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K.g.u Wijayantha - One of the best experts on this subject based on the ideXlab platform.
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electron transport and Back Reaction in electrochemically self assembled nanoporous zno dye hybrid films
Journal of Physical Chemistry B, 2004Co-Authors: Torsten Oekermann, K.g.u Wijayantha, Hideki Minoura, Tsukasa Yoshida, L PeterAbstract:Nanoporous ZnO/eosinY films prepared by electrochemical self-assembly have already shown promising characteristics for use in dye-sensitized solar cells, such as ease of preparation (no need for high-temperature sintering) and high dye loading. In this study, electron transport and Back Reaction in these films have been investigated by intensity modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS). In contrast to sintered colloidal ZnO films, electrodeposited ZnO/eosinY films exhibit electron transit times (τD) that are much shorter than electron lifetimes (τn), leading to very efficient electron collection. The shorter transit times in the electrodeposited layers are due in part to the fact that the films are very thin, but in addition the electron diffusion coefficients are higher than in sintered colloidal ZnO films. Although the unusually high dye concentration in the electrochemically self-assembled film allows efficient light harvesting, it was found th...
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electron transport and Back Reaction in dye sensitised nanocrystalline photovoltaic cells
Electrochimica Acta, 2000Co-Authors: Laurence M. Peter, K.g.u WijayanthaAbstract:The transport and Back Reaction of electrons in dye sensitised nanocrystalline solar cells (DSNC) has been studied by frequency resolved optical perturbation techniques. Intensity modulated photocurrent spectroscopy (IMPS) has been used to obtain values of the electron diffusion coefficient, Dn, as a function of illumination intensity. It was found that Dn increased with intensity (Dn∝I−0.5). Intensity-modulated photovoltage spectroscopy (IMVS) has been used to measure the electron lifetime, τn, which is determined by the rate of Back Reaction with I3− ions in the electrolyte. It was found that τn decreased with light intensity (τn∝I−0.5). The electron diffusion length, Ln=(Dnτn)1/2, is therefore only weakly dependent on light intensity. The values of Ln were used to calculate the theoretical IPCE of the cell. Experimental measurements confirmed the prediction that the IPCE should remain almost constant over five orders of magnitude of light intensity. Possible reasons for the opposite trends in Dn and τn are discussed and related to the fundamental processes taking place in the DSNC.
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a novel charge extraction method for the study of electron transport and interfacial transfer in dye sensitised nanocrystalline solar cells
Electrochemistry Communications, 2000Co-Authors: Noel W Duffy, Laurence M. Peter, R M G Rajapakse, K.g.u WijayanthaAbstract:Abstract A novel charge extraction method has been developed to study the transport, trapping and Back Reaction of photogenerated electrons in dye sensitised nanocrystalline cells (Gratzel cells). The cell is illuminated at open circuit until a steady state is reached in which the rates of photogeneration and of Back Reaction of electrons with tri-iodide are equal. The illumination is then interrupted, and the electron density is allowed to decay for a given time in the dark before short circuiting the cell using a solid state switch. For high efficiency cells, the integrated current measured at short circuit corresponds closely to the remaining electronic charge in the film. Small corrections are required to account for Back Reaction and substrate charging. The delay time between interruption of the illumination and short circuit charge extraction is varied systematically to follow the decay of electron concentration. Analysis of the time dependence of the electron charge indicates that the Back Reaction of electrons with I 3 − is second order in electron density, which is consistent with the formation of I 2 −. as an intermediate. Simultaneous measurement of the charge and photovoltage decay curves shows that the density of trap states decreases exponentially with trap depth.
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investigation of the kinetics of the Back Reaction of electrons with tri iodide in dye sensitized nanocrystalline photovoltaic cells
Journal of Physical Chemistry B, 2000Co-Authors: Noel W Duffy, L.m Peter, And R M G Rajapakse, K.g.u WijayanthaAbstract:A novel technique has been developed to study the kinetics of the Back Reaction of electrons with I3- in dye-sensitized nanocrystalline cells. A solid-state switched operational amplifier feedBack circuit controlling the cell is used to alternate open-circuit and short-circuit conditions. In the experiment, electrons are injected by the photoexcited dye during illumination at open circuit, and then the subsequent decay of the open-circuit photovoltage in the dark is recorded up to a given time, at which the cell is short-circuited abruptly. The electron charge extracted at short circuit is measured by a current amplifier and integrator device. The kinetics of electron decay at open circuit has been studied by varying the delay between interrupting the illumination and short-circuiting the cell. Analysis of the time dependence of the electron density has established that the decay process is second-order in the total electron concentration. This is consistent with a mechanism involving the formation of I2-...
