2-Dimensional MoS2 nanosheets as transparent and highly electrocatalytic counter electrode in dye-sensitized solar cells: Effect of thermal treatments

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JIEC-2453; No. of Pages 7 Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx

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2-Dimensional MoS2 nanosheets as transparent and highly electrocatalytic counter electrode in dye-sensitized solar cells: Effect of thermal treatments Seok-Soon Kim a,*, Jin-Won Lee b, Jin-Mun Yun c, Seok-In Na b a

Department of Nano & Chemical Engineering, Kunsan National University, 558 Daehangno, Kunsan-si, Jeollabuk-do 573-701, Republic of Korea Department of Flexible and Printed Electronics, Jeonbuk National University, 664-14 Deokjin-dong, Deokjin-ku, Jeonju-si, Jeollabuk-do 561-756, Republic of Korea c Radiation Research Division for Industry and Environment, Korea Atomic Energy Research Institute (KAERI), 29 Geumgu-gil, Jeongeup-si, Jeollabuk-do 580-185, Republic of Korea b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 11 November 2014 Received in revised form 30 January 2015 Accepted 15 February 2015 Available online xxx

MoS2 counter electrodes obtained via spin-coating of MoS2 nanosheets followed by thermal treatment at varied temperatures were used as highly transparent counter electrodes in dye-sensitized solar cells, and the effect of temperatures on electrocatalytic activity as well as overall power conversion efficiency (PCE) of DSSCs was investigated. DSSC with thermally treated MoS2 at 100 8C exhibited comparable PCE of 7.35% to conventional Pt contained DSSC showing 7.53%, whereas cell performance with MoS2 thermally treated at higher temperature over 300 8C showed significant decrease in PCE due to the chemical change of MoS2 to poor electrocatalytic MoO3. ß 2015 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.

Keywords: Dye-sensitized solar cells Transparent Counter electrode MoS2 nanosheets

Introduction DSSCs based on mesoporous TiO2 have attracted considerable attention due to their simple fabrication process, low production costs, relatively high conversion efficiency, and environmental suitability [1–3]. In conventional DSSCs, Platinum (Pt) that fulfills two requirements for efficient counter electrode (high electrocatalytic activity toward the I3 + 2e = 3I reaction and excellent electrical conductivity to transport electrons from external circuit) is regarded as outstanding counter electrode material [4]. However, since Pt is a limited resource to induce increase in cost, replacement of Pt has emerged as important issue for further development of DSSCs. As alternative cost-efficient materials, various conducting polymers and carbon-based materials such as carbon black, activated carbon, and single- or multi-wall carbon nanotubes (CNT) have been demonstrated by many groups [5–10]. Although carbon-based materials are cost-efficient and resistant to corrosion, several mm thick film is necessary for comparable efficiency to Pt and thus they are not suitable to be used in transparent solar cells for special applications such as power producing windows

* Corresponding author. Tel.: +82 634694772. E-mail address: [email protected] (S.-S. Kim).

and metal-foil-based DSSCs [11]. To demonstrate transparent or metal substrate based DSSCs, 2-dimensional carboneous materials such as graphene and graphene oxide are considered as attractive materials owing to its high optical transparency, high carrier mobility, and chemical stability [11–15]. However, DSSCs using these 2-dimensional carboneous counter electrodes usually exhibited lower efficiency than conventional Pt counter electrode due to the poor electrocatalytic characteristics. Hence, several groups have reported graphene-Pt or GO-Pt composites as counter electrodes for transparent and efficient DSSCs [16–18]. More recently, several inorganic compounds such as metal oxides, metal carbides, metal nitrides, and metal sulfides have been developed as electrocatalytic counter electrodes in DSSCs because of unique properties of inorganic materials such as material diversity and abundance, low cost, and ease modification [19–33]. Among various materials, molybdenum sulfide (MoS2), which has layered structures similar to graphite, has been studied for the first time by M. Wu et al. at 2011, and several reports on the MoS2 or composite of MoS2 with carboneous materials have been published as summarized in Table 1 [23–33]. In this report, to demonstrate highly transparent counter electrode having excellent electrocatalytic activity, atomically thin 2-dimensional (2-D) MoS2 nanosheets were synthesized by simple Li intercalation/exfoliation method and their structural, optical,

