Figure 4 - uploaded by R. Sh. Ikhsanov
Content may be subject to copyright.
As figure 3, except that F 0 = 10 6 (1), 10 7 (2), 3 × 10 7 (3) and 10 8 V m −1 (4). L = 3 μm for all curves.
Source publication
We examine the published data concerning the shape of the current transients obtained by the time of flight (TOF) technique for molecularly doped polymers and polyvinylcarbazole (PVK) and compare it with the predictions from the existing theories of charge carrier transport in disordered organic solids. We show that TOF current shapes frequently ru...
Context in source publication
Context 1
... L variation (curves 1 to 4 in figure 3) reduces W from 0.285 to 0.030 (theory predicts a 10-fold decrease), ˜ t being unchanged. Increasing F 0 ( figure 4) results in the plateau shortening (curves 1-3) and eventually in its disappearance (curve 4) as Gaussian transport gradually gives way to a dispersive one. ...
Similar publications
In order to elucidate hierarchically self-assembled structure, which is composed of multi- components of soft matters, Ibaraki university promotes “small-angle scattering project” covering methods with electron, X-ray and neutron beams. In this paper, we report a variety of small-angle neutron scattering (SANS) instruments, which have been establis...
The novel BioRef reflectometer currently under construction in neutron guidehall-I of the BER-2 reactor at the Helmholtz Centre Berlin is designed for applications in soft matter science at solid-liquid interfaces especially under dynamic conditions. In addition to a time-of-flight (TOF) approach to be realised with three choppers in order to adapt...
Skeletal muscle is a classic example of a biological soft matter . At both macro and microscopic levels, skeletal muscle is exquisitely oriented for force generation and movement. In addition to the dynamics of contracting and relaxing muscle which can be monitored with ultrasound, variations in the muscle force are also expected to be monitored. T...
Citations
... We name but a few, which in our opinion are most pressing. First of all, the true relation of the quasi-band and hopping carrier transport in polymers needs clarification, just as the dispersive-versus-Gaussian transport dilemma should be finally resolved at least in MDPs along the lines suggested in [57] and [58]. We would like to note that at large ratios of the total disorder energy σ to the thermal energy σ/kT (≥6) [33], TOF transients as given by the MTMg calculations very much resemble curves for the dispersive transport (for σ = 0.165, an equivalent α ≈ 0.5 at 295 K [59]) to such an extent that the sum rule of the latter is satisfied with a good accuracy [20], [46]. ...
... Therefore, a clear turning point between the plateau and the tail section well defineed the transit time either in a linear or log-log plot, when the degree of dispersion was reduced. In order to determine the dispersion property, we used the general expression describing the dispersivity as follows 55,82,83 : ...
... A lower W indicates a less spread of the carrier transport through materials, thus leading to an abrupt drop in the tail section instead of a slow descent. In general, the main idea of estimating the W parameter is for observing the effects of traps on molecularly doped materials because carrier spread was brought about by these trap states that may dominate the dispersivity of materials 55,82,83 . However, the carrier spread during the transport can be influenced by the photogenerated carrier profile, because the distance that required for reaching the Al electrode for the carriers generated at different positions cannot approximate to be constant when the charge-generation width is wide and comparable to the thickness of the material under test. ...
Time-of-flight (TOF) measurements typically require a sample thickness of several micrometers for determining the carrier mobility, thus rendering the applicability inefficient and unreliable because the sample thicknesses are orders of magnitude higher than those in real optoelectronic devices. Here, we use subphthalocyanine (SubPc):C70 as a charge-generation layer (CGL) in the TOF measurement and a commonly hole-transporting layer, N,N'-diphenyl-N,N'-bis(1,1'-biphenyl)-4,4'-diamine (NPB), as a standard material under test. When the NPB thickness is reduced from 2 to 0.3 μm and with a thin 10-nm CGL, the hole transient signal still shows non-dispersive properties under various applied fields, and thus the hole mobility is determined accordingly. Only 1-μm NPB is required for determining the electron mobility by using the proposed CGL. Both the thicknesses are the thinnest value reported to data. In addition, the flexibility of fabrication process of small molecules can deposit the proposed CGL underneath and atop the material under test. Therefore, this technique is applicable to small-molecule and polymeric materials. We also propose a new approach to design the TOF sample using an optical simulation. These results strongly demonstrate that the proposed technique is valuable tool in determining the carrier mobility and may spur additional research in this field.
... The transport molecule has been varied quite freely. Some examples are a pyrazoline compound (DEASP) [3,4], p-diethylaminobenzaldehyde-diphenyl hydrazone (DEH) [5,6], triphenylamine (TPA) [7,8], triphenylaminobenzene derivatives (EFTP) [9][10][11], and N-isopropyl Introduction carbazole (NIPC) [7,12,13]. A lot of the results are reviewed in Ref. [14] and [15]. ...
