Imaging of the 3D Nanostructure of aPolymer Solar Cell by ElectronTomographyB. Viktor Andersson,*,† Anna Herland,† Sergej Masich,‡ and Olle Inganas†

Biomolecular and Organic Electronics, IFM, Linkoping UniVersity, SE-581 83Linkoping, Sweden, and Department of Cell and Molecular Biology, KarolinskaInstitutet, 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 copolymerof fluorene and a derivative of fullerene. The method supplies a three-dimensional representation of the morphology of the film, where domainswith different scattering properties may be distinguished. The reconstruction shows good contrast between the two phases included in thefilm and demonstrates that electron tomography is an adequate tool for investigations of the three-dimensional nanostructure of the amorphousmaterials used in polymer solar cells.

Polymer solar cells are promising for future solar energyconversion on the condition that the manufacturing complex-ity can be kept low together with a further increase in powerconversion efficiency (PCE). A successful class of activelayer materials have been blends of polymer and fullerenederivatives, for example, phenyl-C61-butyric acid methylester (PCBM).1 These blends form bulk heterojunctions ofelectron donors (polymer) and acceptors (PCBM). As thetwo phases are separated in the blends, a network of donorsand acceptors is produced forming a large interfacial areawhere charge separation may occur. For efficient use of theproduced free charge carriers, these must be transported tothe electrodes for extraction to the outer circuit. Thus themorphology of the active layer plays a crucial role both inthe free charge carrier generation process and in the processof delivering these to the electrodes. The morphologicaldemand on the active layer is to combine a large donor-acceptor interface area and simultaneously free pathways tothe electrodes for holes and electrons. The large interfaceensures that all excitons generated can reach an interface;the interconnectedness ensures charge collection. This de-mand implies that a phase separation of intermediate extentis desired. The phase separation is visible by scanning forcemicroscopy (SFM) investigations, which have the appropriateresolution, and SFM is now the standard technique toinvestigate the nanostructure of donor/acceptor blends.2-4

Investigations by SFM microscopy show that solar cell

performance is largely dependent on the phase separation.5-7

Here the amplitude and phase of the SFM signal report thetopography and the mechanical properties of the nanostruc-tured material, but does not allow an identification of thechemical species under investigation. Conductive SPM hasbeen used to measure the local current on active layer blendsto distinguish areas with different electrical properties.8

Transmission electron microscopy9 and X-ray scattering inwide and narrow angles10 has been used to study thenanostructure of the partially crystalline bulk heterojunctionP3HT/PCBM with a resolution in the appropriate lengthrange. Information about chemical composition is availablewith secondary ion mass spectroscopy (SIMS) methodswhere the material is ablated with an ion beam.11,12 Verticalprofiling of the chemical composition is possible here buthas not been combined with lateral resolution in films fororganic photovoltaics. This method therefore does not givea microscopic image, but rather averages over lateraldimensions while giving vertical resolution of the sample.Cross-sectional studies of polymer-fullerene blends withscanning electron microscopy,7,13 as well as with transmissionelectron microscopy14 have been performed, but moreinformation could be gained from a full three-dimensional(3D) morphology study and with a resolution matching thatof the desired dimensions of phase separation, which isexpected to be 5-10 nm.

A method that may give this information is electrontomography (ET).15 In ET, three-dimensional reconstructionsare made from micrographs obtained from transmissionelectron microscopy. The method has mostly been used in

* To whom correspondence should be addressed. E-mail: vikan@ifm.liu.se.† Linkoping University.‡ Karolinska Institutet.

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10.1021/nl803676e CCC: $40.75 2009 American Chemical SocietyPublished on Web 01/02/2009

the biological field from the beginning16 but it is also a well-suited technique for investigating other amorphous materials,such as polymers.17-19 The use of the method to visualizethe active layer in solar cells have only been brieflyreported.20 Here we present results from ET, where materialsused 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 withphotovoltages of 1 V and AM1.5 efficiencies of 3-4%.

The examined films were prepared on glass substrates. Alayer of poly(ethylenedioxy thiophene)/polystyrene sulfonate(PEDOT/PSS) was first deposited by spincoating, uponwhich a layer of the material to be examined was deposited,also by spincoating. The PEDOT/PSS layer was used as asacrificial layer during the lift off procedure, where the glasssubstrate was immersed in water, causing the PEDOT/PSSlayer to dissolve. The lift off procedure leaves the samplefilm floating, which can then be placed on the copper gridused as sample support in the transmission electron micro-scope (TEM). The PEDOT/PSS film is water soluble, butthe existence of residues thereof on the examined film cannotbe discarded. The thickness of both the examined layer andthe PEDOT/PSS layer was measured with a Veeco Dektaksurface profiler and was compared with the thicknesses givenby the reconstructions.

Measurements of the electron scattering properties of thethin organic films were done at 200 kV with several exposuretimes, and the exponential attenuation of electron beams withfilm thickness were 4.2 × 10-3 nm-1, 1.5 × 10-3 nm-1, and2.3 × 10-3 nm-1 for the PCBM, neat polymer, and polymerblend, respectively.

