L. De Schepper

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.

Observation of the subgap optical absorption in polymer-fullerene blend solar cells

This letter reports on highly sensitive optical absorption measurements on organic donor-acceptor solar cells, using Fourier-transform photocurrent spectroscopy (FTPS). The spectra cover an unprecedented dynamic range of eight to nine orders of magnitude making it possible to detect defect and disorder related sub-band gap transitions. Direct measurements on fully encapsulated solar cells with an active layer of poly[2-methoxy-5-(3′,7′-dimethyl-octyloxy)]-p-phenylene-vinylene:(6,6)-phenyl-C61-butyric-acid (1:4 weight ratio) enabled a study of the intrinsic defect generation due to UV illumination. Solar cell temperature annealing effects in poly(3-hexylthiophene):PCBM (1:2 weight ratio) cells and the induced morphological changes are related to the changes in the absorption spectrum, as determined with FTPS.

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.

Characterization of TiN coatings deposited on plasma nitrided tool steel surfaces

Abstract Wear-resistant TiN coatings deposited on tool steels are used frequently in industry. There is a trend towards further optimizing these coatings, e.g. by plasma nitriding the tool surface prior to TiN deposition. In this work the influence of the nitriding conditions on the surface properties of AISI 304 and ASP 23 tool steels was investigated. The plasma nitriding was carried out in a triode ion plating configuration normally used to deposit TiN coatings. At the surface of AISI 304 stainless steel, only a thin compound layer (Fe4N, Fe3N) was found, probably as a consequence of the rather slow nitrogen diffusion in the austenite matrix. For ASP 23 high speed steel, the different nitriding behaviour of the martensitic matrix causes the formation of a diffusion layer which results in an increasein hardness at the surface. On an analogous set of specimens the TiN deposition was started immediately after the plasma nitriding. To optimize this combined treatment, the influence of the nitrogen content of the nitriding medium and that of the thickness of an intermediate titanium layer were examined. By means of X-ray diffraction only an influence on the lattice parameter of TiN and the nitriding layers was found, as compared with the lattice parameter for the separate treatments. Bad adhesion of TiN coatings on plasma nitrided AISI 304 was observed and was explained in terms of the high internal stresses in the compound layer on the nitrided surface. For the ASP 23 substrate a better coating adhesion was observed, probably due to a different structure of the nitriding layer and to the enhanced substrate harness. At this stage of the research, none of the above-mentioned nitriding treatments results in a sufficient adhesion of the TiN layer, although some useful facts could be established for further research.

The stability of Pt heater and temperature sensing elements for silicon integrated tin oxide gas sensors

Abstract Good performance of alarm systems and environmental as well as industrial control methods requires an optimal operation of the gas sensors involved. An essential point related to this challenge is an accurate control of the sensor working temperature. Therefore, the stability of the Ti/Pt heater element used in tin oxide thin film gas sensors has been investigated. It turns out that the heater element is influenced by diffusion and oxidation processes taking place during thermal treatment. This is illustrated by RBS, AES and XPS measurements. The modifications in chemical composition are also found to influence the electrical characteristics R 0 and TCR of the heater.

An interface study of various PVD TiN coatings on plasma-nitrided austenitic stainless steel AISI 304

Abstract The adhesion between a TiN coating and a plasma-nitrided austenitic stainless steel is influenced by using different process steps. In order to obtain some information about the effects of various parameters, different process steps are used during the coating of austenitic stainless steel AISI 304. The deposition of a TiN layer with a Ti intermediate layer results in bad adhesion, especially when a thicker TiN coating has been deposited. X-Ray diffraction measurements indicate the presence of a martensitic phase on top of the plasma-nitrided surface after deposition of the TiN coating. This martensitic phase is also observed if after plasma nitriding only an additional heat treatment is carried out. A transmission electron microscopy study is also performed to reveal the microstructure.

A new degradation model and lifetime extrapolation technique for lightly doped drain nMOSFETs under hot-carrier degradation

Abstract The hot-carrier degradation of lightly doped drain nMOSFETs is studied in detail. The degradation proceeds in a two-stage mechanism, involving first a series resistance increase and saturation, followed by a carrier mobility reduction. The degradation behaviour of a characteristic MOSFET parameter is modelled over the complete degradation range, from 0.02 up to more than 10%. Furthermore, the introduction of a simultaneous non-linear least-square fit of the degradation curves has been successful for predicting the complete degradation behaviour at normal operating conditions.

Interface study of TiN- and TiTiN-coated stainless steel AISI 304 with asymmetric glancing angle X-ray diffraction and classical Bragg-Brentano X-ray diffraction

Abstract The effect of an intermediate titanium layer between a physically vapour deposited TiN coating and an AISI 304 austenitic stainless steel substrate is studied by means of classical Bragg-Brentano X-ray powder diffractometry (XRD) and asymmetric glancing angle X-ray diffraction. The latter makes it possible to study very thin layers on polycrystalline materials. Both X-ray diffraction (XRD) techniques reveal the presence of a γ′-Fe 4 N phase in the top layer of the substrate when no intermediate titanium layer is deposited on the stainless steel substrate. The results from XRD analysis are correlated with results obtained earlier with scratch tests on identically prepared samples with and without an intermediate titanium layer and coated with a 2μm thick TiN layer. The early failure of the adhesion of the TiN coating when no intermediate titanium layer is deposited can partially be explained in terms of the formation of a γ′-Fe 4 N phase in the top layer of the substrate.

Characterization by glancing angle X-ray diffraction of the interface between austenitic stainless steel AISI 304 substrates and a TiN or Ti/TiN physical vapour deposition coating

MET WORKING IND,CTR SCI & TECH,B-3610 DIEPENBEEK,BELGIUM.QUAEYHAEGENS, C, LIMBURGS UNIV CENTRUM,MAT PHYS RES GRP,UNIV CAMPUS,B-3610 DIEPENBEEK,BELGIUM.

Poly (5,6-dithiooctylisothianaphtene), a new low band gap polymer : spectroscopy and solar cell construction

To enhance the efficiency of polymer photovoltaics, much effort is put into synthesis of novel compounds which show a better harvesting of solar light. In this respect, a new low band gap polymer, namely, poly(5,6-dithiooctylisothianaphtene), was synthesised. This work focusses on the spectroscopic characterisation of the material. The dynamics of the photoexcitations were studied by monitoring the dependence of the photoinduced absorption band on the laser modulation frequency and indicated dispersive recombination kinetics in this material. An appropriate model was used to describe the observed behaviour. To investigate the nature of the photogenerated species more profound, photoinduced absorption spectroscopy in the infrared was performed, showing an infrared active vibrational pattern (IRAV). Solar cells were constructed with an active layer consisting of the pristine material or a mixture with an electron accepting moiety. For this purpose, (6,6)-phenyl-C61-butyric-acid (PCBM) in 1:1 a (w/w) ratio with respect to the polymer was used. A clear improvement of the diode behaviour was observed going from the pristine material to the mixture. Photocurrent action spectra of the solar cells with the polymer:PCBM-mixture indicated an active contribution of the polymer to the photocurrent.