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Distribution by Scientific Domains

Polymers and Materials Science	50%

Selected Abstracts

The Effect of Polymer Optoelectronic Properties on the Performance of Multilayer Hybrid Polymer/TiO2 Solar Cells

ADVANCED FUNCTIONAL MATERIALS, Issue 4 2005
P. Ravirajan
Abstract We report a study of the effects of polymer optoelectronic properties on the performance of photovoltaic devices consisting of nanocrystalline TiO2 and a conjugated polymer. Three different poly(2-methoxy-5-(2,-ethylhexoxy)-1,4-phenylenevinylene) (MEH-PPV)-based polymers and a fluorene,bithiophene copolymer are compared. We use photoluminescence quenching, time-of-flight mobility measurements, and optical spectroscopy to characterize the exciton-transport, charge-transport, and light-harvesting properties, respectively, of the polymers, and correlate these material properties with photovoltaic-device performance. We find that photocurrent is primarily limited by the photogeneration rate and by the quality of the interfaces, rather than by hole transport in the polymer. We have also studied the photovoltaic performance of these TiO2/polymer devices as a function of the fabrication route and device design. Including a dip-coating step before spin-coating the polymer leads to excellent polymer penetration into highly structured TiO2 networks, as was confirmed through transient optical measurements of the photoinduced charge-transfer yield and recombination kinetics. Device performance is further improved for all material combinations studied, by introducing a layer of poly(ethylene dioxythiophene) (PEDOT) doped with poly(styrene sulfonic acid) (PSS) under the top contact. Optimized devices incorporating the additional dip-coated and PEDOT:PSS layers produced a short-circuit current density of about 1,mA,cm,2, a fill factor of 0.50, and an open-circuit voltage of 0.86,V under simulated AM,1.5 illumination (100,mW,cm,2, 1,sun). The corresponding power conversion efficiency under 1,sun was ,,0.4,%. [source]

Progress with Molecular Electronic Junctions: Meeting Experimental Challenges in Design and Fabrication

ADVANCED MATERIALS, Issue 43 2009
Richard L. McCreery
Abstract Molecular electronics seeks to incorporate molecular components as functional elements in electronic devices. There are numerous strategies reported to date for the fabrication, design, and characterization of such devices, but a broadly accepted example showing structure-dependent conductance behavior has not yet emerged. This progress report focuses on experimental methods for making both single-molecule and ensemble molecular junctions, and highlights key results from these efforts. Based on some general objectives of the field, particular experiments are presented to show progress in several important areas, and also to define those areas that still need attention. Some of the variable behavior of ostensibly similar junctions reported in the literature is attributable to differences in the way the junctions are fabricated. These differences are due, in part, to the multitude of methods for supporting the molecular layer on the substrate, including methods that utilize physical adsorption and covalent bonds, and to the numerous strategies for making top contacts. After discussing recent experimental progress in molecular electronics, an assessment of the current state of the field is presented, along with a proposed road map that can be used to assess progress in the future. [source]

Influence of top layer geometries on the electronic properties of pentacene and diindenoperylene thin films

PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 3 2008
M. Scharnberg
Abstract Top layers have a pronounced influence on the electronic properties of molecular organic thin films. Here, we report about the changes induced by metallic and polymeric top layers and contacts. As test structures, model systems of diindenoperylene and pentacene crystalline molecular organic thin films are used. A very sensitive radiotracer technique is introduced to study the details of metal penetration during top contact formation. The influence of temperature, evaporation time, adhesion promoter and grain size of the organic film were examined. The electric currents passing through metal top contacts were found to vary by more than a factor of three, depending on the preparation conditions of the metal contact. Furthermore, the series resistance of chemically identical contacts that only differed in the morphology of the interface were found to show pronounced asymmetric conductivity behaviour. We also show that with the help of electret top layers, based on the Teflon-AF fluropolymer, the threshold voltage of an organic field effect transistor can be tuned by several volts. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Potentiometry on pentacene OFETs: Charge carrier mobilities and injection barriers in bottom and top contact configurations

PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 3 2008
R. Scholz
Abstract In a combination of experimental techniques including electrical probes, potentiometry, and charge transient spectroscopy (QTS), we develop concepts how to quantify the potential drops at the contacts, the mobility in the channel region, and the density of states of deep traps in pentacene OFETs. For OFETs grown from unpurified pentacene on pre-patterned Au bottom contacts, a comparison between potentiometry and two-dimensional device simulations determines an injection barrier of 0.73 eV at the source contact and a hole mobility of 0.014 cm2 V,1 s,1 in the pentacene channel. Temperature-dependent QTS data reveal a trap level at about 125 meV from the hole transport band, indicating a relatively high density of unintentional dopants and therefore a high background density of majority charge carriers. In OFETs grown from purified pentacene onto a SiO2 gate dielectric and Au top contacts evaporated onto the pentacene channel without breaking the vacuum, potentiometry reveals a nearly perfect alignment of the metal work function with the hole transport level in the organic layer. The much lower density of deep traps in these samples raises the hole mobility to the range 0.1,0.2 cm2 V,1 s,1. A further improvement of the hole mobility and the resulting device performance can be achieved by a chemical treatment of the gate oxide with n-octadecytrichlorosilane (OTS). (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]