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Morphology controllable time-dependent CoS nanoparticle thin films as efficient counter electrode for quantum dot-sensitized solar cells
Accepted Manuscript Research paper Morphology controllable time-dependent CoSnanoparticle thin films as efficient counterelectrodeforquantumdot-sensitizedsolarcells Araveeti Eswar Reddy, S. Srinivas Rao, Chandu V.V.M. Gopi, Tarugu Anitha, Chebrolu Venkata Thulasi-Varma, Dinah Punnoose, Hee-Je Kim PII: DOI: Reference:
S0009-2614(17)30844-8 http://dx.doi.org/10.1016/j.cplett.2017.09.001 CPLETT 35081
To appear in:
Chemical Physics Letters
Received Date: Revised Date: Accepted Date:
5 July 2017 10 August 2017 4 September 2017
Please cite this article as: A.E. Reddy, S. Srinivas Rao, C.V.V. Gopi, T. Anitha, C.V. Thulasi-Varma, D. Punnoose, H-J. Kim, Morphology controllable time-dependent CoSnanoparticle thin films as efficient counterelectrodeforquantumdot-sensitizedsolarcells, Chemical Physics Letters (2017), doi: http://dx.doi.org/ 10.1016/j.cplett.2017.09.001
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Morphology controllable time-dependent CoS nanoparticle thin films as efficient counter electrode for quantum dot-sensitized solar cells Araveeti Eswar Reddy, S. Srinivas Rao, Chandu V.V.M. Gopi, Tarugu Anitha, Chebrolu Venkata Thulasi-Varma, Dinah Punnoose, Hee-Je Kim* School of Electrical Engineering, Pusan National University, Busandaehak-ro 63beon-gil, Geumjeong-gu, Busan, 46241, South Korea *Corresponding authors. Tel.: +8251 510 2364; fax: +82 51 513 0212 E-mail: [email protected] (Hee-Je Kim) Abstract Cobalt sulfide (CoS) agglomerated nanoparticle thin films obtained by a facile chemical bath method at different deposition times. The CoS counter electrode (CE) deposited at 3 h deposition time (CC-3h) based quantum dot sensitized solar cells (QDSSCs) achieves higher power conversion efficiency (η) of 3.67% than those of CC-2h (1.83%), CC-4h (2.52%), and Pt (1.48%) CEs, under one sun illumination (100 mW cm-2, AM 1.5 G). The electrochemical analysis revealed that CC-3h CE shows a smaller charge transfer resistance (9.22 Ω) at the CE/electrolyte interface than the CC-2h (23.34 Ω), CC-4h (19.73 Ω) and Pt (139.92 Ω) CEs, respectively. Keywords: Time-dependent; CoS; Nanoparticle; Electrocatalytic activity; QDSSC; Introduction Over the past two decades, dye-sensitized solar cells (DSSCs) have been studied extensively due to their facile deposition approach and relatively large efficiency. As an alternative, quantum dot sensitized solar cells (QDSSCs) have gained tremendous interest because of outstanding properties of quantum dots (QDs), such as tunable bandgap [1], possibility of hot electron injection [2], multiple exciton generation [3], high absorption
coefficient [4], and easy fabrication. A classic configuration of QDSSCs is assembled from three constituents: a QD sensitized photoanode, an electrolyte, and a counter electrode (CE). Several studies have focused on discovering effective methods to enhance the performance of QDSSCs, which are developing new kind of QD sensitizers [5,6], optimization and development of CEs, and modifications in electrolyte [7]. The most efficient and commonly used electrolyte in DSSCs is I3-/I-, but this I3-/I- redox electrolyte is unsuitable for QDSSCs due to corrosion and the photodegradation of QDs [8]. Thus far, the polysulfide redox electrolyte (S2-/Sn2-) is most suitable for QDSSCs over I3-/I- to diminish photocorrosion [9,10]. In addition, a conventional platinum (Pt) CE in DSSCs shows high catalytic activity with the I3-/I- redox electrolyte [11], and Pt is unsuitable with S2/Sn2- redox couple for QDSSCs due to the chemisorption with sulfur species on Pt surface [12]. In this regard, alternative Pt CE, several types of QDSSC CEs for the polysulfide electrolyte have been reported, such as CoS [13], Cu2S [14], PbS [15], Cu2ZnSnS4 [16], NiS [17], and carbonaceous materials [18,19]. Among them, Cu2S exhibits the best catalytic activity toward the reduction of the polysulfide electrolyte [20]. On the other hand, the Cu2S CE can poisoning the surface of photoanode and contaminate the polysulfide electrolyte, resulting in a gradual decrease in the performance of the QDSSC. However, cost-effective, environment friendly, facile preparation and high electrocatalytic activity of CEs are essential to further improve the performance of QDSSCs. Therefore, CoS not only a high natural abundance element but also translates to a low material cost and exhibits superior electrocatalytic activity [13]. As a result, this paper reports the cost-effective and facile preparation of highly catalytic CoS CE for QDSSCs. CoS CEs were fabricated on FTO substrate at various deposition durations (2, 3 and 4 h) using a simple chemical bath deposition (CBD) method. This time-dependent CoS electrodes exhibited the highest electrocatalytic activity compared to Pt CE. Based on electrochemical