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characterisation of electron transport and Back Reaction in dye sensitised nanocrystalline solar cells by small amplitude laser pulse excitation
Electrochemistry Communications, 2000Co-Authors: Noel W Duffy, Laurence M. Peter, K.g.u WijayanthaAbstract:The characterisation of the transport and interfacial Reaction of electrons in dye-sensitised nanocrystalline solar cells is complicated by the non-linearity of these processes. This problem has been overcome by superimposing small amplitude pulsed laser excitation on steady Background illumination. The laser perturbation of the photostationary state is sufficiently small that the photocurrent and photovoltage responses can be fitted using constant values of the electron diffusion coefficient Dn and electron lifetime τn. Analytical and finite difference solutions of the continuity equation have been used to analyse the experimental photocurrent, photocharge and photovoltage transients, and the intensity dependence of Dn and of τn has been established by varying the bias illumination level, and hence the dc photocurrent density, jdc. The intensity dependence of Dn (Dn∝jdc0.68) is attributed to trapping/detrapping involving a distribution of trapping levels. The intensity dependence of τn (τn∝jdc−0.62) may indicate that the Back Reaction of electrons with I3− is not first order in electron concentration. Other possible explanations are that the interfacial electron transfer rate constant depends on trap occupancy or on the rate of surface or bulk electron diffusion.
Laurence M. Peter - One of the best experts on this subject based on the ideXlab platform.
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how does Back Reaction at the conducting glass substrate influence the dynamic photovoltage response of nanocrystalline dye sensitized solar cells
Journal of Physical Chemistry B, 2005Co-Authors: Petra J Cameron, Laurence M. PeterAbstract:In dye-sensitized nanocrystalline solar cells (DSC), the transfer of electrons from the conducting glass substrate to triiodide ions in solution is an important loss mechanism that can be suppressed by using thin compact blocking layers of TiO(2). Whereas Back-Reaction at the substrate is relatively unimportant under short circuit conditions, it must be taken into account at the maximum power point or at open circuit. The influence of the Back-Reaction on open circuit photovoltage decay measurements and on intensity modulated photovoltage (IMVS) measurements has been studied by model simulations and by experimental measurements. The simulations demonstrate that reliable information about DSC properties such as trapping distributions can only be derived from transient or periodic photovoltage responses if the Back-Reaction is suppressed by the use of suitable blocking layers.
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how important is the Back Reaction of electrons via the substrate in dye sensitized nanocrystalline solar cells
Journal of Physical Chemistry B, 2005Co-Authors: Petra J Cameron, Laurence M. Peter, Sarmimala HoreAbstract:The role of the conducting glass substrate (fluorine-doped tin oxide, FTO) in the Back Reaction of electrons with tri-iodide ions in dye-sensitized nanocrystalline solar cells (DSCs) has been investigated using thin-layer electrochemical cells that are analogues of the DSCs. The rate of Back Reaction is dependent on the type of FTO and the thermal treatment. The results show that this Back-Reaction route cannot be neglected in DSCs, particularly at lower light intensities, where it is the dominant route for the Back transfer of electrons to tri-iodide. This conclusion is confirmed by measurements of the intensity dependence of the photovoltages of DSCs with and without blocking layers. It follows that blocking layers should be used to prevent the Back Reaction in mechanistic studies in which the light intensity is varied over a wide range. Conclusions based on studies of the intensity dependence of the parameters of DSCs such as photovoltage and electron lifetime in cells without blocking layers, must be critically re-examined.
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electron transport and Back Reaction in dye sensitised nanocrystalline photovoltaic cells
Electrochimica Acta, 2000Co-Authors: Laurence M. Peter, K.g.u WijayanthaAbstract:The transport and Back Reaction of electrons in dye sensitised nanocrystalline solar cells (DSNC) has been studied by frequency resolved optical perturbation techniques. Intensity modulated photocurrent spectroscopy (IMPS) has been used to obtain values of the electron diffusion coefficient, Dn, as a function of illumination intensity. It was found that Dn increased with intensity (Dn∝I−0.5). Intensity-modulated photovoltage spectroscopy (IMVS) has been used to measure the electron lifetime, τn, which is determined by the rate of Back Reaction with I3− ions in the electrolyte. It was found that τn decreased with light intensity (τn∝I−0.5). The electron diffusion length, Ln=(Dnτn)1/2, is therefore only weakly dependent on light intensity. The values of Ln were used to calculate the theoretical IPCE of the cell. Experimental measurements confirmed the prediction that the IPCE should remain almost constant over five orders of magnitude of light intensity. Possible reasons for the opposite trends in Dn and τn are discussed and related to the fundamental processes taking place in the DSNC.
