High performance dye-sensitized solar cell based ...

High performance dye-sensitized solar cell based on hydrothermally deposited multiwall carbon nanotube counter electrode Sumeth Siriroj, Samuk Pimanpang, Madsakorn Towannang, Wasan Maiaugree, Santi Phumying et al. Citation: Appl. Phys. Lett. 100, 243303 (2012); doi: 10.1063/1.4726177 View online: http://dx.doi.org/10.1063/1.4726177 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v100/i24 Published by the AIP Publishing LLC.

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APPLIED PHYSICS LETTERS 100, 243303 (2012)

High performance dye-sensitized solar cell based on hydrothermally deposited multiwall carbon nanotube counter electrode Sumeth Siriroj,1 Samuk Pimanpang,1,2,3,a) Madsakorn Towannang,2 Wasan Maiaugree,2 Santi Phumying,1 Wirat Jarernboon,4 and Vittaya Amornkitbamrung1,2,3

1 Material Science and Nanotechnology Program, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand 2 Department of Physics, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand 3 Thailand Center of Excellence in Physics, CHE, Ministry of Education, Bangkok 10400, Thailand 4 College of Nanotechnology, King Mongkut’s Institute of Technology Ladkrabang, Chalongkrung Rd., Ladkrabang, Bangkok 10520, Thailand

(Received 21 January 2012; accepted 20 May 2012; published online 12 June 2012) Conductive glass was coated with multiwall carbon nanotubes (MWCNTs) by a hydrothermal method. MWCNTs films were subsequently used as dye-sensitized solar cell (DSSC) counter electrodes. The performance of hydrothermal MWCNT DSSC was 2.37%. After film annealing in an Ar atmosphere, annealed-hydrothermal MWCNT (AHT-CNT) DSSC efficiency was significantly increased to 7.66%, in comparison to 8.01% for sputtered-Pt DSSC. Improvement of AHT-CNT DSSC performance is attributed to a decrease in charge-transfer resistance from C 2012 American 1500 X to 30 X as observed by electrochemical impedance spectroscopy. V Institute of Physics. [http://dx.doi.org/10.1063/1.4726177]

Dye-sensitized solar cells (DSSC) exhibit high potential as a replacement technology for commercial silicon solar cells. This is due to its promising power conversion efficiency, simple manufacturing procedures, and low production costs.1 Currently, maximum observed efficiency is 11%.2 DSSC technology is being developed for commercial markets. In order to be attractive to potential customers, DSSC price should be low. To ensure low cost, all materials used in such devices should be inexpensive and widely available. Although, platinum (Pt) is a good catalyst for reducing I3 to I (I3 þ2e ! 3I ),3 Pt is expensive and in limited supply. This limits its use in mass production. Alternative materials are being studied as substitutes for Pt catalyst, including conductive polymers, carbon black, graphite, and carbon nanotube. Many groups used carbon nanotubes (CNTs) as cell counter electrodes (CEs) obtaining very promising energy conversion efficiency. High efficiency of CNTs-based cells is attributed to their large surface area, good conductivity, and good catalytic activity. CNT film can be coated on conductive glass by various methods: lamination of nanotube paste, spraying a CNT solution, or growth of a precursor. Lamination of CNT pastes on a substrate requires a polymer for binding nanotubes. A secondary effect of polymer binding is a decrease of CNT active area. To compromise for active site reduction, a conductive and catalytic polymer is used as binder.3–5 In contrast, spraying CNT solution or direct CNT growth from precursors on conductive glass exposes more CNT active area. Spraying methods normally use an easily evaporated liquid as solvent for airbrushing the solution onto the heated substrate. For example, Trancik et al. sprayed a CNT solution on conductive glass. Good contact between CNT and glass was obtained as well as high a)

Author to whom correspondence should be addressed. Electronic mail: [email protected].

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film catalytic activity.6 Ramasamy et al. sprayed CNT solution on conductive glass and used this material as DSSC CEs.7 Promising efficiency of 7.59% was observed. In case of CNT growth from precursors, chemical vapor deposition (CVD) is a common method of fabricating CNTs. Nam et al.8 grew CNTs on conductive glass by CVD and used as DSSC CEs. They obtained very high efficiency 10.04%. Choi et al. grew nanotubes by CVD on graphene and then transferred them onto conductive glass.9 They obtained 3% cell efficiency. Hence, direct contact or growth of CNTs onto conductive substrate greatly favors high electrode catalytic activity and high DSSC performance. Hydrothermal deposition is an interesting method of preparing nanoparticles or coating film onto conductive or non-conductive substrates. It is a simple and low-cost process.10–12 Hydrothermal deposition uses moderate temperature and high pressure to promote chemical bonding. Charinpanitkul et al. prepared composited TiO2-CNTs nanoparticles (diameter 80-250 nm) by hydrothermal deposition and used for fabricating the DSSC working electrodes (WEs).13 The promising energy conversion efficiency 3.92% was obtained, which was higher than the anatase TiO2 cell (2.93%). In analogue, direct CNT-F-doped SnO2 (FTO) bonding should be initiated via this process leading to full utilization of CNT active area. Performance of DSSC counter electrode produced with hydrothermally deposited CNT films has not been characterized in detail. In this work, our focus was on investigating the morphology of MWCNT films coated onto conductive glass (FTO) via a hydrothermal process and their potential use as DSSC CEs. The influence of calcinations on film structure, film catalytic activity, and DSSC performance was characterized and is discussed. CNT films were coated on FTO (8 X/sq, Solarnix) by a hydrothermal method (PARR 5500 series compact reactor). A solution of 0.02 g MWCNTs (Chaimai University) in 50 ml deionized water was reacted at a temperature of

