Single-Carrier Structures

The device model describes transport in the organic device by the time-dependent continuity equation, with a drift-diffusion form for the current density, coupled to Poisson s equation. To be specific, consider single-carrier structures with holes as the dominant carrier type. In this case. 

The boimdary conditions are given by specifying the particle 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 back into the metal, a process usually called interface 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, take the contact at X=0 as the hole injecting contact and consider the hole current density at this contact. 

With this value for v, the thermionic current density can be written as 

For typical polymer LED device parameters, current is space charge limited if the energy barrier to injection is less than about 0.3-0.4 eV and contact limited if it is larger than that. Injection currents have a component due to thermionic emission and a component due to tunneling. Both thermionic emission and tun- 

---

In this part, we prepared and studied the Ag/Si02 catalyst by one-step and two-step sol-gel methods. The results show that the Ag/Si02 catalyst prepared here is one kind of bulk material which has a high surface area. The Ag/Si02 catalyst is made up with functional component of Ag or silver oxide in 20 to 30 nm and carrier Si02. Moreover, we found that the different preparation methods have great effect on crystal structure of the samples. The structure of the sample prepared by the one-step method is always a single crystal structure. And the structure of the sample prepared by the two-step method is always a mixed crystal structure. 

Finally, we should mention that the above-mentioned emitter materials are occasionally useful in a DH structure. Moreover, it is possible to form a single-layer EM if we could achieve balanced hole and electron injection and transport by equipping adequate carrier injection electrodes. In fact, BAS96 and BczVBi100 were known to show a fairly high EL efficiency in a single-layer structure. 

An accurate description of a single-layer LED should be obtained by using the injection and transport properties of electrons and holes, determined independently from the Schottky energy barrier and single-carrier device measurements, to describe the two carrier LED structure. To test this procedure consider structures fabricated from the conjugated oligomer 2-metooxy-5-(2 -etoylhexyloxy)-... 

Fig. 12.23 Structure of strapped calix[4]pyrroles 36 and the potassium cation carrier valinomycin (top), the pyridine amide strapped calix[4]pyrrole 37, and single crystal structure of its [37 NaCl] complex (middle). TBA cation, water, and hexane molecules in the crystal structure were omitted for clarity. This X-ray figure was reproduced using the data downloaded from the Cambridge Crystallographic Data Centre. The Na transporter monensin, and the Na" channel blocker amiloride (bottom)...

---