ADDRESSING PROBLEMS AND TAILORING PERFORMANCE

Among major problems with CP LEDs are adjusting hole and electron currents for balance. This problem can be somewhat dealt with by selecting the metal electrode used, especially that for electron injection. However, this leads to much reduced efficiency of emission. 

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%. 

Structure of CN-P(PV) (left) and typical characterization data for LED constructed using this (right). After Reference [490], reproduced with permission. 

Schematic stmcture (a), luminance (b) and tum-on time (c) data for LEDs using cyano-P(PV) s. After Reference [763], reproduced with permission. 

Ostergard et al. [764] attempted to tailor performance of CP-LEDs by employing Langmuir-Blodgett (L-B) films of poly(3-hexylthiophene) derivatives, one of the first such studies. They claimed that this led to better balancing of injected holes and electrons, leading to improved quantum efficiency. 

---