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180 C for 5 h. Prior to hydrothermal deposition, MWCNTs were modified in a mixed-acid solution (H2SO4:HNO3 at a volume ratio of 3:1) for 30 min to generate carboxylic groups on the CNT surfaces. Optical and scanning electron microscope (SEM) images of hydrothermally deposited CNT (HTCNT) film are presented in Fig. 1(a). The optical image shows CNT coating onto FTO substrate. The SEM image, Fig. 1(a), reveals non-uniform CNT deposition with impurities. Impurities were likely generated by CNT decomposition during acid treatment and/or hydrothermal process. They can also arise from CVD preparation. CNTs bond to FTO glass through functional group interaction as illustrated in Fig. 1(c). Modified CNT powders (M-CNT) and HT-CNT film were analyzed by Raman spectroscopy (DXR SmartRaman Thermo SCIENTIFIC Inc., laser wavelength 532 nm). Raman spectra for M-CNT and HT-CNT show two strong peaks at 1345 and 1580 cm1, as seen in Fig. 2(a). These peaks are assigned to the order and disordered graphite modes, respectively.14 Moreover, the D0 band at 1605 cm1 is clearly distinct from the G band. A strong disordered mode is observed for M-CNT because of the breakdown of sp2 carbon-carbon bonds. This breakdown was compelled by the acid treatment. It is also from carbon impurities of the CVD process. The area ratio of the disorder peak (ID) over the order peak (IG) of HT-CNT film (1.17) is higher than that of M-CNT (1.00). This means that hydrothermal deposition induces more defects to CNTs (impurities increase 17%). These defects are likely the particles observed in SEM image, Fig. 1(a). DSSCs were assembled using dye-sensitizer coated TiO2 films as working electrodes (WEs) and HT-CNT films as CEs. Carbon film was removed to maintain an active area of 1.5 cm 0.3 cm as shown in optical image, Fig. 1(a). Porous TiO2 film, an active area of 0.25 cm 1 cm, was prepared by a screen printing method.15 Briefly, the transparent and scattering TiO2 films were fabricated using commercial TiO2 pastes, PST-18NR, and PST-400C (JGC Catalysts and Chemicals Company, Japan). TiO2 films were sintered at 500 C for 1 h and treated with UV radiation for 10 min. TiO2 films were immersed in a 0.3 mM N-719 (Solaronix)

FIG. 1. (a) Optical and SEM images of HT-CNT, (b) optical and SEM images of AHT-CNT, and (c) schematic presenting the functional groups of CNTs and FTO interaction.

Appl. Phys. Lett. 100, 243303 (2012)

FIG. 2. (a) Raman spectra of M-CNT, HT-CNT, and AHT-CNT. (b) CV curves of HT-CNT, AHT-CNT, and Pt films at a scan rate of 20 mV/s in 10 mM LiI, 1 mM I2, and 0.1 M LiClO4 in an acetonitrile solution.

solution for 24 h. The electrolyte, a mixture of 0.05 M I2, 0.10 M LiI, 0.0025 M Li2CO3, 0.50 M TBP, and 0.60 M MPI in acetonitrile, was filled into the semi-closed cells. Cell performance was measured using a solar simulator (PEC-L11, Japan) under light intensity of 100 mW/cm2. The photocurrent-density (J) vs. photovoltage (V) curves are shown in Fig. 3(a). Photoelectric parameters such as shortcircuit current density (Jsc), open-circuit voltage (Voc), fill factor (FF), and energy conversion efficiency (g) were extracted from J-V curves and are summarized in Table I. It is seen that FF and g of HT-CNT DSSC (0.21 and 2.37%) are much lower than those of sputtered-Pt DSSC (0.73 and 8.01%). Since the WEs and electrolyte of these two devices were fabricated using the same conditions, the factors impacting performance differences should be from the CEs. Electrochemical impedance spectroscopy (EIS) was conducted to analyze DSSC impedance. EIS was done using Gamry Instrument Reference 3000 system under a light intensity of 100 mW/cm2, frequency ranging from 0.1 Hz to 100 kHz and AC amplitude of 10 mV. Fig. 3(b) shows Nyquist spectra of DSSCs. It is seen that the impedance of HT-CNT DSSC (1500 X) is much larger than for Pt DSSCs (35 X). The inset of Fig. 3(b) shows that the Pt DSSC


