Emissive layer hole injection barrier

The first single-layer PF-based PLED with polymer 1 as emitting layer was reported by Yoshino and coworkers [17]. For this device with ITO and magnesium-indium alloy (Mg In) as anode and cathode, respectively, its main EL emission peak locates at 470 nm, but no data on efficiency was given. However, the authors pointed out that electron injection is difficult because of the large energy barrier (1 eV) between 1 (LUMO 2.9 eV) and the Mg fii cathode (work function of Mg 3.9 eV), while the hole-injection barrier [0.2 eV, as estimated from the work function of ITO (5.5 eV) and the HOMO level of 1 (5.7 eV)] is relatively low as compared to the electron-injection barrier (1 eV). Bradley and coworkers reported that the device with PFO as emit-... 

A polymer layer al a contact can enhance current How by serving as a transport layer. The transport layer could have an increased carrier mobility or a reduced Schottky barrier. For example, consider an electron-only device made from the two-polymer-layer structure in the top panel of Figure 11-13 but using an electron contact on the left with a 0.5 eV injection barrier and a hole contact on the right with a 1.2 cV injection barrier. For this case the electron current is contact limited and thermionic emission is the dominant injection mechanism for a bias less than about 20 V. The electron density near the electron injecting contact is therefore given by... 

Electrical measurements on devices with different layer thickness have shown that the diode current depends on the applied field rather than the drive voltage. This is similar to what has been observed with our alternating PPV copolymers [68]. It indicates that field-driven injection determines the electrical characteristics. From Figure 16-39 it is evident that U-OPV5 has the lowest onset for both current and emission. By means of Fowler-Nordhcini analysis of the /-V -charac-teristics and optical absorption measurements, wc estimated the injection barrier for holes and the HOMO-LUMO gap, respectively [119]. The results of... 

The first quantitative study of the device efficiency of polyfluorene LEDs was reported by Grice and co-workers in 1998.5 The LED used ITO-coated glass as the anode, a hole-transport layer of a polymeric triphenyldiamene (poly-TPD), an emissive layer of PFO, and a calcium cathode. A schematic energy-level structure for the device is inset in Fig. 10.14. The injection barrier for holes into poly-TPD is approximately 0.3 eV, much less than that for PFO (0.8 eV), and the injection barrier for electrons into PFO is approximately 0.1 eV. Poly-TPD had excellent film-forming properties, was transparent to PFO emission, and was insoluble in... 

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