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Current-Voltage Characteristics of Organic CURRENT-VOLTAGE CHARACTERISTICS OF ORGANIC SEMICONDUCTORS: INTERFACIAL CONTROL BETWEEN ORGANIC LAYERS AND ELECTRODES A Thesis Presented to The Academic Faculty by Takeshi Kondo In Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the School of Chemistry and Biochemistry Georgia Institute of Technology August, 2007 Copyright © Takeshi Kondo 2007 CURRENT-VOLTAGE CHARACTERISTICS OF ORGANIC SEMICONDUCTORS: INTERFACIAL CONTROL BETWEEN ORGANIC LAYERS AND ELECTRODES Approved by: Dr. Seth R. Marder, Advisor Dr. Joseph W. Perry School of Chemistry and Biochemistry School of Chemistry and Biochemistry Georgia Institute of Technology Georgia Institute of Technology Dr. Bernard Kippelen, Co-Advisor Dr. Mohan Srinivasarao School of Electrical and Computer School of Textile and Fiber Engineering Engineering Georgia Institute of Technology Georgia Institute of Technology Dr. Jean-Luc Brédas School of Chemistry and Biochemistry Georgia Institute of Technology Date Approved: June 12, 2007 To Chifumi, Ayame, Suzuna, and Lintec Corporation ACKNOWLEDGEMENTS I wish to thank Prof. Seth R. Marder for all his guidance and support as my adviser. I am also grateful to Prof. Bernard Kippelen for serving as my co-adviser. Since I worked with them, I have been very fortunate to learn tremendous things from them. Seth’s enthusiasm about and dedication to science and education have greatly influenced me. It is always a pleasure to talk with Seth on various aspects of chemistry and life. Bernard’s encouragement and scientific advice have always been important to organize my research. I have been fortunate to learn from his creative and logical thinking. I must acknowledge all the current and past members of Prof. Marder’s and Prof. Kippelen’s research groups for their collaborations and friendships. Also I am thankful for my committee members providing valuable suggestions on my thesis. Some of the works presented in this thesis were the results of collaborations among different groups. I am grateful to Dr. Qing Zhang in Prof. Marder’s group for teaching synthesis techniques. I appreciate some materials provided by Dr. Shijun Zheng and Michal Malicki in Prof. Marder’s group, PES and IPES data provided by Prof. Antoine Kahn at Princeton University, and DFT data provided by Prof. Jean-Luc Brédas at Georgia Institute Technology. I must thank LINTEC Corporation for supporting my family’s life and funding for the project related with the work in this thesis. I have had this important period of time with my family. I am deeply grateful to my wife Chifumi who supports me with love and tender heart, and to my lovely children Ayame and Suzuna who always give me sweetest smiles. iv TABLE OF CONTENTS Page ACKNOWLEDGEMENTS iv LIST OF TABLES viii LIST OF FIGURES ix LIST OF SCHEMES xvii LIST OF SYMBOLS xviii LIST OF ABBREVIATIONS xx SUMMARY xxii CHAPTER 1 INTRODUCTION 1 1.1 Organization of the thesis 1 1.2 I-V characteristics of metal/semiconductor contacts 3 1.3 Charge transport in insulating films 11 1.4 Charge transport in organic semiconductors 14 1.5 Electronic traps in organic semiconductors 18 1.6 Amorphous molecular materials 22 1.7 State-of-the-art of two-terminal organic memory devices 32 1.8 References 44 2 CHARGE CARRIER MOBILITIES OF ORGANIC SEMICONDUCTORS INVESTIGATED BY I-V CHARACTERISTICS AND COMPARED TO DIFFERENT MEASUREMENT METHODS 52 2.1 Introduction 52 2.2 Theories of charge carrier mobility characterization techniques 56 2.3 Comparison of hole mobilities and disorder parameters 65 v 2.4 Three different mobility measurement techniques 71 2.5 Concluding remarks 82 2.6 Experimental 84 2.7 References 86 3 HIGH CHARGE CARRIER MOBILITIES IN TRI-SUBSTITUTED HEXAAZATRINAPHTHYLENE DERIVATIVES 89 3.1 Introduction 89 3.2 Syntheses and separations 95 3.3 Estimation of electron affinities and ionization potentials 101 3.4 Thermal analyses and X-ray diffraction studies 106 3.5 Influence of cooling rate on film morphologies of pure isomer b 113 3.6 Charge transport properties 118 3.7 Concluding remarks 125 3.8 Experimental 126 3.9 References 133 4 SILVER NANODOTS ATTACHED ON ITO SURFACE AS A TRAP LAYER IN ORGANIC MEMORY DEVICES 137 4.1 Introduction 137 4.2 Ag-NDs attached to an ITO surface 140 4.3 Nonvolatile organic memory device using the Ag-NDs attached on ITO surfaces 152 4.4 Studies of the switching mechanism 162 4.5 Examination of different organic materials 167 4.6 Concluding remarks 170 4.7 References 171 vi 5 ELECTRICAL SWITCHING EFFECTS IN AN AIR-STABLE SOLUTION- PROCESSED DEVICE CONTAINING SILVER ION AND A HEXAAZATRINAPHTHYLENE DERIVATIVE 175 5.1 Introduction 175 5.2 Operation hypothesis and device design 178 5.3 Film morphologies of the AgNO3:4a/b complex 185 5.4 Dependence of switching characteristics on Ag concentration 186 5.5 Dependence of switching characteristics on PVP thickness 190 5.6 Reliability of switching 192 5.7 Concluding remarks 199 5.8 References 200 6 CONCLUSIONS AND OUTLOOK 202 vii LIST OF TABLES Page Table 1.1: Summary of basic conduction processes in insulators 13 Table 1.2: Summary of charge carrier mobilities of molecular glasses. 