Three-Dimensional Nanoscale Imaging of Polymer Bulk

Anal. Chem. 2009, 81, 8936–8941

Three-Dimensional Nanoscale Imaging of Polymer Bulk-Heterojunction by Scanning Electrical Potential Microscopy and C60+ Cluster Ion Slicing Bang-Ying Yu,†,‡ Wei-Chun Lin,† Jen-Hsien Huang,§ Chih-Wei Chu,†,| Yu-Chin Lin,† Che-Hung Kuo,⊥ Szu-Hsian Lee,⊥ Ken-Tseng Wong,‡ Kuo-Chuan Ho,§ and Jing-Jong Shyue*,†,⊥ Research Center for Applied Sciences, Academia Sinica, Taipei 115, Taiwan, Department of Materials Science and Engineering, Department of Chemistry, and Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan, and Department of Photonics, National Chiao Tung University, Hsinchu 300, Taiwan Solution-processable fullerene and copolymer bulk-heterojunctions are widely used as the active layer of solar cells. It is known that the controlled phase-separation in the film provides a pathway for carrier transportation and is crucial to efficiency. In this work, scanning electrical potential microscopy (SEPM) is used to examine the surface distribution of [6,6]phenyl-C61-butyric acid methyl ester and poly(3-hexylthiophene), which form the bulkheterojunction. Because the two components have different energies in the highest occupied molecular orbital (HOMO), the differences in contact potential yield strong contrast in SEPM. A cluster ion beam (C60+) is used to remove the surface in order to determine the structure below, and SEPM is used to analyze the newly exposed surface. With the SEPM images acquired from different depth through the material stacked, a 3D volume image is obtained. It is demonstrated that using SEPM with cluster ion slicing is an effective tool for studying the 3D nanostructures of soft materials. Nanostructured polymer solar cells are a promising renewable energy source due to their simple fabrication process and relatively low cost compared to silicon-based solar cells. The key component of polymer solar-cells is the bulk-heterojunction,1 which consists of a mixture of polymeric/organic n- and p-type semiconductors. The most common polymer photovoltaic cells are based on regionregular poly(3-hexylthiophene) (P3HT, the p-type material) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM, the n-type material).1,2 The bicontinuous network formed by the mixture ensures a large interfacial contact area between the two components. It also provides an adequate path for charge carriers and thus significantly enhances the charge separation capability. Because of the limited exciton diffusion length, typically in the * To whom correspondence should be addressed. Phone: +866(2)27898000#69. E-mail: [email protected]. † Research Center for Applied Sciences, Academia Sinica. ‡ Department of Chemistry, National Taiwan University. § Department of Chemical Engineering, National Taiwan University. | Department of Photonics, National Chiao Tung University. ⊥ Department of Materials Science and Engineering, National Taiwan University. (1) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789–1791. (2) Takanezawa, K.; Hirota, K.; Wei, Q.-S.; Tajima, K.; Hashimoto, K. J. Phys. Chem. C 2007, 111, 7218–7223.

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Analytical Chemistry, Vol. 81, No. 21, November 1, 2009

