sn2- ions to S2− ions while the positive currents are related to the oxidation of S2− ions [50]. As shown in Fig. 6(a), compared to the CuS/NiS, CuS, CoS and NiS CEs, the CuS/CoS CE shows the higher current density and excellent reversibility, indicating that the electrochemical catalyst of CuS/CoS composite CE is better than the others. Fig. 6(b) is electrochemical impedance spectra. The parameters obtained from impedance spectra are shown in Table 1. The Nyquist plots were measured in the frequency range of 0.01Hz to 100 mHz. The high frequency intercept represents the series resistance (Rs). The semicircle in the middle frequency region represents the charge transfer resistance (Rct) and the corresponding constant phase angle element (Cμ) at the CE/electrolyte interface, while the low frequency region provides the Warburg diffusion impedance (ZW) of the polysulfide electrolyte [45–47]. For CuS/CoS electrode, the charge transfer resistance acquired from the fitting circuit is much smaller than those of NiS, CuS, CuS/NiS, CoS electrodes as shown in Table 1, suggesting that
a large specific surface area [29,30]. Fig. 4 is XRD results of as-prepared samples. There are some unindexed peaks in the diffraction patterns due to the existence of the FTO substrate. It is known that the counter electrode is a thin layer, which results in the existence of FTO substrate. For CoS, the diffraction peaks at 31.39°, 35.27°, 46.78°and 56.09° could be assigned to (100), (101), (102) and (110) lattice planes of the CoS phase (JCPDS PDF NO. 750605), and other sharp diffraction peaks can be ascribed to FTO substrate [44,45]. For NiS, there are five obvious sharp diffraction peaks locating at 31.19°, 34.18°, 51.99°, 61.75° and 65.73° can be observed, corresponding to the (100), (101), (110), (200) and (201) lattice planes (JCPDS NO. 02-1280). The 26.44° and 37.94° can be attributed to the FTO substrate [40,41]. It is known that the counter electrode is a thin layer and the sintered temperature of NiS counter electrode is low, thus the FTO has a high diffraction intensity, which results in the phenomena that the reflection around 31.19° and 34.18° has low angle 202
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Table 3 Result comparisons of present photovoltaic parameters with other reports of different counter electrodes. CE
QD
Preparation
Voc
Jsc(mA cm−2)
η (%)
Refs
CuS/CoS CuS/CoS CuS/CoS CuS NiS CoS CoS CoS CuS CuS CuS CuS Cu1.8S NiS CoS2 Cu2S
CdS/CdSe CdS/CdSe CdS/CdSe CdS/CdSe CdS/CdSe CdS/CdSe CdS CdS CdS/CdSe CdS/CdSe CdS/CdSe CdS/CdSe CdS/CdSe CdS/CdSe CdS/CdSe CdS/CdSe
CBD electrochemical deposition CBD CBD Hydrothermal potentiodynamic electrodeposition CBD SILAR SILAR CBD CBD SILAR CBD CBD Hydrothermal electrodeposition
0.56 0.54 0.48 0.53 0.49 0.52 0.66 0.58 0.55 0.58 0.60 0.59 0.60 0.51 0.53 0.54
19.96 16.09 16.8 14.11 11.67 11.2 1.96 4.07 13.9 13.87 19.92 16.3 19.10 10.38 10.86 19.70
5.22 5.03 4.1 4.02 3.03 1.9 0.64 0.95 2.7 4.01 4.06 4.27 5.16 2.97 2.27 4.68
this work [33] [56] [57] [41] [58] [59] [60] [58] [45] [26] [3] [51] [40] [61] [4]
curve is divided into three major zones. The low potential in left is the polarization zone. The middle potential of curve with a sharp slope is attributed to the Tafel zone, which represents the catalytic ability of the CEs. The right zone at high potential named the diffusion zone. At the high potential, the limiting current density (Jlim) can be derived from the diffusion zone of the Tafel curve. The exchange current density (J0) is attaining from the zero intercept of the approximately linear region of the curve. The electrochemical catalytic activity of CEs was measured by these two parameters. J0 depends on the Rct value, and Rct can be obtained from the EIS analysis according to Eq. (1):
J0 =
RT nFR ct
(1)
where T is temperature, F is the Faraday constant, R is the gas constant, n is the number of electrons involved in the reduction of disulfide at the CE, and Rct is the charge transfer resistance at the CE/electrolyte interface [50]. In the plot of Fig. 6(c), due to higher J0 represents a larger reduce ability, the J0 value of CuS/CoS CE is obviously higher than the other counter electrodes (CuS/NiS, CuS, CoS and NiS CEs), indicating that it has better reduction of the polysulfide electrolyte at the CE/ electrolyte interface [53]. CuS/CoS shows greater Jlim values compared to CuS/NiS, CuS, CoS and NiS CEs from the diffusion zone of the curve. It reveals a higher diffusion velocity of the CuS/CoS CE in the S2−/Sn2− electrolyte [54]. Therefore, because of the highest J0 value with the lowest Rct value, CuS/CoS electrode exhibits the best electrocatalytic
Fig. 8. The error of the PCE with CuS/CoS, CuS/NiS,CuS, CoS and NiS CEs based on QDSSCs under one Sun.
the CuS/CoS electrode could facilitate the electron transport from the CE to the S3− species [51,52]. Fig. 6(c) is the Tafel polarization curves. The Tafel plot can be used to measure the interfacial charge-transfer properties and electrochemical catalytic activity of CEs using S2−/Sn2− couple. The Tafel
Fig. 9. A contrast map of single metal sulfides and composite metal sulfides.
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based on the CuS/CoS CE (TiO2 was used as photoanode with CdS/ CdSe/ZnS core/shell quantum dots) had been shown. Compared to CuS, CoS and NiS, the different metal sulfide composite could enhance the value of Voc2, resulting a noticeable change of the Voc for QDSSCs, because the quasi-Fermi level of metal sulfide composite is closer to the vacuum Fermi level. Furthermore, the photoactive properties of the CuS/CoS electrode enhanced electron injection into the electrolyte and subsequently improved the Jsc and FF values of the cell which could increase the overall performance of QDSSCs to the next level.
activity among as-prepared CEs, agreeing with the CV and EIS results. The optical absorption spectra of counter electrodes are shown in Fig. 6(d), which play an important role CEs in QDSSCs [53]. The optical absorption spectra were found to decrease with the sequence of the CuS/CoS, CuS/NiS, CuS, CoS and NiS CEs in the range of 300–800 nm wavelength, and the peak centered in the spectra is located around 550 nm. The increased adsorption of CuS/CoS leads to improved absorption spectra for counter electrode. It shows that as prepared CuS/ CoS composite could be an appropriate material for harvesting residual light that penetrates a photoanode [55].
