One-step synthesized CuS and MWCNTs composite as a highly efficient counter electrode for quantum dot sensitized solar cells

Materials and Design 160 (2018) 870–875

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Materials and Design journal homepage: www.elsevier.com/locate/matdes

One-step synthesized CuS and MWCNTs composite as a highly efficient counter electrode for quantum dot sensitized solar cells Yinan Zhang a, Di Wang a, Qiming Wang a, Wei Zheng a,b,⁎ a b

School of Material Science and Engineering, Harbin University of Science and Technology, Harbin 150040, China Centre énergie Matériaux et Télécommunications, Institut National de la Recherche Scientifique, QC J3X1S2, Canada

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• CuS/MWCNTs composite in treelike structure is prepared in solutions with one-step method. CuS nanoparticles are grown on MWCNTs branches in tree-like structure. • 100% CuS/MWCNTs counter electrode possesses high photovoltaic performance (Voc = 0.618 V, Jsc = 18.680 mA/cm2, FF = 0.455, η = 5.254 %). Its power conversion efficiency decreases only 3 %(from 5.254 % to 5.086 %) after 24 h illumination.

a r t i c l e

i n f o

Article history: Received 6 June 2018 Received in revised form 13 October 2018 Accepted 15 October 2018 Available online 17 October 2018 Keywords: CuS/MWCNT CEs QDSCs PCE Electrocatalytic activity Photovoltaic stability

a b s t r a c t CuS/MWCNTs counter electrode (CE) in high efficiency and stability is designed that CuS nanoparticles are grown on multi-wall carbon nanotubes (MWCNTs) in solutions to form CuS and MWCNTs composite in treelike structure that dispersed MWCNTs as branches support CuS nano-particles. The quantum dot sensitized solar cells (QDSCs) are assembled with CuS/MWCNT CE prepared with above composite, CdS and ZnS sensitized TiO2/ RGO (reduced graphene oxide) photoanode and polysulfide electrolyte. The electrocatalytic activity of CEs can be analyzed through Nyquist curves and Tafel curves and typical photovoltaic parameters of QDSCs based on different CEs are obtained from J-V curves. CuS nanoparticles aggregate more severely with increase of CuS mass percentage according to TEM images and 100% CuS/MWCNTs CE within all samples exhibits the highest electrocatalytic activity and the power conversion efficiency (PCE) of QDSCs. Besides that, CuS/MWCNT CE exhibits the best photovoltaic stability. PCE of QDSCs with 100% CuS/MWCNTs CE decreases only 3%(from 5.254% to 5.086%) after 24 h illumination. © 2018 Elsevier Ltd. All rights reserved.

1. Introduction

⁎ Corresponding author at: School of Material Science and Engineering, Harbin University of Science and Technology, Harbin 150040, China. E-mail address: [email protected] (W. Zheng).

https://doi.org/10.1016/j.matdes.2018.10.022 0264-1275/© 2018 Elsevier Ltd. All rights reserved.

The quantum dot sensitized solar cells (QDSCs) as candidate for solar cells have received a lot of attention lately due to low cost and simple preparation process. The QDSCs possess similar sandwich structure with dye sensitized solar cells (DSSCs) [1–5]. Dye of DSSCs is displaced by inorganic semiconductor quantum dots (QDs) of QDSCs due to

