Injection limited current

For higher barrier heights at the interface (0B>O.2eV) [85] the overall current flow in the device injection-limited current flow is strongly determined by the injection, especially at low applied voltages (see Fig. 9-27). 

Currem field characteristics measured wiih conjugated polymers sandwiched between an indium-tin oxide (ITO) anode and an aluminum cathode are usually hole dominated and are, consequently, appropriate for testing injection/lransport models for the case of unipolar current How. Data shown in Figure 12-1 refer to injection-limited currents recorded on typically 100 nm thick spin-coated films of derivatives of poly(y d/"fi-phenylenevinylene) (PPV) and a planarized poly(/ /" -pheny-leue) employing a Keilhley source measure unit. The polymers were ... 

A textbook example for the successful application of the model of Arkhipov et al. is the work of van Woudenbergh et al. [173]. More recently, Agrawal et al. [106] compared injection limited currents and space-charge-limited currents in a copper-phthalocyanine sandwich cell with TTO and Al electrodes. An analysis of experimental data yields consistent values for the width of the DOS distribution as well as for inter-site separation [174]. These studies support the model of thermally activated injection into a Gaussian DOS distribution of hopping sites and confirm the notion that disorder facilitates injection because it lowers the injection barrier, although the transport velocity decreases with increasing disorder. 

Agrawal R, Kumar P, Ghosh S, Mahapatro AK (2008) Thickness dependence of space charge limited current and injection limited current in organic molecular semiconductors. Appl Phys Lett 93 073311... 

Accordingly, the recombination zone generally varies with electric field. For a strongly field-dependent injection-limited-currents (ILC) (Sec. 4.3.2), Equation (154)... 

Figure 90 Injection-limited current j (normalized to jo) vs. applied electric field Fo for (a) l = 0.01, and (b) 0.5 nm, and different surface recombination rates tir(0)/is (as given in the figure). The tunneling constant for the surface recombination o<2 = 10 nm-1, other parameters as in Fig. 87. After Ref. 408. Copyright 1994 Wiley-VCH, with permission.

The voltage dependence of the injection-limited current resulting from this treatment, as well as experimentally observed I(V) characteristics are Fowler-Nordheim (FN)-like, i.e., similar to that obtained by tunneling through a triangular barrier. This similarity suggested a number of treatments that analyzed injection into OLEDs in terms of this model, which predicts that... 

Space-Charge Limited Versus Injection-Limited Current Mechanisms... 

Charge-Carrier Injection - Injection-limited Currents... 

In contrast to a direct injection of dc or ac currents in the sample to be tested, the induction of eddy currents by an external excitation coil generates a locally limited current distribution. Since no electrical connection to the sample is required, eddy current NDE is easier to use from a practical point of view, however, the choice of the optimum measurement parameters, like e.g. the excitation frequency, is more critical. Furthermore, the calculation of the current flow in the sample from the measured field distribution tends to be more difficult than in case of a direct current injection. A homogenous field distribution produced by e.g. direct current injection or a sheet inducer [1] allows one to estimate more easily the defect geometry. However, for the detection of technically relevant cracks, these methods do not seem to be easily applicable and sensitive enough, especially in the case of deep lying and small cracks. 

A.J. Campbell, D.D.C. Bradley, H. Antoniadis, M. Inbasekaran, W.W. Wu, and A.P. Woo, Transient and steady-state space-charge-limited currents in polyfluorene copolymer diode structures with ohmic hole injection contacts, Appl. Phys. Lett., 76 1734-1736, 2000. 

For p-type electrodes, the cathodic current is carried at low overvoltages by the minority carriers (electrons) in the conduction band and is controlled at high overvoltages by the limiting current of electron diffusion the anodic current is carried by the mtqority carriers (holes) in the valence band and the concentration of interfacial holes increases with increasing anodic overvoltage until the Fermi level is pinned in the valence band at the electrode interface, where the anodic current finally becomes an electron injection current into the electrode. 

An example of the difficulties encountered when trying to fabricate an ohmic electrode, able to sustain a space-charge-limited current, is the recent work of the Neher group [179]. The authors deposited barium as an electron injection cathode on top of an electron transporting polymer based on a naphthalene diimide core whose LUMO is as low as 4 eV below vacuum level. Although the Fermi level of barium should be above the LUMO of the polymer, the electron current is. 

Koo Y-M, Choi S-J, Chu T-Y, Song O-K, Shin W-J, Lee J-Y, Kim JC, Yoon T-H (2008) Ohmic contact probed by dark injection space-charge-limited current measurements. J Appl Phys 104 123707... 

As soon as the concentration of free carriers (n ) injected from Ohmic contacts exceeds the concentration of carriers (no) produced by intrinsic or extrinsic excitation, space-charge-limited currents (SCLC) appear. In this case the Ohmic current... 

Ohmic contacts. An ohmic contact is defined as one which supplies a particular crystal with an infinite supply of either electrons or positive holes. Under an applied field these charge carriers are drawn into the material setting up a space charge. The subsequent currents are thus termed space-charge limited currents. In general the activation energy required to inject a positive hole from an electrode of work function W into a crystal is Ic — W and that to inject an electron W — Ac. Thus for ohmic contacts the conditions to be satisfied for holes and electrons are respectively (12a) and (12b). Although... 

The enclosed area of the curve is a measure of the reversible charge delivery capacity of the electrode. For fast current pulses, a different measurement method is used to determine the charge injection limits without gassing. Current pulses are applied and the voltage over two electrodes is monitored just below the gassing limits (Fig. 24). Once having passed the extensive in vitro testing, the electrodes were tested in chronic animal experiments. 

---