Direct Photocurrent Mapping of Organic Solar Cells Using a Near

Jan 14, 2004 - Conductive Scanning Probe Characterization and Nanopatterning ... High-Resolution Photocurrent Imaging of Bulk Heterojunction Solar Cel...
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NANO LETTERS

Direct Photocurrent Mapping of Organic Solar Cells Using a Near-Field Scanning Optical Microscope

2004 Vol. 4, No. 2 219-223

Christopher R. McNeill, Holger Frohne, John L. Holdsworth, John E. Furst, Bruce V. King, and Paul C. Dastoor* School of Mathematical and Physical Sciences, UniVersity of Newcastle, UniVersity DriVe, Callaghan, NSW, 2308 Australia Received October 27, 2003; Revised Manuscript Received December 16, 2003

ABSTRACT Direct photocurrent mapping of organic solar cells (OSCs) using a novel implementation of a near-field scanning optical microscope (NSOM) is described. By rastering the light output from the NSOM through a semitransparent electrode across the OSC surface, it is possible to collect height and photocurrent images simultaneously with a lateral resolution that is governed by the NSOM aperture. The photocurrent images demonstrate that film inhomogeneities and segregation effects strongly influence OSC device performance.

Solar cells based on conjugated polymers blended with an electron-accepting agent are promising alternatives to conventional inorganic-based solar cells. Using electron-accepting agents such as functionalized fullerenes1 or semiconducting nanoparticles2 with soluble derivatives of the polymer poly(p-phenylene vinylene) (PPV), power conversion efficiencies of up to 3% under AM 1.5 conditions have been achieved.3 The active layer in such bulk heterojunction devices4 is typically spin coated from a solution containing the polymer and electron acceptor, and the power conversion efficiency of such devices depends on forming a homogeneous blend of the two components. It has long been recognized that morphology and device performance are intimately related.5 Indeed, the role of film morphology in device performance was recently highlighted by the work of Shaheen et al., who demonstrated a significant increase in the power conversion efficiency (from 1 to 2.5%) for the MDMO-PPV (poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene]) and PCBM ((6,6)-phenylC61-butyric acid) system simply by optimizing the spincasting solvent.1 Atomic force microscopy (AFM) indicated that spin coating from chlorobenzene instead of toluene resulted in a more intimate mixing between polymer and fullerene. Traditionally, AFM and transmission electron microscopy (TEM) have been used to characterize the morphology of polymer-based donor-acceptor blend systems.1,2,6-9 NSOM is a powerful technique that scans a tapered optical fiber * Corresponding author. E-mail: [email protected]. Tel: +61 2 4921 5426. Fax: +61 2 4921 6907. 10.1021/nl0349496 CCC: $27.50 Published on Web 01/14/2004

© 2004 American Chemical Society

across the sample, illuminating only that area of the sample that lies directly under the tip aperture, which is typically 50 to 500 nm in diameter. Shear-force or traditional AFM feedback modes are used to keep the tip within 10 nm of the surface, and the sample is always illuminated in the nearfield, thus allowing the optical properties of the surface to be probed with a resolution better than the diffraction limit of the probing light.10 As such, NSOM provides 2D maps of both optical and topographic information simultaneously, and by operating in different detection modes, the absorption, fluorescence, and reflection properties of surfaces can be probed with a resolution of 50 to 500 nm.10 NSOM has been used to probe the optical properties of conjugated polymers11-15 and polymer blends.16,17 Fluorescence mapping of certain polymer blends in particular has been successful in allowing for the identification of the composition of the different phases.16,17 However, there have been no reports in the literature of NSOM fluorescence mapping of the more technologically interesting PPVPCBM composite systems. In this case, PCBM is so efficient at quenching the PPV fluorescence that fluorescence mapping provides insufficient optical contrast to distinguish between PCBM-rich and polymer-rich regions, especially given that phase separation into domains with different donor/acceptor ratios is more likely than the formation of regions of pure constituents.18 Moreover, even if optical contrast could be achieved in these composite systems, fluorescence mapping ultimately provides only indirect information about the efficiency of photocurrent generation efficiency because high fluorescence quenching does not necessarily imply a high efficiency of current collection. Indeed, not only is sufficient

