Quantitative nanoscale surface voltage measurement

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Quantitative nanoscale surface voltage measurement on organic semiconductor blends

This content has been downloaded from IOPscience. Please scroll down to see the full text. 2012 Nanotechnology 23 045703 (http://iopscience.iop.org/0957-4484/23/4/045703) View the table of contents for this issue, or go to the journal homepage for more

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IOP PUBLISHING

NANOTECHNOLOGY

Nanotechnology 23 (2012) 045703 (7pp)

doi:10.1088/0957-4484/23/4/045703

Quantitative nanoscale surface voltage measurement on organic semiconductor blends 1 , Miguel Munoz-Rojo 1, ˜ ˜ Alexandre Cuenat1 , Andr´es Muniz-Piniella 2 1 Wing C Tsoi and Craig E Murphy 1

Materials Division, National Physical Laboratory, Hampton Road, Teddington Middlesex, TW11 0LW, UK 2 Department of Physics and Centre for Plastic Electronics, Imperial College London, Prince Consort Road, London SW7 2AZ, UK E-mail: [email protected]

Received 3 August 2011, in final form 6 December 2011 Published 6 January 2012 Online at stacks.iop.org/Nano/23/045703 Abstract We report on the validation of a method based on Kelvin probe force microscopy (KPFM) able to measure the different phases and the relative work function of polymer blend heterojunctions at the nanoscale. The method does not necessitate complex ultra-high vacuum setup. The quantitative information that can be extracted from the topography and the Kelvin probe measurements is critically analysed. Surface voltage difference can be observed at the nanoscale on poly(3-hexyl-thiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) blends and dependence on the annealing condition and the regio-regularity of P3HT is observed. (Some figures may appear in colour only in the online journal)

1. Introduction

organic semiconductors to the device electrodes is strongly influenced by the local work function of the materials. It is therefore vital to measure the distribution of phases and their electronic properties down to these scales to develop valid structure–property relationships. This paper demonstrates that quantitative information can be extracted from Kelvin probe force microscopy (KPFM) operated in air down to the nanoscale. KPFM is a well developed method [4, 5] that can measure the work function of polymer blends with an energy resolution of a few milli-electronvolt [6]. This scanning probe technique is an adaptation of the well known vibrating Kelvin probe method [7] used widely by the semiconductor industry to measure the surface voltage of materials. However, conventional scanning Kelvin probes at the micro- and macro-scale operate through the detection and nulling of a current produced by the probe vibration, while the nanoscale technique nullifies the electrical forces acting on the probe. Some of the limitations of the scanning Kelvin probe method are well documented: the depth of probing, the

In order to meet the challenges of climate change and energy supply, alternative energy sources are urgently needed. One potential solution is photovoltaic solar cells and, in particular, the sub-class of bulk heterojunction organic devices [1, 2]. Great progress has been made recently in increasing the power conversion efficiency of such solar cells, however challenges remain to enable these systems to enter the market. One crucial aspect that is lacking is easy-to-use validated measurement techniques for electrical properties of organic thin film at nanometre length scales. The main systems of interest are blended systems of polymers and small molecules which form hierarchical phase-separated structures at a length scale that depends on the miscibility of the different components and on the processes used to produce the devices [3]. These structures range from the micrometre down to the nanometre in size and their composition and spatial arrangement control the functional properties of the devices. In particular, the extraction of charges from the 0957-4484/12/045703+07$33.00

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c 2012 IOP Publishing Ltd Printed in the UK & the USA