Y. Zhang et al. / Materials and Design 160 (2018) 870–875

outstanding optical and electrical properties of QDs, such as extremely high molar extinction coefficient [6], tunable size-dependent bandgap [7], large intrinsic dipole moment [8] and a splendid feature of multiple exciton generation (MEG) [9,10]. The counter electrode (CE) can exert great influence in QDSCs, which collects the electrons from the external circuit and realizes regeneration of electrolytes. At present, one of the most common CE materials is the precious metal Pt which is reactive to electrolytes leading to a series of consequences, for example, low stability in cell applications [11–13]. Hence, many new materials are studied to replace precious metals Pt. Carbon materials are promising materials to take the place of precious metals Pt due to lower cost, good electrical conductivity and high chemical stability [14,15]. Sahasrabudhe et al. reported the application of graphitic porous carbon CE as a commercial charcoal CE in QDSCs [16]. Multi-wall carbon nanotubes (MWCNTs) is one of candidate CE materials because of its high specific surface area and rapid electron transfer nature [17,18]. Memon et al. reported the short circuit current density of DSSC with MWCNTs CE was more than that of the conventional Pt-based DSSC [19]. MWCNTs-based composites CE also exhibited excellent photoelectric property through introducing Pt [20], polymer [21–23] and carbon materials [24–26] into MWCNTs. In recent years, besides carbon materials, metal sulfides are often used as CE material of QDSCs [27–31]. Especially, CuS exhibits excellent electrocatalytic activity in polysulfide electrolyte systems [32–34]. Ghosh et al. revealed the Cu1.18S-GOR CE can decrease the loss of charge carriers, leading to a higher power conversion efficiency (PCE) [35]. In this work CuS nanoparticles and MWCNTs are composited in onestep synthesized way. This CuS/MWCNTs composite CE in tree-like structure is designed that MWCNTs serve as framework to support CuS nano-particles in advantage of their individual characteristics which are beneficial to the photovoltaic performance and stability of QDSCs. CuS clusters with high electrocatalytic activity as active sites were grown on the MWCNT branches in three dimensional porous system in favor of penetration of more electrolytes and higher photovoltaic performance of QDSCs. 2. Experimental 2.1. The preparation of CEs The acid-treated MWCNTs were dispersed into 5 mL 0.01 M Cu (CH3COO)2 ethanol in ultrasonic vibration under strong agitation. The 4.16 mL 0.01 M Na2S dilute solution was added into the above suspension with a drop every 2 s. After separating, cleaning and drying, the CuS and MWCNTs composite powder can be obtained. Four CuS/ MWCNTs powders were prepared with varying the mass percentage of CuS to MWCNTs (0, 50, 100 and 200 wt%). 0.1 g composite powder was introduced into 50 mL anhydrous ethanol with dispersant and adhesive, which was deposited on the FTO (Fluorine-doped Tin Oxide) conductive glass (200 × 150 mm, thickness of 2.2 mm, resistance of 14 Ω, transmittance of 90%) and then was sintered at 400 °C for 2 h in nitrogen atmosphere to obtain four CEs (MWCNTs, 50% CuS/MWCNTs, 100% CuS/MWCNTs, 200% CuS/MWCNTs). The Pt CE was prepared with magnetron sputtering on the FTO conductive glass (Shanghai Xingdian Co, China).

