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Improvement of the efficiency of dye-sensitized solar cells with fluorinated carbon-based liquid crystal dopant
Accepted Manuscript
Improvement of the efficiency of dye-sensitized solar cells with fluorinated carbon-based liquid crystal dopant Chang-Feng You , Cheng-En Cheng , Bi-Cheng Lei , Jia-Cih Jhang , Fang-Cheng Yu , Chen-Shiung Chang , Forest Shih-Sen Chien , Chi-Yen Huang PII: DOI: Reference:
S0577-9073(16)30809-7 10.1016/j.cjph.2017.05.028 CJPH 277
To appear in:
Chinese Journal of Physics
Received date: Revised date: Accepted date:
2 December 2016 18 April 2017 12 May 2017
Please cite this article as: Chang-Feng You , Cheng-En Cheng , Bi-Cheng Lei , Jia-Cih Jhang , Fang-Cheng Yu , Chen-Shiung Chang , Forest Shih-Sen Chien , Chi-Yen Huang , Improvement of the efficiency of dye-sensitized solar cells with fluorinated carbon-based liquid crystal dopant, Chinese Journal of Physics (2017), doi: 10.1016/j.cjph.2017.05.028
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.
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Highlights An electrolyte with a liquid crystal is developed for a solar cell. The polar fluoro group of the liquid crystal enhances the stability of the solar cell. The viscosity of the liquid crystal affects the efficiency of the solar cell.
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IMPROVEMENT OF THE EFFICIENCY OF DYE-SENSITIZED SOLAR CELLS WITH FLUORINATED CARBON-BASED LIQUID CRYSTAL DOPANT
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Chang-Feng You*, Cheng-En Cheng†, ‡, Bi-Cheng Lei‡, Jia-Cih Jhang*, Fang-Cheng Yu§, Chen-Shiung Chang†, Forest Shih-Sen Chien‡, and Chi-Yen Huang*,** *
Graduate Institute of Photonics, National Changhua University of Education, Changhua 500, Taiwan
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Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan ‡
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Department of Applied Physics, Tunghai University, Taichung 40704, Taiwan
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AU Optronics Corp., No.23, Li-Hsin Rd., Hsinchu Science Park, Hsinchu 300, Taiwan **[email protected]
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Abstract
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Long-term stability of dye-sensitized solar cells (DSSCs) can be improved using a liquid crystal (LC) dopant. However, the high viscosity of the LC dopant reduces the power conversion efficiency (PCE) of
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these cells. In this study, low-viscosity LC is doped into the DSSCs. Results indicate that the polar fluoro groups of the LCs effectively interact with the cyano groups of the organic solvent in the liquid electrolyte, enhancing the long-term stability of the DSSCs. The viscosity of the LC dopant is a key
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factor that affects the light–to–electrical energy conversion efficiency of the LC-doped DSSCs. The short-current density and related PCE of the DSSCs is not reduced because of the low viscosity of the
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doped fluorinated carbon-based LCs.
Keywords: Liquid crystal; Solar energy; dye-sensitized solar cells
1. Introduction
Liquid crystals (LCs) have become a hot research topic for flat panels since 1962 at the RCA Laboratories1. LC-based electro-optical devices, such as displays2, gratings3, and micro-lens arrays4,5,
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have been extensively studied. LC dispersions have also attracted much attention scientifically because of their potential applications in electro-optical devices and displays4. Addition of nanoparticles5-8 or polymer9 alters the various physical parameters of LCs. This alteration results in promising characteristics, such as fast response, low operation voltage, and bistable characteristics. Photovoltaic applications of dye-sensitized solar cells (DSSCs) have been evaluated because of the
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high conversion efficiency and low cost of these cells10. A typical DSSC comprises a dye-sensitized metal oxide semiconductor film as photoactive anode, a good ion-transporting electrolyte, and an efficient counter electrode. Upon photoexcitation, the photoelectrons are injected from excited dyes into the conducting band of the metal oxide. The oxidized dyes can be reduced by a redox mediator in the electrolyte located between two electrodes. Moreover, the oxidized electrolyte can be reduced by the
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electrons injected from the counter electrode. The most commonly used electrolyte in high-efficiency DSSCs usually consists of a triiodide/iodide (I3−/I−) redox couple dissolved in a volatile organic solvent, which has the potential for advanced commercialization11, 12. However, the liquid electrolytes used in DSSCs present several technological problems, such as leakage and volatility of organic solvent13, which should be alleviated. Replacing the liquid electrolyte with a solid-state or a quasi-solid-state
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electrolyte, such as p-type semiconductors, organic hole transport medium, polymer electrolytes, and
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alternatives are not significant.