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influence of the substrate and the variation between surface voltage and bulk work function are common to all surface voltage measurements [8]. Atomic force microscopy introduces an extra layer of complexity: operation in air may give a value with limited accuracy due to the presence of a water layer on the sample or a water meniscus at the tip; reviews of many of these issues when relevant to organic semiconductors are available elsewhere [9, 10]; double-pass techniques suffer from the increased distance between the tip and the surface during the second pass, which decreases the lateral resolution; amplitude modulation, the ‘standard’ operating mode in air, also gives values that are often smaller than those of the frequency modulation technique used in ultra-high vacuum, the latter technique being sensitive to the gradient of force rather than the electrostatic force directly [11]. The accuracy of the amplitude and frequency modulation methods has been recently discussed [12] and the potential effect of any coupling between the topography and the potential signal demonstrated in some setups [13]. At the nanoscale, the convolution of the electrical field at the end of the tip with the materials under study is important [14] and will depend on the topography, the tip shape and the electrical properties of the material. There is also a series of questions related to the accuracy of the measurement at the nanoscale due to the presence of image forces and the poor mobility of charges in some of the materials studied resulting in trapped charges or pinning of the Fermi level at the material surface [15]. In this report only single-pass techniques are used, this has the main advantage of improving lateral resolution by scanning as close as possible to the surface and by ensuring that the topographic and electrical signals are perfectly aligned as no effect of the stage drift needs to be compensated. The aim of this work is to study the validity of a method to measure the surface voltage of polymer blends at the nanoscale on relatively large (>1 cm2 ) samples for a promising solar cell candidate: poly(3hexyl-thiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM). As well as the size of the samples, some of the objectives of the final method are ease of use and reduced acquisition time, which all point to KPFM measurement in air. In order to validate this method with an accurate state-of-the-art reference, comparison is made with ultra-high vacuum measurement on a polymer blend (poly(9,90 -dioctylfluorene-co-benzothiadia zole):poly(9,90 -dioctylfluorene-co-bis-N,N 0 - (4-butylphenyl)bis-N,N 0 -phenyl-1,4-phenylenediamine) (F8BT:PFB) with larger phases (∼1 µm versus tens of nanometres); the effect of tip convolution and variation of surface voltage measured in air and in vacuum is computed and compared.

cleaned indium tin oxide coated glass slides (Sigma-Aldrich) with a surface resistivity of 30–60 /sq were used as substrates. The poly(9,90 -dioctylfluorene-co-benzothiadia zole):poly(9,90 -dioctylfluorene-co-bis-N,N 0 -(4-butylphenyl)bis-N,N 0 -phenyl-1,4-phenylenediamine) system (F8BT:PFB) was obtained from American Dye Source (Baie d’Urf´e, Quebec, Canada) and the starting solutions were prepared by dissolving 10 mg of the polymers in 1 ml each of toluene. A 1:1 volume ratio solution was filtered using a 0.45 µm PTFE syringe filter and spin coated at 2000 rpm for 30 s onto the substrate. The thickness of the film was measured using atomic force microscopy to be ∼180 nm. The P3HT:PCBM thin films were produced to mimic existing device configurations [16]. Indium tin oxide coated glass substrates with similar resistivity were also used. An underlayer of poly(3,4-ethylenedioxythiophene):poly(styrenesu lfonate) (PEDOT:PSS) was first spin coated on the substrate. Regio-random P3HT (RRa-P3HT) was purchased from Aldrich and regio-regular P3HT (RR-P3HT) was synthesized by Merck Chemicals with a 94.2% regio-regularity. The PCBM was purchased from API Service, Inc. The RRaP3HT:PCBM solution was prepared by dissolving 8.3 mg of RRa-P3HT with 8.3 mg of PCBM in 1 ml of chlorobenzene solution. The RR-P3HT:PCBM solution was prepared by dissolving 10 mg of RR-P3HT with 10 mg of PCBM in 1 ml of chlorobenzene solution. Films of ∼50 nm thickness (as measured by a step profiler) were prepared by spin-coating solutions at 1500 rpm for 60 s. For some of the samples, annealing was carried out in N2 at 140 ◦ C for 30 min. 2.2. Scanning probe microscope KPFM is a nulling technique where a feedback loop minimizes the electrostatic forces—in the amplitude modulation (AM) mode—or the electrostatic force gradient—in the frequency modulation (FM) mode—between the tip and the sample. Under some assumptions [17], this allows one to determine the contact potential difference (CPD) between the tip and the sample. An AC bias voltage VAC sin(ωt) at frequency ω and a DC voltage (VDC ) difference between the tip and the sample induce an electrostatic force [18]. Considering the tip–sample system as a capacitor, the electrostatic force is Fel = −