Gold particles (10 nm) were applied to one side of thespecimens and used as fiducial markers for image alignmentduring the reconstruction procedure. The images of the TEMtilt series were collected using a FEI CM200 FEG micro-scope in a tilt angle range of approximately -60 to 60°.During data collection, the magnification was 20 000 timeswith a postmagnification of 1.531. The underfocus was setto 1.5 µm and a CCD with 24 µm pixel size was used fordata recording. The first zero of the contrast transfer functioncorresponds to a resolution of 1.9 nm. Software developedin house at Karolinska Institutet (Stockholm, Sweden) wasused for reconstruction and analysis, and the reconstructionswere visualized with the volumetric renderer bob.21 Thereconstructions were made using filtered backprojection, andthe resulting data were lowpass filtered during the analysis.In the analysis process, the intensity threshold is set at alevel appropriate to suppress noise and to facilitate thelocalization of volumes with different scattering properties.The aim is to be able to distinguish between the differentphases in the active layer blends. A source of erroneousconclusions is that no sharp edges may be seen between thephases. However, good contrast between regions is observedin the examined samples, and though it is difficult todefinitely assign chemical identities to these regions, the

scattering is sufficient for further morphology analysis ofactive blends.

The reconstruction of the neat APFO-3 film is seen inFigure 1. In the reconstructed images, the lighter volumescorresponds to well scattering material and dark volumes toless scattering material. The box defines the boundaries ofthe reconstructed volume. The distance between the edges(≈130 nm) corresponds well with that of profilometermeasurements of thickness on films produced in the samemanner. The image shows a reconstruction made with filteredbackprojection and low pass filtered at 15 nm. The lowpassfiltering is done in Fourier space by removing the frequencieshigher than the cutoff frequency, which produces a smootherimage, where much of the noise is removed. Notable is thedenser film structure seen at the edges. This is probablybecause the material within the film is homogenouslydistributed. Thus, there is a large contrast between thevacuum surrounding the film and the edges of the film, buta much smaller contrast between domains within the film.The film is oriented such that the sacrificial PEDOT/PSSlayer was below the lower surface and with gold particlesdeposited on the top surface. The gold particles are wellscattering objects, and reconstructions of regions containinggold particles confirm the upper edge to be the one seen inFigure 1.

In Figure 2, the reconstruction of the neat PCBM film isdisplayed. The thickness here is about 130 nm. Denserstructures close to the surfaces are seen also here, while nolarge structures seem to be present in the volume in betweenthe surfaces. The same explanation as in the case with neatAPFO-3 may be used here, to explain this feature.

Figure 1. Filtered back projection reconstruction of pure APFO-3film. Well scattering domains are seen as light volumes.

Figure 2. Filtered back projection reconstruction of pure PCBMfilm. Well scattering domains are seen as light volumes.

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The electron scattering from films of the pure PCBM andpolymer 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 theblend material should be the differing content of PCBM.

The image in Figure 3 shows the reconstruction of theblend material. The thickness of the film is around 160 nm.Compared to the reconstructions of the neat materials thereis a difference. Contrast differences are seen throughout thefilm due to the variation of scattering properties in the blend.There are some blobs of more scattering material seen inFigure 3. The diameter of these ranges up to roughly 100nm. It is also seen that these domains are not solid but aredomains with higher density of well scattering material. Fromthe TEM measurements, we have reason to believe thatPCBM is the more scattering material of the two. The threeinvestigated films have the thicknesses in the same range,but the pure PCBM film was observed as the most scatteringone during data collection. This points toward the conclusionthat the highlighted (scattering) domains in Figure 3 arePCBM rich volumes with APFO-3 rich volumes as sur-rounding material. As the reconstructions are lowpass filteredand the contrast threshold adjusted to suppress noise, thefiner part of the morphology is not displayed.

In Figure 4, a cross sectional view of the blend materialis provided. Lowpass filtering and contrast threshold adjust-ments have been deployed also here. The lowpass filteringis done at a lower level (10 nm), which allows more featuresto be seen. In this figure, the division of material into volumesrich of well scattering material and volumes poor of the sameis seen.

In conclusion we have used electron tomography tovisualize the three-dimensional morphology of an active layerused in polymer solar cells. Further investigations andanalyses need to be performed to give more detailedinformation of the properties of the phases, such as volumeand interface area. The possibility to obtain contrast fromthe samples and use the micrographs for ET make the methodwell suited for mapping the 3D structure of the active layerin polymer solar cells.

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Figure 3. Filtered back projection reconstruction of APFO-3/PCBM(1:4) blend. Well scattering domains are seen as light volumes.

Figure 4. Cross section (width ca. 16 nm) of reconstruction ofAPFO-3/PCBM (1:4) film. Lowpass filtered at 10 nm. Wellscattering domains are seen as light volumes.

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