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a novel charge extraction method for the study of electron transport and interfacial transfer in dye sensitised nanocrystalline solar cells
Electrochemistry Communications, 2000Co-Authors: Noel W Duffy, Laurence M. Peter, R M G Rajapakse, K.g.u WijayanthaAbstract:Abstract A novel charge extraction method has been developed to study the transport, trapping and Back Reaction of photogenerated electrons in dye sensitised nanocrystalline cells (Gratzel cells). The cell is illuminated at open circuit until a steady state is reached in which the rates of photogeneration and of Back Reaction of electrons with tri-iodide are equal. The illumination is then interrupted, and the electron density is allowed to decay for a given time in the dark before short circuiting the cell using a solid state switch. For high efficiency cells, the integrated current measured at short circuit corresponds closely to the remaining electronic charge in the film. Small corrections are required to account for Back Reaction and substrate charging. The delay time between interruption of the illumination and short circuit charge extraction is varied systematically to follow the decay of electron concentration. Analysis of the time dependence of the electron charge indicates that the Back Reaction of electrons with I 3 − is second order in electron density, which is consistent with the formation of I 2 −. as an intermediate. Simultaneous measurement of the charge and photovoltage decay curves shows that the density of trap states decreases exponentially with trap depth.
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characterisation of electron transport and Back Reaction in dye sensitised nanocrystalline solar cells by small amplitude laser pulse excitation
Electrochemistry Communications, 2000Co-Authors: Noel W Duffy, Laurence M. Peter, K.g.u WijayanthaAbstract:The characterisation of the transport and interfacial Reaction of electrons in dye-sensitised nanocrystalline solar cells is complicated by the non-linearity of these processes. This problem has been overcome by superimposing small amplitude pulsed laser excitation on steady Background illumination. The laser perturbation of the photostationary state is sufficiently small that the photocurrent and photovoltage responses can be fitted using constant values of the electron diffusion coefficient Dn and electron lifetime τn. Analytical and finite difference solutions of the continuity equation have been used to analyse the experimental photocurrent, photocharge and photovoltage transients, and the intensity dependence of Dn and of τn has been established by varying the bias illumination level, and hence the dc photocurrent density, jdc. The intensity dependence of Dn (Dn∝jdc0.68) is attributed to trapping/detrapping involving a distribution of trapping levels. The intensity dependence of τn (τn∝jdc−0.62) may indicate that the Back Reaction of electrons with I3− is not first order in electron concentration. Other possible explanations are that the interfacial electron transfer rate constant depends on trap occupancy or on the rate of surface or bulk electron diffusion.
R P Woodard - One of the best experts on this subject based on the ideXlab platform.
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causality implies inflationary Back Reaction
Journal of High Energy Physics, 2017Co-Authors: S Basu, N C Tsamis, R P WoodardAbstract:There is a widespread belief among inflationary cosmologists that a local observer cannot sense super-horizon gravitons. The argument goes that a local observer would subsume super-horizon gravitons into a redefinition of his coordinate system. We show that adopting this view for pure gravity on de Sitter Background leads to time variation in the Hubble parameter measured by a local observer. It also leads to a violation of the gravitational field equation R = 4Λ because that equation is obeyed by the full metric, rather than the one which has been cleansed of super-horizon modes.
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classical gravitational Back Reaction
arXiv: General Relativity and Quantum Cosmology, 2014Co-Authors: N C Tsamis, R P WoodardAbstract:The quantum gravitational Back-Reaction on inflation is based on the self-gravitation of infrared gravitons which are ripped out of the vacuum during inflation. The only quantum part of this process is the creation of gravitons; after they have emerged from the vacuum their behaviour is essentially classical. To test the thesis that a sufficiently dense ensemble of classical gravitons can hold the universe together in pure gravity with a positive cosmological constant, we compute the initial value and first time derivative of an invariant measure of the expansion rate for arbitrary classical initial value data. Our result is that the self-gravitation from the kinetic energy of an initial ensemble of gravitons can indeed slow expansion enough to hold the universe together.
