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Langmuir 2007, 23, 6424-6430
Nanostructured Copper Phthalocyanine-Sensitized Multiwall Carbon Nanotube Films Ross A. Hatton,* Nicholas P. Blanchard, Vlad Stolojan, Anthony J. Miller, and S. Ravi P. Silva Nanoelectronics Centre, AdVanced Technology Institute, UniVersity of Surrey, Guildford, Surrey GU2 7XH, United Kingdom ReceiVed January 18, 2007. In Final Form: March 1, 2007 We report a detailed study of the interaction between surface-oxidized multiwall carbon nanotubes (o-MWCNTs) and the molecular semiconductor tetrasulfonate copper phthalocyanine (TS-CuPc). Concentrated dispersions of o-MWCNT in aqueous solutions of TS-CuPc are stable toward nanotube flocculation and exhibit spontaneous nanostructuring upon rapid drying. In addition to hydrogen-bonding interactions, the compatibility between the two components is shown to result from a ground-state charge-transfer interaction with partial charge transfer from o-MWCNT to TS-CuPc molecules orientated such that the plane of the macrocycle is parallel to the nanotube surface. The electronegativity of TS-CuPc as compared to unsubsubtituted copper phthalocyanine is shown to result from the electron-withdrawing character of the sulfonate substituents, which increase the molecular ionization potential and promote cofacial molecular aggregation upon drying. Upon spin casting to form uniform thin films, the experimental evidence is consistent with an o-MWCNT scaffold decorated with phthalocyanine molecules self-assembled into extended aggregates reminiscent of 1-D linearly stacked phthalocyanine polymers. Remarkably, this self-organization occurs in a fraction of a second during the spin-coating process. To demonstrate the potential utility of this hybrid material, it is successfully incorporated into a model organic photovoltaic cell at the interface between a poly(3hexylthiophene):[6,6]-phenyl-C61 butyric acid methyl ester bulk heterojunction layer and an indium-tin oxide-coated glass electrode to increase the light-harvesting capability of the device and facilitate hole extraction. The resulting enhancement in power conversion efficiency is rationalized in terms of the electronic, optical, and morphological properties of the nanostructured thin film.
Introduction Research pertaining to the interaction between carbon nanotubes and conjugated organic molecules is attracting growing interest as a new topic of fundamental research with numerous potential applications ranging from sensing1 to photovoltaics.2 Combining the remarkable electrical, thermal, and mechanical properties of carbon nanotubes3 with the optoelectronic properties of conjugated organic compounds is also a promising path to realizing hybrid composite materials for utilization in emerging disruptive technologies such as nanoelectronics.2,4-6 Increasingly, materials systems exhibiting spontaneous nanostructuring upon processing from solution have also come to the fore as an economical path to realizing optimized device structures.7,8 An excellent exampleswhich has achieved widespread utilization in organic photovoltaicssis blends of regioregular poly(3hexylthiophene) and [6,6]-phenyl-C61 butyric acid methyl ester which spontaneously phase separate into domains with dimensions * To whom correspondence should be addressed. E-mail: r.hatton@ surrey.ac.uk. Fax: +44 (0)1483 689404. (1) Guo, X.; Huang, L.; O’Brien, S.; Kim, P.; Nuckolls, C. J. Am. Chem. Soc. 2005, 127, 15045-15047. (2) Guldi, D. M.; Rahman, G. M. A.; Prato, M.; Jux, N.; Qin, S.; Ford, W. Angew. Chem., Int. Ed. 2005, 44, 2015-2018. (3) Harris, P. J. F. Carbon Nanotubes and Related Structures; 1st ed.; Cambridge University Press: Cambridge, U.K., 1999. (4) Miller, A. J.; Hatton, R. A.; Silva, S. R. P. Appl. Phys. Lett. 2006, 89, 123115 (1-3). (5) Kymakis, E.; Koudoumas, E.; Franghiadakis, I.; Amaratunga, G. A. J. J. Phys. D 2006, 39, 1058-1062. (6) Curran, S. A.; Ajayan, P. M.; Blau, W. J.; Carroll D. L.; Coleman, J. N.; Dalton, A. B.; Davey, A. P.; Drury, A.; McCathy, B.; Maier, S.; Strevens, A. AdV. Mater. 1998, 10, 1091-1093. (7) Elemans, J. A. A. W.; van Hameren, R.; Nolte, R. J. M.; Rowan A. E. AdV. Mater. 2006, 18, 1251-1266. (8) Cheng, J. Y.; Ross, C. A.; Smith, H. I.; Thomas, E. L. AdV. Mater. 2006, 18, 2505-2521.
