Three-Dimensional Inhomogeneities in PEDOT:PSS Films - The

Nov 13, 2004 - Spin-cast PEDOT/PSS film are investigated using scanning probe-based spectroscopic techniques. The films are found to consist of relati...
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J. Phys. Chem. B 2004, 108, 18820-18825

Three-Dimensional Inhomogeneities in PEDOT:PSS Films M. Kemerink,*,† S. Timpanaro,‡,§ M. M. de Kok,‡ E. A. Meulenkamp,‡ and F. J. Touwslager‡ Molecular Materials and Nano-Systems, EindhoVen UniVersity of Technology, Department of Applied Physics, P.O. Box 513, 5600 MB EindhoVen, The Netherlands and Philips Research Laboratories, Prof. Holstlaan 4, 5656 AA EindhoVen, The Netherlands ReceiVed: August 6, 2004; In Final Form: September 23, 2004

Spin-cast PEDOT/PSS film are investigated using scanning probe-based spectroscopic techniques. The films are found to consist of relatively well-conducting islands that are embedded in a less conductive matrix. A scanning-tunneling microscope-based method is developed to obtain three-dimensional conductivity images with nanometer resolution. Order-of-magnitude variations in the film conductivity are observed in all spatial directions. In addition, charge transport in the normal direction depends on the injection spot and is found to be ohmic or space-charge-limited. We found that both the morphology and the conductivity of the top layer differ substantially from those in the bulk, which is attributed to an enhanced PSS content in the top layer.

1. Introduction PEDOT/PSS (poly(3,4-ethylenedioxythiophene) stabilized with poly(4-styrenesulfonic acid)) is a transparent and conductive material that is widely used as an antistatic coating, electrical interconnect, and electrode material in organic semiconductor devices such as light-emitting diodes (LED) and photovoltaic cells.1 Application in novel memory devices has recently also been demonstrated.2,3 One of the strengths of PEDOT/PSS is that topographically smooth films can be spin cast from aqueous dispersions. However, for applications it is very important that also the electrical properties show little variations within a film. When PEDOT/PSS is used as the anode in organic or polymer LEDs or solar cells, the presence of lateral conductance and injection variations may easily result in an inhomogeneous current density, leading to lower efficiency. Despite the presence of a large number of papers addressing the morphology of PEDOT/PSS layers4-8 and the effect of the chemical composition,9 the ‘functional homogeneity’, i.e., (in)homogeneity of properties such as conductivity, has hardly been addressed.10 Given the fact that PEDOT/PSS films are believed to consist of ‘conducting’ or ‘polaronic’ PEDOT-rich particles embedded in a quasi-insulating11 PSS matrix4,6 and that charge transport seems to occur via hopping or tunneling between these polaronic islands,12,13 large spatial variations in conductivity may be anticipated.14,15 Very recently, Ionescu et al. used conductive atomic force microscopy (AFM) to study the local chargetransport properties at the surface of a 1 µm PEDOT/PSS film.16 Unfortunately, their preparation method (solution casting, followed by a 1 h anneal at 100 °C) resulted in rather rough films, which have a polycrystalline appearance that cannot directly be compared to the films used in typical applications. In the present work we present the first local study of the conductivity of spin-cast PEDOT/PSS films. By combining various scanning-tunneling microscope (STM) based spectro* To whom correspondence should be addressed. E-mail: m.kemerink@ tue.nl. † Eindhoven University of Technology. ‡ Philips Research Laboratories. § Present address: Institu ¨ t fu¨r Physik, LS PKM, D14469 Potsdam, Germany.