Conflicts of interest 3.3. Photovoltaic performance of the QDSSCs Manuscript does not include any content with conflict of interest. The cell performances were investigated under AM 1.5 G irradiation as shown in Fig. 7(a) and the cell parameters such as short-circuit current density (Jsc), open-circuit voltage (Voc), fill factor (FF), and power conversion efficiency (η) extracted from the J–V curves are listed in Table 2. It has been shown that the QDSSC with the CuS/CoS CE presents the best photovoltaic property with PCE of 5.22%, Jsc of 19.96 mA cm−2, Voc of 0.56V, and FF of 0.47, while that the PCE of the CuS, CuS/NiS, CoS and NiS CEs are 4.73%, 2.56%, 2.23% and 1.62%, respectively. Obviously, as shown in Fig. 7(a) and Table 2, all the performance indexes of the solar cell are increased with the CuS/CoS CE of QDSSC, which lead to the improvement of the overall efficiency, because the excellent electrical conductivity and catalytic activity of the CuS and the CoS, respectively [37,38]. However, NiS shows very low performance due to poor catalytic activity in the S2-/Sn2- electrolyte. These data revealed a better electrocatalytic activity for CuS/CoS CE than CuS/NiS, CuS, CoS and NiS CEs, as indicated by the high Jsc and FF. Compared to the similar metal sulfides in previous reports as shown in Table 3, the PCE of CuS/CoS CE achieve the best value in present work. In order to exam the different photovoltaic performance of CuS/ CoS, CuS/NiS, CuS, CoS and NiS CEs, IPCE spectra as a function of wavelength were measured, as shown in Fig. 7(b). Although the IPCE spectrum of CuS based QDSSC was higher, it is lower than CuS/CoS. The IPCE of CuS/CoS counter electrode has been shown the largest value (78%) through the whole measured range. The photovoltaic performance of CuS/CoS composite CE for QDSSCs is improved because it has superior electrocatalytic activity for the reduction of polysulfide electrolyte, which is proved by the electrochemical analysis. The distribution of the PCE curves with CuS/CoS, CuS/NiS, CuS, CoS and NiS CEs based on QDSSCs were shown in Fig. 8. From PCE curves, we found the error of the cell efficiencies were less than 1%. The possible mechanism of the energy bands and the related charge transfer processes of the CuS/CoS and CuS CEs in the QDSSC is supposed, as shown Fig. 9. The Voc of the QDSSCs device is determined by the energy difference between the quasi-Fermi levels (qEf) of the anode and cathode [62]. Generally, the photoactive electrode moves to the quasi-Fermi level near the vacuum Fermi level of the anode or the cathode, which depends on the equilibrium position between the electrode and electrolyte solution [63,64]. In Fig. 9, the photo-voltaic response of QDSSCs based on the anode and CuS/CoS CE: Voc=Voc1 (anode)+Voc2 (CE), the photo-voltaic response of QDSSCs based on the anode and CuS CE: Voc′=Voc1 (anode)+Voc2′ (CE), respectively. Moreover, the single metal sulfides (CuS as an example), the open-circuit voltage of Voc2′ is smaller than the Voc2 of CuS/CoS, the relationship between Voc2′ and Voc2 is Voc2 (CuS/CoS CE)=ΔVoc+Voc2′ (CuS CE). So the density of the CoS which was deposited at the surface of CuS may increase the Voc of QDSSCs due to the higher Voc2 value.
Acknowledgements The authors thank funding by the National Natural Science Foundation of China (Grant Nos. 61574021, 61376020, 11474035, 61774022 and 61604017) and the Education Department of Jilin Province. References [1] C.V.V.M. Gopi, M. Venkata-Haritha, Y.-S. Lee, H.-J. Kim, Correction: ZnO nanorods decorated with metal sulfides as stable and efficient counter-electrode materials for high-efficiency quantum dot-sensitized solar cells, J. Mater. Chem. A 5 (2017) 428–429. [2] J.P. Holdren, Science and technology for sustainable well-being, Science 320 (2008) 179. [3] D. Punnoose, C.S.S.P. Kumar, S.S. Rao, C.V.T. Varma, B. Naresh, A.E. Reddy, N. Kundarala, Y.-S. Lee, M.-Y. Kim, H.-J. Kim, In situ synthesis of CuS nano platelets on nano wall networks of ni foam and its application as an efficient counter electrode for quantum dot sensitized solar cells, Org. Electron. 42 (2017) 115–122. [4] C.K. Kamaja, R.R. Devarapalli, Y. Dave, J. Debgupta, M.V. Shelke, Synthesis of novel Cu2S nanohusks as high performance counter electrode for CdS/CdSe sensitized solar cell, J. Power Sources 315 (2016) 277–283. [5] P.K. Santra, P.V. Kamat, Tandem-layered quantum dot solar cells: tuning the photovoltaic response with luminescent ternary cadmium chalcogenides, J. Am. Chem. Soc. 135 (2013) 877–885. [6] P.V. Kamat, K. Tvrdy, D.R. Baker, J.G. Radich, Beyond photovoltaics: semiconductor nanoarchitectures for liquid-junction solar sells, Chem. Rev. 110 (2010) 6664–6688. [7] M. Wang, W. Chen, J. Zai, S. Huang, Q. He, W. Zhang, Q. Qiao, X. Qian, Hierarchical Cu7S4 nanotubes assembled by hexagonal nanoplates with high catalytic performance for quantum dot-sensitized solar cells, J. Power Sources 299 (2015) 212–220. [8] H. Hu, J. Ding, J. Qian, Y. Li, L. Bai, N. Yuan, Layered CoS/graphene nanocomposite as high catalytic counter electrodes for quantum dot-sensitized solar cells, Mater. Lett. 114 (2014) 7–10. [9] Z.Y. Peng, X.F. Chen, Y.L. Liu, J.L. Chen, J. Chen, Balance on the charge generation, separation and transfer performance of different TiO2 nanostructures in quantum dot sensitized solar cells, Mater. Res. Bull. 94 (2017) 463–471. [10] B. O’Regan, M. Grätzel, A low-cost, high-efficiency solar cells based on dye-sensitized colloidal TiO2 films, Nature 414 (2001) 338–344. [11] H. Wang, Y. Bai, H. Zhang, Z. Zhang, J. Li, L. Guo, J. Mater. Chem. C 114 (2010) 16451–16455. [12] Y. Jiang, B.-B. Yu, J. Liu, Z.-H. Li, J.-K. Sun, X.-H. Zhong, J.-S. Hu, W.-G. Song, L.J. Wan, Boosting the open circuit voltage and fill factor of QDSSCs using hierarchically assembled ITO@Cu2S nanowire array counter electrodes, Nano Lett. 15 (2015) 3088–3095. [13] H. Salaramoli, E. Maleki, Z. Shariatinia, M. Ranjbar, CdS/CdSe quantum dots cosensitized solar cells with Cu2S counter electrode prepared by SILAR, spray pyrolysis and Zn–Cu alloy methods, J. Photochem. Photobiol. A 271 (2013) 56–64. [14] Q. Zhang, Y. Zhang, S. Huang, X. Huang, Y. Luo, Q. Meng, D. Li, Application of carbon counterelectrode on CdS quantum dot-sensitized solar cells (QDSSCs), Electroche. Commun. 