impedance spectroscopy (EIS) and Tafel polarization techniques, the CC-3h counter electrode/electrolyte interface showed low charge-transfer resistance (Rct) and extraordinary catalytic ability for the reduction of Sn2-. As a result, the TiO2/CdS/CdSe/ZnS based CC-3h CE shows a higher PCE of 3.67% than the CC-2h (1.83%), CC-4h (2.52%), and Pt (1.48%) CEs. Experimental section 2.1 Materials Commercially available TiO2 paste (Ti-Nanoxide HT/SP), Cd(CH3COO)2.2H2O (cadmium acetate dihydrate), Zn(CH3COO)2.2H2O (zinc acetate dihydrate), Na2S (sodium sulfide), Se (selenium), S (sulfur), CoCl2.6H2O (cobalt chloride hexahydrate), C2H5NS (thioacetamide), and CH3COOH (acetic acid) were purchased from Sigma-Aldrich and used without further purification. 2.2 Preparation of the CoS and Pt CEs Firstly, FTO substrates were ultrasonically cleaned with acetone, ethanol and DI water for 15 min each. The CoS CEs were deposited by a simple CBD route at different time intervals. The precursor solution for depositing the CoS thin film was prepared by dissolving 0.1 M CoCl2.6H2O and 0.4 M thioacetamide in 100 ml of DI water, followed by the addition of 0.8 M acetic acid under magnetic stirring. FTO substrates were immersed horizontally in the growth solution and for different durations of 1, 2, 3 and 4 h at 90 oC in a hot air oven. After deposition, the coated substrates were cleaned several times with DI water, ethanol. There is no CoS film deposited at the time interval of 1 h and the CoS electrodes prepared at 2, 3 and 4 h are denoted as CC-2h, CC-3h and CC-4h, respectively. The preparation of CoS CE at different deposition times using CBD is shown in scheme 1. For comparison, Pt CE was prepared by depositing Pt paste (Pt-catalyst T/SP,
Solaronix) on FTO in the active area of ~ 0.7 cm2 using the doctor blade method and sintered at 450 oC in air for 10 min. 2.3 Preparation of TiO2/CdS/CdSe/ZnS photoanodes and assembly of QDSSCs and symmetric cells TiO2 coated electrodes were prepared using a commercial 20 nm TiO2 paste (Tinanoxide HT/SP, Solaronix) using the doctor blade method with an active area of 0.25 cm2, and were followed by annealing at 450 oC for 30 min in air. Subsequently, CdS, CdSe QDs, and ZnS passivation layer were prepared according to the methodology in a previous study by the successive ion layer adsorption and reaction (SILAR) method [15]. The QDSSCs were prepared by sandwiching a CEs (CoS and Pt) and TiO2/CdS/CdSe/ZnS photoelectrode using a 60 µm sealant (SX 1170-60, Solaronix) at 100 oC. The internal space between the electrodes was filled with a polysulfide electrolyte (2 M Na2S, 2 M S and 0.2 M KCl in methanol: water is 7:3). Symmetrical CoS and Pt electrodes (CoS/CoS and Pt/Pt) were sealed using a sealant at 100 oC and filled with a polysulfide electrolyte. 2.4 Characterization The crystalline structures of the electrodes was examined by X-ray diffraction (XRD, D/Max2400, Rigaku) using a Cu Kα source operated at 40 kV and 30 mA. The morphology and elemental composition of the samples were evaluated by scanning electron microscopy (SEM, S-2400, Hitachi) equipped with energy-dispersive X-ray spectroscopy (EDX). The chemical composition was investigated using X-ray photon spectroscopy (XPS, VG Scientific ESCALAB 250). EIS with a frequency ranging from 0.1 Hz to 500 kHz and Tafel polarization at a scan rate of 10 mV s -1 measurements were conducted on symmetrical cells using a BioLogic potentiostat/galvanostat/EIS analyzer (SP-150, France) under dark conditions. The current density-voltage (J-V) curves of QDSSCs were recorded using an
ABET Technologies (USA) solar simulator under one sun illumination (AM 1.5G, 100 mW cm-2). 3. Results and discussion Fig. 1 shows the low-magnification (a, b, c) and high-magnification (a1, b1, c1) SEM images of CoS electrode at different time intervals of 2h, 3h and 4 h, respectively. There was no CoS thin film deposition observed on the FTO substrate at a deposition time was less than 1 h. The SEM image of CC-2h in Fig.1a exhibits the nanoparticle morphology with sizes varying from 192 to 577 nm. When the deposition duration was increased from 2 to 3 h, the nanoparticles combined and formed larger particles with sizes between 230 and 654 nm. Upon further increases in growth time from 3 to 4 h, the sizes of the particles were between 239 and 730 nm. Based on this study, the size of the CoS particles gradually increased with increasing deposition time. The improved amount of CoS material covered on FTO surface would have a more catalytic activity, due to the reduction process is responsible for the injection of electrons from the CE to electrolyte. The SEM cross-section images in the inset of Fig. 1 (a1, b1, c1) showed that the thickness for CC-2h, CC-3h, and CC-4h were 454 nm, 590 nm, and 681 nm, respectively. It can be deduced from the surface morphologies that the CC-3h and CC-4h agglomerated nanoparticle thin films with particularly large surface can reduce the charge transfer resistance at the interface of CE/electrolyte and accelerate the diffusion of electrolyte, which are determining factors affecting the QDSSC performance. EDX analysis was conducted to determine the elemental composition in the CoS CE at different times, and the results are depicted in Fig.2. The data shows that the atomic percentage of Co:S was 56.28:43.72, 47.69:52.31, and 51.94:48.06 for the CC-2h, CC-3h, and CC-4h electrodes, respectively. Based on these results, the S percentage increased from 43.72 to 52.31 with increasing deposition time (2h to 3h). In the case of CC-4h, the S content was reduced slightly. The increase of S % in Co: S plays a crucial role in improving the CE