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Table 1 Recent reports on the DSSCs with MoS2-based counter electrodes. Material

Transparency

h (%)

MoS2 powder MoS2/graphene flake MoS2/graphene (RGO*) MoS2/MWCNT* MoS2/graphene nanosheet MoS2/carbon MoS2/CNT* MoS2 nanoparticles Porous MoS2 sheets MoS2 film MoS2/SWCNT*

– – – –

7.59 5.98 6.04 6.45 5.81 7.69 7.92 5.41 6.35 7.01 8.14

– – – – –

(Pt: (Pt: (Pt: (Pt: (Pt: (Pt: (Pt: (Pt: (Pt: (Pt: (Pt:

7.64) 6.23) 6.38) 6.41) 6.24) 6.74) 7.11) 6.58) 6.19) 7.31) 7.78)

Author

Year

Ref.

Mingxing Wu et al. Gentian Yue et al. Chia-Jui Liu et al. Sheng-Yen Tai et al. Jeng-Yu Lin et al. Gentian Yue et al. Gentian Yue et al. B. Lei et al. Wenhong Liu et al. Supriya A. et al. Gentian Yue et al.

2011 2012 2012 2012 2012 2013 2013 2014 2014 2014 2014

[23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]

RGO*: reduced graphene oxide. MWCNT*: multi-walled carbon nanoutbes. CNT*: carbon nanotubes. SWCNT*: single-walled carbon nanoutbes.

and electrochemical properties were investigated. Mainly, we studied the effect of thermal treatments of solution processed MoS2 thin films on the electrocatalytic activity as well as performance of DSSCs for better understanding and optimization of MoS2 counter electrodes.

Experimental 2-Dimensional (2-D) MoS2 nanosheets were synthesized by a simple method published elsewhere [34]. As shown in Fig. 1a, to make a Li intercalated LixMoS2, MoS2 (0.3 g) purchased from

Fig. 1. Schematics of (a) synthesis of 2-D MoS2 nanosheets and (b) AFM image of MoS2 films on glass.

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Sigma–Aldrich was mixed into 2.0 M n-buthyl lithium in a cyclohexane (4 mL) for 2 days, and then washed with hexane for the removal of un-intercalated n-buthyl lithium and organic residues. Next, MoS2 nanosheets were obtained by exfoliation of resultant LixMoS2 via ultrasonication in distilled water for 1 h followed by addition of LiCl. Here, LiCl was added in order to precipitate final MoS2. After washing with DI water several times, produced MoS2 was filtered and re-dispersed in dimethylformamide (DMF) at a concentration of 1 mg/ml for the preparation of MoS2 films. For use as counter electrode and characterization, MoS2 films were prepared on cleaned fluorine-doped tin oxide (FTO) coated glass substrates (Philkington, sheet resistance: 8 V/&) by spincoating at 3000 rpm for 60 s, and thermally treated at varied temperature of 100, 200, 300, and 400 8C in air for 10 min. General Pt counter electrode was also prepared by spin-coating of 0.5 mM chloroplatinic acid (H2PtCl6) in isopropanol following by thermal treatment at 450 8C for 30 min. As photoelectrode in DSSCs, TiO2 nanoporous films were prepared on cleaned FTO substrate by the spreading of TiO2 paste (ENB Korea, TTP-20N) using doctor blade technique and subsequently sintered at 450 8C for 30 min. Then, the nanostructured TiO2 films were kept in an ethanolic solution of 5  104 M Ru 535 dye (Solaronix Co., Ltd.) for 12 h and resultant dye-sensitized nanocrystalline TiO2 electrodes were sandwiched with various counter electrodes using a 25-mmthick hot-melt sealing material. Finally, the inner space was filled with an ionic liquid containing electrolyte solution composed of 0.6 M 1-butyl-3-methylimidazolium iodide (BMII), 0.03 M I2, 0.1 M LiI, 0.1 M guanidium thiocyanate (GSCN), and 0.5 M 4-tertbutylpyridine (TBP) in a mixture of acetonitrile and valeronitrile (85:15 v/v). Photocurrent density–voltage (J–V) characteristics were performed using a Keithley 2400 instrument under 100 mW/cm2 illumination from a xenon light source with an AM 1.5 global filter. A reference Si solar cell certified by the International System of Units (SI) (SRC-1000-TC-KG5-N, VLSI Standards, Inc.) was used for accurate measurement. Structural properties were examined through the measurement of field emission scanning electron microscope (FE-SEM, Hitachi S-4800) and X-ray photoelectron spectroscopy (XPS, AXIS-NOVA, Kratos). Optical transparencies of various counter electrodes were investigated via. UV/vis. absorption spectroscopy (Varian, AU/DMS-100S) and cyclic voltammetry measurements were carried out in an acetonitrile solution of 10 mM LiI, 1 mM I2 and 0.1 M LiClO4 with Pt wire counter electrode and Ag/AgCl reference electrode. In addition, typical electrochemical impedance spectra (EIS) were measured with electrochemical cells that consist of two identical counter electrodes using a Frequency Response Analyzer 1255B (Solartron Co. Ltd.) over a frequency range from 0.1 Hz to 100 kHz with an a.c. amplitude of 5 mV. Results and discussion As mentioned earlier, except for high electrocatalytic activity and electrical conductivity, optical transparency is also necessary for counter electrodes to be used in special applications of DSSCs such as decorative facilities, power-producing windows, and metal-foil-based power source. As shown in Fig. 1a, because MoS2, which is known as highly catalytic material, has layered structure, atomically thin 2-D MoS2 can be easily prepared by simple Li-intercalation and exfoliation process. Hence, by using very thin 2-D MoS2 nanosheet as counter electrode, we can demonstrate highly transparent and efficient DSSCs. Formation of 2-D MoS2 nanosheets with a sub-micron width and 1–1.2 nm thickness was confirmed through the AFM height image shown in Fig. 1b. These values are well consistent with other previous values

