Molecular Rectification: Application in Dye-Sensitized Solar Cells M. K. I. Senevirathna, P. K. D. D. P. Pitigala, V. P. S. Perera, and K. Tennakone* Institute of Fundamental Studies, Hantana, Kandy, Sri Lanka Received December 9, 2004 A dye-sensitized heterojunction of configuration n-TiO2/PD-CuPC-MV/p-CuSCN (where PD ) 3,4- pyridinedicarboxylic acid anchored to TiO2, CuPC ) copper(II) phthallocyanine tetrasulfonic acid ionically linked to PD, and MV ) Methyl Violet complexed to CuPC) is developed to demonstrate the applicability of molecular rectification to dye-sensitized solar cells as a strategy of suppressing recombination. Short- circuit photocurrent, open-circuit voltage, energy conversion efficiency, and incident photon to photocurrent conversion of this system are higher than that of the heterojunctions of configurations n-TiO2/PD-MV/ p-CuSCN, n-TiO/CuPC-MV/p-CuSCN, and n-TiO2/MV/p-CuSCN. The impressively high rectification ratio and the mode of anchorage of CuPC toTiO2 are suggested as the cause of superior photovoltaic action of the cell TiO2/PD-CuPC-MV/p-CuSCN. In a dye-sensitized solar cell (DSC), dye molecules covalently anchored to an n-type semiconductor surface inject electrons to its conduction band leaving dye cations on the semiconductor surface. Transport of the positive charges on the dye cations to a counter electrode across a suitable hole-conducting medium generates a voltage enabling passage of current through an external circuit. 1,2 The impressive performance of DSCs relies on fast electron injection and slow recombination of the dye cation- electron geminate pair. 3-5 The properties of the semi- conductor, the mode of bonding of the dye to the semi- conductor surface, and the molecular structure of the dye determine the injection and recombination rates (k i and k r ). The contribution from molecular structure to k i and k r relates to conductance properties of the dye molecule. Molecular rectification, i.e., high electron conductance in the forward direction and low conductance in the backward direction, could favor a k i greater than k r . Molecular arrangements leading to rectification and model systems that exhibit this property continue to receive much attention. 6-14 We point out that dye molecular structures with rectification properties would be of tremendous value in improving the performance of DSCs, especially dye- sensitized solid-state solar cells (DSSSCs). A DSSSC is a structure of configuration N/D/P (Figure 1), where N and P-type semiconductors sandwich a monolayer of dye D and each dye molecule anchors to both N and P surfaces. 15-19 When the conduction and valence band edges of the two semiconductors and the ground and excited levels of the dye are positioned as in Figure 1, the excited dye will inject electrons to the N material, following by hole injection to P material by the dye cation. Both dark and photocurrents involve electron transport to TiO 2 through CuPC and PD. Therefore the good dark rectification is an indication of back reaction suppression under illumination. If the two semiconductor surfaces are not touching each other, recombination could occur only via conduction through the dye molecule. Thus rectification by the dye molecule has a profound effect in preventing recombination, while allowing fast injection of carriers to the two semiconductors. The fact that some dyes used as sensitizers for solar cells yield photon to photocurrent conversion efficiencies (IPCEs) exceeding 80% 1,20 is an indication that these molecules possess * Corresponding author’s email: tenna@ifs.ac.lk. (1) Gratzel, M. Nature 2001, 338, 414. (2) Hagfeldt, A.; Gratzel, M. Chem. Rev. 1995, 95, 49. (3) Ellingson, R. J.; Asbury, J. B.; Ferrere, S.; Ghosh, H. N.; Sprague, J.; Lian, T.; Nozik, A. J. J. Phys. Chem. B 1998, 102, 10505. (4) Bauer, C.; Boschloo, G.; Mukhtar, E.; Hagfeldt, A. J. Phys. Chem. B 2002, 106, 12693. (5) Tachibana, Y.; Haque, S. A.; Mercer, I. P.; Durrant, J. R.; Klug, D. R. J. Phys. Chem. B 2000, 104, 1198. (6) Molecular Electronics; Jortner, J., Ratner, M., Eds.; Balckwell: London, 1997. (7) Aviram, A.; Ratner, M. Chem. Phys. Lett. 1974, 29, 277. (8) Metzger, R. M. Acc. Chem. Res. 1999, 32, 950. (9) Metzger, R. M.; Xu, T.; Peterson, I. R. J. Phys. Chem. B 2001, 105, 7280. (10) Joachim, C.; Gimzewski, J. K.; Aviram, A. Nature 2000, 408, 541. (11) Ashwell, G. J.; Tyrrell, W. D.; Whittam, A. J. J. Am. Chem. Soc. 2004, 126, 7102. (12) Okazaki, N.; Sambles, J. R.; Jory, M. J.; Aswell, G. J. Appl. Phys. Lett. 2000, 81, 2300. (13) Kushmerick, J. G.; Holt, D. B.; Pollack, S. K.; Ratner, M. A.; Yang, J. C.; Schull, T. L.; Naeiri, J.; Moore, H. M.; Shashidhar, R. J. Am. Chem. Soc. 2002, 124, 10654. (14) McCreeny, R.; Diaringer, J.; Solak, A. O.; Synder, B.; Nowak, A. M.; McGovern, W. R.; DuVall, S. J. Am. Chem. Soc. 2003, 125, 10748. (15) Tennakone, K.; Hewaparakkrama, K. P.; Dewasurendra, M. D.; Jayatissa, A. H.; Weerasena, L. K. Semicond. Sci. Technol. 1988, 3, 382. (16) Tennakone, K.; Kumara, G. R. R. A.; Kottegoda, I. R. M.; Wijayantha, K. G. U.; Sirimanne, P. M. Semicond. Sci. Technol. 1995, 10, 1689. (17) O’Regan, B.; Schwartz, D. T. Chem. Mater. 1995, 7, 1349. (18) O’Regan, B.; Schwartz, D. T. Chem. Mater. 1995, 10, 1501. (19) Bach, U.; Lupo, D.; Compte, P.; Moser, J. E.; Weissortel, F.; Salbeck, S.; Spreitzer, H.; Gratzel, M. Nature 1998, 395, 583. (20) Nazzerudddin, M. K.; Gratzel, M. In Semiconductor Electrodes and Photochemistry; Licht, S., Ed.; Wiley-VCH: Weinheim, 2002. Figure 1. A schematic energy level diagram showing band edge positions of n- and p-type semiconductors and ground (S 0 ) and excited (S*) levels of dye in a N/D/P heterojunction. 2997 Langmuir 2005, 21, 2997-3001 10.1021/la0469710 CCC: $30.25 © 2005 American Chemical Society Published on Web 02/15/2005