in TiO2 photoanode film, RGO was doped into TiO2 matrix because of its excellent electrical property and unique structural characteristic [37]. CdS and ZnS sensitized TiO2/RGO photoanode was prepared with 0.1 M Na2S·9H2O methanol, 0.1 M Cd(NO3)2·4H2O ethanol and 0.1 M Zn(CH3COO)2 ethanol through successive ionic layer adsorption and reaction (SILAR). 2.3. The fabrication of QDSCs QDSCs were assembled with above-mentioned CEs (MWCNTs, 50% CuS/MWCNTs, 100% CuS/MWCNTs, 200% CuS/MWCNTs and Pt), CdS and ZnS sensitized TiO2/RGO photoanode and polysulfide electrolyte which contained Na2S·9H2O (Tianli Chemistry, China), S (99.5%, Tianli Chemistry, China), NaOH and deionized water. 2.4. Characterization The crystal structure was analyzed by X-ray diffraction (XRD-6000, Japan). TEM (JEM-2100, Japan) images were observed for the morphology and microstructure of CuS and MWCNTs composite powder. The Nyquist and Tafel curves of CE dummy cells and J-V curves of QDSCs were measured through an electrochemical workstation (LK98B, Tianjin) and Xenon lamp illumination (AM 1.5, 100 mW cm−2). The illuminated active area of the cell sample is 0.2 mm2. 3. Results and discussion XRD patterns of 100% CuS/MWCNTs and MWCNTs were shown in Fig. 1. The new diffraction peaks of 100% CuS/MWCNTs appeared at 2θ = 32°, 38°, 48°, 55°, 61°, 69°, 75°, corresponding to the (006), (105), (110), (108), (116), (207), (208) planes of CuS crystal [38], which illustrated successful introduction of crystalline CuS. In addition, the coexistence of (002) plane diffraction peak (2θ = 26°) from carbon structure and CuS characteristic peaks indicated the CuS/MWCNTs composite was mixed without phase transformation [39]. Fig. 2(a), (b) and (c) exhibits TEM image of 50% CuS/MWCNTs, 100% CuS/MWCNTs and 200% CuS/MWCNTs composite powder, respectively. As shown in Fig. 2(a), there is a little aggregation of CuS nanoparticles grown on MWCNTs. With the increase of CuS mass percentage, more and more CuS nanoparticles have attached to MWCNTs and form the clusters as shown in Fig. 2(b) and (c). A large amount of CuS nanoparticles can provide enough active sites to reduce electrolyte. The amount, distribution and grain size of CuS nanoparticles are crucial factors affecting regeneration rate of polysulfide electrolytes. As shown in Fig. 2(c), CuS nanoparticles have covered the surface of MWCNTs completely, which will impede the penetration of electrolyte and then degrade the electrocatalytic activity of CE and electron transportation in CE. Fig. 2 (d) exhibits TEM image of 100% CuS/MWCNTs composite powder in

2.2. The preparation of photoanodes The TiO2 sol was prepared with titanium isopropoxide, glacial acetic acid, deionized water, and polyethylene glycol and then doped with RGO (reduced graphene oxide) with RGO to TiO2 volume ratio of 5:95. The TiO2/RGO gol was coated on the FTO conductive glass with electrospinning technique and then was sintered in N2 atmosphere at 450 °C for 1 h. There formed small cracks in film due to uneven being heated in sintering process which is disadvantageous to carrier transportation [36]. In order to improve electron transportation efficiency

871

Fig. 1. XRD patterns of 100% CuS/MWCNTs (a) and MWCNTs (b).

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Y. Zhang et al. / Materials and Design 160 (2018) 870–875

(b)

(a)

(c)

100 nm

(d)

100 nm

100 nm

(f)

(e)

0.306 nm (102) 5 nm

50 nm

5 nm

Fig. 2. TEM images of 50% (a), 100% (b) and 200% CuS/MWCNTs (c); TEM in higher magnification (d), HRTEM image (e) and EDX result (f) of 100% CuS/MWCNTs.

higher magnification to further observe the detail of clustered CuS on MWCNTs. Fig. 2(e) illustrates the lattice spacing of 0.306 nm in HRTEM image of CuS nanoparticles, which corresponds to the (102) planes of hexagonal CuS crystal. Fig. 2(f) is EDX (Energy Dispersive Xray) result of marked area in Fig. 2(d), which also proves the existence of Cu and S elements. The apparent value of Cu/S molar ratio will be higher than the true, since all samples were loaded on Cu mesh for TEM and EDX measurement. Nyquist curves as the common form of electrochemical impedance spectroscopy (EIS) are performed to explain the charge transfer process and electrocatalytic activity. Fig. 3 shows the Nyquist curves of the symmetrical dummy cells including two identical CEs separated by polysulfide electrolyte. The equivalent electrical circuit is inserted in Fig. 3. The arc in the high-frequency range reveals the charge transfer resistance (Rct) at the CE/electrolyte interface which can be calculated according to fitted Nyquist curves. Theoretically, there is another arc in lowfrequency range, corresponding to Nernst diffusion impedance of the S2−/Sn2− redox couple transport in the electrolyte. Here, the arc in low-frequency range is hidden, which is maybe caused from low viscosity of polysulfide electrolyte [40]. The series resistance (Rs) also can be obtained from the intercept on the real axis, indicating electrical conductivity of whole devices. Rs and Rct values calculated from fitted

8

-Z''(Ohm)

6

MWCNTs 50% CuS/MWCNTs 100% CuS/MWCNTs 200% CuS/MWCNTs Pt fitting curves of MWCNTs fitting curves of 50% CuS/MWCNTs fitting curves of 100% CuS/MWCNTs fitting curves of 200% CuS/MWCNTs fitting curves of Pt

4

Table 1 EIS calculating results for dummy cells of different CEs.

