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Charge Carriers (charge + carrier)
Kinds of Charge Carriers Terms modified by Charge Carriers Selected AbstractsPhoto-induced Charge Transfer and Relaxation of Persistent Charge Carriers in Polymer/Nanocrystal Composites for Applications in Hybrid Solar CellsADVANCED FUNCTIONAL MATERIALS, Issue 23 2009Marc Daniel Heinemann Abstract The photo-induced charge transfer and the dynamics of persistent charge carriers in blends of semiconducting polymers and nanocrystals are investigated. Regioregular poly(3-hexylthiophene) (P3HT) is used as the electron donor material, while the acceptor moiety is established by CdSe nanocrystals (nc-CdSe) prepared via colloidal synthesis. As a reference system, organic blends of P3HT and [6,6]-phenyl C61 -butyric acid methyl ester (PCBM) are studied as well. The light-induced charge transfer between P3HT and the acceptor materials is studied by photoluminescence (PL), photo-induced absorption (PIA) and light-induced electron spin resonance spectroscopy (LESR). Compared to neat P3HT samples, both systems show an intensified formation of polarons in the polymer upon photo-excitation, pointing out successful separation of photogenerated charge carriers. Additionally, relaxation of the persistent charge carriers is investigated, and significant differences are found between the hybrid composite and the purely organic system. While relaxation, reflected in the transient signal decay of the polaron signal, is fast in the organic system, the hybrid blends exhibit long-term persistence. The appearance of a second, slow recombination channel indicates the existence of deep trap states in the hybrid system, which leads to the capture of a large fraction of charge carriers. A change of polymer conformation due to the presence of nc-CdSe is revealed by low temperature LESR measurements and microwave saturation techniques. The impact of the different recombination behavior on the photovoltaic efficiency of both systems is discussed. [source] Thermodynamics of Nanosystems with a Special View to Charge CarriersADVANCED MATERIALS, Issue 25-26 2009Joachim Maier Abstract Basic ingredients of interfacial thermodynamics are recapitulated with a special eye on the nanometer-size regime. Questions are then briefly tackled that arise if, in heterogeneous systems, the constituent phases shrink to atomistic dimensions. Particularly helpful in this context are thermodynamic approaches, in which the introduction of interfacial tension is avoided. While the first part addresses ground structure quantities, the second part deals with questions of size and confinement effects on entropy and energy of ionic and electronic defects. These defects represent the respective excitations within this ground structure. The article emphasizes the similarities between ions and electrons manifested in the statistics rather than elaborating on the discrepancies that are primarily reflected by different densities of states and mobilities. It is, therefore, not the intention of the article to address aspects of nanoelectronics that rely on quantum transport for which many reviews are available. Nonetheless all these discussed aspects are directly relevant for both nanoionics and nanoelectronics. [source] Confinement of Charge Carriers and Excitons in Electrophosphorescent Devices: Mechanism of Light Emission and Degradation,ADVANCED MATERIALS, Issue 16 2007D. Chin Charge-carrier and exciton confinement is essential for efficiency and stability enhancment of electrophosphorescent devices. Emission-layer lifetimes of a 4,4,- N,N,-dicarbazole-biphenyl host doped with either a red- or green- emitting dye (upper and lower figures) show a strong dependence and near independence, respectively, on the type of exciton blocking layer used (four are shown). This is explained using energy- level differences and corresponding charge-trapping behavior. [source] Gating of the expressed T-type Cav3.1 calcium channels is modulated by Ca2+ACTA PHYSIOLOGICA, Issue 4 2006L. Lacinová Abstract Aim:, We have investigated the influence of Ca2+ ions on the basic biophysical properties of T-type calcium channels. Methods:, The Cav3.1 calcium channel was transiently expressed in HEK 293 cells. Current was measured using the whole cell patch clamp technique. Ca2+ or Na+ ions were used as charge carriers. The intracellular Ca2+ was either decreased by the addition of 10 mm ethyleneglycoltetraacetic acid (EGTA) or increased by the addition of 200 ,m Ca2+ into the non-buffered intracellular solution. Various combinations of extra- and intracellular solutions yielded high, intermediate or low intracellular Ca2+ levels. Results:, The amplitude of the calcium current was independent of intracellular Ca2+ concentrations. High levels of intracellular Ca2+ accelerated significantly both the inactivation and the activation time constants of the current. The replacement of extracellular Ca2+ by Na+ as charge carrier did not affect the absolute value of the activation and inactivation time constants, but significantly enhanced the slope factor of the voltage dependence of the inactivation time constant. Slope factors of voltage dependencies of channel activation and inactivation were significantly enhanced. The recovery from inactivation was faster when Ca2+ was a charge carrier. The number of available channels saturated for membrane voltages more negative than ,100 mV for the Ca2+ current, but