Conventional nanorod-based dye-sensitized solar cells (DSSCs) are fabricated by growing nanorods on top of a transparent conducting oxide (TCO, typically fluorine-doped tin oxide—FTO). The heterogeneous interface between the nanorod and TCO forms a source for carrier scattering. This work reports on a new DSSC architecture without a TCO layer. The TCO-less structure consists of ZnO nanorods grown on top of a ZnO film. The ZnO film replaced FTO as the TCO layer and the ZnO nanorods served as the photoanode. The ZnO nanorod/film structure was grown by two methods: (1) one-step chemical vapor deposition (CVD) (2) two-step chemical bath deposition (CBD). The thicknesses of the nanorods/film grown by CVD is more uniform than that by CBD. We demonstrate that the TCO-less DSSC structure can operate properly as solar cells. The new DSSCs yield the best short-current density of 3.96？mA/ and a power conversion efficiency of 0.73% under 85？mW/ of simulated solar illumination. The open-circuit voltage of 0.80？V is markedly higher than that from conventional ZnO DSSCs. 1. Introduction Dye-sensitized solar cells (DSSC) are a promising low-cost, green energy source [1, 2]. A power conversion efficiency of 11.18% has been achieved in 2005 [3]. The high efficiency of DSSCs can be attributed to the structure of a photoelectrode which consists of a layer of nanoparticle TiO2 sintered to a transparent-conducting oxide (TCO). The mesoporous TiO2 nanoparticles increase the surface area for dye chemisorptions to a thousand folds over that of a flat electrode of the same size [4]. The progress in enhancing the performance of DSSCs has been slow over the last decade. One of the main problems is the limited diffusion length of the photogenerated electrons. The photogenerated carriers conduct via random hopping through a percolated path in a three-dimensional network of nanoparticles. Previous studies have shown that the photogenerated carriers must undergo 103–106 hoppings (trapping and detrapping) before they reach the collecting electrodes [5]. Carrier trapping, presumably by defect states at the surface of nanoparticles [6, 7], leads to a low electron diffusion coefficient ( ) [8], which is several orders of magnitude smaller than that of single-crystal [9]. To improve the electron transport, researchers have tried to design DSSCs without a nanoparticulate structure. One promising approach is to replace the nanoparticles with crystalline TiO2 nanorods (or nanowires, nanotubes), thereby eliminating the grain boundaries between nanoparticles. TiO2-nanorod DSSCs have yielded
References
[1]
B. O'Regan and M. Gr？tzel, “A low-cost, high-efficiency solar cell based on dye-sensitized colloidal films,” Nature, vol. 353, no. 6346, pp. 737–740, 1991.
[2]
M. Gr？tzel, “Perspectives for dye-sensitized nanocrystalline solar cells,” Progress in Photovoltaics, vol. 8, no. 1, pp. 171–185, 2000.
[3]
M. K. Nazeeruddin, F. de Angelis, S. Fantacci, et al., “Combined experimental and DFT-TDDFT computational study of photoelectrochemical cell ruthenium sensitizers,” Journal of the American Chemical Society, vol. 127, no. 48, pp. 16835–16847, 2005.
[4]
K. Kalyanasundaram, N. Vlachopoulos, V. Krishnan, A. Monnier, and M. Gr？tzel, “Sensitization of in the visible light region using zinc porphyrins,” Journal of Physical Chemistry, vol. 91, no. 9, pp. 2342–2347, 1987.
[5]
K. D. Benkstein, N. Kopidakis, J. van de Lagemaat, and A. J. Frank, “Influence of the percolation network geometry on electron transport in dye-sensitized titanium dioxide solar cells,” Journal of Physical Chemistry B, vol. 107, no. 31, pp. 7759–7767, 2003.
[6]
A. C. Fisher, L. M. Peter, E. A. Ponomarev, A. B. Walker, and K. G. U. Wijayantha, “Intensity dependence of the back reaction and transport of electrons in dye-sensitized nanocrystalline solar cells,” Journal of Physical Chemistry B, vol. 104, no. 5, pp. 949–958, 2000.