5-15 nm range,3 the electron acceptors must be intermixed with polymers on the nanometer scale in order to obtain a high charge separation yield.4 Therefore, control of the nanostructure is crucial to the development of polymer solar cells.5-7 In order to study the nanostructure, scanning probe microscopy (SPM) was used. SPM is widely used to study structural organic materials due to its excellent lateral resolution and the wealth of information it collects about the material nanostructure.8-10 Although the amplitude and phase of the atomic force microscope data (AFM, the most commonly used variety of SPM) revealed the topography and mechanical properties, it cannot identify the chemical species. In addition, the phase image is heavily coupled with the topography, so that differences in properties could be buried by topographical contrast. Recently, conductive SPM has also been used to study the nanostructure of the bulk-heterojunction.11,12 However, since the probe only interacts with the outermost surface, SPM can only provide information about the surface and only a limited amount of information along the vertical axis can be obtained. As the result, SPM is not used to detect 3D nanostructures inside specimens. Electron tomography (ET), based on transmission electron microscopy (TEM), is used to examine 3D structures inside specimens at a series of tilt angles. This technique has been widely used in structural biology13,14 and is extended to study other (3) Dennler, G.; Sariciftci, N. S. Proc. IEEE 2005, 8, 1432. (4) Coakley, K. M.; McGehee, M. D. Appl. Phys. Lett. 2003, 83, 3380–3382. (5) Zhang, F. L.; Jespersen, K. G.; Bjorstrom, C.; Svensson, M.; Andersson, M. R.; Sundstrom, V.; Magnusson, K.; Moons, E.; Yartsev, A.; Inganas, O. Adv. Funct. Mater. 2006, 16, 667–674. (6) Yang, X.; Loos, J. Macromolecules 2007, 40, 1353–1362. (7) van Bavel, S. S.; Sourty, E.; With, G. d.; Loos, J. Nano Lett. 2009, 9, 507– 513. (8) Reyes-Reyes, M.; Kim, K.; Carroll, D. L. Appl. Phys. Lett. 2005, 87, 083506. (9) Ma, W. L.; Yang, C. Y.; Gong, X.; Lee, K.; Heeger, A. J. Adv. Funct. Mater. 2005, 15, 1617–1622. (10) Moule, A. J.; Meerholz, K. Adv. Mater. 2008, 20, 240. (11) Bull, T. A.; Pingree, L. S. C.; Jenekhe, S. A.; Ginger, D. S.; Luscombe, C. K. ACS Nano 2009, 3, 627–636. (12) Lin, Y. Y.; Chu, T. H.; Li, S. S.; Chuang, C. H.; Chang, C. H.; Su, W. F.; Chang, C. P.; Chu, M. W.; Chen, C. W. J. Am. Chem. Soc. 2009, 131, 3644– 3649. (13) McEwena, B. F.; Marko, M. J. Histochem. Cytochem. 2001, 49, 553–563. (14) Downing, K. H.; Sui, H.; Auer, M.; Berkeley, L.; Lab, N. Anal. Chem. 2007, 79, 7949–7957. 10.1021/ac901588t CCC: $40.75  2009 American Chemical Society Published on Web 09/29/2009

Figure 1. (a) Schematic drawing of creation of a crater with an ion beam (not drawn to scale). (b) Optical image of the edge of the crater. (c) SEPM image at the bottom of the crater (5 µm × 5 µm, contrast range is 10 mV). (d, e, and f) The SEPM images (25 µm × 5 µm, contrast range is 50 mV) taken from area 1, 2, and 3 of part b, respectively.

amorphous materials.15,16 With the use of ET, the 3D structure of bulk-heterojunction has recently been reported.6,7,17 However, TEM depends on electron scattering to generate contrast. Therefore, for amorphous organic materials with similar scattering factors, the contrast is very low without staining or adding defocus.18 In addition, inelastic electron scattering and dynamic scattering in thick samples generates a diffused background in the image, which further diminishes the contrast. As a result, the thickness of samples for TEM study is limited to a few hundred nanometers and it is challenging to prepare such specimens for TEM. In addition, besides the need for complicated data reconstruction, multiple tilting axes and large tilting angles are required to minimize the missing wedge/pyramid of data in Fourier space that cause artifacts.19 Furthermore, since high-energy electrons are transmitted through the delicate specimen, a cryogenic sample holder and a low-dose technique are often required to prevent

irradiation damage20 during prolonged tilt-series imaging. These issues make the electron tomography for amorphous organic materials difficult to master. Another emerging technique for obtaining 3D distributions of organic (biological) molecules is scanning time-of-flight secondary ion mass spectrometry (TOF-SIMS) with cluster ion sputtering.21-25 Although cluster ion sputtering significantly alters the outermost surface of ionic-bonded inorganic materials,26 it causes an insignificant amount of alternation to organic surfaces27,28 because of the shallow damage range29 and enhanced sputtering rate.30 Although cluster ion sputtering can still alter the surface morphol(20) (21) (22) (23) (24) (25)

(15) Jinnai, H.; Hasegawa, H.; Nishikawa, Y.; Sevink, G. J. A.; Braunfeld, M. B.; Agard, D. A.; Spontak, R. J. Macromol. Rapid Commun. 2006, 27, 1424– 1429. (16) Sugimori, H.; Nishi, T.; Jinnai, H. Macromolecules 2005, 38, 10226–10233. (17) Andersson, B. V.; Herland, A.; Masich, S.; Ingans, O. Nano Lett. 2009, 9, 853–855. (18) Moon, J. S.; Lee, J. K.; Cho, S.; Byun, J.; Heeger, A. J. Nano Lett. 2009, 9, 230–234. (19) Arslan, I.; Tong, J. R.; Midgley, P. A. Ultramicroscopy 2006, 106, 994– 1000.

