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carbon allotropes; efficient electrocatalyst counter electrodes for high performance quantum dot-sensitized solar cells
Accepted Manuscript Composite films of metal doped CoS/carbon allotropes; efficient electrocatalyst counter electrodes for high performance quantum dot-sensitized solar cells Seyede Sara Khalili, Hossein Dehghani, Malihe Afrooz PII: DOI: Reference:
S0021-9797(17)30005-X http://dx.doi.org/10.1016/j.jcis.2017.01.005 YJCIS 21911
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
16 October 2016 28 December 2016 3 January 2017
Please cite this article as: S. Sara Khalili, H. Dehghani, M. Afrooz, Composite films of metal doped CoS/carbon allotropes; efficient electrocatalyst counter electrodes for high performance quantum dot-sensitized solar cells, Journal of Colloid and Interface Science (2017), doi: http://dx.doi.org/10.1016/j.jcis.2017.01.005
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Composite films of metal doped CoS/carbon allotropes; efficient electrocatalyst counter electrodes for high performance quantum dot-sensitized solar cells Seyede Sara Khalili, Hossein Dehghani*, Malihe Afrooz Department of Inorganic Chemistry, Faculty of Chemistry, University of Kashan, P.O. Box 87317-51167, Kashan, Iran Email: [email protected], [email protected], [email protected] *Corresponding author, Tel: +98 315 591 2386. Fax: +98 315 591 2397, Iran, Area cod Box 8731751167.
Abstract This study reports the enhanced catalytic ability of metal ions-doped CoS and CoS/carbon allotrope counter electrodes (CEs) (synthesized using a successive ionic layer adsorption and reaction (SILAR) method) to improve the power conversion efficiency (η) in quantum dot solar cells (QDSSCs). Firstly, doping effects of different metal ions (Mg2+, Ca2+, Sr2+ and Ba2+) in the CoS CE on QDSSC performance have been investigated. Overall, among the different metal doped CoS CEs, the best energy conversion efficiency of 2.19%, achieved for Sr, is the highest reported for QDSSCs constructed with metal doped CoS. A sandwich structural Sr- and BaCoS/carbon allotrope (graphene sheet (GS), graphene oxide (GO) and carbon nanotube (CNT)) composite CEs have been prepared by repeating electrophoretic deposition (EPD) of carbon materials and deposition of CoS nanoparticles. Dramatic enhancements of η have been observed with the Sr- and Ba-CoS/GO CEs based QDSSCs (∼76% and ∼41%, respectively), which is
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higher than that of the bare CoS CE. Because of the large specific surface area and superior electrical conductivity of GS, GO and CNT and the high electrocatalytic activity of CoS, these CEs show an improvement in the photocurrent density in the cells, as revealed from electrochemical and spectral data. Keywords: Quantum dot-sensitized solar cells, Counter electrode, Efficiency enhancement, Metal ion-doped cobalt sulfide, Electrocatalytic activity
Introduction Semiconductor quantum dot-sensitized solar cells (QDSSCs) have attracted extensive attention as promising alternatives to dye-sensitized solar cells (DSSCs) because of their unique optoelectronic properties such as low cost, environmental friendliness, simple production processes, and relatively high energy conversion efficiency [1-3]. The attractive properties of semiconductor quantum dots include higher extinction coefficients, multiple exciton generation [4], direct hot carrier transfer and tunable band gaps [5, 6]. In quantum dot (QD) sensitizers, CdS and CdSe are the most efficient and widely used ones. Electrolyte acts as a vital component in QDSSCs, is due to the effective scavenging of holes to regenerate the sensitizer and the resistivity against photoanodic corrosion [7]. The iodide/triiodide electrolyte used in DSSCs is not a suitable candidate for QDSSCs, because it causes photocorrosion to the QDs [8]. Polysulfide (S2-/Sn2-) redox couple aqueous solution electrolyte is commonly used in QDSSCs because this electrolyte media can stabilize chalcogenide QD sensitizers and can also offer an acceptable photovoltaic performance of the resultant QDSSC [9-12]. Counter electrode (CE), where electron flow back into the electrolyte solution from external circuit, is an important part of solar cells and is responsible for catalysis and reduction of the oxidized redox species. In the
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past decade, the QDSSC efficiency has seen significant enhancements with the advances in CEs research [13]. In a QDSSC, the CE plays a crucial role in collecting electrons from the external circuit and reducing polysulfide (S2-/Sn-2) electrolytes. Polysulfide electrolyte and Pt counter electrode are the most commonly used in QDSSCs, however, the Pt-based CE shows reduced activity in the polysulfide electrolyte, probably due to the decreased charge transfer rate at CE/electrolyte interfaces. Recently, to overcome this challenge, a variety of alternative CE materials such as CoS [14], PbS [15], NiS [16] and CuS [17], with higher electrocatalytic activity and lower charge transfer resistance at the CE/electrolyte interface, have been widely investigated for the polysulfide electrolyte in the QDSSCs. CoS electrodes can be easily prepared by successive ionic layer adsorption and reaction (SILAR) method and can replaced with Pt electrode. Using of CoS reduces the whole device cost due to the extremely low plenty of platinum compared with cobalt. Since the specific surface area of catalyst is another key factor of the CE, numerous studies have been made on an alternative CE including carbon materials [1823]. Herein, we focus on the QDSSC photovoltaic performances constructed by diff erent CoS CEs. For this purpose, we have developed a facile synthesis of the composites of CoS with diff erent carbon allotropes (graphene sheet (GS), graphene oxide (GO) and carbon nanotube (CNT)) on fluorine doped SnO2-coated glass (FTO). Carbon materials such as graphene and carbon nanotube display some of the most potential for using as an alternative CE due to its highly specific surface, superior electronic conductivity and excellent mechanical strength [2426]. These carbon materials provide a high surface area for the deposition of CoS nanoparticles [2728]. The composite films are fabricated using an electrophoresis of carbon materials onto a FTO
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substrate and then CoS deposit onto the FTO coated with carbon materials by the SILAR method [29-33]. Our findings in the present study indicate that doping of the CoS and CoS/carbon allotrope composite CEs with difference metal ions such as Mg2+, Ca2+, Sr2+ and Ba2+ in a QDSSC improves the electrocatalytic activity of CE compared to the bare CoS. As a result, among the metal doped CoS CEs, the Sr-doped CoS and Ba-doped CoS CEs show excellent catalytic activity for polysulfide reduction than the bare CoS CE, consequently, the cells based on these CEs have the best photovoltaic performance; Sr-doped CoS CE has delivered an efficiency of 2.19%, which is comparable to that of the bare CoS electrode (1.60%) and power conversion efficiency of 2.00% has been achieved with the Ba-doped CoS CE. Also, the effect of amount of metal ion doped in CoS on the QDSSC performance has been discussed and the maximum energy conversion efficiency has been achieved in 10% of metal ions (Sr 2+ and Ba2+). Then, this amount of Sr2+ and Ba2+ ions have been doped in CoS/GO, CoS/GS and CoS/CNT matrix by repeating electrophoretic deposition (EPD)
of carbon materials and
deposition of Sr-/Ba-doped CoS nanoparticles on the FTO [27]. It is expected that the structure of graphene oxide, graphene sheets and carbon nanotube provide a high surface area for the deposit of Sr-/Ba-doped CoS nanoparticle; thus, these carbon nanomaterials play a main role in supporting material and supplying a large number of active sites for nucleation so that leads to a significant increase in the efficiency of a QDSSC. By analyzing electrochemical and spectral data, it is found that the incorporation of metal ions can help to enhance the catalytic property of the metal sulfide CE by promoting electron production, thereby increasing the efficiency of the QDSSCs. Using of the new composite CEs provides a simple approach towards the goal of high performance QDSSCs.
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2. Experimental Section 2.1. Materials and Instrumentation All chemical materials and solvents used for synthesis were purchased from Merck and SigmaAldrich Companies. The fluorine-doped tin oxide (FTO) glass substrates (transmission >90% in the visible, sheet resistance 8 Ωper square, 2.3 mm thickness) and surlyn spacer (50 µm) were obtained from Dyesol (Australia). The photocurrent-voltage (I–V) characteristics of the cells were measured using a Keithley model 2400 digital source meter (Keithley, USA). The electrochemical impedance spectroscopy (EIS) measurements of the cells achieve at a forward bias of 0.46 V under AM 1.5 G simulated light (Luzchem) using potantiostat/galvanostat (PGSTAT 100, Autolab, Eco-Chemie), at an AC amplitude of 5 mV within the frequency range from 0.01 Hz to 500 kHz. The surface features of the CEs were observed using a field emission scanning electron microscope (Philips XL-30FESEM) equipped with energy dispersive spectrometer (EDS) to confirm elements loading. Also, the surface roughness of the samples was investigated using an AFM, NT-MDT, building 167, zeleno grad, by tapping mode. Cyclic voltammograms (CVs) were performed in a three-electrode system with SCE used as the reference in 0.1 M Na2S/0.1 M S electrolyte. Diffuse reflectance spectrum (DRS) were obtained from Uv-1800 spectrophotometry. Confirmation of the more detail morphology of the sample can be seen in transmission electron microscopy TEM images by Zeiss-EM10C-80 KV
2.2. Fabrication of CoS counter electrode At first, the FTO glass substrate was cleaned in water, acetone and ethanol using an ultrasonic bath for 10 min. The SILAR method was used for the deposition of CoS on the FTO. In this method, the FTO glass substrates were then immersed into 0.5 M aqueous Co(CH3COO)2 for 30 s, rinsed with de-ionized (DI) water and dehydrated with a dryer, and then dipped in 0.5 M Na2S 5
aqueous solution for another 30 s followed by rinsing. This constituted one cycle and the process was repeated for four cycles. The prepared CE was dried at 60°C for 10 min.