[7]
J. van de Lagemaat and A. J. Frank, “Nonthermalized electron transport in dye-sensitized nanocrystalline films: transient photocurrent and random-walk modeling studies,” Journal of Physical Chemistry B, vol. 105, no. 45, pp. 11194–11205, 2001.
[8]
Th. Dittrich, E. A. Lebedev, and J. Weidmann, “Electron drift mobility in porous (anatase),” Physica Status Solidi A, vol. 165, no. 2, pp. R5–R6, 1998.
[9]
L. Forro, O. Chauvet, D. Emin, L. Zuppiroli, H. Berger, and F. Lévy, “High mobility n-type charge carriers in large single crystals of anatase ( ),” Journal of Applied Physics, vol. 75, no. 1, pp. 633–635, 1994.
[10]
G. K. Mor, K. Shankar, M. Paulose, O. K. Varghese, and C. A. Grimes, “Use of highly-ordered nanotube arrays in dye-sensitized solar cells,” Nano Letters, vol. 6, no. 2, pp. 215–218, 2006.
[11]
K. Zhu, N. R. Neale, A. Miedaner, and A. J. Frank, “Enhanced charge-collection efficiencies and light scattering in dye-sensitized solar cells using oriented nanotubes arrays,” Nano Letters, vol. 7, no. 1, pp. 69–74, 2007.
[12]
M. Adachi, Y. Murata, I. Okada, and S. Yoshikawa, “Formation of titania nanotubes and applications for dye-sensitized solar cells,” Journal of the Electrochemical Society, vol. 150, no. 8, pp. G488–G493, 2003.
[13]
M. Dürr, A. Schmid, M. Obermaier, S. Rosselli, A. Yasuda, and G. Nelles, “Low-temperature fabrication of dye-sensitized solar cells by transfer of composite porous layers,” Nature Materials, vol. 4, no. 8, pp. 607–611, 2005.
[14]
M. Guo, P. Diao, X. Wang, and S. Cai, “The effect of hydrothermal growth temperature on preparation and photoelectrochemical performance of ZnO nanorod array films,” Journal of Solid State Chemistry, vol. 178, no. 10, pp. 3210–3215, 2005.
[15]
J. B. Baxter and E. S. Aydil, “Nanowire-based dye-sensitized solar cells,” Applied Physics Letters, vol. 86, no. 5, Article ID 045004, pp. 1–3, 2005.
[16]
H. Chen, A. Du Pasquier, G. Saraf, J. Zhong, and Y. Lu, “Dye-sensitized solar cells using ZnO nanotips and Ga-doped ZnO films,” Semiconductor Science and Technology, vol. 23, no. 4, Article ID 045004, 2008.
[17]
K. Vanheusden, W. L. Warren, C. H. Seager, D. R. Tallant, J. A. Voigt, and B. E. Gnade, “Mechanisms behind green photoluminescence in ZnO phosphor powders,” Journal of Applied Physics, vol. 79, no. 10, pp. 7983–7990, 1996.
[18]
C. Y. Jiang, X. W. Sun, G. Q. Lo, D. L. Kwong, and J. X. Wang, “Improved dye-sensitized solar cells with a ZnO-nanoflower photoanode,” Applied Physics Letters, vol. 90, no. 26, Article ID 263501, 2007.
[19]
J.-J. Wu, G.-R. Chen, H.-H. Yang, C.-H. Ku, and J.-Y. Lai, “Effects of dye adsorption on the electron transport properties in ZnO-nanowire dye-sensitized solar cells,” Applied Physics Letters, vol. 90, no. 21, Article ID 213109, 2007.
[20]
I.-D. Kim, J.-M. Hong, B. H. Lee, et al., “Dye-sensitized solar cells using network structure of electrospun ZnO nanofiber mats,” Applied Physics Letters, vol. 91, no. 16, Article ID 163109, 2007.
