PhD thesis - Evans Research Group - University of Wisconsin-Madison

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10The metal evaporation was followed by the lift-off process (Fig. 1.3(f)). The sample wasdipped in acetone under sonication for 10 min to remove PR.SiO 2Si(a)SiO 2Si(d)PhotoresistSiO 2Si(b)UVAuSiO 2Si(e)AuMaskPhotoresistSiO 2Si(c)AuSiO 2Si(f)AuFig. 1.3: Fabrication steps of a bottom contact-FET device. (a) thermally grown oxide, (b)spinning the PR, (c) patterning and exposing the PR, (d) developing the PR, (e) depositingmetal electrodes and (f) removing the PR (lift-off)1.3 Physics of organic semiconductor1.3.1 IntroductionOrganic semiconductor molecules have conjugated structures with alternating single and
11double bonds of carbon-carbon bonds. The molecules have π-orbitals delocalized along theface of a molecule. This orbital delocalization allows electrons to move within a molecule.Organic semiconductors are structurally and chemically quite different from inorganicsemiconductors. Inorganic semiconductors consist of atoms that are covalently or ionicallybonded. The strong interaction between atoms by covalent boding leads to thedelocalization of the individual orbitals of atoms. The interaction between orbitals createsmore energy states and forms a quasi-continuous band in which the energy differencebetween individual energy levels is very small.In organic semiconductors, molecules are linked to each other by comparatively weakvan der waals interactions [38]. Therefore, electrons are largely localized to individualmolecules except electrons in the π orbital, and the weak intermolecular interactions causea narrow electronic bandwidth in molecular solids. The charge transport mechanism inorganic semiconductor is not well understood. As the interaction between π orbitalsincreases the degree of π-π overlap increases. This condition is favorable for the formationof energy bands. Due to the narrow bandwidth arising from the weak interactions betweenmolecules in organic semiconductors the mobilities of charge carriers in organicsemiconductors are low, with typical values of 10 –2 cm 2 /Vs, in comparison with values of100-1000 cm 2 /Vs or more in inorganic semiconductors [40, 49-50].1.3.2 Organic semiconductorsOrganic semiconductor can be divided in two groups. Low molecular weightsemiconductors based on small organic molecules as in Fig. 1.4, and polymers with higher
Page 1: ELECTRONIC AND STRUCTURAL PROPERTIE
Page 6 and 7: vTable of ContentsAbstract………
Page 8 and 9: 7.4 Electrical properties of rubren
Page 10 and 11: 2the interface. For efficient charg
Page 12: 4change the surface energy. The sur
Page 15 and 16: 7A larger negative gate voltage ind
Page 17: 9[46]. The charge transport mechani
Page 21 and 22: 131.3.2.1 PentaceneThe pentacene mo
Page 23 and 24: 15rubrene molecule in the gas phase
Page 25 and 26: 17[17] H. Ishii, K. Sugiyama, E. It
Page 27 and 28: 19[46] M. Vissenberg and M. Matters
Page 29 and 30: 21Chapter 2Channel Formation in Sin
Page 31 and 32: 23This value was comparable to thos
Page 33 and 34: 25The sheet resistance in the four-
Page 35 and 36: 27representing mobility of pentacen
Page 37 and 38: 29to 1.2 ML. After the coverage exc
Page 39 and 40: 31trap density and the two dimensio
Page 41 and 42: 33experiment. The drain current at
Page 43 and 44: 35Increasing the pentacene coverage
Page 45 and 46: 37(a)sheet conductance (1/Ω)2.5 10
Page 47 and 48: 392.5(b)). The change in mobility a
Page 49 and 50: 412.7 References[1] J. M. Shaw and
Page 51 and 52: 43[33] J. Takeya, C. Goldmann, S. H
Page 53 and 54: 45surface normal in the two phases.
Page 55 and 56: 47next higher orbital. Each peak ca
Page 57 and 58: 49Here, θ is the photon incidence
Page 59 and 60: 51plane of the surface we would obs
Page 61 and 62: 53deposited at a low deposition rat
Page 63 and 64: 5521.5M B/M TF10.500.01 0.1 1Deposi
Page 65 and 66: 57Chapter 4Functional Self-Assemble
Page 67 and 68: 59Einμmol= N ( )(4.1)εε d0 molHe
Page 69 and 70:
61Fig. 4.1 shows AFM images of an a
Page 71 and 72:
63(a) (a)(b)Flourinated SAMPentacen
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65Debye and the length of the molec
Page 75 and 76:
67the literature [24, 26]. The turn
Page 77 and 78:
69form the conducting channel. A po
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71(a)(b)80-NH 2+C 60SAM60- drain cu
Page 81 and 82:
732 10 -8 -20 0 20 40 60-drain curr
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75As summarized in Table 4.1, scan
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77drain voltage. The change in the
Page 87 and 88:
79Pentacene thin films deposited on
Page 89 and 90:
81[2] S. H. Kim, J. H. Lee, S. C. L
Page 91 and 92:
83Chapter 5Self-assembled Dipolar C
Page 93 and 94:
85Fig. 5.2: XPS spectra of function
Page 95 and 96:
87In Fig. 5.4(b), the drain current
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89directions. In the scan from 50 V
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91reaction of the device in the DR1
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93100 V, the polarization of the DR
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95gate voltage can cause the photog
Page 105 and 106:
97Chapter 6Ambipolar Rubrene Thin F
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99from the C-H bending bands (out o
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101the size of rubrene islands.6.2.
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103region is 0.67 based on the perc
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105three dimensional growth of rubr
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107(a)(b)0.000110 -80.0001210 -78 1
Page 117 and 118:
109(a)(b)-1 10 -10 0-20 V-30 V-2 10
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111Phys. Rev. Lett. 93, 086602 (200
Page 121 and 122:
113Chapter 7Enhanced Hole Mobility
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115(a)(b)SourceRubrenePolystyreneDr
Page 125 and 126:
117(a)(b)Fig. 7.3: AFM image of rub
Page 127 and 128:
119small average thickness play a v
Page 129 and 130:
121electrodes is much smaller than
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PhD thesis - Evans Research Group - University of Wisconsin-Madison

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