Injection, carrier

Information on charge carrier injection was obtained from analysing contact resistances in OFETs. In contrast to Section 17.5, we now use transfer curves 

Elaborating upon these ideas Arkhipov et al. calculated the dependence of the injection current on electric field temperature of the DOS and strength of intersite coupling. 

The reason for the success of this type of data fitting is that for moderately large barriers it becomes unimportant whether escape for the image potential is treated within the framework of the Onsager or the RS model. An indication that die Onsager description is, nevertheless, more appropriate is the intersection of 7(F) curves for variable temperature at high electric fields. This is a characteristic feature of Onsager processes [24]. 

For PPV-imine and PPV-ether the oxidation potential, measured by cyclic voltammetry using Ag/AgCl as a reference are o v=0.8eV and 0.92 eV, respectively. By adopting the values 4.6 eV and 4.8 eV for the tvork functions of a Ag/ AgCl and an ITO electrode, respectively, one arrives at zero field injection barriers of 0.4 and 0.55 eV. These values represent lower bounds because cyclic voltammetry is carried out in polar solvents in which the stabilization energy of radical ions exceeds that in a polymer film, where only electronic polarization takes place. values for LPPP and PPPV are not available but in theory they should exceed those of PPV-imine and PPV-ether. 

The above strategy was tested [27] with a 3-layer LED consisting of a poly(2,5-thienylene vinylene) (PTV) layer, known to have particularly low oxidation potential [28], followed by a layer of l,4-fciT-(4 -diphenylaminostyryl)-2,5-di-methoxy-benzene (DASMB) [29] and a layer of 2-(4-biphenyl)-5-(4-tert-butyl-pheenyl)-l,3,4-oxadiazol (PBD) dispersed in polystyrene (PS) in a 20 80 ratio. Films of poly-(2,5-thienylene-a-bromoethylene) were obtained by vapor phase pyrolysis of 2,5-fcA-(bromomethyl)thiophene and subsequent vapor deposition of the quinoid monomers onto a cold substrate following a previously published procedure [30]. They were converted to PTV by temperature-induced elimination of HBr. 

One can, nevertheless, conclude that (i) there is only a very small barrier for hole injection from ITO to PTV, if any barrier at all, (ii) a finite energy should exist for hole transport across the PTVIDASMB interface, and (iii) PBD should act as an efficient internal blockade for hole transport towards the cathode. 

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A band diagram of a biased n-p-n BIT is shown in figure C2.16.8. Under forward bias, electrons are injected from tlie n type emitter, giving rise to tlie current 7. flowing into tlie p type base. Some of tlie carriers injected into tlie base recombine in tlie base or at tlie surface. This results in a reduction of tlie base current by 7, tlie lost recombination current, and tlie base current becomes 7g = At tlie same time, holes are injected from tlie... 

The metallic electrode materials are characterized by their Fermi levels. The position of the Fermi level relative to the eneigetic levels of the organic layer determines the potential barrier for charge carrier injection. The workfunction of most metal electrodes relative to vacuum are tabulated [103]. However, this nominal value will usually strongly differ from the effective workfunction in the device due to interactions of the metallic- with the organic material, which can be of physical or chemical nature [104-106]. Therefore, to calculate the potential barrier height at the interface, the effective work function of the metal and the effective ionization potential and electron affinity of the organic material at the interface have to be measured [55, 107],... 

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. 

In electroluminescence devices (LEDs) ionized traps form space charges, which govern the charge carrier injection from metal electrodes into the active material [21]. The same states that trap charge carriers may also act as a recombination center for the non-radiative decay of excitons. Therefore, the luminescence efficiency as well as charge earner transport in LEDs are influenced by traps. Both factors determine the quantum efficiency of LEDs. 

In the following section an overview, of several models describing the charge carrier injection and transport of LEDs based on polymers and organic materials, is presented. The focus will be set on mctal/polymer (organic material)/nictal contacts based on a polymer with a low defect concentration will be discussed. A description of LEDs, based on iolymers with a high defect concentration e-m U>... 

Blom et al. [85] stated that the l/V characteristics in LEDs based on ITO/di-alkoxy-PPVs/Ca are determined by the bulk conductivity and not by the charge carrier injection, which is attributed to the low barrier heights at the interface ITO/PPV and PPV/Ca. They observed that the current flow in so called hole-only devices [80], where the work function of electrodes are close to the valence band of the polymer, with 1TO and Au as the electrodes, depends quadratically on the voltage in a logl/logV plot and can be described with following equation, which is characteristic for a space-charge-limitcd current (SCL) flow (s. Fig. 9-26) ... 

Charle, K. P. Spin-Dependent Kinetics in Dye-Sensitized Charge-Carrier Injection into Organic Crystal Electrodes 19... 

Charge-Carrier Injection into Organic Crystal Electrodes... 

The photocurrent is cathodic or anodic depending on the sign of the minority charge carriers injected from the semiconductor electrode into the electrolyte, i.e. the n-semiconductor electrode behaves as a photoanode and... 

Wt Activation energy for carrier injection from the contact into the dielectric 0.7 0.3-1.0 eV... 

The net carrier concentration, shown in Fig. 7.8, was obtained at a frequency of 100 kHz. DLTS spectra were recorded using reverse- and forward-bias modes in the temperature range of 80-350 K. In the re verse-bias mode, the devices were reverse biased from -1.2V to -0.2V, with a pulse width of 1 ms. Two hole (majority-carrier) trap levels were found in all the devices. These levels were designated as Hi at I iv+0.26 and H2, for which an activation energy could not be resolved. Upon minority-carrier injection (forward-bias mode), DLTS showed two additional electron (minority-carrier) traps, which are labeled Ei (Ec-0.1eV) and E2 (Ec-0.83eV) in Table 7.1. The spectra were measured at an emission time of 465.2 s and the width of the... 

No charge injection Minimum bias for charge Carriers injected... 

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