2.3. Fabrication of Mg2+-, Ca2+-, Sr2+- and Ba2+-doped CoS counter electrodes Mg2+-, Ca2+-, Sr2+- and Ba2+-doped CoS were in situ grown on FTO glass substrate by the SILAR method. To incorporate Mg2+, Ca2+, Sr2+ and Ba2+ ions into the CoS electrode, molar percentages of 10% (0.05 M) of Mg(CH3COO)2.4H2O, Ca(CH3COO)2.4H2O, Sr(NO3)2 and BaCl2.2H2O were blended with 0.5 M Co(CH3COO)2. The experimental molar ratios of Co(CH3COO)2 to the above mentioned salts are 10:1. The substrate was then immersed horizontally into the growth solutions of the doped metal ions of Co2+ for 30 s, rinsed with DI water and dehydrated with a dryer. They were immersed in a 0.5 M Na2S aqueous solution for 30 s, rinsed with DI water and dehydrated with a dryer. These processes were repeated for four times. The CEs were dried at 60 °C for 10 min. 2.4. Fabrication of different molar percentages (5%, 10% and 20%) Ba- and Srdoped CoS counter electrodes Appropriate molar percentages of 5% (0.025 M), 10% (0.05 M) and 20% (0.1 M), Sr(NO3)2 and BaCl 2.2H2O were blended with 0.5 M Co(CH 3COO)2. The experimental molar ratios of Co(CH 3COO)2 to the above mentioned salts are 20:1, 10:1 and 1:5, respectively. The cleaned FTO glass substrates were immersed horizontally into solutions of the doped metal ions of Co 2+ (1:20, 1:10, 1:5) for 30 s, rinsed with deionized water and dehydrated with a dryer then were immersed in a 0.5 M Na 2S aqueous solution for another 30 s and rinsed with DI water and dehydrated with a dryer. These processes were repeated for four times and the prepared CEs were dried at 60 °C for 10 min.
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2.5. Preparation of Sr-doped CoS and Ba-doped CoS based GS, GO and CNT counter electrodes Graphene sheet and carbon nanotubes composite films have been successfully fabricated by electrophoretic deposition and used as counter electrodes of QDSSCs. EPD is a twostep process: in the first step, particles suspended in a liquid are forced to move toward an electrode by applying an electric field to the suspension (electrophoresis); in the second step, the particles pull together at one of the electrode and form a coherent deposit on. It is an economical technique that can be applied to any powdered solid that forms a stable suspension [34,35]. Thus, in this paper, to maintain good contact between graphene and FTO-coated glass, this approach has been applied [19]. Graphene oxide were prepared via a modified Hummers' method [36]. The Sr-CoS/GO and Sr-CoS/GS CEs were prepared by repeating EPD of GS or GO and deposition of Sr-CoS nanoparticles on the FTO via the SILAR method. Also, Ba-CoS/GO and Ba-CoS/GS CEs were prepared similar to what mentioned above. EPD process was performed using constant current density of 0.5 mA cm-2 in 0.2 mg mL-1 graphene aqueous solution. After deposition for 3 min, the graphene-deposited FTO was withdrawn from the solution and dried at room temperature. The Sr-CoS electrode was prepared by repeating the SILAR. Briefly, the electrode was immersed in the Sr-doped CoS (1:10) solution and rinsed with ethanol, and then immersed in 0.5 M Na 2S methanol solution. As mentioned, an EPD process followed by a SILAR process was indicated as 1 cycle of Sr-CoS/GS or Sr-CoS/GO deposition. To fabrication of Sr- and Ba-doped CoS based CNT CEs, at the first functionalized MWCNTs were suspended in a 1:1 mixture of acetone and ethanol by ultrasonication for 2 h. The electrophoretic deposition of CNT was carried out directly in the aqueous
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suspension of 5 mg mL−1 concentration. Cleaned FTO glass substrates and a Pt sheet with a distance of 0.7 cm apart from each other were used as a working electrode and a counter electrode, respectively. A constant potential of 18 V vs. open-circuit potential was employed for the EPD. After deposition of CNT, the Sr-CoS nanoparticles was deposited on the CNT-deposited FTO by the SILAR method. This process was performed for 4 times. The resulting Sr-CoS/CNT and Ba-CoS/CNT CEs were dried at 50°C for 5 min. 2.6. Fabrication of TiO2 photoanode FTO glass substrates were cleaned with DI water, acetone and ethanol in an ultrasonic bath. A standard TiO 2 (P25, Degussa, Germany) paste was prepared following procedure [37]. To prepare the scattering layer in the photoanode, the TiO 2 nanosphere was synthesized according to the literature [38]. Typically, FTO glass were first immersed in 40 mM aqueous TiCl 4 solution at 70°C for 30 min and washed with DI water and dried in air. A double-layer TiO 2 photoelectrodes were coated onto the FTO plates by using doctor blade method and dried at 120°C for 5 min. The deposited bilayer films were gradually heated to 500°C and were calcined for 15 min. The scattering layer was coated on doublelayer TiO 2 photoelectrodes. This layer dried at 120 °C for 5 min and was gradually heated to 500°C for 15 min. Subsequently, for surface modification, the electrodes were immersed into the TiCl 4 aqueous as described previously and then were calcined at 500 °C for 15 min. 2.7. Fabrication of CdS/CdSe/ZnS QDSSCs The CdS/CdSe/ZnS QDSSCs were deposited onto the TiO 2 photoanodes by the SILAR method [38,39]. First, the TiO 2 film was dipped in an ethanol solution containing 0.1 M Cd(NO3)2 for 1 min to allow Cd 2+ to adsorb onto the TiO 2, rinsed in methanol and dried
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with a dryer, dipped for 1 min in 0.1 M Na 2S methanol/water (1:1) solution, where the pre-adsorbed Cd2+ reacts with S2− to form the desired CdS. The film was rinsed again with DI water and dried with a drier. The two-step dipping procedure corresponded to a single SILAR cycle. The process was repeated 5 times. The CdS-coated TiO 2 films were heated at 300°C for 30 min. The TiO 2/CdS electrodes were dipped in the ethanol solution of 0.5 M, Cd(NO 3)2 for 5 min at 55°C and then immersed in aqueous solution of Na2SeSO3 for 30 min at 55°C, followed by rinsing with deionized water and drying with the drier. The two-step dipping is called a single SILAR cycle. This process was repeated for four times to obtain a suitable amount of CdSe on the CdS-coated film and then was heated at 150°C for 30 min. An aqueous Na 2SeSO3 solution was prepared by mixing 0.3 M selenium powder and 0.6 M Na 2SO3 at 70°C for 7 h. The SILAR method was also used to deposit the ZnS passivation layer. The CdS/ CdSe coated TiO 2 films were deposited with a ZnS layer, by dipping into 0.1 M Zn(CH 3COO)2 and 0.1 M Na2S solutions for 1 min, followed by rinsing with DI water and drying with a drier and this process was repeated for two times. Finally, the CdS/CdSe/ZnS-coated TiO2 films were heated at 300°C for 3 min. 2.7. Device fabrication CdS/CdSe/ZnS photoanodes and different CEs (metal ions-doped CoS and metal ions-doped CoS/carbon allotropes CEs) were assembled using 50 μm surlyn spacer. The polysulfide electrolyte was a water/methanol (3:7) solution containing of 1 M Na2S and 1 M S. The active area of the devices was 0.16 cm2.
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3. Results and Discussion 3.1. Spectral and electrochemical properties of CEs Theoretical studies show that porosity affect the catalytic properties of an electrode [40, 41]. So, FESEM was used to evaluate the surface morphology of the CEs on FTO substrate. Fig. 1 shows the SEM images and EDS of the CoS, Sr-CoS, Sr-CoS/CNT, Sr-CoS/GS and Sr-CoS/GO thin film samples. Whereas the electrocatalytic active materials may be released into the electrolyte solution with resulting reduction in the power conversion efficiency, adhesion of the electrocatalytic active materials on the FTO is an important factor in the efficiency of QDSSCs. As seen in Figs. 1a and b (left), when the Sr ion was added to the bare CoS, surface morphology changes. Applying a Sr-doped CoS CE in a QDSSC promotes electron production that leads to an increase in the catalytic activity of the metal sulfide CE. Figs. 1c-e (left) illustrates a surface morphology of Sr-CoS/CNT, Sr-CoS/GS and Sr-CoS/GO CEs with high porosity. It is note that there are some differences in the morphology and porosity of composite films compared to the CoS and Sr-doped CoS films which are effective on the CE performance. The CNT, GS and GO make a large number of active sites for the nucleation, which is more suitable for electrolyte injection. It will help CE performance that leads to an improvement in J sc. It should be considered that because these materials have been stoutly attached to the substrate, they are not soluble in the electrolyte and it is expected that this CE will demonstrate good performance in the QDSSC. The EDS analyses were carried out to identify the elemental compositions of the CoS, Sr-CoS, Sr-CoS/GS, Sr-CoS/CNT and Sr-CoS/GO thin films (Fig. 1 (right)). The EDS results show the presence of Co and S in the bare CoS, Sr-CoS, Sr-CoS/CNT, Sr-CoS/GS and SrCoS/GO CEs. Figs. 1b-e (right) confirms Sr doped with CoS and in Figs. 1c-e (right), the presence of carbon in addition to the other elements can be observed. 10
Fig. 2 shows typical TEM images of Sr-CoS/GO in the different scales on the FTO substrate. According to the TEM result, the GO hybrid showed a morphology in which Sr-CoS was attached to the surface of the GO. As this image was observed the Sr-CoS nanoparticle to be flat on the GO nanosheet. The larger magnification confirm Sr 2+ loading into CoS like to tiny particles with estimated size of about 10 nm. Cyclic voltammetry was performed to investigate the electrocatalytic activity at the electrode/electrolyte interface in a three-electrode system consisting of a working electrode, Pt wire CE, and an SCE (saturated calomel electrode) reference electrode with the polysulfide electrolyte at a scan rate of 30 mV s -1. Fig. 3 shows the CVs of CoS, 10% Sr-CoS, 10% SrCoS/CNT, 10% Sr-CoS/GS and 10% Sr-CoS/GO. Generally, the negative currents of CV plots represent the reduction of S x2- ions to S2- ions while their positive currents are related to the oxidation of S2- ions in the polysulfide electrolyte. A main parameter for comparing catalytic activities of diff erent CEs is the peak currents in CVs [42]. A higher peak current shows a higher active catalytic reaction of the CE for the redox couples in the electrolyte. The magnitude of current density is conspicuously larger for the composite CEs compared to the bare CoS CE. The increase of electro-catalytic activity of the CEs can be attributed to the porous structure of the Srand Ba-CoS/carbon allotropes (GS, GO CNT) composite films. It is obvious that the composite films exhibit a faster charge transfer kinetics than the bare CoS CE under the same QDSSC condition [37]. Fig. 4 presents the UV-Vis absorption spectra for the obtained thin films of bare CoS, 10% SrCoS, 10% Sr-CoS/GO CEs over the range of 350 to 750 nm. Among the spectra, the CoS thin film shows the lowest absorbance intensity. The absorbance increases with doping of Sr to CoS. As seen in Fig. 6, the Sr-CoS/GO thin film shows the highest absorbance intensity which is due