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ionic liquids, has improved the conversion efficiencies of DSSCs14-17. However, the effects of these
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Fig. 1 Structure of a rod-like liquid crystal (LC) molecule.
Figure 1 shows a typical rod-like LC structure consisting of ring systems connected by linking
group A, a side chain terminal group X, and a terminal group Y1. The rings provide the short-range intermolecular forces needed to form the nematic phase. The length of the side chain terminal group X can strongly influence the elastic constants of the nematic phase and the phase transition temperature of the LC. The terminal group Y can either be polar or nonpolar and plays an important role in determining the polarity and dielectric properties of the LCs. A polar terminal group, such as cyano (CN), F, and Cl, can contribute significantly to the dielectric anisotropy and birefringence of LC materials. Among the polar terminal groups, CN has the highest polarity, leading to high dielectric anisotropy and
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birefringence. However, a CN terminal also exhibits high viscosity and insufficient resistivity. In addition, LC molecules with CN terminal group Y can be dissociated under UV illumination. By contrast, LC molecules with the F terminal group exhibit low viscosity, high resistivity, and relatively high stability under UV illumination. We previously demonstrated that addition of nematic E7 effectively impeded the degradation rate of liquid electrolyte-based DSSCs because of the interaction
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between the common CN groups of E7 and liquid electrolytes18. Nonetheless, E7 was highly viscous because of the employed CN terminal group. Thus, the short-circuit current density (JSC) and power conversion efficiency (PCE) of E7-doped DSSCs decreased with the increasing LC concentration. In this study, the effects of a commercially available LC on the electrical properties of the DSSCs were investigated. The employed LC with fluorinated carbon (CF) groups was often used in active matrix LC
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displays because of its low viscosity, high resistivity, and high stability under UV illumination.
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Fig. 2 (a) Schematic diagram of dye-sensitized solar cells (DSSCs) containing LC molecules; (b)
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molecular structure of 3-methoxypropionitrile (3-MPN).
Experiment details
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DSSC cells were fabricated similar to the procedure described in our previous paper18. Figure 2(a) shows the structure of the DSSCs. The electrolyte was prepared using 0.5 M LiI and 0.05 M I2 in 3-methoxypropionitrile [3-MPN; Fig. 2(b), Sigma-Aldrich]. Commercially available LC with CF terminals (defined as CF-LC thereafter, the chemical structure was held confidential by the supplier) was mixed ultrasonically with the electrolyte at ratios of 5 wt%, 10 wt%, 15 wt%, and 30 wt%. The CF-LC exhibited a low viscosity of approximately 11 cps, extraordinary dielectric constant of 9.9, and ordinary dielectric constant of 3.3 (provided by the supplier). The current density–voltage (J–V) characteristics of the DSSCs were recorded on a current–voltage source meter (Jiehan 5000 Electrochemical Workstation). The electrochemical impedance spectra (EIS) of the DSSCs were
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examined in the frequency range of 1 Hz to 100 kHz using a LCR meter (HIOKI 3522-50). The magnitude of the alternative signal was 50 mV. The J–V curve and electrochemical impedance spectral measurement were performed under illumination from a xenon light source with an AM 1.5 G filter. For long-term stability measurement, the DSSC cells were left unsealed and stored in a fume hood in the dark. The thermal property of the LC-doped electrolyte was evaluated using a thermogravimetric
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analyzer (TGA, TI Instrument). The TGA traces for the thermal stability of the LC-doped electrolytes were measured at a heating rate of 10 C/min under air flow. The Fourier transform infrared (FT-IR) spectra of the LC materials in the range of 4000–600 cm-1 were recorded using a Perkin Elmer 2000 FT-IR spectrophotometer.
Results and discussion
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Figure 3 shows the FT-IR absorption spectra of E7 and CF-LC. Several studies have indicated that the broad absorption peak of FT-IR at 3000–2800 cm−1 corresponds to the aromatic-ring-based mesogenic core. The absorption peak at 2226 cm−1 is assigned to the CN groups that are not found in the CF-LC19. In the CF-LC, the peaks at 1300–1000 cm−1 are attributed to the CF stretching20, which causes
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the CF-LC to elicit a significant steric effect21.