(26) (27) (28) (29) (30)

Egerton, R. F.; Li, P.; Malac, M. Micron 2004, 35, 399–409. Parry, S.; Winograd, N. Anal. Chem. 2005, 77, 7950–7957. Wucher, A.; Cheng, J.; Winograd, N. Anal. Chem. 2007, 79, 5529–5539. Ostrowski, S. G.; Kurczy, M. E.; Roddy, T. P.; Winograd, N.; Ewing, A. G. Anal. Chem. 2007, 79, 3554–3560. Ostrowski, S. G.; Szakal, C.; Kozole, J.; Roddy, T. P.; Xu, J.; Ewing, A. G.; Winograd, N. Anal. Chem. 2005, 77, 6190–6196. Fletcher, J. S.; Lockyer, N. P.; Vaidyanathan, S.; Vickerman, J. C. Anal. Chem. 2007, 79, 2199–2206. Lin, Y.-C.; Chen, Y.-Y.; Yu, B.-Y.; Lin, W.-C.; Kuo, C.-H.; Shyue, J.-J. Analyst 2009, 134, 945–951. Yu, B.-Y.; Chen, Y.-Y.; Wang, W.-B.; Hsu, M.-F.; Tsai, S.-P.; Lin, W.-C.; Lin, Y.-C.; Jou, J.-H.; Chu, C.-W.; Shyue, J.-J. Anal. Chem. 2008, 80, 3412–3415. Chen, Y.-Y.; Yu, B.-Y.; Wang, W.-B.; Hsu, M.-F.; Lin, W.-C.; Lin, Y.-C.; Jou, J.-H.; Shyue, J.-J. Anal. Chem. 2008, 80, 501–505. Yu, B.-Y.; Chen, Y.-Y.; Lin, W.-C.; Lin, Y.-C.; Shyue, J.-J. Appl. Surf. Sci. 2008, 255, 2490–2493. Cheng, J.; Wucher, A.; Winograd, N. J. Phys. Chem. B 2006, 110, 8329– 8336.


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Figure 2. SEPM image of the same 10 µm × 10 µm region after different sputtering time. The contrast range is 50 mV.

ogy and can deposit carbon in the remaining surface, it is a promising technique for analyzing organic materials. For an example, SIMS patterns of organic materials before and after cluster ion sputtering remain the same. The m/z is preserved near 1000 indicating that sputtering does not alter the molecular structure in the remaining material.31 It is also shown that preferential sputtering does not occur for organic-inorganic nanocomposites.28 As a result, it is now an accepted new technique for profiling organic materials. For the high depth resolution and minimal alteration of the remaining surface, it has been used to study the depth distribution of molecules in organic LEDs and to examine the microstructure31 and degradation mechanism.32 Currently, application of this sputtering-based technique in 3D volume imaging is limited to biological applications because the spot size of cluster primary ions may be too large for the lateral resolution required in organic electronics. For example, a focused Au3+ or Bi32+ primary ion beam has a specification of a few hundred nanometers in pulse mode for imaging. This means that the lateral resolution of the image is on the order of a few hundred nanometers, while features in organic electronics are on the order of 10 nm. Therefore, sputter-based techniques have yet been used to provide 3D volume images of organic electronics due to their limited lateral resolutions. In this work, a C60+ cluster ion beam is used to sputter away surface materials to generate a crater or prepare slices of the specimen. Because of its high lateral resolution, SPM is used to observe the newly exposed surfaces. SEPM data obtained in this work provides a contrast in an additional physical (31) Lin, W.-C.; Lin, Y.-C.; Wang, W.-B.; Yu, B.-Y.; Iida, S.-i.; Tozu, M.; Hsu, M.F.; Jou, J.-H.; Shyue, J.-J. Org. Electron. 2009, 10, 459–464. (32) Lin, W.-C.; Wang, W.-B.; Lin, Y.-C.; Yu, B.-Y.; Chen, Y.-Y.; Hsu, M.-F.; Jou, J.-H.; Shyue, J.-J. Org. Electron. 2009, 10, 459–464.

