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36 polymer/semiconductor interfaceinto account by heating and equilibrating the interfaces atroom temperature.3.5 p3ht/zno interface: low deposition rateIn the regime of low deposition rate (LDR) the polymertend to organize parallel to the substrate forming s-foils [3].This has been already discussed chapter 2 (see panel a ofFigure 3.4).The atomistic models generated during the LDR assemblingprocedure are reported in Figure 3.4.za)zb)yyc)d)zzyyFigure 3.4.: Assembling of P3HT layers on the ZnO surface.In each layer (s-foil) deposited, the polymer moleculesare aligned with the backbone parallel to rows of Zn-Odimers and the molecule-molecule interdigitation distanceis controlled by its matching with the lattice spacing of theZnO surface, particularly for the first layers. Given the sensitivityof the polymer crystal structure on synthesis conditions,the above mismatch can be important in drivingthe final structure of the polymer at interface. The experimentalinterdigitation distance in the P3HT is reportedto be 16.8 Å. Since the ZnO surface lattice parameter inthe y direction is 5.20 Å, the best matching between thepolymer and the ZnO surface is obtained by putting onepolymer chain every three rows (interdigitation distanceof 15.6 Å corresponds to three times 5.20). For ideally perfectpolymer structure, the calculated interdigitation distancein perfect crystals is smaller than the experimentalone and it can assume two values (as already discussed insection 2.1): 13.6 Å for high density phase and 15.8 Å forthe lower dense phase. Both experimental and high densityideal phase values gives a sizable mismatch with the ZnO