Fig. 3 Fourier transform infrared spectra of E7 and CF-LC.
Figure 4 shows the J–V curves of the CF-LC-doped DSSCs at various CF-LC concentrations. The EIS of the DSSCs under illumination are recorded to identify the electrochemical behavior and analyze the carrier dynamics of the cells. The obtained Nyquist plots of the CF-LC-doped DSSCs are shown in Figure 5, which are fitted with the equivalent circuit (inset in Fig. 5) in the frequency range from 1 Hz to
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100 kHz using commercially available software Zview. The calculated photovoltaic parameters from Fig. 4 and the fitted impedance parameters from Fig. 5 are summarized in Table 1. In the equivalent circuit, the bulk resistance24 (Rs) comprises the resistances of the electrodes, electrolyte, and contacts. The charge-transfer resistance22,
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(Rct) and recombination resistance24,
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(Rrec) describe the
electrochemical reactions of I3−/I− at the counter electrode and related to the electron transport process at
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the TiO2/electrolyte and TiO2/dye interfaces. Rct and Rrec are represented by the radii of the semicircles on the left (>100 Hz) and right (<100 Hz) side in Fig. 5, respectively. The error between the fitted and measured results from Fig. 5 is less than 3%. The radius of left semicircle in Fig. 5 is slightly enlarged with CF-LC concentration when the doped CF-LC concentration is under 15 wt%, indicating that Rct of DSSCs slightly increases with LC dopant. Notably, when the CF-LC concentration is increased ~ 30
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wt%, Rct is increased significantly. The addition of LC dopant increases the viscosity of the liquid electrolyte and decreases the diffusion velocity, which reduces the supply amount of I3− to the Pt counter electrode. These processes thereby deplete the I3− concentration at the electrode and decrease the electrochemical reaction rate and PCEs of the DSSCs, and therefore increase Rct. The radius of right semicircle in Fig. 5 is nearly independent of CF-LC concentration, indicating that the doping of CF-LC
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does not affect Rrec , i.e., the electron transport at the TiO2/electrolyte and TiO2/dye interfaces. The addition of high-viscosity E7 was previously reported to significantly decrease the PCE of the DSSC
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cell, because the doped E7 significantly increased the viscosity of the liquid electrolyte mixture. By contrast, in the present experiment, because of the low viscosity of the doped CF-LC, the addition of
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LCs in this experiment only slightly decreases the PCEs of the DSSC cell, significantly improving the deficiency of low PCE of the E7-doped DSSC cell in previous study20. This result indicates that the
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viscosity of the LC dopant is a key factor that affects the light–to–electrical energy conversion efficiency of the LC-doped DSSCs. Notably, we also observe that the open-circuit voltage (VOC) is
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nearly independent of LC concentration, but detailed mechanisms require further investigations. The decrease in VOC at 30 wt% CF-LC-doped cell could be attributed to the precipitate found in the electrolyte owing to the high LC dopant concentration.
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Fig. 4 Photovoltaic characteristic curves of the CF-LC-doped DSSCs at 1 sun condition.
Fig. 5 Nyquist plots of the CF-LC-doped DSSCs with various CF-LC concentrations. The inset presents
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the equivalent circuit of the CF-LC-doped DSSCs.
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Table1 Simulated photovoltaic parameters of CF-LC-doped DSSCs. PCE (%) Jsc (mA/cm2) Voc (mV) 0 wt% 5 wt% 10 wt% 15 wt% 30 wt%
6.0 5.8 5.8 5.5 4.9
12.8 12.5 12.4 12.1 12.1
717 716 723 717 686
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0.65 0.65 0.65 0.64 0.59
Rs (Ω) Rct (Ω) Rrec (Ω) 4 6 6 6 5
6 8 7 8 10
84 97 88 82 83
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Fig. 6(a) Normalized PCEs of the CF-LC-doped DSSCs with various CF-LC concentrations and (b)
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thermogravimetry curves of the CF-LC-doped electrolytes with various CF-LC concentrations.
Figures 6(a) and 6(b) show the time-dependent normalized PCEs of the DSSCs and the thermogravimetry curves of the CF-LC-doped electrolytes, respectively. The electrolyte continuously lost weight as temperature increases, because the 3-MPN solvent in the electrolyte is evaporated26.
