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P 3 H T - P O LY ( 3 - H E X Y LT H I O P H E N E )2Contents2.1 Mechanism of assembling and morphologyof crystalline P3HT 172.2 P3HT assembling and intermolecular forces 212.3 P3HT crystalline bulk phases 222.4 P3HT surfaces 242.5 Nanocrystalline P3HT 242.6 Conclusions 282.1 mechanism of assembling and morphologyof crystalline p3htThe understanding of the polymer/metal oxide hybridinterface, final aim of this thesis, requires first of all thestudy of the polymeric phase alone. In particular we investigatethe polymer assembling and the mechanisms ofmolecular aggregation, in order to eventually characterizeboth infinite periodic bulks (perfectly crystalline or quasiordered)and finite size nanocrystalline structures.One of the most commonly used conjugated polymer inphotovoltaics is the Poly-3-hexylthiophene (P3HT) since itsunique combination of high carrier mobility (0.1 cm 2 V −1s −1 ), high environmental/thermal stability, electrical conductivity,processability, and synthetic versatility [81].When cast from solvents into thin films, P3HT self-assemblesinto oriented microcrystalline domains (10-60 nm) and amorphousregions [82, 83, 3]. The crystallinity of P3HT thinfilms has considerable impact on the charge-carrier mobility[84] but it is still under debate. The detailed knowledgeof the polymer structure is therefore fundamental and requiresan in-depth investigation.A single P3HT is formed by a π-conjugated thiophenebackbone and alkyl side chains (Figure 2.1). The unit cell ofdimension (7.75 Å) contains two consecutive thiophenesrings and two hexyl side chains each formed by six sp 317
18 p3ht - poly(3-hexylthiophene)carbon atoms. Its regioregular form is the most used variantof the polymer in optolectronic applications [85].Figure 2.1.: P3HT molecule composed by 16 thiophenes.The first step in the study of P3HT is to validate themodel interaction. Specifically, we focus on the π − π interactionthat is dominated by long-range van der Waals dispersiveforces and we adopt as a test case a pair of simplethiophene rings for which accurate first-principles calculationsbeyond Hartree-Fock theory are available (CCSD(T)and MP2 [86]). In Figure 2.2 the calculated MPMD results(symbols) are reported together with ab initio results (continousand dotted lines for CCSD(T) and MP2, respectively).Our model potential reproduce quite well the first-principlesresults being in between the CCSD(T) and MP2 curves, inparticular for the dispersive R −6 tail and for the estimationof the minimum energies [3].We generate two P3HT molecules each formed by 8 monomers(16 thiophenes) with periodic boundary conditions alongthe backbone direction. The equilibrium lattice parameteralong the backbone is 7.75 Å [3].The assembling of the P3HT chains can be driven bytwo main contributions: the π − π interaction between thearomatic rings of the backbones of neighboring molecules,promoting the parallel stacking of different chains, and thechain interdigitation, inducing molecules alignment in thesame plane [3] (see Figure 2.3).As for the π − π binding energy, we consider two polymerchains at varying distance in a face-to-face configurationand we found a minimum at 4 Å (see Figure 2.4).The interactions between backbones are dominated by thethiophene-thiophene interactions calculated above. Additionalsmaller dispersive and electrostatic contributions due
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 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
Page 82 and 83: 5.2 solvent thf interaction with zn
Page 84 and 85: 5.2 solvent thf interaction with zn
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5.2 solvent thf interaction with zn
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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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