tesi R. Miscioscia.pdf - EleA@UniSA

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44 State of the Art and other techniques which are inherently expensive, difficult to maintain and scale-up and down. For this reason, as an instance, starting from promising molecule pentacene (or other acene) through a process of molecular functionalization, Anthony and others [16], obtained high hole mobilities in field effect transistors thanks to the high molecular order which the interaction of the functional groups added to pentacene imposed to the molecules of the material deposited (see figure 13). figure 13: comparison between OFET devices performances obtained in functionalized pentacene in relationship to ordering characteristics of modified molecules deposited films [16]. This depends on the degree of packing and order of the molecules and, therefore, on how the π orbitals overlap along the channel direction. The goal of processes of functionalization of small molecules for organic semiconductors is to insert functional groups and to use solution processing allowing crystalline structures to self-assemble into lattices increasingly extended thus improving the order and the mobility and reducing the amount of defects. An example of this is given by the study of Garnier on a still widely used class of oligomers (thiophenes)[17]. Authors attempted the formation of films with high mobility and, therefore, higher order
Chapter 2 45 factor analysing, in particular, the hexathiophene (sexithiophenes) by means of XRD (X-Ray Diffraction) characterizations. 2.4.3 High performance n-type materials in OTFT processing Studies on thiophenes (by Garnier et Al.) [17] have been also analyzed and enriched by Facchetti and Marks through the insertion of additional functional groups in the aim of reducing the energy levels of HOMO and LUMO (Highest Occupied Molecular Orbital / Lowest Unoccupied Molecular Orbital) of these materials [18] and thus to make possible their operation as semiconductors in n-channel TFTs (n-TFT). In the referred studies, the best results were obtained on a standard topology (SiO2 dielectric on a silicon gate, gold contacts underneath the channel) with the DFH-4T deposited on an heated substrate at temperatures T> 60 ° C. Device exhibited a mobility of 0.24 cm 2 / (V *s ) and on/off ratios of 10 7 . In further works, other analyses and improvements have been obtained by means of the optimization and characterization on the molecular scale [19][20] by studying the variation of the electrical characteristics of films at different deposition temperature reaching outstanding results for such devices: µDFHCO-4T = 0.6 cm 2 /Vs (n-type), Ion/off > 10 7 , VT = 10V). Since the characteristics of the interface and the deposition process have to be considered a key factor in device’s operation, in other cited studies, the mobility, threshold voltages and ratio on / off are compared versus the type of dielectric, the deposition conditions materials, etc. [21] to investigate the link between device processing and measured performances. Because of the availability of a plethora of OSCs having good holes transport properties, many efforts have been concentrated on the achievement of OSCs having enough n-type conduction capabilities to be employed as channel materials in n-TFTs in enhancement operation (this operation mode is the only possible to achieve channel charge modulation in TFTs because of the lack of the possibility to perform in the inversion mode). As we told n-type
Page 1 and 2: Università degli Studi di Salerno
Page 3 and 4: Acknowledgments It is a pleasure to
Page 5 and 6: In loving memory of my uncle, Profe
Page 7 and 8: Introduction Nowadays, it has becom
Page 9 and 10: Introduction 3 The drawbacks of org
Page 11 and 12: Introduction 5 Artificial Skin (OTF
