Unraveling the luminescence signatures of chemical defects in polyethylene

Abstract

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.

Electronic Structure of Polyethylene: Role of Chemical, Morphological and Interfacial Complexity

“…The electronic structure of PE with these defects were examined by computing the Kohn-Sham energy levels, and thermodynamic and optical charge transition levels involving different charged states. These characteristic signatures can be related to measured luminescences spectra, as done previously 3, 26, 37 .…”

“…The differences between and are due to the structural relaxation of PE in the vicinity of the defects during charging and discharging which are included in but not in . Because and were computed from the total energy, they are physically relevant and thus, can be used to unveil the origins of electro-, photo-, and thermo-luminescences 3, 37 as well as other experimentally measured properties of PE.…”

“…It is given by Ref. 37 Here, is the formation energy of the q -charged defect at its equilibrium structure R q , which can be obtained from DFT calculations. The Fermi energy is taken from the VBM to the CBM of the defect-free PE.…”

“…Computations have not been easy to perform at the requisite level of theory for such large systems, and so are typically undertaken for parts of (idealized versions of) the real system 33, 35, 38, 39 . Available luminescence and charge injection barrier measurements are hard to unravel due to the multitude of physical and chemical complexities involved, and are hard to correlate to available computational work 37 .

Figure 1( a ) Schematic structure of an Al/PE interface, containing crystalline and amorphous regions, and populations of chemical defects. Here, C, H, and O atoms are shown in black, white, and red, respectively.

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Structural and Electronic Properties of Defect‐Free and Defect‐Containing Polypropylene: A Computational Study by van der Waals Density‐Functional Method

“…Dienone (CH 2 CHCHO) is the combination of a double bond and a carbonyl defect. These types of defects typically occur in polyethylene (PE), in which context they have been studied both computationally and experimentally . Given the similar nature of chemical bonds in PE and PP, these types of defects are likely to occur in PP.…”