SpringerOpen Newsletter

Receive periodic news and updates relating to SpringerOpen.

Open Access Highly Accessed Nano Express

Improving the performance of dye-sensitized solar cells with TiO2/graphene/TiO2 sandwich structure

Lung-Chien Chen*, Chih-Hung Hsu, Po-Shun Chan, Xiuyu Zhang and Cing-Jhih Huang

Author Affiliations

Department of Electro-optical Engineering, National Taipei University of Technology, 1, 3 Sec., Chung-Hsiao E. Rd., Taipei 106, Taiwan

For all author emails, please log on.

Nanoscale Research Letters 2014, 9:380  doi:10.1186/1556-276X-9-380

The electronic version of this article is the complete one and can be found online at: http://www.nanoscalereslett.com/content/9/1/380


Received:24 June 2014
Accepted:24 July 2014
Published:3 August 2014

© 2014 Chen et al.; licensee Springer.

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited.

Abstract

This study investigates the extent to which the TiO2/graphene/TiO2 sandwich structure improves the performance of dye-sensitized solar cells (DSSCs) over that of DSSCs with the traditional structure. Studies have demonstrated that the TiO2/graphene/TiO2 sandwich structure effectively enhances the open circuit voltage (Voc), short-circuit current density (Jsc), and photoelectrical conversion efficiency (η) of DSSCs. The enhanced performance of DSSCs with the sandwich structure can be attributed to an increase in electron transport efficiency and in the absorption of light in the visible range. The DSSC with the sandwich structure in this study exhibited a Voc of 0.6 V, a high Jsc of 11.22 mA cm-2, a fill factor (FF) of 0.58, and a calculated η of 3.93%, which is 60% higher than that of a DSSC with the traditional structure.

Keywords:
Graphene; Solar cells; Sandwich structure; TiO2

Background

Dye-sensitized solar cells (DSSCs) are attracting attention globally because of their low cost, high energy conversion efficiency and potential applications [1-4]. Graphene has been extensively utilized in organic photovoltaic (PV) cells owing to its excellent optical and electrical characteristics, which are exploited in transparent conductive films or electrodes [5-8]. Some researchers have reported on composite graphene-TiO2 photoelectrodes in DSSCs [9-12]. Fang et al. [9,10] discussed the effect of the amount of graphene on the structures and properties of DSSCs. DSSCs with the optimal composite TiO2 film can achieve a photoelectrical conversion efficiency of 7.02%. Graphene is also commonly used in graphene-based counter electrodes in DSSCs [13-15]. The conventional counter electrode is platinum (Pt) because of its outstanding conductivity, catalytic activity, and stability when in contact with an iodine-based electrolyte. The expensive Pt can be replaced with graphene films in DSSCs without significantly sacrificing photoelectrical efficiency. This replacement can simply reduce the cost of the fabrication process [13]. Zhang et al. [14] grew DSSCs with graphene-based counter electrodes, which exhibited a photoelectrical conversion efficiency of as high as 6.81%. Double-layer photoelectrodes have been used to increase the photoelectrical conversion efficiency of DSSCs. Many investigations have focused on modifying the nanostructures of TiO2 photoelectrodes to nanospheres, nanospindles, nanorods, nanowires, and others [16-20]. Many special nanostructures of photoelectrodes can increase the scattering of light and improve the performance of DSSCs [16,17].

This work develops a new TiO2/graphene/TiO2 sandwich structure for photoelectrodes. A thin layer of graphene was inserted into the traditional TiO2 photoelectrode layer, making it a double layer. DSSCs with the traditional structure were also fabricated and the characteristics of the prepared DSSCs were compared. The DSSC with the TiO2/graphene/TiO2 sandwich structure exhibited excellent performance and higher photoelectrical conversion efficiency. This improvement is associated with the increase in electron transport efficiency and the absorption of light in the visible range.

Methods

Preparation of TiO2 photoelectrodes

The TiO2 slurry was prepared by mixing 6 g of nanocrystalline powder (P25 titanium oxide; Evonik Degussa Japan Co., Ltd., Tokyo, Japan), 0.1 mL Triton X-100, and 0.2 mL acetylacetone. The slurry was then stirred for 24 h before being spin-coated on ITO glass substrate at a rotation rate of 2,000 or 4,000 rpm. Following the deposition of graphene, the above procedure was carried out in the fabrication of DSSCs with the TiO2/graphene/TiO2 sandwich structure. The as-prepared TiO2 photoelectrodes were dried and annealed at 450°C for 30 min.

Preparation of graphene

The graphene film was prepared using a radio-frequency magnetron sputtering system with a carbon target (99.99%, Optotech Materials Co., Ltd, Taichung, Taiwan). The graphene film was deposited on the surface of the first photoelectrode layer. The working pressure of the chamber was maintained at 3 mTorr. The constant RF power was 90 W; the flow rate of argon was 90 sccm, and the deposition time was 2 min.