... 6 shows the mean energy of an ensemble of charge carriers as a function of time for a uniform, exponential, and Gaussian DOS distribution. All charges are inserted at random sites in the DOS at t = 1 and since all the DOS distributions have been chosen to have a mean value of zero, so will the charge carrier energy have at the time of insertion. ...
The transient current curves for a polar molecularly doped polymer were compared for the nearsurface and bulk generation of charge carriers. The horizontal plateau on the first curve was expected to transform into a tilted straight line under the quasiequilibrium transport conditions, but it did not. Instead, for bulk generation, the curve was a hyperbola, on which the slope decreased almost sevenfold during the existence of the plateau. This behavior points to nonequilibrium charge carrier transport. The appearance of a plateau in our case is explained by the effect of the defective surface layer, as indicated in our previous communications.
Transient conductivity measurements have been carried out in a hydrazone-polycarbonate molecularly doped polymer over wide dynamic ranges of time and current as a function of the applied electric field. These data are plotted using both linear-linear and log-log axes. The plots on linear-linear axes give familiar results: an initial spike followed by a relatively flat current before the transit time; a mobility that equals published values and precisely follows the Poole-Frenkel law, being exponential in the square root of the electric field; and a current after the transit time that falls much more slowly than can be accounted for using Gaussian statistics. The same data plotted using log-log axes reveals the behavior over long times and low currents. Before the transit time, the decrease of the current can be characterized by two power laws, but it only decreases by about a factor of 2 from 10(-2) of the transit time to the transit time and is almost independent of electric field from 5 to 80 V/mu m. After the transit time the current decreases algebraically as t(-1.75) to a current value of about 10(-2) of the value at the transit time and is also almost independent of electric field. Available theoretical models are not consistent with these data. We note a surprising coincidence: the current transients are virtually identical to the current transients obtained in molecular crystals, as if disorder does not play a role in determining the shape of the current transients in molecularly doped polymers.
Theoretical and experimental studies of the carrier transport in molecularly doped polymers (MDPs) have been reported. Theoretical analysis uses the multiple trapping (MT) model with an exponential and Gaussian trap distributions. Experimental technique is based on an electron gun technology enabling one to conduct time of flight measurements using the surface and the bulk carrier generation. The list of MDPs tested includes both polar and non-polar systems, some with varying dopant concentration. Experimental results are compared to the MT model predictions as well as the mainstream theories of the hopping conduction in MDPs.
A numerical packet has been developed to solve multiple-trapping equations with an exponential trap distribution without any restrictions on the dispersion parameter, which can be both smaller and greater than unity. Attention has been focused on current transients recorded upon the near-surface, as well as bulk, excitation of the polymer in the small-signal mode. The results have been compared with published data. The program has been used for a critical review of published experimental results obtained by the time-of-flight technique.
We continue to develop a new approach to description of charge kinetics in
disordered semiconductors. It is based on fractional diffusion equations. This
article is devoted to transient processes in structures under dispersive
transport conditions. We demonstrate that this approach allows us (i) to take
into account energetic and topological types of disorder in common, (ii) to
consider transport in samples with spatial distributions of localized states,
and (iii) to describe transport in non-homogeneous materials with distributed
dispersion parameter. Using fractional approach provides some specifications in
interpretation of time-of-flight experiments in disordered semiconductors.
We have investigated the bimolecular recombination of holes and electrons in a typical hole-conducting molecularly doped polymer (DEH-doped polycarbonate). The method used is a variant of the time-of-flight technique with the bulk generation of charge carriers. A small signal regime has been used to extract parameters of the multiple trapping model with an exponential as well as the Gaussian trap distributions. Then, using a large signal regime with the generation rate increasing sequentially in a power of 10, we compared model predictions with experiment. It was found that the best agreement is achieved once the Langevin recombination coefficient is used, this being defined by the microscopic mobility featuring in the models. The relevance of this fact to the published data has been addressed as well.
Using Monte Carlo simulation we investigated time of flight current transients predicted by the dipolar glass model for a random spatial distribution of hopping centers. Behavior of the carrier drift mobility was studied at room temperature over a broad range of electric field and sample thickness. A flat plateau followed by j∝t-2j∝t-2 current decay is the most common feature of the simulated transients. Poole–Frenkel mobility field dependence was confirmed over 5–200 V/μμm as well as its independence of the sample thickness. Universality of transients with respect to both field and sample thickness has been observed. A simple phenomenological model to describe simulated current transients has been proposed. Simulation results agree well with the reported Poole–Frenkel slope and shape of the transients for a prototype molecularly doped polymer.
