M. Garriga

Photoinduced p- to n-type Switching in Thermoelectric Polymer-Carbon Nanotube Composites

UV-induced switching from p- to n-type character is demonstrated during deposition of carbon-nanotube-conjugated polymer composites. This opens the possibility to photopattern n-type regions within an otherwise p-type film, which has a potential for complementary circuitry or, as shown here, thermoelectric generators made from a single solution.

Organic solar cells based on nanoporous P3HT obtained from self-assembled P3HT:PS templates

We demonstrate a solution-based method to create vertical nanoporous structures of semiconducting polymer poly(3-hexylthiophene) over large areas by taking advantage of the spontaneous phase segregation between poly(3-hexylthiophene) and polystyrene deposited from a single solution and, in a second step, removing polystyrene by selectively dissolving it. Nanoporous films with pore diameters which can be tuned down to 120 nm are produced by varying the composition. The potential of the method is further demonstrated by fabricating fully operational solar cells after deposition of [6,6]-phenyl-C61-butyric acid methyl ester from an adequate solvent. Under optimized conditions, the devices based on nanostructured thin films exhibit enhanced efficiencies with respect to graded bilayers and bulk heterojunction organic photovoltaic devices. We relate the increase in fill factor observed in the nanostructured devices to changes in the orientation of poly(3-hexylthiophene) chains induced by nanoconfinement and self-assembly with polystyrene resulting from this simple solution process without the use of any elaborate chemistry or soft lithography.

Chalcogenide glass-based rib ARROW waveguide

Abstract This work is focused on the fabrication of a chalcogenide rib ARROW structure. Three chalcogenide films of two different compositions (Ge33As12Se55 and Ge28Sb12Se60) were stacked on a silicon substrate, and the upper layer etched by a reactive physical process to ensure linear behavior. Cross-section (or lateral) light confinement at 1.55 μm was obtained and gross optical loss measurements yielded 6 dB on a 1 cm long sample.

Tailoring thermal conductivity by engineering compositional gradients in Si1-xGex superlattices

The transport properties of artificially engineered superlattices (SLs) can be tailored by incorporating a high density of interfaces in them. Specifically, SiGe SLs with low thermal conductivity values have great potential for thermoelectric generation and nano-cooling of Si-based devices. Here, we present a novel approach for customizing thermal transport across nanostructures by fabricating Si/Si1−xGex SLs with well-defined compositional gradients across the SiGe layer from x = 0 to 0.60. We demonstrate that the spatial inhomogeneity of the structure has a remarkable effect on the heat-flow propagation, reducing the thermal conductivity to ∼2.2 W·m−1·K−1, which is significantly less than the values achieved previously with non-optimized long-period SLs. This approach offers further possibilities for future applications in thermoelectricity.

Influence of alloy inhomogeneities on the determination by Raman scattering of composition and strain in Si1–xGex/Si(001) layers

In this work, we investigate the influence of alloy composition inhomogeneities on the vibrational properties of strained Si1−xGex/Si layers with x ranging from 0 to 0.5. We show that the frequencies of the principal alloy vibrational modes (Ge-Ge, Si-Ge, and Si-Si) are strongly influenced by the distribution of Ge atoms within the alloy layers, which becomes gradually random following a series of sequential annealing steps. Our measurements suggest that the composition dependence of the optical phonon frequencies in fully random and unstrained alloys is well described by the results previously published by Alonso and Winer [Phys. Rev. B 39, 10056 (1989)]. In the general case of an alloy layer with unknown degree of compositional inhomogeneity and/or strain relaxation, though the analysis of the Raman spectra is not straightforward. Therefore, we propose an analytical/graphical method to accurately estimate the Ge content and residual strain of SiGe layers exhibiting any level of compositional disorder or...

Vapour printing: patterning of the optical and electrical properties of organic semiconductors in one simple step

We have exploited the effective and versatile concept of post-deposition treatment using vapour and added a new dimension, space, in order to form patterns of organic thin films with modified optical and electrical properties in a low cost and straightforward fashion. To achieve this feat, we developed a vapour printer which locally exposes an organic film to low concentrations of common organic solvents. This technique readily enables the generation of spatially resolved patterns through three different approaches: (i) local changes in molecular conformation; (ii) local changes in the degree of crystallinity; and (iii) local chemical reactions. In order to demonstrate the potential of vapour printing, we have fabricated a range of proof-of-concept optoelectronic devices, including integrated optical interferometers, photodiodes with position dependent response, organic solar cells and conducting wires.

Optical studies of gap, hopping energies, and the Anderson-Hubbard parameter in the zigzag-chain compound SrCuO 2

We have investigated the electronic structure of the zigzag ladder (chain) compound SrCuO2 combining polarized optical absorption, reflection, photoreflectance and pseudo-dielectric function measurements with the model calculations. These measurements yield an energy gap of 1.42 eV (1.77 eV) at 300 K along (perpendicular) to the Cu-O chains. We have found that the lowest energy gap, the correlation gap, is temperature independent. The electronic structure of this oxide is calculated using both the local-spin-density-approximation with gradient correction method, and the tight-binding theory for the correlated electrons. The calculated density of electronic states for non-correlated and correlated electrons shows quasi-one-dimensional character. The correlation gap values of 1.42 eV (indirect transition) and 1.88 eV (direct transition) have been calculated with the electron hopping parameters t = 0.30 eV (along a chain), t_yz = 0.12 eV (between chains) and the Anderson-Hubbard repulsion on copper sites U= 2.0 eV. We concluded that SrCuO_2 belongs to the correlated-gap insulators.

Uniaxial macroscopic alignment of conjugated polymer systems by directional crystallization during blade coating

We have developed a one step method for the deposition of uniaxially aligned conjugated polymer layers by directional epitaxial crystallization directly from solution. Oriented, square centimeter sized samples of poly(3-hexylthiophene) (P3HT) deposited from a chlorobenzene (CB) solution containing the additional crystallizable solvent 1,3,5-trichlorobenzene (TCB) are obtained by blade coating. Moreover, we show that the developed technique is not restricted to this specific material combination, but is instead applicable to a range of conjugated polymers and crystallizable solvents. Finally, we demonstrate the potential of this technique by realizing an organic photovoltaic device that exhibits a polarization dependent photoresponse.

Patterned optical anisotropy in woven conjugated polymer systems

Weaving of highly oriented conjugated polymer/polyethylene tapes is demonstrated to permit the generation of concealed patterns that can be detected under appropriate polarized light illumination. This is achieved by exploiting the fact that the amount of transmitted light varies with the superposition sequence of semi-transparent objects that feature a high degree of linear birefringence as well as linear dichroism. An analysis based on Muller calculus provides a theoretical description of the observed optical behavior.

Ultrathin Semiconductor Superabsorbers from the Visible to the Near-Infrared

The design of ultrathin semiconducting materials that achieve broadband absorption is a long-sought-after goal of crucial importance for optoelectronic applications. To date, attempts to tackle this problem consisted either of the use of strong-but narrowband-or broader-but moderate-light-trapping mechanisms. Here, a strategy that achieves broadband optimal absorption in arbitrarily thin semiconductor materials for all energies above their bandgap is presented. This stems from the strong interplay between Brewster modes, sustained by judiciously nanostructured thin semiconductors on metal films, and photonic crystal modes. Broadband near-unity absorption in Ge ultrathin films is demonstrated, which extends from the visible to the Ge bandgap in the near-infrared and is robust against angle of incidence variation. The strategy follows an easy and scalable fabrication route enabled by soft nanoimprinting lithography, a technique that allows seamless integration in many optoelectronic fabrication procedures.