NANO LETTERS
Imaging of the 3D Nanostructure of a Polymer Solar Cell by Electron Tomography
2009 Vol. 9, No. 2 853-855
B. Viktor Andersson,*,† Anna Herland,† Sergej Masich,‡ and Olle Ingana¨s† Biomolecular and Organic Electronics, IFM, Linko¨ping UniVersity, SE-581 83 Linko¨ping, Sweden, and Department of Cell and Molecular Biology, Karolinska Institutet, 171 77 Stockholm, Sweden Received December 5, 2008; Revised Manuscript Received December 18, 2008
ABSTRACT Electron tomography has been used for analyzing the active layer in a polymer solar cell, a bulk heterojunction of an alternating copolymer of fluorene and a derivative of fullerene. The method supplies a three-dimensional representation of the morphology of the film, where domains with different scattering properties may be distinguished. The reconstruction shows good contrast between the two phases included in the film and demonstrates that electron tomography is an adequate tool for investigations of the three-dimensional nanostructure of the amorphous materials used in polymer solar cells.
Polymer solar cells are promising for future solar energy conversion on the condition that the manufacturing complexity can be kept low together with a further increase in power conversion efficiency (PCE). A successful class of active layer materials have been blends of polymer and fullerene derivatives, for example, phenyl-C61-butyric acid methyl ester (PCBM).1 These blends form bulk heterojunctions of electron donors (polymer) and acceptors (PCBM). As the two phases are separated in the blends, a network of donors and acceptors is produced forming a large interfacial area where charge separation may occur. For efficient use of the produced free charge carriers, these must be transported to the electrodes for extraction to the outer circuit. Thus the morphology of the active layer plays a crucial role both in the free charge carrier generation process and in the process of delivering these to the electrodes. The morphological demand on the active layer is to combine a large donoracceptor interface area and simultaneously free pathways to the electrodes for holes and electrons. The large interface ensures that all excitons generated can reach an interface; the interconnectedness ensures charge collection. This demand implies that a phase separation of intermediate extent is desired. The phase separation is visible by scanning force microscopy (SFM) investigations, which have the appropriate resolution, and SFM is now the standard technique to investigate the nanostructure of donor/acceptor blends.2-4 Investigations by SFM microscopy show that solar cell * To whom correspondence should be addressed. E-mail:
[email protected]. † Linko¨ping University. ‡ Karolinska Institutet. 10.1021/nl803676e CCC: $40.75 Published on Web 01/02/2009
2009 American Chemical Society
performance is largely dependent on the phase separation.5-7 Here the amplitude and phase of the SFM signal report the topography and the mechanical properties of the nanostructured material, but does not allow an identification of the chemical species under investigation. Conductive SPM has been used to measure the local current on active layer blends to distinguish areas with different electrical properties.8 Transmission electron microscopy9 and X-ray scattering in wide and narrow angles10 has been used to study the nanostructure of the partially crystalline bulk heterojunction P3HT/PCBM with a resolution in the appropriate length range. Information about chemical composition is available with secondary ion mass spectroscopy (SIMS) methods where the material is ablated with an ion beam.11,12 Vertical profiling of the chemical composition is possible here but has not been combined with lateral resolution in films for organic photovoltaics. This method therefore does not give a microscopic image, but rather averages over lateral dimensions while giving vertical resolution of the sample. Cross-sectional studies of polymer-fullerene blends with scanning electron microscopy,7,13 as well as with transmission electron microscopy14 have been performed, but more information could be gained from a full three-dimensional (3D) morphology study and with a resolution matching that of the desired dimensions of phase separation, which is expected to be 5-10 nm. A method that may give this information is electron tomography (ET).15 In ET, three-dimensional reconstructions are made from micrographs obtained from transmission electron microscopy. The method has mostly been used in
the biological field from the beginning16 but it is also a wellsuited technique for investigating other amorphous materials, such as polymers.17-19 The use of the method to visualize the active layer in solar cells have only been briefly reported.20 Here we present results from ET, where materials used in the active layer of solar cells have been investigated. Two materials, poly [2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4′,7′di-2-thienyl-2′,1′3′-benzothia-diazole)] (APFO-3) and PCBM, and a blend of these (1:4 by weight) have been examined. APFO-3 blended with the acceptor PCBM give devices with photovoltages of 1 V and AM1.5 efficiencies of 3-4%. The examined films were prepared on glass substrates. A layer of poly(ethylenedioxy thiophene)/polystyrene sulfonate (PEDOT/PSS) was first deposited by spincoating, upon which a layer of the material to be examined was deposited, also by spincoating. The PEDOT/PSS layer was used as a sacrificial layer during the lift off procedure, where the glass substrate was immersed in water, causing the PEDOT/PSS layer to dissolve. The lift off procedure leaves the sample film floating, which can then be placed on the copper grid used as sample support in the transmission electron microscope (TEM). The PEDOT/PSS film is water soluble, but the existence of residues thereof on the examined film cannot be discarded. The thickness of both the examined layer and the PEDOT/PSS layer was measured with a Veeco Dektak surface profiler and was compared with the thicknesses given by the reconstructions. Measurements of the electron scattering properties of the thin organic films were done at 200 kV with several exposure