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44 polymer/semiconductor interfacesuch a result to the poor quality of the crystalline ZnOsurface and to the strong polymer substrate interactionthat induces disorder in the polymer backbones. Furthermeasurements on more controlled ZnO samples would allowfor a better comparison of our finding with experiments.However, the P3HT disorder at the inteface is consistentwith our findings of polymer disorder induced bythe strong polymer/ZnO interaction that favors the 010/ZnOinterface.The morphological features discussed above, are expectedto modify the transport properties of the polymer. For example,in the case of a crystalline polymer, if the π − πchannels are orthogonal to the substrate, the carriers caneasily move away from the interface before recombining.This corresponds to the most favorable case for photovoltaicefficiency. In the opposite case, when the π − π channelsare parallel to the interface or when the polymer is disordered,the carriers cannot easily move away from the interface.In conclusion, for transport and performances, theorder in the direction normal to the interface is a key property.In the next section we will discuss an effective methodto evaluate the charge mobility at the interface.3.7 effective model for the transport propertiesThe stacking and the transport properties of the systemsdescribed above can be studied by using the concept of effectivearea. The idea is to represent each thiophene ring byan elliptical shape in the plane of the molecule (see inset ofFigure 3.15). The projected overlap area Θ ⊥ (in the normalx-y plane) between pairs of neighboring molecules along zcan be then calculated. This quantity is related to the crystallineorder of the system and it is small in disordered oramorphous polymer films. In particular, Θ ⊥ is maximumwhen the thiophenes of two neighboring molecules are perfectlyaligned and parallel to the x-y plane. Conversely, Θ ⊥is smaller when thiophenes are shifted in the x or y directionor when the molecules are tilted with respect to z.By referring to the Marcus theory [92, 93], the mobilityµ in the polymer is given considering the local probabilityk αβ that a hole hops between neighboring molecules α and