PCBM to harvest the photogenerated excitons required, but there also needs to be an efficient percolation network of PCBM for the electrons to be efficiently transported to the appropriate electrode.19 Recently, Snaith et al. had to deploy additional techniques in addition to AFM and fluorescence NSOM, namely, farfield fluorescence quenching, time-resolved photoluminescence decay, and far-field device quantum efficiency, to determine indirectly which regions in their polyfluorene blend films actually produce current.9 In this communication, we present a technique that allows for the direct mapping of efficiency by probing a PPV-PCBM bulk heterojunction device in situ. Using the light from an NSOM fiber and scanning it across the device, we show that it is possible to observe the effect of phase segregation and inhomogeneous blending on charge generation and collection directly, without having to infer it from other techniques. Although NSOM has been applied previously for photocurrent mapping studies (for example, to study GeSi dislocations20,21 and the photoconductivity of stretch-oriented PPV planar gap devices22), we believe that this study is the first application of the technique to probe organic solar cells in situ directly. The use of the NSOM technique directly on a fully fabricated device is achieved by illuminating through a semitransparent metal electrode that is only 30 nm thick. The photovoltaic devices imaged in this study were fabricated as follows: Prior to spin coating the active layer, a film of PEDOT-PSS (BAYTRON P from Bayer) was spin coated onto an indium tin oxide (ITO)-coated glass slide and dried at 120 °C under flowing nitrogen to give a thickness of 80 nm. The active poly[2-methoxy-5-(2′-ethyl-hexyloxy)1,4-phenylene vinylene] (MEH-PPV)-PCBM layer of thickness 85 nm was then deposited by spin coating onto the PEDOT-PSS film from a solution of MEH-PPV (125 000 Mw from Sigma-Aldrich at 8 g/L) and PCBM (16 g/L, supplied by the Hummelen group at the University of Groningen) in chlorobenzene. For far-field characterization, photovoltaic devices were completed by vacuum evaporating (at 10-7 mbar) a 40 nm calcium electrode followed by a 60 nm film of silver to prevent the degradation of the calcium in air. These far-field devices were then encapsulated with epoxy resin and tested in air. The devices for the near-field measurements were fabricated with a semitransparent metal electrode, which consisted of approximately 10 nm of calcium capped with 20 nm of silver. The thicknesses of all of the films were measured with a Tencor Alpha-Step 500 surface profilometer. The far-field devices were characterized by measuring their IV characteristics under illumination from a 532 nm laser with incident power of 4 mW. Photocurrent action spectra were collected using a lock-in amplifier (Ithaco Dynatrac) to measure the short-circuit current signal from the devices when illuminated by chopped monochromatic light from a tungsten halogen lamp passed through a monochromator (Oriel Cornerstone 130). Near-field scanning measurements were performed with a modified Nanonics AFM/NSOM 100 system, and the experimental setup is shown schematically in Figure 1. A 220

Figure 1. Experimental setup of our NSOM current mapping technique. Also shown are the chemical structures of PCBM and MEH-PPV and the device architecture used.

904 nm IR laser was used as the feedback laser to prevent any light from the feedback laser producing current in the device. Tips with apertures of 200 nm diameter drawn from multimode fiber were used for this study. The images were collected in contact mode with light from a Coherent Vioflame 409 nm diode laser chopped at a frequency of 220 Hz used to probe the surface of the semitransparent metal electrode. The photocurrent line profiles were the same in the forward and reverse scan directions, indicating that the MEH-PPV-PCBM blend films exhibit no photoinduced degradation under the influence of the incident light beam. The devices were always measured under short-circuit conditions using a homemade current-to-voltage converter with the output then fed into the lock-in amplifier and the lock-in output fed into the auxiliary port of the Nanonics system, which allowed the analogue short-circuit current signal to be converted into a digital signal that can be stored. The bandwidth of the lock-in amplifier was chosen to ensure that its response time was sufficiently short to ensure no broadening of the current images, which was also confirmed by observing that the current profiles in the forward and reverse scans were identical. The far-field characterization of the fabricated photovoltaic devices is shown in Figure 2. The devices exhibit an opencircuit voltage of 0.83 V, a fill factor of 0.33, and a monochromatic power conversion efficiency of 1.1% at a wavelength of 532 nm. The inset in Figure 2 also shows the incident photon collected electron (IPCE) efficiency spectra. Figure 3 shows a typical AFM scan and its associated 2D current map observed using the new photocurrent mapping technique. In the photocurrent map, two main features are observed: (a) large (dark) regions with dimensions on the order of 1 to 5 µm where little or no current is produced and (b) smaller (bright) regions with dimensions of the order of 200 to 500 nm that exhibit higher photocurrent efficiency. A careful inspection of these large dark regions reveals that they contain a smaller bright region at their center. These central brighter regions in the photocurrent map correspond precisely to the small bright spots in the AFM scan, Nano Lett., Vol. 4, No. 2, 2004

Figure 2. Far-field current-voltage characteristics of the ITO/ PEDOT-PSS/MEH-PPV-PCBM/Ca/Ag devices fabricated under 4 mW 532 nm illumination. Inset: Corresponding incident photon collected electron (IPCE) efficiency spectra.

