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

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viii Introduction ties of these materials. Even more controlled and defect-free structures can be obtained with single crystals. The understanding gained by studying the properties of single crystals can then be exploited for the optimization of the properties of crystalline thin films. Among small molecule organic semiconductors, one of the most promis- ing is rubrene (5,6,11,12-tetraphenyltetracene), which in the single crystal form reaches the highest charge carrier mobility values among organic semiconductors, up to ≈ 40 cm 2 /Vs, a value comparable to that of amorphous silicon[14, 15]. Rubrene single crystals show also an extremely high photo- conductivity and large exciton diffusion length[16, 17]. While rubrene single crystals with a size up to few centimeters can be prepared quite easily by sublimation methods[18], the growth of crystalline thin films with the same structural properties (and thus the same transport properties) of rubrene single crystals is not as straightforward[19–24]. While the choice of the right substrate and deposition technique for the growth of rubrene thin films can lead to crystalline thin films, they always consists of polycrystalline layers with multiple in-plane orientations[25–28]. The high anisotropy of rubrene transport properties make it thus impossible to achieve mobility values close to those found for rubrene single crystals. Rubrene molecules are also strongly affected by oxidation, particularly in the presence of light, leading to the formation of rubrene endoperoxide[29]. Several theoretical and experimental studies have been carried out in order to understand the effects of oxidation over transport and optical properties of crystalline rubrene, but a clear picture is still missing and lots of contra- dictory results have been reported[30–35]. In order to exploit rubrene as the active layer of electronic devices operating in ambient atmosphere a deeper understanding of its oxidation mechanism and of its effects over crystalline rubrene transport properties is thus mandatory. The aim of the experimental work described in this thesis has been to gain more control over the growth of rubrene in the thin film form, by exploiting epitaxy with proper substrates and then to study the effects of the interaction with oxygen over the structural and transport properties of crystalline rubrene. In chapter 1, a general overview of the properties of organic molecular solids is presented, with a focus over the physical properties of organic small molecule crystals.
Chapter 2 is devoted to a thorough description of rubrene physical and chemical properties, with a presentation of the state of the art and of the main open problems related to this material. In chapter 3 the experimental techniques exploited for the preparation and characterization of the samples studied in the following chapters are presented. The growth and structural/morphological characterization of rubrene thin films grown by means of organic molecular beam epitaxy on two different substrates, namely tetracene single crystals and α-quaterthiophene crystalline thin films grown on potassium hydrogen phthalate single crystals, are presented in chapter 4. The latter heterostructure is of particular interest regarding its use in the fabrication of organic cascade solar cells. Here, thanks to the adoption of several characterization techniques it will be shown that rubrene thin films grown on the (0 0 1) surface of tetracene single crystals are crystalline and, even more important, grow according to a unique epitaxial relationship with the substrate, which in turn leads to a unique crystalline orientation of the film in the growth plane. Rubrene thin films grown on α-quaterthiophene still grow according to a unique epitaxial relationship but, due to symmetry reasons, show four different in-plane orientations of their crystalline lattice. Rubrene thin films grown on tetracene, whose structural characterization has been carried out in chapter 4, are then studied with regard to their morphological and structural evolution upon oxidation. The results of this characterization are reported in chapter 5. There, thanks to a combination of various experimental techniques, it is shown that the exposition of rubrene thin films to ambient air leads to the formation of a stable crystalline layer of rubrene endoperoxide molecules, packed according to a specific crystalline structure, on top of the pristine rubrene film. This process somehow resem- bles what happens for silicon with the formation of a native oxide layer. Finally, in chapter 6 the results of dark and photo-conductivity measure- ments carried out on rubrene and rubrene derivatives single crystals, aimed at determining the effects of interaction with oxygen over rubrene transport properties, are presented. There, it is demonstrated that the interaction with oxygen plays an essential role in enhancing crystalline rubrene transport properties. In particular the experimental results suggest that what is important for electrical transport in crystalline rubrene is the interstitial ix
Page 1: Università degli Studi di Milano-B
Page 4 and 5: iv Contents 4 Rubrene thin films 47
Page 6 and 7: vi Contents
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
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Page 50 and 51: 40 Experimental techniques Fω =
Page 52 and 53: 42 Experimental techniques In order
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48 Rubrene thin films for a long ti
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50 Rubrene thin films Parameter Val
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52 Rubrene thin films Figure 4.3: M
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54 Rubrene thin films series of dif
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56 Rubrene thin films present in th
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58 Rubrene thin films Figure 4.6: T
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60 Rubrene thin films Figure 4.7: M
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62 Rubrene thin films for a rubrene
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64 Rubrene thin films Figure 4.9: (
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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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