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4.3 electronic and optical properties of the system 57acting oligomer (column H in Figure 4.6), indicates that thepresence of the ZnPc layer, hinders the P3HT/ZnO chargerecombination. The tendency of the electrons to drop intothe ZnO conduction band minimum while the holes remainin the P3HT can be further confirmed by ROKS calculations(column E in Figure 4.6).The previous analysis can be summarized as follows:• The e-h pairs can be generated both in the ZnPc moleculesand in the P3HT oligomers thanks to their comparablyhigh absorption coefficients.• Due to the existence of strongly mixed ZnO/ZnPclevels, the electrons reach easily the ZnO conductionband. The ZnPc HOMO is lowered (Figure 4.6, columnC) but its potential energy difference with theP3HT HOMO is raised (Figure 4.6, column D), favouringthe injection of the hole into the P3HT.• The injection is supported also by the close “face-toface”proximity of the organic moieties.• The presence of the ZnPc interlayer causes higherpotential energy of an excited electron in the P3HTLUMO (Figure 4.6, colum G) with respect to the P3HT/ZnOsystem (Figure 4.6, colum D). This results again in abetter injection of electrons into the ZnO conductionband through the ZnPc layer.• On the other hand, the hole transfer from the P3HTlayer to the ZnPc layer is not likely to occur due tothe lowering of the P3HT HOMO (Figure 4.6, columE).4.3.2 Charge densities and recombinationAs widely discussed in chapter 1, one of the major limitationsto the efficiency of hybrid interfaces is the recombinationbetween the charges. In the ternary system herestudied, the ZnPc layer act as an electronic spacer thathinders the e-h recombination. This assertion is shown inFigure 4.7, where the electrons and holes charge density(calculated by using the ROKS method) of the P3HT/ZnOand P3HT/ZnPc/ZnO systems are reported in panels C, D
58 ternary zno/znpc/p3ht systemFigure 4.7.: Photogenerated electron and hole displacementsin the cases of binary P3HT/ZnO and ternaryP3HT/ZnPc/ZnO interfaces. A (B): z-projections ofthe e and h charge densities in the case of a P3HT/ZnO(P3HT/ZnPc/ZnO double) interface; C and D (E andF): Electronic density plots of singly occupied ROKSorbitals, see the text, containing a photogenerated holeand electron, respectively, in the case of a P3HT/ZnO(P3HT/ZnPc/ZnO double) interface. Charge densitiesrelated to holes (electrons) are sampled at 0.0005 (0.0001)e/a.u. 3 (figure from [6]).and E, F. The projections of the same densities along the zaxis are reported in panels A and B. In the case of the binarysystem, a 12% overlap between the electron and holecharge densities has been found. This overlap is mainlydue to the partial electrons delocalisation on the P3HTbackbone (light blue isosurface in Figure 4.7 D), while theholes are almost fully confined in the P3HT backbone (greenisosurface in Figure 4.7 C). In the case of the ternary system,the overlap is reduced to 4%, due to a major localizationof the electrons on the ZnO surface (light blue isosurfacein Figure 4.7 F), while the holes are mainly presentin the P3HT backbone, with a smaller contribution of theZnPc layer (green isosurface in Figure 4.7 E).4.3.3 Absorption spectraAs anticipated, an important characteristic of the ternarysystem under consideration are its peculiar optical proper-
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Università degli Studi di Cagliari
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A C K N O W L E D G M E N T SI woul
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example of a novel class of systems
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C O N T E N T S1 introduction 11.1
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L I S T O F F I G U R E SFigure 1.1
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List of FiguresxiiiFigure 2.15 Init
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List of FiguresxvFigure 4.6Figure 4
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Figure 5.7Figure 5.8Figure A.1Figur
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2 introduction1-3 eV) and the trans
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4 introductionorder in such a compl
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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 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 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
Page 102 and 103: B I B L I O G R A P H Y[1] http://w
Page 104 and 105: ibliography 85[17] J. D. Servaites,
Page 106 and 107: ibliography 87Zakeeruddin, and M. G
Page 108 and 109: ibliography 89Phys. Chem. Chem. Phy
Page 110 and 111: ibliography 91[65] D. J. Binks and
Page 112 and 113: ibliography 93cells,” Energy Envi
Page 114 and 115: ibliography 95[97] C. Ingrosso, A.
Page 116 and 117: ibliography 97298.15 k,” Journal
Page 118 and 119: ibliography 99charges. am1-bcc mode
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