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Notably, the addition of CF-LC impedes the evaporative rate and inhibits the weight loss of the electrolytes, thereby improving the long-term stability of the DSSCs. The improvement is due to the
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strong interaction of the CF group of the doped LC with the CN group of the 3-MPN in the electrolyte. Usually, the liquid electrolyte in the DSSCs easily volatilizes and leaks out during operation27-29. The
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evaporation of liquid electrolytes often causes practical limitations during the sealing of DSSC cells. Therefore, the evaporation rate of liquid electrolytes should be suppressed to improve the lifetime and
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yield rate of DSSC cells. From our experiments, we do not find any phase separation in mixing CF-LC with liquid electrolyte. However, in the high concentration CF-LC-doped samples, the CF-LC-doped
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electrolytes generates some precipitate a week after mixing. The precipitate might be the main cause of the high Rct when the doped CF-LC concentration exceeds 30 wt%. Nonetheless, this precipitate phenomenon is not found in the E7-doped liquid electrolytes reported previously20. The interaction between CF-LC and 3-MPN in the liquid electrolyte is a complex condition. CF terminal group in the CF-LC and CN terminal group in the 3-MPN are polar terminal groups with different polarity. Owing to this difference in polarity, in the DSSCs, the CF molecules have the tendency to bind with one another, thereby suppress the interaction between 3-MPN and CF-LCs. Consequently, some precipitate appears in the high concentration CF-LC-doped samples. The precipitation phenomenon of 3-MPN and CF-LC mixture may reduce the long term stability of DSSCs when the doped LC concentration is high or a long
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time after mixing. On the contrary, in our previous study20, the doped nematic E7 has the terminal group CN, which is also appeared in 3-MPN liquid electrolyte. Therefore, precipitates are not found in the high concentration E7-doped samples. Notably, the chemical structure and component of CF-LC are held confidential by the supplier. Therefore, the detail interaction mechanisms between LC and liquid electrolyte in DSSCs need more investigations.
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In this study, the addition of CF-LC effectively decreases the evaporation rate of the liquid electrolyte without significantly reducing the PCE of the DSSCs owing to the low viscosity of the employed CF-LC. The binding force between the polar terminal groups of the LC and CN groups of 3-MPN in the electrolyte is likely the key factor that influences the long-term stability of the DSSCs. After solidly binding the liquid electrolyte and LC by polar groups, the viscosity of the doped LCs may
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reduce the diffusion velocity and the electrochemical reaction rate and PCEs of the DSSCs. Consequently, the use of low-viscosity LC reasonably avoids the reduction of PCE of the DSSCs.
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Conclusions
In this study, we have investigated the effects of CF-LC dopant on the electrical properties of the
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DSSCs. The obtained results indicate that the binding between the polar terminal groups of LCs and the CN groups of 3-MPN in the electrolyte improves the long-term stability of the DSSCs. The
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low-viscosity CF-LC dopant effectively decreases the evaporation rate of the liquid electrolyte without reducing the PCE of the DSSCs. The optimization of CF-LC for DSSC application is under way.
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Furthermore, the liquid electrolyte used in this study contains CN groups, which may be harmful to the environment. The improvement of the stability and efficiency of LC-based DSSCs and the development
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of environment-friendly materials for DSSC applications are currently being investigated. Acknowledgments
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This work was supported by the Ministry of Science and Technology of Taiwan (Contract Nos. MOST 101-2112-M-018-002-MY3, 104-2811-M-018-001, 103-2622-E-018-007-CC3, 102-2112-M-029-005MY3, and 104-2112-M-018-003-MY3).
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E. Lueder, Liquid crystal displays : addressing schemes and electro-optical effects, 2nd ed. (Wiley, Chichester, West Sussex, U.K., 2010). C. T. Kuo, R. H. Chiang, C. Y. Wang, P. H. Hsieh, Y. T. Lin, C. H. Lin and C. Y. Huang, Opt. Express 22, 9759 (2014).
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19. C. A. McFarland, J. L. Koenig and J. L. West, Appl. Spectrosc. 47, 321 (1993). 20. F. Peng, Y. Chen, S. T. Wu, S. Tripathi and R. J. Twieg, Liq. Cryst. 41, 1545 (2014). 21. M. Hird, Chemical Society Reviews 36, 2070 (2007). 22. Q. W. Jiang, G. R. Li and X. P. Gao, Chem. Comm. 44, 6720 (2009). 23. G. R. Li, F. Wang, Q. W. Jiang, X. P. Gao and P. W. Shen, Angew. Chem. Int. Ed. 49, 3653 (2010). 24. Y. J. Chang and T. J. Chow, J. Mater. Chem. 21, 9523 (2011).