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[35,36]. To evaluate the potential of 2-D MoS2 nanosheets as counter electrode, re-dispersed 2-D MoS2 nanosheets were spin coated on FTO substrate and assembled with conventional TiO2 nanoparticles-based photoelectrode. Thermal treatments at varied temperatures of 100, 200, 300, and 400 8C were applied to 2-D MoS2 films, and their effect on performance was investigated. Before applying 2-D MoS2 films to DSSCs, to confirm successful formation of 2-D MoS2 on FTO, surface morphologies of FTO and 2-D MoS2 coated FTOs, which are thermally treated at various temperatures, were observed using FE-SEM. Fig. 2a shows irregular surface of the FTO composed of many grains with variable shapes and sizes, indicating that FTO we used in this study was produced by sputtering method. Compared to Fig. 2a showing clearly visible grainy structure, the images of FTO substrates become unclear after the coating of 2-D MoS2 as shown in Fig. 2b–e. Unclear images are attributed to the coating of FTO surface with atomically thin 2-D MoS2 nanosheets. Meanwhile, thermal treatments at varied temperatures have no critical influence on morphology of 2-D MoS2 films. To evaluate the optical properties of 2-D MoS2 films, optical transparency was measured. As shown in transmittance spectra in Fig. 3, 2-D MoS2 counter electrodes showed higher transmittance in the all range of solar spectrum than conventional Pt counter electrodes with a thickness of 80–100 nm and do not significantly alter the transparency of FTO electrode. Interestingly, although thickness and morphologies of every 2-D MoS2 films are similar, in case of 2-D MoS2 films thermally treated at 300 and 400 8C showed slightly higher transmittance comparing to 2-D MoS2 films thermally treated at 100 and 200 8C. It might be attributed to the chemical change from MoS2 to MoO3 resulting from thermal treatment at high temperature in air. The possibility of 2-D MoS2 as counter electrode and the effect of thermal treatments on performance were investigated through the measurements of J–V characteristics. As a reference, typical Pt counter electrode produced by thermal decomposition was also applied to DSSC. Fig. 4 shows representative J–V data of the DSSCs, and the performance characteristics were summarized in Table 1. As shown in Fig. 4 and summarized in Table 2, when we applied MoS2 counter electrode thermally treated at 100 8C, comparable PCE of 7.35% with VOC of 0.67 V, JSC of 17.09 mA/cm2, and F.F of 62.52% to conventional Pt contained DSSC showing 7.53% was obtained. In contrast, cell performance with MoS2 counter electrode thermally treated at higher temperature showed decrease in performance. In particular, DSSCs with MoS2 counter electrodes thermally treated at 300 and 400 8C showed significant decrease to 4.15 and 1.13%, respectively. To find the reasons for poor performance of DSSCs with MoS2 counter electrodes thermally treated at 300 and 400 8C, electrochemical characteristics and changes in chemical composition were investigated. First, because electrocatalytic activity toward the I3  2e $ 3I reaction strongly affect the performance of DSSC, electrochemical characteristics of various counter electrodes toward the I/I3 redox reaction were investigated through the measurements of CVs. Although the exact charge transfer mechanisms are not fully understood, many literatures report that two pairs of redox peaks (the pair of peaks at low potential: I3  2e $ 3I, and the pair of peaks at high potential: 3I2  2e $ 2I3) can be observed in the CVs of excellent counter electrodes for the iodide species [13,16]. As shown in Fig. 5a, two prominent pairs of redox peaks were observed in case of conventional Pt counter electrode, similar to previous other reports. When we thermally treated MoS2 at 100 and 200 8C, two prominent peaks were also observed. However, the current density at the second reduction peak, which is directly proportional to the ability of counter electrode to reduce I3, was relatively lower than conventional Pt. Furthermore, peak to peak separation of pair of peaks at lower potential, which is negatively correlated with electrochemical