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to adding modified GO layers on the CoS layer that was described in the experimental section. So, the absorbance of a layer of CoS or Sr-CoS with the same thickness is lower than Sr-CoS/GO CE. 10% Sr-CoS/GS CE contains two different layers of Sr-doped CoS and graphene oxide that both of them improve catalytic behavior of CE. The strong absorption in the UV-Vis range implicates the potential application in light energy harvesting devices. To study the surface roughness of different CEs, high-resolution tapping mode atomic force microscopy (AFM) was applied, and the corresponding 3-dimensional (3D) images are shown in Figs. 5a-e. The smoothest film had a root-mean square (RMS) roughness value of 16.73 nm and was obtained from the CoS electrode. Sr doped CoS shows roughness value of 31.52 nm. The films of the Sr-CoS/CNT, Sr-CoS/GS and Sr-CoS/GO electrodes had higher surface roughness (RMS = 35.023 nm, RMS = 74.73 nm and 145.95 nm). Among these composite films, the highest RMS value (145.95 nm) was observed for the Sr-CoS/GO film. These results indicate that the area between the electrode and electrolyte increases in Sr-CoS/GO CE. The CE with the high superficial roughness would enhance its electrocatalytic activity for a polysulfide/sulfide or triiodide/iodide electrolyte system, and the charge transfer resistance at the CE/electrolyte interface would be lower in the cases of the Sr-CoS/CNT, Sr-CoS/GS and Sr-CoS/GO CEs, because they offer more electrocatalytic active sites for the reaction of polysulfide redox couple in the electrolyte. 3.2. Photovoltaic performance and electrical impedance analysis The current density–voltage (J-V) characteristics of the sensitized QDSSCs using CoS, 10% BaCoS, 10% Mg-CoS, 10% Ca-CoS, 10% Sr-CoS under one sun illumination (100 mWcm-2) have been shown in Fig. 6. Their calculated photovoltaic parameters from J–V curves (short circuit current density (Jsc), open circuit voltage (Voc), fill factor (FF) and efficiency (η) have been
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summarized in Table 1. Interestingly, doping of different metal ions (Mg2+, Ca2+, Sr2+ and Ba2+) in CoS CE can effectively catalyze the polysulfide electrolyte in the cells. As a whole, the addition of different metal ions to CoS CE leads to an improvement in the J sc, while slight change in Voc and FF occurs. In the case of CoS CE, the values of Jsc, Voc and FF of the QDSSC were 12.10 mA cm-2, 0.46 V and 0.29, respectively, yielding an efficiency of 1.60%. According to photovoltaic results in Table 1, among different QDSSCs based on the metal doped CoS CEs, the best performance is obtained for Sr doped CoS with Jsc of 15.90 mA cm-2, a Voc of 0.46 V, a FF of 0.30 and η of 2.19%. Also, a Jsc of 15.56 mA cm-2, a Voc of 0.46 V, a FF of 0.28 and η of 2.00% for Ba doped CoS are observed. The addition of 10% Sr2+ and 10% Ba2+ ions in the CoS CE shows 37% and 25% enhancements in efficiency compared to the bare CoS, respectively. After determining the best metal ion for doping in the CoS, we studied the effect of doped metal ion values in the CoS films on the cell performance. In a semiconductor, doping intentionally introduces impurities into an extremely pure intrinsic semiconductor for the purpose of modulating its electrical properties. Our purpose of addition of metal ions in CoS and creating defect in CoS lattice is increasing conductivity of CoS semiconductor. If amount of doped metal ion be less (5%) or more (20%) than a specified amount, a decrease in the cell performance will be observed. The photovoltaic parameters and the J–V curves of the QDSSCs based on different amounts of Ba2+ and Sr2+ ions (0%, 5%, 10% and 20%) doped CoS CEs under AM1.5 irradiation condition are presented in Table 2 and Figs. 7a and b, respectively. It is found that the QDSSCs based on CEs with different amount of doped metal ions exhibit different values of Jsc while the Voc remained almost unchanged. In result, the best photovoltaic performances obtained for 10% metal ion doped CoS CEs in the QDSSCs, so this amount of metal ion was used for preparing
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different composite CEs in the cells. A further increase of the metal ion amount (20%) leads to a decrease in the Jsc that is due a lower conductivity of CE compared to the 10% metal ion doped CoS. The solar cell using a 20% Sr-CoS as the CE gives a Jsc of 14.86 mA cm-2, a Voc of 0.46 V, and a FF of 0.28, yielding a η of 1.90%; these results show that a decrease in the photovoltaic performance has occurred compared to the cell based on 10% Sr-CoS CE. Also, for the cell based on different amounts of Ba2+ doped in CoS CEs, similar results are obtained. To evaluate the performances of the M-CoS/CNT, M-CoS/GS and M-CoS/GO CEs, the CdS/CdSe QDSSC using these electrodes were compared to the devices based on the CoS and M-CoS CEs. Figs. 8a and b display the J-V characteristics of cells composed of different kind of composite counter electrodes. The corresponding photovoltaic parameters of the cells have been summarized in Table 3. The cell with CoS counter electrode (Jsc = 12.10 mA cm-2, Voc = 0.46, FF = 0.29, η = 1.6) possess low FF, J sc and η due to the poor catalytic properties of this CE. The addition of Sr2+ ion (10%) leads to an improvement in the J sc and η (Jsc = 15.90 mA cm-2, Voc = 0.46, FF = 0.30, η = 2.19) that confirms a better catalytic properties of 10% Sr-CoS CE than the bare CoS. To improve the catalytic activity of CE, we attempted to use the CNT, GS and GO with the Sr- and Ba-CoS. The 10% Sr-CoS/CNT CE in QDSSC exhibits η of 2.32%, Jsc of 14.56 mA cm-2 and FF of 0.34, respectively, that shows a 45% improvement compared to the bare CoS CE. The photovoltaic parameters of the 10% Sr-CoS/GS CE are Jsc of 16.14 mA cm-2, Voc of 0.47 V, FF of 0.31 and η of 2.35%. In this cell, noticeable improvements appear in the η and J sc. In this case, a 33% increase in Jsc is observed. These increase is responsible for obtained improvement which is about 47% increase in efficiency (2.35%) compared to standard cell (1.60%). Furthermore, the addition of GO to 10% Sr-CoS leads to an improvement in the Jsc, so that the best performance with J sc of 16.17 mA cm-2, Voc of 0.47 V, FF of 0.37, and η of 2.81%
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are obtained. The Jsc increases from 12.10 mA cm-2 for the bare CoS to 16.17 mA cm-2 for 10% Sr-CoS/GO. This enhancement for Jsc for the cell with 10% Sr-CoS/GO CE is responsible for obtained efficiency improvement which is about 76% (2.81%) in comparison with the standard cell (1.60%). As shown in Fig. 8b, the similar photovoltaic performance have been observed for QDSSCs based on the Ba-CoS/carbon allotrope CEs. Therefore, the CoS CE has the lowest photovoltaic performance among the CEs, which originated from the lack of electrocatalytic activity toward polysulfide reduction. The CNT, GS and special GO are as a supporting material and supply a large number of active sites for nucleation that leads to a fast charge transfer at the CE/electrolyte interface. Based on the different morphologies of the composite films (Figs. 1cd), it is reasonable to expect that electrodes with different porosity exhibit different photovoltaic performance. As seen in table 3, among the devices based on the composite CEs, the cells with 10% Sr-CoS/GS and 10% Sr-CoS/GO CEs present Jsc of 16.14 and 16.17 mA cm-2, respectively, higher than that of device based on the 10%Sr-CoS/CNT CE (14.56 mA cm-2). That is due to GS and GO composites have almost similar structure and morphology that leads to closed together photocurrent density. Besides, GS and GO have higher intrinsic conductivity compared to CNT and using of them as CE in the QDSSC leads to lower charge transfer resistance and thereby higher Jsc . To investigate the electrochemical properties of different CEs toward polysulfide electrolyte interfacial charge transport processes in the QDSSCs, electrochemical impedance spectroscopy (EIS) were employed on the symmetric cells. The EIS spectra were recorded at a forward bias of ~ 0.46 V near the Voc. The Nyquist plots and equivalent circuit diagram (inset) of the QDSSCs based on different CEs have been shown in Figs. 9-11. The resistance at high frequency defined as RS, is related to the ohmic resistance of the FTO layer in photoanode and the CE. The middle
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frequency gives information of the charge transfer resistance (R ct) at the CE/electrolyte interface and in the lower frequency, Zw is the Warburg impedance of the redox electrolyte. These values are obtained by fitting with an equivalent circuit using Z-VIEW software [43,44]. Fig. 9 shows the Nyquist plots for the symmetrical cells fabricated with CoS, Mg-doped CoS, Ca-doped CoS, Sr-doped CoS and Ba-doped CoS CEs. As shown in this figure, the Rs values of bare CoS, 10% Mg-doped CoS, 10% Ca-doped CoS, 10% Sr-doped CoS and 10% Ba-doped CoS were obtained as 5.6, 4.8, 4.5, 3.1 and 4.4 Ω cm2, respectively. The low Rs value of the 10% SrCoS provides good bonding strength between the Sr-CoS film with the FTO substrate, which in turn promotes the collection of more electrons from the external circuit. Also, the R ct values of bare CoS, 10% Mg-doped CoS, 10% Ca-doped CoS, 10% Sr-doped CoS and 10% Ba-doped CoS were calculated as 60, 55, 58, 47 and 45 Ω cm2, respectively. Rct is responsible for the charge exchange between CE and electrolyte, with its value closely related to the electrocatalytic activity of the CE. So, the lower values of R ct of the 10% Sr- and Ba-CoS CEs reflect the superior charge transfer and electrocatalytic ability at the CE/electrolyte interface in these cells. The EIS spectra of the QDSSCs organized by bare 0% CoS, 5% Sr-doped CoS, 10% Sr-doped CoS and 20% Sr-doped CoS CEs have been presented in Fig. 10. Rs values of bare CoS, 5% Srdoped CoS, 10% Sr-doped CoS and 20% Sr-doped CoS are 5.6, 3.8, 3.1 and 2.2 Ω cm2, respectively. A smaller Rs values of 10% and 20% Sr-doped CoS confirms good bonding strength between the Sr-CoS with the FTO. The Rct value for 10% Sr-doped CoS (47 Ω cm2) is much lower than the Rct values for 20% Sr -doped CoS (64 Ω cm2), 5% Sr-doped CoS (61 Ω cm2), and CoS (60 Ω cm2). 10% Sr-CoS has the lowest Rct value, indicating more conductivity and electrocatalytic activity compared to the CoS. In general, in the Nyquest plots, the CE with higher conductivity shows a lower Rct. In the cell with 20% Sr-CoS, conductivity of CE
16
decreases compared to 10% Sr-CoS CE because Rct increases and thereby Jsc decreases. Fig. 11 displays the Nyquist plots of the EIS results for CoS, 10% Sr-CoS, 10% Sr-CoS/CNT, 10% Sr-CoS/GS and 10% Sr-CoS/GO. The Rs values of bare CoS, 10% Sr-CoS, 10% SrCoS/CNT, 10% Sr-CoS/GS and 10% Sr-CoS/GO were obtained as 5.6, 3.1, 4.5, 2.7, and 3.3 Ω cm2. The lower Rs values of the cells based on the composite CEs mean that the catalytic materials are more firmly attached to the FTO substrate which in turn promotes the electron collection from the external circuit in the QDSSC. Rct values of CoS, 10% Sr-CoS, 10% SrCoS/CNT, 10% Sr-CoS/GS and 10% Sr-CoS/GO are 60, 47, 36, 45 and 22 Ω cm2, respectively, indicating that compounds possess good catalytic activity. The R ct value for Ca-CoS/GO is much smaller than that of the bare CoS indicating that Ca-CoS/Go has excellent charge transfer at the interface of the electrolyte and CE. The EIS results show that carbon allotropes (CNT, GS and GO) are promising material for CEs in QDSSC because of their excellent conductivity that can decrease Rct. GS and GO have better intrinsic conductivity compared to CNT and using of them as CE in the QDSSC leads to lower R ct (as seen in Fig 11) and thereby higher photocurrent density. The observations of EIS and CV measurements conform well to the QDSSC performance of the CEs. In a word, metal doped CoS CEs display a higher catalytic activity toward the reduction of polysulfide electrolyte than the CoS CE, as revealed from photovoltaic and EIS data. Also, the power conversion efficiency of QDSSC using metal doped CoS could be further improved by optimizing value of doped metal ion. It seems very possible that by doping of metal ion in the CoS/carbon allotrope (GS, GO and CNT) composite films, a significant improvement in J sc is obtained compared to the metal doped CoS CEs. SEM, AFM and electrochemical results show that these composite films have a more porosity and conductivity compared to other CEs that can
17
improve electrocatalytic activity surface and electron transport, corresponding to enhancement of photocurrent density and thereby QDSSC efficiency. By comparing the photovoltaic parameters of composite CEs, it is found that 10% Sr-CoS/GS and 10% Sr-CoS/GO CEs have higher Jsc compared to the 10% Sr-CoS/CNT that is due to more conductivity of them.
