Hole-blocking layer doping effect

Several different materials can be used as transparent electrodes, most of them as anode material ITO [113,194,195,200], pofyaniline and polyaniline blends [206, 207, 209, 210], TO [201, 202,204], and F-doped TO [112,205]. The use of transparent material as a cathode has also been reported [202]. In the case of PPV and several of its derivatives, the effective mobility of the electrons is lower than that of the holes, implying a reduction in the extent of the recombination zone in the electroluminescent polymer layer, as observed in PPP LEDs [164]. Further, the values of polymer electroaffinity and ionization potential make the injection and transport of holes etisier than that of electrons in single-polymer-layer devices. The injection dynamics also depends on the injected carrier that remains in the polymer (space charge), modifying the electric field distribution in the device [217]. For these reasons, different materials are tested as cathode and anode and, in several cases, intermediate layers are also introduced in order to improve the injection of a specific charge carrier type or to block its transport through the device [212,213,218-220]. 

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