1 ∂C [VDC − VCPD + VAC sin(ωt)]2 , 2 ∂z

(1)

where ∂C/∂z is the capacitance gradient of the tip–sample system and VCPD is the measured contact potential difference between the tip and the sample. The force can be further separated into different frequency components:

2. Experimental methods

C2 (VDC + VCPD )2 , 20

(2)

C2 (VDC − VCPD )VAC sin(ωt), 0

(3)

C2 2 V cos(ωt). 40 AC

(4)

FDC = −

2.1. Materials Fω = −

The materials are blends of electron transporting and hole transporting materials with lateral phase sizes ranging from micrometres to nanometres. Ultraviolet–ozone

F2ω = − 2

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To obtain sharp contrast, we used HOPG samples (SPI Inc, West Chester, USA), grade ZYH, left in air with about 40% relative humidity for a day. These samples show clear contrast in Kelvin probe measurements between single layer terraces, this allows one to measure the transfer function both in air and in vacuum using the Gwyddion software. The resolution limit of our KPFM systems was estimated by measuring a multi-layer system of AlGaAs and GaAs (BAM-L200 reference materials), the observed spatial resolution corresponding to our scanning parameters was around 18 nm in air and 10 nm in UHV. The contact potential difference between materials is estimated using a watershed approach [23]. Watershed analysis is a useful technique used in segmentation of complex structures; the image is treated as a topographical surface in which the grey level corresponds to elevation. A minimum is a valley surrounded by higher land and the grain size surrounding this minimum is defined by the area where water ‘falling’ on the landscape would flow down to this minimum. Watershed analysis is sensitive to noise and this will produce oversegmentation. We use a median filter as this is known to conserve shapes [24]. To take into account the finite lateral resolution of the measurement, only areas that are at least 5 pixels × 5 pixels size wide, corresponding to an area approximately 20 nm × 20 nm large are considered for analysis. The samples are phase separated in all three dimensions: X, Y and Z. The depth probed by the technique depends to a first approximation on the Debye length of the materials [25]. Assuming an electrical permittivity of 3 for the polymers and a charge density of 1015 cm−3 [26], the Debye length at room temperature is ∼46 nm. The measured surface voltage will thus vary between the two values set by the pure single materials; in other words, the probing depth is such that we certainly measure a mixture of the two materials in the Z direction. Beyond the influence of deeper structures, the effect of the tip convolution with the surface potential is also present. It is difficult to precisely quantify their relative contributions, but it is likely that for domains with equivalent diameter of the order of the Debye length the convolution effect dominates. To measure the work function of single phases, we have set an arbitrary value for the watershed level of these pure phases. The lowest value that fills 5% of the images is defined as one phase and, in a similar way, the value of the second, highest phase is the watershed level corresponding to 95% filling of the potential landscape. Only type A uncertainties, inferred from repeated measured values, are reported with a series of at least five measurements with the same AFM parameters. We note that variation between AFM cantilevers contributes less to the total uncertainty than repeatability variation, when calibration of the cantilever is carried out with an identical gold sample.

