Structural relaxation of E' gamma centers in amorphous silica

We report experimental evidence of the existence of two variants of the E' gamma centers induced in silica by gamma rays at room temperature. The two variants are distinguishable by the fine features of their line shapes in paramagnetic resonance spectra. These features suggest that the two E' gamma differ for their topology. We find a thermally induced interconversion between the centers with an activation energy of about 34 meV. Hints are also found for the existence of a structural configuration of minimum energy and of a metastable state.Comment: 4 pages, 2 figures, submitted to Phys. Rev. Let

Optical properties of wetting layers in stacked InAs/GaAs quantum dot structures

The optical transitions of the wetting layers in two-fold self- assembled InAs/GaAs quantum dot samples are studied as a function of GaAs spacer thickness by various methods. The absorption related studies by photoreflectance and selective photoluminescence excitation spectroscopy reveal already for thick barriers, for which coupling effects can be excluded, two energetically separated heavy-hole transitions. This splitting indicates the formation of two wetting layers during growth with a 10% difference in width and reflects strain field interaction between the island layers. Thin spacer layer samples show in addition the expected wetting layer coupling as confirmed by subband calculations. (C) 2002 Elsevier Science B.V. All rights reserved

Superconducting Layers by Gallium Implantation and Short-Term Annealing in Semiconductors

Superconducting layers in silicon and germanium are fabricated via gallium implantation through a thin $SiO_2$ cover layer and subsequent rapid thermal annealing. Gallium accumulation at the $SiO_2//Si$ and $SiO_2//Ge$ interfaces is observed but no pure gallium phases were found. In both cases superconducting transition occurs around 6-7 K which can be attributed to the metallic conducting, gallium rich interface layer. However, the superconducting as well as the normal-state transport properties in gallium overdoped silicon or germanium are different

ESR and LESR X-band study of morphology and charge carrier interaction in blended P3HT-SWCNT and P3HT-PCBM-SWCNT solid thin films

An electron spin resonance (ESR) X-band study of regio-regular poly(3-hexylthiophene) (RR-P3HT) and RR-P3HT/PCBM ([6,6]-phenyl-C 61-butyric acid methyl ester) composites blended with a low concentration (∼0.1-1%) of single wall carbon nanotubes (SWCNT) is presented. The substantial line-width broadening of polaron and PCBM anion radical ESR spectra was registered after SWCNT incorporation. The possibility of an electron spin exchange interaction between SWCNT conductive electrons and polymer polarons in the RR-P3HT:SWCNT blend without light illumination and between SWCNT conductive electrons and polymer polarons and PCBM anion radical electrons in the RR-P3HT:PCBM:SWCNT blend under light illumination is discussed. In addition, the unusually significant RR-P3HT film morphology (dominant crystalline phase orientation) is substantially altered, in particular a rotation of up to 90° of the P3HT crystalline phase relative to the film plane, due to SWCNT annexation in the polymer, is registered by the ESR spectra external magnetic field angular dependences. © 2011 Elsevier B.V. All rights reserved

Organic photovoltaic solar cells using thin tungsten oxide as interlayer anode contact material

Recent results for single junction bulk heterojunction OPV devices show markedly improved efficiencies above 8 %. However, long term stability remains an open issue. The common ITO anode contact in combination with PEDOT:PSS is known as a severe source of degradation. We investigate P3HT:PCBM based organic solar cells using an alternative PEDOT:PSS free anode contact structure using a thin WO3 interlayer for hole extraction: Glass / ITO / WO3 (0 - 20 nm) / P3HT:PCBM/Al. We report on parameter variation of the device preparation with respect to WO3 interlayer thickness. RF sputter deposition from ceramic ITO and reactive DC sputter deposition from metallic W targets was used for deposition of the Glass / ITO / WO structure. By comparison of the solar cell performance with the conventional device structure, the proof-of-principle and applicability of these potentially long-term stable oxide interlayer contact as PEDOT:PSS replacements is demonstrated. The dependence of solar cell parameters on the oxide layer thickness is analyzed