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25. N. Cho, H. Choi, D. Kim, K. Song, M.-S. Kang, S. O. Kang and J. Ko, Tetrahedron 65, 6236 (2009). 26. D. Xu, X. Chen, L. Wang, L. Qiu, H. Zhang and F. Yan, Electrochimica Acta 106, 181 (2013). 27. J. Xia, F. Li, C. Huang, J. Zhai and L. Jiang, Sol. Energy Mater. Sol. 90, 944 (2006). 28. H. Yang, M. Huang, J. Wu, Z. Lan, S. Hao and J. Lin, Mater. Chem. Phys. 110, 38 (2008).
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29. A. Mathew, G. M. Rao and N. Munichandraiah, Adv. Mater. Lett. 5, 180 (2014).
Improvement of the efficiency of dye-sensitized solar cells with fluorinated carbon-based liquid crystal dopant Chang-Feng You , Cheng-En Cheng , Bi-Cheng Lei , Jia-Cih Jhang , Fang-Cheng Yu , Chen-Shiung Chang , Forest Shih-Sen Chien , Chi-Yen Huang PII: DOI: Reference:
S0577-9073(16)30809-7 10.1016/j.cjph.2017.05.028 CJPH 277
To appear in:
Chinese Journal of Physics
Received date: Revised date: Accepted date:
2 December 2016 18 April 2017 12 May 2017
Please cite this article as: Chang-Feng You , Cheng-En Cheng , Bi-Cheng Lei , Jia-Cih Jhang , Fang-Cheng Yu , Chen-Shiung Chang , Forest Shih-Sen Chien , Chi-Yen Huang , Improvement of the efficiency of dye-sensitized solar cells with fluorinated carbon-based liquid crystal dopant, Chinese Journal of Physics (2017), doi: 10.1016/j.cjph.2017.05.028
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.
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Highlights An electrolyte with a liquid crystal is developed for a solar cell. The polar fluoro group of the liquid crystal enhances the stability of the solar cell. The viscosity of the liquid crystal affects the efficiency of the solar cell.
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IMPROVEMENT OF THE EFFICIENCY OF DYE-SENSITIZED SOLAR CELLS WITH FLUORINATED CARBON-BASED LIQUID CRYSTAL DOPANT
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Chang-Feng You*, Cheng-En Cheng†, ‡, Bi-Cheng Lei‡, Jia-Cih Jhang*, Fang-Cheng Yu§, Chen-Shiung Chang†, Forest Shih-Sen Chien‡, and Chi-Yen Huang*,** *
Graduate Institute of Photonics, National Changhua University of Education, Changhua 500, Taiwan
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Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan ‡
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Department of Applied Physics, Tunghai University, Taichung 40704, Taiwan
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AU Optronics Corp., No.23, Li-Hsin Rd., Hsinchu Science Park, Hsinchu 300, Taiwan **[email protected]
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Abstract
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Long-term stability of dye-sensitized solar cells (DSSCs) can be improved using a liquid crystal (LC) dopant. However, the high viscosity of the LC dopant reduces the power conversion efficiency (PCE) of
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these cells. In this study, low-viscosity LC is doped into the DSSCs. Results indicate that the polar fluoro groups of the LCs effectively interact with the cyano groups of the organic solvent in the liquid electrolyte, enhancing the long-term stability of the DSSCs. The viscosity of the LC dopant is a key
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factor that affects the light–to–electrical energy conversion efficiency of the LC-doped DSSCs. The short-current density and related PCE of the DSSCs is not reduced because of the low viscosity of the
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doped fluorinated carbon-based LCs.