Following a standard procedure for the Kelvin method, the DC voltage level is varied until the AC induced vibration of the cantilever at ω is zero. At this point, VDC = VCPD and the local contact potential difference can be evaluated with high spatial and voltage resolution using VCPD = (9tip −9sample )/e, where 9 is the work function of the materials and e the electron charge. 2.2.1. Ultra-high vacuum environment. Ultra-high vacuum experiments (10−10 mbar) were carried out at room temperature in an RHK UHV-300 AFM (RHK-tech, Michigan, USA). A conducting cantilever (Mikromasch NSC14/Pr–Ir) with a 168.72(1) kHz resonance frequency and 0.02  cm resistivity was used. KPFM measurements were performed in the frequency modulation (FM) mode using a PLLPro FM detector for the topography signal and a Nanonis SPM control system to measure the contact potential difference between the tip and the surface. The voltage modulation (1 kHz) was set at a frequency that was higher than the bandwidth of the topography feedback to minimize the cross-talk between topography and voltage measurement. All images were acquired in the constant detuning mode with the RHK SPM100 system and analysed with Gwyddion software [19]. 2.2.2. In-air environment. For air measurement we used an XE-100 AFM from Park. A series of cantilevers (Mikromasch NSC14/Cr–Au) with a nominal resonance frequency of 160 kHz and 0.01–0.05  cm resistivity were used. For all the images, the oscillation amplitude was 4 nm, the tip–sample distance 10 nm and the setpoint 15% reduction of the free oscillation amplitude with a scan speed of 1 ms/pixel. The tip was biased with the output of a lock-in amplifier (Stanford Research Systems SR830 DSP), which was also used to measure and null the Kelvin force on the cantilever. Tip–sample distances were determined by measuring first frequency shift in vacuum and deflection in air on an NPL traceable steps sample. A force curve was then obtained and the force distance defined using standard procedures [20]. For both air and vacuum measurements, all scans were performed in the forward and reverse directions, displaying quasi-identical features in all the measured channels. 2.3. Analysis Due to the relatively long range of the electrostatic forces, not only the tip apex but also the tip itself and the cantilever may contribute to the Kelvin signal. Analytical [21] as well as numerical [14] approaches to the contributing forces can be found. If the point spread function of the measuring tip is known, the actual surface potential can be restored by deconvolution with the Kelvin probe data. Since the force gradient decays faster than the force, FM–KFM techniques are less influenced by effects from the cone and the cantilever [22]. Images of sharp CPD variations (on HOPG or on a monolayer) can be used to reconstruct the transfer function and, by inversion, deconvolve the image.

3. Results The F8BT:PFB blend is first imaged in ultra-high vacuum (UHV) and the results are presented in figure 1. 3

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(a)

A Cuenat et al

(c)

(b)

Figure 1. Ultra-high vacuum measurement of the topography, potential and overlay (topography on potential) in F8BT:PFB blend film. 1f = −25 Hz, Aosc = 30 nm, scan speed 25 ms/pixel. (a) Topography, Z-range 68 nm peak to peak. (b) Potential, Z-range 700 meV peak to peak. (c) Overlay.

(a)

(b)

(c)

Figure 2. In-air measurement of topography, potential and overlay (topography on potential) in F8BT:PFB blend film. Oscillation amplitude 4 nm, tip–sample distance 10 nm and setpoint 15% reduction of the free oscillation amplitude. (a) Topography, Z-range 60 nm peak to peak. (b) Potential, Z-range 500 meV peak to peak. (c) Overlay.

The maximal contact potential difference between F8BT and PFB, defined as the difference between the 95% whitest watersheds and the 5% darkest, is found to be 655 ± 22 meV. Assuming that there is no pinning of the Fermi level and that equilibrium is reached so that the flat-band condition is achieved, the CPD effectively measures the Fermi level difference between the materials [27]. Assuming that the Fermi level is in the middle of the HOMO–LUMO gap, the result compares favourably with optical spectroscopy data from the literature (665 meV) [28, 29]. The different polymer phases will show topography variation when separated; the overlay of the topography contour at the median height with the mid-level potential shows, as expected, a link between the potential and the topography, although not a one to one relationship. From the measured Fermi level of the tip and the experimental value of the energy gap for F8BT and PFB, it is possible to identify the higher potential region (white on the image) as corresponding to F8BT rich; the F8BT-rich areas are also higher (white) on the topography scan compared to PFB-rich areas.

