NANO LETTERS
Enhancement of Charge-Transport Characteristics in Polymeric Films Using Polymer Brushes
2006 Vol. 6, No. 3 573-578
Gregory L. Whiting,† Henry J. Snaith,‡ Saghar Khodabakhsh,† Jens W. Andreasen,§ Dag W. Breiby,§ Martin M. Nielsen,§ Neil C. Greenham,‡ Richard H. Friend,*,‡ and Wilhelm T. S. Huck*,† MelVille Laboratory for Polymer Synthesis, Department of Chemistry, UniVersity of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K., CaVendish Laboratory, UniVersity of Cambridge, Madingley Road, Cambridge CB3 0HE, U.K., and POL-124 Risoe National Laboratory, P.O. Box 49, DK-4000 Roskilde, Denmark Received September 8, 2005; Revised Manuscript Received January 30, 2006
ABSTRACT We show that charge-transporting polymer chains in the brush conformation can be synthesized from a variety of substrates of interest, displaying a high degree of stretching and showing up to a 3 orders of magnitude increase in current density normal to the substrate as compared with a spin-coated film. These nanostructured polymeric films may prove to be suitable for electronic devices based on molecular semiconductors as current fabrication techniques often provide little control over film structure.
Over the past decade considerable progress has been made in the development of electronic devices based on chargetransporting organic molecules.1 However, the low longrange mobility of charges within these materials has limited the ultimate utility of these devices. To improve the characteristics of such devices further, it is essential to increase the ordering of the components of the organic layer to provide direct pathways for the movement of charge though the film.2 In this report we examine the use of polymer brushes, synthesized via a surface-initiated polymerization,3 as a method for alignment of hole-transporting polymer chains, out of the plane in a thin film. Polymer brushes have previously been used to control surface properties such as adhesion, corrosion resistance, and wettability,4 and recently as dielectric films in field-effect transistors.5 The vertically stretched nature of polymer brushes6 makes them ideal candidates to incorporate as an electroactive component within organic semiconductor devices, potentially improving charge transport properties, providing clear pathways for charge transport in the direction normal to the substrate. Here we describe the synthesis of poly(triphenylamine acrylate) brushes from various surfaces, * Corresponding authors: R.H.F., tel +44 (0)1223 337218, fax +44 (0)1223 353397, e-mail
[email protected]; W.T.S.H., tel +44 (0)1223 334370, fax +44 (0)1223 334866, e-mail
[email protected]. † Melville Laboratory for Polymer Synthesis, Department of Chemistry, University of Cambridge. ‡ Cavendish Laboratory, University of Cambridge. § POL-124 Risoe National Laboratory. 10.1021/nl051803t CCC: $33.50 Published on Web 02/08/2006
© 2006 American Chemical Society
demonstrate the vertically stretched structure of these polymer chains, and show up to a 1000-fold increase in current density through the polymer brush film, as compared to a spin-coated film of the same polymer. In this study we have focused on brushes composed of poly(triphenylamine acrylate) (PTPAA), which has been shown to exhibit a respectable hole transport mobility in amorphous films.7,8 The monomer is prepared following a method similar to that of Tamada, et al.8 The route used for the synthesis of PTPAA brushes via atom transfer radical polymerization (ATRP) and a schematic representation of the polymer brushes are shown in Figure 1. Here, the triphenylamine acrylate monomer was polymerized either in the bulk by a free radical polymerization, using azobisisobutyronitrile (AIBN), or directly from a substrate modified with an initiator-terminated self-assembled monolayer, using ATRP. The surface-bound trichlorosilane ATRP initiator was prepared following published procedures.9 PTPAA brush growth was typically carried out at 90 °C, in N,N-dimethylformamide (DMF) using a CuBr/N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA) catalyst. Figure 2a shows kinetic plots for the synthesis of PTPAA brushes. An inverse relationship between film thickness and catalyst concentration is observed, as is expected from the kinetic expressions and as has previously been shown by Kim et al.10 Polymer brushes up to 80 nm thick, as measured by ellipsometry, were grown on Si/SiO2 and indium tin oxide (ITO) substrates (a commonly used transparent electrode for
Figure 1. Overview of PTPAA brushes: (a) scheme for the surface-initiated synthesis of PTPAA brushes; (b) cartoon representation of a polymer brush and spin-coated film.
