A. Swinnen

Hasselt University, Hasselt, Flemish, Belgium

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Publications (16)41.36 Total impact

  • European Polymer Congress 2009, Graz, Austria; 07/2009
  • Belgian Polymer Group Annual Meeting 2009, Ol Fosse d’Outh, Houffalize, Belgium; 05/2009
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    ABSTRACT: In this work, the phase diagram of poly(3-hexyl thiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) blends is measured by means of standard and modulated temperature differential scanning calorimetry. Blends were made by solvent-casting from chlorobenzene, as blends cast from toluene or 1,2-dichlorobenzene prove to retain effects of phase segregation during casting, hindering the determination of the phase diagram. The film morphology of P3HT/PCBM blends cast from chlorobenzene results from a dual crystallization behavior, in which the crystallization of each component is hindered by the other component. A single glass transition is observed for all compositions. The glass transition temperature (Tg) increases with increasing concentration of PCBM: from 12.1 degrees C for pure P3HT to 131.2 degrees C for pure PCBM. The observed Tg defines the operating window for the thermal annealing and explains the long-term instability of both the morphology and the photovoltaic performance of the P3HT/PCBM solar cells.
    The Journal of Physical Chemistry B 02/2009; 113(6):1587-91. DOI:10.1021/jp804151a · 3.38 Impact Factor
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    ABSTRACT: The disclosure of the nanomorphology of thin films in organic solar cells, prepared from blends of conjugated polymers and PCBM, is of key importance for a better understanding of the occurring photovoltaic (PV) mechanisms. Hereto solid-state NMR relaxometry has been evaluated as a complementary technique to traditional microscopic techniques like atomic force microscopy and transmission electron microscopy. It is demonstrated that proton wide-line solid-state NMR relaxometry is a useful and innovative tool to study the phase morphology of blends used in semi-conducting polymer based PV devices. Attention is focused on the influence of the blend composition and casting conditions on the resulting phase morphology. Two different casting techniques, i.e. spincoating and Doctor Blading, were compared. To demonstrate the applicability of NMR relaxometry in this field, MDMO-PPV/PCBM blends where used, since these are known for their significant phase separation behavior in combination with toluene as solvent. In films prepared from blends in toluene with a PCBM content ≥70 wt %, a fraction of the PCBM is phase separated into crystalline domains, whereas the remaining part remains homogeneously mixed with the MDMO-PPV. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 138–145, 2008
    Journal of Polymer Science Part A Polymer Chemistry 02/2008; 46(1):138 - 145. DOI:10.1002/pola.22365 · 3.54 Impact Factor
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    ABSTRACT: State-of-the-art organic bulk heterojunction (BH) solar cells, also called excitonic solar cells, are based on intimate mixtures of donor and acceptor organic materials of which the nanoscale morphology strongly influences both the photovoltaic performances and the stability of the device. In particular, the form and the size of the three-dimensional (3D) interpenetrating network of donor/acceptor material is shown to be crucial for the electrical transport properties and the resulting photovoltaic properties. Powerful high-resolution characterisation tools to locally map the morphology of these material systems are Scanning and Transmission Electron Microscopy (SEM/TEM), Atomic Force Microscopy (AFM) and Nuclear Magnetic Resonance (NMR). Yet, to correlate morphology with local electrical properties, significant progress has been made by the recent introduction of advanced Scanning Probe Microscopy (SPM) methods based on electrostatic force microscopy (EFM) and conductive atomic force microscopy (C-AFM). EFM related methods measure the electrostatic interaction between the probe and the surface of the organic thin films, hence to derive the variations in the sample surface potential. C-AFM based methods perform two-dimensional (2D) current mapping of sample conductivity and local spectroscopy to analyse transversal charge transport mechanisms in the blends. In case the space charge limiting current (SCLC) regime is dominating the charge transport mechanisms, carrier mobility can also be determined. Finally, the sensitivity of C-AFM to photovoltaic properties is reported. In this paper a review dealing with the different SPM methods currently used and the respective achievements performed on organic blends for BH solar cells is proposed. Copyright © 2007 John Wiley & Sons, Ltd.
