Injected electrodes

The materials used as the electron and hole injecting electrodes play a crucial role in the overall performance of the device and therefore cannot be neglected even in a brief review of the materials used in OLEDs. The primary requirements for the function of the electrodes is that the work function of the cathode be sufficiently low and that of the anode sufficiently high, to enable good injection of electrons and holes, respectively. In addition, at least one electrode must be sufficiently transparent to permit the exit of light from the organic layer. 

Y. Cao, Thin metal-oxide layer as stable electron-injecting electrode for light emitting diodes, PCTInt. Appl., WO 2000022683, pp. 37, (2000). 

The LEC structure that involves the addition of ionic dopants and surfactants to the printable inks enables the ability to print a top electrode without restriction by the work function of the metal. Silver, nickel, or carbon particle-based pastes are generally the preferred printable electron injecting electrodes however, the shape and size of the particles combined with the softening properties of the solvent can create electrical shorts throughout the device when printed over a thin polymer layer that is only several hundred nanometers thick. For optimal performance, the commercially available pastes must be optimized for printing onto soluble thin films to make a fully screen-printed polymer EL display. 

Poly( -phenylene vinylene) (PPV) was the first reported (1990) polymer to exhibit electroluminescence. PPV is employed as a semiconductor layer. As noted earlier, the layer was sandwiched between a hole-injecting electrode and electron-injecting metal on the other. PPV has an energy gap of about 2.5 eV and thus produces a yellow-green luminescence. Today, other materials are available, which give a variety of colors. 

An even simpler duplication process based on the same principle is shown in Figure 5.29(B). If a hole-injecting electrode is used instead of a photosensitive layer, simple electrostatic charging allows the pattern to be read and transferred.148 Other new technologies based on the photopatteming of polysilanes are being developed, and may become commercialized in the future. 

It has been shown that the EL of polysilane-based LEDs is emitted near the interface between the polysilane and the electron injecting electrode, because of the strong unipolar (hole conductive) nature of polysilanes. Defect levels existing at the interface are considered to play an essential role in the emission of EL in the visible region,93 and have both positive and negative effects on the LED characteristics. The positive space charges generated by... 

N-type HgCdTe epitaxial layers 30, surrounded by p-type CdTe resistance layers 20, are formed in an n-type CdTe substrate 10. The imager fiirther comprises storage electrodes 40 and charge injecting electrodes 50. 

The increase in the electric field with increasing distance from the injecting electrode has an important consequence current pulses are accelerated in the bulk of the sample, leading to a faster transit time than under space-charge-free conditions. 

Although LiF/Al electrodes are already widely used for enhancing the efficiency of electron injection electrodes for OLEDs, the underlying mechanisms are worth discussing. Several mechanisms can be suggested ... 

Figure 38 Relative increase in the monomolecular decay rate constant (A/ // t) (decrease in the lifetime) of triplet excitons in three different anthracene crystals under the positive voltage applied to two different hole injecting electrodes Cul (a) and anthracene positive ions (A+) in nitromethane (b). jSt = t 1 is the triplet decay rate constant with no voltage jh = 239 s-1 for the d = 350 pm-thick crystal, / t = 175s 1 for anthracene with d = 625 pm (from Ref. 243) / t = 200s 1 for the d = 320 pm-thick crystal, A+ injecting contact (see Ref. 238). In the right-top scale in part (b) the Aji/U vs. j/U2 is presented (points) to be compared with Eq. (115) (solid line).

Results for a 20 pm thick sample of polycarbonate containing 50 mass% TPD for a field of 1.5 x 107Vm-1 at 296 K with charges injected from an indium tin oxide (ITO) electrode coated with a 0.1 pm thick layer of PPV are shown in Fig. 8.30(b). The limiting current is close to the trap-free SCLC, indicating that the PPV-coated ITO acts as an efficient hole-injecting electrode. The lower curve is the TOF transient recorded under identical conditions. The arrow on the lower curve indicates the transit time and that on the upper curve is 0.8 of this value. The step-voltage response is therefore close to the theoretical prediction. 

Optical quality thin films of metallic polymers are useful, therefore, as transparent electrodes [68]. For example, polyaniline [69], polypyrrole [70] and PEDOT [71] have been used as transparent hole-injecting electrodes in polymer LEDs (the initial demonstration of mechanically flexible polymer LEDs utilized PANl as the anode [69]). Transparent conducting films can be used for a variety of purposes for example, as antistatic coatings on CRT screens, as electrodes in liquid crystal display cells, or for fabricating electrochromic windows. 

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