scopic techniques, order-of-magnitude conductivity variations are found in all spatial directions on length scales ranging from a few to several hundreds of nanometers. 2. Experimental Section 2.1. Film Preparation. The PEDOT/PSS dispersion, containing approximately 0.4 wt % PEDOT and approximately 0.8 wt % PSS, was provided by AGFA. Si substrates were used covered from top to bottom, with 230 Å Au, 40 Å Ti, and 300 Å SiO2. The Ti layer is used for adhesion of the Au layer. Au-covered substrates were used rather than the more common ITO/glass substrates to prevent spurious effects due to diffusion of contaminants from the ITO17 and because of the lower roughness of Au. We used tapping-mode (TM) AFM to verify that no significant differences were present between the topographies of films cast on Au or ITO. Within the sample-to-sample and spot-to-spot variation the RMS roughness values were identical for both substrates. Films were deposited by spin coating to give 200-250 nm thick films, as determined by a Dektak profilometer. The dispersions were filtered prior to deposition using a 5 µm filter to remove particles. After deposition the samples were baked on a hot plate for 2 min at 180 °C to remove water. The lateral bulk conductivity of the resulting films was 1 S/cm, as determined by four-point probe measurements. 2.2. STM Measurements. Scanning-tunneling microscopy and -spectroscopy (STS) experiments were performed in a home-built STM setup using etched Pt tips with an apex radius of about 50 nm.18 All STM and STS experiments are performed in a He atmosphere. During transportation and mounting the samples were exposed to air. The Au bottom electrode is used as sample contact to which the bias is applied. The tip is virtually grounded through the current amplifier. Apart from topographic, constant-current imaging, and current-voltage (I-V) spectroscopy, two less established spectroscopic techniques are extensively used in this work. Both exploit the fact that the metallic STM tip is much stiffer than the conjugated material allowing penetrating of the polymer with the STM tip. Despite the seemingly destructive nature of ‘invasive spectroscopy’, reproducible and meaningful images can be obtained, provided that care is taken in optimizing scan and feedback parameters.

10.1021/jp0464674 CCC: $27.50 © 2004 American Chemical Society Published on Web 11/13/2004

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Figure 1. Topography of the PEDOT/PSS film (a) and Au substrate (b) as obtained by tapping-mode AFM. (c-f) Topography of the PEDOT/PSS film as obtained by STM. Settings: V ) 1.5 V, I ) 30 pA (c); V ) 10 V, I ) 30 pA (d), I ) 0.3 nA (e), I ) 1.0 nA (f). The height scales in a, b, c, d, e, and f are 10, 7, 25, 35, 25, and 25 nm, respectively. The scan area is 500 × 500 nm (a, b, d-f) and 250 × 250 nm (c).

According to Ohm’s law the current in an STM experiment depends on the bias and local resistance: V(z) ) I(z) R(z). In current vs tip height (I-z) spectroscopy the tip-sample current I(z) under constant bias is measured as a function of penetration depth of the tip z, which one immediately gives the local resistance R(z). By taking I-z curves on a lateral grid, a threedimensional resistance (or conductance) map is obtained that can be converted numerically into a conductivity map. In z-V spectroscopy the bias is ramped with the feedback system active, i.e., while maintaining a constant current by adjusting the tip height.19 Assuming planar contacts, it has been shown that the obtained z-V curves reflect the inverted potential distribution V(z) between tip and sample.20 The slope of the z-V curve at a particular height therefore equals the inverse of the local electric field E. Since the current density j ) σ‚E, with σ being the (local) conductivity, is constant because of the active feedback, a large (small) slope of the z-V curve reflects a high (low) local conductivity. Both z-V and I-z spectroscopy are performed with the tip at a fixed lateral position. The starting height of each individual spectroscopic curve is determined by the current and bias set points. AFM images were taken under ambient conditions on a Digital Instruments Dimension 3100 AFM in tapping mode, using Si cantilevers. 3. Results and Discussion 3.1. Topographic Images. Figure 1a and d shows 500 × 500 nm TM-AFM and STM images of the same PEDOT/PSS film taken at different spots. Clearly, there is a qualitative difference in the morphology as observed by both techniques. The AFM image (a) shows a relatively flat surface that is composed of small particles; the STM image (d) has a much larger roughness and shows larger particles (20-60 nm) that seem to have a broader size distribution. Reducing the STM set point bias V from 10 to 1.5 V at I ) 30 pA causes a drastic change in the observed topography, see Figure 1c and d. Approximately 20 nm long elongated features can be discerned in c on a (rather noisy) surface with a roughness that is a factor two less than at 10 V. When, at V ) 10 V, the set point current I is increased, the morphology continuously changes, although