ESR and LESR X-band study of morphology and charge carrier interaction in blended P3HT-SWCNT and P3HT-PCBM-SWCNT solid thin films

An electron spin resonance (ESR) X-band study of regio-regular poly(3-hexylthiophene) (RR-P3HT) and RR-P3HT/PCBM ([6,6]-phenyl-C 61-butyric acid methyl ester) composites blended with a low concentration (∼0.1-1%) of single wall carbon nanotubes (SWCNT) is presented. The substantial line-width broadening of polaron and PCBM anion radical ESR spectra was registered after SWCNT incorporation. The possibility of an electron spin exchange interaction between SWCNT conductive electrons and polymer polarons in the RR-P3HT:SWCNT blend without light illumination and between SWCNT conductive electrons and polymer polarons and PCBM anion radical electrons in the RR-P3HT:PCBM:SWCNT blend under light illumination is discussed. In addition, the unusually significant RR-P3HT film morphology (dominant crystalline phase orientation) is substantially altered, in particular a rotation of up to 90° of the P3HT crystalline phase relative to the film plane, due to SWCNT annexation in the polymer, is registered by the ESR spectra external magnetic field angular dependences. © 2011 Elsevier B.V. All rights reserved

ESR and LESR X-band study of morphology and charge carrier interaction in blended P3HT-SWCNT and P3HT-PCBM-SWCNT solid thin films

An electron spin resonance (ESR) X-band study of regio-regular poly(3-hexylthiophene) (RR-P3HT) and RR-P3HT/PCBM ([6,6]-phenyl-C 61-butyric acid methyl ester) composites blended with a low concentration (∼0.1-1%) of single wall carbon nanotubes (SWCNT) is presented. The substantial line-width broadening of polaron and PCBM anion radical ESR spectra was registered after SWCNT incorporation. The possibility of an electron spin exchange interaction between SWCNT conductive electrons and polymer polarons in the RR-P3HT:SWCNT blend without light illumination and between SWCNT conductive electrons and polymer polarons and PCBM anion radical electrons in the RR-P3HT:PCBM:SWCNT blend under light illumination is discussed. In addition, the unusually significant RR-P3HT film morphology (dominant crystalline phase orientation) is substantially altered, in particular a rotation of up to 90° of the P3HT crystalline phase relative to the film plane, due to SWCNT annexation in the polymer, is registered by the ESR spectra external magnetic field angular dependences. © 2011 Elsevier B.V. All rights reserved

Influence of thermal annealing on PCDTBT:PCBM composition profiles

A variety of measurement techniques including photothermal deflection spectroscopy (PDS), auger electron spectroscopy (AES), (sub-bandgap) external quantum efficiency (EQE), and impedance spectroscopy are applied to poly[N-900-hepta-decanyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10, 30-benzothiadiazole (PCDTBT)/[6,6]-phenyl C71 butyric acid methyl ester (PC71BM) films and devices to probe the stability under thermal annealing. Upon annealing, solar cell performance is drastically decreased for temperatures higher than 140 C. Detailed investigation indicate changes in polymer:fullerene interactions resulting in the formation of a polymer wetting layer upon annealing at temperatures higher than 140 C. Upon device completion this wetting layer is located close to the metal electrode and therefore leads to an increase in recombination and a decrease in charge carrier extraction, providing an explanation for the reduced fill factor (FF) and power conversion efficiency (PCE). Because long-term solar cell stability may be influenced by morphological reorganization, the effect of thermal annealing on bulk heterojunction solar cells composed of poly[N-900-hepta-decanyl-2,7-carbazole- alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole (PCDTBT):[6,6]-phenyl C71 butyric acid methyl ester (PC71BM) is investigated. Using photothermal deflection spectroscopy (PDS), Auger electron spectroscopy, and impedance spectroscopy, it is demonstrated that annealing at high temperatures leads to formation of a polymer wetting layer close to the metal electrode