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Langmuir 2002, 18, 10176-10182
Ultrathin Regioregular Poly(3-hexyl thiophene) Field-Effect Transistors Henrik G. O. Sandberg,† Gitti L. Frey,*,‡ Maxim N. Shkunov,§ Henning Sirringhaus, and Richard H. Friend University of Cambridge, Department of Physics, Cambridge, U.K.
Martin Meedom Nielsen*,| The Danish Polymer Centre, Risø National Laboratory, Denmark
Christian Kumpf Condensed Matter Physics and Chemistry Department, Risø National Laboratory, Denmark Received June 28, 2002. In Final Form: October 1, 2002 Ultrathin films of regioregular poly(3-hexyl thiophene) (RR-P3HT) were deposited through a dip-coating technique and utilized as the semiconducting film in field-effect transistors (FETs). Proper selection of the substrate and solution concentration enabled the growth of a monolayer-thick RR-P3HT film. Atomic force microscopy (AFM), UV-vis absorption spectroscopy, X-ray reflectivity, and grazing incidence diffraction were used to study the growth mechanism, thickness and orientation of self-organized monolayer thick RR-P3HT films on SiO2 surfaces. Films were found to adopt a Stranski-Krastanov-type growth mode with formation of a very stable first monolayer. X-ray measurements show that the direction of π-stacking in the films (the (010) direction) is parallel to the substrate, which is the preferred orientation for high field-effect carrier mobilities. The field-effect mobilities in all ultrathin films prepared in this study are lower than those obtained for thicker films spun on similar substrates. However, monolayer FETs provide a direct experimental system to study the charge transport at the charge accumulation layer.
Introduction The usual architecture of a polymer FET is a thin film MISFET (metal-insulator-semiconductor field-effect transistor) device driven in accumulation mode. Under negative gate bias an accumulation layer of positive charges is formed at the polymer/insulator interface. The gate bias modulates the carrier concentration and hence the conductivity of carriers between the source and drain electrodes (in the semiconducting channel of the FET). The thickness of the accumulation layer in polymer FETs is still subject to some debate, and it has been suggested that the field induced conducting channel formed at the interface is only one or a few monolayers thick.1-3 The most promising and studied family of polymers for FETs are the poly-(3-alkylthiophene)s (PAT). PATs have a structure similar to hairy-rod polymers and form semicrystalline films with crystalline domains embedded in an amorphous matrix. The crystalline domains are formed due to the π-π interactions between adjacent rodlike polymer backbones that self-organize into a lamellar supramolecular assembly.4,5 The high degree of crystallinity and strong interchain interactions in PATs * To whom correspondence should be addressed. † Current address: Åbo Akademi University, Department of Physics, 20500 Åbo, Finland. ‡ E-mail:
[email protected]. § Current address: Merck Chemicals Ltd., Chilworth Science Park, Southampton, SO16 7QD, U.K. | E-mail:
[email protected]. (1) Horowitz, G.; Hajlaoui, R.; Bourguiga, R.; Hajlaoui, M. Synth. Met. 1999, 101, 401. (2) Ziemelis, K. E.; Hussain, A. T.; Bradley, D. D. C.; Friend, R. H.; Ru¨he, J.; Wegner, G. Phys. Rev. Lett. 1991, 66, 2231. (3) Dyreklev, P.; Ingana¨s, O.; Paloheimo, J.; Stubb, H. J. Appl. Phys. 1992, 71, 2816. (4) Aasmundtveit, K. E.; Samuelsen, E. J.; Guldstein, M.; Steinsland, C.; Flornes, O.; Fagermo, C.; Seeberg, T. M.; Pettersson, L. A. A.; Ingana¨s, O.; Feidenhans’l, R.; Ferrer, S. Macromolecules 2000, 33, 3120.