12 (2010) 327–330. [15] T. You, L. Jiang, K.L. Han, W.Q. Deng, Improving the performance of quantum dotsensitized solar cells by using TiO2 nanosheets with exposed highly reactive facets, Nanotechnology 24 (2013) 245401. [16] S. Gorer, G. Hodes, A hydrothermal reaction combined with a post anion-exchange reaction of hierarchically nanostructured NiCo2S4 for high-performance QDSSCs and supercapacitors, Phys. Chem. 98 (2010) 5338–5346. [17] W.A. Tisdale, K.J. Williams, B.A. Timp, D.J. Norris, E.S. Aydil, X.Y. Zhu, Hotelectron transfer from semiconductor nanocrystals, Science 328 (2010) 1543–1547. [18] Z. Yang, C.Y. Chen, P. Roy, H.T. Chang, Quantum dot-sensitized solar cells incorporating nanomaterials, Chem. Commun. 47 (2011) 9561–9571. [19] K. Zhao, Z.X. Pan, I. Mora-Ser´o, E. C´anovas, H. Wang, Y. Song, X.Q. Gong, J. Wang, M. Bonn, J. Bisquert, X.H. Zhong, Boosting power conversion efficiencies of quantum dot sensitized solar cells beyond 8% by recombination control, J. Am.
4. Conclusions The CuS/CoS, CuS/NiS, CuS, CoS and NiS counter electrodes were deposited on FTO conducting glass by a simple chemical bath deposition method. The higher power conversion efficiency (5.22%) of QDSSC 204
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Investigation of metal sulfide composites as counter electrodes for improved performance of quantum dot sensitized solar cells
T
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Beilei Yuana, Lianfeng Duana, , Qiqian Gaoa, Xueyu Zhanga, Xuesong Lia, Yue Yanga, Li Chenb, ⁎ Wei Lüa, a b
Key Laboratory of Advanced Structural Materials, Ministry of Education, Changchun University of Technology, Changchun 130012, PR China School of Basic Sciences & Advanced Institute of Materials Science, Changchun University of Technology, Changchun 130012, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Quantum dot-sensitized solar cell CuS/CoS Counter electrode Open-circuit voltage Power conversion efficiency
The electrochemical performance of counter electrodes (CEs) plays an important role for achieving high efficient quantum dot sensitized solar cells (QDSSCs). Considering the optical band gap energy of semiconductor materials and the Voc of the QDSSCs is determined by the energy difference between the quasi-Fermi levels (qEf) of the anode and cathode, so in this work, we synthesized the different metal sulfides and their composites including CuS/CoS, CuS/NiS, CuS, CoS and NiS by chemical bath deposition (CBD) method and used as CEs of QDSSCs. The effect of different CEs on performance of cells were investigated, and the best power conversion efficiency (η) of 5.22% was achieved by CuS/CoS CE, which is higher than that of CuS/NiS, CuS, CoS and NiS (η = 2.56%, 4.73%, 2.23% and 1.62%). The improved cell efficiency was attributed to the excellent electrical conductivity and catalytic activity of CuS and CoS, respectively. The result indicated that the combination of different metal sulfide composites could increase open-circuit voltage (Voc) value without sacrificing the short-circuit current (Jsc), and leaving good perspectives for significantly higher solar cell performances.
1. Introduction With the increasing problems of energy demands, global warming and emission of greenhouse gases, the development of renewable energy sources has been got much more attention [1–4]. Solar power is becoming a great choice to tackle the crisis and environmental problems. As one of the third generation solar cells, quantum dot-sensitized solar cells (QDSSCs) has attracted increasing attention, because of its relatively low cost, easy fabrication and high-efficiency energy conversion [5–7]. Basically, QDSSCs have similar structures with that of dye-sensitized solar cells (DSSCs). The semiconductor quantum dots (QDs) have presented the possibility of band gap tunability, high extinction coefficient, large intrinsic dipole moments, and potential processes of multiple exciton generation [8–13]. Furthermore, for the sensitizers of DSSCs, QDs also possess higher absorption coefficients [14], multiple exciton generation (MEG) [15], tunable bandgaps due to the quantum confinement effect [16], the possibility of hot electron injection [17], and easy fabrication processes [18]. Although the QDSSCs have been attracted great attention as the excellent properties for QDs, the highest power conversion efficiency of QDSSC is still lower than that of a DSSC in present study [19,20]. QDSSC is mainly composed of a transparent conductive glass, wide
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band gap oxide semiconductor thin films, quantum dots, electrolyte and counter electrode. The efficient charge transfers between sulfide sensitizers and the polysulfide redox couple generates a large short-circuit photocurrent (Jsc) in QDSSCs [21–27]. However, the large charge transfer resistance at the counter electrode results in a low fill factor (FF) and the small difference between the TiO2 Fermi level and the S2−/Sn2− redox level confines the open-circuit photovoltage (Voc). A promising strategy to increase the Voc value without sacrificing the high Jsc is to use a narrow band gap semiconductor as the counter electrode to provide an auxiliary tandem effect. Actually, different kinds of CEs have been found for QDSSCs, such as carbon, carbon derivatives, noble metals and conducting polymers [28–31]. The commonly used noble metal CEs, such as Pt and Au, exhibit poor catalytic property toward polysulfide electrolyte because they usually passivated by sulfide compound absorbed on their surface and the Pt produces a large charge transfer resistance in the polysulfide electrolyte lead to low fill factor [32]. Apart from noble metal, the carbon and carbon derivatives, including activated carbon, mesocellular carbon foam, have no better catalytic activity and there is not enough contact with the base [33]. Conducting polymers have poor power conversion efficiency. So the correlational studies have been reported rarely in CEs for QDSSCs [34]. Therefore, the optimization and
Corresponding authors. E-mail addresses: [email protected] (L. Duan), [email protected] (W. Lü).
