Structure-property relationship of anilino-squaraines in organic solar cells

Soluble molecular semiconductors are a promising alternative to semiconducting polymers in the field of organic photovoltaics. Here, three custom-made symmetric 1,3-bis(N,N-alkylated-2,6-dihydroxy-anilino)squaraines containing systematic variations in their molecular structures are compared regarding their applicability as donor materials in bulk-heterojunction solar cells. The terminal substitution pattern of the squaraines is varied from cyclic over linear to branched including a stereogenic center. Single crystal structures are determined, and, in the case of chiral squaraine, unusual formation of stereoisomer co-crystals is revealed. The thin film absorbance spectra show characteristic signatures of H- and J-bands or hint at the formation of tautomers. The general feasibility of these model compounds for photovoltaic applications is studied by light-induced electron spin resonance spectroscopy. The impact of the different molecular substitution patterns on aggregation behavior and, consequently, their optoelectronic solid state properties including charge carrier mobility and finally the solar cell performance are investigated.

Impact of the incorporation of Au nanoparticles into polymer/fullerene solar cells

The addition of small amounts of dodecylamine-capped Au nanoparticles into the active layer of organic bulk heterojunction solar cells consisting of poly(3-octylthiophene) (P3OT) and C(60) was recently suggested to have a positive impact on device performance due to improved electron transport. This issue was systematically further investigated in the present work. Different strategies to incorporate colloidally prepared Au nanoparticles with a narrow size distribution into organic solar cells with the more common donor/acceptor system consisting of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C(61)-butyric acid methyl ester (PCBM) were pursued. Au nanoparticles were prepared with either P3HT or dodecylamine as ligands. Additionally, efforts were undertaken to incorporate nearly ligand-free Au nanoparticles into the system. Therefore, a procedure was successfully developed to remove the dodecylamine ligand shell by a postpreparative ligand exchange with pyridine, a much smaller molecule that can later partly be removed from solid films by annealing. However, for all types of nanoparticles studied here, the performance of the P3HT/PCBM solar cells was found to decrease with the Au particles as an additive to the active layer, meaning that adding Au nanoparticles is not a suitable strategy in the case of the P3HT/PCBM system. Possible reasons are discussed on the basis of detailed investigations of the structure, photophysics and charge transport in the system.

Colloidal synthesis of pt nanoparticles: on the formation and stability of nanowires

Catalytic properties of Pt nanoparticles can significantly depend on the crystallite shape, which renders shape control an important aim in the chemical synthesis. Starting from a colloidal synthesis of quasispherical Pt nanocrystals capped with dodecylamine ligands, systematic variations of different synthesis parameters were performed in the present work in order to obtain Pt nanowires. Mechanistic investigations revealed that nanowires can form by aggregation of quasispherical particles. The process of wire formation was found to be influenced by parameters such as the concentration of the stabilizing ligands on the particle surface. Furthermore, the thermal stability of the obtained nanoparticles was examined. The nanowires were found to be stable up to approximately 140-160 degrees C. In this temperature range a structural transition to a more spherical crystallite shape occurred, which can be understood by thermodynamic considerations.

Photoelectron Spectroscopic Investigations of Chemical Bonding in Organically Stabilized PbS Nanocrystals

The surface structure of organically capped PbS nanocrystals using synchrotron radiation excited core-level photoelectron spectroscopy has been studied. The nanocrystallites prepared by methods of colloidal chemistry have average diameters of 3.1, 3.9, 4.6, and 7.6 nm with narrow size distributions and are stabilized either with oleic acid only or with a combination of trioctylphosphine and oleic acid as ligands. High resolution photoelectron spectroscopy measurements allowed the surface structure to be studied and in particular how the organic ligands bind to the surface of the PbS nanocrystals to be elucidated. The results indicate that the trioctylphosphine ligands passivate only the surface S sites while oleic acid ligands appear to bind mainly to Pb sites.