did not reach steady state even at ,150 mV for the Na+ current. Conclusions:, Ca2+ ions facilitate transitions of Cav3.1 channel from open into closed and inactivated states as well as backwards transition from inactivated into closed state, possibly by interacting with its voltage sensor. [source] High-Density Carrier Accumulation in ZnO Field-Effect Transistors Gated by Electric Double Layers of Ionic LiquidsADVANCED FUNCTIONAL MATERIALS, Issue 7 2009Hongtao Yuan Abstract Very recently, electric-field-induced superconductivity in an insulator was realized by tuning charge carrier to a high density level (1,×,1014 cm,2). To increase the maximum attainable carrier density for electrostatic tuning of electronic states in semiconductor field-effect transistors is a hot issue but a big challenge. Here, ultrahigh density carrier accumulation is reported, in particular at low temperature, in a ZnO field-effect transistor gated by electric double layers of ionic liquid (IL). This transistor, called an electric double layer transistor (EDLT), is found to exhibit very high transconductance and an ultrahigh carrier density in a fast, reversible, and reproducible manner. The room temperature capacitance of EDLTs is found to be as large as 34,µF cm,2, deduced from Hall-effect measurements, and is mainly responsible for the carrier density modulation in a very wide range. Importantly, the IL dielectric, with a supercooling property, is found to have charge-accumulation capability even at low temperatures, reaching an ultrahigh carrier density of 8×1014 cm,2 at 220,K and maintaining a density of 5.5×1014 cm,2 at 1.8,K. This high carrier density of EDLTs is of great importance not only in practical device applications but also in fundamental research; for example, in the search for novel electronic phenomena, such as superconductivity, in oxide systems. [source] Electrical properties of modified-grafted polypropyleneJOURNAL OF APPLIED POLYMER SCIENCE, Issue 6 2007Naeem M. El-Sawy Abstract The electrical properties of polypropylene (PP), grafted polypropylene (PP- g -PVP), and modified-grafted PVP with ,-cyano-,-(2-thienyl) crotononitrile were investigated. Also, the electrical characteristic of the modified-grafted PVP subjected to ,-irradiation (60 kGy) was studied. The results show that the , of trunk polymer undergoing different degree of grafting generally increases as function of the grafting yield. The grafting yield between 64.1 and 149% resulted in a progressive decrease in ,E, value. Inclusion of sulfur-containing substrate in different films, having various grafting yields, leads to both increase and decrease in , values. A significant increase in , values is observed upon inclusion of sulfur-containing substrate having maximum grafting yield (149%). These changes are accompanied by fluctuation in , values. The exposure of sulfur-containing substrate in grafting film to a dose of 60 kGy results in a significant decrease in ,E, values for the films undergoing a grafting yield between 64.1 and 149%. The observed changes in ,E, of different films investigated could be attributed mainly to corresponding changes in , values. The observed improvement in electrical properties is mainly because of possible increase in concentration of charge carrier and/or their mobilities. The scanning electron micrographs of some selected films show significant changes in the morphology of the films investigated due to changing the grafting yield, inclusion of sulfur-containing substrate, and exposure to ,-irradiation. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 3797,3803, 2007 [source] Photoinduced splitting of bipolarons in polymersPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 13 2004Xiao Shen Abstract In conducting polymers, a polaron carries charge with spin and a bipolaron carries charge without spin. Hence, the polaron is a spin carrier but the bipolaron is not. Through a dynamical simulation, our study of photoinduced phenomena shows that, by absorbing a photon, a bipolaron is split into two polarons. This photoinduced splitting converts a charge carrier (bipolaron) into two spin carriers (polarons). (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Ionic conductivity in poly (L-leucine)1,3-diamino propane,lithium iodide solid polymer electrolytePOLYMERS FOR ADVANCED TECHNOLOGIES, Issue 3 2009N. H. Kaus Abstract The pelletized Poly(L-Leucine)-1,3-diamino propane,lithium iodide (LiI) samples have been prepared by using a low temperature sintering method. Results from impedance spectroscopy have proven this mixture to be a superionic material with maximum conductivity obtained in the range of 10,3,S/cm for the samples containing 50,wt% LiI. The high ionic conductivity achieved was due to the increased number of charge carrier from LiI. Improved conductivity could also be due to hopping of lithium ion through the side chain of polymer. Infrared spectroscopy showed that both LiI and poly amino acid may co-exist together. From the spectra it is revealed that the CO band at 1643,cm,1 shifted to higher wave number indicating that chelation of Li+ may have occurred at oxygen atom. Results from X-ray diffraction show that the prepared samples were partially crystalline in nature. Some of the peaks have disappeared and this confirmed that some complexation has occurred within the sample. Copyright © 2009 John Wiley & Sons, Ltd. [source] Model of the influence of energetic disorder on inter-chain