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Single Molecular Junction Study for Understanding Effect of Intrachain Charge Transport in Diketopyrrolopyrrole Oligomers

  • Hyun Ju Lee (이현주, 대학원 화학과 유기화학전공)

초록/요약

Molecular tunnel junctions are organic devices miniaturized to the molecular scale. They serve as a versatile toolbox that can systematically examine charge transport behaviors at the atomic level. The electrical conductance of the molecular wire that bridges the two electrodes in a junction is significantly influenced by its chemical structure, and an intrinsically poor conductance is a major barrier for practical applications toward integrating individual molecules into electronic circuitry. Therefore, highly conjugated molecular wires are attractive as active components for the next-generation electronic devices, owing to the narrow highest occupied molecular orbital (HOMO)–lowest occupied molecular orbital (LUMO) gaps provided by their extended π-building blocks. In Chapter 1, We aims to highlight the significance of highly conductive molecular wires in molecular electronics, the structures of which are inspired from conductive organic polymers, and presents a body of discussion on molecular wires exhibiting ultralow, zero, or inverted attenuation of tunneling probability at different lengths, along with future directions. In Chapter 2, thiophene bridging units have remarkably enhanced charge mobility in polydiketopyrrolopyrroles (polyDPPs), a class of widely studied organic polymer semiconductors in organic electronics and optoelectronics. This paper examines the role of thiophene bridging unit in intrachain charge transport through incorporation of a series of molecules comprising DPP units, and β-unsubstituted oligothiophene (Tn), and ethylene bridging units into single-molecule junctions. Junction measurements indicated that the length dependence of rate of charge transport by tunneling relies significantly on the magnitude of applied voltage and the chemical structure of bridging units. Upon transition from low to high bias regimes, an exponential decay of conductance was changed into a nearly decay-less one as the number of -(T-DPP-T)- units increased, while the insertion of ethylene bridging moiety between the Tn units led to an increase in tunneling probability with increasing the molecular length. Results of single-molecule study were reconciled with charge mobility performance in polyDPP-based electronic devices.

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Contents
Abstract Ⅰ
Contents III

Chapter 1.
Achieving Ultralow, Zero, and Inverted Tunneling
Attenuation Coefficients in Molecular Wires with Extended
Conjugation
1.1 Molecular electronics versus inorganic-based commercial devices 2
1.2 Length-dependence model 6
1.3 Molecular structures that exhibit ultralow attenuation coefficients 11
1.3.1 Thiophenes 11
1.3.2 Enes 18
1.3.3 Ynes 20
1.3.4 Phenylene ethynylene 23
1.3.5 Porphyrins 25
1.3.6 Acenes 28
1.3.7 Fluorenes 30
1.3.8 Diketopyrrolopyrroles (DPPs) 32
1.4 Polarity inversion of the attenuation coefficient 34
1.4.1 Thiophenes 34
1.4.2 Fused porphyrins 38
1.4.3 DPPs 40
1.4.4 Polymethines 42
1.4.5 Theoretical models for inverted attenuation 44
1.5 Concluding remarks and outlooks 51
1.6 References 56

Chapter 2.
Single Molecular Junction Study for Understanding Effect of Intrachain Charge Transport in Diketopyrrolopyrrole Oligomers
2.1 Introduction 72
2.2 Experimental section 75
2.2.1 Materials and Characterization 75
2.2.2 Synthesis 76
2.2.3 UV-vis-NIR measurements 80
2.2.4 CV measurements 82
2.2.5 Junction formation 84
2.2.6 Electrical measurements and data analysis 84
2.2.7 Measurements of I-V traces 85
2.3 Results and discussion 87
2.3.1 Synthesis and characterization 87
2.3.2 Junction fabrication and measurements 88
2.3.3 Exponential decay at low bias 89
2.3.4 Length dependence at high bias 92
2.3.5 Upward length dependence 93
2.3.6 Band gaps and energy levels of frontier orbitals 94
2.3.7 Linking my findings with bulk device studies 99
2.4 Conclusion 102
2.5 References 103

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