electrocatalytic activity. XPS is useful for identifying the oxidation states of elements. Fig.3a shows the complete survey spectrum of the CoS (CC-3h) CE films. The survey spectrum shows the elements S, C, N, Sn, O, and Co. The Co2p spectrum shown in Fig.3b has two peaks. A main peak at 779.44 eV for Co2p3/2 and another shoulder peak at 794.48 eV for Co2p1/2, indicating the presence of the Co+2 oxidation state [21]. The peaks at 161.92 eV and 166.29 eV in Fig.3c were assigned to S2p1/2 and S2p3/2, respectively. The S2p peak at 161-163 eV suggests that the S species exist as S-2 in the composite, corresponding to a binding energy Co-S [22]. Therefore, the EDX and XPS results suggest that the CoS had been deposited successfully onto the FTO substrate. EIS is used to investigate the charge transport processes at the interface of the CEelectrolyte [23]. Fig. 4 presents Nyquist plots obtained from various dummy cells (under dark condition) under bias potential of 0.6 V and the frequency of 0.1 Hz to 500 kHz and the corresponding amplitude was kept at 10 mV in all cases. The inset in Fig. 4 also shows the equivalent circuit and magnified plot for Pt CE. The plots were fitted with Z-view software using the equivalent circuit and results are summarized in Table1. The high-frequency intercept on the real axis indicates the series resistance (Rs), the left arc in the middlefrequency region represents the charge transfer resistance (Rct) with chemical capacitance (Cµ) and the small semicircle in the low frequency region denotes the Warburg diffusion impedance (Zw) of the electrolyte. The Rs values of the CC-2h, CC-3h and CC-4h electrodes were 9.42, 7.26, and 9.39 Ω, respectively, which are much smaller than the Pt (12.14 Ω) CE. The lowest Rs value of CC-3h CE could be attributed to the improved bonding strength between FTO and the CC-3h film, which promotes improved charge transfer mechanism at the CE-electrolyte interface [24]. The Rct value of the CC-3h CE (9.22 Ω) is much lower than that of the CC-2h (23.34 Ω), CC-4h (19.73 Ω), and Pt (139.92 Ω) CEs, demonstrating that CC-3h CE shows increased
electrocatalytic activity in the reduction of polysulfide electrolyte. The Cµ values of the CC2h, CC-3h, CC-4h, and Pt CEs were 159.61 µF, 387.19 µF, 296.75 µF, and 46.35 µF, respectively. The higher Cµ value is due to the higher surface area and better catalytic activity of CE [24]. The different morphologies of the CoS affect the Warburg diffusion coefficient of the electrodes. The agglomerated nanoparticles in the CC-3h CE is obviously beneficial for the diffusion of polysulfide redox couple, which leads to the lower Zw value. As a result, CC3h provides a lower Zw value of 2.41 Ω than the other CEs, which indicates that facile diffusion of the polysulfide electrolyte at the CC-3h-electrolyte interface. To further confirm the electrocatalytic activity of the CoS CEs, Tafel polarization analysis was conducted using dummy cells under dark conditions. Fig. 5 shows the Tafel curves of the CoS and Pt CEs as a function of logarithmic current density (log J) vs. potential (V). The exchange current density (J0) can be derived from the intercept of the Tangent line in of the Tafel zone when the overpotential is zero. In Tafel curve, the slope of CC-3h CE is larger than that of Pt, CC-2h, and CC-4h CEs, suggesting CC-3h show higher J0. J0 can also be calculated using Eq (2) [25];
where Rct is the charge transfer resistance derived from the EIS measurement, R is the gas constant, T is the temperature, n is the number of electrons participated in the reduction of disulfide at the CE and F is Faraday’s constant. The relationship between the two parameters Jo and Rct were inversely proportional to each other. Furthermore, the limiting diffusion current density (Jim) can be extracted from the diffusion zone of the Tafel plot. Jlim depends on the diffusion coefficient (D) of the polysulfide redox couple, which can be related by Eq. (3):