indicating the presence of topographical protrusions. Moreover, the lateral dimensions of the central bright features in the photocurrent map are similar to the lateral size of the topographical features seen in the AFM scan. We interpret the bright spots in the AFM image as PCBM-rich islands that have segregated from the MEH-PPV-PCBM matrix. Phase segregation is known to occur when PCBM is blended with a variety of PPV-based semiconducting polymer systems (e.g., MDMO-PPV) and depends strongly upon the solvent system from which the thin films are spun.1 In particular, films spun from chlorobenzene show much reduced phase segregation compared to films spun from toluene. When the same films are produced via screen printing, AFM imaging of the surface revealed the presence of large (∼0.5 µm) structural features that were not observed during spin coating and that were also associated with phase segregation in PCBM-rich regions.18 The nucleation of these larger islands was attributed to the slower time scale for evaporation during screen printing and indicates that as the system approaches thermodynamic equilibrium a phaseseparated structure is energetically most stable.23 Moreover, the interpretation that these bright spots in the AFM image are associated with phase-segregated PCBM-rich islands is in excellent agreement with recent transmission electron microscopy (TEM) images where such PCBM-rich islands are directly observed in 150 nm thick MDMO-PPV-PCBM films.8 To form the PCBM-rich islands observed in the AFM images, mass transport of PCBM from the region around the island must occur during the spin-coating process, thus depleting the surrounding region of PCBM. The magnitude of the photocurrent produced by the active layer is very sensitive to PCBM concentration.24 In particular, it is well established that the short-circuit current of these bulk heterojunction devices rises rapidly for PCBM concentrations above approximately 10 to 20 wt % and reaches a maximum at a PCBM concentration of approximately 75%.19,24,25 This improvement in short-circuit current is attributed to the formation of a percolation network of C60 that enhances electron transport through these devices.19 Hence, the PCBMdepleted region will exhibit a much lower photocurrent than Nano Lett., Vol. 4, No. 2, 2004

Figure 3. AFM (top) and current (bottom) images of an ITO/ PEDOT-PSS/MEH-PPV-PBCM/Ca/Ag device. Two main features are observed: large dark regions (marked A) and small bright regions (marked B). The feature marked C corresponds to a small bright feature in the center of the large dark region (A) of the current image that is also observed in the AFM image. The inset in the bottom image shows the height (black, solid) and current (red, dotted) traces taken along the white line. The scale bar is 5 µm in length.

the MEH-PPV-PCBM blend, thus producing the large dark regions that are observed in the photocurrent image. However, increasing the PCBM concentration necessarily reduces the semiconducting polymer content of the film. As the PCBM concentration is increased further, the enhancement in electron transport is insufficient to compensate for the reduced semiconducting polymer content in the film, and the short-circuit current decreases. Because the MEH-PPVPCBM ratio is optimized for the bulk film, the PCBM-rich islands are likely to have a PCBM concentration that is much higher than the optimum value and hence a response that is lower than that of the bulk film. This segregation hypothesis was tested by comparing the volume of the PCBM-rich island with the volume of its surrounding PCBM-depleted region for a number of different samples. Line scans were collected across several large dark regions for a number of samples, 221

Figure 4. Composite 3D image of the AFM and current map images of Figure 3. The height is given by the AFM scan, and the grayscale, by the current map. Height is in nanometers, and length is in micrometers.

and the diameters and heights of the PCBM-rich island (dA and hA) and the PCBM-depleted region (dB and hB) were measured from the AFM and photocurrent traces, respectively. Assuming that the PCBM-rich islands are formed primarily from the segregation from the PCBM-depleted regions suggests that the PCBM-rich/PCBM-depleted volume ratio, hAd2A/hBd2B, should be constant. This analysis reveals that the PCBM-rich/PCBM-depleted volume ratios are indeed approximately constant (to within (15%), indicating that the source of material in the island observed in the AFM map does most likely originate from the surrounding dark region observed in the photocurrent map. The small bright regions in the photocurrent map do not appear to correlate with any corresponding features in the AFM image. The regions where these bright spots are observed in the photocurrent map are effectively featureless in the AFM map, as shown in the composite 3D image shown in Figure 4. These “hot spots” in the photocurrent map are much smaller than the PCBM-depleted dark regions and appear to be approximately evenly distributed over the image. The size of these photocurrent “hot spots” varies from 200 nm to approximately 1 µm and demonstrates that the resolution of our photocurrent mapping technique is determined by the aperture size of the fiber optic probe, which in this case was 200 nm. Moreover, it seems unlikely that the observed photocurrent contrast is due to variations in tipsurface separation (or other artifacts of the AFM imaging process) because there are no corresponding features in the AFM image. It seems reasonable to conclude, therefore, that the small, bright photocurrent hot spots are due to regions of locally higher PCBM concentration. Furthermore, this conclusion is consistent with the recent observations of Martens et al., who reported that phase segregation of PCBM and MDMO-PPV is observed on the submicrometer level, with the extent of segregation being greater for films spun from toluene than from chlorobenzene.8 However, in both cases, contrast was always simultaneously observed in both the TEM and AFM images, suggesting that phase segregation is detected via TEM only at the point at which there are comparable changes in surface topology as measured by AFM. Interestingly, for the direct photocurrent mapping technique presented here, contrast in the photocurrent map is observed in the absence of any corresponding changes in surface topology as probed by AFM. In summary, we have demonstrated the technique of using a near-field scanning optical microscope to map the mono222