2

0 35

Nyquist curves are listed in Table 1. The MWCNTs CE exhibits the largest Rs (48.252 Ω) and Rct (38.352 Ω). Rs and Rct of CuS/MWCNTs composite CE obviously reduce, which indicates CuS and MWCNTs composite powder can improve electrocatalytic activity of CEs efficiently. The 100% CuS/MWCNTs CE exhibits the least Rs (22.265 Ω) and Rct (7.338 Ω), which reflects the highest electrocatalytic activity on polysulfide electrolyte and enhanced charge transfer process at the CE/electrolyte interface. CuS nanoparticles with high electrocatalytic activity can contribute active sites to improve the reduction rate of polysulfide species. However, with the increase of CuS mass percentage, Rs and Rct of 200% CuS/MWCNTs CE increase inversely compared with those of 100% CuS/ MWCNTs CE, which is thought that a number of CuS particles cover the surface of MWCNTs shown in Fig. 2(c) and then impede the penetration of electrolyte into CE, degrading the electrocatalytic activity and electron transportation in CE. What is emphasized is that the CuS/MWCNTs CE possesses lower Rs and Rct values than Pt CE, which proves CuS/ MWCNTs CE has higher electrocatalytic activity on the polysulfide electrolyte and lower CE/electrolyte interface resistance, leading to enhanced charge transfer process at the CE/electrolyte interface than Pt CE. To further verify the analysis of electrocatalytic activity and interfacial charge transfer property of CE/electrolyte interface, Tafel curves are recorded in Fig. 4. The limiting current density (Jlim) and exchange current density (J0) reflect diffusion velocity and reduction rate of polysulfide electrolyte species, which can be calculated through the high potential region of the Tafel curve and the extrapolated intercepts of the cathodic and anodic branches of the Tafel curves to zero overpotential, respectively. The J0 and Jlim values show the order of 100% CuS/MWCNTs N 200% CuS/MWCNTs N 50% CuS/MWCNTs N Pt N MWCNTs. The 100% CuS/MWCNTs displays highest J0 and Jlim values and the lowest Rct that approves the results of Nyquist curve analysis [41]. All calculating results of Tafel curves are also listed in Table 1 for comparison. J0 and Jlim values increase firstly and then decrease with increase of CuS

40

45

50

55

60

Z'(Ohm) Fig. 3. Nyquist curves of different CEs.

65

70

CE

Rs (Ω)

Rcta (Ω)⁎

Rctb (Ω)⁎

J0 (mA/cm2)

Jlim (mA/cm2)

MWCNTs 50% CuS/MWCNTs 100% CuS/MWCNTs 200% CuS/MWCNTs Pt

48.252 43.383 38.352 40.280 45.274

22.265 10.626 7.338 11.335 16.281

22.241 10.661 7.330 11.416 16.329

0.058 0.121 0.176 0.113 0.079

4.498 7.368 9.046 7.990 4.549

⁎Rcta calculated from Nyquist curves; Rctb calculated from Tafel curves.

Y. Zhang et al. / Materials and Design 160 (2018) 870–875 Table 2 Calculating results of J-V curves for QDSCs with different CEs.

