Excitons metal/polymer interfaces

The generation of photoexcited species at a particular position in the film structure has been shown in (6.19) and (6.20) to be proportional to the product of the modulus squared of the electric field, the refractive index, and the absorption coefficient. The optical electric field is strongly influenced by the mirror electrode. In order to illustrate the difference between single (ITO/polymer/Al) and bilayer (ITO/polymer/Ceo/Al) devices, hypothetical distributions of the optical field inside the device are indicated by the gray dashed line in Fig. 6.1. Simulation of a bilayer diode (Fig. 6.1b) clearly demonstrates that geometries may now be chosen to optimize the device, by moving the dissociation region from the node at the metal contact to the heterojunction. Since the exciton dissociation in bilayer devices occurs near the interface of the photoactive materials with distinct electroaffinity values, the boundary condition imposed by the mirror electrode can be used to maximize the optical electric field E 2 at this interface [17]. 

The presence of interfaces within a polymer LED can also introduce additional nonradiative decay channels. This is particularly important in proximity to a metal electrode. Excitons which are able to diffuse to the metal surface are liable to be quenched directly by interaction with the metal wave function. This mechanism is therefore active only within a few nanometers of the interface. At larger distances (up to about 100 nm), excited molecules can couple to the surface plasmon excitations in the metal, thus providing a further nonradiative decay channel. The combined effects of changes in the radiative and nonradiative rates in two-layer LED structures have been modelled by Becker et al.,83 who have been able to model the variation in EL efficiency with layer thickness due to changes in the efficiency of exciton decay. 

The functioning of such a device is illustrated schematically in Fig. 16-6. Here, holes injected from the ITO electrode are blocked at the interface with the electrontransporting polymer layer, which comprises a 1 1 blend of PMMA with 2-(4 biphenylyl)-5-(4-ter-butylphenyl)-l,3,4-oxadiazole)(calledbutyl-PBD).Thisblocking causes increased electron injection from the other electrode, forcing a balance in electron and hole currents. Additionally, excitons formed at the PPV/PBD-PMMA interface are kept away from the other electrode. Such tailoring allows the use of less reactive metal cathodes, such as Mg, in place of Ca, and yields quantum efficiencies as high as 0.4%. 

PSS-rich layer blocks electrons at the interface and prevents the recombination of electrons at the anode (Figure 10.35). The trapped electrons themselves promote hole injection due to coulombic attraction. Additionally, the PSS interlayer acts as a barrier for excitons being formed close to the anode s interface. Because of this wide band gap polymer the electron-hole pairs will not quench at the metal-like PEDOTPSS layer but will preferentially recombine under the emission of light. 

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