Keywords: Liquid crystal; Solar energy; dye-sensitized solar cells
1. Introduction
Liquid crystals (LCs) have become a hot research topic for flat panels since 1962 at the RCA Laboratories1. LC-based electro-optical devices, such as displays2, gratings3, and micro-lens arrays4,5,
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have been extensively studied. LC dispersions have also attracted much attention scientifically because of their potential applications in electro-optical devices and displays4. Addition of nanoparticles5-8 or polymer9 alters the various physical parameters of LCs. This alteration results in promising characteristics, such as fast response, low operation voltage, and bistable characteristics. Photovoltaic applications of dye-sensitized solar cells (DSSCs) have been evaluated because of the
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high conversion efficiency and low cost of these cells10. A typical DSSC comprises a dye-sensitized metal oxide semiconductor film as photoactive anode, a good ion-transporting electrolyte, and an efficient counter electrode. Upon photoexcitation, the photoelectrons are injected from excited dyes into the conducting band of the metal oxide. The oxidized dyes can be reduced by a redox mediator in the electrolyte located between two electrodes. Moreover, the oxidized electrolyte can be reduced by the
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electrons injected from the counter electrode. The most commonly used electrolyte in high-efficiency DSSCs usually consists of a triiodide/iodide (I3−/I−) redox couple dissolved in a volatile organic solvent, which has the potential for advanced commercialization11, 12. However, the liquid electrolytes used in DSSCs present several technological problems, such as leakage and volatility of organic solvent13, which should be alleviated. Replacing the liquid electrolyte with a solid-state or a quasi-solid-state
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electrolyte, such as p-type semiconductors, organic hole transport medium, polymer electrolytes, and
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alternatives are not significant.
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ionic liquids, has improved the conversion efficiencies of DSSCs14-17. However, the effects of these
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Fig. 1 Structure of a rod-like liquid crystal (LC) molecule.
Figure 1 shows a typical rod-like LC structure consisting of ring systems connected by linking
group A, a side chain terminal group X, and a terminal group Y1. The rings provide the short-range intermolecular forces needed to form the nematic phase. The length of the side chain terminal group X can strongly influence the elastic constants of the nematic phase and the phase transition temperature of the LC. The terminal group Y can either be polar or nonpolar and plays an important role in determining the polarity and dielectric properties of the LCs. A polar terminal group, such as cyano (CN), F, and Cl, can contribute significantly to the dielectric anisotropy and birefringence of LC materials. Among the polar terminal groups, CN has the highest polarity, leading to high dielectric anisotropy and
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birefringence. However, a CN terminal also exhibits high viscosity and insufficient resistivity. In addition, LC molecules with CN terminal group Y can be dissociated under UV illumination. By contrast, LC molecules with the F terminal group exhibit low viscosity, high resistivity, and relatively high stability under UV illumination. We previously demonstrated that addition of nematic E7 effectively impeded the degradation rate of liquid electrolyte-based DSSCs because of the interaction
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between the common CN groups of E7 and liquid electrolytes18. Nonetheless, E7 was highly viscous because of the employed CN terminal group. Thus, the short-circuit current density (JSC) and power conversion efficiency (PCE) of E7-doped DSSCs decreased with the increasing LC concentration. In this study, the effects of a commercially available LC on the electrical properties of the DSSCs were investigated. The employed LC with fluorinated carbon (CF) groups was often used in active matrix LC
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displays because of its low viscosity, high resistivity, and high stability under UV illumination.
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Fig. 2 (a) Schematic diagram of dye-sensitized solar cells (DSSCs) containing LC molecules; (b)
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molecular structure of 3-methoxypropionitrile (3-MPN).
Experiment details
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DSSC cells were fabricated similar to the procedure described in our previous paper18. Figure 2(a) shows the structure of the DSSCs. The electrolyte was prepared using 0.5 M LiI and 0.05 M I2 in 3-methoxypropionitrile [3-MPN; Fig. 2(b), Sigma-Aldrich]. Commercially available LC with CF terminals (defined as CF-LC thereafter, the chemical structure was held confidential by the supplier) was mixed ultrasonically with the electrolyte at ratios of 5 wt%, 10 wt%, 15 wt%, and 30 wt%. The CF-LC exhibited a low viscosity of approximately 11 cps, extraordinary dielectric constant of 9.9, and ordinary dielectric constant of 3.3 (provided by the supplier). The current density–voltage (J–V) characteristics of the DSSCs were recorded on a current–voltage source meter (Jiehan 5000 Electrochemical Workstation). The electrochemical impedance spectra (EIS) of the DSSCs were
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examined in the frequency range of 1 Hz to 100 kHz using a LCR meter (HIOKI 3522-50). The magnitude of the alternative signal was 50 mV. The J–V curve and electrochemical impedance spectral measurement were performed under illumination from a xenon light source with an AM 1.5 G filter. For long-term stability measurement, the DSSC cells were left unsealed and stored in a fume hood in the dark. The thermal property of the LC-doped electrolyte was evaluated using a thermogravimetric
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analyzer (TGA, TI Instrument). The TGA traces for the thermal stability of the LC-doped electrolytes were measured at a heating rate of 10 C/min under air flow. The Fourier transform infrared (FT-IR) spectra of the LC materials in the range of 4000–600 cm-1 were recorded using a Perkin Elmer 2000 FT-IR spectrophotometer.