charge carrier mobility in poly[2-methoxy-5-(2,-ethylhexyloxy)- p -phenylene vinylene]POLYMERS FOR ADVANCED TECHNOLOGIES, Issue 3 2009Petr Toman Abstract The theoretical model of the inter-chain charge carrier mobility in poly[2-methoxy-5-(2,-ethylhexyloxy)- p -phenylene vinylene] (MEH,PPV) doped with polar additive is put forward. The polymer chain states of a charge carrier were calculated by means of diagonalization of a tight-binding Hamiltonian, which includes disorder in both the local energies and transfer integrals. Consequently, the inter-chain charge carrier transport is taking place on a spatially and energetically disordered medium. Because it is believed that the additive does not significantly influence the polymer supramolecular structure, the polymer conformations were simplified as much as possible. On the other hand, the energetic disorder is rigorously described. The transfer rates between the polymer chains were determined using the quasi-classical Marcus theory. The model considered the following steps of the charge carrier transport: the charge carrier hops to a given polymer chain. Then, the charge carrier thermalizes to the Boltzmann distribution over all its possible states on this chain. After that, the charge carrier hops to any possible state on one of the four nearest neighboring chains. The results showed that the inter-chain charge carrier mobility is very strongly dependent on the degree of the energetic disorder. If the energetic disorder is doubled from 0.09 to 0.18,eV, the mobility decreases by two or three orders of magnitude. Copyright © 2008 John Wiley & Sons, Ltd. [source] Endothelin-1 activates a Ca2+ -permeable cation channel with TRPC3 and TRPC7 properties in rabbit coronary artery myocytesTHE JOURNAL OF PHYSIOLOGY, Issue 3 2007C. M. Peppiatt-Wildman In the present work we used patch pipette techniques to study the properties of a novel Ca2+ -permeable cation channel activated by the potent coronary vasoconstrictor endothelin-1 (ET-1) in freshly dispersed rabbit coronary artery myocytes. With cell-attached recording bath application of 10 nm ET-1 evoked cation channel currents (Icat) with subconductance states of about 18, 34 and 51 and 68 pS, and a reversal potential of 0 mV. ET-1 evoked channel activity when extracellular Ca2+ was the charge carrier, illustrating significant Ca2+ permeability. ET-1-induced responses were inhibited by the ETA receptor antagonist BQ123 and the phospholipase C (PLC) inhibitor U73122. The diacylglycerol analogue 1-oleoyl-2-acetyl- sn -glycerol (OAG) also stimulated Icat, but the protein kinase C (PKC) inhibitor chelerythrine did not inhibit either the OAG- or ET-1-induced Icat. Inositol 1,4,5-trisphosphate (IP3) did not activate Icat, but greatly potentiated the response to OAG and this effect was blocked by heparin. Bath application of anti-TRPC3 and anti-TRPC7 antibodies to inside-out patches markedly inhibited ET-1-evoked Icat, but antibodies to TRPC1, C4, C5 and C6 had no effect. Immunocytochemical studies demonstrated preferential TRPC7 expression in the plasmalemma, whereas TRPC3 was distributed throughout the myocyte, and moreover co-localization of TRPC3 and TRPC7 signals was observed at, or close to, the plasma membrane. Flufenamic acid, Gd3+, La3+ and extracellular Ca2+ inhibited Icat with IC50 values of 2.45 ,m, 3.8 ,m, 7.36 ,m and 22 ,m, respectively. These results suggest that in rabbit coronary artery myocytes ET-1 evokes a Ca2+ -permeable non-selective cation channel with properties similar to TRPC3 and TRPC7, and indicates that these proteins may be important components of this conductance. [source] Functional Studies of Synthetic Gramicidin Hybrid Ion Channels in CHO CellsCHEMBIOCHEM, Issue 5 2007Ryszard Wesolowski Abstract The function of a gramicidin hybrid ion channel in living Chinese hamster ovary (CHO) cells was investigated by the patch clamp method. The synthetic ion channel 1 consists of two cyclohexyl ether amino acids that link two minigramicidin strands. With 1 at a concentration of 1.0 ,M, an increase in the whole-cell membrane conductance was observed after 1.37 min. The conductance showed larger currents when Cs+ was used as charge carrier than when Na+ and K+ were used. In single-channel recordings with Cs+ as charge carrier, the substance showed comparable single-channel amplitudes in the membrane of living cells and artificial black lipid bilayers. In addition to functioning as a cation channel, compound 1 appeared to be a water channel. Exposure of the CHO cells to an extracellular hypoosmotic solution did not substantially change the cell volume. Extracellular hypoosmotic conditions in the presence of 1 increased the cell size to 146.5,% that of the control. Thus, the synthetic hybrid channel 1 can function as a cation channel with some Cs+ specificity, and as a water channel in CHO cells. [source] Gating of the expressed T-type Cav3.1 calcium channels is modulated by Ca2+ACTA PHYSIOLOGICA, Issue 4 2006L. Lacinová Abstract Aim:, We have investigated the influence of Ca2+ ions on the basic biophysical properties of T-type calcium channels. Methods:, The Cav3.1 calcium channel was transiently expressed in HEK 293 cells. Current was measured using the whole cell patch clamp technique. Ca2+ or Na+ ions were used as charge carriers. The intracellular Ca2+ was either decreased by the addition of 10 mm ethyleneglycoltetraacetic acid (EGTA) or increased by the addition of 200 ,m Ca2+ into