0

-2

lg(J/mA.cm )

-1

-2

-3

MWCNTs 50% CuS/MWCNTs 100% CuS/MWCNTs 200% CuS/MWCNTs Pt

-4

-1.0

-0.5

0.0

0.5

1.0

Potential(V) Fig. 4. Tafel curves of different CEs.

introduction, which means proper mass ratio of CuS to MWCNTs is beneficial to charge transfer and electrolyte regeneration efficiency in cells. In Fig. 5 photocurrent density-voltage (J-V) curves of QDSCs based on different CEs are measured under one-sun illumination (AM 1.5, 100 mW cm−2). The corresponding photovoltaic parameters including open-circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF), and PCE (η) are summarized in Table 2. QDSC of MWCNTs possesses the poorest photovoltaic parameters within all cell samples (Voc = 0.552 V, Jsc = 13.980 mA/cm−2, FF = 0.334, η = 2.577%). After the introduction of CuS nanoparticles the photovoltaic property improves significantly, which is attributed to the introduction of CuS as active sites, boosting reduction of electrolyte. However, overall photovoltaic parameters of 200% CuS/MWCNTs based device get worse compared with those of 100% CuS/MWCNTs cell. The possible reason is that excessive CuS nano-particles aggregates severely, leading to decrease of active sites. On the other hand, excessive CuS nano-particles will further form a dense layer on the FTO substrate which could be peeled off in the electrolyte immersion, resulting in the lower cell efficiency [42]. Jsc becomes larger continuously with the increase of CuS mass percentage, which states more and more CuS is beneficial to the electron collection in spite of aggregation. Among all cell samples, the QDSC with 100% CuS/MWCNTs CE exhibits the most optimal photovoltaic parameters (Voc = 0.618 V, Jsc = 18.680 mA cm−2, FF = 0.455, η =

20

MWCNTs 50% CuS/MWCNTs 100% CuS/MWCNTs 200% CuS/MWCNTs Pt

Current Density(mA.cm

-2

15

10

5

0 0.0

873

0.1

0.2

0.3

0.4

0.5

Voltage(V) Fig. 5. J-V curves of QDSCs of different CEs.

0.6

QDSC

Voc (V)

Jsc(mA cm−2)

FF

η (%)

MWCNTs 50% CuS/MWCNTs 100% CuS/MWCNTs 200% CuS/MWCNTs Pt

0.552 0.592 0.618 0.583 0.557

13.980 17.960 18.680 19.403 16.771

0.334 0.443 0.455 0.433 0.287

2.577 ± 0.053 4.708 ± 0.022 5.254 ± 0.013 4.983 ± 0.035 2.680 ± 0.101

5.254%). This higher Voc could be attributed to the larger electron density in the conduction band of TiO2 contributed by RGO doping, since the Fermi level rises in this situation [43]. Photovoltaic parameters of QDSC with Pt as a traditional CE in DSSCs and QDSCs are just better than those of MWCNTs, which proves CuS/MWCNT composite can substitute Pt for CE material undoubtedly. Zhang et al. reported bare CuS served as a more satisfied CE than Pt that exhibited the poor FF in polysulfide, typical electrolyte of QDSCs, which is similar to our work [44]. The composites of CuxS and carbon series, such as CNT, GO and RGO show higher photovoltaic performance than bare CuxS and Pt as CE materials of QDSCs because of their advantage of component and structure designability owned by composites [45]. To investigate the photovoltaic stability of different CEs, Fig. 6(a), (b) and (c) recorded J-V curves at 0, 12 and 24 h of QDSC samples based on MWCNTs, 100% CuS/MWCNTs and Pt CE, respectively. The comparison of η, Jsc and Voc of three cell samples are shown in Fig. 6 (d), (e) and (f), respectively. Moreover, the corresponding photovoltaic parameters are recorded in Table 3. After 24 h illumination, the QDSC with 100% CuS/MWCNTs CE reveals the highest stability with almost unvaried Voc, Jsc and η. The photovoltaic parameters of QDSC with MWCNTs CE change slightly. In contrast, the stability of QDSC with Pt CE is the worst since Jsc and η decrease more than 30% after 24 h illumination. Pt CE would be prone to adsorb S atoms from the polysulfide electrolytes into its surface like a slow process of nonmetallic-ion surface injection [46], leading to the decrease of conductivity and electrocatalytic reducing property and thereby affecting the PCE of QDSCs. Jsc of MWCNTs CE QDSC is lower than that of Pt at the beginning due to its second-class electrocatalytic activity [47] and then is close to that of Pt CE after 24 h illumination, which further illustrates the photovoltaic stability of MWCNTs. In MWCNT CE cells, the efficiency of electron collection becomes slower and slower because of retardation of electrolyte reducing rate and “harmful” reversed recombination caused from its limited electrocatalytic activity. Hence, MWCNTs are not suitable as CE in QDSC, which is why CuS nano-particles are introduced to form composite CE.