the topography on a larger length scale seems to remain unchanged, see Figure 1d-f. The number of ‘large’, structureless particles decreases with increasing current set point, whereas the diameter of the smallest particles remains unchanged at about 20 nm. This strongly suggests that larger particles are actually clusters of multiple smaller ones, separated by a thin PSS layer.4 The observed minimal particle size of 20 nm compares favorably with the 18-30 nm particle size in solution, as measured using a disk centrifuge. The topographic changes with both increasing current set point and decreasing bias strongly suggest that the tip penetrates further into the film when the conductance associated with the STM set point, i.e., I/V, is increased. The ‘surface of equal conductance’, or isoconductance surface, which is visualized by the STM, has to lie closer to the substrate when a higher conductance is required, i.e., when the current is increased or the bias is decreased. The remarkable difference between AFM and STM images can now be explained by realizing that this isoconductance surface does not necessarily coincide with the mechanical sample surface that is probed by AFM. Apparently the PEDOT/PSS film has a top layer with a strongly reduced conductivity, which may be related to a difference between bulk and surface morphology (cf. Figure 1a and d) or composition. The latter cannot be concluded from our measurements but is consistent with the results reported in refs 6, 21, and 22, who reported a PSS-rich top layer for similar films on the basis of energy-dependent XPS measurements6,21 and neutron reflectometry.22 Since PSS is hardly conductive, PSS-rich material is ‘not seen’ by the STM and the tip will penetrate the film to reach more PEDOT-rich regions.23 The particles that appear in the topographic STM images can thus be interpreted as PEDOT-rich conducting, or polaronic, islands. All insufficiently conducting material is invisible and pushed aside by the scanning tip. It should be pointed out that these results are entirely consistent with our observations on another set of PEDOT/PSS films.10 In the latter work, the morphological effects of a treatment with a high-boiling solvent are investigated by STM. 3.2. z-V Spectroscopy. To quantify the remarkable bias dependence that was found in topographic imaging, z-V spectra

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Figure 2. z-V curves taken on PEDOT/PSS. Thin solid lines are individual z-V spectra; the thick solid line is their average. Dashed lines are calculated z-V spectra, assuming a homogeneous PEDOT/ PSS film and a parabolic tip with apex radius 50 nm. The current and bias set point are V ) 10 V and I ) 30 pA.

were taken on the same sample. Figure 2 shows a number of typical z-V curves together with their average (bold line). The vertical range of the z-V curve agrees well with the nominal layer thickness of 200-250 nm. Clearly, all curves show a sudden change in slope at a tip height of about 20 nm below the starting point of the measurement. Note that the starting point does not have to coincide with the ‘true’ sample surface, as discussed in section 3.1. The change in slope of the z-V curve indicates a rather abrupt change in conductivity. On the basis of numerical simulations, shown as dashed lines in Figure 2, the conductivity in the top layer and bulk are estimated to be 2.5 × 10-5 and 1.3 × 10-4 S/m, respectively. These simulations calculate the potential distribution and the charge transport between a parabolic tip with apex radius R and a planar metallic substrate. The charge transport in the intermediate medium is assumed to be ohmic. More details can be found in ref 24. The validity of the assumption that PEDOT/PSS can be modeled as a homogeneous ohmic medium will be discussed in section 4. The bias-dependent morphology observed by STM, cf. Figure 1c-f, can now be understood. At high bias the tip is in the less conductive top layer; below 4-5 V, the tip penetrates the better conducting bulk of the film and a different morphology is observed. The relatively high noise level in Figure 1c is most likely due to the fact that the tip has to move through an approximately 200 nm thick layer. It should be pointed out that the topography in Figure 1c is still markedly different from that of the Au substrate, which is shown in Figure 1b, indicating that the tip is still in the PEDOT layer. Due to the intrinsic inhomogeneity of the PEDOT/PSS film and the large vertical distances needed to compensate for small bias changes (i.e., dz/dV is large due to the relatively high conductivity, as compared to undoped organic semiconductors19,20), proper operation of the feedback loop is extremely difficult. This problem is circumvented by I-z spectroscopy, in which the feedback loop is disabled. 3.3. I-z Spectroscopy. Figure 3a displays I-z curves that are taken in different regions of a 500 × 500 nm area. There are large qualitative and quantitative differences between the I-z curves obtained at different positions, indicating variations in the conductivity in both the lateral and normal directions. This is illustrated in Figure 3c, in which qualitative I-z curves are constructed for an STM tip penetrating an inhomogeneous medium at two different positions. The regions of different conductivity through which the current passes can be regarded

Figure 3. (a) I-z curves taken in different regions of a 500 × 500 nm area at V ) 10 V and I ) 30 pA. Curves are taken on a 16 × 16 grid and averaged over subareas. The solid black line is a calculation, assuming a homogeneous PEDOT/PSS film of 270 nm thickness with σ ) 2.3 × 10-5 S/m and a parabolic tip with apex radius 50 nm. S ) 5.5, zstart ) -20 nm. (b) Same data as in a, transformed into local conductivity using eq 1. The thin solid lines are obtained by fitting a ninth-order polynomial through the data of panel a and should be regarded as a guide to the eye. The dash-dotted line indicates the conductivity that is used as input for the numerical simulation. (c) Schematic illustration of the effect of inhomogeneous conductivity on I-z curves. The hatched (white) areas denote regions of low (high) conductivity; see text for further explanation.