of ∼10 nm.9 By judicious selection of the solvent a high degree of favorable intermolecular order between polymer molecules in the poly(3-hexylthiophene) phase is also realized, maximizing the overlap between frontier molecular orbitals on adjacent molecules.10,11 For photovoltaic device applications the former is important since it ensures that all photoexcited molecules are within an exciton diffusion length of a dissociating interface, while the latter ensures that the positive charge carriers formed upon exciton dissociation are efficiently transported to the external circuit.10,11 To date the research effort in hybrid carbon nanotubeconjugated molecule systems has largely focused on the use of single-wall carbon nanotubes (SWCNTs).2,5,12-14 A major complication with SWCNTs is that they are a mixture of metallic and semiconducting tubes, complicating the interpretation of experimental data.3 Conversely, as a result of their larger diameter and more complex multilayered structure, multiwall carbon nanotubes (MWCNTs) are invariably metallic, offering far more predictable functionality.3 Notably, both types of carbon nanotubes exhibit poor solubility in common solvents unless chemically functionalized or stabilized by a physical interaction with a soluble molecule. (9) Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J. AdV. Funct. Mater. 2005, 15, 1617-1622. (10) Li, G.; Shrotriya, V.; Huang, J. S.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Nat. Mater. 2005, 4, 864-868. (11) Kim, Y.; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C.; Giles M.; Mcculloch I.; Ha C. S.; Ree, M. Nat. Mater. 2006, 5, 197-203. (12) Hasobe, T.; Fukuzumi S.; Kamat, P. V. J. Am. Chem. Soc. 2005, 127, 11884-11885. (13) Steuerman, D. W.; Star, A.; Narizzano, R.; Choi, H.; Ries, R. S.; Nicolini, C.; Stoddart, J. F.; Heath, J. R. J. Phys. Chem. B 2002, 106, 3124-3130. (14) Keogh, S. M.; Hedderman, T. G.; Gregan, E.; Farrell, G.; Chambers, G.; Byrne H. J. J. Phys. Chem. B 2004, 108, 6233-6241.
10.1021/la070156d CCC: $37.00 © 2007 American Chemical Society Published on Web 04/18/2007
Nanostructured CuPc-Sensitized MWCNT Films
Langmuir, Vol. 23, No. 11, 2007 6425
Unlike the vast majority of reports pertaining to carbon nanotube-conjugated organic molecule systems, in this study water is used as the solvent. As such this work contributes to an increasingly important and rapidly growing body of research pertaining to the nanostructured materials process from aqueous solutions motivated by the requirement for environmental compatibility and growing interest in the nascent field of bionanotechnology. Experimental Methods
Figure 1. A typical TEM image of a drop-cast film of an o-MWCNT: TS-CuPc composite supported on a nickel grid. Inset: TS-CuPc.
Thus far, the interactions between MWCNTs and small conjugated molecules (molecular semiconductors) such as phthalocyanines,15 porphyrins,16 and perylenes17 have received relatively little research attention. Unlike conjugated polymers, molecular semiconductors have well-defined electronic structures since they comprise a fixed number and arrangement of atoms. Furthermore, by peripheral functionalization with polar moieties it is possible to tune the energy of the frontier molecular orbits of small conjugated molecules by as much as 1 eV.18 Peripheral functionalization of the conjugated macrocycle is also an effective means of tuning the intermolecular arrangement and the extent of spontaneous aggregation upon removal of the solvent.19 These properties, combined with enhanced stability toward photobleaching and thermally induced degradation as compared to those of most conjugated polymers, make MWCNT-conjugated small molecule systems interesting from both a fundamental and an applied perspective. In this study the soluble molecular semiconductor 3,4,4,4tetrasulfonic acid tetrasodium salt copper phthalocyanine (TSCuPc) (Figure 1 (inset)) is blended with surface-oxidized MWCNTs (o-MWCNTs) in water to form a stable composite ink. Upon rapid drying via spin coating, the resulting film is shown to comprise an o-MWCNT scaffold decorated with phthalocyanine molecules assembled into extended aggregates and orientated perpendicular to the nanotube surface. This propensity for self-organization is rationalized in terms of the electronic properties of the two components and the nature of the molecule-nanotube and intermolecular interactions. Furthermore, to demonstrate the utility and probe the dynamic properties of this hybrid system, it must be incorporated into a model organic photovoltaic device. Cell performance is rationalized in terms of the interfacial valance state alignment and optical and morphological properties of the two components. (15) Wang, Y.; Chen, H-Z.; Li, H-Y.; Wang, M. Mater. Sci. Eng., B 2005, 117, 296-301. (16) Guldi, D. M.; Rahman, G. M. A.; Jux, N.; Balbinot, D.; Tagmatarchis, N.; Prato, M. Chem. Commun. 2005, 2038-2040. (17) Feng, W.; Fujii, A.; Ozaki, M.; Yoshino, K. Carbon 2005, 43, 25012507. (18) Peisert, H.; Knupfer, M.; Schwieger, T.; Fuentes, G. G.; Olligs, D.; Fink, J.; Schmidt, Th. J. Appl. Phys. 2003, 93, 9683-9692. (19) Hassan, B.; Li, H.; McKeown, N. B. J. Mater. Chem. 2000, 10, 39-45.
