Development of Molecular Switch Embedded Multifunctional Organic Opto-electronic Devices

Abstract

Recently, based on the characteristics of low cost, physical flexibility, lightness, large area processability, and solution processing of organic semiconductor materials, they are attracting attention in various electronic devices as next-generation semiconductor materials. The development of organic semiconductors with these unique characteristics has been accomplished as follows. 1) an understanding of the underlying physics of mechanisms such as charge generation and transport: 2) optimization of electronic/opto-electronic performance based on the development of new materials and/or device structures: 3) development of devices that utilize additional functions by embedding functional materials. In the third stage, the most frequently used group of materials is a molecular switch. A molecular switch is a material in which two or more metastable states can be isomerized. External stimuli that induce this isomerization include light, temperature, pH, and pressure. Among them, photochromic molecular switches are the most actively studied field in the field of molecular switches because it can build a non-destructive and remote controllable system. Representative photochromic molecular switches include diarylethene utilizing conjugation length change, azobenzene utilizing trans/cis structural change, spiropyran and spirooxazine utilizing ionic and polarity change. Therefore, molecular-switch-embedded organic electronic device has the potential to realize multi-functionality through non-destructive and effective optical control. Such molecular-switch-embedded opto-electronic devices are being used for diodes, conductors, and transistors, and among them, organic field-effect transistors (OFETs) are spotlighted as a platform that can easily utilize changes in the intrinsic characteristics of molecular switches. Chapter 1 introduces the basic background knowledge of molecular switches and the physical/chemical properties of representative molecular switches, and introduces the semiconductor charge trap control and semiconductor junction control that can be expected from OFETs with molecular switches. In Chapter 2, the materials used in Chapters 3, 4, and 5, as well as device fabrication methods and measurements are comprehensively introduced. Chapter 3 introduces the study of controlling the junction characteristics between the electrode and semiconductor interface using a self-assembled monolayer (SAM) based on azobenzene molecular switch. The change in photoswitching performance using azobenzene SAM-based OFETs were investigated. In order to control the work function of an electrode such as gold, a SAM structure incorporating -SH groups are required, and the uniformity, surface coverage, and film quality of the final SAM may vary depending on the polarity of the processing solvent. At this time, the molecular structure maximized the change in the work function of the electrode by introducing fluorobenzene into the outermost part of the azobenzene SAM structure to maximize the dipole moment that can be caused by the trans/cis change according to light control. In the polar processing solvent as ethanol, aggregation between adjacent molecules can be effectively screened by the solvent when the SAM is formed, and as a result, a dense and high-quality SAM with high coverage can be formed. However, in the case of such a dense SAM, steric hindrance with adjacent molecular structures increases due to trans/cis photoisomerization of azobenzene by light control, making photoswitching difficult. Finally, the surface coverage could be reduced when forming the SAM in a processing solvent condition with low polarity such as chloroform, but the optical switching performance of the opto-electronic device using this could be maximized. Chapters 4, and 5 introduce the study of realizing OFETs using a system capable of photo-controlling the charge traps in semiconductors using diarylethene molecular switches. Diarylethene has two isomers, an open-ring and a closed-ring. When exposed to UV light, closed-ring isomerization occurs, and as the conjugate length decreases, it has a narrow band gap and a high HOMO level compared to the open-ring isomer. When exposed to visible light wavelengths, it is restored to an open-ring isomer and has a wide bandgap and a deep HOMO level. At this time, if the HOMO level of the p-type organic semiconductor is positioned between the HOMO levels of two isomers, a reversible light-controlled charge trap system can be constructed in which hole carriers can be trapped in a closed-ring isomer but are released in an open-ring isomer. In order to realize this system, an OFETs is manufactured by using a blended semiconductor film with an organic semiconductor and a molecular switch. Therefore, I can expect improvements for optical switching performance by controlling the characteristics of the corresponding blending film through chemical modification of a molecular switch and enhancing the crystallographic compatibility between organic semiconductor and diarylethenes by optimizing the chemical structure of organic semiconductor. In Chapter 4, the photoisomerization of molecular switches was investigated, and it was found through single crystal analysis that the open-ring isomer requires about twice the unit volume of the closed-ring isomer. Therefore, in a blending film with an organic semiconductor, which is a solid phase with limited molecular free volume, it is necessary to have a larger molecular free volume for effective isomerization of diarylethene. To this end, diarylethene having different molecular sizes was optimized by adding a longer alkyl chain to the terminal group of the diarylethene. Diarylethene with a long alkyl chain can have a larger molecular free volume in the film, so it was shown that the switching ratio and stability were improved even in the optical switching performance of the actual device. In Chapter 5, control of crystallographic compatibility with organic semiconductors through control of molecular free volume expected through control of diarylethene chemical structure, control of isomerization conversion efficiency (photoelectric efficiency) through control of antiparallel structure of open ring isomer, to control fatigue stability.