as a series network of high and low resistances. When a high (low) resistance is ‘removed’ by pushing the tip through, a large (small) increase in the current results. The slope of the I-z curve can thus be interpreted as a measure of the local resistivity. A more quantitative estimate of the local resistivity or conductivity can be obtained by using the expression

F)

1 d(V/I) z2 ) 2πSR σ dz z0

(1)

in which F and σ are the local resistivity and conductivity and z and z0 are the tip depth and the layer thickness, respectively. Equation 1 is easily derived from Ohm’s law, assuming a parabolic tip, so z ) r2/2R, with r being the radius of the section through the tip at the film surface. This yields a contact area A that, in good approximation, follows A ) 2πRz. In eq 1 S is a correction factor for the divergence of the current. For a onedimensional problem, S ) 1. From our numerical simulations we find that S may be assumed to be constant for the penetration depths used. I-z curves, which depend on the particular tip shape and film thickness, can now be transformed into σ-z curves, which reflect the local variation of a key material

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Figure 4. Conductivity maps on a 32 × 32 grid at increasing relative tip depth. V ) 10 V, I ) 30 pA. Brighter colors indicate higher conductivity; the (logarithmic) conductivity scale ranges from 10-5 to 10-4 S/m. The scan area is 1.5 × 1.5 µm; the relative tip depth is, left to right, top to bottom, 0, -5, -10, -20, -30, and -40 nm.

property, see Figure 3b. The starting depth of the tip is estimated from Figure 3a by extrapolation of the I-z curves to zero current, which gives a starting value for z, zstart ≈ -20 nm. The total layer thickness z0 is estimated by adding zstart to the thickness of the PEDOT layer under the tip at the start of the I-z measurement, which can be obtained from z-V curves with the same set point, see Figure 2, giving z0 ≈ 270 nm. The applicability of eq 1 is confirmed by the relatively small difference between the conductivity as obtained by applying eq 1 to the numerically calculated I-z curve and the actual input value of the conductivity, shown by the dash-dotted line. Figure 3b shows the results of the transformation from I-z to σ-z. Significant conductivity variations in both lateral and normal directions are seen. The conductivity of the top layer of the PEDOT/PSS film that is found from the analysis of z-V curves (Figure 2) and I-z curves is quite similar. Analysis of I-z curves taken at a set point bias and current of 1.5 V and 30 pA, i.e., the conditions of Figure 1c where the tip is deep in the PEDOT film, yields a conductivity on the order of 7 × 10-4 S/m. This is in reasonable agreement with the analysis of the z-V data at low bias (Figure 2), where a value of 1.3 × 10-4 S/m was found. Unfortunately, the relatively large uncertainty in the tip height at this bias makes a more accurate determination of the conductivity from I-z curves impossible. At positive tip height, i.e., when the tip is retracted, the current drops to zero in 10-30 nm, depending on position. This proves our notion that even at the highest biases the tip is inside the PEDOT/PSS film, as the presence of a vacuum gap would cause the tunneling current to drop to zero within a few Å. At V ) 1.5 V (not shown) the current drops to zero in a few nanometers, indicating that, in this case, the excess material is mostly wiped aside by the scanning motion of the tip. The latter observation seems to exclude the possibility that the retracting tip pulls material out of the film for more then a few nanometers. The topographic images displayed in Figure 1c-f confirm the anticipated morphology for PEDOT-PSS films by showing the presence of conductive particles in a quasi-insulating matrix.4 It seems reasonable to interpret these particles as the ‘polaronic islands’ between which the transport-limiting tunneling or hopping takes place.12 Then, conductance variations on a length

