Advances in Materials Science and Engineering

Advances in Materials Science and Engineering / 2014 / Article
Special Issue

Nanotechnology for Energy and Environment

View this Special Issue

Research Article | Open Access

Volume 2014 |Article ID 107352 | 4 pages | https://doi.org/10.1155/2014/107352

Enhanced Performance of Dye-Sensitized Solar Cells with Nanostructure Graphene Electron Transfer Layer

Academic Editor: J. M. P. Q. Delgado
Received08 Nov 2013
Revised03 Feb 2014
Accepted03 Feb 2014
Published05 Mar 2014

Abstract

The utilization of nanostructure graphene thin films as electron transfer layer in dye-sensitized solar cells (DSSCs) was demonstrated. The effect of a nanostructure graphene thin film in DSSC structure was examined. The nanostructure graphene thin films provides a great electron transfer channel for the photogenerated electrons from TiO2 to indium tin oxide (ITO) glass. Obvious improvements in short-circuit current density of the DSSCs were observed by using the graphene electron transport layer modified photoelectrode. The graphene electron transport layer reduces effectively the back reaction in the interface between the ITO transparent conductive film and the electrolyte in the DSSC.

1. Introduction

Dye-sensitized solar cells (DSSCs), also known as Grätzel’s cell, are of particular interest in the field of solar energy owing to their low cost and simplicity of fabrication [15]. The DSSCs have a basic structure that comprises two conductive substrates, an absorbing layer of semiconductor materials with wide band gap, dye molecules, and a redox electrolyte.

The basic principle of operation of DSSCs is that electrons are injected from the photoexcited dye into the conductive band of the semiconductor under illumination; meanwhile, the electrolyte reduces the oxidized dye and transports the positive charges to the counter electrode. However, one of the major issues hindering the rapid commercialization of DSSCs is their lower conversion efficiency compared to conventional p-n junction solar cells [6]. That may be attributed to poor charge separation in DSSC structure. Therefore, charge transfer structure, such as Au nanoparticles and quantum dots, has been employed in a DSSC to improve the device performance through charge separation in the photoelectrodes [710]. Graphene is a potential material for many applications due to their high electron mobility, outstanding optical properties, and thermal, chemical, and mechanical stability [1115]. Therefore, this study investigates the effect on the graphene layer as electron transport layer in the DSSC structure deposited by the magnetron sputtering method; in particular, it examines the performance of the DSSCs with the graphene electron transport layer.

2. Experimental

A 60 nm thick graphene layer was sputtered on indium tin oxide (ITO) conductive glass substrate by radio-frequency magnetron sputtering using a graphite target as an electron transport layer to improve the electron transfer in the DSSC structure. Next, the solution consisting of 1 g TiO2 nanocrystalline powder with diameter ~25 nm, 1 mL of triton X-100, acetic acid, and deionized water were mixture as colloidal solution, and the colloidal solutions were daubed uniformly onto the graphene electron transfer layer to form a thick film. The films were annealed at 450°C for 10 min. Thereafter, the photoelectrode with the graphene layer was immersed in a 3 × 10−4 M solution of N719 dye adsorption ((Bu4N)2-[Ru(dcbpyH)2(NCS)2] complex) in ethanol for 24 hr, before being sintered at 450°C for 30 min, to increase its anatase content. The electrolyte was composed of 0.05 M iodide and 0.5 M lithium iodide with and without 0.5 M 4-tert-butylpyridine (TBP) in propylene carbonate. Then, a 100 nm thick layer of platinum was sputtered onto ITO substrate as a counter electrode. Cells were fabricated by placing sealing films (SX1170-60, SOLARONIX) between the two electrodes and leaving just two via-holes for injection electrolyte. The sealing process was carried out on a hot plate at 100°C for 3 min. Then, the electrolyte was injected into the space between the two electrodes through the via-holes. Finally, the via-holes were sealed using the epoxy with low vapor transmission rate. Figure 1 schematically depicts the complete structure. Figure 1 shows the cross-section of the completed structure. The current density-voltage characteristics were measured using a Keithley 2420 programmable source meter under irradiation by a 1000 W xenon lamp. Finally, the irradiation power density on the surface of the sample was calibrated at 100 mW/cm2.

3. Results and Discussion

Figure 2 shows the absorption of TiO2 DSSCs with and without the graphene electron transfer layer. Absorption of the DSSC with the 60 nm thick graphene electron transport layer has obviously higher absorption intensity than the DSSC with the 100 nm thick graphene electron transport layer and without the graphene electron transport layer in visible range, especially in the range of 310–400 nm. That means the graphene electron transport layer has an increased absorption coefficient. Therefore, the graphene electron transport layer is also as an absorption layer to improve the absorption of the solar cells.

