Double carrier injection

Studies of double carrier injection and transport in insulators and semiconductors (the so called bipolar current problem) date all the way back to the 1950s. A solution that relates to the operation of OLEDs was provided recently by Scott et al. [142], who extended the work of Parmenter and Ruppel [143] to include Lange-vin recombination. In order to obtain an analytic solution, diffusion was ignored and the electron and hole mobilities were taken to be electric field-independent. The current-voltage relation was derived and expressed in terms of two independent boundary conditions, the relative electron contributions to the current at the anode, jJfVj, and at the cathode, JKplJ. 

Figure 64 Current-voltage characteristics for single-layer (SL) EL cells based on anthracene (a) and tetracene (b) single crystals with unipolar and double injection contacts. Na/K-Na/K mononegative carrier injection, Au-H20 mono-positive carrier injection, Au-Na/K double injection. The crystal thicknesses are 98 and 108 pm for anthracene and tetracene, respectively. The slopes of the straight-line segments for tetracene characteristics are given nearby the curves. After Ref. 51.

Thus, the key feature of photocurrent multiplication is a majority carrier injection step (reaction 34b or 34d) from a reaction intermediate (usually a free radical) into the semiconductor CB or VB, respectively. Thus, in the examples above, each photon generates two carriers in the external circuit, affording a quantum yield (in the ideal case) of 2. This is the current-doubling process. 

Based on the three basic OLED structures, the new multilayer structures (MH1-3) are proposed as summarized in Fig. 2.5b. MH-1 and MH-2 structures are composed of double HTLs54-56 and ETLs,57,58 as shown in Figs. 2.5a and 2.5b. The meaning of the double layers is a separation of carrier injection from an electrode and carrier transport. The following are five major requirements that the HTL should have ... 

The final configuration (Fig. 5.9) allows double sample injection, which can be used for similar purposes as the previous configuration or for injection of two different reagents. Thus a triple zone, very rich on information (which advantageously might be decoded with the aid of che-mometrics) is obtained by a mutual dispersion of sample and reagents in an inert carrier stream. 

Figure 8.1. Random access FI A system furnished with a double barrelled injection valve. In the load position top) sample (5) and reagent (/ ) are by means of pump P2 aspirated into the injection valve. During this sequence the carrier stream (C) propelled by pump FI is by-passing the valve. In the inject position bottom) the two zones are by means of stream C carried into the FI A manifold, chasing and penetrating into each before reaching the detector (D). By this approach a composite readout due to sample alone, reaction product, and reagent blank is obtained.

A more effective carrier confinement is offered by a double heterostructure in which a thin layer of a low-gap material is sandwiched between larger-gap layers. The physical junction between two materials of different gaps is called a heterointerface. A schematic representation of the band diagram of such a stmcture is shown in figure C2.l6.l0. The electrons, injected under forward bias across the p-n junction into the lower-bandgap material, encounter a potential barrier AE at the p-p junction which inliibits their motion away from the junction. The holes see a potential barrier of... 

Fig. 2. Schematic diagram of active layer stmctures employed in LEDs under forward bias showing the conduction band (CB) and valence band (VB). The simplest devices employ (a) a homostmcture active layer wherein the bandgap is constant throughout the device. More advanced stmctures consist of (b) single and (c) double heterostmctures. Heterostmctures faciUtate the confinement and injection of carriers in the active region where the carriers may...

Volume of analyte injected As the volume of sample and standard solution injected increases, the peak height and duration increase. Nevertheless, if the volume increases too much, dispersion of the carrier into the analyte is limited and the concentration gradient within the flow is distorted resulting in very high dispersion values at the edges and very low values in the center. As a result, double peaks are recorded that are not appropriate for analytical measurement. 

There have been many investigations of photoinduced effects in -Si H films linked to material parameters. Changes have been observed in the carrier diffusion length, unpaired spin density, density of states in the gap, and infrared transmission. The transition from state A to B seems to be induced by any process that creates free carriers, including x-ray radiation and injection (double) from the electrodes. Because degradation in a solar cell is accentuated at the open-circuit voltage conditions, the A to B transition occurs upon recombination of excess free carriers in which the eneigy involved is less than the band gap. It has been pointed out that this transition is a relatively inefficient one and the increase in spin density takes place at a rate of 10-8 spins per absorbed photon. 

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