4. Conclusions In summary, this study was performed on counter electrode system in QDSSC to improving photovoltaic performance. Firstly, we doped different metal ions in the CoS CE and optimized the doped metal ion amount. After that, the metal ions-doped CoS/carbon allotrope CEs were prepared and their performance were investigated in the cells. The results showed that the doping of metal ions-doped in the CoS and CoS/carbon allotrope composites CEs leads to an enhancement in the Jsc and a decrease in the Rct. It is expected that the photovoltaic performance based on metal ions-doped CoS/carbon allotrope CEs will be better that metal ions-doped CoS CEs because of their higher porosity and thereby better catalytic activity. Also, GS and GO have better intrinsic conductivity compared to CNT and using of their composites as CE in the QDSSC leads to lower RCT and higher Jsc. Finally, a 76% higher power conversion efficiency was achieved by applying a 10% Sr-CoS/GO CE than the bare CoS CE, a 41% higher power conversion efficiency was observed by 10% Ba-CoS/GO CE. The findings exposes that incorporating M2+ in the CoS and utilizing of M-CoS nanocomposite with some carbon allotropes are as a simple and low cost method to improve the overall power conversion efficiency of QDSSC. As it is demonstrated herein, the findings indicate that this kind of electrodes are a promising generation of CEs, which deserve further investigation.
Acknowledgements
18
The authors gratefully acknowledge from the University of Kashan for supporting this project by Grant No. 159183/30.
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Table legends: Table 1. Photovoltaic performance of QDSSCs based on CoS, Mg-CoS, Ca-CoS, Sr-CoS and Ba-CoS CEs under AM 1.5 illumination with an active area of 0.18 cm2. Table 2 Photovoltaic performance of QDSSCs based on different amounts of Ba 2+ and Sr2+ ions (0%, 5%, 10% and 20%) doped CoS CEs under AM 1.5 illumination with an active area of 0.18 cm2. Table 3 Photovoltaic performance of QDSSCs based on 10% Ba2+ and Sr2+-doped CoS/carbon allotrope CEs under AM 1.5 illumination with an active area of 0.18 cm2.
22
Figure captions: Fig. 1. FESEM images of CEs: (a) CoS, (b) 10% Sr-CoS, (c) 10% Sr-CoS/CNT, (d) 10% SrCoS/GS and (c) 10% Sr-CoS/GO thin films on the FTO. Fig. 2. The TEM images of 10% Sr-CoS/GO sample in different scales. Fig. 3. Cyclic voltammetry spectra of various counter electrodes measured in a three electrode configuration. Fig. 4. UV-Vis absorption spectra of CoS, 10% Sr-doped CoS and 10% Sr-doped CoS/GO thin films. Fig. 5. 3-Dimensional (3D) AFM images of the CEs: (a) CoS, (b) 10% Sr-CoS, (c) 10% SrCoS/CNT, (d) 10% Sr-CoS/GS and (c) 10% Sr-CoS/GO thin films on the FTO. Fig. 6. J-V curves of QDSSCs constructed with CoS, Ba-CoS, Mg-CoS, Ca-CoS, Sr-CoS CEs and tested under simulated AM1.5 solar irradiation (100 mW cm-2). Fig. 7. J-V curves of QDSSCs constructed with different amounts of (a) Ba2+ and (b) Sr2+ ions (0%, 5%, 10% and 20%) doped CoS CEs under simulated AM1.5 solar irradiation (100 mW cm-2). Fig. 8. J-V curves of QDSSCs constructed with different amounts of (a) 10% Ba2+ and (b) Sr2+doped CoS/carbon allotrope CEs under simulated AM1.5 solar irradiation (100 mW cm-2). Fig. 9. Nyquist plots of QDSSCs constructed with CoS, Ba-CoS, Mg-CoS, Ca-CoS, Sr-CoS CEs and tested under simulated AM1.5 solar irradiation (100 mW cm-2). Fig. 10. Nyquist plots of QDSSCs constructed with different amounts of Sr2+ ion (0%, 5%, 10% and 20%) doped CoS CEs under simulated AM1.5 solar irradiation (100 mW cm-2). Fig. 11. Nyquist plots of QDSSCs constructed with different amounts of 10% Sr 2+-doped CoS/carbon allotropes CEs under simulated AM1.5 solar irradiation (100 mW cm-2).
23
Tables: Table 1. Jsc [mA cm-2] 12.10
Voc [V] 0.46
0.29
η [%] 1.60
10% Mg-CoS
14.37
0.44
0.30
1.89
10% Ca-CoS
15.04
0.46
0.25
1.72
10% Sr-CoS
15.90
0.46
0.30
2.19
10% Ba-CoS
15.56
0.46
0.28
2.00
Counter electrode CoS
24
FF
Table 2. Counter electrode CoS
Jsc [mA cm-2] 12.10
Voc [V] 0.46
0.29
η [%] 1.60
FF
5% Sr-CoS
10.17
0.46
0.32
1.49
10% Sr-CoS
15.90
0.46
0.30
2.19
20% Sr-CoS
14.86
0.46
0.28
1.90
5% Ba-CoS
13.21
0.40
0.32
1.69
10% Ba-CoS
15.56
0.46
0.28
2.00
20% Ba-CoS
14.41
0.45
0.26
1.68
25
Table 3. Jsc [mA cm-2] 12.10
Voc [V] 0.46
0.29
η [%] 1.60
10% Sr-CoS
15.90
0.46
0.30
2.19
10%Sr-CoS/CNT
14.56
0.47
0.34
2.32
10% Sr-CoS/GS
16.14
0.47
0.31
2.35
10% Sr-CoS/GO
16.17
0.47
0.37
2.81
10% Ba-CoS
15.56
0.46
0.28
2.00
10% Ba-CoS/CNT
15.46
0.45
0.30
2.08
10% Ba-CoS/GS
17.35
0.45
0.28
2.11
10% Ba-CoS/GO
17.60
0.46
0.28
2.26
Counter electrode CoS
26
FF
Figures:
Fig. 1.
27
Fig. 2.
28
25000
Current density (µA)
20000 15000
10000 5000 0
CoS 10% Sr-CoS 10% Sr-CoS/CNT 10% Sr-CoS/GS 10% Sr-CoS/GO
-5000 -10000
-15000 -20000 -2
-1
0
1
Potential (V)
Fig. 3.
29
2
Absorbance
1.4
CoS
1.2
10% Sr-CoS 10% Sr-CoS/GO
1 0.8 0.6 0.4
0.2 0 350
450
550
Wavelength (nm)
Fig. 4.
30
650
750
Fig. 5.
31
Fig. 6.
32
Current density (mA cm-2)
18
(a)
CoS 5% Sr-CoS 10% Sr-CoS 20% Sr-CoS
16
14 12 10 8
6 4 2
0 0
0.1
0.2
0.3
Voltage (V)
Fig. 7.
33
0.4
0.5
Fig. 8.
34
35
CoS 10% Sr-CoS 10% Ba-CoS 10% Ca-CoS 10% Mg-CoS
-Z" (Ohm cm2)
30
25 20 15 10 5 0 0
20
40
Z' (Ohm cm2)
Fig. 9.
35
60
80
35
CoS 5% Sr-CoS 10% Sr-CoS 20% Sr-CoS
30
-Z" (Ohm cm2)
25 20 15 10
5 0 0
20
40
Z' (Ohm
60
cm2)
Fig. 10.
