Charge carrier injection and transport

When we apply 10 V to a typical 100-nm-thick organic layer, we need to watch for an unexpected high electric field (106 V/cm2), which enables us to induce carrier injection and SCLC. First, we turn our attention to the behavior of current injection from electrodes. We have two possible mechanisms to inject charge carriers Schottky thermal emission and the tunneling injection processes, both of which are based on the theory of inorganic semiconductors. The Schottky emission process is described by41 

FIGURE 2.1. Two possible carrier injection mechanisms at the organic/metal electrode interface (a) Schottky-type carrier injection via impurity or structural disordered levels with thermal assistance and (b) Fowler-Nordheim tunneling carrier injection with the assistance of a local high electric field (106—107 V/cm). 

On the basis of this equation, even with the applied electric field of 106 V/cm, the estimated tunneling distance exceeds 10 nm, which is not a realistic value. Thus, we have to anticipate the application of local high electric field (= 107 V/cm), between a molecule and a metal electrode otherwise, the practical energy barrier must be lower than the estimated value if we want to use the tunneling mechanism. 

Esclc = external field (V/d) + internal built in field (Eint) Ohm current 

FIGURE 2.2. (a) Schematic view of SCLC. With an external electric field (V/d), an additional internal field (E ) induced by injected excess charge carriers plays important role for the achievement of high current density. The total electric field, Etotai = V/d + E, significantly enhances current flow. Open symbols (o) corresponds to excess holes, closed symbols ( ) correspond to excess electrons, (b) Comparison of J—V characteristics based on ohmic current (dashed line) and SCLC (solid line). The meshed area shows the J—V requisite for practical devices. 

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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>... 

The electroluminescence was located in the part of the anthracene crystal next to the hole-injecting anode, which suggests an imbalance of charge-carrier injection and transport. The intensity of light was linearly proportional to the... 

In a number of studies of carrier injection and transport in DNA, optical excitation of Ti02 quantum dots has been used to inject carriers into dna. " " In many of these cases, the subsequent transport of charge in DNA wires has been studied using gel... 

The analytic theory outlined above provides valuable insight into the factors that determine the efficiency of OI.EDs. However, there is no completely analytical solution that includes diffusive transport of carriers, field-dependent mobilities, and specific injection mechanisms. Therefore, numerical simulations have been undertaken in order to provide quantitative solutions to the general case of the bipolar current problem for typical parameters of OLED materials [144—1481. Emphasis was given to the influence of charge injection and transport on OLED performance. 1. Campbell et at. [I47 found that, for Richardson-Dushman thermionic emission from a barrier height lower than 0.4 eV, the contact is able to supply... 

In this chapter, we showed that OLED performance is clearly defined based on the simplified working mechanism of carrier injection and successive SCLC. The balance of holes and electrons injected within an emitter layer is a major factor contributing to overall EL quantum efficiency. The unipolar-charged-transport layers, HTL and ETL, contribute to the increase of quantum efficiency. 

Although the optical properties of polyfluorenes are attractive, the electrical properties are, if anything, even more so. Charge carrier mobilities are a key factor in determining the performance of polymer LEDs because of the requirement of both balanced injection and transport of electrons and holes. Charge carrier mo-... 

As for a6T, Mg led to superior rectifying behaviour for ECnT thin films, whereas the electroluminescence yields are not enhanced for lower workfunction electrodes [313], The latter unexpected effect could be due to different indiflusion of the metals or chemical reactions at the interface, as discussed in Section 6.1. From asymmetric cells comprising two different ECnTs it can be concluded that electroluminescence always takes place in the layer at the electron-injecting electrode. Thus the hole-injection and transport seems to be more efficient if compared with the electron transfer. The majority charge carriers are therefore holes injected at the ITO electrode. 

The material is assumed to be an insulator, i.e. the thermally-generated charge-carrier density is so small that it makes no noticeable contribution to the transport and therefore can be neglected in the model. All the charge carriers which participate in the transport or are captured in traps are excess charge carriers injected from the contacts. 

When in the stationary state equal numbers of holes and electrons are injected per unit hme, but e.g. the hole mobility is much higher than the electron mobility, then in a single-layer OLED, the recombination occurs very near the cathode. This leads as a rule to an increase in non-radiative recombination and thus to a reduction of the Hght yield. If different numbers of holes and electrons are injected and transported per unit time, then that part of the charge carriers which is in excess, i.e. a part of the current, cannot contribute to the production of Ught This too reduces the efficiency of the OLED. 

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