Charge Carrier Transport in a Molecularly Doped Polymer: Dispersive versus Gaussian

Abstract

A typical molecularly doped polymer (bisphenol- A-polycarbonate containing 30 mass% of hydrazone DEH) has been put to thorough experimental examination by both time of flight and radiationinduced conductivity methods. A much improved measurement technique incorporating a computerassisted registration scheme that allows one to record a complete current transient over up to five decades in time in one single shot was employed. Studies of radiation-induced conductivity (conductivity proper as well as transit effects) uniquely prove the dispersive rather than the Gaussian transport of holes (majority carriers) in this polymer. The shape of the time-of flight current transients is strongly influenced by some extraneous factor, presumably surface traps, whose role in radiation-induced conductivity could hardly be detected. We describe the phenomenon quantitatively using a multiple trapping formalism. A critical discussion of the present situation in the field of charge carrier transport in disordered solids is also included.

Two-Layer Mutiple Trapping Model for Universal Current Transients in Molecularly Doped Polymers

“…The maximum charge generation rate occurs at a distance approximately equal to one-third of the maximum electron range, and is a factor of 2.3 larger than at the surface. The maximum electron range l m is given by the empirical formula , l m = l 0 ( E e E 0 ) 1.67 where E e is the energy of the electron beam, and E 0 = 43 keV and l 0 = 31 μm are empirical constants. The charge generation depth l is assumed equal to l m until l m exceeds the sample thickness, at which point l is taken to be equal to the sample thickness L .…”

“…Instead, we investigate here the idea that the flat plateau, universality, and the approximately t −2 current decay in the tails, as well as the cusps that occasionally occur, may all be caused by a delay of the charge carriers which arises, not just from traps right at the interface but from a trap-rich surface layer that penetrates from the interface into the bulk of the film. The idea that cusps may be caused by the presence of a surface layer in MDPs that has different trapping characteristics than the bulk has been suggested before. − Tyutnev, et al , have suggested that there is a thin, approximately one micrometer thick, trap-rich surface layer that causes the charge to be delayed before it crosses into the bulk. Indeed, according to this proposal it is the interplay between the depth of the charge generation region and the thickness of the surface region that determines the particular transient shape seen in any given time-of-flight measurement.…”

The role played by a polymer matrix in the transfer of charge carriers in molecularly doped polumers

“…The signal to be measured was recorded using a universal device for analog and digital information input, output, and processing aug mented by an electronic block for high frequency noise filtration. The experimental data were processed using the approach described in [2][3][4]. RUSSIAN …”

“…Extension of the experimental apparatus of studies with invoking the radiation induced variant of the time of flight method showed that the transport of charge carriers in several molecularly doped polymers was non equilibrium [2][3][4], and a plateau of time of flight curves was an artifact of the surface method for the generation of charge carriers [5]. These findings required a refinement of the Gaussian disorder model conclusions concerning the form of time of flight curves [6].…”

Hole transport in polystyrene and polycarbonate doped with polar dopants

“…In PC doped with polar molecules of p diethylami nobenzaldehyde diphenylhydrazone (DEH) with a dipole moment of 3.16 D, the hole transport is non equilibrium [3][4][5][6]. Note that a flat plateau is often observed on time of flight curves, a phenomenon that is considered evidence for the achievement of quasi equilibrium transport in the system.…”