4. Conclusion CuS nanoparticles and MWCNTs composite is prepared with a facile one-step synthesis method and compared with MWCNTs and Pt CE from photovoltaic properties and stability based on electrocatalytic activity and electrical conductivity of materials in QDSCs mainly. CuS/ MWCNTs composite in tree-like 3D structure facilitates more electrolyte penetration to improve its regeneration efficiency. CuS nanoparticles acted as active sites are added into MWCNTs in order to improve electrocatalytic activity, but the introduction of excessive CuS is harmful to photovoltaic property of QDSCs due to aggregation of CuS nanoparticles and the decrease of active sites in CE according to TEM analysis. Through EIS and J-V curve analysis, it is concluded that 100% CuS/MWCNTs CE presents the best photoelectric parameters within all cell samples and surpass others on photovoltaic property, stability and commercial cost. The CuS/MWCNTs composite is one of promising materials for mass production of CEs including flexible counter electrode for threegeneration solar cells due to its high efficiency, low cost and facile preparation method.

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15

(b)

10

5

MWCNTs(0h) MWCNTs(12h) MWCNTs(24h)

0 0.0

0.1

0.2

0.3

0.4

0.5

10

0 0.0

0.6

100 % CuS/MWCNTs(0h) 100 % CuS/MWCNTs(12h) 100 % CuS/MWCNTs(24h) 0.2

Voltage(V)

0.4

(c)

15

Current Density(mA.cm-2

Current Density(mA.cm-2)

Current Density(mA.cm-2)

(a)

10

5

Pt(0h) Pt(12h) Pt(24h)

0 0.0

0.6

0.2

0.4

0.6

Voltage(V)

Voltage(V)

5.5

(d)

19

(e)

18

4.5

(f)

0.60

17

3.5 3.0 2.5 2.0

MWCNTs 100% CuS/MWCNTs Pt

1.5 1.0 0

16

Voltage(V)

4.0

Current Density(mA.cm-2)

Power conversion efficiency(%)

5.0

15 14 13 12

MWCNTs 100% CuS/MWCNTs Pt

11 10

12

24

0.55

0

12

Reaction time(h)

Reaction time(h)

MWCNTs 100% CuS/MWCNTs Pt

0.50

24

0

12

24

Reaction time(h)

Fig. 6. J-V curves and photovoltaic parameter change of QDSCs with three CEs under 0, 12 and 24 h illumination.

Table 3 Photovoltaic parameters of QDSCs with three CEs under 0, 12 and 24 h illumination. QDSC

Voc (V)

Jsc (mA cm−2)

η (%)

MWCNTs(0 h) MWCNTs(12 h) MWCNTs(24 h) 100% CuS/MWCNTs(0 h) 100% CuS/MWCNTs(12 h) 100% CuS/MWCNTs(24 h) Pt(0 h) Pt(12 h) Pt(24 h)

0.522 0.513 0.513 0.617 0.609 0.604 0.557 0.539 0.538

13.980 13.081 12.076 18.680 18.393 18.159 16.770 13.116 11.705

3.882 ± 0.059 3.614 ± 0.086 3.485 ± 0.041 5.254 ± 0.083 5.159 ± 0.026 5.086 ± 0.037 2.680 ± 0.110 2.062 ± 0.057 1.828 ± 0.064

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