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3.8 conclusions 47As for the low temperature cases, in the LDR model (inred in Figure 3.16) the effective mobility turns out to beabout one half that of a perfect s-crystal (represented inblue) for the first two layers. Starting from the third layerthe mobility strongly decreases due to the mismatch betweenthe layers and, eventually, drops to zero. In the HDRcase (in green in Figure 3.16), the behavior is opposite. Infact, the strong tilt of the π − π channels in the first layersreduces the mobility, which is, though, partially recoveredfor the last two ones.Figure 3.17.: Normal mobility obtained at 300 K by approximating thethiophene rings with ellipses of eccentricity ɛ = 1.15.Figure 3.17 shows taht the effect of temperature (300 K)for both the two models is to further reduce the mobility,but preserving the overall behavior found at 1 K.3.8 conclusionsIn conclusion, the polymer crystal is highly affected atthe interface with ZnO. The 010/ZnO interface is foundto be the most favorable, with polymer thiophenes facingthe ZnO surface due to the high molecule/surface interaction.Due to disorder at the interface, polymer chains arelikely misaligned close to the ZnO surface thus reducingthe normal carrier mobility in the first layers. Holes that aregenerated at the interface are not able to diffuse throughthe polymer and, as a consequence, they likely recombinewith electrons. Similarly, excitons photogenerated withinthe polymer cannot easily move to the interface in order tobe separated.