6598
Langmuir 1999, 15, 6598-6600
Electrochemically Active Surface Zirconium Complexes on Indium Tin Oxide Susan K. VanderKam, Ellen S. Gawalt, Jeffrey Schwartz,* and Andrew B. Bocarsly* Department of Chemistry, Princeton University, Princeton, New Jersey 08544-1009 Received January 28, 1999. In Final Form: May 11, 1999
Indium tin oxide (ITO) is a common transparent conductor1 which is widely used as an electrode material in, for example, organic light-emitting diode2-4 (OLED) displays. It is generally acknowledged3,5-8 that inefficient carrier injection from an ITO electrode into an organic layer of an OLED can lead to device failure; it may be that appropriate surface modification of ITO can be accomplished to enhance such carrier injection and, consequently, device performance. Commercial ITO is usually characterized1 according to its physical parameters, including surface resistance, grain size, direct optical band gap, and etch capabilities, but not by its chemical ones. Indeed, despite the large number of reports concerning device use of ITO,3,5-8 only few studies have focused on a possibly key issue of ITO surface chemistry: surface hydroxyl group density. It seems unlikely that interface design involving these surface OH groups can be optimized for device applications until this simple aspect of the surface chemistry of ITO, at least, is better described. Attachment of ferrocenyl compounds onto ITO via a silyl9,10 or alkanecarboxylic acid tethering functionality11 has been reported as a method both to enhance the electrochemical performance of ITO and to determine the number of electrochemically reactive sites bound per nominal unit area of ITO. Self-assembly of ferrocenylpyrrole and -thiophene derivatives through hydrogen bonding with surface OH groups has also been described.12 In each case, electrochemical determinations indicated surface coverage of ferrocenyl groups to be on the order of 10-10 mol/cm2 (attributed to a “monolayer”10,11). However, the surface density of reactive OH functionality on the ITO could not be estimated using these systems, since no information is available on the stoichiometry of the reaction between such functionality and the derivatization reagents. (1) Chopra, K. L.; Major, S.; Pandya, D. K. Thin Solid Films 1983, 102, 1-46. (2) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913-915. (3) Forrest, S. R.; Burrows, P. E.; Thompson, M. E. In Laser Focus World; PennWell Publishing Company: Tulsa, OK, 1995; Vol. 31, pp 99-107. (4) Burrows, P. E.; Gu, G.; Bulovic´, V.; Shen, Z.; Forrest, S. R.; Thompson, M. E. IEEE Transactions on Electron Devices; Institute of Electrical and Electronics Engineers: New York, 1997; Vol. 44, pp 11881203. (5) Greenham, N. C.; Moratti, S. C.; Bradley, D. D. C.; Friend, R. H.; Holmes, A. B. Nature 1993, 365, 628-630. (6) Gautier, E.; Lorin, A.; Nunzi, J. M.; Schalchli, A.; Benattar, J. J.; Vital, D. Proc. SPIEsInt. Soc. Opt. Eng. 1997, 3002, 104-107. (7) Scott, J. C.; Carter, S. A.; Karg, S.; Angelopoulos, M. Proc. SPIEs Int. Soc. Opt. Eng. 1997, 3002, 86-91. (8) Antoniadis, H.; Miller, J. N.; Roitman, D. B.; Cambell, I. H. IEEE Trans. Electron Devices 1997, 44, 1289-1294. (9) Chen, K.; Herr, B.; Singewald, E. T.; Mirkin, C. A. Langmuir 1992, 8, 2585-2587. (10) A similar study has been done for a cobalt complex. See, Herr, B.; Mirkin, C. A. J. Am. Chem. Soc. 1994, 117, 6927-6933. (11) Gardner, T. J.; Frisbie, C. D.; Wrighton, M. S. J. Am. Chem. Soc. 1995, 117, 6927-6933. (12) Zotti, G.; Schiavon, G.; Zecchin, S.; Berlin, A.; Pagani, G.; Canavesi, A. Langmuir 1997, 13, 2964-2968.
We have previously exploited the reactivity of surface OH groups of metal oxides for surface functional group quantification and modification using organometallic reagents,13-16 and we have reported that bonding of carboxylic acids to oxide surfaces can be stabilized using zirconium alkoxide complex-derived interfaces.16,17 Because of the ease of ligand metathesis in these surfacebound alkoxides, they represent a versatile interface for formation of families of surface-bound materials from a common, easy-to-prepare precursor.16,18,19 We now report that these synthetic methodologies can be used to bond a simple, electrochemically active ferrocenecarboxylate derivative to the ITO surface via ligand metathesis at Zr in a process that obviates tedious tethering group synthesis and attachment. Happily, we find that the Zr complex interface does not deleteriously affect the electrochemical behavior of the ligated ferrocenecarboxylate species. Importantly, because the stoichiometries of Zr complex surface chemisorption and subsequent ligand metathesis are known, electrochemical quantitation of surface complex-bound ferrocenecarboxylate ligand groups provides an indirect measure of ITO surface OH coverage. Experimental Part Reagents. ITO-coated (one side only) glass slides (Delta Technologies) had a resistance
