Multilayer Devices The Incorporation of Charge-Transporting Layers

1 Multilayer Devices The Incorporation of Charge-Transporting Layers 

Electron-hopping is the main charge-transport mechanism in ECHB materials. There is precedence in the photoconductivity field for improved charge transport by incorporating a number of redox sites into the same molecule. A number of attempts to adapt this approach for ECHB materials have been documented. Many use the oxadiazole core as the electron-transport moiety and examples include radialene 40 and dendrimer 41. However, these newer systems do not offer significant improvements in electron injection over the parent PBD. 

An alternative approach utilizes polymeric analogs of PBD. The oxadiazole unit may be in the polymer main chain or attached as a side chain. A reasonable device performance has been demonstrated in poly(aromatic oxadiazolejs [71- 

A hyperbranched polymer 42 comprising oxadiazole subunits has been synthesized, but defect formation in such a structure appears to limit its use as a hole-blocking material [74], 

Other electron-deficient heterocyclic systems have also been investigated as electron-transporting materials, hi particular, devices employing poly(phenyl qui-noxaline) 43 as an ECHB layer have shown improvements in device efficiency when used in conjunction with an emissive PPV layer [75]. 

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Refining the Properties of PPV - Multilayer Devices 13 Multilayer Devices The Incorporation of Charge-Transporting Layers 14... 

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