DSSC assembly

The electrolyte was composed of 0.05 M iodide, 0.5 M lithium iodide, and 0.5 M 4-tert-butylpyridine (TBP) in propylene carbonate. A 100-nm-thick layer of platinum was sputtered onto the ITO substrate as an electrochemical catalyst to form the counter electrode. Cells were fabricated by placing sealing films between the two electrodes, leaving two via holes through which the electrolyte could be injected. The sealing process was performed on a hot plate at 100°C for 3 min. Then, the electrolyte was injected into the space between the two electrodes through via holes. Finally, the via holes were sealed using epoxy with a low-vapor transmission rate. DSSCs with different structures were prepared to examine the effect of structure on the properties of the DSSC. Sample 1 was fabricated with a traditional structure and a single TiO2 photoelectrode layer, which was spin-coated at a rotation rate of 4,000 rpm. Sample 2 also had the traditional structure with a single TiO2 photoelectrode layer, which was spin-coated at a rotation rate of 2,000 rpm. Sample 3 had the sandwich structure of TiO2/graphene/TiO2 on ITO glass, and the deposition of the TiO2 photoeletrodes was performed at rotation rate of 4,000 rpm.

Characterization

The crystalline microstructure of the products was elucidated using a PANalytical X'Pert Pro DY2840 X-ray diffractometer (PANalytical B.V., Almelo, The Netherlands) with Cu-Kα radiation (λ = 0.1541 nm) in the scanning range 2θ = 30° and 70°. The surface morphology and vertical structure were analyzed using a LEO 1530 field-emission scanning electron microscope (One Zeiss Drive Thornwood, New York, USA). The optical absorption properties were measured in the range of 300 to 900 nm using a Hitachi U-2001 ultraviolet-visible spectrophotometer (Chiyoda, Tokyo, Japan). The photocurrent voltage (I-V) characteristics were measured using a Keithley 2420 programmable source meter under 100 mW cm-2 irradiation (Keithley Instruments Inc., Cleveland, OH, USA). Simulated sunlight was provided by a 500-W xenon lamp (Hong Ming Technology Co, Ltd, Taiwan) that had been fitted with an AM-1.5 filter. The active area of each DSSC, which was exposed to the light, was 0.3 × 0.3 cm2.

Results and discussion

Figure  1 presents the phase structure of the TiO2 photoelectrodes in the samples. Clearly, most peaks were indexed to anatase TiO2 (JCPDS No. 21-1271). Only one peak, at θ = 27.41°, corresponded to rutile TiO2 (JCPDS No. 76-0317). The strong similarity of the patterns of that were obtained from the samples indicates that the phase structures of the samples were all the same although the structures of the DSSCs differed.

thumbnailFigure 1. XRD patterns of TiO2 photoelectrodes used in DSSCs.

Figure  2a shows the surface morphology of the TiO2 photoelectrode. The TiO2 nanoparticles have a mean diameter of 50 nm. Sufficient interspaces in the photoelectrode layer facilitated the loading of dye into the film. Figure  2b,c,d shows the cross-sectional scanning electron microscopy (SEM) images of the three prepared DSSCs - samples 1, 2, and 3, respectively. The thicknesses of the photoeletrodes in samples 1 and 2 were 4 and 9.5 μm, respectively, as presented in Figure  2b,c. However, the thickness of the first TiO2 layer in sample 3 was 4 μm and that of the second layer was 6.5 μm. The thickness of the two photoelectrode layers differed although the spin-coating parameters were the same because different substrates were used during spin-coating. The graphene layer served as the substrate when the second photoelectrode layer had been deposited. The thickness of the photoelectrode of sample 3 is almost the same as the one of sample 2.

thumbnailFigure 2. SEM images of TiO2 nanoparticles. (a) Nanoparticles in structures of DSSCs. (b) Sample 1. (c) Sample 2. (d) Sample 3.

Figure  3a,b presents the Raman scattering spectra of the graphene film that was deposited on the glass substrate using the process that was described in the ‘Preparation of graphene’ section. The spectra include important peaks that correspond to the D band (approximately 1,350 cm-1), the G band (approximately 1,580 cm-1), and the 2D band (approximately 2,700 cm-1) [21]. The D band originates from defects owing to the disorder of the sp2-hybridized carbon atoms. The G band is associated with the doubly degenerate E2g mode. The 2D peak is associated with the second-order modes of the D band. The Raman spectra indicate that the prepared graphene layer exhibits two-dimensional properties.

thumbnailFigure 3. Raman scattering spectra of graphene film deposited on glass substrate (a,b).