the non-buffered intracellular solution. Various combinations of extra- and intracellular solutions yielded high, intermediate or low intracellular Ca2+ levels. Results:, The amplitude of the calcium current was independent of intracellular Ca2+ concentrations. High levels of intracellular Ca2+ accelerated significantly both the inactivation and the activation time constants of the current. The replacement of extracellular Ca2+ by Na+ as charge carrier did not affect the absolute value of the activation and inactivation time constants, but significantly enhanced the slope factor of the voltage dependence of the inactivation time constant. Slope factors of voltage dependencies of channel activation and inactivation were significantly enhanced. The recovery from inactivation was faster when Ca2+ was a charge carrier. The number of available channels saturated for membrane voltages more negative than ,100 mV for the Ca2+ current, but did not reach steady state even at ,150 mV for the Na+ current. Conclusions:, Ca2+ ions facilitate transitions of Cav3.1 channel from open into closed and inactivated states as well as backwards transition from inactivated into closed state, possibly by interacting with its voltage sensor. [source] The Effect of Nanoparticle Shape on the Photocarrier Dynamics and Photovoltaic Device Performance of Poly(3-hexylthiophene):CdSe Nanoparticle Bulk Heterojunction Solar CellsADVANCED FUNCTIONAL MATERIALS, Issue 16 2010Smita Dayal Abstract The charge separation and transport dynamics in CdSe nanoparticle:poly(3-hexylthiophene) (P3HT) blends are reported as a function of the shape of the CdSe-nanoparticle electron acceptor (dot, rod, and tetrapod). For optimization of organic photovoltaic device performance it is crucial to understand the role of various nanostructures in the generation and transport of charge carriers. The sample processing conditions are carefully controlled to eliminate any processing-related effects on the carrier generation and on device performance with the aim of keeping the conjugated polymer phase constant and only varying the shape of the inorganic nanoparticle acceptor phase. The electrodeless, flash photolysis time-resolved microwave conductivity (FP-TRMC) technique is used and the results are compared to the efficiency of photovoltaic devices that incorporate the same active layer. It is observed that in nanorods and tetrapods blended with P3HT, the high aspect ratios provide a pathway for the electrons to move away from the dissociation site even in the absence of an applied electric field, resulting in enhanced carrier lifetimes that correlate to increased efficiencies in devices. The processing conditions that yield optimum performance in high aspect ratio CdSe nanoparticles blended with P3HT result in poorly performing quantum dot CdSe:P3HT devices, indicating that the latter devices are inherently limited by the absence of the dimensionality that allows for efficient, prolonged charge separation at the polymer:CdSe interface. [source] Enhanced Charge Transportation in Semiconducting Polymer/Insulating Polymer Composites: The Role of an Interpenetrating Bulk InterfaceADVANCED FUNCTIONAL MATERIALS, Issue 11 2010Guanghao Lu Abstract The charge transportation in poly(3-butylthiophene) (P3BT)/insulating polymer composites is studied both microscopically and macroscopically. The increased mobility of free charge carriers, in particular hole mobility, contributes to the enhanced electrical conductivity of this semiconductor/insulator composite. The conductivity origin of the composite, as revealed by conductive-atomic force microscopy (C-AFM), comes mainly from the P3BT network, whose carrier mobility has been improved as a result of reduced activation energy for charge transportation upon forming an interface with the insulating matrix. Both the huge interfacial area and interconnected conductive component are morphologically required for the enhanced electrical property of the composite. An increased size of the P3BT domains, which correspondingly reduces the interfacial area between the two components, ruins the enhancement. This study clarifies the mechanism of the higher electrical properties achieved in a semiconducting polymer upon blending with an insulating polymer, which will further promote the development of these low-cost, easily processable, and environmentally stable composites. [source] On the Design of High-Efficiency Thermoelectric Clathrates through a Systematic Cross-Substitution of Framework ElementsADVANCED FUNCTIONAL MATERIALS, Issue 5 2010Xun Shi Abstract Type I clathrates have recently been identified as prospective thermoelectric materials for power generation purposes due to their very low lattice thermal conductivity values. The maximum thermoelectric figure of merit of almost all type I clathrates is, however, less than 1 and occurs at, or above, 1000,K, making them unfavorable especially for intermediate temperature applications. In this report, the Zintl,Klemm rule is demonstrated to be valid for Ni, Cu, and Zn transition metal substitution in the framework of type I clathrates and offers many degrees of freedom for material modification, design, and optimization. The cross-substitution of framework elements introduces ionized impurities and lattice defects into these materials, which optimize the scattering of charge carriers by the substitution-induced ionized impurities and the scattering of heat-carrying