Figure 3 shows the characteristics of the DSSCs. The cell performance was measured under AM 1.5 illumination with a solar intensity of 100 mW/cm2 at 25°C. The cell has an active area of 3 × 3 mm2 and no antireflective coating. The measured cell parameters, open-circuit voltage , short-circuit current , fill factor , and energy conversion efficiency () are shown in Table 1. As shown in Figure 3, TiO2 DSSCs with graphene electron transfer layer exhibited the following static parameters: of 0.5 V and of 17.5 mA/cm2. The fill factor can be estimated by [16] where is the maximum output current density and is the maximum output voltage. Therefore, the value of results is equal to 0.456. Similarly, the energy conversion efficiency can be calculated by [16] with as the incident power and results to be 3.98%, respectively. The improvement may be attributed to the incorporation of the graphene electron transport layer. Figure 4 shows the energy level diagram and mechanism of photocurrent generation in TiO2 DSSCs with the graphene layer. The work function of the graphene layer is around 4.5 eV [17, 18]. Graphene has a work function similar to that of the ITO (4.8 eV) electrode. The graphene layer does not prevent the flow of injected electrons down to the ITO electrode because its work function exceeds that of the ITO electrode [1921]. The CB and VB are the conduction band and valence band, respectively. The LUMO and HOMO are the lowest unoccupied molecular orbit and highest occupied molecular orbit, respectively. Therefore, the brief operating process is as follows. Dye N719 was excited by incident light, and electrons transit from HOMO to LUMO. Electrons are injected into the graphene electron transport layer via the TiO2 photoelectrode. The electrons transferred to the graphene electron transport layer were collected at the back contact to generate a photocurrent. Therefore, the inserted graphene layer collects electrons and acts as a transporter in the effective separation of charge and rapid transport of the photogenerated electrons. According to Figures 2 and 3, the enhanced performance of DSSCs with a graphene was attributed to the increase in electron transport efficiency and light absorption in visible range.


TiO2Graphene + TiO2

(mA/cm2)6.917.5
(V)0.50.5
FF0.4190.456
(%)1.453.98

4. Conclusions

DSSCs with a graphene electron transport layer were prepared on an indium tin oxide glass substrate by radiofrequency magnetron sputtering. This work discusses the improvement associated with the introduction of a graphene layer in DSSCs. The enhanced performance of DSSCs with a graphene may be attributed to the increase in electron transport efficiency and light absorption in visible range, especially in the range of 310–400 nm. Therefore, the short-circuit current density and efficiency of conversion of solar energy to electricity were increased from 6.9 mA/cm2 and 1.45% to 17.5 mA/cm2 and 3.98%, respectively, under simulated full sun illumination. By incorporation of the graphene electron transport layer, the device efficiency can be increased by over 170%.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgment

