Charge Trapping in Polymers

Abstract

An investigation is described of the role of unsaturated groups in the molecular structure on charge trapping in synthetic polymers. An attempt has also been made to quantify the effect on carrier migration of doping these materials with halogens. Using a surface charge decay technique carrier mobility has been determined in vacuo for a number of synthetic polymers and also for both high-and low-density polyethylenes doped with halogens. These data are presented together with an account of concurrent measurements of optical absorption in the same materials. The data indicate a tendency for "pure polymers" with known higher unsaturation content to exhibit a lower carrier mobility. Increased mobility was observed with either bromine or iodine impregnation of polyethylene. The optical data show that the iodine enters the polymer as neutral molecules most of which emerge under atmospheric conditions. The remainder, however, form charge exchange complexes at unsaturation sites in the polymer. The role of complexes in carrier transport is discussed.The charge storage property of insulating materials--as exemplified in the electret--is becoming of widespread technological importance from microphones (6; and references therein) to prostheses (1). The elucidation of the underlying charge injection and carrier Key words: polymers, carrier mobility, conduction. migration processes in synthetic polymers is vital to the future utility of these materials in almost any capacity.Modern experimental techniques are yielding increasingly more definitive models of the basic mechanisms. It has been shown (3, 7) that carrier mobility in low conductivity polymers is extremely small, the carriers being predominantly located in traps. Carrier migration is described by a hopping process. Recently published data (2) indicate that both polymer crystallinity and molecular symmetry significantly affect the electret stability, that is, charge trapping, and further, that phenyl and unsaturated groups in the molecular structure can form traps. Previous work (4) has also shown that carrier mobility in these materials may be greatly increased by impregnating the polymer with iodine. This effect has been interpreted by postulating charge exchange complex formation in the polymer, the complexes effectively reducing trapping effects by "handing-on" the carriers. A charge exchange complex in the present context means an interaction between two species, the strength of which lies between a full chemical bond and a small orbital overlap.The present paper describes an investigation of the role of unsaturated groups in carrier trapping and an attempt to quantify the effects of doping on the transport mechanism. Using a surface-charge decay technique, carrier mobility has been determined in vacuo for a number of synthetic polymers and also for both high-density (HDPE) and low-density (LDPE) polyethylenes impregnated with halogens. An account of concurrent measurements of optical absorption in the same materials is also presented.The experimental r...

High-field electrical conduction in polyimide films

“…Since the substrate is a p-type silicon, it is likely that holes from silicon may be trapped in the surface states at the PI/Si interface. [17][18][19][20] These holes can be easily transferred to the PI film via a tunneling process with or without applied fields 21 since polyimide may have a variety of donor-like and acceptor-like states in the PI bulk, as well as at the PI/Si interface. After the first cycle of the I -F and C -F measurements, the second, third and fourth cycles were immediately followed.…”

PEO‐PVP‐NaIO4 Ion‐Conducting Polymer Electrolyte: Inspection for Ionic Space Charge Polarization and Charge Trapping

“…In the steady state after application of an external electric field to the electrodes, the mobile charge carriers can be trapped in the bulk of PEO close to the electrode . Note that such CT/SCP differs from the CT in localized traps, well known for the low‐ and high‐density polyethylenes where the CT and the lower mobility of the charge carriers are due to the unsaturated groups in the molecular structure of these polymers . Further, the CT process considered here also differs from the CT in PEO‐based hybrid electret systems when this polymer is modified by incorporating of functional chemical moieties in the repeating subunits of ethylene oxide composing PEO .…”

“…For this, a significant role plays the formation of coordination complex between Na + ion and the network of the PEO‐PVP polymer blend. Such conclusion is consistent with the common conception that the molecular complexes formed in polymeric materials reduce the CT effects . For the solid ion‐conductive polymer‐salt electrolyte materials, like the coupled Na + ‐polymer systems inspected here, the strongly reduced charge trapping and space charge polarization are highly desirable in view of the enhancement of the ion mobility, ion conductivity and electrochemical performance.…”

Structural characterization and thermally stimulated discharge conductivity (TSDC) study in polymer thin films

“…As the dopant concentration is increased, the dopant molecules start bridging the gap separating the two localized states and lowering the potential barrier between them, thereby facilitating the transfer of charge carrier between two localized states. Work reported earlier (Herman and Rembaum 1967;Davies and Lock 1973) suggests the formation of 1 : 1 charge transfer (CT) complex between donor naphthalene and acceptor polystyrene.…”