Results and discussion
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2.
Figure 3 shows the FT-IR absorption spectra of E7 and CF-LC. Several studies have indicated that the broad absorption peak of FT-IR at 3000–2800 cm−1 corresponds to the aromatic-ring-based mesogenic core. The absorption peak at 2226 cm−1 is assigned to the CN groups that are not found in the CF-LC19. In the CF-LC, the peaks at 1300–1000 cm−1 are attributed to the CF stretching20, which causes
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the CF-LC to elicit a significant steric effect21.
Fig. 3 Fourier transform infrared spectra of E7 and CF-LC.
Figure 4 shows the J–V curves of the CF-LC-doped DSSCs at various CF-LC concentrations. The EIS of the DSSCs under illumination are recorded to identify the electrochemical behavior and analyze the carrier dynamics of the cells. The obtained Nyquist plots of the CF-LC-doped DSSCs are shown in Figure 5, which are fitted with the equivalent circuit (inset in Fig. 5) in the frequency range from 1 Hz to
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100 kHz using commercially available software Zview. The calculated photovoltaic parameters from Fig. 4 and the fitted impedance parameters from Fig. 5 are summarized in Table 1. In the equivalent circuit, the bulk resistance24 (Rs) comprises the resistances of the electrodes, electrolyte, and contacts. The charge-transfer resistance22,
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(Rct) and recombination resistance24,
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(Rrec) describe the
electrochemical reactions of I3−/I− at the counter electrode and related to the electron transport process at
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the TiO2/electrolyte and TiO2/dye interfaces. Rct and Rrec are represented by the radii of the semicircles on the left (>100 Hz) and right (<100 Hz) side in Fig. 5, respectively. The error between the fitted and measured results from Fig. 5 is less than 3%. The radius of left semicircle in Fig. 5 is slightly enlarged with CF-LC concentration when the doped CF-LC concentration is under 15 wt%, indicating that Rct of DSSCs slightly increases with LC dopant. Notably, when the CF-LC concentration is increased ~ 30
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wt%, Rct is increased significantly. The addition of LC dopant increases the viscosity of the liquid electrolyte and decreases the diffusion velocity, which reduces the supply amount of I3− to the Pt counter electrode. These processes thereby deplete the I3− concentration at the electrode and decrease the electrochemical reaction rate and PCEs of the DSSCs, and therefore increase Rct. The radius of right semicircle in Fig. 5 is nearly independent of CF-LC concentration, indicating that the doping of CF-LC
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does not affect Rrec , i.e., the electron transport at the TiO2/electrolyte and TiO2/dye interfaces. The addition of high-viscosity E7 was previously reported to significantly decrease the PCE of the DSSC
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cell, because the doped E7 significantly increased the viscosity of the liquid electrolyte mixture. By contrast, in the present experiment, because of the low viscosity of the doped CF-LC, the addition of
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LCs in this experiment only slightly decreases the PCEs of the DSSC cell, significantly improving the deficiency of low PCE of the E7-doped DSSC cell in previous study20. This result indicates that the
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viscosity of the LC dopant is a key factor that affects the light–to–electrical energy conversion efficiency of the LC-doped DSSCs. Notably, we also observe that the open-circuit voltage (VOC) is
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nearly independent of LC concentration, but detailed mechanisms require further investigations. The decrease in VOC at 30 wt% CF-LC-doped cell could be attributed to the precipitate found in the electrolyte owing to the high LC dopant concentration.
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Fig. 4 Photovoltaic characteristic curves of the CF-LC-doped DSSCs at 1 sun condition.
Fig. 5 Nyquist plots of the CF-LC-doped DSSCs with various CF-LC concentrations. The inset presents
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the equivalent circuit of the CF-LC-doped DSSCs.