Figure  4 displays the UV-vis spectra of photoelectrodes with different structures before and after they were loaded with dye. Clearly, the photoelectrode with the TiO2/graphene/TiO2 sandwich structure has a higher absorption than those with the traditional structure both before and after loading with dye. Dye loading substantially increases the absorption in the short wavelength region (400 to 600 nm) perhaps because of the absorption of light by the N719 dye. The DSSC with the TiO2/graphene/TiO2 sandwich structure exhibited the greatest increase in absorption after dye loading perhaps because of the interface between the graphene and the TiO2 film and the upper photoelectrode with more porous structure, which retained more dye.

thumbnailFigure 4. UV-vis absorption spectra of DSSCs with different structure (a) before and (b) after dye loading.

Figure  5 presents the energy level diagram of the DSSC with the TiO2/graphene/TiO2 sandwich structure. Under illumination, electrons from the photoexcited dye are transported to the conduction band (CB) of TiO2 via the CB of the graphene and TiO2. The transportation path via the CB of graphene is in addition to the traditional path. Owing to the excellent electrical conduction of the graphene, the graphene layer bridges behave as a channel for transferring electrons and rapidly transport the photoexcited electrons [22]. The graphene is homogeneous throughout the system, and the excited electrons are captured by the graphene without any obstruction. The collected electrons can be rapidly and effectively transported to the CB of TiO2 through graphene bridges. In the interface of graphene and TiO2, the resistance through which charges are transported is reduced relative to the DSSC without graphene bridge and the recombination and back-reaction processes are suppressed.

thumbnailFigure 5. Energy level diagram and mechanism of photocurrent generation in DSSCs with TiO2/graphene/TiO2 sandwich structure.

Figure  6 plots the photovoltaic performance of the DSSCs that were fabricated with the traditional structure and the sandwich structure on ITO substrate. Table  1 summarizes the photovoltaic parameters of these fabricated DSSCs. The model used to calculate shunt resistance (Rsh) and series resistance (Rs) is taken from [23]. Clearly, the DSSCs with the sandwich structure have higher photoelectrical conversion efficiency (3.93%) than those with the traditional structure (2.46%). This improvement in photoelectrical conversion efficiency in the DSSCs arises mainly from increases in Jsc and Voc. The sandwich structure also slightly increases FF. The recombination of the electrons is suppressed and an additional path for the transportation of photogenerated electrons is available, increasing Jsc. Moreover, the photoelectrodes with the TiO2/graphene/TiO2 sandwich structure have a smaller absorption edge, as presented in Figure  3, so the DSSC with the TiO2/graphene/TiO2 sandwich structure can absorb light over a wide range of wavelengths and, therefore, has a higher Voc.

thumbnailFigure 6. Photovoltaic performance of DSSCs fabricated with different structures.

Table 1. Photovoltaic parameters of DSSCs fabricated with different structures

Conclusions

This work proposed a simple and convenient method to enhance the performance of DSSCs using a low-cost and easy fabrication process. DSSCs with three structures were fabricated, and the characteristics of these DSSCs, including the Jsc, Voc, and photoelectrical conversion η of these DSSCs, were investigated. Clearly, the induced graphene film and sandwich structure markedly improve the performance of the DSSCs. This improvement in performance is associated with an increase in the absorption of light, a wide range of absorption wavelengths, shorter charge transportation distances, and the suppression of charge recombination when the graphene is applied.

Competing interests

The authors declare that they have no competing interests.

Authors’ contributions

LCC wrote the paper and designed the experiments. CHH prepared the samples. PSC, XYZ, and CJH did all the measurements and analyzed the data. All authors read and approved the final manuscript.

Acknowledgements

The financial support of this paper was provided by the National Science Council of the Republic of China under Contract No. NSC 102-2622-E-027-021-CC3.

References

  1. Grätzel M: Perspectives for dye-sensitized nanocrystalline solar cells.

    Prog Photovolt Res Appl 2000, 8:171-185. Publisher Full Text OpenURL

  2. O’Regan B, Gratzel M: A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films.

    Nature 1991, 353:737. Publisher Full Text OpenURL

  3. Gratzel M: Solar energy conversion by dye-sensitized photovoltaic cells.

    Inorg Chem 2005, 44:6841. PubMed Abstract | Publisher Full Text OpenURL

  4. Grätzel M: Photoelectrochemical cells.

    Nature 2001, 414:338-344. PubMed Abstract | Publisher Full Text OpenURL

  5. Du X, Skachko I, Barker A, Andrei EY: Approaching ballistic transport in suspended graphene.

    Nat Nanotechnol 2008, 3:491. PubMed Abstract | Publisher Full Text OpenURL

  6. Nair RR, Blake P, Grigorenko AN, Novoselov KS, Booth TJ, Stauber T, Peres NMR, Geim AK: Fine structure constant defines visual transparency of graphene.

