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1.3 theoretical modeling of hybrid interfaces 13acting atoms representing the material of interest by solvingthe Newton’s Equation of motions (F = ma). Forcesare derived from a suitable model potential of the atomicpositions that is calibrated in such a way to reproduce a setof physical properties of the material (see Appendix A).The relatively low computational workload associated toMPMD, allows to obtain predictive informations regardingthermodinamics and microcrystalline evolution over the 10ns timescale of systems as large as 10 nm.Furthermore, the molecular dynamics approach makespossible to easily take into account long range dispersiveinteractions, by using simple Lennard-Jones type potential.The accurate description of interatomic forces in hybridsis however challenging. A general model potential for thehybrid system is not available, but there are reliable potentialsfor the organic and inorganic phases separately. Organicpolymers can be described by means of the “triedand true” Amber force field [62], while in the case of metaloxide the modeling is slightly more complicated. In particular,the ZnO description must take into account its partiallyionic and partially covalent nature [63]. A simpleand succesfull solution is the use of pair interactions consistingof a short-range part (usually a Buckingham interaction[64]) and long-range Coulombic terms employingfixed charges. This method, however, does not take into accountthe charge redistribution around a defect or at thesurface [63]. In the more advanced shell-model description[63, 65, 66], the electronic polarizability is includedadding an additional charged site to each ion connected viaa spring [67]. The shell models however do not properly describethe covalent character of ZnO [63], problem that canbe aided by using higher-order terms in the many-bodyexpansion [68], or by neglecting also the ionic characterand using a bond-order potential [69], but these solutionshave also several drawbacks [63], including larger computationalcosts. Finally, the reactive force field (ReaxFF)[63, 70, 71] is also a bond-order interaction model consistingof the two-body, three-body and four-body short-rangeinteraction terms. It allows the redistribution of charges,can simulate the breaking and reforming of bonds and canreproduce the structures and mechanical properties of con-
14 introductiondensed phases [63, 70, 71] but requires an high computationalcost and a very large number of fitting parameters.In the present work, we focus on a planar ideally perfectmetal oxide surface (ZnO), so that the role of defectsand its evolution is not critical. Furthermore, at room temperaturemost of the microstructure evolution is expectedin the softer organic part of the system. For these reasonswe adopt the simple combination of Buckingham plus longrange Coulombic interatomic potentials, that represents acompromise between computational cost and accuracy, thereliability of this description being confirmed by severalworks [72, 73].As for the electronic properties of PV interfaces, the DFTapproach provides very good choice but its heavier computationalcost limits the analysis to small portions of theMPMD generated system. The DFT method require somecare in the choice of exchange-correlation functional used.In particular, both the LDA [74] and the GGA [75] functionalssuffer from the problem of the underestimation ofthe band gap for the semiconductors (including the ZnO)[76]. This problem can be partially overcome by using theLDA+U approach [77, 78] or hybrid funcionals (such as theB3LYP [79, 80]). This latter, however, severely increases thecomputational cost.1.4 aims and outline of this thesisThe aim of this thesis work is to generate realistic atomisticmodel for hybrid interfaces and supply informationsfor their design and optimization.In particular, a major emphasis will be given to the morphologicalaspects of the investigated systems, while theelectronic properties will be addressed mainly as a reviewcontribution framed in a more general discussion.The understanding of the hybrid interface requires firstof all the study of the polymer alone, its structure andmechanisms of aggregation. Therefore, the first chapter ofthis work concerns the P3HT polymer studied as singlemolecule (dimer and oligomer) and aggregated bulk includingcrystalline and nanocrystalline phases. Its crystallineproperties and self assembling mechanism are investigated,as well as the structure of polymer nanoclusters.
Page 1 and 2: Università degli Studi di Cagliari
Page 4: A C K N O W L E D G M E N T SI woul
Page 7 and 8: example of a novel class of systems
Page 10 and 11: C O N T E N T S1 introduction 11.1
Page 12 and 13: L I S T O F F I G U R E SFigure 1.1
Page 14 and 15: List of FiguresxiiiFigure 2.15 Init
Page 16 and 17: List of FiguresxvFigure 4.6Figure 4
Page 18: Figure 5.7Figure 5.8Figure A.1Figur
Page 21 and 22: 2 introduction1-3 eV) and the trans
Page 23 and 24: 4 introductionorder in such a compl
Page 25 and 26: 6 introductionprocesses that need t
Page 27 and 28: 8 introduction1.2 physical factors
Page 29 and 30: 10 introductionrecovered by the int
Page 31: 12 introductionA cheap and environm
Page 36 and 37: P 3 H T - P O LY ( 3 - H E X Y LT H
Page 38 and 39: 2.1 mechanism of assembling and mor
Page 40 and 41: 2.2 p3ht assembling and intermolecu
Page 42 and 43: 2.3 p3ht crystalline bulk phases 23
Page 44 and 45: 2.5 nanocrystalline p3ht 25Figure 2
Page 46 and 47: 2.5 nanocrystalline p3ht 27Table 2.
Page 48 and 49: 2.6 conclusions 29Figure 2.13.: Ini
Page 50 and 51: P O LY M E R / S E M I C O N D U C
Page 52 and 53: 3.3 adhesion of a single p3ht molec
Page 54 and 55: 3.4 p3ht/zno interface 35surface (t
Page 56 and 57: 3.5 p3ht/zno interface: low deposit
Page 58 and 59: 3.6 p3ht/zno interface: high deposi
Page 64 and 65: 3.7 effective model for the transpo
Page 66 and 67: 3.8 conclusions 47As for the low te
Page 68 and 69: T E R N A RY Z N O / Z N P C / P 3
Page 70 and 71: 4.1 self assembling of znpcs on zno
Page 72 and 73: 4.2 polymer interaction with znpcs
Page 74 and 75: 4.3 electronic and optical properti
Page 76 and 77: 4.3 electronic and optical properti
Page 78 and 79: 4.4 conclusions 59ties. The absorpt
Page 80 and 81: I N T E R A C T I O N B E T W E E N
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5.2 solvent thf interaction with zn
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Page 86 and 87:
Page 88 and 89:
5.3 conclusions 69γ L/L ∼ 0.112
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C O N C L U S I O N SIn this thesis
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M O L E C U L A R D Y N A M I C SAa
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A.1 molecular dynamics 75a.1.2The t
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A.2 the force field 77Finally, τ t
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A.2 the force field 79Waals interac
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A.3 methods 81tional Theory (DFT) l
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B I B L I O G R A P H Y[1] http://w
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ibliography 85[17] J. D. Servaites,
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ibliography 87Zakeeruddin, and M. G
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ibliography 89Phys. Chem. Chem. Phy
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ibliography 91[65] D. J. Binks and
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ibliography 93cells,” Energy Envi
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ibliography 95[97] C. Ingrosso, A.
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ibliography 97298.15 k,” Journal
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ibliography 99charges. am1-bcc mode
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colophonThis document was typeset u
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