lead to high charge mobility since the carriers are no longer confined to a single chain.5 However, the charge transport in the polymer FET channel is highly dependent on the orientation of the crystalline domains relative to the carrier transport direction (source to drain). The optimal polythiophene chain orientation in a FET is with the plane of the conjugated rings perpendicular to the transport direction so that carrier transport is along the π-π stacking direction as shown in Figure 1a. However, the charge carrier mobility is also influenced by the lateral offset between different chains as well as interchain distance (distance b in Figure 1a) and tilt between neighboring chains. FETs using highly regioregular headto-tail coupled P3HT, Figure 1b, as the active material and controlling the orientation of the lamellar structure showed the highest field-effect mobility in polymer FETs so far, 0.1 cm2/(V s).6,7 There is growing interest in polymer field-effect transistors due to their inexpensive fabrication process and prospective for integration into large area and flexible electronic applications such as displays and smart cards.8-10 However, to achieve high efficiency and performance of these devices, it is necessary to understand the fundamental electronic processes occurring in the polymers under device conditions. (5) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; de Leeuw, D. M. Nature 1999, 401, 685. (6) Bao, Z.; Dodabalapur, A.; Lovinger, A. J. Appl. Phys. Lett. 1996, 69, 4108. (7) Sirringhaus, H.; Tessler, N.; Friend, R. H. Science 1998, 280, 1741. (8) Sirringhaus, H.; Kawase, T.; Friend, R. H.; Shimoda, T.; Inbasekaran, M.; Wu, W.; Woo, E. P. Science 2000, 290, 2123. (9) Gelinck, G. H.; Geuns, T. C. T.; de Leeuw, D. M. Appl. Phys. Lett. 2000, 77, 1487. (10) Huitema, H. E. A.; Gelinck, G. H.; van der Putten, J. B. P. H.; Kuijk, K. E.; Hart, C. M.; Cantatore, E.; Herwig, P. T.; van Breemen, A. J. M. M.; de Leeuw, D. M. Nature 2001, 414, 599.
10.1021/la0261444 CCC: $22.00 © 2002 American Chemical Society Published on Web 11/19/2002
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Figure 1. (a) The preferred orientation of RR-P3HT in a thin film transistor is with the π-π stacking parallel to the substrate and (b) the molecular structure of RR-P3HT.
Monolayer polythiophene films have been previously deposited by the Langmuir-Blodgett technique,11,12 and by self-assembly on solution/HOPG interface.13 The first LB FET used regiorandom poly-(3-alkylthiophene)s (RIPAT) as the semiconducting material.14 The random position of the side chains in the RI-PAT inhibits the π-π stacking and reduces the crystallinity of the film. Consequently, the field-effect mobility in the RI-PAT FET is low, 10-7-10-4 cm2/(V s). Amphiphilic PAT form stable LB films with large ordered domains at the air-water interface.15 RR-P3HT is also likely to form stable Langmuir films due to its hairy-rodlike polymer structure.16,17 However, the films formed were rigid and stiff and a watersoluble amphiphile spread-aiding compound was introduced in order to produce homogeneous and flexible films. The transferred LB films were oriented with the polymer backbone parallel to the substrate in an edge-on conformation, which is favorable for the operation of a FET. The absolute thickness of a single LB layer was not determined directly and was estimated to contain a double molecular layer.17 Monolayers of highly regioregular oligo-alkylthiophenes were also prepared through self-assembly onto a solution/ HOPG interface.13 The alkyl side chains are ordered epitaxially on the HOPG structure forcing the rodlike segments to lay flat on the surface. The rodlike segments are ordered in a parallel lamella-type stack with interdigitated alkyl side chains to achieve maximum van der Waals interactions. Monolayers are also observed for R-substituted18 and β-substituted19 oligothiophenes while multilayers are very rarely seen and also only for nearsaturated solutions. In this paper, ultrathin RR-P3HT films are deposited through a dip-coating technique. In dip-coating the system (11) Wegner, G. Thin Solid Films 1992, 216, 105. (12) Reitzel, N.; Greve, D. R.; Kjaer, K.; Howes, P. B.; Jayamaram, M.; Savoy, S.; McCullough, R. D.; McDevitt, J. T.; Bjo¨rnholm, T. J. Am. Chem. Soc. 2000, 122, 5877. (13) Kirschbaum, T.; Azumi, R.; Mena-Osteritz, E.; Ba¨uerle, P. New J. Chem. 1999, 23, 241. (14) Paloheimo, J.; Kuivalainen, P.; Stubb, H.; Vuorimaa, E.; YliLahti, P. Appl. Phys. Lett. 1990, 56, 1157. (15) Bøggild, P.; Rey, F.; Hassenkam, T.; Greve, D. R.; Bjo¨rnholm, T. Adv. Mater. 2000, 12, 947. (16) Ochiai, K.; Tabuchi, Y.; Rikukawa, M.; Sanui, K.; Ogata, N. Thin Solid Films 1998, 327, 454. (17) Xu, G.; Bao, Z.; Groves, J. T. Langmuir 2000, 16, 1834. (18) Azumi, R.; Go¨tz, G.; Ba¨uerle, P. Synth. Met. 1999, 101, 569. (19) Azumi, R.; Go¨tz G.; Debaerdemaeker, T.; Ba¨uerle, P. Chem. Eur. J. 2000, 6, 735.