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. B. O'Regan and M. Gratzel, “A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films,” Nature, vol. 353, no. 6346, pp. 737–740, 1991. View at: Google Scholar
  2. B. O'Regan, J. Moser, M. A. Anderson, and M. Gratzel, “Vectorial electron injection into transparent semiconductor membranes and electric field effects on the dynamics of light-induced charge separation,” The Journal of Physical Chemistry, vol. 94, no. 24, pp. 8720–8726, 1990. View at: Google Scholar
  3. N. G. Park, J. van de Lagemaat, and A. J. Frank, “Comparison of dye-sensitized rutile- and anatase-based TiO2 solar cells,” The Journal of Physical Chemistry B, vol. 104, no. 38, pp. 8989–8994, 2000. View at: Google Scholar
  4. M. Y. Song, D. K. Kim, K. J. Ihn, S. M. Jo, and D. Y. Kim, “Electrospun TiO2 electrodes for dye-sensitized solar cells,” Nanotechnology, vol. 15, no. 12, pp. 1861–1865, 2004. View at: Publisher Site | Google Scholar
  5. J. Bisquert, D. Cahen, G. Hodes, S. Rühle, and A. Zaban, “Physical chemical principles of photovoltaic conversion with nanoparticulate, mesoporous dye-sensitized solar cells,” The Journal of Physical Chemistry B, vol. 108, no. 24, pp. 8106–8118, 2004. View at: Publisher Site | Google Scholar
  6. T. Bora, H. H. Kyaw, S. Sarkar, S. K. Pal, and J. Dutta, “Highly efficient ZnO/Au Schottky barrier dye-sensitized solar cells: role of gold nanoparticles on the charge-transfer process,” Beilstein Journal of Nanotechnology, vol. 2, no. 1, pp. 681–690, 2011. View at: Publisher Site | Google Scholar
  7. C. Hagglund, M. Zach, and B. Kasemo, “Enhanced charge carrier generation in dye sensitized solar cells by nanoparticle plasmons,” Applied Physics Letters, vol. 92, no. 1, Article ID 013113, 2008. View at: Publisher Site | Google Scholar
  8. S. Barazzouk and S. Hotchandani, “Enhanced charge separation in chlorophyll a solar cell by gold nanoparticles,” The Journal of Applied Physics, vol. 96, no. 12, pp. 7744–7746, 2004. View at: Publisher Site | Google Scholar
  9. S. W. Tong, C. F. Zhang, C. Y. Jiang et al., “Improvement in the hole collection of polymer solar cells by utilizing gold nanoparticle buffer layer,” Chemical Physics Letters, vol. 453, no. 1–3, pp. 73–76, 2008. View at: Publisher Site | Google Scholar
  10. L. C. Chen, C. C. Wang, and B. S. Tseng, “Enhancement in nanocrystalline TiO2 solar cells sensitized with ZnPc by nanoparticles,” Journal of Optoelectronic and Biomedical Materials, vol. 1, no. 3, pp. 249–254, 2009. View at: Google Scholar
  11. X. Du, I. Skachko, A. Barker, and E. Y. Andrei, “Approaching ballistic transport in suspended grapheme,” Nature Nanotechnology, vol. 3, no. 8, pp. 491–495, 2008. View at: Publisher Site | Google Scholar
  12. R. R. Nair, P. Blake, A. N. Grigorenko et al., “Fine structure constant defines visual transparency of graphene,” Science, vol. 320, no. 5881, p. 1308, 2008. View at: Publisher Site | Google Scholar
  13. X. Wang, L. Zhi, N. Tsao, Z. Tomovic, J. Li, and K. M. Müllen, “Transparent carbon films as electrodes in organic solar cells,” Angewandte Chemie International Edition, vol. 47, no. 16, pp. 2990–2992, 2008. View at: Publisher Site | Google Scholar
  14. X. Wang, L. Zhi, and K. Müllen, “Transparent, conductive graphene electrodes for dye-sensitized solar cells,” Nano Letters, vol. 8, no. 1, pp. 323–327, 2008. View at: Publisher Site | Google Scholar
  15. S. Bae, H. Kim, Y. Lee et al., “Roll-to-roll production of 30-inch graphene films for transparent electrodes,” Nature Nanotechnology, vol. 5, no. 8, pp. 574–578, 2010. View at: Publisher Site | Google Scholar
  16. S. M. Sze, Physics of Semiconductor Devices, John Wiley & Sons, New York, NY, USA, 2nd edition, 1981.
  17. M. Koshino and T. Ando, “Electronic structures and optical absorption of multilayer graphenes,” Solid State Communications, vol. 149, no. 27-28, pp. 1123–1127, 2009. View at: Publisher Site | Google Scholar
  18. G. Giovannetti, P. A. Khomyakov, G. Brocks, V. M. Karpan, J. van den Brink, and P. J. Kelly, “Doping graphene with metal contacts,” Physical Review Letters, vol. 101, no. 2, Article ID 026803, 2008. View at: Publisher Site | Google Scholar
  19. R. Czerw, B. Foley, D. Tekleab, A. Rubio, P. M. Ajayan, and D. L. Carroll, “Substrate-interface interactions between carbon nanotubes and the supporting substrate,” Physical Review B, vol. 66, Article ID 033408, 2002. View at: Google Scholar
  20. A. Kongkanand, R. Martinez-Dominguez, and P. V. Kamat, “Single wall carbon nanotube scaffolds for photoelectrochemical solar cells. Capture and transport of photogenerated electrons,” Nano Letters, vol. 7, no. 3, pp. 676–680, 2007. View at: Publisher Site | Google Scholar
  21. F. Xu, J. Chen, X. Wu et al., “Graphene scaffolds enhanced photogenerated electron transport in ZnO photoanodes for high-efficiency dye-sensitized solar cells,” The Journal of Physical Chemistry C, vol. 117, pp. 8619–8627, 2013. View at: Publisher Site | Google Scholar

Copyright © 2014 Chih-Hung Hsu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


More related articles

1436 Views | 915 Downloads | 7 Citations
 PDF  Download Citation  Citation
 Download other formatsMore
 Order printed copiesOrder

Related articles

We are committed to sharing findings related to COVID-19 as quickly and safely as possible. Any author submitting a COVID-19 paper should notify us at help@hindawi.com to ensure their research is fast-tracked and made available on a preprint server as soon as possible. We will be providing unlimited waivers of publication charges for accepted articles related to COVID-19. Sign up here as a reviewer to help fast-track new submissions.