https://doi.org/10.1016/j.materresbull.2017.12.021 Received 6 October 2017; Received in revised form 11 December 2017; Accepted 15 December 2017 Available online 20 December 2017 0025-5408/ © 2017 Elsevier Ltd. All rights reserved.
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sodium sulfite (Na2SO3) were supplied by Sigma-Aldrich. Acetic acid (C2H4O2), cadmium acetate (Cd(CH3COO)2·2H2O), L-cysteine (C3H7NO2S), zinc acetate (CH3(COOH)2Zn) and potassium chloride (KCl) were bought from Aladdin. Sodium sulfide (Na2S·9H2O), copper sulfate pentahydrate (CuSO4·5H2O), selenium (Se), sodium hyposulfite (Na2S2O3) and urea (CH4N2O) were purchased from Tianjin Guangfu Technology Development Co., Ltd. All chemicals were used without further purification.
fabrication of CEs with high electrocatalytic activity and good stability are still very important for improving the performances of QDSSCs. Recently, for improving electrocatalytic activity and low fill factor of the CE, metal sulfides have been got much attention due to its low cost, low resistance, good electrocatalytic activity and facile fabrication [5,35]. Furthermore, the metal sulfides would be good choice as CEs for covering related problems discussed above. Several works have reported the preparation and application of metal sulfides as CEs in QDSSCs. Lin et al. applied PbS thin films as CEs for QDSSCs by SILAR method and these exhibited the highest η of 4.7% [36]. Savariraj et al. prepared CuS thin films using a facile CBD method and got the highest η of 4.53% [37]. Yuan et al. reported that CoS thin films be synthesized and applied to the CEs of QDSSCs and achieved the highest η of 5.20% [38]. However, single metal sulfides as CE generally suffers poor electrical conductivity or catalytic activity. Therefore, we use metal sulfide composites as CEs of QDSSCs and these different metal sulfide composites may change the value of Voc2 (the difference between the potential of redox/reduction of electrolyte and the Fermi level of CE), resulting in a noticeable change in the Voc of QDSSCs. In this paper, inspired by above discussion, CuS/CoS, CuS/NiS, CuS, CoS and NiS thin films are deposited on fluorine doped tin oxide (FTO) glass substrates by chemical bath deposition (CBD), which are further used as CEs of QDSSCs, and the cell performances based on these CEs are systematically investigated. In Fig. 1, it has been described that the transport pathways of electrons and photoelectrical conversion configuration of a TiO2/CdS/CdSe/ZnS for anode with the counter electrode of CuS/CoS for QDSSCs. The cell using CuS/CoS CE show a 5.22% power conversion efficiency (PCE) under one sun illumination, which is higher than that of CuS/NiS, CuS, CoS and NiS (η = 2.56%, 4.73%, 2.23% and 1.62%, respectively). The enhanced PCE of CuS/CoS CE is attributed to the combination of different metal sulfides, for that CuS shows the exceptional electrical conductivity and CoS shows the excellent catalytic activity. The result indicates that the combination of different metal sulfide composites could increase open-circuit voltage (Voc) value without sacrificing the short-circuit current (Jsc), and leaving good perspectives for significantly higher solar cell performances.
2.2. Preparation of counter electrodes The CBD method was used to deposit metal sulfide (CuS/CoS, CuS/ NiS, CuS, CoS and NiS) to serve as CEs. For CuS CE, 1 M Na2S2O3 aqueous solution and 1 M CuSO4 aqueous solution were mixed with the volume ratio of 4:1, and the pH value was adjusted to 2 by Acetic acid. Then, the FTO glasses were immersed into as-prepared mixed solution and heated to 70 ℃ for 3 h. After cooling down to the room temperature, the substrates were washed with DI and then heated to 130 ℃ and kept for 30 min. For NiS CE, a solution included 8 g NiSO4·6H2O and 2 g C3H7NO2S in 40 ml deionized water was prepared. The solution was stirred vigorously for 10 min to get a bottle-green solution. Under constant stirring, 4 g CH3CSNH2 and 12 g C2H4O2 were added to the above solution and stirred ultrasonically for another 15 min. Then, the FTO substrates were put into the solution and kept at 90 ℃ for 120 min. For CoS CE, a solution included 5 g CoCl2 and 5 g CH3CSNH2 in 30 ml deionized water was prepared. The solution was stirred vigorously for 10 min to acquire a amaranth solution. Next, 12 g of C2H4O2 was added to the above amaranth solution, and the reaction mixture was stirred vigorously for a further 15 min. To prepare CuS/CoS and CuS/NiS CEs, 0.1 M CuSO4·5H2O, 0.5 M CH4N2O and 0.5 M CH3CSNH2 were dissolved in 30 ml deionized water, and the FTO glasses were immersed into as-prepared mixed solution and heated to 70 ℃ for 2 h. Then the as-prepared CuS substrates were put into a solution of containing 0.1 M CoCl2·6H2O, 1 M CH4N2O, 0.5 M CH3CSNH2 and 0.6 M C2H4O2 for further deposition of CoS. This film substrate named CuS/CoS. After cooling down to the room temperature, the substrates were washed with DI water and heated to 60 ℃ for 30 min. To deposit the CuS/NiS, the as-prepared CuS substrates were put into a solution that consisted of 0.1 M NiSO4·6H2O, 1 M CH4N2O, 0.5 M CH3CSNH2 and 0.6 M C2H4O2 and kept at 90℃ for 2 h.