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is near equilibrium and the nature of the substrate/ polymer interface strongly influences the film formation. This techniqe provides fast and simple means for the preparation of ultrathin and undoped films in a controlled atmosphere. Moreover, this technique combines the tendency of the polymer to form stable monolayers at the solution/substrate interface as observed on HOPG, with the stability of the monolayer in air, as observed for the LB films. We studied the fabrication and performance of a polythiophene monolayer-thick FET. By proper selection of the substrate and control of the solution concentration, self-organized monolayers of RR-P3HT are deposited through dip coating. The growth mechanism and properties of the films were studied in various techniques including AFM, X-ray, and UV-vis absorption. The films are utilized as the semiconducting component in FETs. Monolayer FETs provide a direct experimental system to study the charge transport at the charge accumulation layer. Experimental Section Materials and Film Deposition. Ultrathin films of RR-P3HT were prepared by dipping hydrophilic Si/SiO2 (oxide thickness 2000 Å) and glass substrates into a dilute solution of RR-P3HT (McCullough route, MA ) 80k) in xylene (Aldrich 99%, anhydrous). The concentration of the solution was 95-09, North Carolina State University: Raleigh, 1995. (26) Press, W. H.; Flannery, B. P.; Teukolsky, S. A.; Vetterling, W. T. Robust Estimation; Cambridge University Press: Cambridge, 1988; pp 539-46. (27) Sze, S. M. Physics of Semiconductor Devices; John Wiley & Son: New York, 1981.
Figure 3. (a) Optical absorption spectra of glass substrates dipped 1, 2, 3, and 5 times into a 0.25 mg/mL RR-P3HT in xylene solution. (b) The absorption intensity of the (0-0) peak (∼550 nm) as a function of number of dip/dry processes. (c) Smoothed absorption (normalized) of samples prepared by one and three dip/dry processes.
and a smaller nearly linear intensity increase in the following processes. This reveals that a larger amount of material is deposited in the first dip/dry process than in the following processes. The steady increase in the peak intensity in the two-to-five dip/dry processes indicates that a similar or slightly increasing amount of material is deposited in each of these processes, but still much less for each dip/dry process than the first initial deposition. Note that the spot size of the absorption measurement is on the order of 1 mm and reflects an average over that area. In addition to an increase in the peak intensity in the two-to-five dip/dry films, the features become more pronounced indicating that a high degree of order is maintained in the film as more material is deposited on the surface. Figure 3c shows the smoothed absorption curves for samples prepared by one and three dip/dry processes. After the three dip/dry process, a noticeable separation of the (0-0) peak at ∼550 nm and the (0-1) at ∼525 nm as well as a slight increase in the shoulder at ∼600 nm is observed. Both features are associated with increased molecular order in the material. Recently, the intensity of the lowest energy feature in the absorption spectrum (∼600 nm) was correlated with the degree of order in the polymer film.28 The fact that this feature is noticeable as a small shoulder in the spectrum of the ultrathin film indicates that the film is well ordered. If the polymer film were deposited by a layer-by-layer growth mechanism, this system could provide a direct means to study the charge carrier mobility as a function of the number of molecular layers in the channel. AFM Measurements. AFM was used to determine the morphology and topography of the deposited film in order to shed light on the film formation mechanism. The surface of the ultrathin film formed after one dip/dry process is very smooth with no noticeable pinholes or defects over the whole measured areas as shown in Figure 4A. The area probed by AFM, though, is much smaller than the sample area probed in the absorption measurements. By scratching the film surface and measuring the height of the step, the film thickness was found to be approximately 20 Å as presented in Figure 5. This enabled us to measure the thickness of the film in several locations on the film including in the channel of the FET. Parts B and C of Figure 4 show the AFM micrograph for samples after three and five dip/dry processes, respectively, with correspond(28) Brown, P. J.; Thomas, D. S.; Ko¨hler, A.; Wilson, J. S.; Kim, J.-S.; Ramsdale, C. M.; Sirringhaus, H.; Friend, R. H. Submitted for publication.