    Science 2008, 320:1308. PubMed Abstract | Publisher Full Text OpenURL

  7. Bae S, Kim H, Lee Y, Xu XF, Park JS, Zheng Y, Balakrishnan J, Lei T, Kim HR, Song YI, Kim YJ, Kim KS, Ozyilmaz B, Ahn JH, Hong BH, Iijima S: Roll-to-roll production of 30-inch graphene films for transparent electrodes.

    Nat Nanotechnol 2010, 5:574-578. PubMed Abstract | Publisher Full Text OpenURL

  8. Wang X, Zhi L, Tsao N, Tomovic Z, Li J, Müllen K: Transparent carbon films as electrodes in organic solar cells.

    Angew Chem Int Ed 2008, 47:2990-2992. Publisher Full Text OpenURL

  9. Fang X, Li M, Guo K, Zhu Y, Zhongqiang H, Liu X, Chen B, Zhao X: Improved properties of dye-sensitized solar cells by incorporation of graphene into the photoelectrodes.

    Electrochim Acta 2012, 65:174-178. OpenURL

  10. Fang X, Li M, Guo K, Liu X, Zhu Y, Sebo B, Zhao X: Graphene-compositing optimization of the properties of dye-sensitized solar cells.

    Sol Energy 2014, 101:176-181. OpenURL

  11. Sun S, Gao L, Liu Y: Enhanced dye-sensitized solar cell using graphene-TiO2 photoanode prepared by heterogeneous coagulation.

    Appl Phys Lett 2010, 96:083113. Publisher Full Text OpenURL

  12. Tsai T-H, Chiou S-C, Chen S-M: Enhancement of dye-sensitized solar cells by using graphene-TiO2 composites as photoelectrochemical working electrode.

    Int J Electrochem Sci 2011, 6:3333-3343. OpenURL

  13. Gong F, Wang H, Wang Z-S: Self-assembled monolayer of graphene/Pt as counter electrode for efficient dye-sensitized solar cell.

    Phys Chem Chem Phys 2011, 13:17676-17682. PubMed Abstract | Publisher Full Text OpenURL

  14. Zhang DW, Li XD, Li HB, Chen S, Sun Z, Yin XJ, Huang SM: Graphene-based counter electrode for dye-sensitized solar cells.

    Carbon 2011, 49:5382-5388. Publisher Full Text OpenURL

  15. Choi H, Kim H, Hwang S, Han Y, Jeon M: Graphene counter electrodes for dye-sensitized solar cells prepared by electrophoretic deposition.

    J Mater Chem 2011, 21:7548. OpenURL

  16. Park JT, Roh DK, Chi WS, Patel R, Kim JH: Fabrication of double layer photoelectrodes using hierarchical TiO2 nanospheres for dye-sensitized solar cells.

    J Ind Eng Chem 2012, 18:449-455. Publisher Full Text OpenURL

  17. Qiu Y, Chen W, Yang S: Double-layered photoanodes from variable-size anatase TiO2 nanospindles: a candidate for high-efficiency dye-sensitized solar cells.

    Angew Chem 2010, 122:3757-3761. Publisher Full Text OpenURL

  18. Lin XP, Song DM, Gu XQ, Zhao YL, Qiang YH: Synthesis of hollow spherical TiO2 for dye-sensitized solar cells with enhanced performance.

    Appl Surf Sci 2012, 263:816-820. OpenURL

  19. Kim A-Y, Kang M: High efficiency dye-sensitized solar cells based on multilayer stacked TiO2 nanoparticle/nanotube photoelectrodes.

    J Photochem Photobiol A Chem 2012, 233:20-23. OpenURL

  20. Bakhshayesh AM, Mohammadia MR, Dadar H, Fray DJ: Improved efficiency of dye-sensitized solar cells aided by corn-like TiO2 nanowires as the light scattering layer.

    Electrochim Acta 2013, 90:302-308. OpenURL

  21. Ferrari AC, Meyer JC, Scardaci V, Casiraghi C, Lazzeri M, Mauri F, Piscanec S, Jiang D, Novoselov KS, Roth S, Geim AK: Raman spectrum of graphene and graphene layers.

    Phys Rev Lett 2006, 97:187401. PubMed Abstract | Publisher Full Text OpenURL

  22. Yang N, Zhai J, Wang D, Chen Y, Jiang L: Two-dimensional graphene bridges enhanced photoinduced charge transport in dye-sensitized solar cells.

    ACS Nano 2010, 4:887-894. PubMed Abstract | Publisher Full Text OpenURL

  23. Murayama M, Mori T: Evaluation of treatment effects for high-performance dye-sensitized solar cells using equivalent circuit analysis.

    Thin Sol Film 2006, 509:123-126. Publisher Full Text OpenURL