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P O LY M E R / S E M I C O N D U C T O R I N T E R FA C E3Contents3.1 Hybrid Interfaces 313.2 Zinc Oxide 323.3 Adhesion of a single P3HT molecule onthe Zinc Oxide surface 333.4 P3HT/ZnO interface 343.5 P3HT/ZnO interface: Low DepositionRate 363.6 P3HT/ZnO interface: High DepositionRate 383.7 Effective model for the transport properties443.8 Conclusions 473.1 hybrid interfacesIn this chapter, the interface between the metaloxide ZnOand the polymer P3HT is investigated by means of atomisticsimulations based on model potential molecular dynamics.Such an interface is the core of the hybrid P3HT/ZnOsolar cell whose efficiencies are typically too low for practicalapplications, with a record of 2% [28] in bulk heterojunctionarchitectures. The different behavior of the samepolymer P3HT in combination with ZnO or with the organicPCBM (for which relatively high efficiencies of 5%are possible [88]), shows the need of a better understandingof the main physical concepts controlling the interfacestructure at the atomic scale.In this chapter, after discussing the most stable and abundantZnO surface, we set up the force model describing thepolymer/ZnO interaction and we generate several modelsof P3HT/ZnO interfaces. Our goal is to understandthe polymer organization at the interface in terms of crystallinityand disorder by including different kinetic andthermodynamic conditions. The implications of morphol-31
32 polymer/semiconductor interfaceogy on the transport properties are investigated as well interms of effective models.3.2 zinc oxideZinc oxide (ZnO) is a wide band gap semiconductor(3.37 eV) [27] that provides very good electron mobility(205 cm 2 V −1 s −1 ), it is non-toxic, and it can be grownin a variety of highly crystalline nanostructures [27, 89]which are commonly used as electron acceptors. In combinationwith organic donors (e.g. conjugated polymers ormolecules), ZnO nanostructures have been used to synthesizehybrid bulk heterojunctions. In particular, nanorodshave attracted great attention as elongated nanostructuresthat could contribute to improve the charge transport inthe hybrids.Zinc Oxide crystallizes in two main forms, hexagonalwurtzite and cubic zincblende. The wurtzite structure ismost stable at ambient conditions and thus most common.The lattice parameters of the zinc oxide are a = 3.25 Å andc = 5.20 Å (see Figure 3.1).Figure 3.1.: ZnO wurtzite structure.The most energetically stable surface of crystalline ZnOis the non-polar (10¯10) and, hereafter, we will focus on itsince it is the most common in ZnO. For example, ZnOtypical nanorods used in hybrid bulk heterojunctions [90]exhibit six equivalent (10¯10) surfaces of lateral size largerthan 10 nm.The atomic scale model of the ideal ZnO surface is generatedby cutting a wurtzite ZnO crystal along the (10¯10)plane and by relaxing it at low temperature. The atomic
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 and 32: 12 introductionA cheap and environm
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 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
Page 82 and 83: 5.2 solvent thf interaction with zn
Page 88 and 89: 5.3 conclusions 69γ L/L ∼ 0.112
Page 90 and 91: C O N C L U S I O N SIn this thesis
Page 92 and 93: M O L E C U L A R D Y N A M I C SAa
Page 94 and 95: A.1 molecular dynamics 75a.1.2The t
Page 96 and 97: A.2 the force field 77Finally, τ t
Page 98 and 99: A.2 the force field 79Waals interac
Page 100 and 101:
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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