Multiple-Layer Polymer LEDs

The first two-layer device based on a conjugated polymer was demonstrated by Brown et al.39 They introduced an electron-transporting layer (ETL) between PPV and the negative electrode, as shown in Fig. 5.14. The electron-transporting layer used was the molecule 2-(4-biphenylyl)-5-(4-terf-butylphenyl)-l,3,4-oxadiazole, dispersed in a matrix of polymethylmethacrylate. Quantum efficiencies were typically increased by a factor of 10 compared to devices without the electron-transport 

FIGURE 5.14. Schematic structure of a PPV LED incorporating an electron-transporting layer. 

FIGURE 5.15. Schematic energy-level diagram for the device shown in Fig. 5.14. 

Electron-transporting layers have been used in conjunction with conjugated polymers by several other groups, with a similar effect.64,65 Unfortunately, the stability of LEDs using molecular electron-transporting materials dispersed in polymeric matrices tends to be poor, probably due to slow recrystallization of the electron-transporting molecules. This problem has led to the development of various polymeric electron-transporting materials, which show improved stability.66 

---

This formalism has been applied to polymer LEDs to estimate the position of the recombination zone. Particularly useful experimental data here come from studies of multiple-layer LEDs, where the emission spectmm can be used to determine the amount of recombination occurring in each layer.76 The simple theory has been extended by Gartstein et al. to take into account the discrete nature of the hopping process and the distribution in site energies.77... 

---