28 Table 1.3: Summary of the literature survey for organic memory devices. 38 Table 2.1: Summary of the mobilities and disorder parameters extrapolated from the TOF and SCLC measurements by using the same ITO/PS:TPD=1:1 molar ratio (d = 20 µm)/ITO sample. The values of µ are at V/d = 1.0 × 105 V/cm and room-temperature. The values of µ0 and γ are at room-temperature. 69 Table 2.2: Electrochemical data and electronic spectroscopic data for compound I and TPD. 78 Table 2.3: Summary of the comparison between three different measurements. 83 Table 3.1: Vertical and adiabatic electron affinities (VEA and AEA, respectively) and vertical and adiabatic ionization potentials (VIP and AIP, respectively), relaxation energies for the neutral [λ1] and radical-anion/cation [λ2] potential wells, and the intramolecular reorganization [λ] energy for reduction/oxidation for 1, 2b, and 3b. All energies are in eV. 104 Table 3.2: Summary of electrochemical potential (±0.02 V, V vs. ferrocenium/ferrocene) for 1, 2, 3, and 4 in CH2Cl2 / 0.1 M n + – [ Bu4N] [PF6] . 105 Table 3.3: Electronic absorption data for 1, 3, and 4. 106 Table 3.4: Glass-transition temperatures (Tg), crystallization temperatures (Tc), and melting points (Tm) of the compounds 3 and 4. 109 Table 3.5: Summary of the carrier mobility characteristics at room-temperature for films of 3 and 4. 124 Table 4.1: Atomic compositions derived from the survey spectra in Figure 4.3. 145 Table 4.2: Summary of the contact angles for the five different samples. 151 Table 4.3: Summary of the switching properties of the devices evaluated in this chapter. 169 viii LIST OF FIGURES Page Figure 1.1: Energy-band diagrams of metal-semiconductor contacts. q = magnitude of electronic charge, φm = work function of metal, φBn = barrier height of metal-semiconductor, χ = electron affinity of semiconductor, EC = bottom of conduction band, EF = Fermi energy level, EV = top of valence band, Vn = EC – EF, Vbi = built-in potential, and W = depletion width. 6 Figure 1.2: Energy-band diagrams of metal/n-type and p-type semiconductors under different biasing conditions: (a) thermal equilibrium; (b) forward bias; and (c) reverse bias. 8 Figure 1.3: Four basic transport processes under forward bias. 10 Figure 1.4: Distribution of HOMO and LUMO levels in amorphous organic semiconductors. 15 Figure 1.5: Schematic representation of (a) a hole transfer from molecule A to molecule B going through a transition state, as assumed in the semiclassical model, and (b) the two energetic terms λ1 and λ2 defining the inner reorganization energy. 17 Figure 1.6: (a) Most probable jump of a carrier from a tail state to the transport energy Et. (b) Trap states in the tail, additional trap states and regular transport states. 20 Figure 1.7: Schematic crosspoint memory architecture. 34 Figure 2.1: (a) Evolution of a theoretical signal in drift hole mobility experiments. (b) Pulse shape in case of deep traps conditions. 58 Figure 2.2: OFET device configurations: (a) Top-contact device, with source and drain electrodes evaporated onto the organic semiconducting layer. (b) Bottom-contact device, with the organic semiconducting deposited onto prefabricated source and drain electrodes. 64 Figure 2.3: (a) Transient photocurrents observed for an ITO/PS: TPD (20 µm)/ITO device at different temperatures. The applied field is 1.75 × 105 V/cm. (b) I-V characteristics of the same device in (a) at different temperatures. The dashed lines represent the prediction from the SCLC model characterizing the field-dependent mobility of Eq. 2.10. 67 ix Figure 2.4: Electric-field dependence of the hole mobilities measured for the ITO/PS: TPD = 1:1 molar ratio (d = 20 µm)/ITO device at different temperatures. Symbols represent the TOF experimental data. Lines represent the calculated data obtained from the SCLC experiments. 68 Figure 2.5: (a) Temperature dependence of the zero-field mobility values for the ITO/PS: TPD = 1:1 molar ratio (d = 20 µm)/ITO device measured by two different methods: TOF and SCLC. The lines represent a linear regression according to the disorder formalism. (b) The coefficient 2 of field dependent mobility versus σˆ (σˆ = σ / k BT ) measured by two different methods. The lines are linear fits according to the disorder formalism (Eq. 2.6). 68 Figure 2.6: Current density, J, versus electric field V/d of ITO/PS:TPD = 1:1 molar ratio (d = 20 µm)/ITO device at room-temperature. The open circles represent the experimental current density, Jdc, in the SCLC experiments.
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