organic electronic devices). Both surfaces gave very similar thicknesses, as measured by atomic force microscopy (AFM, scratch test). Polymer brushes were also synthesized from initiator-modified Spectrosil substrates and from spin-coated poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) films. PEDOT/PSS is commonly used as an electrode, or an electron blocking layer, in organic electronic devices. As received, PEDOT/PSS is soluble in N,Ndimethylformamide (the solvent used for the polymer brush synthesis). However, the PEDOT/PSS film can be made highly insoluble by cross-linking the PSS with multivalent ions such as Fe2+/Fe3+.11,12 After a brief (30 s) exposure to a dry air plasma (which does not alter the electronic properties of PEDOT/PSS), it is possible to contact print13 (with either a flat or patterned stamp) the silane ATRP initiator and grow the polymer brushes from the PEDOT/ PSS surface. These films were also found to be of similar thickness to those grown from modified silicon, Spectrosil, and ITO substrates. Figure 2b shows UV-vis absorption spectra indicating the presence of PTPAA brushes on both ITO and PEDOT/PSS surfaces. Figure 2c shows an AFM image of PTPAA brushes synthesized from a cross-linked PEDOT/PSS surface, where the silane initiator has been printed with a patterned poly(dimethylsiloxane) (PDMS) stamp (2 × 2 µm lines). These results for different surfaces show that the polymer brush synthesis is broadly applicable for the types of substrates commonly used in organic electronic devices. In surface-initiated polymerizations, the polymer chains are grown directly from surface-tethered initiator sites. If the density of attachment sites (grafting density) is sufficiently 574
Figure 2. Synthesis of PTPAA brushes. (a) Plots of PTPAA brush film thickness versus time for 50:1 (squares, solid line), 75:1 (circles, dashed line), 100:1 (upward triangles, dotted line), and 300:1 (downward triangles, dot-dash line) monomer:catalyst ratios. All polymerizations were carried out under N2, at 90 °C, with a 5:1 ligand:catalyst ratio, and had a 1:1 monomer (g):solvent (mL) ratio. (b) UV-vis absorption spectra showing the presence of PTPAA brushes on ITO (45 nm film) (dot-dash line) and PEDOT/ PSS (58 nm film) (dashed line). Solution spectra of PTPAA in CHCl3 (solid line) is presented for comparison. (c) Tapping mode AFM image of 2 × 2 µm lines of PTPAA brushes on a crosslinked PEDOT/PSS surface.
high, then the polymer chains are forced into a conformation where they stretch away from the surface, the “polymer brush” regime.14 To probe the composition and structure of these polymer brushes, total reflection X-ray fluorescence Nano Lett., Vol. 6, No. 3, 2006
(TXRF) was used. In TXRF a surface is irradiated with X-rays at an angle less than that required for total external reflection. The excited atoms then fluoresce, with emitted X-rays characteristic of the particular elements. By analysis of these characteristic fluorescence X-rays, surface concentrations (for the first few nanometers) of the elements can be quantified. For a polymer synthesized via ATRP, the termini of the polymer chains will be bromine capped (as CuBr was used as a catalyst),15 so an estimation of the grafting density of the chains can be found from the surface concentration of bromine. Using TXRF, a surface concentration (C) of 1.4 × 1013 atoms cm-2 was found for bromine on a 35 nm PTPAA brush film, giving an estimated grafting density of 0.14 chains nm-2. For a spin-coated PTPAA film of similar thickness (synthesized via an uncontrolled freeradical polymerization) the surface concentration of bromine was found to be about a factor of 40 lower, confirming that the increased bromine concentration of the brush film is due to the bromine end groups expected when synthesizing a polymer using ATRP. Using the bulk film density (F) of a PTPAA brush film (1.19 g cm-3, as measured by X-ray reflectivity), the average molecular weight (MW(P)), given by MW(P) ) hdryFNA/C (where NA is