36
80
35
CoS 10% Sr-CoS 10% Sr-CoS/GS 10% Sr-CoS/GO 10% Sr-CoS/CNT
30
-Z" (Ohm cm2)
25 20 15 10 5 0 0
20
40
Z' (Ohm
60
cm2)
Fig. 11.
Graphical Abstract:
37
80
38
S0021-9797(17)30005-X http://dx.doi.org/10.1016/j.jcis.2017.01.005 YJCIS 21911
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
16 October 2016 28 December 2016 3 January 2017
Please cite this article as: S. Sara Khalili, H. Dehghani, M. Afrooz, Composite films of metal doped CoS/carbon allotropes; efficient electrocatalyst counter electrodes for high performance quantum dot-sensitized solar cells, Journal of Colloid and Interface Science (2017), doi: http://dx.doi.org/10.1016/j.jcis.2017.01.005
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Composite films of metal doped CoS/carbon allotropes; efficient electrocatalyst counter electrodes for high performance quantum dot-sensitized solar cells Seyede Sara Khalili, Hossein Dehghani*, Malihe Afrooz Department of Inorganic Chemistry, Faculty of Chemistry, University of Kashan, P.O. Box 87317-51167, Kashan, Iran Email: [email protected], [email protected], [email protected] *Corresponding author, Tel: +98 315 591 2386. Fax: +98 315 591 2397, Iran, Area cod Box 8731751167.
Abstract This study reports the enhanced catalytic ability of metal ions-doped CoS and CoS/carbon allotrope counter electrodes (CEs) (synthesized using a successive ionic layer adsorption and reaction (SILAR) method) to improve the power conversion efficiency (η) in quantum dot solar cells (QDSSCs). Firstly, doping effects of different metal ions (Mg2+, Ca2+, Sr2+ and Ba2+) in the CoS CE on QDSSC performance have been investigated. Overall, among the different metal doped CoS CEs, the best energy conversion efficiency of 2.19%, achieved for Sr, is the highest reported for QDSSCs constructed with metal doped CoS. A sandwich structural Sr- and BaCoS/carbon allotrope (graphene sheet (GS), graphene oxide (GO) and carbon nanotube (CNT)) composite CEs have been prepared by repeating electrophoretic deposition (EPD) of carbon materials and deposition of CoS nanoparticles. Dramatic enhancements of η have been observed with the Sr- and Ba-CoS/GO CEs based QDSSCs (∼76% and ∼41%, respectively), which is
1
higher than that of the bare CoS CE. Because of the large specific surface area and superior electrical conductivity of GS, GO and CNT and the high electrocatalytic activity of CoS, these CEs show an improvement in the photocurrent density in the cells, as revealed from electrochemical and spectral data. Keywords: Quantum dot-sensitized solar cells, Counter electrode, Efficiency enhancement, Metal ion-doped cobalt sulfide, Electrocatalytic activity
Introduction Semiconductor quantum dot-sensitized solar cells (QDSSCs) have attracted extensive attention as promising alternatives to dye-sensitized solar cells (DSSCs) because of their unique optoelectronic properties such as low cost, environmental friendliness, simple production processes, and relatively high energy conversion efficiency [1-3]. The attractive properties of semiconductor quantum dots include higher extinction coefficients, multiple exciton generation [4], direct hot carrier transfer and tunable band gaps [5, 6]. In quantum dot (QD) sensitizers, CdS and CdSe are the most efficient and widely used ones. Electrolyte acts as a vital component in QDSSCs, is due to the effective scavenging of holes to regenerate the sensitizer and the resistivity against photoanodic corrosion [7]. The iodide/triiodide electrolyte used in DSSCs is not a suitable candidate for QDSSCs, because it causes photocorrosion to the QDs [8]. Polysulfide (S2-/Sn2-) redox couple aqueous solution electrolyte is commonly used in QDSSCs because this electrolyte media can stabilize chalcogenide QD sensitizers and can also offer an acceptable photovoltaic performance of the resultant QDSSC [9-12]. Counter electrode (CE), where electron flow back into the electrolyte solution from external circuit, is an important part of solar cells and is responsible for catalysis and reduction of the oxidized redox species. In the
2
past decade, the QDSSC efficiency has seen significant enhancements with the advances in CEs research [13]. In a QDSSC, the CE plays a crucial role in collecting electrons from the external circuit and reducing polysulfide (S2-/Sn-2) electrolytes. Polysulfide electrolyte and Pt counter electrode are the most commonly used in QDSSCs, however, the Pt-based CE shows reduced activity in the polysulfide electrolyte, probably due to the decreased charge transfer rate at CE/electrolyte interfaces. Recently, to overcome this challenge, a variety of alternative CE materials such as CoS [14], PbS [15], NiS [16] and CuS [17], with higher electrocatalytic activity and lower charge transfer resistance at the CE/electrolyte interface, have been widely investigated for the polysulfide electrolyte in the QDSSCs. CoS electrodes can be easily prepared by successive ionic layer adsorption and reaction (SILAR) method and can replaced with Pt electrode. Using of CoS reduces the whole device cost due to the extremely low plenty of platinum compared with cobalt. Since the specific surface area of catalyst is another key factor of the CE, numerous studies have been made on an alternative CE including carbon materials [1823]. Herein, we focus on the QDSSC photovoltaic performances constructed by diff erent CoS CEs. For this purpose, we have developed a facile synthesis of the composites of CoS with diff erent carbon allotropes (graphene sheet (GS), graphene oxide (GO) and carbon nanotube (CNT)) on fluorine doped SnO2-coated glass (FTO). Carbon materials such as graphene and carbon nanotube display some of the most potential for using as an alternative CE due to its highly specific surface, superior electronic conductivity and excellent mechanical strength [2426]. These carbon materials provide a high surface area for the deposition of CoS nanoparticles [2728]. The composite films are fabricated using an electrophoresis of carbon materials onto a FTO
3
substrate and then CoS deposit onto the FTO coated with carbon materials by the SILAR method [29-33]. Our findings in the present study indicate that doping of the CoS and CoS/carbon allotrope composite CEs with difference metal ions such as Mg2+, Ca2+, Sr2+ and Ba2+ in a QDSSC improves the electrocatalytic activity of CE compared to the bare CoS. As a result, among the metal doped CoS CEs, the Sr-doped CoS and Ba-doped CoS CEs show excellent catalytic activity for polysulfide reduction than the bare CoS CE, consequently, the cells based on these CEs have the best photovoltaic performance; Sr-doped CoS CE has delivered an efficiency of 2.19%, which is comparable to that of the bare CoS electrode (1.60%) and power conversion efficiency of 2.00% has been achieved with the Ba-doped CoS CE. Also, the effect of amount of metal ion doped in CoS on the QDSSC performance has been discussed and the maximum energy conversion efficiency has been achieved in 10% of metal ions (Sr 2+ and Ba2+). Then, this amount of Sr2+ and Ba2+ ions have been doped in CoS/GO, CoS/GS and CoS/CNT matrix by repeating electrophoretic deposition (EPD)
of carbon materials and
deposition of Sr-/Ba-doped CoS nanoparticles on the FTO [27]. It is expected that the structure of graphene oxide, graphene sheets and carbon nanotube provide a high surface area for the deposit of Sr-/Ba-doped CoS nanoparticle; thus, these carbon nanomaterials play a main role in supporting material and supplying a large number of active sites for nucleation so that leads to a significant increase in the efficiency of a QDSSC. By analyzing electrochemical and spectral data, it is found that the incorporation of metal ions can help to enhance the catalytic property of the metal sulfide CE by promoting electron production, thereby increasing the efficiency of the QDSSCs. Using of the new composite CEs provides a simple approach towards the goal of high performance QDSSCs.
4
2. Experimental Section 2.1. Materials and Instrumentation All chemical materials and solvents used for synthesis were purchased from Merck and SigmaAldrich Companies. The fluorine-doped tin oxide (FTO) glass substrates (transmission >90% in the visible, sheet resistance 8 Ωper square, 2.3 mm thickness) and surlyn spacer (50 µm) were obtained from Dyesol (Australia). The photocurrent-voltage (I–V) characteristics of the cells were measured using a Keithley model 2400 digital source meter (Keithley, USA). The electrochemical impedance spectroscopy (EIS) measurements of the cells achieve at a forward bias of 0.46 V under AM 1.5 G simulated light (Luzchem) using potantiostat/galvanostat (PGSTAT 100, Autolab, Eco-Chemie), at an AC amplitude of 5 mV within the frequency range from 0.01 Hz to 500 kHz. The surface features of the CEs were observed using a field emission scanning electron microscope (Philips XL-30FESEM) equipped with energy dispersive spectrometer (EDS) to confirm elements loading. Also, the surface roughness of the samples was investigated using an AFM, NT-MDT, building 167, zeleno grad, by tapping mode. Cyclic voltammograms (CVs) were performed in a three-electrode system with SCE used as the reference in 0.1 M Na2S/0.1 M S electrolyte. Diffuse reflectance spectrum (DRS) were obtained from Uv-1800 spectrophotometry. Confirmation of the more detail morphology of the sample can be seen in transmission electron microscopy TEM images by Zeiss-EM10C-80 KV
2.2. Fabrication of CoS counter electrode At first, the FTO glass substrate was cleaned in water, acetone and ethanol using an ultrasonic bath for 10 min. The SILAR method was used for the deposition of CoS on the FTO. In this method, the FTO glass substrates were then immersed into 0.5 M aqueous Co(CH3COO)2 for 30 s, rinsed with de-ionized (DI) water and dehydrated with a dryer, and then dipped in 0.5 M Na2S 5
aqueous solution for another 30 s followed by rinsing. This constituted one cycle and the process was repeated for four cycles. The prepared CE was dried at 60°C for 10 min.
