Eunhee Lim

Bio: Eunhee Lim is an academic researcher from KAIST. The author has contributed to research in topics: Fluorene & Polyfluorene. The author has an hindex of 14, co-authored 20 publications receiving 1065 citations. Previous affiliations of Eunhee Lim include KITECH & Kyonggi University.

Synthesis and characterization of a new light-emitting fluorene-thieno[3,2-b]thiophene-based conjugated copolymer

Abstract: A new alternating polyfluorene copolymer, poly(9,9‘-dioctylfluorene-alt-thieno[3,2-b]thiophene) (PFTT), containing a thiophene-condensed thieno[3,2-b]thiophene moiety has been synthesized via a palladium-catalyzed Suzuki coupling reaction. The synthesized polymer was successfully characterized by 1H NMR, 13C NMR, and elemental analysis. It shows good thermal stability and displays unique phase transition behavior between the crystalline and liquid-crystalline states. The ionization potential and electron affinity of PFTT are −5.38 eV and −2.40 eV, respectively, as determined by cyclic voltammetry. Thus, PFTT has an electrochemical band gap of approximately 2.98 eV, which is smaller than that of common polyfluorene (PF) homopolymers. As a film, PFTT exhibits UV−vis and photoluminescence maxima at 471 and 511 nm, respectively. A light-emitting diode device fabricated with an ITO/PEDOT/PFTT/LiF/Al configuration exhibits pure green light emission with the full width at half-maximum (fwhm) of only 57 nm and a ...

High-photosensitivity p-channel organic phototransistors based on a biphenyl end-capped fused bithiophene oligomer

Abstract: We report highly photosensitive organic phototransistors (OPTs) based on a 2,5-bis-biphenyl-4-yl-thieno[3,2-b]thiophene (BPTT). The measured maximum sensitivity and the ratio of photocurrent to dark current (Iph∕Idark) in BPTT OPTs were 82A∕W and 2.0×105 under 380nm UV light with 1.55mW∕cm2, respectively. The prepared OPTs show a photocurrent response similar to the absorption spectrum of BPTT. The major mechanisms for photocurrent amplification in this device were verified from experimental results as photovoltaic (turn-on) and photocurrent effect (turn-off) by a fitting to theoretic equations.

Synthesis, Characterization, and Electroluminescence of New Conjugated Polyfluorene Derivatives Containing Various Dyes as Comonomers

Abstract: Four new fluorene-based alternating polymers (PFR1-S, PFR2-S, PFR3-S, and PFR4-S) containing different comonomers (R1, R2, R3, and R4) have been designed, synthesized, and characterized. These polymers were found to be thermally stable and readily soluble in common organic solvents. Single layer LED devices fabricated from these polymers emitted bluish green to pure red light. Our results show that the color of the light emitted by the homopolymer, poly(9,9-dioctylfluorene-2,7-diyl) (PDOF), can be tuned by incorporating R1, R2, R3, and R4 comonomers, which have narrower band gaps. The absorption and emission maxima of the copolymers varied according to the position of the cyano group in the vinylene unit (α- or β-position) and the type of incorporated aromatic group (thiophene or phenylene). Notably, PFR4-S showed pure red emission (CIE coordinate values x = 0.66, y = 0.33) that is almost identical to the standard red (0.66, 0.34) demanded by the National Television System Committee. PFR3-S also exhibited...

Solution-processable field-effect transistor using a fluorene- and selenophene-based copolymer as an active layer

Abstract: We have synthesized a new p-type polymer, poly(9,9‘-n-dioctylfluorene-alt-biselenophene) (F8Se2), via the palladium-catalyzed Suzuki coupling reaction. The number-average molecular weight (Mn) of F8Se2 was found to be 72 600. F8Se2 dissolves in common organic solvents such as chloroform and chlorobenzene. The PL emission peak of a film of F8Se2 is clearly red-shifted with respect to that of its sulfur analogue, poly(9,9‘-n-dioctylfluorene-alt-bithiophene) (F8T2), due to the electron-donating properties of selenium and the strong interactions between the biselenophene moieties in neighboring copolymer chains. We confirmed that F8Se2 is a thermotropic liquid crystalline polymer with an aligned structure by carrying out DSC, PLM, and XRD measurements. The introduction of the selenophene moiety into the liquid-crystalline polymer system results in better field-effect transistor (FET) performance than that of F8T2. A solution-processed F8Se2 FET device with a bottom contact geometry was found to exhibit a hole...

Semiconducting π-conjugated systems in field-effect transistors: a material odyssey of organic electronics.