2. Experimental details 2.1. Materials FTO glasses were purchased from Zhuhai Kaivo Optoelec-tronic Technology Co., Ltd., cobalt chloride hexahydrate (CoCl2·6H2O), nickel sulfite hexahydrate (NiSO4·6H2O), thioacetamide (CH3CSNH2) and
2.3. Preparation of photoanode (TiO2 /CdS/CdSe/ZnS) TiO2 films were made using the doctor-blade method by coating Fig. 1. Schematic structure of QDSSCs based CuS/ CoS CE.
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Fig. 2. SEM images of the (a) CuS; (b) CoS and (c) NiS thin films deposited on FTO substrates.
Fig. 3. SEM images of the (a) CuS/NiS; (c) CuS/CoS thin films deposited on FTO substrates; (b) and (d) are the magnified images of (a) and (c), respectively.
2.4. Fabrication of qdsscs The as-prepared CuS/CoS, CuS/NiS, CuS, CoS and NiS counter electrodes and TiO2/CdS/CdSe/ZnS photoanode were assembled to a sandwich-type cell and penetrated with a polysulfide electrolyte of containing 2 M Na2S and 2 M S in methanol and H2O solution (v/v= 7:3). 2.5. Characterization The morphology and structure of counter electrodes were observed by scanning electron microscopy (SEM) (S4800, Hitachi). X-ray diffraction (XRD) was performed on D-MAX II A X-ray diffractometer. The electrochemical impedance spectroscopy (EIS), Cyclic voltammetry (CV) and Tafel polarization curve were obtained with Solartron Impedance Analyzer. A VG ESCALAB MKII spectrometer can be used to measure the XPS spectra. The incident photon-to-current conversion efficiency (IPCE) was measured by a solar cell scan 100 (Zolix, Beijing). AM 1.5 solar simulator (Zolix, Beijing) was used to measure the performances of the cell.
Fig. 4. XRD spectra ofCuS/CoS, CuS/NiS, CuS, CoS and NiS thin film on FTO substrates.
TiO2 paste on FTO glass substrates, followed by sintering at 450 ℃ for 30 min, and obtain a film of ∼7.5 μm [39]. Next, CdS QDs were deposited on the TiO2 films by the successive ionic layer adsorption and reaction (SILAR) method [40,41]. For deposition of CdSe QDs on TiO2 films, chemical bath deposition (CBD) method was used to deposit CdSe QDs [1,42,43]. Finally, ZnS passivation layer was deposited by SILAR method and it can avoid corrosion caused by the polysulfide electrolyte and prevent the recombination of electrons in the electrolyte [14,26,28].
3. Results and discussion 3.1. Synthesis and characterization of the counter electrodes Fig. 2 shows SEM images of the CuS, CoS and NiS thin films prepared separately on FTO substrate, respectively. It is obvious that the three CEs have different morphologies. The CuS thin film consists of many nanoparticles with spheres-like structures as shown in Fig. 2(a). The NiS thin film in Fig. 2(c) is more like nano-flowers, and the CoS film in Fig. 2(b) shows the intermediate structures. For CuS/NiS film as shown in Fig. 3(a), many cracks between 200
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Fig. 5. XPS spectra of CuS/CoS thin film on FTO surface: (a) survey scan, high resolution spectrum of the (b) Co 2p; (c) Cu 2p; and (d) S 2p.
Fig. 6. (a) CV curves for the CuS/CoS, CuS/NiS, CuS, CoS and NiSCEs (b) Nyquist plots of EIS for the symmetric cells. (c) Tafel polarization curves of the symmetric cells. (d) UV–vis results of the symmetric cells.
adjacent NiS structures could be observed, which are more obvious compared with that of pure NiS deposited on FTO substrate (as seen in Fig. 2(c)). The enlarged SEM image in Fig. 3(b) could clearly observe the morphology, which may be due to the different substrates for deposition. For CuS/NiS film, the NiS layer is deposited onto the CuS layer, and the CuS film is composed of nanoparticles as shown in Fig. 3(a), the uneven surface of substrate induces the large space between adjacent NiS crystal particles. For CuS/CoS film, the top CoS layer also exhibits different morphology with that of CoS film on FTO as shown in Fig. 2(b). The CoS on CuS layer shows porous structure composed of many thin nanosheets. As it could be expected to yield electrodes, the highly porous structure of the CuS/CoS CE would provide more catalytically active sites for the redox process, which is with
Table 1 Photovoltaic parameters of QDSSCs based on various counter electrodes. Counter electrodes
Rs(Ω)
Rct(Ω)
Zw(Ω)
CPE(μF)
CuS/NiS CuS/CoS CuS CoS NiS
2.491 2.203 2.284 3.017 3.021
55.61 22.17 37.75 55.72 78.11
1.9 0.9 1.4 2.1 2.5
37.27 41.88 38.87 37.01 32.99
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peak in the diffraction pattern of NiS [40,45]. The sharp and strong diffraction peaks of CuS at 27.70°, 29.57°, 32.05°, 48.61° and 59.30° can be observed, corresponding to the (101), (102), (006), (110) and (116) lattice planes (JCPDS NO. 06-0464). Similarly, the XRD patterns of as prepared CuS/CoS film can be attributed to the diffraction on (101), (102), (006), (110), (116) lattice planes of CuS, (101) and (112) lattice planes of CoS, which can be well indexed to CuS/CoS phases (JCPDS PDF NO. 75-0605 and JCPDS NO. 06-0464). The XRD patterns of the CuS/NiS film are resulted from (101), (102), (006), (110) and (116) lattice planes of CuS, (100), (101), and (110), (116) lattice planes of NiS, which can be well indexed to CuS/NiS phases (JCPDS NO. 06-0464 and JCPDS NO. 02-1280). For CuS/CoS composites, there are some peaks at around 20 degrees and 78 degrees can be observed, this peaks can be ascribed to FTO substrate. For CuS/ NiS composites, there are some peaks at around 20-25 degrees and 62–65° can be observed, this peaks can be attributed to the FTO substrate [40,41]. These results confirm the successful deposition of CuS/CoS and CuS/NiS compounds on the FTO substrates. Fig. 5 is the XPS spectrum of the CuS/CoS thin film on FTO surface for examining the chemical valence state and elemental compositions of the as prepared CEs. Fig. 5(a) is overall scan of XPS and all elements involved in synthesis could be observed. In the high-resolution