Measurements in air present similar topography and potential distribution as can be seen in figure 2. The contact potential difference is now 386 ± 39 meV, a value about 40% smaller than the UHV result. The P3HT:PCBM blend is also imaged in air; the topography, potential and the mid-range equipotential contour for the as-prepared and annealed samples are presented in figure 3. An analysis similar to that for the F8BT samples allows one to identify the higher value (white in the images) of the topography and the potential with P3HT. The difference in potential between P3HT and PCBM, set at the 95% and 5% watershed levels respectively, is 228 ± 54 mV on the annealed sample. For comparison, samples made of regio-random P3HT were also imaged under the same conditions and results for the potential are presented in figure 4. The difference in potential between the regio-random P3HT and PCBM was now increased to 288 ± 75 mV for the annealed sample, with the increase mostly due to an increase in the measured value of the Fermi level of regio-random P3HT compared to the regio-regular one. 4

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(a)

(b)

(c)

(d)

(e)

(f)

Figure 3. In-air measurement of topography and potential on regio-regular P3HT:PCBM, as deposited and annealed. Oscillation amplitude 4 nm, tip–sample distance 10 nm and setpoint 15% reduction of the free oscillation amplitude. (a) Regio-regular (RR) topography, Z range 3 nm peak to peak. (b) RR potential, Z range 300 meV peak to peak. (c) RR watershed contour. (d) RR-annealed topography, Z range 3 nm peak to peak. (e) RR-annealed potential, Z range 300 meV peak to peak. (f) RR-annealed watershed contour.

(b)

(a)

Figure 4. In-air measurement of potential on regio-random P3HT:PCBM, as deposited and annealed. Z-range 200 meV, oscillation amplitude 4 nm, tip–sample distance 10 nm and setpoint 15% reduction of the free oscillation amplitude. (a) Regio-random as produced potential, range 350 meV peak to peak. (b) Regio-random-annealed potential, range 350 meV peak to peak.

4. Discussion

the Kelvin measurement feedback loop. Organic adsorbates have an effect on surface dipoles and this will affect the value of the measured work function [30]. The UHV experiments were repeated after the air experiments and the results were similar to the original ones, which seems to indicate that the surface is stable. To corroborate our experimental findings with theory, we performed a finite element analysis simulation

The difference between air and UHV measurements is not surprising and has been observed in a number of cases [14]. Two reasons exist for this difference, the eventual presence of a water layer or adsorbates in air and the difference between amplitude modulation and frequency modulation for 5

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(COMSOL Multiphysics), modelling the tip apex as a sphere of radius of 20 nm with a cone of 15◦ aperture as the tip. The tip is kept at a constant distance (10 nm) from an island of materials at a potential of 1 V and a diameter of 200 nm, with the surrounding materials at 0 V. Assuming a perfectly conducting tip, the electric field will be perpendicular to the normal of the tip n with E = (σ/0 )n, where σ is the charge density. The force on the tip results from the Maxwell stress tensor τij = 0 (Ei Ej − 12 E2 δij ) +

1 (Bi Bj − 12 B2 δij ), µ0

semiconductor—according to the model of Hudlet [17] with a tip–sample distance of 10 nm, a difference between tip and surface potential of 0.1 V and a charge density of 1015 cm−3 . 4.1. Effect of annealing It is well established [32] that in the P3HT:PCBM system a dual crystallization process is observed upon annealing. For short annealing times and at low annealing temperatures only P3HT starts to crystallize. Bright-field TEM images show that P3HT fibre-like structures are formed. The watershed analysis can be used to quantify the effect of the annealing procedure on the domain size. This is only possible on the potential images as the topography of the P3HT samples did not show enough contrast between the components—the surface has a root mean squared roughness of less than a nanometre, see figure 3. For the potential images and at smaller scale, well-defined features are clearly visible for annealed samples in the form of elongated objects with moderate aspect ratio. These features appear for more than one tip and in different scan orientations and are thus not scan artefacts. Recent electron-tomography images [33] allow these objects to be identified as P3HT nanocrystallites formed after annealing. For the analysis of the P3HT mean domain size, the watershed level is again chosen between 5% and 95% in figures 3 and 4; variation of this level does not affect the result qualitatively. The diameter is defined as the equivalent diameter of a circle corresponding to the same area and similar results are found for regio-random and regio-regular samples. The mean diameter of the P3HT domain is found to be around 15 ± 5 nm for the as-prepared samples and around 34 ± 10 nm for the annealed samples. These numbers are indicative only as the convolution of the tip electrostatic potential is certainly still important in this case. The images also show that there is not a one to one correspondence between the topography and the potential, consistent with the phase segregation scale—of the order of 10 nm for P3HT and the vertical segregation observed in these systems. In our experiment, the probing depth is such that we certainly measure a mixture of the two materials in the Z direction. A higher Fermi level (on average by 44 ± 12 mV) is associated with regio-regular P3HT; this value corresponds to the difference of the average potential at the 95% watershed level as measured on the annealed regular and random P3HT samples respectively. The different phases could also be observed using the information in the phase (AM mode) or damping (FM mode) images and higher values of damping are recorded over the P3HT or PFB. Indeed, the phase and damping contrast are affected by the local chemical and mechanical nature of the surface and a higher energy is dissipated in the tip surface interaction over the most flexible component of the blend. However, a detailed modelling of the damping interaction would be needed for a quantitative analysis.