Avogadro’s number), of polymer chains in a brush film with a dry thickness (hdry) of 35 nm is estimated to be 2 × 105 g mol-1. The ratio of the dry thickness to the maximum possible (fully stretched) length, h/hmax, for a polymer chain of this molecular weight, given by hdry/hmax ) CMW(M)/2FNAlC-C sin(0.5τ) (where lC-C is the C-C bond length, τ is the tetrahedral angle, and MW(M) is the molecular weight of the monomer), is estimated as 0.3, in good agreement with previous results for PMMA brushes.16 In a thin film, a completely unperturbed polymer of this molecular weight would occupy a cube of 6 nm height, which means that in these 35 nm thick polymer brush films, the polymers are stretched away from the surface by a factor of approximately 6. To examine the electronic characteristics of the brush films, diodes were fabricated from both brush films and spincoated films of the same polymer. An ITO substrate was used as the anode, and a PEDOT/PSS layer was spin coated on top of the polymer film followed by a thermally evaporated gold cathode (Figure 3a). The current density through brush films (or spin-coated films of identical thickness) was measured, and the current-voltage characteristics are presented in parts b and c of Figure 3. The 80 nm thick PTPAA brush film exhibits 3 orders of magnitude higher current density than the spin-coated PTPAA film (2.2 A m-2 as opposed to 2.5 × 10-3 A m-2 at 3.5 V applied bias) (Figure 3b). We note that this increase is seen in both forward and reverse bias, illustrating that the initiator monolayer itself is not responsible for this effect. Also shown are the current density characteristics for a 35 nm PTPAA brush and spin-coated film (Figure 3c). These data also show an increase in current density for the brush film, with the difference between the brush and spin-coated film being less pronounced at 35 nm than at 80 nm. As compared to more commonly used conjugated hole-transporting polymers, the PTPAA brush diodes perform similarly. A device structured Nano Lett., Vol. 6, No. 3, 2006
Figure 3. Properties of PTPAA brushes. (a) Schematic diagram of the diode structure used for current density testing. (b) Current density versus applied bias for a sandwich structure device of ITO/ PTPAA (80 nm)/PEDOT:PSS/Au, with PTPAA brushes (solid line) and PTPAA spin-coated amorphous film (dashed line). Positive bias corresponds to hole injection from the PEDOT:PSS and negative bias corresponds to hole injection from the ITO. (c) Current density versus applied bias for a unipolar diode made with 35 nm PTPAA films: brush (solid line), spin coated (dashed line).
identically to those above, fabricated with a 80 nm spincoated poly(9,9-dioctylfluorene-co-bis-N,N′-(4-butylphenyl)bis-N,N′-phenyl-1,4-phenylenediamine) (PFB) film (a common hole transporter used in optoelectronic device applications)17,18 exhibited over 1 order of magnitude lower current density than through the PTPAA brush diode (5 × 10-2 A m-2 for PFB as opposed to 2.2 A m-2 for a PTPAA brush at 3.5 V applied bias). To deduce the cause for increased charge transport in the brush film, we need to consider the mechanism by which charge transport occurs in this polymer. PTPAA is comprised of an inert backbone with active TPA side chains. Charge transport is likely to occur predominantly through hopping between adjacent TPA units, which are known to stack or aggregate in solution.7,8 In this manner we expect PTPAA to act much like a conjugated polymer with good “intrachain” transport. However, the hole-mobility in this material is 575
Figure 4. Modeling results. (a) Schematic diagram showing the boundary conditions experienced by the random walker. (b) “IV” curve for cuboid (solid line) and cube (a ) 20) (dashed line) cases. (c) Ratio of N/nab (cuboid case) to N/nab (cube case) versus the volume ratio of each cuboid (VB) to cube (VS), varying VS, at a bias of 2.5. (d) Ratio of the number of jumps to an adjacent shape for the cube case (NjS) to the cuboid case (NjB) versus bias.