XPS spectrum of Co 2p (Fig. 5(b)), there are two peaks of Co 2p3/2 at 781 eV and Co 2p1/2 at 797 eV. For Co 2p3/2 peak at 781 eV, it is assigned to CoS formation as well as a broad peak at higher binding energies (about 781 eV) [46]. Moreover, there is a little shake-up satellite peak of the metallic Co at 778.5 eV, which is contributed to the presence of Co0 species are being formed [47,48]. This could be attributed to the cobalt ion has a very strong affinity to oxygen and it is difficult to exclude it impurities from the resultant materials [31,47]. As shown in Fig. 5(c), the XPS spectrum exhibited two strong peaks at binding energies of 931.3 eV and 951.4 eV, which can be attributed to Cu 2p3/2 and Cu 2p1/2, respectively. Except for them, there are several weak satellite peaks around 943 eV due to the presence of Cu2+ [48,49]. In addition, there are S 2p3/2 and S 2p1/2 peaks at 162.9 eV and 168.36 eV in Fig. 5(d), indicating that the S species exist as S2− in the deposits [48]. From the XPS analyses, it is indicated that CuS/CoS composites had been synthesized successfully onto the FTO surface by chemical bath deposition method.
Fig. 7. (a) J–V and (b) IPCE curves characteristics of the QDSSCs with CuS/CoS, CuS/ NiS,CuS, CoS and NiSCEs.
Table 2 Photovoltaic parameters of CuS/CoS, CuS/NiS, CuS, CoS and NiS CEs based on QDSSCs under one Sun. Samples
Jsc(mA cm−2)
Voc
FF
η (%)
NiS CoS CuS CuS/NiS CuS/CoS
9.52 12.39 17.82 13.09 19.96
0.42 0.42 0.58 0.45 0.56
0.40 0.43 0.45 0.44 0.47
1.62 2.23 4.73 2.56 5.22
3.2. Electrochemical performance of the counter electrodes The electrochemical catalytic activity of as-prepared CuS/CoS, CuS/ NiS, CuS, CoS and NiS CEs in polysulfide electrolyte are investigated. The cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), Tafel polarization and UV–vis measurements are performed, as shown in Fig. 6. In the CV curves, the high current peak indicates active electrocatalytic reaction of the electrode for the S2−/Sn2− redox couples in the polysulfide electrolyte [49]. The negative currents represent the reduction of Sn2- ions to S2− ions while the positive currents are related to the oxidation of S2− ions [50]. As shown in Fig. 6(a), compared to the CuS/NiS, CuS, CoS and NiS CEs, the CuS/CoS CE shows the higher current density and excellent reversibility, indicating that the electrochemical catalyst of CuS/CoS composite CE is better than the others. Fig. 6(b) is electrochemical impedance spectra. The parameters obtained from impedance spectra are shown in Table 1. The Nyquist plots were measured in the frequency range of 0.01Hz to 100 mHz. The high frequency intercept represents the series resistance (Rs). The semicircle in the middle frequency region represents the charge transfer resistance (Rct) and the corresponding constant phase angle element (Cμ) at the CE/electrolyte interface, while the low frequency region provides the Warburg diffusion impedance (ZW) of the polysulfide electrolyte [45–47]. For CuS/CoS electrode, the charge transfer resistance acquired from the fitting circuit is much smaller than those of NiS, CuS, CuS/NiS, CoS electrodes as shown in Table 1, suggesting that
a large specific surface area [29,30]. Fig. 4 is XRD results of as-prepared samples. There are some unindexed peaks in the diffraction patterns due to the existence of the FTO substrate. It is known that the counter electrode is a thin layer, which results in the existence of FTO substrate. For CoS, the diffraction peaks at 31.39°, 35.27°, 46.78°and 56.09° could be assigned to (100), (101), (102) and (110) lattice planes of the CoS phase (JCPDS PDF NO. 750605), and other sharp diffraction peaks can be ascribed to FTO substrate [44,45]. For NiS, there are five obvious sharp diffraction peaks locating at 31.19°, 34.18°, 51.99°, 61.75° and 65.73° can be observed, corresponding to the (100), (101), (110), (200) and (201) lattice planes (JCPDS NO. 02-1280). The 26.44° and 37.94° can be attributed to the FTO substrate [40,41]. It is known that the counter electrode is a thin layer and the sintered temperature of NiS counter electrode is low, thus the FTO has a high diffraction intensity, which results in the phenomena that the reflection around 31.19° and 34.18° has low angle 202
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Table 3 Result comparisons of present photovoltaic parameters with other reports of different counter electrodes. CE
QD
Preparation
Voc
Jsc(mA cm−2)
η (%)
Refs
CuS/CoS CuS/CoS CuS/CoS CuS NiS CoS CoS CoS CuS CuS CuS CuS Cu1.8S NiS CoS2 Cu2S
CdS/CdSe CdS/CdSe CdS/CdSe CdS/CdSe CdS/CdSe CdS/CdSe CdS CdS CdS/CdSe CdS/CdSe CdS/CdSe CdS/CdSe CdS/CdSe CdS/CdSe CdS/CdSe CdS/CdSe
CBD electrochemical deposition CBD CBD Hydrothermal potentiodynamic electrodeposition CBD SILAR SILAR CBD CBD SILAR CBD CBD Hydrothermal electrodeposition
0.56 0.54 0.48 0.53 0.49 0.52 0.66 0.58 0.55 0.58 0.60 0.59 0.60 0.51 0.53 0.54
19.96 16.09 16.8 14.11 11.67 11.2 1.96 4.07 13.9 13.87 19.92 16.3 19.10 10.38 10.86 19.70
5.22 5.03 4.1 4.02 3.03 1.9 0.64 0.95 2.7 4.01 4.06 4.27 5.16 2.97 2.27 4.68
this work [33] [56] [57] [41] [58] [59] [60] [58] [45] [26] [3] [51] [40] [61] [4]
curve is divided into three major zones. The low potential in left is the polarization zone. The middle potential of curve with a sharp slope is attributed to the Tafel zone, which represents the catalytic ability of the CEs. The right zone at high potential named the diffusion zone. At the high potential, the limiting current density (Jlim) can be derived from the diffusion zone of the Tafel curve. The exchange current density (J0) is attaining from the zero intercept of the approximately linear region of the curve. The electrochemical catalytic activity of CEs was measured by these two parameters. J0 depends on the Rct value, and Rct can be obtained from the EIS analysis according to Eq. (1):
J0 =
RT nFR ct
(1)