where C is concentration of electrolyte, l is thickness of spacer, n, F and D have their usual meanings. As shown in Fig.5, the Jlim value follows the order Pt < CC-2h < CC-4h < CC-3h, yielding higher diffusion velocity of CC-3h CE in polysulfide electrolyte. The decrease of Rct in the EIS measurements and higher Jo and Jlim values from Tafel plot provided clear evidence that CC-3h CE shows the best electrocatalytic activity. Fig. 6 shows the J-V plots for the QDSSCs based on CoS CEs at 100 mW cm-2 light intensity and the corresponding photovoltaic parameters were listed in Table 1. The photovoltaic performance of the QDSSC based CC-3h was superior to the devices based on the CC-2h, CC-4h, and Pt CEs. The Voc, Jsc, FF and η values for the Pt CE were 0.507 V, 10.24 mA cm-2, 0.285 and 1.48%, respectively. The open-circuit voltage (Voc) of the QDSSC was not affected significantly by the CE. On the other hand, the fill factor (FF) of the QDSSCs with CC-3h (0.526) was much higher than that of CC-2h (0.301) and CC-4h (0.436). Consequently, the power conversion efficiency of the QDSSCs with CC-3h CE (3.67%) was much higher than that with the CC-2h (1.83%) and CC-4h (2.52%) CEs, which was attributed to the higher electrocatalytic ability of CC-3h in the reduction of polysulfide electrolytes used in QDSSCs. The electrocatalytic ability of CC-3h was discussed in the Tafel polarization EIS sections above. These two experiment results fully support the increased power conversion efficiency of CC-3h CE in the J-V plots. The electron recombination process of CC-2h, CC-3h, CC-4h and Pt CEs in QDSSCs were studied by open-circuit voltage decay (OCVD) technique (Fig. 7), which provides the information about electron life time (τe). It was noticed that QDSSC based CC-3h CE decays more slowly than that of the devices with CC-2h, CC-4h, and Pt CEs, suggesting that CC-3h has a larger τe. The longer electron lifetime and slower Voc decay rate of the QDSSCs based on the CC-3h CE productively collect electrons from TiO2 and transfer them to the redox
couple, which enhances the performance of the QDSSCs. The Voc decay rate is and τe are related to the according to following Eq (3) [26]:
where KB is the Boltzmann constant, T is the absolute temperature and e is the positive elementary charge. 4. Conclusions This paper reported the fabrication of highly electrocatalytic active CoS CEs for high performance QDSSCs by chemical bath deposition for different durations (2, 3, and 4 hrs). SEM results exhibited that the CoS particles sizes increased gradually with growth durations. Interestingly, the growth time affects surface morphology, thickness and the atomic percentage of Co:S, which are relatively affected the performance of QDSSCs. Impressively, the highest conversion efficiency of 3.67% is achieved by the QDSSCs based on the CC-3h CE, much higher than the CC-2h (1.83 %), CC-4h (2.52 %) and Pt (1.48 %) CEs. In EIS studies of symmetrical cells, CC-3h CE exhibited a lower Rct of 9.22 Ω for the polysulfide redox couple than the CC-2h (23.34 Ω), CC-4h (19.73 Ω), and Pt (139.92 Ω) CEs. Therefore, improved agglomerated nanoparticle surface morphology and higher electrocatalytic activity of CC-3h CE is an effective and low-cost CE for QDSSCs. These results suggest that the CoS CE can be an alternative to Pt CEs. Acknowledgements This research was supported by the Basic Research Laboratory through the National Research Foundation of Korea funded by the Ministry of Science, ICT and Future Planning (NRF2016K2A9A2A08003717). References: [1] W. Yu, L. Qu, W. Guo, X. Peng, Chem. Mater. 15 (2003) 2854-2860.
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Table Captions Table 1 Performance of the CoS and Pt CE based QDSSCs and their respective symmetrical half-cell impedance data.
Figure Captions Scheme. 1 Schematic of the CoS CE manufacturing process at different deposition times using CBD method. Fig. 1 FE-SEM micrographs of CoS thin films based on various deposition times (2, 3, and 4 h) on an FTO substrate: low magnification ((a) CC-2h, (b) CC-3h, and (c) CC-4h ) and high magnification (a1) CC-2h, (b1) CC-3h, and (c1) CC-4h )) FE-SEM images. The inset of (a1, b1 and C1) shows the cross-sectional images of CoS. Fig. 2 EDX analysis of (a) CC-2h, (b) CC-3h, and (c) CC-4h on the FTO surface. Fig. 3 XPS analysis of CC-3h electrode (a) survey spectra; high-resolution spectra of (b) S2p and (c) Co2p. Fig. 4 Nyquist plots for the symmetric cell configuration consisting of two identical (a) CC2h, (b) CC-3h, (c) CC-4h and (d) Pt CEs. The inset shows the magnified plot for the Pt CE and the equivalent circuit to fit Nyquist plots. Fig. 5 Tafel polarization plots of CoS and Pt CEs at a scan rate of 10 mV s -1. Fig. 6 J–V curves of QDSSCs assembled with CoS or Pt CEs under one sun illumination (100 mW cm-2). Fig. 7 Open-circuit voltage decay (OCVD) characteristics of the CoS and Pt CEs.
Figures
Scheme 1
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
Fig. 7
Table 1 Performance of the CoS and Pt CE based QDSSCs and their respective symmetrical half-cell impedance data. CE
Voc
Jsc
FF
(V)
(mA cm-2)
CC-2h
0.580
10.45
0.301
CC-3h
0.599
11.62
CC-4h
0.592
Pt
0.507
η%
Rs
Rct
Cμ
Zw
(Ω)
(Ω)
(µm)
(Ω)
1.83
9.42
23.34
159.61
6.25
0.526
3.67
7.26
9.22
387.19
2.41
10.72
0.436
2.52
9.39
19.73
296.75
6.18
10.24
0.285
1.48
12.14
139.92
46.35
71.66
Highlights CoS thin film is deposited on FTO glass by a facile chemical bath deposition method. The surface of the CoS films exhibit uniform nanoparticle morphology. Morphology and electrocatalytic activity of CoS CE are varied with deposition time. The power conversion efficiency of 3.67% with CoS 3 h CE based QDSSCs.