lattice phonons by point defects, respectively, leading to an enhanced power factor, reduced lattice thermal conductivity, and therefore improved thermoelectric figure of merit. Most importantly, the bandgap of these materials can be tuned between 0.1 and 0.5,eV by adjusting the cross-substitution ratio of framework elements, making it possible to design clathrates with excellent thermoelectric properties between 500 and 1000,K. [source] Near IR Sensitization of Organic Bulk Heterojunction Solar Cells: Towards Optimization of the Spectral Response of Organic Solar CellsADVANCED FUNCTIONAL MATERIALS, Issue 2 2010Markus Koppe Abstract The spectroscopic response of a poly(3-hexylthiophene)/[6,6]-phenyl-C61 -butyric acid methyl ester (P3HT/PCBM)-based bulk heterojunction solar cell is extended into the near infrared region (NIR) of the spectrum by adding the low bandgap polymer poly[2,6-(4,4-bis-(2-ethylhexyl)-4H -cyclopenta[2,1- b;3,4- b´]-dithiophene)- alt -4,7-(2,1,3-benzothiadiazole)] [PCPDTBT] to the blend. The dominant mechanism behind the enhanced photosensitivity of the ternary blend is found to be a two-step process: first, an ultrafast and efficient photoinduced charge transfer generates positive charges on P3HT and PCPDTBT and a negative charge on PCBM. In a second step, the positive charge on PCPDTBT is transferred to P3HT. Thus, P3HT serves two purposes. On the one hand it is involved in the generation of charge carriers by the photoinduced electron transfer to PCBM, and, on the other hand, it forms the charge transport matrix for the positive carriers transferred from PCPDTBT. Other mechanisms, such as energy transfer or photoinduced charge transfer directly between the two polymers, are found to be absent or negligible. [source] Evaluating Carrier Accumulation in Degraded Bulk Heterojunction Organic Solar Cells by a Thermally Stimulated Current TechniqueADVANCED FUNCTIONAL MATERIALS, Issue 24 2009Kenji Kawano Abstract Here, the initial photo-degradation of encapsulated P3HT:PCBM bulk heterojunction organic solar cells is investigated. The degraded device is recovered by thermal annealing treatment. Thermally stimulated current measurements reveal that the cause of photo-degradation is carrier accumulation and that the degraded organic solar cell has two broad trap levels, of 0.71 and 0.81,eV. These traps are independent of the thickness of the photoactive layers, the mixing ratio of the photoactive materials and the cathode materials. In addition, it is confirmed that there is a close relationship between the degree of degradation and the amount of accumulated charge carriers. [source] Photo-induced Charge Transfer and Relaxation of Persistent Charge Carriers in Polymer/Nanocrystal Composites for Applications in Hybrid Solar CellsADVANCED FUNCTIONAL MATERIALS, Issue 23 2009Marc Daniel Heinemann Abstract The photo-induced charge transfer and the dynamics of persistent charge carriers in blends of semiconducting polymers and nanocrystals are investigated. Regioregular poly(3-hexylthiophene) (P3HT) is used as the electron donor material, while the acceptor moiety is established by CdSe nanocrystals (nc-CdSe) prepared via colloidal synthesis. As a reference system, organic blends of P3HT and [6,6]-phenyl C61 -butyric acid methyl ester (PCBM) are studied as well. The light-induced charge transfer between P3HT and the acceptor materials is studied by photoluminescence (PL), photo-induced absorption (PIA) and light-induced electron spin resonance spectroscopy (LESR). Compared to neat P3HT samples, both systems show an intensified formation of polarons in the polymer upon photo-excitation, pointing out successful separation of photogenerated charge carriers. Additionally, relaxation of the persistent charge carriers is investigated, and significant differences are found between the hybrid composite and the purely organic system. While relaxation, reflected in the transient signal decay of the polaron signal, is fast in the organic system, the hybrid blends exhibit long-term persistence. The appearance of a second, slow recombination channel indicates the existence of deep trap states in the hybrid system, which leads to the capture of a large fraction of charge carriers. A change of polymer conformation due to the presence of nc-CdSe is revealed by low temperature LESR measurements and microwave saturation techniques. The impact of the different recombination behavior on the photovoltaic efficiency of both systems is discussed. [source] Electronic Structure and Geminate Pair Energetics at Organic,Organic Interfaces: The Case of Pentacene/C60 HeterojunctionsADVANCED FUNCTIONAL MATERIALS, Issue 23 2009Stijn Verlaak Abstract Organic semiconductors are characterized by localized states whose energies are predominantly determined by electrostatic interactions with their immediate molecular environment. As a result, the details of the energy landscape at heterojunctions between different organic semiconductors cannot simply be deduced from those of the individual semiconductors, and they have so far remained largely unexplored. Here, microelectrostatic computations are performed to clarify the nature of the electronic structure and geminate pair energetics at the pentacene/C60 interface, as archetype for an interface between a donor molecule and a fullerene electron acceptor. The size and orientation of the molecular quadrupole moments, determined by material choice, crystal orientation, and