thought to be limited by conformational traps, due to “misaligned” TPA units.19 The much-improved conductivity for the brush layers suggests that the different structure of the PTPAA brush film provides clearer pathways for charge transport than a spin-coated film. In the spin-coated films, the polymers are mostly disordered and do not present pathways where charges can travel from one electrode to the other along one polymer chain. Spin-coated films are in fact more likely to exhibit a certain degree of chain alignment in the plane of the film.20 Thus, the charges will need to hop between chains much more often than those in the brush system, which will significantly impede charge transport, and this effect will become amplified as film thickness increases. To further our understanding of these systems, we have developed a simple model for charge transport to compare the brush and the spin-coated polymer regimes, based upon the film structures observed and described above. In our model a random walker (charge) starts at a position on the xy plane described by z ) 0 (injecting electrode) and always makes steps of a unit distance in bounded three-dimensional (3D) space. The random walker is bounded by a reflecting barrier at z ) 0, and an absorbing barrier at z ) h (collecting electrode), where h is the total allowed z travel for the random walk. To simulate the morphology of the polymer chains in the film, the walker is confined, by partially reflecting barriers, either to a cuboid (brush case) with a ) b * c (where c ) h) or to a cube (spin-coated case), Figure 4a. At some defined transfer probability (pt) the barrier between cubes or cuboids will be reflecting (a step over the 576
barrier is not allowed) or absorbing (a step over the barrier is allowed, and the walker moves to an adjacent cube or cuboid). These partially reflecting barriers allow the walker to “see” the structure of its environment, which is designed to roughly simulate the structure of polymer chains in a brush or spin-coated film. At a transfer probability of 1 the cube or cuboid barriers will not exist, the walker will not experience any internal structure within the film, and both experiments will be identical unbounded random walks in 3D space. As the transfer probability is decreased, the walker will increasingly be influenced by the structure of its internal environment. To simulate current-voltage data, a bias is applied to the walker using a simple Metropolis Monte Carlo method, where steps against the field are tested using a Boltzmann probability (p), such that p ) e-Vq∆z/hkT, where V is bias, q is the charge of the electron, k is the Boltzmann constant, and T is temperature (kept at 298 K). For each defined bias the model outputs the number of jumps (Nj) to an adjacent cube or cuboid, as well as the ratio of the total number of walkers to reach the collecting barrier (N, which is directly proportional to charge) to the total number of steps taken to reach the collecting barrier (nab, which is directly proportional to time). We therefore consider N/nab to be the simulated current density Js. Figure 4 shows some results generated using this model. For each of these experiments N ) 1000 and pt ) 0.001. These values were used as they led to reasonable averages and computing times, and we found that this value for transfer probability suitably allowed the walker to experience Nano Lett., Vol. 6, No. 3, 2006
its internal environment. Also, for all of these simulations a(cuboid) ) 27 and h ) 350, as these are the correct scaled values for the polymer chains in the brush film based on the observed value for grafting density found for a 35 nm PTPAA brush film. Figure 4b shows a comparison of the results obtained for running the models using the values given above for a polymer brush film, and spin-coated film with a(cube) ) 20. This value is used as it is the correct scaled value (relative the size of the cuboids in the brush film), for a polymer with a molecular weight of 1 × 104 g mol-1, which was typical of the polymers used to create the spin-coated films. These data show that using these input values, the “current” through the simulated polymer brush film is over an order of magnitude higher than that through the simulated spin-coated film. The effect of changing the molecular weight, and therefore the volume of the cube, for the polymer chains in the spin-coated film is displayed in Figure 4c. As the molecular weight of the spin-coated polymer chains is decreased relative to the molecular weight of the polymer brush chains, it takes more time for the walker to reach the collection barrier and leads to larger divergence in simulated current densities between the brush and spin-coated cases. It is also noted, that when identical molecular volumes are used (VB/VS ) 1), there is still roughly an order of magnitude difference in current density between the two cases, suggesting that the geometry of the two films themselves leads to higher currents in the polymer brush films. These data show that not only the geometry of polymer brushes but also the ability to fabricate films containing high molecular weights (not easily achievable with a spin-coated polymer film, due to