S0009-2614(17)30844-8 http://dx.doi.org/10.1016/j.cplett.2017.09.001 CPLETT 35081
To appear in:
Chemical Physics Letters
Received Date: Revised Date: Accepted Date:
5 July 2017 10 August 2017 4 September 2017
Please cite this article as: A.E. Reddy, S. Srinivas Rao, C.V.V. Gopi, T. Anitha, C.V. Thulasi-Varma, D. Punnoose, H-J. Kim, Morphology controllable time-dependent CoSnanoparticle thin films as efficient counterelectrodeforquantumdot-sensitizedsolarcells, Chemical Physics Letters (2017), doi: http://dx.doi.org/ 10.1016/j.cplett.2017.09.001
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Morphology controllable time-dependent CoS nanoparticle thin films as efficient counter electrode for quantum dot-sensitized solar cells Araveeti Eswar Reddy, S. Srinivas Rao, Chandu V.V.M. Gopi, Tarugu Anitha, Chebrolu Venkata Thulasi-Varma, Dinah Punnoose, Hee-Je Kim* School of Electrical Engineering, Pusan National University, Busandaehak-ro 63beon-gil, Geumjeong-gu, Busan, 46241, South Korea *Corresponding authors. Tel.: +8251 510 2364; fax: +82 51 513 0212 E-mail: [email protected] (Hee-Je Kim) Abstract Cobalt sulfide (CoS) agglomerated nanoparticle thin films obtained by a facile chemical bath method at different deposition times. The CoS counter electrode (CE) deposited at 3 h deposition time (CC-3h) based quantum dot sensitized solar cells (QDSSCs) achieves higher power conversion efficiency (η) of 3.67% than those of CC-2h (1.83%), CC-4h (2.52%), and Pt (1.48%) CEs, under one sun illumination (100 mW cm-2, AM 1.5 G). The electrochemical analysis revealed that CC-3h CE shows a smaller charge transfer resistance (9.22 Ω) at the CE/electrolyte interface than the CC-2h (23.34 Ω), CC-4h (19.73 Ω) and Pt (139.92 Ω) CEs, respectively. Keywords: Time-dependent; CoS; Nanoparticle; Electrocatalytic activity; QDSSC; Introduction Over the past two decades, dye-sensitized solar cells (DSSCs) have been studied extensively due to their facile deposition approach and relatively large efficiency. As an alternative, quantum dot sensitized solar cells (QDSSCs) have gained tremendous interest because of outstanding properties of quantum dots (QDs), such as tunable bandgap [1], possibility of hot electron injection [2], multiple exciton generation [3], high absorption
coefficient [4], and easy fabrication. A classic configuration of QDSSCs is assembled from three constituents: a QD sensitized photoanode, an electrolyte, and a counter electrode (CE). Several studies have focused on discovering effective methods to enhance the performance of QDSSCs, which are developing new kind of QD sensitizers [5,6], optimization and development of CEs, and modifications in electrolyte [7]. The most efficient and commonly used electrolyte in DSSCs is I3-/I-, but this I3-/I- redox electrolyte is unsuitable for QDSSCs due to corrosion and the photodegradation of QDs [8]. Thus far, the polysulfide redox electrolyte (S2-/Sn2-) is most suitable for QDSSCs over I3-/I- to diminish photocorrosion [9,10]. In addition, a conventional platinum (Pt) CE in DSSCs shows high catalytic activity with the I3-/I- redox electrolyte [11], and Pt is unsuitable with S2/Sn2- redox couple for QDSSCs due to the chemisorption with sulfur species on Pt surface [12]. In this regard, alternative Pt CE, several types of QDSSC CEs for the polysulfide electrolyte have been reported, such as CoS [13], Cu2S [14], PbS [15], Cu2ZnSnS4 [16], NiS [17], and carbonaceous materials [18,19]. Among them, Cu2S exhibits the best catalytic activity toward the reduction of the polysulfide electrolyte [20]. On the other hand, the Cu2S CE can poisoning the surface of photoanode and contaminate the polysulfide electrolyte, resulting in a gradual decrease in the performance of the QDSSC. However, cost-effective, environment friendly, facile preparation and high electrocatalytic activity of CEs are essential to further improve the performance of QDSSCs. Therefore, CoS not only a high natural abundance element but also translates to a low material cost and exhibits superior electrocatalytic activity [13]. As a result, this paper reports the cost-effective and facile preparation of highly catalytic CoS CE for QDSSCs. CoS CEs were fabricated on FTO substrate at various deposition durations (2, 3 and 4 h) using a simple chemical bath deposition (CBD) method. This time-dependent CoS electrodes exhibited the highest electrocatalytic activity compared to Pt CE. Based on electrochemical
