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Charge Generation (charge + generation)

Distribution by Scientific Domains

Polymers and Materials Science	64%
Chemistry	28%

Selected Abstracts

Nanomorphology and Charge Generation in Bulk Heterojunctions Based on Low-Bandgap Dithiophene Polymers with Different Bridging Atoms

ADVANCED FUNCTIONAL MATERIALS, Issue 7 2010
Mauro Morana
Abstract Carbon bridged (C-PCPDTBT) and silicon-bridged (Si-PCPDTBT) dithiophene donor,acceptor copolymers belong to a promising class of low bandgap materials. Their higher field-effect mobility, as high as 10,2,cm2 V,1,s,1 in pristine films, and their more balanced charge transport in blends with fullerenes make silicon-bridged materials better candidates for use in photovoltaic devices. Striking morphological changes are observed in polymer:fullerene bulk heterojunctions upon the substitution of the bridging atom. XRD investigation indicates increased ,,, stacking in Si-PCPDTBT compared to the carbon-bridged analogue. The fluorescence of this polymer and that of its counterpart C-PCPDTBT indicates that the higher photogeneration achieved in Si-PCPDTBT:fullerene films (with either [C60]PCBM or [C70]PCBM) can be correlated to the inactivation of a charge-transfer complex and to a favorable length of the donor,acceptor phase separation. TEM studies of Si-PCPDTBT:fullerene blended films suggest the formation of an interpenetrating network whose phase distribution is comparable to the one achieved in C-PCPDTBT:fullerene using 1,8-octanedithiol as an additive. In order to achieve a balanced hole and electron transport, Si-PCPDTBT requires a lower fullerene content (between 50 to 60,wt%) than C-PCPDTBT (more than 70,wt%). The Si-PCPDTBT:[C70]PCBM OBHJ solar cells deliver power conversion efficiencies of over 5%. [source]

Charge Generation and Photovoltaic Operation of Solid-State Dye-Sensitized Solar Cells Incorporating a High Extinction Coefficient Indolene-Based Sensitizer

ADVANCED FUNCTIONAL MATERIALS, Issue 11 2009
Henry J. Snaith
Abstract An investigation of the function of an indolene-based organic dye, termed D149, incorporated in to solid-state dye-sensitized solar cells using 2,2,,7,7,-tetrakis(N,N -di- p -methoxypheny-amine)-9,9,-spirobifluorene (spiro-OMeTAD) as the hole transport material is reported. Solar cell performance characteristics are unprecedented under low light levels, with the solar cells delivering up to 70% incident photon-to-current efficiency (IPCE) and over 6% power conversion efficiency, as measured under simulated air mass (AM) 1.5 sun light at 1 and 10,mW cm,2. However, a considerable nonlinearity in the photocurrent as intensities approach "full sun" conditions is observed and the devices deliver up to 4.2% power conversion efficiency under simulated sun light of 100,mW cm,2. The influence of dye-loading upon solar cell operation is investigated and the thin films are probed via photoinduced absorption (PIA) spectroscopy, time-correlated single-photon counting (TCSPC), and photoluminescence quantum efficiency (PLQE) measurements in order to deduce the cause for the non ideal solar cell performance. The data suggest that electron transfer from the photoexcited sensitizer into the TiO2 is only between 10 to 50% efficient and that ionization of the photo excited dye via hole transfer directly to spiro-OMeTAD dominates the charge generation process. A persistent dye bleaching signal is also observed, and assigned to a remarkably high density of electrons "trapped" within the dye phase, equivalent to 1.8,×,1017,cm,3 under full sun illumination. it is believed that this localized space charge build-up upon the sensitizer is responsible for the non-linearity of photocurrent with intensity and nonoptimum solar cell performance under full sun conditions. [source]

Correlation of Heterojunction Luminescence Quenching and Photocurrent in Polymer-Blend Photovoltaic Diodes

ADVANCED MATERIALS, Issue 38-39 2009
Astrid Gonzalez-Rabade
Charge generation in organic solar cells proceeds via photogeneration of excitons in the bulk that form geminate electron,hole pairs at the heterojunction formed between electron donor and acceptors. It is shown that an externally applied electric field increases the number of free charges formed from the geminate pair, and quenches the luminescence from the relaxed exciplex with one-to-one correspondence. [source]

Charge generation during filling of insulated tanks

PROCESS SAFETY PROGRESS, Issue 3 2002
Migvia Vidal
This research involves charge generation measurements for various flammable fluids during filling of insulated tanks and relating static charging with flow rate and physicochemical parameters, especially changes in electrical conductivity. The objective is to correlate static charge measured inside baffled metal and polyethylene tanks as a function of impeller, Reynolds number, and the electrical conductivity of both the hydrocarbon and the dispersed water phase for a variety of flammable liquids. A product of the research is a correlation for polyethylene and other plastic insulated tanks for transfers of low conductivity liquid fuels. Also, a universal (dimensionless) correlation that relates the charging data to the colloidal nature of the mixture will be developed for static charge generation during impeller mixing of light hydrocarbons containing various concentrations of water in cylindrical tanks. The correlation of electrostatic data from this research will help make it possible to control electrostatic charges, and, as a result, greatly improve safety of operations involving flammable fluids in industry. [source]