where T is temperature, F is the Faraday constant, R is the gas constant, n is the number of electrons involved in the reduction of disulfide at the CE, and Rct is the charge transfer resistance at the CE/electrolyte interface [50]. In the plot of Fig. 6(c), due to higher J0 represents a larger reduce ability, the J0 value of CuS/CoS CE is obviously higher than the other counter electrodes (CuS/NiS, CuS, CoS and NiS CEs), indicating that it has better reduction of the polysulfide electrolyte at the CE/ electrolyte interface [53]. CuS/CoS shows greater Jlim values compared to CuS/NiS, CuS, CoS and NiS CEs from the diffusion zone of the curve. It reveals a higher diffusion velocity of the CuS/CoS CE in the S2−/Sn2− electrolyte [54]. Therefore, because of the highest J0 value with the lowest Rct value, CuS/CoS electrode exhibits the best electrocatalytic
Fig. 8. The error of the PCE with CuS/CoS, CuS/NiS,CuS, CoS and NiS CEs based on QDSSCs under one Sun.
the CuS/CoS electrode could facilitate the electron transport from the CE to the S3− species [51,52]. Fig. 6(c) is the Tafel polarization curves. The Tafel plot can be used to measure the interfacial charge-transfer properties and electrochemical catalytic activity of CEs using S2−/Sn2− couple. The Tafel
Fig. 9. A contrast map of single metal sulfides and composite metal sulfides.
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based on the CuS/CoS CE (TiO2 was used as photoanode with CdS/ CdSe/ZnS core/shell quantum dots) had been shown. Compared to CuS, CoS and NiS, the different metal sulfide composite could enhance the value of Voc2, resulting a noticeable change of the Voc for QDSSCs, because the quasi-Fermi level of metal sulfide composite is closer to the vacuum Fermi level. Furthermore, the photoactive properties of the CuS/CoS electrode enhanced electron injection into the electrolyte and subsequently improved the Jsc and FF values of the cell which could increase the overall performance of QDSSCs to the next level.
activity among as-prepared CEs, agreeing with the CV and EIS results. The optical absorption spectra of counter electrodes are shown in Fig. 6(d), which play an important role CEs in QDSSCs [53]. The optical absorption spectra were found to decrease with the sequence of the CuS/CoS, CuS/NiS, CuS, CoS and NiS CEs in the range of 300–800 nm wavelength, and the peak centered in the spectra is located around 550 nm. The increased adsorption of CuS/CoS leads to improved absorption spectra for counter electrode. It shows that as prepared CuS/ CoS composite could be an appropriate material for harvesting residual light that penetrates a photoanode [55].
Conflicts of interest 3.3. Photovoltaic performance of the QDSSCs Manuscript does not include any content with conflict of interest. The cell performances were investigated under AM 1.5 G irradiation as shown in Fig. 7(a) and the cell parameters such as short-circuit current density (Jsc), open-circuit voltage (Voc), fill factor (FF), and power conversion efficiency (η) extracted from the J–V curves are listed in Table 2. It has been shown that the QDSSC with the CuS/CoS CE presents the best photovoltaic property with PCE of 5.22%, Jsc of 19.96 mA cm−2, Voc of 0.56V, and FF of 0.47, while that the PCE of the CuS, CuS/NiS, CoS and NiS CEs are 4.73%, 2.56%, 2.23% and 1.62%, respectively. Obviously, as shown in Fig. 7(a) and Table 2, all the performance indexes of the solar cell are increased with the CuS/CoS CE of QDSSC, which lead to the improvement of the overall efficiency, because the excellent electrical conductivity and catalytic activity of the CuS and the CoS, respectively [37,38]. However, NiS shows very low performance due to poor catalytic activity in the S2-/Sn2- electrolyte. These data revealed a better electrocatalytic activity for CuS/CoS CE than CuS/NiS, CuS, CoS and NiS CEs, as indicated by the high Jsc and FF. Compared to the similar metal sulfides in previous reports as shown in Table 3, the PCE of CuS/CoS CE achieve the best value in present work. In order to exam the different photovoltaic performance of CuS/ CoS, CuS/NiS, CuS, CoS and NiS CEs, IPCE spectra as a function of wavelength were measured, as shown in Fig. 7(b). Although the IPCE spectrum of CuS based QDSSC was higher, it is lower than CuS/CoS. The IPCE of CuS/CoS counter electrode has been shown the largest value (78%) through the whole measured range. The photovoltaic performance of CuS/CoS composite CE for QDSSCs is improved because it has superior electrocatalytic activity for the reduction of polysulfide electrolyte, which is proved by the electrochemical analysis. The distribution of the PCE curves with CuS/CoS, CuS/NiS, CuS, CoS and NiS CEs based on QDSSCs were shown in Fig. 8. From PCE curves, we found the error of the cell efficiencies were less than 1%. The possible mechanism of the energy bands and the related charge transfer processes of the CuS/CoS and CuS CEs in the QDSSC is supposed, as shown Fig. 9. The Voc of the QDSSCs device is determined by the energy difference between the quasi-Fermi levels (qEf) of the anode and cathode [62]. Generally, the photoactive electrode moves to the quasi-Fermi level near the vacuum Fermi level of the anode or the cathode, which depends on the equilibrium position between the electrode and electrolyte solution [63,64]. In Fig. 9, the photo-voltaic response of QDSSCs based on the anode and CuS/CoS CE: Voc=Voc1 (anode)+Voc2 (CE), the photo-voltaic response of QDSSCs based on the anode and CuS CE: Voc′=Voc1 (anode)+Voc2′ (CE), respectively. Moreover, the single metal sulfides (CuS as an example), the open-circuit voltage of Voc2′ is smaller than the Voc2 of CuS/CoS, the relationship between Voc2′ and Voc2 is Voc2 (CuS/CoS CE)=ΔVoc+Voc2′ (CuS CE). So the density of the CoS which was deposited at the surface of CuS may increase the Voc of QDSSCs due to the higher Voc2 value.