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Fig. 2. SEM image of (a) FTO, thermally treated 2-D MoS2 at (b) 100, (c) 200, (d) 300, and (e) 400 8C.

rate constant of redox reaction of I/I3, was larger than conventional Pt counter electrode [37]. These results indicate that thermally treated MoS2 at 100 and 200 8C has slightly lower electrocatalytic activity toward the reduction reaction of redox mediator, and thus overall PCE was slightly lower than conventional Pt-based DSSC. In contrast, when

the temperature was increased to 300 and 400 8C, two pairs of peaks were not clear. It means that thermal treatment at higher temperature over 300 8C results in significant degradation in electrocatalytic activity to the reduction of triiodide ion and overall PCE of DSSCs.

o

Pt

Current density (mA/cm2)

Transmittance (a.u)

100

80

60

FTO Pt o MoS2-100 C

40

o

MoS2-200 C o

MoS2-300 C o

20

0 300

MoS2-100 C o

MoS2-400 C

400

500

600

700

800

Wavenumber(nm) Fig. 3. Transmittance spectra of various counter electrodes.

o

MoS2-200 C

24

MoS2-300 C

o

MoS2-400 C

21 18 15 12 9 6 3 0 -3 -0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Voltage (V) Fig. 4. Representative J–V characteristics of DSSCs using various counter electrodes.

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JIEC-2453; No. of Pages 7 S.-S. Kim et al. / Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx Table 2 Representative cell performance of DSSCs with various counter electrodes.

Pt MoS2 MoS2 MoS2 MoS2

100 8C 200 8C 300 8C 400 8C

Voc (V)

Jsc (mA/cm2)

F.F. (%)

PCE (%)

RCT (V)

0.68 0.67 0.66 0.61 0.59

17.80 17.09 17.02 17.32 10.36

61.50 62.52 58.56 39.61 18.27

7.53 7.35 6.73 4.15 1.13

7 12 48 294 5332

For better understanding on the counter electrode performance, EIS measurements of symmetrical cells consisting of two identical MoS2 counter electrodes were carried out. In the equivalent circuit for this type of cell, the series resistance (RS) mainly describing the

resistance of substrate can be determined to the impedance for high frequency region, where the phase is zero. In the middle frequency range, the impedance was dominated by the RC network of counter electrode and electrolyte interface, which is composed of the charge transfer resistance (RCT) and capacitance of electrical double layer (Cdl), and the impedance in the low frequency can be interpreted to the Nernst diffusion impedance (ZN). As shown in Fig. 5b, in case of Pt system, two semicircles were observed and from the semicircle on the middle frequency, RCT of 7 V was evaluated. The RCT value for MoS2 that thermally treated at 100 8C was slightly higher than that of Pt, whereas RCT values were considerably increased with the increase of temperature (12, 48, 294, and 5332 V for MoS2 thermally treated at 100, 200, 300, and