(5)

with i, j = x, y, z, E, B the electric and magnetic field respectively on the tip–cantilever system, 0 the vacuum permittivity and µ0 the vacuum permeability. The force is given by fi =

3 Z X j=1

τij nj dA,

(6)

At

where the integration is carried out on the area At using the element dA of the tip–cantilever, nj being the normal of its surface. For the frequency of the applied voltage, the magnetic induction can be neglected. Furthermore, assuming the probe oscillation is small compared to the probe–sample separation, the force can be decomposed into frequency dependent components and Z EDC EAC (n) dS. (7) fω = 0 sin(ωt) tip

The contact potential difference corresponds to the minimum of either fω for the air experiment or ∂fω /∂z for the UHV experiment. For the chosen tip–sample distance (10 nm), a reduction of around 55% is observed in the simulation. This is in line with the observed reduction (40%) between UHV and air measurement for the F8BT sample; the discrepancy is likely to be due to the uncertainty in the measured tip–sample distance and the potential presence of a water meniscus in air. We also note that accurate amplitude modulation measurements are usually dependent on the chosen setpoint and initial amplitude [12]; these variations are highly system (AFM, materials under test, tip) dependent [31]. Our experiment used the non-contact mode with an oscillation amplitude as small as possible to avoid an averaging effect of the long range electrostatic forces. The non-contact mode also has the advantage of avoiding tip change due to potential contamination when the tip is in contact with the surface. The magnitude of the contrast in UHV is consistent with previous KPFM reports on similar organic blends [5]. Due to the very small distance between the tip and the sample we have investigated the possibility of field emission between the tip and the surface; this would obviously modify the electronic properties of the sample under test. Using an external amplifier (FEMTO, DLPCA-200) no current was measured above the 1.8 pA noise threshold for a bias voltage of 0.1 V. We also note that the potential band bending is well below 0.1 mV when calculated—for an inorganic

5. Summary The limit of validity of Kelvin probe force microscopy used in air to measure the phase separation and local work function of 6

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polymer blends down to around 20 nm lateral resolution has been presented. The lateral resolution is sufficient to observe a difference in domain sizes for un-annealed and annealed regio-regular P3HT:PCBM samples, but the measured work function values are not accurate: a reduction of about 40% is observed compared to bulk work function measurements. This is due to a combination of the tip convolution effect that averages values between adjacent zones with different work functions and a reduction in value due to the feedback loop method used for the Kelvin measurement in air-amplitude modulation. However, the potential resolution is good enough to measure variation of Fermi levels between P3HT regio-regular and P3HT regio-random areas. Despite the commonly accepted statement that frequency modulation should provide better lateral resolution due its dependence on the gradient of the forces, we show that similar lateral resolution can be achieved with amplitude modulation if minimal tip–sample distance and small cantilever oscillation are used. The method applied here to passive thin film structures can be readily extended to the analysis of fully working device structures, including cross-sections.