impedance spectroscopy (EIS) and Tafel polarization techniques, the CC-3h counter electrode/electrolyte interface showed low charge-transfer resistance (Rct) and extraordinary catalytic ability for the reduction of Sn2-. As a result, the TiO2/CdS/CdSe/ZnS based CC-3h CE shows a higher PCE of 3.67% than the CC-2h (1.83%), CC-4h (2.52%), and Pt (1.48%) CEs. Experimental section 2.1 Materials Commercially available TiO2 paste (Ti-Nanoxide HT/SP), Cd(CH3COO)2.2H2O (cadmium acetate dihydrate), Zn(CH3COO)2.2H2O (zinc acetate dihydrate), Na2S (sodium sulfide), Se (selenium), S (sulfur), CoCl2.6H2O (cobalt chloride hexahydrate), C2H5NS (thioacetamide), and CH3COOH (acetic acid) were purchased from Sigma-Aldrich and used without further purification. 2.2 Preparation of the CoS and Pt CEs Firstly, FTO substrates were ultrasonically cleaned with acetone, ethanol and DI water for 15 min each. The CoS CEs were deposited by a simple CBD route at different time intervals. The precursor solution for depositing the CoS thin film was prepared by dissolving 0.1 M CoCl2.6H2O and 0.4 M thioacetamide in 100 ml of DI water, followed by the addition of 0.8 M acetic acid under magnetic stirring. FTO substrates were immersed horizontally in the growth solution and for different durations of 1, 2, 3 and 4 h at 90 oC in a hot air oven. After deposition, the coated substrates were cleaned several times with DI water, ethanol. There is no CoS film deposited at the time interval of 1 h and the CoS electrodes prepared at 2, 3 and 4 h are denoted as CC-2h, CC-3h and CC-4h, respectively. The preparation of CoS CE at different deposition times using CBD is shown in scheme 1. For comparison, Pt CE was prepared by depositing Pt paste (Pt-catalyst T/SP,
Solaronix) on FTO in the active area of ~ 0.7 cm2 using the doctor blade method and sintered at 450 oC in air for 10 min. 2.3 Preparation of TiO2/CdS/CdSe/ZnS photoanodes and assembly of QDSSCs and symmetric cells TiO2 coated electrodes were prepared using a commercial 20 nm TiO2 paste (Tinanoxide HT/SP, Solaronix) using the doctor blade method with an active area of 0.25 cm2, and were followed by annealing at 450 oC for 30 min in air. Subsequently, CdS, CdSe QDs, and ZnS passivation layer were prepared according to the methodology in a previous study by the successive ion layer adsorption and reaction (SILAR) method [15]. The QDSSCs were prepared by sandwiching a CEs (CoS and Pt) and TiO2/CdS/CdSe/ZnS photoelectrode using a 60 µm sealant (SX 1170-60, Solaronix) at 100 oC. The internal space between the electrodes was filled with a polysulfide electrolyte (2 M Na2S, 2 M S and 0.2 M KCl in methanol: water is 7:3). Symmetrical CoS and Pt electrodes (CoS/CoS and Pt/Pt) were sealed using a sealant at 100 oC and filled with a polysulfide electrolyte. 2.4 Characterization The crystalline structures of the electrodes was examined by X-ray diffraction (XRD, D/Max2400, Rigaku) using a Cu Kα source operated at 40 kV and 30 mA. The morphology and elemental composition of the samples were evaluated by scanning electron microscopy (SEM, S-2400, Hitachi) equipped with energy-dispersive X-ray spectroscopy (EDX). The chemical composition was investigated using X-ray photon spectroscopy (XPS, VG Scientific ESCALAB 250). EIS with a frequency ranging from 0.1 Hz to 500 kHz and Tafel polarization at a scan rate of 10 mV s -1 measurements were conducted on symmetrical cells using a BioLogic potentiostat/galvanostat/EIS analyzer (SP-150, France) under dark conditions. The current density-voltage (J-V) curves of QDSSCs were recorded using an
ABET Technologies (USA) solar simulator under one sun illumination (AM 1.5G, 100 mW cm-2). 3. Results and discussion Fig. 1 shows the low-magnification (a, b, c) and high-magnification (a1, b1, c1) SEM images of CoS electrode at different time intervals of 2h, 3h and 4 h, respectively. There was no CoS thin film deposition observed on the FTO substrate at a deposition time was less than 1 h. The SEM image of CC-2h in Fig.1a exhibits the nanoparticle morphology with sizes varying from 192 to 577 nm. When the deposition duration was increased from 2 to 3 h, the nanoparticles combined and formed larger particles with sizes between 230 and 654 nm. Upon further increases in growth time from 3 to 4 h, the sizes of the particles were between 239 and 730 nm. Based on this study, the size of the CoS particles gradually increased with increasing deposition time. The improved amount of CoS material covered on FTO surface would have a more catalytic activity, due to the reduction process is responsible for the injection of electrons from the CE to electrolyte. The SEM cross-section images in the inset of Fig. 1 (a1, b1, c1) showed that the thickness for CC-2h, CC-3h, and CC-4h were 454 nm, 590 nm, and 681 nm, respectively. It can be deduced from the surface morphologies that the CC-3h and CC-4h agglomerated nanoparticle thin films with particularly large surface can reduce the charge transfer resistance at the interface of CE/electrolyte and accelerate the diffusion of electrolyte, which are determining factors affecting the QDSSC performance. EDX analysis was conducted to determine the elemental composition in the CoS CE at different times, and the results are depicted in Fig.2. The data shows that the atomic percentage of Co:S was 56.28:43.72, 47.69:52.31, and 51.94:48.06 for the CC-2h, CC-3h, and CC-4h electrodes, respectively. Based on these results, the S percentage increased from 43.72 to 52.31 with increasing deposition time (2h to 3h). In the case of CC-4h, the S content was reduced slightly. The increase of S % in Co: S plays a crucial role in improving the CE