Acknowledgements The authors thank funding by the National Natural Science Foundation of China (Grant Nos. 61574021, 61376020, 11474035, 61774022 and 61604017) and the Education Department of Jilin Province. References [1] C.V.V.M. Gopi, M. Venkata-Haritha, Y.-S. Lee, H.-J. Kim, Correction: ZnO nanorods decorated with metal sulfides as stable and efficient counter-electrode materials for high-efficiency quantum dot-sensitized solar cells, J. Mater. Chem. A 5 (2017) 428–429. [2] J.P. Holdren, Science and technology for sustainable well-being, Science 320 (2008) 179. [3] D. Punnoose, C.S.S.P. Kumar, S.S. Rao, C.V.T. Varma, B. Naresh, A.E. Reddy, N. Kundarala, Y.-S. Lee, M.-Y. Kim, H.-J. Kim, In situ synthesis of CuS nano platelets on nano wall networks of ni foam and its application as an efficient counter electrode for quantum dot sensitized solar cells, Org. Electron. 42 (2017) 115–122. [4] C.K. Kamaja, R.R. Devarapalli, Y. Dave, J. Debgupta, M.V. Shelke, Synthesis of novel Cu2S nanohusks as high performance counter electrode for CdS/CdSe sensitized solar cell, J. Power Sources 315 (2016) 277–283. [5] P.K. Santra, P.V. Kamat, Tandem-layered quantum dot solar cells: tuning the photovoltaic response with luminescent ternary cadmium chalcogenides, J. Am. Chem. Soc. 135 (2013) 877–885. [6] P.V. Kamat, K. Tvrdy, D.R. Baker, J.G. Radich, Beyond photovoltaics: semiconductor nanoarchitectures for liquid-junction solar sells, Chem. Rev. 110 (2010) 6664–6688. [7] M. Wang, W. Chen, J. Zai, S. Huang, Q. He, W. Zhang, Q. Qiao, X. Qian, Hierarchical Cu7S4 nanotubes assembled by hexagonal nanoplates with high catalytic performance for quantum dot-sensitized solar cells, J. Power Sources 299 (2015) 212–220. [8] H. Hu, J. Ding, J. Qian, Y. Li, L. Bai, N. Yuan, Layered CoS/graphene nanocomposite as high catalytic counter electrodes for quantum dot-sensitized solar cells, Mater. Lett. 114 (2014) 7–10. [9] Z.Y. Peng, X.F. Chen, Y.L. Liu, J.L. Chen, J. Chen, Balance on the charge generation, separation and transfer performance of different TiO2 nanostructures in quantum dot sensitized solar cells, Mater. Res. Bull. 94 (2017) 463–471. [10] B. O’Regan, M. Grätzel, A low-cost, high-efficiency solar cells based on dye-sensitized colloidal TiO2 films, Nature 414 (2001) 338–344. [11] H. Wang, Y. Bai, H. Zhang, Z. Zhang, J. Li, L. Guo, J. Mater. Chem. C 114 (2010) 16451–16455. [12] Y. Jiang, B.-B. Yu, J. Liu, Z.-H. Li, J.-K. Sun, X.-H. Zhong, J.-S. Hu, W.-G. Song, L.J. Wan, Boosting the open circuit voltage and fill factor of QDSSCs using hierarchically assembled ITO@Cu2S nanowire array counter electrodes, Nano Lett. 15 (2015) 3088–3095. [13] H. Salaramoli, E. Maleki, Z. Shariatinia, M. Ranjbar, CdS/CdSe quantum dots cosensitized solar cells with Cu2S counter electrode prepared by SILAR, spray pyrolysis and Zn–Cu alloy methods, J. Photochem. Photobiol. A 271 (2013) 56–64. [14] Q. Zhang, Y. Zhang, S. Huang, X. Huang, Y. Luo, Q. Meng, D. Li, Application of carbon counterelectrode on CdS quantum dot-sensitized solar cells (QDSSCs), Electroche. Commun. 12 (2010) 327–330. [15] T. You, L. Jiang, K.L. Han, W.Q. Deng, Improving the performance of quantum dotsensitized solar cells by using TiO2 nanosheets with exposed highly reactive facets, Nanotechnology 24 (2013) 245401. [16] S. Gorer, G. Hodes, A hydrothermal reaction combined with a post anion-exchange reaction of hierarchically nanostructured NiCo2S4 for high-performance QDSSCs and supercapacitors, Phys. Chem. 98 (2010) 5338–5346. [17] W.A. Tisdale, K.J. Williams, B.A. Timp, D.J. Norris, E.S. Aydil, X.Y. Zhu, Hotelectron transfer from semiconductor nanocrystals, Science 328 (2010) 1543–1547. [18] Z. Yang, C.Y. Chen, P. Roy, H.T. Chang, Quantum dot-sensitized solar cells incorporating nanomaterials, Chem. Commun. 47 (2011) 9561–9571. [19] K. Zhao, Z.X. Pan, I. Mora-Ser´o, E. C´anovas, H. Wang, Y. Song, X.Q. Gong, J. Wang, M. Bonn, J. Bisquert, X.H. Zhong, Boosting power conversion efficiencies of quantum dot sensitized solar cells beyond 8% by recombination control, J. Am.
4. Conclusions The CuS/CoS, CuS/NiS, CuS, CoS and NiS counter electrodes were deposited on FTO conducting glass by a simple chemical bath deposition method. The higher power conversion efficiency (5.22%) of QDSSC 204
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