thermodynamic growth parameters of the semiconductors, dominate the interface energetics. Not only do quadrupoles produce direct electrostatic interactions with charge carriers, but, in addition, the discontinuity of the quadrupole field at the interface induces permanent interface dipoles. That discontinuity is particularly striking for an interface with C60 molecules, which by virtue of their symmetry possess no quadrupole. Consequently, at a pentacene/C60 interface, both the vacuum-level shift and geminate pair dissociation critically depend on the orientation of the pentacene ,-system relative to the adjacent C60. [source] Anhydrous Polymeric Proton Conductors Based on Imidazole Functionalized PolysiloxaneFUEL CELLS, Issue 3-4 2006G. Scharfenberger Abstract Intrinsically proton conducting polymers with imidazole as proton solvent tethered to a polysiloxane backbone via a flexible spacer have been synthesized. Apart from the standard characterization also their thermal properties and transport behavior have been investigated. The materials exhibit proton conductivity as a consequence of self-dissociation of the imidazole moieties and "structure diffusion" of the resulting defects. In particular, no liquid phase such as water or monomeric imidazole is needed for the observed proton conductivities. To study the influence of the tether structure on the transport properties, cyclic oligomers and open chain polymers with different spacer lengths have been synthesized. The materials are thermally stable up to 200,°C and become soft around room temperature. The conductivity exhibits VTF and WLF behavior with maximum conductivities around ,,=,1.5.10,3,S,cm,1 at T,=,160,°C. The activation volume of the conductivity as derived from pressure dependent measurements is found to be unusually high. The lowest activation volumes and the highest conductivities are observed for the materials with the highest segmental mobilities, i.e. the longest spacers. Proton self-diffusion coefficients as obtained from PFG NMR diffusion measurements are significantly higher than expected from the proton conductivities obtained by dielectric spectroscopy. This corresponds to unusually high Haven ratios which have been interpreted by correlated proton transfers allowing for fast proton diffusion while minimizing the separation of ionic charge carriers. [source] Observation of a Charge Transfer State in Low-Bandgap Polymer/Fullerene Blend Systems by Photoluminescence and Electroluminescence StudiesADVANCED FUNCTIONAL MATERIALS, Issue 20 2009Yi Zhou Abstract The presence of charge transfer states generated by the interaction between the fullerene acceptor PCBM and two alternating copolymers of fluorene with donor,acceptor,donor comonomers are reported; the generation leads to modifications in the polymer bandgap and electronic structure. In one of polymer/fullerene blends, the driving force for photocurrent generation, i.e., the gap between the lowest unoccupied molecular orbitals of the donor and acceptor, is only 0.1,eV, but photocurrent is generated. It is shown that the presence of a charge transfer state is more important than the driving force. The charge transfer states are visible through new emission peaks in the photoluminescence spectra and through electroluminescence at a forward bias. The photoluminescence can be quenched under reverse bias, and can be directly correlated to the mechanism of photocurrent generation. The excited charge transfer state is easily dissociated into free charge carriers, and is an important intermediate state through which free charge carriers are generated. [source] Analysis of Nanostructuring in High Figure-of-Merit Ag1,xPbmSbTe2+m Thermoelectric MaterialsADVANCED FUNCTIONAL MATERIALS, Issue 8 2009Bruce A. Cook Abstract Thermoelectric materials based on quaternary compounds Ag1,xPbmSbTe2+m exhibit high dimensionless figure-of-merit values, ranging from 1.5 to 1.7 at 700,K. The primary factor contributing to the high figure of merit is a low lattice thermal conductivity, achieved through nanostructuring during melt solidification. As a consequence of nucleation and growth of a second phase, coherent nanoscale inclusions form throughout the material, which are believed to result in scattering of acoustic phonons while causing only minimal scattering of charge carriers. Here, characterization of the nanosized inclusions in Ag0.53Pb18Sb1.2Te20 that shows a strong tendency for crystallographic orientation along the {001} planes, with a high degree of lattice strain at the interface, consistent with a coherent interfacial boundary is reported. The inclusions are enriched in Ag relative to the matrix, and seem to adopt a cubic, 96 atom per unit cell Ag2Te phase based on the Ti2Ni type structure. In-situ high-temperature synchrotron radiation diffraction studies indicated that the inclusions remain thermally stable to at least 800,K. [source] Monitoring the Channel Formation in Organic Field-Effect Transistors via Photoinduced Charge TransferADVANCED FUNCTIONAL MATERIALS, Issue 5 2009Thokchom Birendra Singh Abstract Conducting channel formation in organic field-effect transistors (OFETs) is considered to happen in the organic semiconductor layer very close to the interface with the gate dielectric. In the gradual channel approximation, the local density of accumulated charge carriers varies as a result of applied gate bias, with the majority of the charge carriers being localized in the first