The Role of Transition Metal Oxides in Charge-Generation Layers for Stacked Organic Light-Emitting Diodes

ADVANCED FUNCTIONAL MATERIALS, Issue 11 2010
Sami Hamwi
Abstract The mechanism of charge generation in transition metal oxide (TMO)-based charge-generation layers (CGL) used in stacked organic light-emitting diodes (OLEDs) is reported upon. An interconnecting unit between two vertically stacked OLEDs, consisting of an abrupt heterointerface between a Cs2CO3 -doped 4,7-diphenyl-1,10-phenanthroline layer and a WO3 film is investigated. Minimum thicknesses are determined for these layers to allow for simultaneous operation of both sub-OLEDs in the stacked device. Luminance,current density,voltage measurements, angular dependent spectral emission characteristics, and optical device simulations lead to minimum thicknesses of the n-type doped layer and the TMO layer of 5 and 2.5,nm, respectively. Using data on interface energetic determined by ultraviolet photoelectron and inverse photoemission spectroscopy, it is shown that the actual charge generation occurs between the WO3 layer and its neighboring hole-transport material, 4,4',4"-tris(N -carbazolyl)-triphenyl amine. The role of the adjacent n-type doped electron transport layer is only to facilitate electron injection from the TMO into the adjacent sub-OLED. [source]

Ultrafast Hole-Transfer Dynamics in Polymer/PCBM Bulk Heterojunctions

ADVANCED FUNCTIONAL MATERIALS, Issue 10 2010
Artem A. Bakulin
Abstract Ultrafast dynamics of the hole-transfer process from methanofullerene to a polymer in a polymer/PCBM bulk heterojunction are directly resolved. Injection of holes into MDMO-PPV is markedly delayed with respect to [60]PCBM excitation. The fastest component of the delayed response is attributed to the PCBM,polymer hole-transfer process (30,±,10,fs), while the slower component (,150,fs) is provisionally assigned to energy transfer and/or relaxation inside PCBM nanoclusters. The charge generation through the hole transfer is therefore as fast and efficient as through the electron-transfer process. Exciton harvesting efficiency after PCBM excitation crucially depends on the concentration of the methanofullerene in the blend, which is related to changes in the blend morphology. Ultrafast charge generation is most efficient when the characteristic scale of phase separation in the blend does not exceed ,20,nm. At larger-scale phase separation, the exciton harvesting dramatically declines. The obtained results on the time scales of the ultrafast charge generation after PCBM excitation and their dependence on blend composition and morphology are instrumental for the future design of fullerene-derivative-based photovoltaic devices. [source]

Enhanced Thermal Stability and Efficiency of Polymer Bulk-Heterojunction Solar Cells by Low-Temperature Drying of the Active Layer

ADVANCED FUNCTIONAL MATERIALS, Issue 5 2010
Ching Lin
Abstract This study addresses two key issues, stability and efficiency, of polymer solar cells based on blended poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) by demonstrating a film-forming process that involves low-temperature drying (,5,°C) and subsequent annealing of the active layer. The low-temperature process achieves 4.70% power conversion efficiency (PCE) and ,1250,h storage half-life at 65,°C, which are significant improvements over the 3.39% PCE and ,143,h half-life of the regular room-temperature process. The improvements are attributed to the enhanced nucleation of P3HT crystallites as well as the minimized separation of the P3HT and PCBM phases at the low drying temperature, which upon post-drying annealing results in a morphology consisting of small PCBM-rich domains interspersed within a densely interconnected P3HT crystal network. This morphology provides ample bulk-heterojunction area for charge generation while allowing for facile charge transport; moreover, the P3HT crystal network serves as an immobile frame at heating temperatures less than the melting point (Tm) of P3HT, thus preventing PCBM/P3HT phase separation and the corresponding device degradation. [source]

Photophysics and Photocurrent Generation in Polythiophene/Polyfluorene Copolymer Blends