solubility issues) allow charges to transport more quickly through the polymer brush film, as compared with a spin-coated film of the same polymer. Figure 4d shows that due to the geometry of the films, a charge in the spincoated film will have to make many more hops to adjacent chains in order to reach the collection electrode as compared with charges confined to a polymer brush morphology. From this, it is expected that slower interchain hopping will play more of a role in charge-transport through a spin-coated film, as opposed to a brush film. Overall, these models demonstrate that charge transport through “polymer brush morphology” is expected to be significantly higher than charge transport through a film composed of polymer chains in a morphology similar to that of a spin-coated film. To conclude that the observed increase in current density is due to the polymer brush morphology, we must also rule out other possible origins of this effect. In particular, an increase in the film roughness, or contamination from the metal catalyst used to synthesize the brushes via ATRP (Cu), could possibly influence the charge transport through the film. AFM results over a 100 µm2 area show that the root mean square (rms) roughness of a PTPAA brush film on ITO is similar to bare ITO itself (rms roughness ∼2 nm), which demonstrates that the roughness of the brush film is determined primarily by the roughness of the underlying substrate. X-ray reflectivity was used to study the nature of the brush films in more depth. This technique gives information on properties such as film thickness, roughness, and Nano Lett., Vol. 6, No. 3, 2006
Figure 5. PTPAA brush film profiles. (a) X-ray reflectivity data for a 50 nm PTPAA brush film on Si/SiO2 (open squares) and fit (gray line). (b) X-ray reflectivity depth profile (thick black line) with 90% confidence boundaries (thin lines with gray shading) for a 50 nm PTPAA brush film. Gray shading below 0 Å indicates the substrate. (c) SIMS depth profile for a 45 nm PTPAA brush film (square points), including a linear regression line (solid line).
density. It can also be used for depth profiling of polymeric films. Interference data (Figure 5a) and depth profiling data (Figure 5b) are shown for a 50 nm PTPAA brush film on Si/SiO2. These data give a roughness for the brush film of 1.2 nm over a large area (X-ray spot size ∼1 cm2), confirming that the polymer brush films are very smooth on both the micro- and macroscale. An increase in surface roughness is therefore most likely not the cause of the increased charge density. 577
Secondary ion mass spectrometry (SIMS) and TXRF were used to quantify copper contamination in the brush films due to the ATRP catalyst. SIMS experiments confirm the presence of copper contamination in a brush film. Depth profiling SIMS shows that the copper levels within the PTPAA brush film decrease through the film, with the highest level at the polymer air interface (Figure 5c). Using TXRF the level of copper contamination within the film is roughly 1.5 copper atoms per polymer chain (0.05 wt %, 0.08 vol %). It is known, however, that for effective percolation pathways between spheres in a matrix, 3 orders of magnitude higher loadings of conducting material are required.21,22 In our films, even if well dispersed, the distance between the atoms will be too large for effective hopping to occur. Thus, the level of copper contamination in the PTPAA brush films will not facilitate effective charge transport, especially since we expect copper to be in its Cu(II) oxidation state, which would not facilitate hole transport. Furthermore, spin-coated PTPAA films (synthesized without a copper catalyst) show relatively similar concentrations (0.02 wt %) of copper contamination, and experiments on deliberately copper-contaminated spin-coated PTPAA films (not shown) did not show increases in current density at these concentration levels. Thus, it is highly unlikely that the copper contamination is causing the increased current density observed through the brush film. In conclusion, we have shown that charge-transporting polymer brushes can be synthesized from a variety of surfaces relevant for organic electronic device fabrication. These polymer brush films contain a greater level of ordering at the molecular level and display significantly higher characteristics of charge mobility compared to a spin-coated film of the same polymer. We believe that polymer brush layers are well suited for use in electronic devices such as photovoltaics, light emitting diodes (LEDs), and vertical transistors. Recently, we have examined the use of these polymeric layers as a hole-accepting component for a hybrid photovoltaic device, where we observe increased charge collection efficiency with a polymer brush layer as compared to a spin-coated layer. Experimental results on these devices are reported elsewhere.23 Acknowledgment. This work was supported by The Engineering and Physical Sciences Research Council (EPSRC) (GR/R97122/01) and EU Integrated Project NAIMO (NMPCT-2004-500355). The authors acknowledge Richard Chater, Paul Ebblewhite, Steve Edmondson, Chris Mulcahy, and Carlos Silva for valuable discussions and experimental assistance.