2.3. Fabrication of Mg2+-, Ca2+-, Sr2+- and Ba2+-doped CoS counter electrodes Mg2+-, Ca2+-, Sr2+- and Ba2+-doped CoS were in situ grown on FTO glass substrate by the SILAR method. To incorporate Mg2+, Ca2+, Sr2+ and Ba2+ ions into the CoS electrode, molar percentages of 10% (0.05 M) of Mg(CH3COO)2.4H2O, Ca(CH3COO)2.4H2O, Sr(NO3)2 and BaCl2.2H2O were blended with 0.5 M Co(CH3COO)2. The experimental molar ratios of Co(CH3COO)2 to the above mentioned salts are 10:1. The substrate was then immersed horizontally into the growth solutions of the doped metal ions of Co2+ for 30 s, rinsed with DI water and dehydrated with a dryer. They were immersed in a 0.5 M Na2S aqueous solution for 30 s, rinsed with DI water and dehydrated with a dryer. These processes were repeated for four times. The CEs were dried at 60 °C for 10 min. 2.4. Fabrication of different molar percentages (5%, 10% and 20%) Ba- and Srdoped CoS counter electrodes Appropriate molar percentages of 5% (0.025 M), 10% (0.05 M) and 20% (0.1 M), Sr(NO3)2 and BaCl 2.2H2O were blended with 0.5 M Co(CH 3COO)2. The experimental molar ratios of Co(CH 3COO)2 to the above mentioned salts are 20:1, 10:1 and 1:5, respectively. The cleaned FTO glass substrates were immersed horizontally into solutions of the doped metal ions of Co 2+ (1:20, 1:10, 1:5) for 30 s, rinsed with deionized water and dehydrated with a dryer then were immersed in a 0.5 M Na 2S aqueous solution for another 30 s and rinsed with DI water and dehydrated with a dryer. These processes were repeated for four times and the prepared CEs were dried at 60 °C for 10 min.
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2.5. Preparation of Sr-doped CoS and Ba-doped CoS based GS, GO and CNT counter electrodes Graphene sheet and carbon nanotubes composite films have been successfully fabricated by electrophoretic deposition and used as counter electrodes of QDSSCs. EPD is a twostep process: in the first step, particles suspended in a liquid are forced to move toward an electrode by applying an electric field to the suspension (electrophoresis); in the second step, the particles pull together at one of the electrode and form a coherent deposit on. It is an economical technique that can be applied to any powdered solid that forms a stable suspension [34,35]. Thus, in this paper, to maintain good contact between graphene and FTO-coated glass, this approach has been applied [19]. Graphene oxide were prepared via a modified Hummers' method [36]. The Sr-CoS/GO and Sr-CoS/GS CEs were prepared by repeating EPD of GS or GO and deposition of Sr-CoS nanoparticles on the FTO via the SILAR method. Also, Ba-CoS/GO and Ba-CoS/GS CEs were prepared similar to what mentioned above. EPD process was performed using constant current density of 0.5 mA cm-2 in 0.2 mg mL-1 graphene aqueous solution. After deposition for 3 min, the graphene-deposited FTO was withdrawn from the solution and dried at room temperature. The Sr-CoS electrode was prepared by repeating the SILAR. Briefly, the electrode was immersed in the Sr-doped CoS (1:10) solution and rinsed with ethanol, and then immersed in 0.5 M Na 2S methanol solution. As mentioned, an EPD process followed by a SILAR process was indicated as 1 cycle of Sr-CoS/GS or Sr-CoS/GO deposition. To fabrication of Sr- and Ba-doped CoS based CNT CEs, at the first functionalized MWCNTs were suspended in a 1:1 mixture of acetone and ethanol by ultrasonication for 2 h. The electrophoretic deposition of CNT was carried out directly in the aqueous
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suspension of 5 mg mL−1 concentration. Cleaned FTO glass substrates and a Pt sheet with a distance of 0.7 cm apart from each other were used as a working electrode and a counter electrode, respectively. A constant potential of 18 V vs. open-circuit potential was employed for the EPD. After deposition of CNT, the Sr-CoS nanoparticles was deposited on the CNT-deposited FTO by the SILAR method. This process was performed for 4 times. The resulting Sr-CoS/CNT and Ba-CoS/CNT CEs were dried at 50°C for 5 min. 2.6. Fabrication of TiO2 photoanode FTO glass substrates were cleaned with DI water, acetone and ethanol in an ultrasonic bath. A standard TiO 2 (P25, Degussa, Germany) paste was prepared following procedure [37]. To prepare the scattering layer in the photoanode, the TiO 2 nanosphere was synthesized according to the literature [38]. Typically, FTO glass were first immersed in 40 mM aqueous TiCl 4 solution at 70°C for 30 min and washed with DI water and dried in air. A double-layer TiO 2 photoelectrodes were coated onto the FTO plates by using doctor blade method and dried at 120°C for 5 min. The deposited bilayer films were gradually heated to 500°C and were calcined for 15 min. The scattering layer was coated on doublelayer TiO 2 photoelectrodes. This layer dried at 120 °C for 5 min and was gradually heated to 500°C for 15 min. Subsequently, for surface modification, the electrodes were immersed into the TiCl 4 aqueous as described previously and then were calcined at 500 °C for 15 min. 2.7. Fabrication of CdS/CdSe/ZnS QDSSCs The CdS/CdSe/ZnS QDSSCs were deposited onto the TiO 2 photoanodes by the SILAR method [38,39]. First, the TiO 2 film was dipped in an ethanol solution containing 0.1 M Cd(NO3)2 for 1 min to allow Cd 2+ to adsorb onto the TiO 2, rinsed in methanol and dried
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with a dryer, dipped for 1 min in 0.1 M Na 2S methanol/water (1:1) solution, where the pre-adsorbed Cd2+ reacts with S2− to form the desired CdS. The film was rinsed again with DI water and dried with a drier. The two-step dipping procedure corresponded to a single SILAR cycle. The process was repeated 5 times. The CdS-coated TiO 2 films were heated at 300°C for 30 min. The TiO 2/CdS electrodes were dipped in the ethanol solution of 0.5 M, Cd(NO 3)2 for 5 min at 55°C and then immersed in aqueous solution of Na2SeSO3 for 30 min at 55°C, followed by rinsing with deionized water and drying with the drier. The two-step dipping is called a single SILAR cycle. This process was repeated for four times to obtain a suitable amount of CdSe on the CdS-coated film and then was heated at 150°C for 30 min. An aqueous Na 2SeSO3 solution was prepared by mixing 0.3 M selenium powder and 0.6 M Na 2SO3 at 70°C for 7 h. The SILAR method was also used to deposit the ZnS passivation layer. The CdS/ CdSe coated TiO 2 films were deposited with a ZnS layer, by dipping into 0.1 M Zn(CH 3COO)2 and 0.1 M Na2S solutions for 1 min, followed by rinsing with DI water and drying with a drier and this process was repeated for two times. Finally, the CdS/CdSe/ZnS-coated TiO2 films were heated at 300°C for 3 min. 2.7. Device fabrication CdS/CdSe/ZnS photoanodes and different CEs (metal ions-doped CoS and metal ions-doped CoS/carbon allotropes CEs) were assembled using 50 μm surlyn spacer. The polysulfide electrolyte was a water/methanol (3:7) solution containing of 1 M Na2S and 1 M S. The active area of the devices was 0.16 cm2.
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3. Results and Discussion 3.1. Spectral and electrochemical properties of CEs Theoretical studies show that porosity affect the catalytic properties of an electrode [40, 41]. So, FESEM was used to evaluate the surface morphology of the CEs on FTO substrate. Fig. 1 shows the SEM images and EDS of the CoS, Sr-CoS, Sr-CoS/CNT, Sr-CoS/GS and Sr-CoS/GO thin film samples. Whereas the electrocatalytic active materials may be released into the electrolyte solution with resulting reduction in the power conversion efficiency, adhesion of the electrocatalytic active materials on the FTO is an important factor in the efficiency of QDSSCs. As seen in Figs. 1a and b (left), when the Sr ion was added to the bare CoS, surface morphology changes. Applying a Sr-doped CoS CE in a QDSSC promotes electron production that leads to an increase in the catalytic activity of the metal sulfide CE. Figs. 1c-e (left) illustrates a surface morphology of Sr-CoS/CNT, Sr-CoS/GS and Sr-CoS/GO CEs with high porosity. It is note that there are some differences in the morphology and porosity of composite films compared to the CoS and Sr-doped CoS films which are effective on the CE performance. The CNT, GS and GO make a large number of active sites for the nucleation, which is more suitable for electrolyte injection. It will help CE performance that leads to an improvement in J sc. It should be considered that because these materials have been stoutly attached to the substrate, they are not soluble in the electrolyte and it is expected that this CE will demonstrate good performance in the QDSSC. The EDS analyses were carried out to identify the elemental compositions of the CoS, Sr-CoS, Sr-CoS/GS, Sr-CoS/CNT and Sr-CoS/GO thin films (Fig. 1 (right)). The EDS results show the presence of Co and S in the bare CoS, Sr-CoS, Sr-CoS/CNT, Sr-CoS/GS and SrCoS/GO CEs. Figs. 1b-e (right) confirms Sr doped with CoS and in Figs. 1c-e (right), the presence of carbon in addition to the other elements can be observed. 10
Fig. 2 shows typical TEM images of Sr-CoS/GO in the different scales on the FTO substrate. According to the TEM result, the GO hybrid showed a morphology in which Sr-CoS was attached to the surface of the GO. As this image was observed the Sr-CoS nanoparticle to be flat on the GO nanosheet. The larger magnification confirm Sr 2+ loading into CoS like to tiny particles with estimated size of about 10 nm. Cyclic voltammetry was performed to investigate the electrocatalytic activity at the electrode/electrolyte interface in a three-electrode system consisting of a working electrode, Pt wire CE, and an SCE (saturated calomel electrode) reference electrode with the polysulfide electrolyte at a scan rate of 30 mV s -1. Fig. 3 shows the CVs of CoS, 10% Sr-CoS, 10% SrCoS/CNT, 10% Sr-CoS/GS and 10% Sr-CoS/GO. Generally, the negative currents of CV plots represent the reduction of S x2- ions to S2- ions while their positive currents are related to the oxidation of S2- ions in the polysulfide electrolyte. A main parameter for comparing catalytic activities of diff erent CEs is the peak currents in CVs [42]. A higher peak current shows a higher active catalytic reaction of the CE for the redox couples in the electrolyte. The magnitude of current density is conspicuously larger for the composite CEs compared to the bare CoS CE. The increase of electro-catalytic activity of the CEs can be attributed to the porous structure of the Srand Ba-CoS/carbon allotropes (GS, GO CNT) composite films. It is obvious that the composite films exhibit a faster charge transfer kinetics than the bare CoS CE under the same QDSSC condition [37]. Fig. 4 presents the UV-Vis absorption spectra for the obtained thin films of bare CoS, 10% SrCoS, 10% Sr-CoS/GO CEs over the range of 350 to 750 nm. Among the spectra, the CoS thin film shows the lowest absorbance intensity. The absorbance increases with doping of Sr to CoS. As seen in Fig. 6, the Sr-CoS/GO thin film shows the highest absorbance intensity which is due
