Tom Martens

Disclosure of the nanostructure of MDMO-PPV:PCBM bulk hetero-junction organic solar cells by a combination of SPM and TEM

Abstract The microstructure of MDMO-PPV:PCBM blends as used in bulk hetero-junction organic solar cells is studied by atomic force microscopy (AFM) to image the surface morphology and by means of transmission electron microscopy (TEM) to disclose the bulk nanostructure of the film. Typical thin films, as used for state-of-the-art organic bulk hetero-junction solar cells consist of a 1:4 ratio by weight of MDMO-PPV as electron donating polymer and PCBM, a soluble electron accepting C 60 derivative. For these films it is found, using both TEM an AFM, that phase separation occurs. A two-phase system is observed that consists of PCBM-rich domains that are embedded in a matrix consisting of a mixture of MDMO-PPV and PCBM. By combining planar and cross-sectional views, three-dimensional information is obtained on the phase separated PCBM-rich regions, formed during spincoating. Changing the solvent is found to influence the size of the phase separated PCBM-rich domains. But not only the dimensions of the phase separated regions are affected by changing the solvent. Also the composition of the matrix is found to be determined by the choice of solvent. This was studied by changing the ratio of PCBM compared to MDMO-PPV. Since it is commonly believed that the morphology of the active layer influences electrical properties and photovoltaic performance, the nanostructural information obtained with the presented analytical techniques will attribute to a better understanding and improvement of present organic photovoltaic devices.

A comparison between state-of-the-art ‘gilch’ and ‘sulphinyl’ synthesised MDMO-PPV/PCBM bulk hetero-junction solar cells

Abstract To obtain photovoltaic devices based on electron donating conjugated polymers with a higher efficiency, a major breakthrough was realised by mixing the polymers with a suitable electron acceptor, thereby enhancing the rate for photo-induced charge generation by several orders. State-of-the-art organic bulk hetero-junction photovoltaic cells are based on an interpenetrating donor–acceptor network in the bulk to form efficient nanostructured p–n junctions in the organic materials. Devices made with ‘Gilch’ poly(2-methoxy-5-(3′,7′-dimethyl-octyloxy))- p -phenylene vinylene, (MDMO-PPV), as an electron donor and (6,6)-phenyl-C 61 -butyric-acid (PCBM) (a soluble C60 derivative) as an electron acceptor yielded the highest efficiency until now in this class of devices. A power conversion efficiency of approximately η e ≥2.5% (electrical power out/incident light power) under AM 1.5 illumination was reported. The ‘gilch’ route is a direct synthetic route. The ‘sulphinyl’ route is a promising, indirect precursor-route towards MDMO-PPV. Due to the non-symmetric monomer, so-called ‘head-to-head’ and ‘tail-to-tail’ additions are excluded to a higher level in comparison to the ‘gilch’ route. This difference between both materials makes them interesting candidates to compare them in the state-of-the-art photovoltaic devices. Preliminary results indicate that the ‘sulphinyl’ MDMO-PPV/PCBM bulk hetero-junction solar cells attain a power conversion efficiency of nearly η e =3% (electrical power out/incident light power), have a higher fill factor, incident photon per converted electron value (IPCE) and short circuit current. It is indicated that the observed solar cell characteristics are related to the defect level of the conjugated polymer used.

Morphology of MDMO-PPV:PCBM bulk heterojunction organic solar cells studied by AFM, KFM, and TEM

The microstructure of MDMO-PPV:PCBM blends as used in bulk hetero-junction organic solar cells was studied by Atomic Force Microscopy (AFM) and Kelvin Force Microscopy (KFM) to image the surface morphology and by means of Transmission Electron Microscopy (TEM) to reveal images of the films interior. By introducing KFM, it was possible to demonstrate that phase separated domains have different local electrical properties than the surrounding matrix. Since blend morphology clearly influences global electrical properties and photovoltaic performance, an attempt to control the morphology by means of casting conditions was undertaken. By using AFM, it has been proven that not only the choice of solvent, but also drying conditions dramatically influence the blend structure. Therefore, the possibility of discovering the blend morphology by AFM, KFM and TEM makes them powerful tools for understanding todays organic photovoltaic performances and for screening new sets of materials.

The influence of the microstructure upon the photovoltaic performance of MDMOPPV: PCBM bulk hetero-junction organic solar cells

In this paper, a clear view on the bulk microstructure of MDMO-PPV:PCBM blends as used in bulk hetero-junction organic solar cells is obtained by means of TEM (Transmission Electron Microscopy). Using TEM, 3-dimensional information is acquired on phase separated regions, formed during casting. Particle statistics illustrate quantitatively that a.o. drying conditions and choice of solvent dramatically influence the blend structure. More information about the lateral blend structure and distribution is obtained in cross-sectional view. Since blend morphology is strongly related to photovoltaic performance, TEM can be a powerful tool for understanding todays photovoltaic performances and screening new sets of materials.

In-situ sem observation of electromigration in thin metal films at accelerated stress conditions

Abstract With so-called in-situ SEM experiments, electromigration experiments are performed in a SEM (scanning electron microscope) equipped with a heating stage. BSE (back scattered electron) images are taken continuously over the entire length of a metal line submitted to high current and temperature stress, monitoring in detail the microstructure. Comparing the electrical resistance curves with the corresponding SEM micrographs and with ex-situ AFM measurements leads to detailed qualitative and quantitative information about the occurring electromigration and precipitation / dissolution effects in the metal lines.

In-situ electrical and spectroscopical techniques for the study of degradation mechanisms and life time prediction of organic based electronic material systems

In order to tailor the synthesis of new robust organic materials for electronic applications and to guarantee the required life time for the emerging commercial plastic electronic applications it is of key importance to understand the underlying degradation mechanisms. Since plastic electronics is a rather young technology introducing new material systems, its reliability is characterized by new failure and degradation mechanisms, a relatively high amount of early failures and multi-modal failure distributions. To understand the mechanism responsible for a given failure or degradation mode, it is essential to study it separately, through appropriate test structures and test techniques. Powerful techniques for this purpose are a.o. analytical techniques (SEM, TEM, SPM,…), in-situ electrical measurement techniques and spectroscopical techniques ( in-situ FTIR, in-situ UV-Vis, PDS). The benefits of these in-situ techniques in the reliability study of organic based electronics will be illustrated in this contribution.