Regioregular Field-Effect Transistors
Figure 4. AFM micrograph showing topography of films formed by (A) one dip/dry process, (B) three dip/dry processes, and (C) five dip/dry processes on Si/SiO2 substrates. The bar length is 1 micron.
Figure 5. Sections of mechanical scratches in the films presented in Figure 4.
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Figure 6. X-ray reflectivity (circles) and model fits (lines) of films prepared by one and three dip/dry processes on an FET substrate (thick oxide) and films prepared by three and five dip/dry processes on a Si substrate with a native oxide. The spectra are offset vertically for clarity. The inset shows that also the thickness fringes from a FET SiO2 layer of 2038 Å are reproduced by the calculations.
Figure 7. Electron density distribution as function of depth into the substrate of the samples presented in Figure 6. The spectra are offset vertically for clarity.
ing cross sections in Figure 5. As can be seen, material has aggregated into islands on the surface of the underlying initial film. The grains are typically 500-1 000 Å wide and of roughly the same height throughout the dip/dry processes, however, with a large distribution in the height of the aggregates. The density of the grains on the surface increases with the number of dip/dry processes. The AFM measurements reveal that the repetitive dipping does not result in a layer-by-layer deposition of homogeneous monolayers but in aggregation of material into islands on top of a flat background layer. The film thickness of the background layer is, as obtained for the film formed by one dip/dry process, ∼20 Å (Figure 5). In P3AT, the crystalline domains are formed due to the π-π interactions between adjacent rodlike polymer backbones that self-organize into a lamellar supramolecular assembly. The lamella can be formed by close-packing of the flat polymer strands of the regioregular material. The interlamellar distance (a-axis) depends on the length of the alkyl side chain and on the conformation of the side chains. For RR-P3HT the a-axis parameter is 16 Å.29 The a-axis increases with increasing the length of the alkyl side chain and amounts to 2.1 Å per CH2 unit for butyl to hexyl and 1.4 Å per unit for octyl to decyl.30 Two models for the packing of the alkyl side chains have been suggested for PATs. In the first, the side chains tilt away from the plane of the conjugated backbone to eliminate the interactions between side chains of adjacent polymer strands. In the second, the side chains are interdigitated. Direct
observation of two-dimensional crystals of RR-P3HT at the solution/HOPG interface by STM agrees well with the latter model. However, the epitaxial effect of the underlying graphite “compresses” the interdigitation by two methylene groups compared to the bulk and the a-axis is reduced to 13.3 Å.31 The thickness of ∼20 Å of the film formed after one dipping is in good agreement with the thickness of a single lamella of RR-P3HT stacked perpendicular to the substrate. The 4 Å increase in the a-axis (see Figure 1a) compared to the bulk is due to the fact that there is no interdigitation and hence the side-chain is “effectively” 2 methylene groups longer than that in the bulk. Indeed, the thickness of a P3OT lamella in bulk (including interdigitation) is about 20 Å.30 To confirm that the ultrathin film is a true RR-P3HT monolayer, the film was characterized by X-ray measurements. X-ray Reflectivity. Using X-ray reflectivity it was possible to characterize the film thickness and roughness over large areas. To facilitate data analysis, two samples (three and five dip/dry) are measured on a Si/SiO2 wafer with only the native oxide, while the other three dip/dry sample and a one dip/dry sample are on a FET substrate with a thick oxide layer. The results are summarized in Figures 6 and 7. The thickness of the polymer films deposited on the native oxide substrates (three and five dip/dry processes) is found to be 18.5 Å with an RMS
(29) Kobashi, M.; Takeuchi, H. Macromolecules 1998, 31, 7273. (30) Samuelsen, J.; Mårdalen, J. Handbook of Organic Conductive Molecules and Polymers; Nalwa, H. S., Ed.; Wiley: 1997; Vol. 3, p 87.
