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3.4 p3ht/zno interface 35surface (that have different lattice parameters). In case (ii)there are surfaces in the ZnO cluster with sizable effectson the crystal slab structure (unless fixing the atomic positions,that is not compatible with finite temperature simulations).In this work, we prefer to adopt the boundary conditionsof type (iii) where the interface is obtained by puttinga non periodic finite size polymer nanocrystal (up to 10 4atoms) on a periodic ZnO surface. In this way the polymerlattice spacing is not constrained by boundary conditions.If the polymer nanocrystal is large enough, the results thatare calculated under these boundary conditions can be appliedto real polymer/ZnO interfaces.As for the polymer crystalline structure, it is experimentallyknown that the polymer is highly sensitive to the synthesisconditions [84]. Accordingly, within the conditionsdescribed above, we explore two different ways of generatingthe hybrid interfaces hereafter named Low DepositionRate (LDR) and High Deposition Rate (HDR). In the LDRthe polymer nanocrystal is assembled on ZnO layer bylayer at a low rate while fully relaxing the atomic positionsat each step. In the HDR case, the polymer nanocrystal iscut from an ideal infinite ordered bulk and it is merged andrelaxed on the ZnO surface. The above two cases are representativeof two opposite experimental regimes; the LDRcorresponds to the case where the substrate-molecule interactionis the ruling assembling mechanism; in this case thepolymer molecules can face on the surface (see section 3.3)forming successive s-foils (see chapter 2). The HDR casecorresponds to the physical regime in which the P3HTmolecules are likely to aggregate before interacting withthe surface; in this situation the polymer-polymer forcescontrols the assembling of the interface.In both LDR and HDR, the P3HT nanocrystals are chosenof dimensions 6 nm x 11 nm x 4 nm and are formed by30 molecules of length 6 nm with backbones oriented alongthe x direction, in agreement with [91], in which a preferentialorientation of the P3HT along the dimer rows of ZnO isfound. The size of these nanocrystals is comparable withP3HT crystalline domains in real samples (10-50 nm [3]).Atomic relaxations are always obtained by extensive lowtemperature annealings followed by conjugated gradientsenergy optimizations. Temperature effects are also taken