Page 13 and 14: Introduction 7 structure has been d
Page 15 and 16: Chapter 1 Working principles An Org
Page 17 and 18: Chapter 1 11 1.1 A model for DC beh
Page 19 and 20: Chapter 1 13 hp. 4) Decoupling betw
Page 21 and 22: Chapter 1 15 figure 2: integration
Page 23 and 24: Chapter 1 17 eq. 11 − = Then, bei
Page 25 and 26: Chapter 1 19 The formula in eq. 18
Page 27 and 28: Chapter 1 21 and behaves as a resis
Page 29 and 30: Chapter 1 23 eq. 22 I DS = μ C ins
Page 31 and 32: Chapter 1 25 In eq. 27, kB is the B
Page 33 and 34: Chapter 1 27 eq. 30 · Thu
Page 35 and 36: Chapter 2 A survey on the state of
Page 37 and 38: Chapter 2 31 emphasizing the role p
Page 39 and 40: Chapter 2 33 Only after those years
Page 41 and 42: Chapter 2 35 In fact, usually the m
Page 43 and 44: Chapter 2 37 Examples of top-gate b
Page 45 and 46: Chapter 2 39 deposition process of
Page 47 and 48: Chapter 2 41 region is patterned by
Page 49: Chapter 2 43 figure 11: Non-Relief-
Page 53 and 54: Chapter 2 47 In literature results,
Page 55 and 56: Chapter 2 49 It is clear that the c
Page 57 and 58: Chapter 2 51 In figure 17 the impro
Page 59 and 60: Chapter 2 53 figure 19: OFET trans-
Page 61 and 62: Chapter 2 55 figure 22: Comparison
Page 63 and 64: Chapter 2 57 channel and gate in Pe
Page 65 and 66: Chapter 2 59 figure 28: Pentacene b
Page 67 and 68: Chapter 2 61 figure 31: OTFT realiz
Page 69 and 70: Chapter 2 63 materials, the higher
Page 71 and 72: Chapter 2 65 The effect of the barr
Page 73 and 74: Chapter 2 67 figure 36: potential p
Page 75 and 76: Chapter 2 69 From table 2, it is ea
Page 77 and 78: Chapter 2 71 thermal-budget treatme
Page 79 and 80: Chapter 2 73 in particular in OLEDs
Page 81 and 82: Chapter 2 75 One interesting charac
Page 83 and 84: Chapter 2 77 figure 48: electrical
Page 85 and 86: Chapter 2 79 The research on the OE
Page 87 and 88: Chapter 2 81 photolysis). In this p
Page 89 and 90: Chapter 2 83 [26] Facchetti, Wasiel
Page 91 and 92: Chapter 3 Gate-leakages in polymeri
Page 93 and 94: Gate-leakages 87 determined by XRD
Page 95 and 96: Gate-leakages 89 The results are:
Page 97 and 98: Gate-leakages 91 It has to be consi
Page 99 and 100: Gate-leakages 93 figure 8: Output c
Page 101 and 102:
Gate-leakages 95 figure 12: electri
Page 103 and 104:
Gate-leakages 97 3.4 Advanced circu
Page 105 and 106:
Gate-leakages 99 figure 16: SEM ima
Page 107 and 108:
Gate-leakages 101 modeled with a co
Page 109 and 110:
Gate-leakages 103 eq. 11 • Shadow
Page 111 and 112:
Gate-leakages 105 References [1] R.
Page 113 and 114:
Chapter 4 Morphology-mobility relat
Page 115 and 116:
Chapter 4 109 insulators to increas
Page 117 and 118:
Chapter 4 111 3.2 General device st
Page 119 and 120:
Chapter 4 113 Pentacene islands are
Page 121 and 122:
Chapter 4 115 For the preparation o
Page 123 and 124:
Chapter 4 117 figure 3: PFTEOS:TEOS
Page 125 and 126:
Chapter Chapter 4 4 119 (a): (a):
Page 127 and 128:
Chapter 4 121 3.4 Device’s charac
Page 129 and 130:
Chapter 4 123 mobility with the dim
Page 131 and 132:
Chapter 4 125 As it can be observed
Page 133 and 134:
Chapter 4 127 eq. 4 Where r
Page 135 and 136:
Chapter 4 129 3.4.3 Thermal charact
Page 137 and 138:
Chapter 4 131 Ea(eV) 0.50 0.45 0.40
Page 139 and 140:
Chapter 4 133 I D (A) 100.0n 80.0n
Page 141 and 142:
Chapter 4 135 μ (cm 2 V -1 s -1 )
Page 143 and 144:
Chapter 4 137 T MN (K) 2400 2200 20
Page 145 and 146:
Chapter 4 139 Such results, supply
Page 147 and 148:
Chapter 4 141 References [1] O. D.
Page 149 and 150:
Chapter 4 143 [26] P. Palestri, D.
Page 151 and 152:
Conclusions In this PhD dissertatio
Page 153 and 154:
Introduction 147 characterizing mor
Page 155 and 156:
Table of acronyms and symbols Symbo
Page 157:
Table of acronyms and symbols _____
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