[6] Palermo V, Palma M and Samori P 2006 Adv. Mater. 18 145–64 [7] Schr¨oder D K 2001 Meas. Sci. Technol. 12 R16 [8] Kronik L and Shapira Y 1999 Surf. Sci. Rep. 37 1–206 [9] Pingree L S C, Reid O G and Ginger D S 2009 Adv. Mater. 21 19–28 [10] Berger R, Butt H J, Retschke M B and Weber S A L 2009 Macromol. Rapid Commun. 30 1167–78 [11] Zerweck U, Loppacher C, Otto T, Grafstr¨om S and Eng L M 2005 Phys. Rev. B 71 125424 [12] Ziegler D and Stemmer A 2011 Nanotechnology 22 075501 [13] M´elin T, Barbet S, Diesinger H, Th´eron D and Deresmes D 2011 Rev. Sci. Instrum. 82 036101 [14] Jacobs H O, Leuchtmann P, Homan O J and Stemmer A 1998 J. Appl. Phys. 84 1168–73 [15] Ishii H, Sugiyama K, Ito E and Seki K 1999 Adv. Mater. 11 605–25 [16] Tsoi W C et al 2011 Adv. Funct. Mater. 13 66 [17] Jean M S, Hudlet S, Guthmann C and Berger J 1999 J. Appl. Phys. 86 5245–8 [18] Bonaccurso E, Sch¨onfeld F and Butt H J 2006 Phys. Rev. B 74 085413 [19] Klapetek P, Neˇcas D and Anderson C 2004–2009 Gwyddion User Guide http://gwyddion.net/ [20] Cappella B and Dietler G 1999 Surf. Sci. Rep. 34 1–104 [21] Colchero J, Gil A and Bar´o A M 2001 Phys. Rev. B 64 245403 [22] Glatzel T, Sadewasser S and Lux-Steiner M C 2003 Appl. Surf. Sci. 210 84–9 [23] Beucher S and Lantuejoul C 1979 Use of watersheds in contour detection Int. Workshop on Image Processing: Real-time Edge and Motion Detection/Estimation (Rennes) [24] Barner K and Arce G 2004 Nonlinear Signal and Image Processing: Theory, Methods, and Applications (Electrical Engineering and Applied Signal Processing Series) (Boca Raton, FL: CRC Press) [25] Huang H, Wang H, Zhang J and Yan D 2009 Appl. Phys. A 95 125–30 [26] Lonergan M 2004 Annu. Rev. Phys. Chem. 55 257–98 [27] Tal O and Rosenwaks Y 2006 J. Phys. Chem. B 110 25521–4 [28] Morteani A C, Sreearunothai P, Herz L M, Friend R and Silva C 2004 Phys. Rev. Lett. 92 247402 [29] Schmidtke J P, Kim J S, Gierschner J, Silva C and Friend R H 2007 Phys. Rev. Lett. 99 167401 [30] Hoppe H et al 2005 Nano Lett. 5 269 [31] Garia R and Perez R 2002 Surf. Sci. Rep. 47 197–301 [32] Dennler G, Scharber M C and Brabec C J 2009 Adv. Mater. 21 1323–38 [33] Oosterhout S D, Wienk M M, van Bavel S S, Thiedmann R, Koster L J A, Gilot J, Loos J, Schmidt V and Janssen R A J 2009 Nature Mater. 8 818

Acknowledgments This work was supported by the National Measurement Office through the IRD project CO2 ‘Nanostructured Multilayer Characterisation for Plastic Electronics’ and by the Technology Strategy Board project K2536J ‘Thin film organic displays’. The authors thank Tony Samano for helping to prepare the F8BT:PFB samples and Patrick Nicholson for helpful discussion.

References [1] Yu G, Gao J, Hummelen J C, Wudl F and Heeger A 1995 Science 270 1789 [2] Nicholson P G and Castro F A 2010 Nanotechnology 21 492001 [3] Kim J S, Ho P K H, Murphy C E and Friend R H 2004 Macromolecules 37 2861–71 [4] Nonnenmacher M, O’Boyle M P and Wickramasinghe H K 1991 Appl. Phys. Lett. 58 2921–3 [5] Chiesa M, B¨urgi L, Kim J S, Shikler R, Friend R H and Sirringhaus H 2005 Nano Lett. 5 559–63

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