few semiconductor monolayers close to the dielectric interface. In this report, a new concept is employed which enables the accumulation of charge carriers in the channel by photoinduced charge transfer. An OFET employing C60 as a semiconductor and divinyltetramethyldisiloxane-bis(benzocyclobutene) as the gate dielectric is modified by a very thin noncontinuous layer of zinc-phthalocyanine (ZnPc) at the semiconductor/dielectric interface. With this device geometry, it is possible to excite the phthalocyanine selectively and photogenerate charges directly at the semiconductor/dielectric interface via photoinduced electron transfer from ZnPc onto C60. Thus the formation of a gate induced and a photoinduced channel in the same device can be correlated. [source] Influence of Electric Field on Microstructures of Pentacene Thin-Films in Field-Effect Transistors,ADVANCED FUNCTIONAL MATERIALS, Issue 2 2008L. Cheng Abstract We report on electric-field-induced irreversible structural modifications in pentacene thin films after long-term operation of organic field-effect transistor (OFET) devices. Micro-Raman spectroscopy allows for the analysis of the microstructural modifications of pentacene in the small active channel of OFET during device operation. The results suggest that the herringbone packing of pentacene molecules in a solid film is affected by an external electric field, particularly the source-to-drain field that parallels the a,b lattice plane. The analysis of vibrational frequency and Davydov splitting in the Raman spectra reveals a singular behavior suggesting a reduced separation distance between pentacene molecules after long-term operations and, thus, large intermolecular interactions. These results provide evidence for improved OFET performance after long-term operation, related to the microstructures of organic semiconductors. It is known that the application of large electric fields alters the semiconductor properties of the material owing to the generation of defects and the trapping of charges. However, we first suggest that large electric fields may alter the molecular geometry and further induce structural phase transitions in the pentacene films. These results provide a basis for understanding the improved electronic properties in test devices after long-term operations, including enhanced field-effect mobility, improved on/off current ratio, sharp sub-threshold swing, and a slower decay rate in the output drain current. In addition, the effects of source-to-drain electric field, gate electric field, current and charge carriers, and thermal annealing on the pentacene films during OFET operations are discussed. [source] The Effect of Thermal Treatment on the Morphology and Charge Carrier Dynamics in a Polythiophene,Fullerene Bulk Heterojunction,ADVANCED FUNCTIONAL MATERIALS, Issue 8 2005J. Savenije Abstract The influence of various thermal treatment steps on the morphology and the photoconductive properties of a non-contacted, 50,nm thick blend (50:50,wt.-%) of [6,6]-phenyl C61 -butyric acid methyl ester (PCBM) and poly(3-hexyl thiophene) (P3HT) spin-coated from chloroform has been studied using transmission electron microscopy (TEM) and the electrodeless time-resolved microwave conductivity technique. After annealing the film for 5,min at 80,°C, TEM images show the formation of crystalline fibrils of P3HT due to a more ordered packing of the polymer chains. The thermal treatment results in a large increase of the photoconductivity, due to an enhancement of the hole mobility in these crystalline P3HT domains from 0.0056,cm2,V,1,s,,1 for the non-annealed sample to 0.044,cm2,V,1,s,,1 for the sample annealed at 80,°C. In contrast, the temporal shape of the photoconductivity, with typical decay half-times, ,1/2, of 1,,s for the lowest excitation intensities, is unaffected by the temperature treatment. Further annealing of the sample at 130,°C results in the formation of three different substructures within the heterojunction: a PCBM:P3HT blend with PCBM-rich clusters, a region depleted of PCBM, and large PCBM single crystals. Only a minor increase in the amplitude, but a tenfold rise of the decay time of the photoconductivity, is observed. This is explained by the formation of PCBM-rich clusters and large PCBM single crystals, resulting in an increased diffusional escape probability for mobile charge carriers and hence reduced recombination. [source] Improved Film Morphology Reduces Charge Carrier Recombination into the Triplet Excited State in a Small Bandgap Polymer-Fullerene Photovoltaic CellADVANCED MATERIALS, Issue 38 2010Daniele Di Nuzzo The use of diiodooctane as processing additive for construction of PCPDTBT:PCBM solar cells results in a profound change in photophysical behavior of this blend. In the improved morphology obtained with the additive, recombination of charge carriers to the lowest triplet excited state is suppressed. This contributes to the boost in solar power conversion efficiency induced by the use of the processing agent. [source] Effect of Traps on Carrier Injection and Transport in Organic Field-effect TransistorIEEJ TRANSACTIONS ON ELECTRICAL AND ELECTRONIC ENGINEERING, Issue 4 2010Martin Weis Non-member Abstract This study illustrates effect of traps on the charge injection and transport in the organic field-effect transistor (OFET). Here are included silicon nanoparticles (NPs) on a semiconductor-gate insulator interface, which work as trapping