2

2

Current density (mA/cm )

(a)

5

1

0

-1

o

Pt

MoS2-100 C

MoS2-200oC

MoS2-300oC

o

MoS2-400 C -2 -0.6

-0.3

0.0

0.3

0.9

0.6

1.5

1.2

Potential (V) (b)

RCT

ZN

RS Cdl 2000

MoS2-200oC

30000

MoS2-300oC MoS2-400oC

20000

Pt

1500

300

-Z'' (Ohm)

MoS2-100oC

40000

-Z'' (Ohm)

-Z'' (Ohm)

50000

1000

150

0

20

40

60

Z' (Ohm)

500

10000 0

2000

4000

6000

Z' (Ohm)

8000

10000

0

0

30

60

90

120

150

180

210

Z' (Ohm)

Fig. 5. (a) Cyclic voltammograms of various counter electrodes in acetonitrile solution of 10 mM LiI, 1 mM I2 and 0.1 M LiClO4 and (b) equivalent circuit for the impedance spectrum and impedance spectra of the symmetrical cells.

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in SEM images showing unclear grainy structure of FTO after coating of MoS2, and higher transmittance than conventional Pt counter electrodes was observed in the all range of solar spectrum. DSSC with thermally treated MoS2 nanosheets at 100 8C exhibited comparable PCE of 7.35% to conventional Pt contained DSSC showing 7.53%. In constrast, cell performance with MoS2 counter electrode thermally treated at higher temperature over 300 8C showed significant decrease in PCE. Dramatic decrease in efficiency is attributed to the chemical change of MoS2 to poor electrocataytic MoO3 during thermal treatment at high temperature in air. In XPS spectra of thermally treated MoS2 at 300 and 400 8C, decrease in intensity of Mo4+ 3d5/2 and S2 2p3/2 peaks at 229 and 162 eV and more prominent peak at 236 eV for Mo6+ 3d5/2, indicating chemical change to MoO3, was observed. For better counter electrodes, studies on synthetic strategy to restrict partial oxidation of MoS2 during Li intercalation/exfoliation process and additional post treatment to recover oxidized MoS2 are currently underway. Acknowledgement This research was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (12A12852361).

References Fig. 6. (a) Mo 3d and (b) S 2p XPS spectra of thermally treated 2-D MoS2.

400 8C). In good agreement with the CV measurements, this indicates that electrocatalytic activity of MoS2 thermally treated at 100 8C is slightly lower comparing to Pt, and the electrocatalytic activity is degraded by the thermal treatment at higher temperature. To find the reason of degradation of electrocatalytic activity, changes in chemical composition after thermal treatments were investigated through the XPS and resultant XPS spectra were shown in Fig. 6a and b. The XPS spectra of thermally treated MoS2 at 100 and 200 8C showed Mo 3d5/2 and Mo 3d3/2 peaks at 229 and 232 eV, respectively, which are the expected values for the Mo4+ in MoS2. The peak corresponding to S 2p3/2 was also clearly appeared at 162 eV, which is consistent with the S2 in MoS2, as shown in Fig. 6b [38]. Additionally, peak at 236 eV corresponding to Mo6+ 3d5/2 was shown in Fig. 6a, suggesting partial oxidation of MoS2 during Li intercalation/exfoliation process [36]. On the other hands, when the temperature was increased to 300 8C, Mo4+ 3d5/2 and S2 2p3/2 peaks at 229 and 162 eV were dramatically decreased, and these peaks were disappeared when the temperature was increased to 400 8C. Furthermore, peak at 236 eV for Mo6+ 3d5/2 of thermally treated MoS2 at 300 and 400 8C was more prominent. These results indicate chemical change of MoS2 to MoO3 as a result of thermal treatment at high temperature in air. As above mentioned, in case of thermally treated MoS2 at 300 and 400 8C showed slight increase in transmittance and this also can be an evidence of chemical change to MoO3. Consequently, change to poor electrocataytic MoO3 resulted in dramatic decrease in efficiency.

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Conclusions We demonstrated highly transparent and electrocatalytic counter electrode using atomically thin 2-D MoS2 nanosheets fabricated by simple intercalation/exfoliation process. Successful formation of 2-D MoS2 on FTO was confirmed through the change

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