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dimension that arises due to differences in the work function, which can be related to the degree of phase separation. The 3D molecular distribution in the volume can be obtained by analyzing the edge of the crater or by stacking images acquired from different slices (depths). Thus, 3D nanostructure can be reconstructed with high resolution in all dimensions and up to four-dimensions of information can be obtained using the presenting technique. EXPERIMENTAL SECTION Specimen. The photoactive films were prepared by spincoating 1,2-dichlorobenzene solution in a 1:1 weight ratio of P3HT: PCBM on a poly(ethylene dioxythiophene):polystyrenesulfonate (PEDOT:PSS)-modified ITO surface. After spin-coating at 600 rpm for 60 s, the specimen was then slowly dried in covered Petri dishes. The final thickness of the active layer was about 200 nm. Ion Sputtering. Cluster ion sputtering was carried out with a PHI 5000 VersaProbe (ULVAC-PHI, Chigasaki, Japan) system. The sputtering rate was determined from the elemental depth profile using X-ray photoelectron spectrometry and was found to be ∼0.6 nm/min for the P3HT:PCBM layer. A charge neutralizer (30 V flooding electron beam) was used to compensate for the charge build-up effect during sputtering. The Ar+ ion source (FIG-5CE), at a fixed incident angle of 45°, was operated at 0.2 kV and 300 nA using a floating voltage of 500 V and a spot size of ∼200 µm. A Wien-filtered C60+ ion source (IOG C60-10, Ionoptika, Chandler’s Ford, UK) was operated at 10 nA and 10 kV (spot size ∼600 µm) at a fixed incident angle of 70° (from the normal to the surface of analysis). The beam current was measured by the sample current on an Au foil, and was controlled by the strength of the condenser lens. The ion beams were rastered over an area of 2 mm × 2 mm. The low-energy Ar+ ion beam

Figure 3. A 3D projection of the reconstructed structure of P3HT:PCBM. The image size is 5.23 µm × 5.23 µm × 198 nm. The arrow in the insets indicates the direction of the projection: (a) 0°, (b) 33°, and (c) 33°-tilting along the viewing direction. (d) 3D-rendered isosurface of the structure.

and high-energy C60+ ion beam were used concurrently to effectively remove the surface with minimal damage.27 Scanning Electrical Potential Microscopy. The surface potential was measured with a Veeco Innova (Woodbury, NY) atomic force microscope using a conductive 75 kHz cantilever in the lift-mode. While the piezo driver was grounded, the magnitude and frequency of the ac field on the tip is 2 V and the mechanical resonate frequency, respectively. A dc bias was then applied to the tip to zero the change in electric-field induced tip oscillation. The magnitude of the dc component reflects the contact potential of the interacting surface and was recorded at each spatial location. In order to aid sample navigation after cluster ion sputtering, a 50 µm × 50 µm box was scratched into the surface using the lithography mode with a 300 kHz cantilever and a force of about 12 mN. Images were collected from the center of the alignment box using a closed-loop scanner. The electric potential image was passed through a low-pass filter in Fourier space to remove highfrequency random noise. Cross-correction was used to fine-align the images using SPIP software. The final image stack was reconstructed into a 3D volume image using ImageJ.

RESULTS AND DISCUSSION Because ion slicing will inevitably influence the morphology of the remaining surface slightly, an SPM technique that can decouple the topography from other properties is required. For a generic polymer with different elastic modules, force modulation microscopy (FMM) may be used to generate the mapping of elastic properties.33 For the organic semiconductors presenting in this work, SEPM (also known as Kelvin probe microscopy, KPM)34 is chosen because the contact potential between the tip and the various materials is different. The resulting image presents a contrast in the local effective work-function. In other words, SEPM data obtained in this work provides a contrast in an additional physical dimension that arises due to differences in the work function, which can be related to the degree of phase separation. Figure 1a describes how the crater is made. Note that the schematic is not drawn to scale. The film thickness is about 200 (33) Troyon, M.; Wang, Z.; Pastre, D.; Lei, H. N.; Hazotte, A. Nanotechnology 1997, 8, 163–171. (34) Nabhan, W.; Equer, B.; Broniatowski, A.; DeRosny, G. Rev. Sci. Instrum. 1997, 68, 3108–3111.


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Figure 4. Slices of reconstructed 3D images. The image size is 5.23 µm × 5.23 µm × 198 nm. The dashed boxes indicate the cross-section of the nanostructure.

nm and the diameter of the crater is about 2 mm. Figure 1b shows the optical image of the crater edge; the vertical feature seen in the optical image is from the sample holder. The SEPM image taken at the bottom of the crater (remaining sputtered glass), shown in Figure 1c, revealed little to no contrast as expected for homogeneous glass, indicating that the topographic information is successfully decoupled. Parts d -f of Figure 1 are the SEPM images along the edge of the crater created by ion sputtering. The image was stitched from five 5 µm × 5 µm images. Because the PCBM has a lower highest occupied molecular orbital (HOMO) (-6.1 eV) than that of P3HT (-5.0 eV),35 PCBM will be darker in the image than P3HT. Ideally, if the PCBM and P3HT are totally separated and each forms a pure phase, the contrast will be 1.1 V. However, the observed contrast is around 50 mV, indicating that total-separation did not occur. The components are still mixed with each other, but about 5% change in concentration can be estimated by approximating the linear relationship between the composition and the effective work-function.36 Since the contrast itself also reveals significant quantitative physical meaning, the resulting image actually contains information in four dimensions. (35) Yu, B. Y.; Tsai, A.; Tsai, S. P.; Wong, K. T.; Yang, Y.; Chu, C. W.; Shyue, J. J. Nanotechnology 2008, 19, 255202. (36) Wu, K.-Y.; Yu, S.-Y.; Tao, Y.-T. Langmuir 2009, 25, 6232–6238.