electrocatalytic activity. XPS is useful for identifying the oxidation states of elements. Fig.3a shows the complete survey spectrum of the CoS (CC-3h) CE films. The survey spectrum shows the elements S, C, N, Sn, O, and Co. The Co2p spectrum shown in Fig.3b has two peaks. A main peak at 779.44 eV for Co2p3/2 and another shoulder peak at 794.48 eV for Co2p1/2, indicating the presence of the Co+2 oxidation state [21]. The peaks at 161.92 eV and 166.29 eV in Fig.3c were assigned to S2p1/2 and S2p3/2, respectively. The S2p peak at 161-163 eV suggests that the S species exist as S-2 in the composite, corresponding to a binding energy Co-S [22]. Therefore, the EDX and XPS results suggest that the CoS had been deposited successfully onto the FTO substrate. EIS is used to investigate the charge transport processes at the interface of the CEelectrolyte [23]. Fig. 4 presents Nyquist plots obtained from various dummy cells (under dark condition) under bias potential of 0.6 V and the frequency of 0.1 Hz to 500 kHz and the corresponding amplitude was kept at 10 mV in all cases. The inset in Fig. 4 also shows the equivalent circuit and magnified plot for Pt CE. The plots were fitted with Z-view software using the equivalent circuit and results are summarized in Table1. The high-frequency intercept on the real axis indicates the series resistance (Rs), the left arc in the middlefrequency region represents the charge transfer resistance (Rct) with chemical capacitance (Cµ) and the small semicircle in the low frequency region denotes the Warburg diffusion impedance (Zw) of the electrolyte. The Rs values of the CC-2h, CC-3h and CC-4h electrodes were 9.42, 7.26, and 9.39 Ω, respectively, which are much smaller than the Pt (12.14 Ω) CE. The lowest Rs value of CC-3h CE could be attributed to the improved bonding strength between FTO and the CC-3h film, which promotes improved charge transfer mechanism at the CE-electrolyte interface [24]. The Rct value of the CC-3h CE (9.22 Ω) is much lower than that of the CC-2h (23.34 Ω), CC-4h (19.73 Ω), and Pt (139.92 Ω) CEs, demonstrating that CC-3h CE shows increased
electrocatalytic activity in the reduction of polysulfide electrolyte. The Cµ values of the CC2h, CC-3h, CC-4h, and Pt CEs were 159.61 µF, 387.19 µF, 296.75 µF, and 46.35 µF, respectively. The higher Cµ value is due to the higher surface area and better catalytic activity of CE [24]. The different morphologies of the CoS affect the Warburg diffusion coefficient of the electrodes. The agglomerated nanoparticles in the CC-3h CE is obviously beneficial for the diffusion of polysulfide redox couple, which leads to the lower Zw value. As a result, CC3h provides a lower Zw value of 2.41 Ω than the other CEs, which indicates that facile diffusion of the polysulfide electrolyte at the CC-3h-electrolyte interface. To further confirm the electrocatalytic activity of the CoS CEs, Tafel polarization analysis was conducted using dummy cells under dark conditions. Fig. 5 shows the Tafel curves of the CoS and Pt CEs as a function of logarithmic current density (log J) vs. potential (V). The exchange current density (J0) can be derived from the intercept of the Tangent line in of the Tafel zone when the overpotential is zero. In Tafel curve, the slope of CC-3h CE is larger than that of Pt, CC-2h, and CC-4h CEs, suggesting CC-3h show higher J0. J0 can also be calculated using Eq (2) [25];
where Rct is the charge transfer resistance derived from the EIS measurement, R is the gas constant, T is the temperature, n is the number of electrons participated in the reduction of disulfide at the CE and F is Faraday’s constant. The relationship between the two parameters Jo and Rct were inversely proportional to each other. Furthermore, the limiting diffusion current density (Jim) can be extracted from the diffusion zone of the Tafel plot. Jlim depends on the diffusion coefficient (D) of the polysulfide redox couple, which can be related by Eq. (3):
where C is concentration of electrolyte, l is thickness of spacer, n, F and D have their usual meanings. As shown in Fig.5, the Jlim value follows the order Pt < CC-2h < CC-4h < CC-3h, yielding higher diffusion velocity of CC-3h CE in polysulfide electrolyte. The decrease of Rct in the EIS measurements and higher Jo and Jlim values from Tafel plot provided clear evidence that CC-3h CE shows the best electrocatalytic activity. Fig. 6 shows the J-V plots for the QDSSCs based on CoS CEs at 100 mW cm-2 light intensity and the corresponding photovoltaic parameters were listed in Table 1. The photovoltaic performance of the QDSSC based CC-3h was superior to the devices based on the CC-2h, CC-4h, and Pt CEs. The Voc, Jsc, FF and η values for