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pure gravitational Back Reaction observables
Physical Review D, 2013Co-Authors: N C Tsamis, R P WoodardAbstract:After discussing the various issues regarding and requirements on pure quantum gravitational observables in homogeneous-isotropic conditions, we construct a composite operator observable satisfying most of them. We also expand it to first order in the loop counting parameter and suggest it as a physical quantifier of gravitational BackReaction in an initially inflating cosmology.
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no one loop Back Reaction in chaotic inflation
Physical Review D, 2002Co-Authors: L R Abramo, R P WoodardAbstract:We use an invariant operator to study the quantum gravitational Back Reaction to scalar perturbations during chaotic inflation. Our operator is the inverse covariant d'Alembertian expressed as a function of the local value of the inflaton. In the slow roll approximation this observable gives $\ensuremath{-}{1/(2H}^{2})$ for an arbitrary homogeneous and isotropic geometry; hence it is a good candidate for measuring the local expansion rate even when the spacetime is not perfectly homogeneous and isotropic. Corrections quadratic in the scalar creation and annihilation operators of the initial value surface are included using the slow-roll and long wavelength approximations. The result is that all terms which could produce a significant secular Back Reaction cancel from the operator, before one even takes its expectation value. Although it is not relevant to the current study, we also develop a formalism for using stochastic samples to study Back Reaction.
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Back Reaction is for real
Physical Review D, 2002Co-Authors: L R Abramo, R P WoodardAbstract:We demonstrate the existence of a secular Back Reaction on inflation using a simple scalar model. The model consists of a massless, minimally coupled scalar with a quartic self-interaction which is a spectator to $\ensuremath{\Lambda}$-driven inflation. To avoid problems with coincident propagators, and to make the scalars interact more like gravitons, we impose a covariant normal ordering prescription which has the effect of removing tadpole graphs. This version of the theory exhibits a secular slowing at three loop order due to interactions between virtual infrared scalars which are ripped apart by the inflating Background. The effect is quantified using an invariant observable and all orders bounds are given. We also argue that, although stochastic effects can have either sign, the slowing mechanism is superimposed upon them.
Noel W Duffy - One of the best experts on this subject based on the ideXlab platform.
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a novel charge extraction method for the study of electron transport and interfacial transfer in dye sensitised nanocrystalline solar cells
Electrochemistry Communications, 2000Co-Authors: Noel W Duffy, Laurence M. Peter, R M G Rajapakse, K.g.u WijayanthaAbstract:Abstract A novel charge extraction method has been developed to study the transport, trapping and Back Reaction of photogenerated electrons in dye sensitised nanocrystalline cells (Gratzel cells). The cell is illuminated at open circuit until a steady state is reached in which the rates of photogeneration and of Back Reaction of electrons with tri-iodide are equal. The illumination is then interrupted, and the electron density is allowed to decay for a given time in the dark before short circuiting the cell using a solid state switch. For high efficiency cells, the integrated current measured at short circuit corresponds closely to the remaining electronic charge in the film. Small corrections are required to account for Back Reaction and substrate charging. The delay time between interruption of the illumination and short circuit charge extraction is varied systematically to follow the decay of electron concentration. Analysis of the time dependence of the electron charge indicates that the Back Reaction of electrons with I 3 − is second order in electron density, which is consistent with the formation of I 2 −. as an intermediate. Simultaneous measurement of the charge and photovoltage decay curves shows that the density of trap states decreases exponentially with trap depth.
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investigation of the kinetics of the Back Reaction of electrons with tri iodide in dye sensitized nanocrystalline photovoltaic cells
Journal of Physical Chemistry B, 2000Co-Authors: Noel W Duffy, L.m Peter, And R M G Rajapakse, K.g.u WijayanthaAbstract:A novel technique has been developed to study the kinetics of the Back Reaction of electrons with I3- in dye-sensitized nanocrystalline cells. A solid-state switched operational amplifier feedBack circuit controlling the cell is used to alternate open-circuit and short-circuit conditions. In the experiment, electrons are injected by the photoexcited dye during illumination at open circuit, and then the subsequent decay of the open-circuit photovoltage in the dark is recorded up to a given time, at which the cell is short-circuited abruptly. The electron charge extracted at short circuit is measured by a current amplifier and integrator device. The kinetics of electron decay at open circuit has been studied by varying the delay between interrupting the illumination and short-circuiting the cell. Analysis of the time dependence of the electron density has established that the decay process is second-order in the total electron concentration. This is consistent with a mechanism involving the formation of I2-...