scale that is comparable to the particle size are to be expected and indeed observed in the normal direction, see Figures 2 and 3. In the lateral directions the smallest observed length scale is on the order of 50-100 nm, which can be regarded as an upper limit, as the lateral resolution is limited by the tip apex radius. However, because of the preferential alignment of the PEDOT chains with the substrate surface,25 the lateral conductivity of PEDOT/PSS films is much larger than the normal conductivity.16,26 In the present case, the in-plane/out-of-plane conductivity ratio is on the order of 104, in good agreement with ref 16. Consequently, the percolating path(s) from a point source on top of the film to a planar bottom contact may very well have a large lateral component to it. Depending on the morphological details and the exact value of the conductivity anisotropy ratio, the lateral component may easily exceed the film thickness. In Figure 4 1.5 × 1.5 µm conductivity maps at increasing depth are displayed. Several subsurface regions of enhanced conductivity, starting at various relative tip heights, are clearly visible. Length and width of these regions exceed both the film thickness and the size of the conducting islands that are observed in the topographic images of Figure 1. In line with the preceding discussion, we tentatively interpret these regions as percolation paths consisting of multiple, strongly coupled conducting particles. It is worthwhile to point out a remarkable topological resemblance between the domains in Figure 4 and the region of enhanced conductivity on a polythiophene film as observed by Bøggild et al. by four-point conductive AFM.27 Despite this resemblance, the origin of the observed domains is different. The locally enhanced conductivity in the polythiophene film of ref 27 is the result of a more favorable stacking of the constituent molecules, whereas in our case the enhanced conductivity results from a strong coupling between neighboring conductive particles. Finally, the reproducibility of the I-z images in Figures 3 and 4 should be addressed. The displayed images are obtained in the forward scan direction, i.e., with the tip moving from left to right. Those obtained in the backward scan direction are virtually identical, apart from a 100 (Figure 3) to 300 (Figure 4) nm displacement in the horizontal direction. This we attribute

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Figure 5. (a) I-V curves taken at different spots of Figure 1d. V ) 10 V, I ) 30 pA. Solid, dashed, and dash-dotted lines are fits to the functional shape of an ohmic conductor, a space-charge-limited conductor, and a space-charge-limited conductor with a field-dependent mobility, respectively. (b) I-V curves taken at different spots of Figure 1f. V ) 10 V, I ) 1 nA; rest as panel a.

to bending of the tip because of the lateral friction due to the physical contact between the scanning tip and the PEDOT/PSS film. The good reproducibility of I-z images strongly suggests that in absence of a lateral tip motion (the scanning is stopped while the I-z curve is taken and only restarted after the tip has reestablished the equilibrium position according to the set point) the movement of PEDOT/PSS material by the penetrating tip is largely reversible. Nevertheless, no meaningful topographic images are obtained in combination with I-z measurements with a vertical tip travel of 40 nm. For penetration depths on the order of 10 nm, this seems possible. In the latter case, a noisy but relatively flat (vertical scale of 10 nm on a 250 × 250 nm scan area) topography is observed that is very comparable to Figure 1c. No signs of indentation of the PEDOT/PSS layer are discernible in the topography, which is consistent with our view that the polymer swiftly relaxes after or during retraction of the tip. 3.4. I-V Spectroscopy. In the preceding two sections we used invasive spectroscopy, i.e., I-z and z-V, to show that large variations in the local conductivity are present in the PEDOT/ PSS film. The underlying conduction mechanism has so far not been addressed. Noninvasive current-voltage spectroscopy can shed some light on the latter issue. Figure 5a shows I-V curves that are taken simultaneously with Figure 1d, i.e., with the tip in the low-conductive surface layer. The exact value of the current depends strongly on position. However, the functional shape seems to indicate that the transport is governed by spacecharge effects. The observed bias dependence is significantly stronger than the linear dependence of an ohmic conductor. The curves are fitted reasonably well by either purely space-chargelimited conduction (SCLC, j ) (9/8)µ0rV2/L3 with L the layer thickness and µ the carrier mobility) or SCLC in combination with a field-dependent mobility, which is commonly observed for conjugated materials (µ ) µ0 exp(γxE) with µ0 being the zero-field mobility and γ a prefactor28). The observation that perpendicular charge transport through the top layer of the