the Pt CE were 0.507 V, 10.24 mA cm-2, 0.285 and 1.48%, respectively. The open-circuit voltage (Voc) of the QDSSC was not affected significantly by the CE. On the other hand, the fill factor (FF) of the QDSSCs with CC-3h (0.526) was much higher than that of CC-2h (0.301) and CC-4h (0.436). Consequently, the power conversion efficiency of the QDSSCs with CC-3h CE (3.67%) was much higher than that with the CC-2h (1.83%) and CC-4h (2.52%) CEs, which was attributed to the higher electrocatalytic ability of CC-3h in the reduction of polysulfide electrolytes used in QDSSCs. The electrocatalytic ability of CC-3h was discussed in the Tafel polarization EIS sections above. These two experiment results fully support the increased power conversion efficiency of CC-3h CE in the J-V plots. The electron recombination process of CC-2h, CC-3h, CC-4h and Pt CEs in QDSSCs were studied by open-circuit voltage decay (OCVD) technique (Fig. 7), which provides the information about electron life time (τe). It was noticed that QDSSC based CC-3h CE decays more slowly than that of the devices with CC-2h, CC-4h, and Pt CEs, suggesting that CC-3h has a larger τe. The longer electron lifetime and slower Voc decay rate of the QDSSCs based on the CC-3h CE productively collect electrons from TiO2 and transfer them to the redox
couple, which enhances the performance of the QDSSCs. The Voc decay rate is and τe are related to the according to following Eq (3) [26]:
where KB is the Boltzmann constant, T is the absolute temperature and e is the positive elementary charge. 4. Conclusions This paper reported the fabrication of highly electrocatalytic active CoS CEs for high performance QDSSCs by chemical bath deposition for different durations (2, 3, and 4 hrs). SEM results exhibited that the CoS particles sizes increased gradually with growth durations. Interestingly, the growth time affects surface morphology, thickness and the atomic percentage of Co:S, which are relatively affected the performance of QDSSCs. Impressively, the highest conversion efficiency of 3.67% is achieved by the QDSSCs based on the CC-3h CE, much higher than the CC-2h (1.83 %), CC-4h (2.52 %) and Pt (1.48 %) CEs. In EIS studies of symmetrical cells, CC-3h CE exhibited a lower Rct of 9.22 Ω for the polysulfide redox couple than the CC-2h (23.34 Ω), CC-4h (19.73 Ω), and Pt (139.92 Ω) CEs. Therefore, improved agglomerated nanoparticle surface morphology and higher electrocatalytic activity of CC-3h CE is an effective and low-cost CE for QDSSCs. These results suggest that the CoS CE can be an alternative to Pt CEs. Acknowledgements This research was supported by the Basic Research Laboratory through the National Research Foundation of Korea funded by the Ministry of Science, ICT and Future Planning (NRF2016K2A9A2A08003717). References: [1] W. Yu, L. Qu, W. Guo, X. Peng, Chem. Mater. 15 (2003) 2854-2860.
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Table Captions Table 1 Performance of the CoS and Pt CE based QDSSCs and their respective symmetrical half-cell impedance data.
Figure Captions Scheme. 1 Schematic of the CoS CE manufacturing process at different deposition times using CBD method. Fig. 1 FE-SEM micrographs of CoS thin films based on various deposition times (2, 3, and 4 h) on an FTO substrate: low magnification ((a) CC-2h, (b) CC-3h, and (c) CC-4h ) and high magnification (a1) CC-2h, (b1) CC-3h, and (c1) CC-4h )) FE-SEM images. The inset of (a1, b1 and C1) shows the cross-sectional images of CoS. Fig. 2 EDX analysis of (a) CC-2h, (b) CC-3h, and (c) CC-4h on the FTO surface. Fig. 3 XPS analysis of CC-3h electrode (a) survey spectra; high-resolution spectra of (b) S2p and (c) Co2p. Fig. 4 Nyquist plots for the symmetric cell configuration consisting of two identical (a) CC2h, (b) CC-3h, (c) CC-4h and (d) Pt CEs. The inset shows the magnified plot for the Pt CE and the equivalent circuit to fit Nyquist plots. Fig. 5 Tafel polarization plots of CoS and Pt CEs at a scan rate of 10 mV s -1. Fig. 6 J–V curves of QDSSCs assembled with CoS or Pt CEs under one sun illumination (100 mW cm-2). Fig. 7 Open-circuit voltage decay (OCVD) characteristics of the CoS and Pt CEs.
Figures
Scheme 1
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
Fig. 7
Table 1 Performance of the CoS and Pt CE based QDSSCs and their respective symmetrical half-cell impedance data. CE
Voc
Jsc
FF
(V)
(mA cm-2)
CC-2h
0.580
10.45
0.301
CC-3h
0.599
11.62
CC-4h
0.592
Pt
0.507
η%
Rs
Rct
Cμ
Zw
(Ω)
(Ω)
(µm)
(Ω)
1.83
9.42
23.34
159.61
6.25
0.526
3.67
7.26
9.22
387.19
2.41
10.72
0.436
2.52
9.39
19.73
296.75
6.18
10.24
0.285
1.48
12.14
139.92
46.35
71.66
Highlights CoS thin film is deposited on FTO glass by a facile chemical bath deposition method. The surface of the CoS films exhibit uniform nanoparticle morphology. Morphology and electrocatalytic activity of CoS CE are varied with deposition time. The power conversion efficiency of 3.67% with CoS 3 h CE based QDSSCs.