Article pubs.acs.org/JPCC
Time-of-Flight Current Shapes in Molecularly Doped Polymers: Effects of Sample Thickness and Irradiation Side and Carrier Generation Width Andrey P. Tyutnev National Research University Higher School of Economics, 20 Miasnitskaya Ulitsa, Moscow 101000, Russia
David S. Weiss Department of Chemical Engineering, University of Rochester, Rochester, New York 14627, United States
David H. Dunlap Department of Physics and Astronomy and Consortium of the Americas for Interdisciplinary Studies, 1919 Lomas Blvd NE, Albuquerque, New Mexico 87131, United States
Vladimir S. Saenko National Research University Higher School of Economics, 20 Miasnitskaya Ulitsa, Moscow 101000, Russia ABSTRACT: The time-of-flight (TOF) current transients from solution-cast, free-standing films of p-diethylaminobenzaldehyde diphenyhydrazone in bisphenol A polycarbonate (DEH:PC) have been studied using electron gun induced charge generation. Changes in the shape of the current transient, cusp formation in particular, with film thickness, electron-beam penetration depth, and the side of the sample irradiated, have been analyzed with a two-layer multiple trapping model, and indicate that the time-of-flight transients of solution-cast films can be problematic in that plateau formation does not necessarily imply nondispersive charge transport. The results are consistent with the existence of thin surface layers which are depleted of the hole transport material. The depleted layer on the surface of the film that was exposed to air during coating/drying is always thicker than it is for the surface contacting the substrate. Depletion could occur through transport material sublimation, surface physical characteristics such as porosity, inhomogeneous solvent evaporation, or some other mechanism. applied field (V/cm). Ideally, the current j(t) will be constant until the sheet of charge reaches the counter electrode, at which time it will drop to zero. The transit time ttr is the time when this change occurs. However, in practice the current transient exhibits an initial spike before equilibration. If the charge motion is nondispersive, this is followed by a flat or gently decreasing plateau and a broad tail.6 If the charge motion is dispersive, the current continues to decay from the initial spike without any distinguishing characteristics.7 The transit time in this case is determined by locating the inflection point in a log− log plot of current vs time. Occasionally, the current increases after the decay of the initial spike, forming a cusp before the
1. INTRODUCTION An understanding of charge transport in amorphous materials is necessary to optimize the performance of organic electronic devices such as light-emitting diodes, electrophotographic copiers and printers, solar cells, and field-effect transistors.1−4 Charge mobility, as determined with the time-of-flight (TOF) methodology, is commonly used to characterize charge transport in thin films.5 Typically in a TOF experiment the thin film sample is treated as a parallel plate capacitor and a small amount of charge, relative to CV (where C is capacitance and V the applied voltage), is generated in a thin layer using light that is strongly absorbed by the charge-transporting material. The sheet of charge drifts under the influence of the applied field to the counter electrode, and the current vs time transient is used to extract the transit time. From the transit time, the mobility is calculated as the velocity (cm/s) per unit © 2014 American Chemical Society
Received: November 19, 2013 Revised: January 28, 2014 Published: February 26, 2014 5150
dx.doi.org/10.1021/jp411377r | J. Phys. Chem. C 2014, 118, 5150−5158
The Journal of Physical Chemistry C
Article
was related to irradiation of the “free” vs the “release” film surface. However, the experiments were performed mainly with 7 keV beam electrons. In the experiments reported in this paper we carried out a systematic study of free vs release surface ebeam exposure as a function of the e-beam energy (low energies in particular) and the film thickness. These experiments have demonstrated that, in fact, cusp formation does depend on the choice of the irradiated film face. Charge generation near the free surface produces cusps for much higher electron energies than charge generation near the release surface.
tail.8−11 In these cases, it is difficult to accurately assign a transit time. Sometimes the cusp is ignored in the experimental fitting of the data. For example, although the “typical photocurrent transient” for EFTP:PC exhibited a cusp, its presence was not discussed in the analysis determining the transit time.10 Cusp formation has received various explanations including space charge perturbation (due to an excess of generated charge modifying the internal field),12 which is why the amount of generated charge is kept very low (