centers of charge carriers. Charge transport and injection phenomena are investigated by electrical measurements in presence of traps with designed densities. We find that OFETs with a low concentration of intrinsic carriers, such as a pentacene, are extremely sensitive to the internal electric fields. A significant threshold voltage shift due to trapped charge is observed, with a possibility to tune it by controlling the NP density. We demonstrate that the NP film can serve to design the amount of the accumulated charge in OFET and thus change the space-charge-limited conditions to the injection-limited conditions. A detailed analysis of pentacene OFET based on dielectric properties and the Maxwell-Wagner model reveals the internal electric field created by NPs. Additionally, the effect of NPs is discussed with respect to effective mobility, and its decrease is related to deceleration of carrier propagation by the trapping effect as well as low injection due to the increase of the carrier injection barrier by the internal field. Copyright © 2010 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc. [source] High-Performance Organic Field-Effect TransistorsADVANCED MATERIALS, Issue 14-15 2009Daniele Braga Abstract With the advent of devices based on single crystals, the performance of organic field-effect transistors has experienced a significant leap, with mobility now in excess of 10,cm2 V,1 s,1. The purpose of this review is to give an overview of the state-of-the-art of these high-performance organic transistors. The paper focuses on the problem of parameter extraction, limitations of the performance by the interfaces, which include the dielectric,semiconductor interface, and the injection and retrieval of charge carriers at the source and drain electrodes. High-performance devices also constitute tools of choice for investigating charge transport phenomena in organic materials. It is shown how the combination of field-effect measurements with other electrical characterizations helps in elucidating this still unresolved issue. [source] Thermally stimulated depolarization study in polyvinylidenefluoride,polysulfone polyblend filmsJOURNAL OF APPLIED POLYMER SCIENCE, Issue 6 2010Pooja Saxena Abstract Thermally stimulated depolarization currents (TSDCs) in short- and open-circuit modes in polyvinylidenefluoride (PVDF),polysulfone (PSF) polyblend have been recorded. The TSDC thermograms of PVDF and PSF in short-circuit mode show two peaks, whereas the polyblend of the two polymers shows a single peak. With the increase in PSF weight percentage in the polyblend, the magnitude of TSDC peak current increased and the peak current position shifted toward the lower temperature side. The single peak in polyblend appears at 165°C ± 10°C, which is at higher temperature than the temperature of low-temperature peak for individual polymers. This suggests that this peak may be due to dipolar polarization. Subsequently, shifting of peak toward higher temperature side with increase in polarizing temperature indicates the space charge peak. This contradiction has been explained on the basis of induced dipole theory. The behavior of short circuit TSDC could be explained in terms of the heterocharge caused by dipole orientation and ionic homocharge drift, together with the injection of charge carriers from electrodes and their subsequent localization in surface and bulk traps. However, two oppositely directed TSDC peaks observed in open-circuit mode in all the polyblend samples could be considered as the result of superposition of two overlapped and oppositely directed peaks, one caused by relaxation of dipole polarization and the other by the space charge. Thus, we have compared TSDC measured in open- and short-circuit modes to distinguish between these two relaxation processes and separate them. There is only one broad peak observed in the short-circuit mode of the polyblend, which entirely corresponds to the relaxation of dipole polarization. Insertion of a dielectric gap in the open-circuit mode does not affect the dipole current, but the space charge component flowing in the opposite direction is added to the former. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010 [source] Optical and thermo electrical properties of ZnO nano particle filled polystyreneJOURNAL OF APPLIED POLYMER SCIENCE, Issue 5 2010Mulayam S. Gaur Abstract The study of optical and thermally stimulated electrical properties such as optical band gap, refractive index, X-ray spectra, SEM spectra, thermally stimulated discharge current (TSDC), differential scanning calorimetry (DSC) have been undertaken in ZnO nanoparicle filled polystyrene nanocomposite thin film of 30 ,m thickness. The appearance of single TSDC peak at temperature 408 ± 5 K in nanocomposite samples shows the charge carriers injected from deeper trapping levels. It is due to the modification of surface and bulk properties of polystyrene by filling of ZnO nanoparticles. In other hand, the strong interaction of nanoparticles with polymer matrix is the expected reason of improvement of crystallite size, optical energy band gap, refractive index, TSDC, glass transition temperature, and charge storage. It is confirmed from SEM images that the modifications of these properties are caused by creation of clusters in amorphous,crystalline boundaries of pristine polystyrene. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010 [source] | ||||||||||||||||||||||||||||||||||