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to adding modified GO layers on the CoS layer that was described in the experimental section. So, the absorbance of a layer of CoS or Sr-CoS with the same thickness is lower than Sr-CoS/GO CE. 10% Sr-CoS/GS CE contains two different layers of Sr-doped CoS and graphene oxide that both of them improve catalytic behavior of CE. The strong absorption in the UV-Vis range implicates the potential application in light energy harvesting devices. To study the surface roughness of different CEs, high-resolution tapping mode atomic force microscopy (AFM) was applied, and the corresponding 3-dimensional (3D) images are shown in Figs. 5a-e. The smoothest film had a root-mean square (RMS) roughness value of 16.73 nm and was obtained from the CoS electrode. Sr doped CoS shows roughness value of 31.52 nm. The films of the Sr-CoS/CNT, Sr-CoS/GS and Sr-CoS/GO electrodes had higher surface roughness (RMS = 35.023 nm, RMS = 74.73 nm and 145.95 nm). Among these composite films, the highest RMS value (145.95 nm) was observed for the Sr-CoS/GO film. These results indicate that the area between the electrode and electrolyte increases in Sr-CoS/GO CE. The CE with the high superficial roughness would enhance its electrocatalytic activity for a polysulfide/sulfide or triiodide/iodide electrolyte system, and the charge transfer resistance at the CE/electrolyte interface would be lower in the cases of the Sr-CoS/CNT, Sr-CoS/GS and Sr-CoS/GO CEs, because they offer more electrocatalytic active sites for the reaction of polysulfide redox couple in the electrolyte. 3.2. Photovoltaic performance and electrical impedance analysis The current density–voltage (J-V) characteristics of the sensitized QDSSCs using CoS, 10% BaCoS, 10% Mg-CoS, 10% Ca-CoS, 10% Sr-CoS under one sun illumination (100 mWcm-2) have been shown in Fig. 6. Their calculated photovoltaic parameters from J–V curves (short circuit current density (Jsc), open circuit voltage (Voc), fill factor (FF) and efficiency (η) have been
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summarized in Table 1. Interestingly, doping of different metal ions (Mg2+, Ca2+, Sr2+ and Ba2+) in CoS CE can effectively catalyze the polysulfide electrolyte in the cells. As a whole, the addition of different metal ions to CoS CE leads to an improvement in the J sc, while slight change in Voc and FF occurs. In the case of CoS CE, the values of Jsc, Voc and FF of the QDSSC were 12.10 mA cm-2, 0.46 V and 0.29, respectively, yielding an efficiency of 1.60%. According to photovoltaic results in Table 1, among different QDSSCs based on the metal doped CoS CEs, the best performance is obtained for Sr doped CoS with Jsc of 15.90 mA cm-2, a Voc of 0.46 V, a FF of 0.30 and η of 2.19%. Also, a Jsc of 15.56 mA cm-2, a Voc of 0.46 V, a FF of 0.28 and η of 2.00% for Ba doped CoS are observed. The addition of 10% Sr2+ and 10% Ba2+ ions in the CoS CE shows 37% and 25% enhancements in efficiency compared to the bare CoS, respectively. After determining the best metal ion for doping in the CoS, we studied the effect of doped metal ion values in the CoS films on the cell performance. In a semiconductor, doping intentionally introduces impurities into an extremely pure intrinsic semiconductor for the purpose of modulating its electrical properties. Our purpose of addition of metal ions in CoS and creating defect in CoS lattice is increasing conductivity of CoS semiconductor. If amount of doped metal ion be less (5%) or more (20%) than a specified amount, a decrease in the cell performance will be observed. The photovoltaic parameters and the J–V curves of the QDSSCs based on different amounts of Ba2+ and Sr2+ ions (0%, 5%, 10% and 20%) doped CoS CEs under AM1.5 irradiation condition are presented in Table 2 and Figs. 7a and b, respectively. It is found that the QDSSCs based on CEs with different amount of doped metal ions exhibit different values of Jsc while the Voc remained almost unchanged. In result, the best photovoltaic performances obtained for 10% metal ion doped CoS CEs in the QDSSCs, so this amount of metal ion was used for preparing
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different composite CEs in the cells. A further increase of the metal ion amount (20%) leads to a decrease in the Jsc that is due a lower conductivity of CE compared to the 10% metal ion doped CoS. The solar cell using a 20% Sr-CoS as the CE gives a Jsc of 14.86 mA cm-2, a Voc of 0.46 V, and a FF of 0.28, yielding a η of 1.90%; these results show that a decrease in the photovoltaic performance has occurred compared to the cell based on 10% Sr-CoS CE. Also, for the cell based on different amounts of Ba2+ doped in CoS CEs, similar results are obtained. To evaluate the performances of the M-CoS/CNT, M-CoS/GS and M-CoS/GO CEs, the CdS/CdSe QDSSC using these electrodes were compared to the devices based on the CoS and M-CoS CEs. Figs. 8a and b display the J-V characteristics of cells composed of different kind of composite counter electrodes. The corresponding photovoltaic parameters of the cells have been summarized in Table 3. The cell with CoS counter electrode (Jsc = 12.10 mA cm-2, Voc = 0.46, FF = 0.29, η = 1.6) possess low FF, J sc and η due to the poor catalytic properties of this CE. The addition of Sr2+ ion (10%) leads to an improvement in the J sc and η (Jsc = 15.90 mA cm-2, Voc = 0.46, FF = 0.30, η = 2.19) that confirms a better catalytic properties of 10% Sr-CoS CE than the bare CoS. To improve the catalytic activity of CE, we attempted to use the CNT, GS and GO with the Sr- and Ba-CoS. The 10% Sr-CoS/CNT CE in QDSSC exhibits η of 2.32%, Jsc of 14.56 mA cm-2 and FF of 0.34, respectively, that shows a 45% improvement compared to the bare CoS CE. The photovoltaic parameters of the 10% Sr-CoS/GS CE are Jsc of 16.14 mA cm-2, Voc of 0.47 V, FF of 0.31 and η of 2.35%. In this cell, noticeable improvements appear in the η and J sc. In this case, a 33% increase in Jsc is observed. These increase is responsible for obtained improvement which is about 47% increase in efficiency (2.35%) compared to standard cell (1.60%). Furthermore, the addition of GO to 10% Sr-CoS leads to an improvement in the Jsc, so that the best performance with J sc of 16.17 mA cm-2, Voc of 0.47 V, FF of 0.37, and η of 2.81%
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are obtained. The Jsc increases from 12.10 mA cm-2 for the bare CoS to 16.17 mA cm-2 for 10% Sr-CoS/GO. This enhancement for Jsc for the cell with 10% Sr-CoS/GO CE is responsible for obtained efficiency improvement which is about 76% (2.81%) in comparison with the standard cell (1.60%). As shown in Fig. 8b, the similar photovoltaic performance have been observed for QDSSCs based on the Ba-CoS/carbon allotrope CEs. Therefore, the CoS CE has the lowest photovoltaic performance among the CEs, which originated from the lack of electrocatalytic activity toward polysulfide reduction. The CNT, GS and special GO are as a supporting material and supply a large number of active sites for nucleation that leads to a fast charge transfer at the CE/electrolyte interface. Based on the different morphologies of the composite films (Figs. 1cd), it is reasonable to expect that electrodes with different porosity exhibit different photovoltaic performance. As seen in table 3, among the devices based on the composite CEs, the cells with 10% Sr-CoS/GS and 10% Sr-CoS/GO CEs present Jsc of 16.14 and 16.17 mA cm-2, respectively, higher than that of device based on the 10%Sr-CoS/CNT CE (14.56 mA cm-2). That is due to GS and GO composites have almost similar structure and morphology that leads to closed together photocurrent density. Besides, GS and GO have higher intrinsic conductivity compared to CNT and using of them as CE in the QDSSC leads to lower charge transfer resistance and thereby higher Jsc . To investigate the electrochemical properties of different CEs toward polysulfide electrolyte interfacial charge transport processes in the QDSSCs, electrochemical impedance spectroscopy (EIS) were employed on the symmetric cells. The EIS spectra were recorded at a forward bias of ~ 0.46 V near the Voc. The Nyquist plots and equivalent circuit diagram (inset) of the QDSSCs based on different CEs have been shown in Figs. 9-11. The resistance at high frequency defined as RS, is related to the ohmic resistance of the FTO layer in photoanode and the CE. The middle
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frequency gives information of the charge transfer resistance (R ct) at the CE/electrolyte interface and in the lower frequency, Zw is the Warburg impedance of the redox electrolyte. These values are obtained by fitting with an equivalent circuit using Z-VIEW software [43,44]. Fig. 9 shows the Nyquist plots for the symmetrical cells fabricated with CoS, Mg-doped CoS, Ca-doped CoS, Sr-doped CoS and Ba-doped CoS CEs. As shown in this figure, the Rs values of bare CoS, 10% Mg-doped CoS, 10% Ca-doped CoS, 10% Sr-doped CoS and 10% Ba-doped CoS were obtained as 5.6, 4.8, 4.5, 3.1 and 4.4 Ω cm2, respectively. The low Rs value of the 10% SrCoS provides good bonding strength between the Sr-CoS film with the FTO substrate, which in turn promotes the collection of more electrons from the external circuit. Also, the R ct values of bare CoS, 10% Mg-doped CoS, 10% Ca-doped CoS, 10% Sr-doped CoS and 10% Ba-doped CoS were calculated as 60, 55, 58, 47 and 45 Ω cm2, respectively. Rct is responsible for the charge exchange between CE and electrolyte, with its value closely related to the electrocatalytic activity of the CE. So, the lower values of R ct of the 10% Sr- and Ba-CoS CEs reflect the superior charge transfer and electrocatalytic ability at the CE/electrolyte interface in these cells. The EIS spectra of the QDSSCs organized by bare 0% CoS, 5% Sr-doped CoS, 10% Sr-doped CoS and 20% Sr-doped CoS CEs have been presented in Fig. 10. Rs values of bare CoS, 5% Srdoped CoS, 10% Sr-doped CoS and 20% Sr-doped CoS are 5.6, 3.8, 3.1 and 2.2 Ω cm2, respectively. A smaller Rs values of 10% and 20% Sr-doped CoS confirms good bonding strength between the Sr-CoS with the FTO. The Rct value for 10% Sr-doped CoS (47 Ω cm2) is much lower than the Rct values for 20% Sr -doped CoS (64 Ω cm2), 5% Sr-doped CoS (61 Ω cm2), and CoS (60 Ω cm2). 10% Sr-CoS has the lowest Rct value, indicating more conductivity and electrocatalytic activity compared to the CoS. In general, in the Nyquest plots, the CE with higher conductivity shows a lower Rct. In the cell with 20% Sr-CoS, conductivity of CE
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decreases compared to 10% Sr-CoS CE because Rct increases and thereby Jsc decreases. Fig. 11 displays the Nyquist plots of the EIS results for CoS, 10% Sr-CoS, 10% Sr-CoS/CNT, 10% Sr-CoS/GS and 10% Sr-CoS/GO. The Rs values of bare CoS, 10% Sr-CoS, 10% SrCoS/CNT, 10% Sr-CoS/GS and 10% Sr-CoS/GO were obtained as 5.6, 3.1, 4.5, 2.7, and 3.3 Ω cm2. The lower Rs values of the cells based on the composite CEs mean that the catalytic materials are more firmly attached to the FTO substrate which in turn promotes the electron collection from the external circuit in the QDSSC. Rct values of CoS, 10% Sr-CoS, 10% SrCoS/CNT, 10% Sr-CoS/GS and 10% Sr-CoS/GO are 60, 47, 36, 45 and 22 Ω cm2, respectively, indicating that compounds possess good catalytic activity. The R ct value for Ca-CoS/GO is much smaller than that of the bare CoS indicating that Ca-CoS/Go has excellent charge transfer at the interface of the electrolyte and CE. The EIS results show that carbon allotropes (CNT, GS and GO) are promising material for CEs in QDSSC because of their excellent conductivity that can decrease Rct. GS and GO have better intrinsic conductivity compared to CNT and using of them as CE in the QDSSC leads to lower R ct (as seen in Fig 11) and thereby higher photocurrent density. The observations of EIS and CV measurements conform well to the QDSSC performance of the CEs. In a word, metal doped CoS CEs display a higher catalytic activity toward the reduction of polysulfide electrolyte than the CoS CE, as revealed from photovoltaic and EIS data. Also, the power conversion efficiency of QDSSC using metal doped CoS could be further improved by optimizing value of doped metal ion. It seems very possible that by doping of metal ion in the CoS/carbon allotrope (GS, GO and CNT) composite films, a significant improvement in J sc is obtained compared to the metal doped CoS CEs. SEM, AFM and electrochemical results show that these composite films have a more porosity and conductivity compared to other CEs that can
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improve electrocatalytic activity surface and electron transport, corresponding to enhancement of photocurrent density and thereby QDSSC efficiency. By comparing the photovoltaic parameters of composite CEs, it is found that 10% Sr-CoS/GS and 10% Sr-CoS/GO CEs have higher Jsc compared to the 10% Sr-CoS/CNT that is due to more conductivity of them.