    Progress in Photovoltaics Research and Applications 12/2007; 15(8):713 - 726. DOI:10.1002/pip.795 · 9.70 Impact Factor
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    ABSTRACT: Control of morphology is a key issue in order to improve the performance of organic bulk heterojunction solar cells. Solar cells consisting of a blend of regioregular P3HT (poly(3-hexylthiophene)) and PCBM ([6-6]-phenyl C61 butyric acid methyl ester) have demonstrated the highest efficiencies until now (up to 5 %). This performance was achieved by applying a post-production annealing, which is considered to induce a dual crystallization behavior. In order to control and tune the morphology, the phase behavior needs to be described in terms of the underlying fundamental thermodynamics. Hence, it is essential to obtain a phase diagram of the blend. In this study, the state diagram of P3HT:PCBM blends is measured by means of standard and modulated temperature differential scanning calorimetry (DSC). For the first time, the glass transition (Tg) of PCBM could be determined. All blends evidenced a single Tg, indicating an homogeneous blend is formed. Phase separation is thus only induced from crystallization and no "intrinsic" phase separation is occurring in the blend.
    Proceedings of SPIE - The International Society for Optical Engineering 09/2007; DOI:10.1117/12.733761 · 0.20 Impact Factor
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    ABSTRACT: A new approach is presented in order to improve the thermal stability of polymer:PCBM bulk heterojunction solar cells. The central idea in this approach is the use of a polymer with high glass transition temperature (T g), well above the normal operating temperatures of the devices. In this paper a PPV-derivative with a T g of 150°C was used as electron donor and the thermal stability of the obtained solar cells was compared with solar cells based on the reference material Poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) with a T g of 45°C. The use of the material with higher glass transition temperature resulted in a significant improvement of the thermal stability of the photovoltaic parameters. Furthermore, a systematic TEM study demonstrates that the better thermal stability of performance coincides with a more stable active layer morphology. Both improvements are attributed to the reduced free movement of the electron donor material ([6-6]-phenyl C 61 butyric acid methyl ester, PCBM) within the active layer of the solar cell.
    Solar Energy Materials and Solar Cells 03/2007; 91(5). DOI:10.1016/j.solmat.2006.10.008 · 5.03 Impact Factor
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    ABSTRACT: In this work, electrostatic force microscopy (EFM) and conductive atomic force microscopy (C-AFM) are applied to perform high-resolution electrical characterisation of organic photovoltaic films. These films are composed of the C60-derivative PCBM blended with hole conductive conjugated polymers PPV derivatives or P3HT. It is demonstrated that both EFM and C-AFM are able to electrically evidence phase separation in the blends, suggesting in addition higher density of carriers along interfaces. Correlation between the EFM contrast and the photovoltaic properties of the blends was observed. Local spectroscopy (I–V curves) completes the C-AFM investigations, analysing charge transport mechanisms in the P3HT:PCBM blend. Significant modifications of the local electrical properties of P3HT are shown to occur upon blending. Space charge limited current is evidenced in the blend and a hole mobility of 1.7×10−2cm2V−1s−1 is determined for P3HT.
    Microelectronic Engineering 03/2007; 84(3):431-436. DOI:10.1016/j.mee.2006.10.056 · 1.34 Impact Factor
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    ABSTRACT: The nanoscale morphology of the active layer in bulk-heterojunction solar cells consisting of regioregular poly(3-hexylthiophene-2, 5-diyl) (P3HT) and methanofullerene([6-6]-phenyl C$_{61}$ butyric acid methyl ester) (PCBM) was extensively studied using Atomic Force Microscopy (AFM), Transmission Electron Microscopy (TEM), X-Ray Diffraction (XRD) and optical microscopy. Different weight ratios of P3HT:PCBM were investigated as a function of annealing temperature and time revealing the occurrence of crystallization of both components. Firstly, the as-prepared films can be described as a semi-crystalline blend. Secondly, it has been demonstrated that for a short annealing time (5 min) at lower annealing temperatures (75–115$^{\circ}\mathrm{C}$) an increased crystallization of P3HT occurs. Thirdly, it has been observed that a prolonged annealing at the given temperature range or a short annealing at higher temperatures ($\geq$ 120$^{\circ}\mathrm{C}$) leads to the formation of a new ordered crystalline structure of PCBM. These new ordered structures, a few $\mu$m up to 100 $\mu$m in length, form a network of needle-like and even fan-shaped crystals. Key-parameters to “tune” this new ordered structure of PCBM are blend ratio and annealing conditions. The growth mechanism of these new PCBM-structures is described by means of diffusion.
    The European Physical Journal Applied Physics 12/2006; DOI:10.1051/epjap:2006138 · 0.79 Impact Factor
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    ABSTRACT: The performance of organic solar cells based on the blend of regioregular poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) is strongly influenced by blend composition and thermal annealing conditions. X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM) diffraction measurements show that in the considered blends, ordering of P3HT plays a key role in understanding the PV-performance. It is demonstrated that the natural tendency of regioregular P3HT to crystallize is disturbed by the addition of PCBM. Annealing however improves the crystallinity, explaining the observed spectral broadening and is also resulting in a higher mobility of the holes in P3HT.