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Table1 Simulated photovoltaic parameters of CF-LC-doped DSSCs. PCE (%) Jsc (mA/cm2) Voc (mV) 0 wt% 5 wt% 10 wt% 15 wt% 30 wt%
6.0 5.8 5.8 5.5 4.9
12.8 12.5 12.4 12.1 12.1
717 716 723 717 686
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0.65 0.65 0.65 0.64 0.59
Rs (Ω) Rct (Ω) Rrec (Ω) 4 6 6 6 5
6 8 7 8 10
84 97 88 82 83
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Fig. 6(a) Normalized PCEs of the CF-LC-doped DSSCs with various CF-LC concentrations and (b)
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thermogravimetry curves of the CF-LC-doped electrolytes with various CF-LC concentrations.
Figures 6(a) and 6(b) show the time-dependent normalized PCEs of the DSSCs and the thermogravimetry curves of the CF-LC-doped electrolytes, respectively. The electrolyte continuously lost weight as temperature increases, because the 3-MPN solvent in the electrolyte is evaporated26.
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Notably, the addition of CF-LC impedes the evaporative rate and inhibits the weight loss of the electrolytes, thereby improving the long-term stability of the DSSCs. The improvement is due to the
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strong interaction of the CF group of the doped LC with the CN group of the 3-MPN in the electrolyte. Usually, the liquid electrolyte in the DSSCs easily volatilizes and leaks out during operation27-29. The
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evaporation of liquid electrolytes often causes practical limitations during the sealing of DSSC cells. Therefore, the evaporation rate of liquid electrolytes should be suppressed to improve the lifetime and
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yield rate of DSSC cells. From our experiments, we do not find any phase separation in mixing CF-LC with liquid electrolyte. However, in the high concentration CF-LC-doped samples, the CF-LC-doped
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electrolytes generates some precipitate a week after mixing. The precipitate might be the main cause of the high Rct when the doped CF-LC concentration exceeds 30 wt%. Nonetheless, this precipitate phenomenon is not found in the E7-doped liquid electrolytes reported previously20. The interaction between CF-LC and 3-MPN in the liquid electrolyte is a complex condition. CF terminal group in the CF-LC and CN terminal group in the 3-MPN are polar terminal groups with different polarity. Owing to this difference in polarity, in the DSSCs, the CF molecules have the tendency to bind with one another, thereby suppress the interaction between 3-MPN and CF-LCs. Consequently, some precipitate appears in the high concentration CF-LC-doped samples. The precipitation phenomenon of 3-MPN and CF-LC mixture may reduce the long term stability of DSSCs when the doped LC concentration is high or a long
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time after mixing. On the contrary, in our previous study20, the doped nematic E7 has the terminal group CN, which is also appeared in 3-MPN liquid electrolyte. Therefore, precipitates are not found in the high concentration E7-doped samples. Notably, the chemical structure and component of CF-LC are held confidential by the supplier. Therefore, the detail interaction mechanisms between LC and liquid electrolyte in DSSCs need more investigations.
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In this study, the addition of CF-LC effectively decreases the evaporation rate of the liquid electrolyte without significantly reducing the PCE of the DSSCs owing to the low viscosity of the employed CF-LC. The binding force between the polar terminal groups of the LC and CN groups of 3-MPN in the electrolyte is likely the key factor that influences the long-term stability of the DSSCs. After solidly binding the liquid electrolyte and LC by polar groups, the viscosity of the doped LCs may
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reduce the diffusion velocity and the electrochemical reaction rate and PCEs of the DSSCs. Consequently, the use of low-viscosity LC reasonably avoids the reduction of PCE of the DSSCs.
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Conclusions
In this study, we have investigated the effects of CF-LC dopant on the electrical properties of the
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DSSCs. The obtained results indicate that the binding between the polar terminal groups of LCs and the CN groups of 3-MPN in the electrolyte improves the long-term stability of the DSSCs. The
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low-viscosity CF-LC dopant effectively decreases the evaporation rate of the liquid electrolyte without reducing the PCE of the DSSCs. The optimization of CF-LC for DSSC application is under way.
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Furthermore, the liquid electrolyte used in this study contains CN groups, which may be harmful to the environment. The improvement of the stability and efficiency of LC-based DSSCs and the development
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of environment-friendly materials for DSSC applications are currently being investigated. Acknowledgments
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This work was supported by the Ministry of Science and Technology of Taiwan (Contract Nos. MOST 101-2112-M-018-002-MY3, 104-2811-M-018-001, 103-2622-E-018-007-CC3, 102-2112-M-029-005MY3, and 104-2112-M-018-003-MY3).
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