(31) Mena-Osteriz, E.; Meyer, A.; Langeveld-Voss, B. M. W.; Janssen, R. A. J.; Meijer, E. W.; Ba¨uerle, P. Angew. Chem., Int. Ed. 2000, 39, 2680.
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Figure 9. Intensity of the (100) peak as a sample prepared by three dip/dry processes is translated in the beam. Figure 8. Grazing incidence X-ray diffraction data showing (a) the out-of-plane scattering and (b) the in-plane scattering of samples prepared by three and five dip/dry processes on native Si and FET Si/SiO2 substrates.
roughness on the order of 3 Å. The thickness of the native oxide layer was found to be ∼7 Å, (comparable to the smallest distance being probed by the measurements), while the oxide layer on the FET-samples was around 2050 Å. These findings are in excellent agreement with the AFM data. The AFM micrographs of the three and five dip/dry samples show a continuous film of 20 Å with aggregates accumulated on the surface. The reflectivity measurement shows the same thickness, but the roughness is increased due to the aggregates. The ∼20 Å polymer film is associated with a monolayer-thick single lamella of RR-P3HT stacked perpendicular to the substrate. Figure 7 shows the electron density distribution as function of depth into the substrate of the samples presented in Figure 6. The two samples prepared on Si substrate with a native oxide exhibit a clear minimum at the interface between the oxide layer and the P3HT. The width of the minimum is of the order of 7 Å, and could be indicative of the lower density side chains being close to the substrate (and hence the π-stack in the plane of the sample). This is not seen so clearly for the samples on FET substrates, which could possibly be due to the thick oxide layer making the fitting less reliable. Grazing Incidence Diffraction. The orientation of the polymer chains with reference to the substrate is studied with grazing incidence diffraction (GID) in the in-plane and out-of-plane directions. GID reveals the presence of crystalline scatterers in all the films measured. (We only measured samples prepared by more than one dip/dry process). The peak at 1.65 ( 0.01 Å-1 in the inplane scans (Figure 8b) is assigned to the (010) reflection, corresponding to a π-π stacking distance of 3.81 ( 0.03 Å. The broad feature around 1.5 Å-1 stems from the packing of the amorphous alkyl side chains, and some scattering from amorphous SiO2. The (010) reflection is not observed in the pseudo-out-of-plane direction (Figure 8a), demonstrating a preferred orientation of the π-stack in the plane of the sample. For the three dip/dry sample on native oxide, it was possible to find regions of macroscopic aggregation of crystallites, as shown in Figure 9. The large density of crystallites made it possible to observe (100), (200), and (300) reflections (see the three dip/dry sample on FET in Figure 8a), indicative of a preferential orientation of the lamella parallel to the surface (i.e., π-stack in the plane of the sample). The position of the peaks corresponds to an inter-lamellar packing distance of 16.02 ( 0.05 Å, while the width corresponds to a Debye-Scherrer dimension of the coherently scattering domains of around 130 Å. We did not find areas of similar crystallite density for the other samples in Figure 8, but they all show the prescence of a
(pronounced) shoulder at the location of the (100) peak. Hence, the grazing incidence diffraction shows that the film is ordered and highly anisotropic with the π-π stacking parallel to the substrate. The apparent discrepancy between the relatively large dimensions of the crystalline domains observed in the pseudo out-of-plane scans, and the film thickness as measured by reflectivity (25.2 Å), is resolved as the intensity of the (100) reflection is monitored as function of horizontal translation of the sample (see Figure 9). By translating the three dip/dry on FET substrate sample during the measurement, it is possible to probe the changes in the crystallinity and thickness of the film as a function of position. The figure shows how the scattering is localized at certain positions along the sample, consistent with a notion of islands of accumulated crystalline grains. The abundance of islands is in good agreement with the AFM micrograph shown in Figure 4 for a similar sample. This also illustrates how diffraction directly probes the crystalline regions of the sample, while reflectivity is probing the average distribution of electron density. However, it is important to note the different lateral scales in the AFM micrograph shown in Figure 4 (microns), and the translated X-ray grazing