Injection limiting

Clearly, the optimal injection condition is for electrons and holes to be in balance, but this does not necessarily guarantee that b- I. For example, when both contacts are injection limited and the injected charge densities arc small, the prob-... 

Another issue that can be clarified with the aid of numerical simulations is that of the recombination profile. Mailiaras and Scott [145] have found that recombination takes place closer to the contact that injects the less mobile carrier, regardless of the injection characteristics. In Figure 13-12, the calculated recombination profiles arc shown for an OLED with an ohmic anode and an injection-limited cathode. When the two carriers have equal mobilities, despite the fact that the hole density is substantially larger than the electron density, electrons make it all the way to the anode and the recombination profile is uniform throughout the sample. 

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-nitrosamines HBr-AcOH with 4-(2-phthalimidyl)benzoyl chloride (pre-column derivatisation) EC Fluorescence 0.4-1.6 pmol per injection limits of detection [79]... 

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... 

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. 

The United States Pharmacopeia (USP) test (467) describes three different approaches to measuring organic volatile impurities in pharmaceuticals. Method I uses a wide-bore coated open tubular column (G-27, 5% phenyl-95 % methylpolysiloxane) with a silica guard column deactivated with phe-nylmethyl siloxane and a flame-ionization detector. The samples are dissolved in water and about 1 p is injected. Limits are set for benzene, chloroform, 1,4-dioxane, methylene chloride, and trichloroethylene. Methods V and VI are nearly identical to method I except for varying the chromatographic conditions. For the measurement of methylene chloride in coated tablets, the headspace techniques described above are recommended. 

FIG. 3.35. Experimental J—V characteristics of an ITO/PEDOT PSS/PCBM/Au injection limited electron current (triangles) and calculated space charge limited hole current in OC4C10-PPV (circles) for a thickness of L = 170 nm and temperature T = 290 K. The inserted figure represents die device band diagram under the flat band condition of a bulk heterojunction solar cell using Au as a top electrode [65]. 

The measured electron current in PCBM injected through Au electrode is shown in Fig. 3.35. Calculated SCL hole current in OC1C10-PPV is shown by circles in Fig. 3.35 for a thickness of L = 170 nm. Even with the high Schottky barrier of 1.4 eV (the barrier determined from the measured current is 0.76 eV) with A1 cathode for PCBM is not sufficient to suppress the electron current in PCBM below the hole current in OCiCio-PPV. With the large injection barrier the electron current is injection limited. This work shows that it is not possible to make hole only devices using OCiCio-PPV PCBM bulk heterostructures. 

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

In the case where the current flowing through the sample is injection limited (cf. Sec. 4.3.2), the recombination zone width becomes a complex function of applied field because the field decreasing F2// factor in Eq. (154) adds to the field-dependent mobility of w and often can dominate the electric field-induced changes in the recombination zone (cf. Sec. 5.4). 

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.

Possible techniques to reduce complications include avoiding Atkinson s position, the classical position for retrobulbar block (309), during injection, limiting the volume of solution injected, and the use of shorter needles (310,311). 

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... 

Black Widow spider venom contains several different protein fractions. The most significant component of the venom is the neurotoxin, a-latrotoxin. This neurotoxin acts at the presynaptic membrane of the neuronal and the neuromuscular junctions. The binding of the a-latrotoxin results in the opening of nonspecific cation channels, a massive influx of calcium, release of acetylcholine and norepinephrine and decreased uptake of the neurotransmitter. The neurotransmitter release is most likely responsible for hypertension, muscle fasciculations, and spasms frequently experienced by victims of a bite. Later, generalized muscle weakness and labored breathing may develop in severe cases. While the venom of the black widow spider has been characterized as being more potent than that of many poisonous snakes, the small amount of venom injected limits the degree of toxicity. 

Peyghambarian et al. modeled the dependence of the current flow and the efficiency of devices on various device parameters as the (balance of the) charge carrier mobility and the barrier height at the interfaces for devices, where the current flow is determined by Fowler-Nordheim tunneling (see Fig. 9-24) [83]. In this case, the current flow through the LEDs is injection limited and dominated by Fowler-Nordheim tunneling and the following characteristics are observed [83] ... 

Figure 13-12. The recombination profiles for an OLED with an ohmic anode and an injection-limited cathode for various ratios of the electron/hole mobilities. Reproduced with permission from [145],...

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