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It is noteworthy that the slope of the crater edge is about 120 µm/200 nm. Because the slope is very flat, the y-axis of the image contains the structural information along both the y-axis and the z-axis of the specimen. The SEPM revealed the structure at different depths of the film by scanning along the edge of the crater. In the image, the bicontinuous network formed by the phase between the P3HT-rich and the PCBMrich regions is obvious. The ∼50 nm width of the network can be clearly observed and is similar to that reported before on the similar outermost surface.11 Such evident bicontinuous structure is known to be the key to the efficiency of bulkheterojunction solar cells. Figure 2 shows SEPM images at the same region of interest after ion sputtering for different lengths of time. The depth difference between adjacent images is about 18 nm (30 min sputtering), and the similar structure observed at different depths (that is, exposed to ion beams for different amounts of time) confirms that the structure was not smeared by cluster ion sputtering. Through the use of the slicing series of the sample, the nanostructure at different depths was revealed directly. Because of a minor offset and rotation present in the images, cross-correlation was used to align the image. Only regions with complete information were kept for the 3D reconstruction.

After 3D stacking carried out in ImageJ, a 3D projection of the structure is generated. Figure 3a-c shows the projection at various viewing angles. In order to retain the physical meaning of the contrast (contact potential), simple interpolation is used to generate the projection image. Furthermore, in order to emphasize the nanostructure, 3D-rendered isosurface at 50% threshold is also presented in Figure 3d using “ImageJ 3D Viewer” and “VolumeJ” plugins. The animated projection and anaglyph in 1° rotation steps is available in the Supporting Information. The structure was then further evaluated using the “Volume Viewer” plugin in ImageJ. Figure 4 shows cross-sectional slices of the reconstructed 3D structure. The interlacing 3D bicontinuous network of PCBM and P3HT segregation is evident. Compared to results from similar materials obtained with techniques like electron tomography,7,17 cross-sectional TEM imaging,18 and conductive AFM,11 a similar nanostructure was observed in this work. In addition, the contrast in the presented work directly relates to the opto-electronic properties of the materials, as opposed to the scattering power of the sample. If other techniques of SPM are used, other properties of the material can also be studied, given that the contrast is not coupled with the topography. Furthermore, although the total thickness presented here is only about 200 nm (the thickness of the P3HT:PCBM layer), sputtering can be continued indefinitely so that there is no physical limit on the thickness of the analyzed region as in TEM based techniques. The specimen can also be studied at any state without sample preparation. In terms of spatial resolution, the lateral resolution of SEPM is on the order of 10 nm and the depth resolution is limited by the ion-beam induced damage, which is estimated to be in the order of nanometers.29 Therefore, high-resolution 3D images can be acquired by the presented method and physical information up to four dimensions can be obtained.

CONCLUSIONS On the basis of the combination of cluster ion sputtering and scanning probe microscopy, two methods are presented in this work to study the 3D nanostructure of the P3HT:PCBM bulkheterojunction. By analyzing the edge of a crater created by ion bombardment using SEPM, the 3D nanostructure is revealed with minimal effort. In order to reconstruct a 3D volume image, SEPM is also used to examine the newly exposed surface after ion sputtering for different times. The resulting image can be stacked, and clear 3D nanostructures can be observed. Both methods reveal the 3D nanostructure in high-resolution. This is a simple and effective tool for studying organic materials. Furthermore, the contrast in the image revealed differences in the work-function; thus, the results presented in this work can provide the information about the specimen in four physical dimensions. ACKNOWLEDGMENT The authors acknowledge sponsorship by Academia Sinica and the Taiwan National Science Council through Grant Numbers 972113-M-001-015 and 97-2120-M-002-014. The authors thank Mr. Mark Chang of Veeco Taiwan for the suggestion of locating the ROI in SPM. SUPPORTING INFORMATION AVAILABLE Animated 3D projection and anaglyph in 1° rotation steps. This material is available free of charge via the Internet at http://pubs.acs.org. Received for review July 16, 2009. Accepted September 14, 2009. AC901588T


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Three-Dimensional Nanoscale Imaging of Polymer Bulk

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