incidence diffraction in Figure 9 (millimeters) reflecting the 1.5 mm width of the X-ray beam. Hence, the intensity variation of the (100) peak in Figure 9 is a measure of the variation in the local density of the aggregates, rather than individual aggregates. The X-ray results also show that the grains are wellordered crystallites with the π-π stacking parallel to the substrate. The monolayer could act as a template for the grains so the orientation of the grains follows that of the monolayer with the π-π stacking parallel to the substrate. This effect is also seen for the β-alkylated quarterthiophenes where the molecular organization of the selfassembled monolayer fully coincides with the packing in one cross section of a 3D crystal of the same material.19 The crystalline grains, on the other hand, could anchor the amorphous monolayer and locally improve its molecular packing or crystallinity. Hence, as observed by AFM, the repetitive dipping does not result in a layerby-layer growth of the film but rather in the accumulation of grains on the ultrathin film that was formed in the first dip/dry process. The thickness of the underlying film calculated from the X-ray reflectivity measurements are in good agreement with the AFM results. The orientation of the polymer chains in the deposited grains is with the π-stacking parallel to the substrate. However, the deposited grain could also improve the crystallinity of the underlying monolayer. The deposition of an initial monolayer followed by the growth of islands on the monolayer is a StranskiKrastanov-type growth mechanism.32 In this growth mode (32) Feldman, L. C.; Mayer, J. W. Fundamentals of Surface and Thin Film Analysis; North-Holland: New York, 1986.
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Table 1. Surface Coverage, Aggregate Density, and Mean Size for Samples Prepared by One, Three, and Five Dip/Dry Processes no. of dip/dry processes
surface coverage (%)
aggregate density (/µm2)
aggregate mean size (nm2)
1 3 5
0.28 1.30 1.70
1.07 4.08 9.56
2629 3184 1783
the first monolayer is deposited due to the large free energy of the surface. The adsorbate-substrate interactions are stronger than adsorbate-adsorbate interactions and hence dominate the growth. However, at a certain critical coverage the free energy of the substrate is reduced and the substrate-polymer interaction is no longer stronger than the polymer-polymer interaction. At this point the growth mode switches from layer-by-layer to island growth. In our case, the physisorption of polymer chains onto the surface of the SiO2 substrates is an equilibrium process balancing the strength of the interactions between adjacent polymer chains and the energy between the polymer and the subtrate. The hydrophilic substrate’s surface energy is strong enough to attract the polymer chains to the surface but not strong enough to overrule the polymer chains’ π-stacking. When the substrate is fully covered by a monolayer the surface energy is reduced and the stronger polymer-polymer interaction result in island formation. Nevertheless, the internal order of the initial monolayer may be modified during the island deposition. We support this model by dipping a hydrophobic hexamethyldisilazane (HMDS) treated substrate into the same polymer solution. The low surface energy of the HMDS treated substrate resulted in a noncontinuous film due to an island growth mechanism. It is important to note that the solution is dilute enough to minimize polymer aggregation in the solution. The stability of the monolayer was verified after dipping the film into a pure solvent by absorption, FTIR, and AFM measurements. Both the monolayer and the accumulated grains were observed after immersion into pure solvent for up to 50 min. Therefore, in the first dip/dry process a film is deposited on the bare SiO2 substrate, but in the following dip/dry processes the material is accumulated on the polymer that was deposited in the first process. Consecutive dipping enlarged the number of nucleation sites and their size, regardless of whether the films were annealed between dipping or not. The largest grains are on the order of 1500 Å (diameter) for both the samples prepared by three and five dip/dry processes. However, the grain density increase but the average grain size decreases in the sample prepared by five dip/dry processes compared to the sample prepared by three dip/dry processes. This indicates that small grains are accumulated on the surface in the later dip/dry processes, decreasing the average grain size. Table 1 shows the number of grains per unit area and the total coverage of the sample surface. The main increase in surface coverage for multiply dipped samples is due to the newly deposited small grains. The increase in absorbance observed in Figure 3 cannot be attributed to the aggregates alone due to the low coverage of the surface. One possible explanation is that the grains anchor the monolayer and when immersed again and again into solution the “free” polymer chains in the monolayer (not anchored by grains) re-organizes. The restructuring improves the order in the monolayer and makes it denser. Large scale discontinuities in film coverage with filling of pinholes accounting for the increase in signal for both UV-vis absorption and X-ray could also
