Hole injection

Hole injection into the / -region similarly results in a positive current flow from to, leading to the Shockley diode equation (eq. 12), with a... 

In bilayer LEDs the field distribution within the device can be modified and the transport of the carriers can be controlled so that, in principle, higher efficiencies can be achieved. On considering the influence of the field modification, one has to bear in mind that the overall field drop over the whole device is given by the effective voltage divided by the device thickness. If therefore a hole-blocking layer (electron transporting layer) is introduced to a hole-dominated device, then the electron injection and hence the efficiency of the device can be improved due to the electric field enhancement at the interface to the electron-injection contact, but only at expense of the field drop at the interface to the hole injection contact This disadvantage can be partly overcome, if three layer- instead of two layer devices are used, so that ohmic contacts are formed at the interfaces [112]. 

The boundary conditions are given by specifying the panicle currents at the boundaries. Holes can be injected into the polymer by thermionic emission and tunneling [32]. Holes in the polymer at the contact interface can also fall bach into the metal, a process usually called interlace recombination. Interface recombination is the time-reversed process of thermionic emission. At thermodynamic equilibrium the rates for these two time-reversed processes are the same by detailed balance. Thus, there are three current components to the hole current at a contact thermionic emission, a backflowing interface recombination current that is the time-reversed process of thermionic emission, and tunneling. Specifically, lake the contact at Jt=0 as the hole injecting contact and consider the hole current density at this contact. 

Figure 11-14 shows the calculated hole density (upper panel) and the electric field (lower panel) as a function of position for the three structures. For the devices with a hole barrier there is a large accumulation of holes at the interface. The spike in the hole density at the interface causes a rapid change in the electric field at the interface. The field in the hole barrier layer is significantly larger than in the hole injection layer. For the 0.5 eV hole barrier structure, almost all of the... 

There are many organic compounds with useful electronic and/or optical properties and with sufficiently high volatility to be evaporable at a temperature well below that at which decomposition occurs. Since thermal evaporation lends itself to facile multilayering, organic compounds may be selected for use in one or more function electron injection, electron transport, hole injection, hole transport, andI or emission. A complete list of materials that have been used in OLEDs is too vast to be included here. Rather, we list those that have been most extensively studied. 

Table 16-6. Electrical properties and efficiencies of single-layer and double-layer OPVS-LEDs with 1TO hole-injecting contacts in forward-bias operation.

PPV and its alkoxy derivatives are /j-type conductors and, as a consequence, hole injection is more facile than electron injection in these materials. Efficient injection of both types of charge is a prerequisite for efficient LED operation. One approach to lowering the barrier for electron injection is the use of a low work function metal such as calcium. Encapsulation is necessary in this instance, however, as calcium is degraded by oxygen and moisture. An alternative approach is to match the LUMO of the polymer to the work function of the cathode. The use of copolymers may serve to redress this issue. 

The dominant parameters for charge earner injection are the height of the potential barrier at the interface polymer/hole injection contact and the hole mobility of the polymer. 

According to that model, the net current flow in the device therefore can be increased in bilayer structures using a hole-transport layer, which possess higher hole mobility than the active polymer layer and which changes the height of the potential barrier at the interface transport layer/hole injection contact [81],... 

Figure 11-8. Linear-linear (upper panel) and log-linear (lower panel) plots of calculated current density as a (unction of bias voltage for 100 nm MliH-PPV devices with a 2.2 eV barrier to electron injection and 0.1, 0.2, 0.3, 0.4. 0.5. and 0.6 eV barriers to hole injection.

Figure 11-9. Measured (solid lines) and calculated (dashed lines) current density us a (unction o( voltage bias for MBH-PPV devices o( about 110 nut in thickness with Au us the electron injecting contact and Pt, Au, Cu. and Al us the hole injecting contact. The upper panel shows a schematic energy level diagram for the structures.

Figure 11-10. Culcululcd values of ihc injection currenl components and the total device current as a function of bias for the Pi (upper panel), Cu (central panel), and Al (lower panel) hole injecting contact devices.

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. 

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. 

Tabic 16-7. Estimated HOMO-LUMO gaps and hole injection barriers. 

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

O Regan B, Schwartz DT (1995) Efficient photo-hole injection from adsorbed cyanine dyes into electrodeposited copper(I) thiocyanate thin films p-type semiconductors. Chem Mater 7 1349-1354... 

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