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2.1 mechanism of assembling and morphology of crystalline p3ht 19Figure 2.2.: Interaction energy of a thiophene dimer as a function ofthe thiophenes distance calculated according to MPMD(symbols) CCSD(T) (continuous line) and MP2 (dottedline) methods (figure from [3]).to the atoms of the alkyl chains increasing the binding energyto 0.3 eV per thiophene.The second driving force for the assembling is associatedto the interdigitation between parallel molecules inan edge-to-edge configuration. We calculate the interactionas a function of distance and we found two minima, oneat 13.6 Å and another at 16.0 Å , separated by an energybarrier as high as 0.3 eV (see Figure 2.5).The molecule-molecule assembling force described aboveis consistent with the results for cohesion in the bulk crystallinephase [3]. The orthorhombic unit cell of crystallineP3HT with cristallographic vectors lying respectively in thealkyl side chains (a), stacking (b) and backbone (c) directionsis represented in Figure 2.6. The bulk energy dependenceon the molecules separation can be calculated by performinga series of geometry optimizations by varying thelattice parameters a and b in the range 13.0-16.2 Å and 6.8-10.0 Å (corresponding to a thiophene-thiophene distanceof 3.4-5.0 Å). The value of c is kept fixed at 7.75 Å.In Figure 2.7 it is reported the corresponding color-mapof energy as a function of a and b. The energy profilealong the π − π direction b shows a larger variation (∼ 0.5eV/thiophene) and a well defined minimum consistent withthe above results for thiophene-thiophene and polymerpolymerface-to-face interactions. The energy variation along