The effect of nanocrystal surface structure on the luminescence properties: Photoemission study of HF-etched InP nanocrystals

InP nanocrystals with narrow size distribution and mean particle diameter tunable from approximately 2 up to approximately 7 nm were synthesized via the dehalosilylation reaction between InCl3 and tris(trimethylsilyl)phosphine. Specific capping of the nanocrystal surface with a shell of organic ligands protects the nanocrystals from oxidation and provides solubility of the particles in various organic solvents. InP nanocrystals with enhanced photoluminescence (PL) efficiency were obtained from the initial nanocrystals by photoassisted etching of the nanocrystal surface with HF. The resulting PL quantum efficiency of InP nanocrystals dispersed in n-butanol is about three orders of magnitude higher when compared to the nonetched InP samples and approaches approximately 40% at room temperature. High-resolution photoelectron spectroscopy with the use of synchrotron radiation was applied to reveal the changes of the nanocrystal surface responsible for the dramatic improvement of the PL efficiency. The analysis of high-resolution P 2p core-level spectra confirmed significant changes of the nanocrystal surface structure induced by the postpreparative treatments and allowed us to propose the description of the etching mechanism. In the nonetched InP nanocrystals, some surface P atoms generate energy states located inside the band gap which provide nonradiative recombination pathways. Photoassisted treatment of InP nanocrystals with HF results in selective removal of these phosphorous atoms from the nanocrystal surface. The reconstructed surface of the etched InP nanocrystals is terminated mainly with In atoms and is efficiently passivated with tri-n-octylphosphine oxide ligands.

Three-dimensional reconstruction and volumetry of intracranial haemorrhage and its mass effect

Intracerebral haemorrhage still causes considerable disability and mortality. The studies on conservative and operative management are inconclusive, probably due to inexact volumetry of the haemorrhage. We investigated whether three-dimensional (3-D), voxel-based volumetry of the haemorrhage and its mass effect is feasible with routine computed tomography (CT) scans. The volumes of the haemorrhage, ventricles, midline shift, the intracranial volume and ventricular compression in CT scans of 12 patients with basal ganglia haemorrhage were determined with the 3-D slicer software. Indices of haemorrhage and intracranial or ventricular volume were calculated and correlated with the clinical data. The intended measures could be determined with an acceptable intra-individual variability. The 3-D volumetric data tended to correlate better with the clinical course than the conventionally assessed distance of midline shift and volume of haemorrhage. 3-D volumetry of intracranial haemorrhage and its mass effect is feasible with routine CT examination. Prospective studies should assess its value for clinical studies on intracranial space-occupying diseases.

Simple invasive fixation device for fractionated stereotactic LINAC based radiotherapy

BACKGROUND

The aim of this study was to develop a relocatable fixation device for linear accelerator (LINAC) based fractionated stereotactic radiotherapy.

METHOD

The device consists of a CT- and MRI-compatible stereotactic frame, monocortical titan bone screws, four frame-posts with a lock for the fixation pins and a modified head clamp with additional arms that allows the exact and rigid placement of the frame in the desired final position prior to the final placement of the bone screws. By simply disconnecting the lock from the posts, the frame can be dismounted after treatment planning and after each treatment session. The accuracy of reposition was assessed prospectively, using phantom studies and also by comparison of isocenter movements during fractionated radiotherapy in 10 patients with an intracranial lesion.

FINDINGS

No adverse events were seen after the surgical procedure and the screws were well tolerated throughout the course of treatment. The mean isocenter shifts observed during phantom reposition studies were x=0.05 mm, y=-0.32 mm, z=0.18 mm and the mean isocenter shifts during fractionated treatment were x=0.67 mm, y=0.65 mm, z=0.44 mm.

INTERPRETATION

This new fixation device provides excellent accuracy of reposition during stereotactic radiotherapy. It appears superior to non-invasive, mask fixation techniques. Safety margins as small as 1-1.5 mm may therefore be sufficient for this method of stereotactic radiotherapy.