Figure 10. FET characteristics for a device with a monolayer forming the active channel in the device. Table 2. Charge Carrier Mobility Calculated for Saturated and Low Field Regime for Samples Prepared by One and Three Dip/Dry Processes no. of dip/dry processes
charge carrier mobility in low field (cm2/Vs)
charge carrier mobility in saturation (cm2/Vs)
1 3
1.09 × 10-4 1.06 × 10-4
4.89 × 10-5 5.23 × 10-5
be a possibility, even though AFM does not show any such effects. (Note the 10-100 times difference in AFM scan size and the abosorption and X-ray spot size.) FET Characterization. The RR-P3HT monolayer FETs showed mobilities on the order of 10-5-10-4 cm2/(V s) as calculated from the output characteristics in Figure 10. (See Table 2) This is almost 2 orders of magnitude lower than the carrier mobility in an FET prepared by spin coating from a more concentrated solution of the same polymer on a similar hydrophilic substrate (µ ) 1.5 × 10-3 cm2/(V s)). The thickness of the monolayer indicates that the orientation of the polymer chains in the film is in the preferred orientation for high carrier mobility in P3HT FETs. Therefore, FETs utilizing the monolayers will probe the carrier mobility in the π-stacking of a single molecular layer. The lower carrier mobility in the monolayer FET is in good agreement with the relatively low mobility reported for RR-P3HT single LB film devices (double layer, 33.4 Å thick). The mobility in devices composed of two to five LB layers (four to 10 layers) was insensitive to the film thickness but was higher than that obtained for a single LB layer.17 Therefore, the charge carriers were assumed to be confined within the first few molecular layers of the films. The charge density in these samples calculated from the conductivity at zero gate bias is on the order of 1 × 10-19 1/cm3. In the monolayer films prepared in this study, only one molecular layer is available for carrier confinement. Several mechanisms could explain the low mobility values obtained for the monolayer: (I) The monolayer is poorly ordered due to the interfacial strain. It is possible that in thicker films the presence of a well ordered bulk of the film results in a higher degree of order in the interfacial monolayer than in a monolayer which is in contact with the substrate and air, respectively. The presence of a highly ordered bulk in thicker films may also exert an ordering effect onto the interfacial layer, that is not present when only a monolayer is deposited. Even though the polymer chains are ordered in the preferred edge-on configuration there might be a slight off-set or tilt between the individual rings/π-orbitals resulting in a reduction of charge carrier mobility. (II) The mobility in the monolayer is inhibited by percolation of charges between well-ordered domains in the amorphous film. In thicker films charge transport at interfacial
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Figure 11. Transfer curve of a RR-P3HT monolayer device.
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solution on a hydrophobic HMDS and other SAM treated substrates. The polymer solution dewets the HMDS surface and during spin-coating the system is not in equilibrium. In the dip-coating technique, the system is in equilibrium and the nature of the substrate/polymer interface strongly influences the film formation and hence the performance of the device. The thickness of the accumulation layer depends on the applied field and monolayers of RR-P3HT may be efficient in submicron devices that require lower operation voltages. Smaller physical dimensions of the FET channel and improvement of the gate insulator will decrease the number of defects and grain boundaries within the FET channel in the monolayer and more readily probe the charge carrier mobility of the π-conjugated system. Conclusions
Figure 12. FET characteristics for a device utilizing a RRP3HT film prepared by three dip/dry processes.