times, and the exponential attenuation of electron beams with film thickness were 4.2 × 10-3 nm-1, 1.5 × 10-3 nm-1, and 2.3 × 10-3 nm-1 for the PCBM, neat polymer, and polymer blend, respectively. Gold particles (10 nm) were applied to one side of the specimens and used as fiducial markers for image alignment during the reconstruction procedure. The images of the TEM tilt series were collected using a FEI CM200 FEG microscope in a tilt angle range of approximately -60 to 60°. During data collection, the magnification was 20 000 times with a postmagnification of 1.531. The underfocus was set to 1.5 µm and a CCD with 24 µm pixel size was used for data recording. The first zero of the contrast transfer function corresponds to a resolution of 1.9 nm. Software developed in house at Karolinska Institutet (Stockholm, Sweden) was used for reconstruction and analysis, and the reconstructions were visualized with the volumetric renderer bob.21 The reconstructions were made using filtered backprojection, and the resulting data were lowpass filtered during the analysis. In the analysis process, the intensity threshold is set at a level appropriate to suppress noise and to facilitate the localization of volumes with different scattering properties. The aim is to be able to distinguish between the different phases in the active layer blends. A source of erroneous conclusions is that no sharp edges may be seen between the phases. However, good contrast between regions is observed in the examined samples, and though it is difficult to definitely assign chemical identities to these regions, the 854
Figure 1. Filtered back projection reconstruction of pure APFO-3 film. Well scattering domains are seen as light volumes.
Figure 2. Filtered back projection reconstruction of pure PCBM film. Well scattering domains are seen as light volumes.
scattering is sufficient for further morphology analysis of active blends. The reconstruction of the neat APFO-3 film is seen in Figure 1. In the reconstructed images, the lighter volumes corresponds to well scattering material and dark volumes to less scattering material. The box defines the boundaries of the reconstructed volume. The distance between the edges (≈130 nm) corresponds well with that of profilometer measurements of thickness on films produced in the same manner. The image shows a reconstruction made with filtered backprojection and low pass filtered at 15 nm. The lowpass filtering is done in Fourier space by removing the frequencies higher than the cutoff frequency, which produces a smoother image, where much of the noise is removed. Notable is the denser film structure seen at the edges. This is probably because the material within the film is homogenously distributed. Thus, there is a large contrast between the vacuum surrounding the film and the edges of the film, but a much smaller contrast between domains within the film. The film is oriented such that the sacrificial PEDOT/PSS layer was below the lower surface and with gold particles deposited on the top surface. The gold particles are well scattering objects, and reconstructions of regions containing gold particles confirm the upper edge to be the one seen in Figure 1. In Figure 2, the reconstruction of the neat PCBM film is displayed. The thickness here is about 130 nm. Denser structures close to the surfaces are seen also here, while no large structures seem to be present in the volume in between the surfaces. The same explanation as in the case with neat APFO-3 may be used here, to explain this feature. Nano Lett., Vol. 9, No. 2, 2009
Figure 3. Filtered back projection reconstruction of APFO-3/PCBM (1:4) blend. Well scattering domains are seen as light volumes.
In Figure 4, a cross sectional view of the blend material is provided. Lowpass filtering and contrast threshold adjustments have been deployed also here. The lowpass filtering is done at a lower level (10 nm), which allows more features to be seen. In this figure, the division of material into volumes rich of well scattering material and volumes poor of the same is seen. In conclusion we have used electron tomography to visualize the three-dimensional morphology of an active layer used in polymer solar cells. Further investigations and analyses need to be performed to give more detailed information of the properties of the phases, such as volume and interface area. The possibility to obtain contrast from the samples and use the micrographs for ET make the method well suited for mapping the 3D structure of the active layer in polymer solar cells. References
Figure 4. Cross section (width ca. 16 nm) of reconstruction of APFO-3/PCBM (1:4) film. Lowpass filtered at 10 nm. Well scattering domains are seen as light volumes.
The electron scattering from films of the pure PCBM and polymer is considerably different, varying by a ratio of 3, as concluded from the scattering property measurements. This suggests that the main element creating contrast in the blend material should be the differing content of PCBM. The image in Figure 3 shows the reconstruction of the blend material. The thickness of the film is around 160 nm. Compared to the reconstructions of the neat materials there is a difference. Contrast differences are seen throughout the film due to the variation of scattering properties in the blend. There are some blobs of more scattering material seen in Figure 3. The diameter of these ranges up to roughly 100 nm. It is also seen that these domains are not solid but are domains with higher density of well scattering material. From the TEM measurements, we have reason to believe that PCBM is the more scattering material of the two. The three investigated films have the thicknesses in the same range, but the pure PCBM film was observed as the most scattering one during data collection. This points toward the conclusion that the highlighted (scattering) domains in Figure 3 are PCBM rich volumes with APFO-3 rich volumes as surrounding material. As the reconstructions are lowpass filtered and the contrast threshold adjusted to suppress noise, the finer part of the morphology is not displayed.
Nano Lett., Vol. 9, No. 2, 2009
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