4. Conclusions In summary, this study was performed on counter electrode system in QDSSC to improving photovoltaic performance. Firstly, we doped different metal ions in the CoS CE and optimized the doped metal ion amount. After that, the metal ions-doped CoS/carbon allotrope CEs were prepared and their performance were investigated in the cells. The results showed that the doping of metal ions-doped in the CoS and CoS/carbon allotrope composites CEs leads to an enhancement in the Jsc and a decrease in the Rct. It is expected that the photovoltaic performance based on metal ions-doped CoS/carbon allotrope CEs will be better that metal ions-doped CoS CEs because of their higher porosity and thereby better catalytic activity. Also, GS and GO have better intrinsic conductivity compared to CNT and using of their composites as CE in the QDSSC leads to lower RCT and higher Jsc. Finally, a 76% higher power conversion efficiency was achieved by applying a 10% Sr-CoS/GO CE than the bare CoS CE, a 41% higher power conversion efficiency was observed by 10% Ba-CoS/GO CE. The findings exposes that incorporating M2+ in the CoS and utilizing of M-CoS nanocomposite with some carbon allotropes are as a simple and low cost method to improve the overall power conversion efficiency of QDSSC. As it is demonstrated herein, the findings indicate that this kind of electrodes are a promising generation of CEs, which deserve further investigation.
Acknowledgements
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The authors gratefully acknowledge from the University of Kashan for supporting this project by Grant No. 159183/30.
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Table legends: Table 1. Photovoltaic performance of QDSSCs based on CoS, Mg-CoS, Ca-CoS, Sr-CoS and Ba-CoS CEs under AM 1.5 illumination with an active area of 0.18 cm2. Table 2 Photovoltaic performance of QDSSCs based on different amounts of Ba 2+ and Sr2+ ions (0%, 5%, 10% and 20%) doped CoS CEs under AM 1.5 illumination with an active area of 0.18 cm2. Table 3 Photovoltaic performance of QDSSCs based on 10% Ba2+ and Sr2+-doped CoS/carbon allotrope CEs under AM 1.5 illumination with an active area of 0.18 cm2.
22
Figure captions: Fig. 1. FESEM images of CEs: (a) CoS, (b) 10% Sr-CoS, (c) 10% Sr-CoS/CNT, (d) 10% SrCoS/GS and (c) 10% Sr-CoS/GO thin films on the FTO. Fig. 2. The TEM images of 10% Sr-CoS/GO sample in different scales. Fig. 3. Cyclic voltammetry spectra of various counter electrodes measured in a three electrode configuration. Fig. 4. UV-Vis absorption spectra of CoS, 10% Sr-doped CoS and 10% Sr-doped CoS/GO thin films. Fig. 5. 3-Dimensional (3D) AFM images of the CEs: (a) CoS, (b) 10% Sr-CoS, (c) 10% SrCoS/CNT, (d) 10% Sr-CoS/GS and (c) 10% Sr-CoS/GO thin films on the FTO. Fig. 6. J-V curves of QDSSCs constructed with CoS, Ba-CoS, Mg-CoS, Ca-CoS, Sr-CoS CEs and tested under simulated AM1.5 solar irradiation (100 mW cm-2). Fig. 7. J-V curves of QDSSCs constructed with different amounts of (a) Ba2+ and (b) Sr2+ ions (0%, 5%, 10% and 20%) doped CoS CEs under simulated AM1.5 solar irradiation (100 mW cm-2). Fig. 8. J-V curves of QDSSCs constructed with different amounts of (a) 10% Ba2+ and (b) Sr2+doped CoS/carbon allotrope CEs under simulated AM1.5 solar irradiation (100 mW cm-2). Fig. 9. Nyquist plots of QDSSCs constructed with CoS, Ba-CoS, Mg-CoS, Ca-CoS, Sr-CoS CEs and tested under simulated AM1.5 solar irradiation (100 mW cm-2). Fig. 10. Nyquist plots of QDSSCs constructed with different amounts of Sr2+ ion (0%, 5%, 10% and 20%) doped CoS CEs under simulated AM1.5 solar irradiation (100 mW cm-2). Fig. 11. Nyquist plots of QDSSCs constructed with different amounts of 10% Sr 2+-doped CoS/carbon allotropes CEs under simulated AM1.5 solar irradiation (100 mW cm-2).
23
Tables: Table 1. Jsc [mA cm-2] 12.10
Voc [V] 0.46
0.29
η [%] 1.60
10% Mg-CoS
14.37
0.44
0.30
1.89
10% Ca-CoS
15.04
0.46
0.25
1.72
10% Sr-CoS
15.90
0.46
0.30
2.19
10% Ba-CoS
15.56
0.46
0.28
2.00
Counter electrode CoS
24
FF
Table 2. Counter electrode CoS
Jsc [mA cm-2] 12.10
Voc [V] 0.46
0.29
η [%] 1.60
FF
5% Sr-CoS
10.17
0.46
0.32
1.49
10% Sr-CoS
15.90
0.46
0.30
2.19
20% Sr-CoS
14.86
0.46
0.28
1.90
5% Ba-CoS
13.21
0.40
0.32
1.69
10% Ba-CoS
15.56
0.46
0.28
2.00
20% Ba-CoS
14.41
0.45
0.26
1.68
25
Table 3. Jsc [mA cm-2] 12.10
Voc [V] 0.46
0.29
η [%] 1.60
10% Sr-CoS
15.90
0.46
0.30
2.19
10%Sr-CoS/CNT
14.56
0.47
0.34
2.32
10% Sr-CoS/GS
16.14
0.47
0.31
2.35
10% Sr-CoS/GO
16.17
0.47
0.37
2.81
10% Ba-CoS
15.56
0.46
0.28
2.00
10% Ba-CoS/CNT
15.46
0.45
0.30
2.08
10% Ba-CoS/GS
17.35
0.45
0.28
2.11
10% Ba-CoS/GO
17.60
0.46
0.28
2.26
Counter electrode CoS
26
FF
Figures:
Fig. 1.
27
Fig. 2.
28
25000
Current density (µA)
20000 15000
10000 5000 0
CoS 10% Sr-CoS 10% Sr-CoS/CNT 10% Sr-CoS/GS 10% Sr-CoS/GO
-5000 -10000
-15000 -20000 -2
-1
0
1
Potential (V)
Fig. 3.
29
2
Absorbance
1.4
CoS
1.2
10% Sr-CoS 10% Sr-CoS/GO
1 0.8 0.6 0.4
0.2 0 350
450
550
Wavelength (nm)
Fig. 4.
30
650
750
Fig. 5.
31
Fig. 6.
32
Current density (mA cm-2)
18
(a)
CoS 5% Sr-CoS 10% Sr-CoS 20% Sr-CoS
16
14 12 10 8
6 4 2
0 0
0.1
0.2
0.3
Voltage (V)
Fig. 7.
33
0.4
0.5
Fig. 8.
34
35
CoS 10% Sr-CoS 10% Ba-CoS 10% Ca-CoS 10% Mg-CoS
-Z" (Ohm cm2)
30
25 20 15 10 5 0 0
20
40
Z' (Ohm cm2)
Fig. 9.
35
60
80
35
CoS 5% Sr-CoS 10% Sr-CoS 20% Sr-CoS
30
-Z" (Ohm cm2)
25 20 15 10
5 0 0
20
40
Z' (Ohm
60
cm2)
Fig. 10.
36
80
35
CoS 10% Sr-CoS 10% Sr-CoS/GS 10% Sr-CoS/GO 10% Sr-CoS/CNT
30
-Z" (Ohm cm2)
25 20 15 10 5 0 0
20
40
Z' (Ohm
60
cm2)
Fig. 11.
Graphical Abstract:
37
80
38