Electrons and holes injection

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. 

Wehrenberg BL, Guyot-Sionnest P (2003) Electron and hole injection in PbSe quantum dot films. J Am Chem Soc 125 7806-7807... 

Scheme 2.14). This suggests that the electron and hole injections (or transport) in PPV are unbalanced and holes are the dominant charge carriers. 

Electron and hole injection from the cathode and the anode... 

In nanocrystaUine semiconductor films (commonly obtained in CD), the crystal size may be too small to support an appreciable space charge layer. Charges in that case are separated by differing kinetics between electron and hole injection into the electrolyte. The upcoming discussion on nonannealed films treats this in somewhat more detail. (Chapter 9 discusses PECs and their principles of operation more fully.)... 

Alteration of the silver plasmon band spectrum upon electron and hole injection has been rationalized in terms of changes in the density, Ne, and conductivity, o, of the electron gas in the metal particles as described by Eqs. (16)—(18) [506]. Thus, a decrease in Ne by electron extraction from the metallic silver particles increases Xc (Eq. 16) and thereby shifting the absorption maximum (Eq. 15) of the plasmon band to a longer wavelength (Fig. 83). A decrease in Ne also decreases a (Eq. 18), which leads, in turn, to an increase of w (Eq. 17) that is, to an increase in the bandwidth of the plasmon band absorption (Fig. 83). Similarly, the increase in Ne by electron transfer to the silver colloids is paralleled by a decrease in Xc (Eq. 16) and, hence, by a decrease in Xm (Eq. 15), as seen by the shift of the plasmon absorption band to a shorter wavelength (Fig. 83). Electron donation to the silver particles also causes an increase in cr (Eq. 18)... 

Fig. 20 Plots of H/kTc against 1 kTQ for electron and hole injection into anthracene crystals grown under different atmospheres open symbols refer to electrons and closed symbols to positive holes. (After Owen et al, 1974)...

Figure 1 The schematic representation of various electronic excitation mechanisms due to ac or dc external electric fields (a) the tuneling electrons from the valence band (VB) to the conduction band (CB) and ionization of an acceptor state (-o-) (Zener effect) followed by electron-hole recombination, indicated by horizontal and vertical arrows, respectively (b) excitation or ionization by electron impact (c) recombination of electrons ( ) and (o) holes at a semiconductor p-n junction and (d) bulk recombination of electrons and holes injected from electrodes. Adapted from Ref. 2...

Interest surged in studies of fluorescence after the discovery of electroluminescence from PPV (Burroughes et al., 1990), because of its potential for practical application in light emitting devices (LEDs). Electroluminescence is fluorescent emission produced by the recombination of electrons and holes injected into a thin film of conjugated polymer, and will be discussed in the next section. If photoluminescent emission from a polymer is weak, then the electroluminescence is unlikely to be of practical significance, and consequently studies of photoluminescence and photoluminescent quantum efficiency have been used as a means of selecting polymers likely to be useful in LEDs. 

Bu-PBD (a well-known electron acceptor and transport molecule) was motivated by the need to achieve better-balanced electron and hole currents see Fig. 4.15. The data from an identical device fabricated with pure OC1C10-PPV (without Bu-PBD) are shown for comparison. For devices containing Bu-PBD, the efficiency increases reversibly with temperature at 85 °C, QEext(EL) =4%. The effect of the Bu-PBD is evidently to fine tune the balance of the electron and hole injection. The acceptor level in Bu-PBD is close to the bottom of the Jt-band of the luminescent polymer (see Fig. 4.15). 

Fig. 10.5 Ene rgy diagram of a CdSe/ZnS NC/PPV device. Whereas the NC acts as an acceptor of electrons, there is a substantial barrier to be overcome for hole injection into the NC. An additional barrier to both electron and hole injection occurs due to the presence of the organic ligands. Reprinted from [14], Copyright 1998, with permission from the American Institute of Physics.

Interestingly, Vg-controlled electroluminescence and ambipolar characteristics have been recently observed in conjugated polymer OFETs [67,68], which indicates a balanced electron and hole injection. However, low hole and electron mobilities ( 10 cmWs), typical for polymer semiconductors, limit the channel current and therefore may present a serious problem for realizatiou of electrically pumped polymer lasers. For this reasou, ordered smaU-molecule orgauic semicouductors with higher mobilities are very promisiug for research in this direction. 

The main drawback of the PLED device with jt-conjugated polymers is that there is an imbalance between the rate of electrons and hole injection near the interface between the polymer and the cathode. Many approaches have been proposed to improve the performance of the devices such as using... 

This diagram illustrates the equivalence of the valence band with the ionization potential (IP) and the HOMO as well as that of the conduction band with the electron affinity (EA) and the LUMO. Notably, electron and hole injection are controlled by the energy barrier between the contact and the organic material. In the absence of surface states and a depletion region due to impurity doping, the energy barriers are given by Eqs. (6-1) and (6-2). 

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