chromatic efficiency of an organic solar cell in two dimensions directly, with a resolution of 200 nm. Using this new technique, we have, for the first time, shown directly that phase segregation in a two-component organic blend system produces local variations in device efficiency. Near-field scanning photocurrent microscopy (NSPM) is a powerful new technique that allows for the imaging of photocurrent conversion efficiency variations directly without having to infer them from other techniques such as fluorescence NSOM. Furthermore, NSPM reveals phase-segregation features in the photocurrent image of the surface that exhibit no contrast when the surface is probed using other scanning probe microscopy techniques such as AFM. Further optimization of the NSPM system is expected to allow us to use smaller aperture tips and hence produce photocurrent images with much improved resolution. Acknowledgment. The Australian Research Council (ARC) is gratefully acknowledged for providing financial support for this project. We thank Erich Ammann (Nanonics) and Jim Cleary (University of Newcastle) for technical assistance. C.R.M. gratefully acknowledges the University of Newcastle for the provision of a research scholarship. References (1) Shaheen, S. E.; Brabec, C. J.; Sariciftci, N. S.; Padinger, F.; Fromherz, T.; Hummelen, J. C. Appl. Phys. Lett. 2001, 78, 841-843. (2) Sun, B.; Marx, E.; Greenham, N. C. Nano Lett. 2003, 3, 961-963. (3) Munters, T.; Martens, T.; Goris, L.; Vrindts, V.; Manca, J.; Lutsen, L.; de Ceuninck, W.; Vanderzande, D.; de Schepper, L.; Gerlan, J.; Sariciftci, N. S.; Brabec, C. Thin Solid Films 2002, 403-404, 247251. (4) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. AdV. Funct. Mater. 2001, 11, 15-26. (5) Wallace, G. G.; Dastoor, P. C.; Officer, D. L.; Too, C. O. Chemical InnoVation 2000, 30, 14-22. (6) Halls, J. J. M.; Walsh, C. A.; Greenham, N. C.; Marseglia, E. A.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Nature 1995, 376, 498500. (7) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425-2427. (8) Martens, T.; D’Haen, J.; Munters, T.; Beelen, Z.; Goris, L.; Manca, J.; D’Olieslaeger, M.; Vanderzande, D.; de Schepper, L.; Andriessen, R. Synth. Met. 2003, 138, 243-247. (9) Snaith, H. J.; Arias, A. C.; Morteani, A. C.; Silva, C.; Friend, R. H. Nano Lett. 2002, 2, 1353-1357. (10) DeAro, J. A.; Weston, K. D.; Buratto, S. K. Phase Transitions 1999, 68, 27-57. (11) Credo, G. M.; Lowman, G. M.; DeAro, J. A.; Carson, P. J.; Winn, D. L.; Buratto, S. K. J. Chem. Phys. 2000, 112, 7864-7872. (12) Nagahara, L. A.; Nakamura, M.; Tokumoto, H. Ultramicroscopy 1998, 71, 281-285. (13) Tan, C. H.; Inigo, A. R.; Hsu, J. H.; Fann, W.; Wei, P. K. J. Phys. Chem. Solids 2001, 62, 1643-1655. (14) Nguyen, T. Q.; Schwartz, B. J.; Schaller, R. D.; Johnson, J. C.; Fee, L. F.; Haber, L. H.; Saykally, R. J. J. Phys. Chem. B 2001, 105, 5153-5160. (15) McNeill, J. D.; O’Connor, D. B.; Barbara, P. F. J. Chem. Phys. 2000, 112, 7811-7821. (16) Arias, A. C.; MacKenzie, J. D.; Stevenson, R.; Halls, J. J. M.; Inbasekaran, M.; Woo, E. P.; Richards, D.; Friend, R. H. Macromolecules 2001, 34, 6005-6013. (17) Chappell, J.; Lidzey, D. G.; Jukes, P. C.; Higgins, A. M.; Thompson, R. L.; O’Connor, S.; Grizzi, I.; Fletcher, R.; O’Brien, J.; Geoghegan, M.; Jones, R. A. L. Nat. Mater. 2003, 2, 616-621. (18) Rispens, M. T.; Meetsma, A.; Rittberger, R.; Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. Chem. Commun. 2003, 2116-2118. (19) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789. Nano Lett., Vol. 4, No. 2, 2004

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