This letter reports on highly sensitive optical absorption measurements on organic donor-acceptor solar cells, using Fourier-transform photocurrent spectroscopy ͑FTPS͒. 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 poly͓2-methoxy-5-͑3Ј ,7Ј -dimethyl-octyloxy͔͒-p-phenylene-vinylene:͑6,6͒-phenyl-C61-butyric-acid ͑1:4 weight ratio͒ enabled a study of the intrinsic defect generation due to UV illumination. Solar cell temperature annealing effects in poly͑3-hexylthiophene͒:PCBM ͑1:2 weight ratio͒ cells and the induced morphological changes are related to the changes in the absorption spectrum, as determined with FTPS.

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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 film's 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 today's organic photovoltaic performances and for screening new sets of materials.

Positron-annihilationS-parameter measurements in thermal equilibrium on pure and carbon-doped (50 and 750 at. ppm) a-iron are presented. It is shown that trapping of positrons in both monovacancies and carbon-vacancy pairs occurs, even far above the dissociation temperature of the vacancy pairs. Therefore, a three-state trapping model is used in the analysis of the measured S curves. The vacancy-formation enthalpy in both the paramagnetic and ferromagnetic state is deduced: It is found to be 1.79%0.10 eV in the paramagnetic state and 2.0+0.2 eV in the ferromagnetic state. These values are larger than those published so far. The activation enthalpy for vacancy migration obtained by combining the values cited above with recently published self-diffusion enthalpy values confirms the applicability of the one-interstitial model in a-iron.