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4.3 electronic and optical properties of the system 554.3 electronic and optical properties of thesystem4.3.1 Electronic level alignmentIn this section we report a review of the electronic andoptical properties of the generated ternary ZnO/ZnPc/P3HT,as in [6]. The morphology and structural properties of thesystem found resulting by MPMD, have been used as startingpoint for a DFT+U optimization. The methods used aredescribed in section A.3.The system studied by DFT is composed by a smallerP3HT oligomer (four thiophenes) and a portion of the previouslydescribed ZnO/ZnPc surface in properly periodicboundary conditions.In Figure 4.6 the electronic calculations are summarized.As for the the ZnPc/ZnO and P3HT/ZnO binary systems,both are able to separate the e − h pair with the electrontranferred on the metal oxide and the hole localized inthe organic molecule. This behavior is confirmed by theoccurence of charge transfer as a result of the interactionbetween the ZnPc and the substrate, found by studing theelectronic ground states of a ZnPc/ZnO systems [4].The charge transfer induces a polarization of the interfacelowering the HOMO and the LUMO of the moleculewith respect to the non interacting cases (compare the columnsA and B and G and H in Figure 4.6). Furthermore, in theZnPc case, a splitting of the LUMO orbitals and a mixingwith the ZnO conduction band can be observed [100, 4],resulting in a favourable injection of electrons in the substrate.The electronic properties of the ZnPc/ZnO and P3HT/ZnOsystems can be further investigated by performing a openshellKohn-Sham (ROKS) calculation [101]. By this calculationwe can obtain an approximate description of the lowestexcited state of the systems by keeping fixed the occupationof the Kohn-Sham levels, in order to force the hole inthe HOMO and the electron in the LUMO [4]. The resultsare shown in columns C (for the ZnPc/ZnO system) and F(for the P3HT/ZnO system) in Figure 4.6), and confirm thepresence of the electrons within the ZnO conduction bandminimum while the holes are in the ZnPc or P3HT layers.
56 ternary zno/znpc/p3ht systemThis strong polarization of the donor-acceptor interface resultsin a further lowering of the HOMO and LUMO levelsfor the ZnPc and the P3HT [100].Figure 4.6.: Electronic eigenvalues calculated at the Γ point in the caseof: (A) ZnPc molecule non bonded to the ZnO surface; (B)ZnPc/ZnO interface (ground state); (C) ZnPc/ZnO interface(ROKS excited state); (D) P3HT/ZnPc/ZnO doubleinterface (ground state); (E) P3HT/ZnPc/ZnO doubleinterface (ROKS excited state); (F) P3HT/ZnO interface(ROKS excited state); P3HT/ZnO interface (groundstate); P3HT oligomer non bonded to the ZnO surface.The electronic eigenvalues have been aligned by using the1s level of a He atom inserted as a reference in all thesupercells. CBM and VBM labels indicate the ZnO conductionband minimum and valence band maximum, respectively(figure from [6]).When a layer of ZnPc is put between the ZnO and theP3HT, it produces a favourable alignment of the electronicground state levels of the ternary system (see column D inFigure 4.6).In detail, the P3HT HOMO represents the highest occupiedelectronic level of the ternary system, with the ZnPcHOMO placed below. The ZnO conduction band minimumrepresents the lowest unoccupied electronic level, followedby the ZnPc and P3HT LUMO, both falling within the ZnOconduction band.The position of the P3HT HOMO an LUMO (column Din Figure 4.6), only slightly lower than that of the non inter-
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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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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 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 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
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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