Charge electroluminescence devices

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. 

The efficient formation of singlet excitons from the positive and negative charge carriers, which are injected via the metallic contacts and transported as positive and negative polarons (P+ and P ) in the layer, and the efficient radiative recombination of these singlet excitons formed are crucial processes for the function of efficient electroluminescence devices. 

U.S. 5,536,949 Charge injection auxiliary material and organic electroluminescence device containing the same... 

This list can be divided into three main classes based mainly on function and redox state. First, applications that utilize the conjugated polymer in its neutral state are often based around their semi-conducting properties, as in electronic devices such as field effect transistors or as the active materials in electroluminescent devices. Secondly, the conducting forms of the polymers can be used for electron transport, electrostatic charge dissipation, and as EMI-shielding mate-... 

Figure 1-1. Schematic drawing of a single-layer electroluminescent device. An applied electric field leads to injection of holes (positive charges the majortiy charge carriers in polymers such as PPV) and electrons (usually the minority charge carriers) into the light-emitting polymer film from the two electrode contacts. Formation of an electron-hole pair within the polymer may then result in the emission of a photon. Since holes migrate much more easily through PPV than electrons, electron-hole recombination takes place in the vicinity of the cathode.

One of the important functions of metal complexes is charge transfer. This is realized mainly by redox reactions of the central metal ion in the ground as well as in excited states. When metal complexes are incorporated in a polymer matrix, the polymer matrix can take on the function of transporting charges in the dark or under light irradiation, which can lead to electronic and photonic devices. Please refer also Chapter 13. Another possibility where metal complexes are involved are electroluminescent devices in which a material is excited by applying a voltage, and lumineseence is observed. 

The optical transparency of poly(vinyl carbazole) films produced by this room temperature process appears to be quite high, although transparency decreases at high conversions. In film form, this material is useful for photoconductors, charge-transfer complexes, and electroluminescent devices. The higher polymer molecular weight typically enhances film mechanical properties [247]. 

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