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22 p3ht - poly(3-hexylthiophene)Figure 2.7.: Energy landscapes obtained by MP for the bulk P3HTstructure. The lattice parameters are referred to the equilibriumvalues a 0 and b 0 while the total energy is referredto the energy of two unbound chains (figure from [3]).a long-range interaction with the other foil (Figure 2.8 toppanels) [3]. In particular, it was found that the thiophenerings belonging to adjacent h-foils formed in a zigzag-likeconfiguration less interdigitated (16.2 Å) with respect tothe ideal minimum energy phase. Such a value is in agreementwith the experimental results [82].On the other hand, in presence of a planar surface stronglyinteracting with the polymer (e.g. with a binding energyfor the face-on polymer comparable with the π − π interaction)it is likely that P3HT molecules will align on the substrateforming s-foils (see Figure 2.3 right) [3]. The polymeris expected to further grow according to a layer-by-layermechanism as depicted in Figure 2.8 bottom panel .2.3 p3ht crystalline bulk phasesIn the present work, in order to study the crystallinityof the polymer bulk phase, we calculate a functional of theatomic positions, hereafter named S(q):S(q) = 1 N∣N∑j=1f j · e −iq·x j∣(2.1)where x j are the coordinates of the j-th atom, N is the numberof atoms and |q| = 2π /λ is any possible wave vector.