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1.2 physical factors relevant for photoconversion at hybrid interfaces 11can be used to cause a band edge shift in titania, resultingin a change in the open-circuit voltage [46].Recently also small molecules, such as catechol or isonicotinicacid, have attracted attention as interface modifiersin hybrid systems [48, 49, 50]. In particular, catechol isused as an anchoring group for organic and organometallicdyes due its efficient adsorption onto TiO 2 via formationof a strong adsorbate-substrate complex [48, 49, 50] and forthe type II hybrid junction that forms in combination withTiO 2 [48, 51, 52].Again, an ordered molecular layer composed by the 4-mercaptopyridine (4-MP) molecules between a TiO 2 surfaceand a polymer, has produced an overall efficiency ofthe device which overcomes the 1% limit [47]. The presenceof the oriented molecular layer, triggered by selective interactionswith the TiO 2 surface, drives local ordering smoothingthe otherwise abrupt interface. This result shows theimportance of molecular interactions and local morphologyin hybrid interfaces and their implications on chargeseparation and recombination [47].High-efficient solid-state hybrid polymer/metal oxide solarcells have been obtained by depositing Sb 2 S 3 as sensitizerand P3HT as hole conductor and light absorber ona titania surface [53]. This cells exhibit good conversionefficiency and it is highly stable in air, even without encapsulation[53].The most important modification of the hybrid polymer/metaloxide interface is obtained by inserting optically active interlayersthat can contribute to light absorption and injection.This is possible by using dyes and sensitizers toload the metal oxide surface. Most of the information inthis approach comes from the research on DSSCs. Amongthe most widely used sensitizers there are the porphyrins,partly because the their structure synthetically analogue ofchlorophyll [54]. Porphyrins have extensively conjugatedπ systems, are favourable to fast electron transfer to an acceptorand absorb light well in the blue and moderately inthe green regions of the visible spectrum with high molarabsorption coefficients [54]. In particular, one of themost notable efficiency improvements in hybrid devices(3%) has been obtained by using porphyrins as dyes in aP3HT/TiO 2 system [55].
12 introductionA cheap and environmentally friendly alternative arephthalocyanines (Pcs) [56, 54]. They are characterized byan intensive absorption in the far-red IR region, by an excellentchemical, light, and thermal stability, a long excitondiffusion length (8-68 nm for CuPc) and a high holeconductivity (2 × 10 −5 to 5 × 10 −4 cm 2 V −1 s −1 ) [54].Furthermore, phthalocyanines offer flexibility in their opticaland electronic properties through synthetic modifications,including the addition of functional groups to themolecule perimeter [54]. The structure of these moleculesis characterized by one or more macrocyclic ligands carryingclouds of delocalized electrons and by a central metalor group [56]. Since Pc aggregates have electrochemical,spectroscopic, photophysical, and conductive properties differentfrom those of the corresponding monomers [56], theabilty to understand and drive their assembling is crucialin order to obtain interlayers that really improve the hybridinterfaces.Finally, another problem to deal with in the hybrid systemsproduction is the influence of the solvent used forspin coating. It was found that the change of solvent (fromchloroform to xylene) yields one to two orders of magnitudeimprovement in a photovoltaic TiO 2 /P3HT cell efficiency[57]. Furthermore, fabrication conditions, as well asthe inorganic nanoparticles concentration, can significantlyaffect the morphology of the interface and the device performance[57]. The presence of residual solvent at the interfacecan affect the polymer deposition, acting as an interfacemodifiers just as in the cases discusses above [58].1.3 theoretical modeling of hybrid interfacesThe previous discussion clearly shows the need of a thoroughtheoretical study of hybrid interfaces in order to clarifytheir properties and the effects of interlayers in improvingtheir photoconversion performances.To this aim, in this work we adopt a combination ofatomic scale methods including Model Potential MolecularDynamics (MPMD) and Density Functional Theory (DFT)calculations.MPMD [59, 60, 61] is a computational technique that consistsin calculating the classical trajectories of a set of inter-
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: 10 introductionrecovered by the int
Page 33 and 34: 14 introductiondensed phases [63, 7
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
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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:
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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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