    Solar Energy Materials and Solar Cells 09/2006; 90(14):2150-2158. DOI:10.1016/j.solmat.2006.02.010 · 5.03 Impact Factor
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    ABSTRACT: Conductive atomic force microscopy (CAFM) is introduced to perform electrical characterization of organic photovoltaic blends with high spatial resolution. Reference blends used in organic bulk heterojunction solar cells are investigated. The ability of CAFM to electrically evidence phase separated donor and acceptor regions is demonstrated. Furthermore, local spectroscopy is performed to analyze charge transport mechanisms in the blends. Significant modifications of the electrical properties of the semiconducting polymers are shown to occur after blending with fullerene derivatives. Finally, the sensitivity of CAFM to photoelectrical phenomena is revealed. Current variations of few picoamperes are locally observed under illumination of P3HT:PCBM.
    07/2006; 89(3-3):032107. DOI:10.1063/1.2227846
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    ABSTRACT: Organic bulk heterojunction solar cells enjoy a well established reputation, reaching conversion efficiencies up to 4% at present. Information on electronic states in the gap, whether or not connected with defects, that can influence the power conversion efficiency, are still lacking. A photoelectrical study of the optical absorption processes in encapsulated cells is presented. Fourier-transform photocurrent spectroscopy is used to measure the transitions connected with free charge carrier generation. The dynamic range obtained with this method reaches over at least eight orders of magnitude. Measurements on solar cells with an active layer of MDMO-PPV:PCBM (1:4 weight ratio) enabled a study of the intrinsic defect generation due to UV-illumination. Temperature annealing effects in P3HT:PCBM (1:2 weight ratio) cells and the induced morphological changes are related with the changes in absorption spectrum. These morphological changes were separately probed with optical and transmission electron microscopy.
    Journal of Non-Crystalline Solids 06/2006; 352(9-20):1656-1659. DOI:10.1016/j.jnoncrysol.2005.09.053 · 1.72 Impact Factor
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    ABSTRACT: A new ordered structure of the C-60 derivative PCBM ([6-6]-phenyl C-61-butyric acid methyl ester) is obtained in thin films based on the blend PCBM:regioregular P3HT (poly(3-hexylthiophene)). Rapid formation of needlelike crystalline PCBM structures of a few micrometers up to 100 mu m in size is demonstrated by submitting the blended thin films to an appropriate thermal treatment. These structures can grow out to a 2D network of PCBM needles and, in specific cases, to spectacular PCBM fans. Key parameters to tune the dimensions and spatial distribution of the PCBM needles are blend ratio and annealing conditions. The as-obtained blended films and crystals are probed using atomic force microscopy, transmission electron microscopy, selected area electron diffraction, optical microscopy, and confocal fluorescence microscopy. Based on the analytical results, the growth mechanism of the PCBM structures within the film is described in terms of diffusion of PCBM towards the PCBM crystals, leaving highly crystalline P3HT behind in the surrounding matrix.
    Advanced Functional Materials 02/2006; 16(6):760 - 765. DOI:10.1002/adfm.200500812 · 10.44 Impact Factor
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    ABSTRACT: This paper investigates the thermal stability of organic bulk heterojunction solar cells, with a special focus on the thermal ageing of both photovoltaic parameters and morphology of the active layer. The photovoltaic parameters of a set of bulk heterojunction solar cells were determined by IV-characterization and their bulk morphology was investigated with transmission electron microscopy (TEM). A link could be made between the degradation of the short circuit current under a thermal treatment and the corresponding change in bulk morphology. A possible improvement of the thermal stability of bulk heterojunction solar cells is presented through the use of a polymer with higher glass transition temperature.
    Proceedings of SPIE - The International Society for Optical Engineering 01/2006; DOI:10.1117/12.663156 · 0.20 Impact Factor
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    ABSTRACT: Nowadays bulk heterojunction polymer:fullerene (PCBM) solar cells reach efficiencies of 5% through the use of high mobility donor polymers (e.g. P3HT) and through a continued nanoscale control of the morphology of the donor-acceptor interpenetrating networks [1]. One of the general bottlenecks of organic solar cells is their poor stability. Organic solar cells have a low resistance towards oxygen, UV-light, high temperatures etc. This work focuses on the thermal stability of organic solar cells.