Growth and physical properties of crystalline rubrene - BOA Bicocca ...

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16 Rubrene: physical and chemical properties Figure 2.5: Polar plot showing the field-effect mobility values of a rubrene single crystal measured along various directions in the (b,c) plane. The b and c lattice directions are labeled on the plot. The different symbols indicate mobility values determined for different source-drain voltages of the field-effect transistor fabricated for the measurements. From [49] mobility values measured in the (b,c) plane of a rubrene single crystal reported in figure 2.5[14, 49]. Rubrene, like most organic semiconductors, is also a unipolar p-type material, since the reported high mobility values re- gard only hole transport, while electron mobility is smaller by several orders of magnitude[50]. The lattice direction of maximum mobility corresponds to the direction of the π − π stacking of the rubrene molecules (see also figure 2.3c). This fact strongly suggests that the conduction mainly originates from the over- lap of molecular π orbitals. Indeed, the perfect alignment along the short molecular axis between adjacent rubrene molecules is not found in the case of other similar materials with a similar molecular structure and crystalline packing, but worse charge transport properties (e.g. tetracene or pentacene, whose molecules are instead slightly displaced along the short molecular axis)[42, 51]. The role of π − π stacking in the electrical transport properties of orthorhombic rubrene has also been confirmed by measuring mobility as a function of the pressure applied to the crystal, showing that to an in- creasing pressure, and thus to a decreasing of the intermolecular distances, corresponds an increase in mobility[52]. The strong anisotropy in the transport properties of rubrene single crystals suggests that the main conduction mechanism in these systems is band- like transport. Even if there is still some debate whether this picture is correct or not, other strong pieces of evidence in its favor have been re-
2.5 Optical properties of crystalline rubrene 17 Figure 2.6: (a) Absorption spectra of a rubrene single crystal collected at normal incidence on the (b,c) surface and with light polarized along the b lattice direction (black) and c lattice direction (red). (b) Photoluminescence spectra of a pristine rubrene crystal collected at normal incidence on the (b,c) surface and with an excitation wavelength of 473 nm at 15 K (black) and 300 K (red).The arrow indicates the 650 nm PL peak attributed to crystal defects (see also section 2.6). ported: the low amount of traps and scattering centers in bulk rubrene single crystals, the results of Hall effect measurements, the inverse depen- dence of the mobility on temperature up to temperatures as high as 300 K, Seebeck effect measurements on rubrene FETs, direct measurements of the HOMO band dispersion and UPS (ultraviolet photoelectron spectroscopy) measurements[14, 53–57]. 2.5 Optical properties of crystalline rubrene The normal incidence absorption spectrum and photoluminescence (PL) spectrum of a rubrene single crystal exposing the (1 0 0) surface are reported in figures 2.6a and b, respectively. The absorption spectrum, reflecting the molecular packing in orthorhombic rubrene, in which the molecules have an herringbone packing motif, with their short axes (M ) all parallel to the a lattice direction, is strongly anisotropic, with maximum absorption in the visible range for light polarized along the b lattice direction. Apart from this anisotropy, the spectrum strongly resembles that of the single molecule, as expected for an organic crystal in which the intermolecular interactions are extremely weak. The absorption spectra are characterized by the vibronic progression with main peaks at 2.50 eV, 2.67 eV and 2.86 eV, attributed to b- and c-polarized vibronic transitions originating from the second, LN - polarized, molecular transition (where the L and N molecular axes lie in the
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Page 4 and 5: iv Contents 4 Rubrene thin films 47
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Page 8 and 9: viii Introduction ties of these mat
Page 10 and 11: x Introduction molecular oxygen inc
Page 12 and 13: 2 Organic semiconductors Figure 1.1
Page 14 and 15: 4 Organic semiconductors 1.3.1 Opti
Page 16 and 17: 6 Organic semiconductors Figure 1.2
Page 18 and 19: 8 Organic semiconductors 1.3.2 Elec
Page 20 and 21: 10 Organic semiconductors
Page 22 and 23: 12 Rubrene: physical and chemical p
Page 38 and 39: 28 Experimental techniques Figure 3
Page 42 and 43: 32 Experimental techniques 3.2 Samp
Page 46 and 47: 36 Experimental techniques the scan
Page 48 and 49: 38 Experimental techniques by repre
Page 50 and 51: 40 Experimental techniques Fω =
Page 52 and 53: 42 Experimental techniques In order
Page 56 and 57: 46 Experimental techniques whole di
Page 58 and 59: 48 Rubrene thin films for a long ti
Page 60 and 61: 50 Rubrene thin films Parameter Val
Page 62 and 63: 52 Rubrene thin films Figure 4.3: M
Page 64 and 65: 54 Rubrene thin films series of dif
Page 66 and 67: 56 Rubrene thin films present in th
Page 68 and 69: 58 Rubrene thin films Figure 4.6: T
Page 70 and 71: 60 Rubrene thin films Figure 4.7: M
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66 Rubrene thin films Figure 4.11:
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72 Rubrene thin films The presence
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74 Rubrene thin films present. Sinc
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76 Rubrene thin films ≈ −33.5 k
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78 Rubrene thin films
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80 Oxidation dynamics of rubrene ep
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100 Oxidation dynamics of rubrene e
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102 Influence of oxygen on electric
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118 Conclusions and Perspectives On
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120 Conclusions and Perspectives
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122 Bibliography [13] D. A. Bernard
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124 Bibliography [41] V. Coropceanu
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126 Bibliography [71] A. J. Maliaka
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128 Bibliography [101] C. Kittel, I
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130 Bibliography [130] D. Beljonne,
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132 Bibliography
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134 PhD activity Attended courses
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