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characterisation of electron transport and Back Reaction in dye sensitised nanocrystalline solar cells by small amplitude laser pulse excitation
Electrochemistry Communications, 2000Co-Authors: Noel W Duffy, Laurence M. Peter, K.g.u WijayanthaAbstract:The characterisation of the transport and interfacial Reaction of electrons in dye-sensitised nanocrystalline solar cells is complicated by the non-linearity of these processes. This problem has been overcome by superimposing small amplitude pulsed laser excitation on steady Background illumination. The laser perturbation of the photostationary state is sufficiently small that the photocurrent and photovoltage responses can be fitted using constant values of the electron diffusion coefficient Dn and electron lifetime τn. Analytical and finite difference solutions of the continuity equation have been used to analyse the experimental photocurrent, photocharge and photovoltage transients, and the intensity dependence of Dn and of τn has been established by varying the bias illumination level, and hence the dc photocurrent density, jdc. The intensity dependence of Dn (Dn∝jdc0.68) is attributed to trapping/detrapping involving a distribution of trapping levels. The intensity dependence of τn (τn∝jdc−0.62) may indicate that the Back Reaction of electrons with I3− is not first order in electron concentration. Other possible explanations are that the interfacial electron transfer rate constant depends on trap occupancy or on the rate of surface or bulk electron diffusion.
Robert H Brandenberger - One of the best experts on this subject based on the ideXlab platform.
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Back Reaction and the trans planckian problem of inflation reexamined
Physical Review D, 2005Co-Authors: Robert H Brandenberger, Jerome MartinAbstract:It has recently been suggested that Planck scale physics may effect the evolution of cosmological fluctuations in the early stages of cosmological inflation in a nontrivial way, leading to an excited state for modes whose wavelength is super-Planck but sub-Hubble. In this case, the issue of how this excited state Back-reacts on the Background space-time arises. In fact, it has been suggested that such Back-Reaction effects may lead to tight constraints on the magnitude of possible deviations from the usual predictions of inflation. In this note we discuss some subtle aspects of this Back-Reaction issue and point out that rather than preventing inflation, the Back-Reaction of ultraviolet fluctuations may simply lead to a renormalization of the cosmological constant driving inflation.
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Back Reaction and local cosmological expansion rate
Physical Review D, 2002Co-Authors: Robert H Brandenberger, Ghazal GeshnizjaniAbstract:We calculate the Back Reaction of cosmological perturbations on a general relativistic variable which measures the local expansion rate of the Universe. Specifically, we consider a cosmological model in which matter is described by a single field. We analyze Back Reaction both in a matter-dominated Universe and in a phase of scalar field-driven chaotic inflation. In both cases, we find that the leading infrared terms contributing to the Back Reaction vanish when the local expansion rate is measured at a fixed value of the matter field which is used as a clock, whereas they do not appear to vanish if the expansion rate is evaluated at a fixed value of the Background time. We discuss possible implications for more realistic models with a more complicated matter sector.
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termination of the phase of quintessence by gravitational Back Reaction
Physical Review D, 2001Co-Authors: Wenbin Lin, Xinmin Zhang, Robert H BrandenbergerAbstract:We study the effects of gravitational Back Reaction in models of quintessence. The effective energy-momentum tensor with which cosmological fluctuations Back react on the Background metric will in some cases lead to a termination of the phase of acceleration. The fluctuations we make use of are the perturbations in our present Universe. Their amplitude is normalized by recent measurements of anisotropies in the cosmic microwave Background; their slope is taken to be either scale invariant or characterized by a slightly blue tilt. In the latter case, we find that the Back-Reaction effect of fluctuations whose present wavelength is smaller than the Hubble radius but which are stretched beyond the Hubble radius by the accelerated expansion during the era of quintessence domination can become large. Since the Back-Reaction effects of these modes oppose acceleration, Back Reaction will lead to a truncation of the period of quintessence domination. This result impacts on the recent discussions of the potential incompatibility between string theory and quintessence.
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energy momentum tensor for cosmological perturbations
Physical Review D, 1997Co-Authors: Raul L Abramo, Robert H Brandenberger, Viatcheslav MukhanovAbstract:We study the effective energy-momentum tensor (EMT) for cosmological perturbations and formulate the gravitational Back-Reaction problem in a gauge-invariant manner. We analyze the explicit expressions for the EMT in the cases of scalar metric fluctuations and of gravitational waves and derive the resulting equations of state. The formalism is applied to investigate the Back-Reaction effects in chaotic inflation. We find that for long wavelength scalar and tensor perturbations, the effective energy density is negative and thus counteracts any preexisting cosmological constant. For scalar perturbations during an epoch of inflation, the equation of state is de Sitter-like.