MWCNTs (>90 wt %) grown by chemical vapor deposition were obtained commercially (Nanocyl) and used as received. The typical diameter and length of the MWCNT were confirmed using transmission electron microscopy (TEM) to be 10 nm and several micrometers, respectively. Stable dispersions of o-MWCNTs in deionized water were prepared by ultrasonically dispersing carbon nanotubes in a 3:1 mixture of concentrated sulfuric and nitric acids for 10 min. The mixture was refluxed at 130 °C for 60 min before dilution with HPLC-grade deionized water. The very low pH of the resulting solution destabilizes the o-MWCNTs, causing them to settle out. To accelerate this process, the dispersion was centrifuged and the acid supernatant decanted off. The remaining solid was repeatedly washed with deionized water over a 50 nm polycarbonate filter until the washings were pH 6-7. Before the o-MWCNTs were allowed to dry, the entire filter paper was submerged in a small quantity of deionized water whereupon the o-MWCNTs spontaneously dispersed to form a concentrated dispersion. The resulting dispersion was centrifuged to remove residual o-MWCNT aggregates. Concentrated aqueous solutions of 3,4,4,4-tetrasulfonic acid tetrasodium salt copper phthalocyanine (Sigma-Aldrich) were filtered using a 0.1 µm mixed cellulose ester filter prior to being blended with concentrated o-MWCNT dispersions of known nanotube loading. To determine the actual o-MWCNT and TS-CuPc loadings in solution prior to blending, a sample of known volume was completely dried and weighed. The resulting nanocomposite solutions required no further preparative treatment and were stable for >6 months. The composite films were characterized using a Philips CM200 ST transmission electron microscope with an LaB6 filament and a 200 kV accelerating voltage. Figure 1 was taken with an objective aperture of 100 µm at a large defocus (∼1.5-2 µm). Absorbance spectra were obtained using a UV-vis Varian Cary 5000 UVvis-NIR spectrometer. The X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) measurements were made using an Omicron multiprobe system, with a base pressure of 0.4 eV) at one or the other electrode. Since the electron-extracting electrode is unchanged in the present case, any change in the energy step to carrier extraction must occur at the interface with the holeextracting electrode. The difference in the current-voltage characteristics between cells incorporating TS-CuPc only and the composite implies that o-MWCNTssas part of a composite with TS-CuPcsare in direct electrical contact with the underlying ITO electrode, since the work function of o-MWCNTs (5.0 eV) is very much larger than that of ITO glass (4.4 eV), greatly reducing the energy step from the HOMO of TS-CuPc. This result highlights the important role played by o-MWCNTs in ensuring that the cell fill factor is not compromised. Overall, cells utilizing this hybrid layer at the ITO/PCBM: P3HT interface exhibit the best cell characteristics, having a power conversion efficiency ∼25% larger than that of any of the reference devices (1.25% vs e1%). These results clearly demonstrate that this material can be processed into thin films suitable for incorporation into thin film optoelectronic devices and has a strong potential to enhance the power conversion efficiency of bulk heterojunction organic photovoltaic cells. The experimental evidence indicates that the TS-CuPc phase enhances light absorption, while the o-MWCNT phase facilitates conduction of holes to the underlying ITO electrode owing to close alignment of the relevant valance states. The maximum power conversion efficiency in optimized organic photovoltaics utilizing this novel hybrid material will be the subject of a future paper.
Conclusions In summary, a detailed study of the interaction between surfaceoxidized multiwall carbon nanotubes and the molecular semiconductor TS-CuPc is reported. The sulfonate moieties attached to the periphery of the phthalocyanine macrocycle are shown to modify the molecular ionization potential, promote the formation of extended cofacial molecular aggregates in the solid state, and play a pivotal role in determining the nature of the charge-transfer interaction with o-MWCNTs. Upon spin casting to form a uniform thin film, the experimental evidence is consistent with an o-MWCNT scaffold decorated with phthalocyanine molecules self-assembled into extended aggregates reminiscent of 1-D linearly stacked phthalocyanine polymers and orientated perpendicular to the o-MWCNT surface. Remarkably, this selforganization occurs in a fraction of a second during the spincoating process. To demonstrate the utility and probe the dynamic properties of this hybrid system, it is incorporated into a model organic photovoltaic device. The improvement in cell performance is rationalized in terms of the interfacial valence-state alignment and optical and morphological properties of the two components. Crucially, this material is processed from an aqueous solution, thereby maximizing its environmental compatibility. Acknowledgment. We acknowledge Dr. Yann Tison for useful discussions pertaining to the XPS analysis and Dr. David Cox for help with the AFM and SEM analysis. We also thank the United Kingdom Engineering and Physical Science Research Council for funding this research via the Portfolio Partnership Award. LA070156D (36) Nelson, J.; Kirkpatrick, J.; Ravirajan P. Phys. ReV. B 2004, 69, 035337, 1-11.