Kemerink et al. PEDOT/PSS film is governed by space-charge effects is consistent with a similar observation by Ionescu et al.,16 who attributed the deviation from ohmic behavior to the high local current density that is typical for SPM-based spectroscopy. At a higher current set point, i.e., with the tip deeper in the PEDOT/PSS film, the character of the I-V curves changes to an ohmic dependence. This is illustrated in Figure 5b, which is taken simultaneously with Figure 1f. This observation is consistent with our finding that two layers of different conductivity are present in the film. Apparently, the charge density and mobility in the PEDOT-poor top layer are insufficient to yield ohmic behavior at the current density used. In the PEDOTrich bulk the product of charge density and mobility (σ ) qnµ) is significantly higher, leading to a reduced importance of spacecharge effects and hence (quasi) ohmic conductance. It is important to note that in both panels of Figure 5 the current is limited by the transport through the PEDOT and not by the injection from the tip, which is an implicit assumption used in the analysis of Figures 2-4. The large offset bias of 2.3 V in the I-V curves of Figure 5 is not understood. When the tip is deeper in the film, the offset is absent. Apparently, its presence is related to a local property of the PEDOT/PSS blend. A tentative explanation is the presence of deep charge traps that are filled or emptied on a time scale that is comparable to the sweep speed at which the I-V curves are taken,29 which results in a nonzero current at zero bias and an apparent built-in voltage. Alternatively, an electrochemical reaction between Pt or PtOx on the tip and the acidic PSS may result in a current offset. As mentioned in section 3.2, the numerical model that is used to compare the experimental results to assumes a homogeneous and ohmic medium. As we have shown, both assumptions are not valid for the PEDOT/PSS films investigated here. Therefore, the model curves in Figures 2 and 3 should be regarded as indicative of how an ideal ohmic conductor of a comparable conductivity would respond in the same experiment. Equation 1 is derived from Ohm’s law, which, from the I-V curves in Figure 5a and b, is not found to hold at low current set points, i.e., in the top layer of our films, but seems to become valid at higher current set points, i.e., deeper in the film. For the interpretation and validity of the data in Figures 3 and 4, it is therefore crucial to make an estimate of the depth at which the conduction becomes ohmic. In principle, the depth at which the transition between ohmic and nonohmic conduction occurs can relatively easily be quantified using depth- (or set-point-) dependent I-V curves, as done in Figure 5. However, because of the invasive nature of that experiment, a subsequent z-V measurement must be performed either on the same, no longer pristine area, or on a different spot. Obviously, since one is interested in intrinsic film properties, the data of Figure 3a are taken on another part of the sample as those in Figures 1 and 5. With the previous remarks in mind, one may tentatively estimate the nonohmic to ohmic transition depth from the topographies in Figure 1. The relatively large resemblance between panels d and f suggests that the transition occurs at a relatively modest depth, i.e., on the order of or less than 10 nm, below the height that is associated with the low-current set point of 10 V and 30 pA that is used for both Figure 1d and Figures 3 and 4. A depth of 10 nm seems to make sense since the nonohmic region is likely to coincide with the lowconductivity top layer, which is most apparently seen in Figure 2. In view of the discussion above, the condition of ohmic conductance is estimated to be fulfilled in at least the lower

Three-Dimensional Inhomogeneities half of the I-z curves. For smaller penetration depths, both the absolute value and the depth dependence of the conductivity should be treated with care. On the other hand, the observed nonmonotonic depth dependence of σ cannot be explained by a shift from space-charge-limited to ohmic conductance. The same holds for the lateral variations. Therefore, our main conclusions concerning variations in conductivity and compositional inhomogeneity remain as previously stated, despite the limitations of our analysis in the upper part of the PEDOT/PSS film. 4. Conclusions By combining several STM-based spectroscopic techniques with standard topographical imaging by AFM and STM, we performed a first investigation to the nanometer-scale chargetransport properties of spin-cast PEDOT/PSS films. The films are found to consist of relatively well-conducting particles with a typical diameter of 20 nm that are embedded in a less conductive matrix. Order-of-magnitude variations in the film conductivity are observed in all spatial directions. In addition, depending on the injection position, the charge transport in the normal direction is found to be either ohmic or space-chargelimited. The results are interpreted in terms of charge transport occurring along a percolating path or network, formed by strongly coupled conductive particles. Moreover, we found that the morphology and conductivity of the top layer differ substantially from those in the bulk. In agreement with recent literature, the reduced top layer conductivity is attributed to an enhanced PSS content. The spectroscopic method that is developed and applied in this paper can, in principle, be used to study the charge-transport properties of any soft (semi)conducting material on a nanometer scale. Acknowledgment. We gratefully acknowledge Frank Louwet from AGFA for the generous gift of the PEDOT/PSS dispersion. References and Notes (1) For a review on PEDOT and its derivatives, see, for example: Groenendaal, L.; Jonas, F.; Freitag, D.; Pielartzik, H.; Reynolds, J. R. AdV. Matter. 2000, 12, 481. (2) Mo¨ller, S.; Perlov, C.; Jackson, W.; Taussig, C.; Forrest, S. Nature 2003, 426, 166. (3) Smits, J. H. A.; Meskers, S. C. J.; Marsman, A. W.; Janssen, R. A. J. Unpublished results.

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