defects and grain boundaries in the first monolayer is facilitated by the presence of covering layers in which lower barriers to transport might be encountered. The monolayer FETs have a rather large positive turnon voltage, varying within +10 and +20 V, as shown in Figure 11. The extremely thin film thickness increases the risk of introducing contamination in the sensitive FET channel region at the interface. The relatively high doping level results in a rather large turn on voltage. FET devices were also fabricated utilizing the RR-P3HT films prepared by repetitive dip/dry processes. These FETs comprise films with well ordered aggregates on top of the underlying monolayer. The field-effect mobility in a threedip/dry-processed film was calculated from the saturated region of the I-V curves presented in Figure 12 and is 5.23 × 10-5 cm2/(V s). This mobility value was also obtained for FETs with films prepared by five and seven dip/dry processes, and is slightly higher (5-10%) than the mobility in the monolayer film (one dip/dry process). (See Table 2.) Calculated from the linear region of the output characteristics, the mobility value was close to 1 × 10-4 cm2/(V s) both for samples prepared by one and several dip/dry processes. Two mechanisms could account for the small enhancement of the mobility in the “aggregated” films: (I) the highly crystalline aggregates formed on the surface offer alternative transport routes through highly crystalline domains and reduce the percolation effect; (II) the aggregates deposited on the monolayer induce densification and crystallization of the monolayer and the transport is through a highly ordered monolayer with improved interchain distance and π-π interaction; (III) in addition to aggregate deposition, in the second dipping process material is deposited in pinholes and defects of the monolayer and results in a better quality and continuity of the underlying monolayer throughout the FET channel improving the transport properties of the device. High mobility FETs presented in the literature are usually prepared by spin-coating from a concentrated
Dipping a high surface energy substrate into a low concentration RR-P3HT solution results in the selfassembly of a stable monolayer polymer film. UV-vis absorption and AFM measurements show that the monolayer is approximately 20 Å thick and well ordered. The thickness of 20 Å for the monolayer is in good agreement with the thickness of a single lamella of RR-P3HT stacked perpendicular to the substrate. The repetitive dip/dry process resulted in the accumulation of crystalline grains on the surface of the initial monolayer in a StranskiKrastanov-type growth mechanism. The monolayer is stable and does not redissolve into solution during the consecutive dipping processes. X-ray measurements show that the π-stacking (the (010) direction) is parallel to the substrate and the plane of the thiophene rings (the a-axis, or (100) direction) is normal to the substrate. The grains have the same orientation as the monolayer with the plane of the polymer backbone perpendicular to the surface of the substrate. This orientation is preferred for high fieldeffect mobilities and in an FET utilizing the monolayer the carriers are accumulated and transported through the π-π stacking of a molecular film. The field-effect mobility in the saturated regime is for the monolayer FET 5 × 10-5 cm2/(V s) and slightly increases in the aggregated films. However, the field-effect mobilities in all ultrathin films prepared in this study are almost 2 orders of magnitude lower than those obtained for thicker films spun on similar substrates, and almost 3 orders of magnitude lower than the highest mobility obtained for RR-P3HT devices (spun on silylated substrates). However, by preparing FETs utilizing a molecular monolayer, we directly studied the carrier transport in the accumulation layer. The low mobility in the ultrathin films prepared in this study, which is not affected by the slight discrepancy between the absorption and AFM measurements on samples prepared by several dip/dry processes, is associated with the poor order in the monolayer. The higher mobility in thicker films could be attributed to either (I) reducing the percolation effect across amorphous regions in the monolayer through three-dimensional transport or (II) the bulk polymer film (accumulated grains in this study) induces order in the underlying interfacial layer (the monolayer in this study) thus improving the carrier transport in that layer. Acknowledgment. H.G.O.S. thanks the Academia Scientiarum Fennica, the GSMR at Åbo Akademi, Magnus Ehrnroths Stiftelse and Stiftelsen fo¨r Teknikens Fra¨mjande for financial support. We acknowledge support from theEuropeanCommission(G.L.F.,Marie-CurieFellowship). LA0261444
