Trapping of excitons at chemical defects in polyethylene

Ceresoli, Davide; Tosatti, Erio; Scandolo, Sandro; Santoro, Giuseppe E.; Serra, Stefano

doi:10.1063/1.1783876

Cited by 44 publications

(37 citation statements)

References 24 publications

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“…Such defects can introduce charge carrier (or defect) states within the band gap of PE, can act as traps and sources of charge carriers, catalyze further damage, and can deleteriously affect the overall conduction behavior of the insulator. [2][3][4][5][6] It is thus critical that a firm understanding of the nature of such defects be obtained.…”

Section: Introductionmentioning

confidence: 99%

Unraveling the luminescence signatures of chemical defects in polyethylene

Chen

Huan

Wang

et al. 2015

The Journal of Chemical Physics

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Chemical defects in polyethylene (PE) can deleteriously downgrade its electrical properties and performance. Although these defects usually leave spectroscopic signatures in terms of characteristic luminescence peaks, it is nontrivial to make unambiguous assignments of the peaks to specific defect types. In this work, we go beyond traditional density functional theory calculations to determine intra-defect state transition and charge recombination process derived emission and absorption energies in PE. By calculating the total energy differences of the neutral defect at excited and ground states, the emission energies from intra-defect state transition are obtained, reasonably explaining the photoluminescence peaks in PE. In order to study the luminescence emitted in charge recombination processes, we characterize PE defect levels in terms of thermodynamic and optical charge transition levels that involve total energy calculations of neutral and charged defects. Calculations are performed at several levels of theory including those involving (semi)local and hybrid electron exchange-correlation functionals, and many-body perturbation theory. With these critical elements, the emission energies are computed and further used to clarify and confirm the origins of the observed electroluminescence and thermoluminescence peaks.

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Section: Introductionmentioning

confidence: 99%

Unraveling the luminescence signatures of chemical defects in polyethylene

Chen

Huan

Wang

et al. 2015

The Journal of Chemical Physics

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“…Although ΔSCF has gained some traction recently as a DFT-based alternative to TDDFT for excited states 24,42,[44][45][46] , the performance and range of validity of the method remain poorly understood. This paper addresses this gap in understanding in two ways: first, by comparing excitation energies computed by TDDFT and ΔSCF with experimental values for a representative set of conjugated organic molecules; and second, by providing new insight into the approximations that are made when computing excitation energies from ΔSCF.…”

Section: Introductionmentioning

confidence: 99%

Assessment of the ΔSCF density functional theory approach for electronic excitations in organic dyes

Kowalczyk¹,

Yost²,

Voorhis³

2011

The Journal of Chemical Physics

174

206

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This paper assesses the accuracy of the ΔSCF method for computing low-lying HOMO→LUMO transitions in organic dye molecules. For a test set of vertical excitation energies of 16 chromophores, surprisingly similar accuracy is observed for time-dependent density functional theory and for ΔSCF density functional theory. In light of this performance, we reconsider the ad hoc ΔSCF prescription and demonstrate that it formally obtains the exact stationary density within the adiabatic approximation, partially justifying its use. The relative merits and future prospects of ΔSCF for simulating individual excited states are discussed.

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“…Previous work has identified physical, (conformational) [52,55] and chemical (impurities and decomposition products) [57][58][59] electron traps in models of polyethylene and characterized them using ab initio methods. Some of this information was used to create a preliminary distribution function representing the density of trap states (DoS) as a function of electron energy and employed in a Monte Carlo simulation to predict the current-voltage [65] characteristics of model polyethylene, showing how once the DoS is known the electrical properties can be estimated.…”

Section: Discussionmentioning

confidence: 99%

“…They also however provide the means to go further and begin to investigate the influence of trapped electrons, the space charge, on the surrounding material. For example there has been very little work on the fate of the energy given up by trapped electrons in polyethylene (but see [58] for trapped excitons) which may alter the environment leading to local damage and ageing. The very recent DFT results [54] suggesting PE degradation by exciton recombination provide an interesting scenario that needs to be confirmed.…”

Section: Discussionmentioning

confidence: 99%

“…A similar methodology was used to estimate trapping energies for chemical defects and an approximate excess electron density of states obtained. Other workers have used similar methods [57][58][59]. Some results for chemical defects are given in table 1, clearly there are some very deep traps (>0.5 eV) but also some shallow traps (<0.5eV) which overlap in energy with those caused by conformational defects.…”

Section: Electronic Properties Of Pementioning

confidence: 98%

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Molecular modeling and electron transport in polyethylene

Wang

Cubero

et al. 2014

IEEE Trans. Dielect. Electr. Insul.

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Polyethylene is commonly used as an insulator for AC power cables. However it is known to undergo chemical and physical change which can lead to dielectric breakdown. Despite almost eighty years of experimental characterization of its electrical properties, very little is known about the details of the electrical behaviour of this material at the molecular level. An understanding of the mechanisms of charge trapping and transport could help in the development of materials with better insulating properties required for the next generation of high voltage AC and DC cables. Molecular simulation techniques provide a unique tool with which to study dielectric processes at the atomic and electronic level. Here we summarise simulation methodologies which have been used to study the properties of PE at the molecular level, elucidating the role of morphology in the trapping of excess electrons. We find that polyethylene has localised states due to conformational trapping extending below the mobility edge (above which the excess electrons are delocalised), at -0.1±0.1eV with respect to the vacuum level. These trap states with localisation lengths between 0.3 and 1.2nm have energies as low as -0.4±0.1eV in the amorphous and interfacial regions of polyethylene with more positive values in lamella structures. Crystalline regions have a mobility edge at +0.46 ±0.1eV, so we would expect transport by electrons excited above the mobility edge to delocalised states to be predominantly through amorphous regions if they percolate the sample.

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Trapping of excitons at chemical defects in polyethylene

Cited by 44 publications

References 24 publications

Unraveling the luminescence signatures of chemical defects in polyethylene

Unraveling the luminescence signatures of chemical defects in polyethylene

Assessment of the ΔSCF density functional theory approach for electronic excitations in organic dyes

Molecular modeling and electron transport in polyethylene

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