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FIG. 3. (a) Photocurrent-density (J) vs. photovoltage (V) curves of HTCNT, AHT-CNT, and Pt DSSCs. (b) Nyquist plots of HT-CNT, AHT-CNT, and Pt DSSCs.

impedance is composed of three semicircles. The highfrequency semicircle represents the charge-transfer resistance at the counter electrode/electrolyte interface (RCE). The middle-frequency semicircle indicates the charge-transfer resistance at the working electrode/electrolyte interface (RWE). The low-frequency semicircle indicates the diffusion rate of the electrolyte (Dion). However, there are only two semicircles visible in HT-CNT DSSC. The absence of the lowfrequency semicircle is explained by the large values of RWE and RCE obscuring the third semicircle. In effect, the low-frequency semicircle is hidden in middle-frequency semicircle. Large HT-CNT DSSC impedance implies a low electron transfer rate and low film catalytic activity. Cyclic voltammogram (CV) was conducted to further analyze the film catalytic activity. CV measurements were performed in the three-electrode cells: Pt plate as a counter electrode, Ag/ AgCl electrode as a reference electrode, and CNT or ther-

TABLE I. Summary of open-circuit voltage (Voc), short-circuit current density (Jsc), FF, and solar cell efficiency (g) of HT-CNT, AHT-CNT, and Pt DSSCs. ID/IG values of HT-CNT and AHT-CNT. J-V

Raman 2

Electrode

Jsc (mA cm )

Voc (V)

FF

g (%)

ID/IG

HT-CNT AHT-CNT Pt

14.40 14.27 14.02

0.79 0.78 0.77

0.21 0.68 0.73

2.37 7.66 8.01

1.17 0.97 N/A

mally deposited Pt films as a working electrode, at a scan rate 20 mV/s. The CV result in Fig. 2(b) reveals that Pt film composes of two oxidation peaks and two reduction peaks. However, HT-CNT film shows no clear oxidation and reduction peaks. This means that HT-CNT film has low catalytic activity, which is well agreed with its large cell impedance. As observed in Fig. 1(a) and Raman spectra, HT-CNT films have abundant impurities on the CNT surface. These impurities should reduce the CNT and electrolyte interface and block direct CNTs-FTO bonding. To reduce impurities, HT-CNT film was annealed at 450 C for 18 h under Ar atmosphere. A low temperature and long duration were used because such conditions tend not to degrade FTO conductivity and allow for a long period to remove impurities. The optical image, Fig. 1(b), reveals that AHT-CNT film has a brighter color than HT-CNT film. This implies removal of impurities and/or CNTs. The SEM image, Fig. 1(b), displays more CNTs and fewer impurities. Reduction of impurities is also confirmed by a decrease in the ID/IG value of AHT-CNT film (0.97) as compared to an ID/IG value of 1.17 for HT-CNT film, estimated from Raman spectra in Fig. 2(a). The impurities decrease 17.1% after film annealing, compared to HT-CNT film. The high disorder mode found in AHT-CNT films is a result of low annealing temperature. Surprisingly, cell performance is significantly improved to 7.66% after film annealing. The g enhancement is subjected to the increment of FF from 0.21 to 0.68. EIS was conducted to characterize changes in cell impedance. It is seen in Fig. 3(b) that AHT-CNT DSSC impedance is dramatically lower, 50 X, than that of HT-CNT cells, 1500 X. Three-semicircle impedance is observed for AHT-CNT DSSCs similarly to Pt DSSC behavior. The reduction of AHT-CNT DSSC impedance has three possible causes: (1) diminishing levels of impurities exposing more nanotube surface area, (2) stronger bonding between CNTsCNTs and CNTs-FTO, and (3) more graphitic-like structure. The trend of cell impedance (impedance of HTCNT > AHT-CNT > Pt) agrees well with efficiency behavior (g of HT-CNT < AHT-CNT < Pt). CV result of AHT-CNT film (Fig. 2(b)) displays two oxidation peaks and two reduction peaks similar to Pt film. The position of oxidation and reduction peaks of AHT-CNT film is about the same as the Pt film. This confirmed that the annealing process greatly promotes the CNT film catalytic activity, leading to the enhancement of DSSC performance. Optimized conditions (including CNT concentration, hydrothermal temperature and duration, annealing temperature and duration) were not determined by this work. It is likely that better performance will be accomplished by optimizing these parameters. This is a fertile area for further study. In conclusion, FTO glass was coated by MWCNT films by hydrothermal method and used as DSSC CEs. The efficiency of HT-CNT DSSC is poor 2.37%. However, AHTCNT DSSC provides much higher efficiency 7.66%. Improvement of cell efficiency is attributed to the reduction of impurities and CE charge-transfer resistance (RCE), as well as exposure of more CNT surface area. This work was supported by the Higher Education Research Promotion and National Research University


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Project of Thailand, Office of the Higher Education Commission, through the Advanced Functional Materials Cluster of Khon Kaen University, The Thailand Research Fund and Khon Kaen University (MRG5480024), The Integrated Nanotechnology Research Center, Khon Kaen University, and The Center of Excellence in Physics (ThEP). 1

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