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Note Added after ASAP Publication. A Supporting Information paragraph was not included with the version published ASAP February 8, 2006; the correct version was published March 8, 2006. Supporting Information Available: Details of the X-ray reflectometry study. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Shaw, J. M.; Seidler, P. F. IBM J. Res. DeV. 2001, 45 (1), 3-9. (2) Dimitrakopoulos, C. D.; Marscaro, D. J. IBM J. Res. DeV. 2001, 45 (1), 11-27. (3) Edmondson, S.; Osborne, V. L.; Huck, W. T. S. Chem. Soc. ReV. 2004, 33 (1), 14-22. (4) Jones, D. M.; Smith, J. R.; Huck, W. T. S.; Alexander, C. AdV. Mater. 2002, 14 (16), 1130-1134. (5) Rutenberg, I. M.; Scherman, O. A.; Grubbs, R. H.; Jiang, W. R.; Garfunkel, E.; Bao, Z. J. Am. Chem. Soc. 2004, 126 (13), 40624063. (6) Milner, S. T. Science 1991, 251 (4996), 905-914. (7) Stolka, M.; Pai, D. M.; Renfer, D. S.; Yanus, J. F. J. Polym. Sci., Polym. Chem. Ed. 1983, 21 (4), 969-983. (8) Tamada, M.; Koshikawa, H.; Hosoi, F.; Suwa, T.; Usui, H.; Kosaka, A.; Sato, H. Polymer 1999, 40 (11), 3061-3067. (9) Husseman, M.; Malmstro¨m, E. E.; McNamara, M.; Mate, M.; Mecerreyes, D.; Benoit, D. G.; Hendrick, J. L.; Mansky, P.; Huang, E.; Russel, T. P.; Hawker, C. J. Macromolecules 1999, 32 (5), 14241431. (10) Kim, J. B.; Huang, W. X.; Miller, M. D.; Baker, G. L.; Bruening, M. L. J. Polym. Sci., Polym. Chem. 2003, 41 (3), 386-394. (11) Ghosh, S.; Rasmusson, J.; Inganas, O. AdV. Mater. 1998, 10 (14), 1097-1099. (12) Vazquez, M.; Danielsson, P.; Bobacka, J.; Lewenstam, A.; Ivaska, A. Sens. Actuators, B 2004, 97 (2-3), 182-189. (13) Fichet, G.; Corcoran, N.; Ho, P. K. H.; Arias, A. C.; MacKenzie, J. D.; Huck, W. T. S.; Friend, R. H. AdV. Mater. 2004, 16 (21), 19081912. (14) Wu, T.; Efimenko, K.; Genzer, J. T. J. Am. Chem. Soc. 2002, 124 (32), 9394-9395. (15) Patten, T. E.; Matyjaszewski, K. AdV. Mater. 1998, 10 (12), 901915. (16) Yamamoto, S.; Ejaz, M.; Tsujii, Y.; Matsumoto, M.; Fukuda, T. Macromolecules 2000, 33 (15), 5602-5607. (17) Arias, A. C.; Corcoran, N.; Banach, M.; Friend, R. H.; MacKenzie, J. D.; Huck, W. T. S. Appl. Phys. Lett. 2002, 80 (10), 1695-1697. (18) Morteani, A. C.; Dhoot, A. S.; Kim, J. S.; Silva, C.; Greenham, N. C.; Murphy, C.; Moons, E.; Cina, S.; Burroughes, J. H.; Friend, R. H. AdV. Mater. 2003, 15 (20), 1708-1712. (19) Slowik, J. H.; Chen, I. Bull. Am. Phys. Soc. 1977, 22 (3), 434. (20) Ramsdale, C. M.; Greenham, N. C. AdV. Mater. 2002, 14 (3), 212215. (21) Toker, D.; Azulay, D.; Shimoni, N.; Balber, I.; Milo, O. Phys. ReV. B 2003, 68 (4), 041403. (22) Pollak, M. In The Metal Non-Metal Transition in Disordered Systems. Proceedings of the 19th Scottish UniVersities Summer School in Physics; Friedman, L. R., Tunstall, D. P., Eds.; SUSSP Publications: Edinburgh, 1978; pp 95-148. (23) Snaith, H. J.; Whiting, G. L.; Sun, B.; Greenham, N. C.; Huck, W. T. S.; Friend, R. H. Nano Lett. 2005, 5 (9), 1653-1657.
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