ADVANCED FUNCTIONAL MATERIALS, Issue 19 2009
Christopher R. McNeill
Abstract Here, studies on the evolution of photophysics and device performance with annealing of blends of poly(3-hexylthiophene) with the two polyfluorene copolymers poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2,,2,,-diyl) (F8TBT) and poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) are reported. In blends with F8TBT, P3HT is found to reorganize at low annealing temperatures (100,°C or below), evidenced by a redshift of both absorption and photoluminescence (PL), and by a decrease in PL lifetime. Annealing to 140,°C, however, is found to optimize device performance, accompanied by an increase in PL efficiency and lifetime. Grazing-incidence small-angle X-ray scattering is also performed to study the evolution in film nanomorphology with annealing, with the 140,°C-annealed film showing enhanced phase separation. It is concluded that reorganization of P3HT alone is not sufficient to optimize device performance but must also be accompanied by a coarsening of the morphology to promote charge separation. The shape of the photocurrent action spectra of P3HT:F8TBT devices is also studied, aided by optical modeling of the absorption spectrum of the blend in a device structure. Changes in the shape of the photocurrent action spectra with annealing are observed, and these are attributed to changes in the relative contribution of each polymer to photocurrent as morphology and polymer conformation evolve. In particular, in as-spun films from xylene, photocurrent is preferentially generated from ordered P3HT segments attributed to the increased charge separation efficiency in ordered P3HT compared to disordered P3HT. For optimized devices, photocurrent is efficiently generated from both P3HT and F8TBT. In contrast to blends with F8TBT, P3HT is only found to reorganize in blends with F8BT at annealing temperatures of over 200,°C. The low efficiency of the P3HT:F8BT system can then be attributed to poor charge generation and separation efficiencies that result from the failure of P3HT to reorganize. [source]

Photocrosslinkable Polythiophenes for Efficient, Thermally Stable, Organic Photovoltaics

ADVANCED FUNCTIONAL MATERIALS, Issue 14 2009
Bumjoon J. Kim
Abstract Photocrosslinkable bromine-functionalized poly(3-hexylthiophene) (P3HT-Br) copolymers designed for application in solution-processed organic photovoltaics are prepared by copolymerization of 2-bromo-3-(6-bromohexyl) thiophene and 2-bromo-3-hexylthiophene. The monomer ratio is carefully controlled to achieve a UV photocrosslinkable layer while retaining the ,,, stacking feature of the conjugated polymers. The new materials are used as electron donors in both bulk heterojunction (BHJ) and bilayer type photovoltaic devices. Unlike devices prepared from either P3HT:PCBM blend or P3HT-Br:PCBM blend without UV treatment, photocrosslinked P3HT-Br:PCBM devices are stable even when annealed for two days at the elevated temperature of 150,°C as the nanophase separated morphology of the bulk heterojunction is stabilized as confirmed by optical microscopy and grazing incidence wide angle X-ray scattering (GIWAXS). When applied to solution-processed bilayer devices, the photocrosslinkable materials show high power conversion efficiencies (,2%) and excellent thermal stability (3 days at 150,°C). Such performance, one of the highest obtained for a bilayer device fabricated by solution processing, is achieved as crosslinking does not disturb the ,,, stacking of the polymer as confirmed by GIWAXS measurements. These novel photocrosslinkable materials provide ready access to efficient bilayer devices thus enabling the fundamental study of photophysical characteristics, charge generation, and transport across a well-defined interface. [source]

Correlation of anatase particle size with photocatalytic properties

PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 10 2010
Sangwook Lee
Abstract The influence of the anatase TiO2 particle size on the photocatalytic properties was systematically investigated on the nanometer-size scale. The anatase TiO2 crystallite size estimated using the Scherrer equation was varied from 8.0 to 16.4,nm, and the photocatalytic properties were significantly deteriorated with decreasing crystallites size. The photocurrent measurements showed that the charge generation diminished with decreasing TiO2 particle size. The bandgap energies were measured as a function of the particle size using UV,Vis spectroscopy in order to investigate the origin of the lowered charge-generation characteristics. The absorption band edge shift was found in the anatase nanocrystallites, which was attributed to the quantum size effect. Therefore, the increased bandgap energy induced the low light absorption capability, which was responsible for the low photoactivity of the very tiny anatase nanoparticles. These findings demonstrated that the particle-size reduction was not the best way to maximize the photocatalytic properties of TiO2 because of the diminished light absorption. [source]

Charge generation during filling of insulated tanks

Photoconduction in Amorphous Organic Solids

CHEMPHYSCHEM, Issue 5 2008
Dirk Hertel Dr.
Abstract Herein, we focus on the principles of photoconduction in random semiconductors,the key processes being optical generation of charge carriers and their subsequent transport. This is not an overview of the current work in this area, but rather a highlight of elementary processes, their involvement in modern devices and a summary of recent developments and achievements. Experimental results and models are discussed briefly to visualize the mechanism of optical charge generation in pure and doped organic solids. We show current limits of models based on the Onsager theory of charge generation. After the introduction of experimental techniques to characterize charge transport, the hopping concept for transport in organic semiconductors is outlined. The peculiarities of the transport of excitons and charges in disorderd organic semiconductors are highlighted. Finally, a short discussion of ultrafast transport and single chain transport completes the review. [source]