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32 polymer/semiconductor interfaceogy on the transport properties are investigated as well interms of effective models.3.2 zinc oxideZinc oxide (ZnO) is a wide band gap semiconductor(3.37 eV) [27] that provides very good electron mobility(205 cm 2 V −1 s −1 ), it is non-toxic, and it can be grownin a variety of highly crystalline nanostructures [27, 89]which are commonly used as electron acceptors. In combinationwith organic donors (e.g. conjugated polymers ormolecules), ZnO nanostructures have been used to synthesizehybrid bulk heterojunctions. In particular, nanorodshave attracted great attention as elongated nanostructuresthat could contribute to improve the charge transport inthe hybrids.Zinc Oxide crystallizes in two main forms, hexagonalwurtzite and cubic zincblende. The wurtzite structure ismost stable at ambient conditions and thus most common.The lattice parameters of the zinc oxide are a = 3.25 Å andc = 5.20 Å (see Figure 3.1).Figure 3.1.: ZnO wurtzite structure.The most energetically stable surface of crystalline ZnOis the non-polar (10¯10) and, hereafter, we will focus on itsince it is the most common in ZnO. For example, ZnOtypical nanorods used in hybrid bulk heterojunctions [90]exhibit six equivalent (10¯10) surfaces of lateral size largerthan 10 nm.The atomic scale model of the ideal ZnO surface is generatedby cutting a wurtzite ZnO crystal along the (10¯10)plane and by relaxing it at low temperature. The atomic