Abstract: Since the discovery of highly conducting polyacetylene by Shirakawa, MacDiarmid, and Heeger in 1977, π-conjugated systems have attracted much attention as futuristic materials for the development and production of the next generation of electronics, that is, organic electronics. Conceptually, organic electronics are quite different from conventional inorganic solid state electronics because the structural versatility of organic semiconductors allows for the incorporation of functionality by molecular design. This versatility leads to a new era in the design of electronic devices. To date, the great number of π-conjugated semiconducting materials that have either been discovered or synthesized generate an exciting library of π-conjugated systems for use in organic electronics. 11 However, some key challenges for further advancement remain: the low mobility and stability of organic semiconductors, the lack of knowledge regarding structure property relationships for understanding the fundamental chemical aspects behind the structural design, and realization of desired properties. Organic field-effect transistors (OFETs) are a kind of device consisting of an organic semiconducting layer, a gate insulator layer, and three terminals (drain, source, and gate electrodes). OFETs are not only essential building blocks for the next generation of cheap and flexible organic circuits, but they also provide an important insight into the charge transport of πconjugated systems. Therefore, they act as strong tools for the exploration of the structure property relationships of πconjugated systems, such as parameters of field-effect mobility (μ, the drift velocity of carriers under unit electric field), current on/off ratio (the ratio of the maximum on-state current to the minimum off-state current), and threshold voltage (the minimum gate voltage that is required to turn on the transistor). 17 Since the discovery of OFETs in the 1980s, they have attracted much attention. Research onOFETs includes the discovery, design, and synthesis of π-conjugated systems for OFETs, device optimization, development of applications in radio frequency identification (RFID) tags, flexible displays, electronic papers, sensors, and so forth. It is beyond the scope of this review to cover all aspects of π-conjugated systems; hence, our focus will be on the performance analysis of π-conjugated systems in OFETs. This should make it possible to extract information regarding the fundamental merit of semiconducting π-conjugated materials and capture what is needed for newmaterials and what is the synthesis orientation of newπ-conjugated systems. In fact, for a new science with many practical applications, the field of organic electronics is progressing extremely rapidly. For example, using “organic field effect transistor” or “organic field effect transistors” as the query keywords to search the Web of Science citation database, it is possible to show the distribution of papers over recent years as shown in Figure 1A. It is very clear

Synthesis of Light-Emitting Conjugated Polymers for Applications in Electroluminescent Devices

Abstract: A review was presented to demonstrate a historical description of the synthesis of light-emitting conjugated polymers for applications in electroluminescent devices. Electroluminescence (EL) was first reported in poly(para-phenylene vinylene) (PPV) in 1990 and researchers continued to make significant efforts to develop conjugated materials as the active units in light-emitting devices (LED) to be used in display applications. Conjugated oligomers were used as luminescent materials and as models for conjugated polymers in the review. Oligomers were used to demonstrate a structure and property relationship to determine a key polymer property or to demonstrate a technique that was to be applied to polymers. The review focused on demonstrating the way polymer structures were made and the way their properties were controlled by intelligent and rational and synthetic design.

Functional oligothiophenes: molecular design for multidimensional nanoarchitectures and their applications.

Abstract: 3.2. Thienothiophenes 1216 3.2.1. Thieno[3,4-b]thiophene Analogues 1216 3.2.2. Thieno[3,2-b]thiophene Analogues 1217 3.2.3. Thieno[2,3-b]thiophene Analogues 1218 3.3. , ′-Bridged Bithiophenes 1219 3.3.1. Dithienothiophene (DTT) Analogues 1220 3.3.2. Dithieno[3,2-b:2′3′-d]thiophene-4,4-dioxides 1221 3.3.3. Dithienosilole (DTS) Analogues 1221 3.3.4. Cyclopentadithiophene (CPDT) Analogues 1221 3.3.5. Nitrogen and Phosphor Atom Bridged Bithiophenes 1222

Organic light detectors: photodiodes and phototransistors.

Abstract: While organic electronics is mostly dominated by light-emitting diodes, photovoltaic cells and transistors, optoelectronics properties peculiar to organic semiconductors make them interesting candidates for the development of innovative and disruptive applications also in the field of light signal detection. In fact, organic-based photoactive media combine effective light absorption in the region of the spectrum from ultraviolet to near-infrared with good photogeneration yield and low-temperature processability over large areas and on virtually every substrate, which might enable innovative optoelectronic systems to be targeted for instance in the field of imaging, optical communications or biomedical sensing. In this review, after a brief resume of photogeneration basics and of devices operation mechanisms, we offer a broad overview of recent progress in the field, focusing on photodiodes and phototransistors. As to the former device category, very interesting values for figures of merit such as photoconversion efficiency, speed and minimum detectable signal level have been attained, and even though the simultaneous optimization of all these relevant parameters is demonstrated in a limited number of papers, real applications are within reach for this technology, as it is testified by the increasing number of realizations going beyond the single-device level and tackling more complex optoelectronic systems. As to phototransistors, a more recent subject of study in the framework of organic electronics, despite a broad distribution in the reported performances, best photoresponsivities outperform amorphous silicon-based devices. This suggests that organic phototransistors have a large potential to be used in a variety of optoelectronic peculiar applications, such as a photo-sensor, opto-isolator, image sensor, optically controlled phase shifter, and opto-electronic switch and memory.