Haijin Shin

NIR-sensitive poly(3,4-ethylenedioxyselenophene) derivatives for transparent photo-thermo-electric converters.

Electrochromism, photothermal effect, and thermoelectric properties of hexyl-derivatized poly(3,4-ethylenedioxyselenophene) are investigated by precisely controlling the morphology. These properties are clearly demonstrated by controlling the applied electrical potential of the polymer films. Especially, the doped polymer film at -0.1 V reveals the highest photothermal conversion efficiency and a power factor of 42.5% and 354.7 μW m(-1) K(-2) , respectively. Efficient visible to near-infrared absorption, photon to heat, and heat to electric conversion has been realized in one film that could benefit in exploiting multifunctional film displays, invisible NIR sensors, photodynamic theragnosis, and thermoelectric devices.

Reversible Full‐Color Generation with Patterned Yellow Electrochromic Polymers

Structural colors in nature are generally much brighter than chemical colors and commonly found in butterflies, beetles, and fish. These colors change in some animals, such as the chameleon, which camouflages itself by blending in with the surrounding colors. However in an artificial chameleon, typically known as chromogenic system, it is difficult to achieve reversible color modulation as nature does, mainly because of the lack of materials or systems that show multicolor under different stimuli in a reproducible way. The structural colors change with respect to the angle of the structure to the eye, or with respect to the depth of the pattern, however, keeping these parameters intact makes reversible color modulation is challenge. In passive gratings, diffractive colors are produced, but color switching is not possible since the refractive index of the grating material is fixed. To generate multicolor and to switch from one color to another, at least three or more gratings with different grating parameters (period and thickness) are required, which are then superimposed to produce a full color scheme. For example, Knop reported multiple-phase gratings using polyvinylchloride with different thickness over 400 nm to generate visible colors. In a recent report, microchannels of poly(dimethylsiloxane) (PDMS) were used as a tunable visual color filter based on microfluidic transmission and a shift from red to blue color was observed. The grating materials with different refractive index were allowed to flow through the microchannels to obtain a different colors. Ge et al. employed colloidal photonic crystals having reversible tunability in response to external magnetic fields. Electroactive thin films patterned by photolithographic or imprint method have been explored for monochromatic diffraction intensity modulation and in a recent report, electroactive subwavelength gratings are used for color and intensity modulation in reflective mode however, there are hardly any reports on the use of patterned conjugated polymers as diffraction gratings for color modulation, despite wide range of conjugated polymers. Thus we considered it challenging to explore electrochemical (EC) gratings as visual color filters and artificial chameleons. Electrochemically active polymers have a great advantage over other materials because of their ability to change redox state under external bias, which results in a change in the refractive index. Herein, we report a new method to obtain an artificial chameleon effect on reversibly electroactive polymer gratings without changing gratings parameters, employing multiple gratings, or changing the surrounding medium. Considering the light dispersion principles, we hypothesized that a color generated from the dispersion of light into a polymer grating can be electrochemically modulated by changing the redox state of the polymer in the grating. Thus, the aim of this study is to demonstrate reversible color modulation by electrochemical reactions using a patterned film of propylenedioxythiophene phenylene copolymer P(ProDOT-Ph). This polymer is cathodically coloring, changes color from yellow to pale blue depending upon applied voltage. We chose the yellow electrochromic polymer because it has a faint color and thus the optical absorption factor change, on applying a voltage, is mainly a result of the change in the refractive index because the extinction coefficient change in the imaginary part is weak. Moreover it shows a greater change in refractive index between the neutral and oxidized state in comparison to P3HT. The change in refractive index was found to be almost two times more than that of P3HT in the small voltage range of 2 to 2 V. The yellow electrochromic polymer P(ProDOT-Ph) was synthesized by a Suzuki coupling reaction following reported procedures (see Supporting Information, Figure S1–S4). A solution with 0.4 wt% of the polymer P(ProDOT-Ph) in chloroform was used for the preparation of the thin film and patterning. The line grating patterns of the polymer were fabricated with the micromolding in capillaries (MIMIC) method. The overall scheme for simple and large area (ca. 1.5 cm) patterning of P(ProDOT-Ph) using the elastomeric PDMS stamp is shown in Figure 1a. An elastomeric PDMS stamp with a line grating with a 10 mm period, 5.6 mm relief feature, and 1 mm pattern depth used was confirmed by scanning electron microscope (SEM) as shown in Figure 1b. The SEM image in Figure 1 c shows the polymer gratings in large area created by MIMIC with an applied pressure of 0.0015 MPa. The patterns were of uniform thickness and were [*] Dr. T. Bhuvana, B. Kim, X. Yang, H. Shin, Prof. E. Kim Department of Chemical and Biomolecular Engineering, Yonsei University 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749 (South Korea) and Active Polymer Center for Pattern Integration (APCPI), Yonsei University 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749 (South Korea) E-mail: eunkim@yonsei.ac.kr Homepage: http://web.yonsei.ac.kr/APCPI [] These authors contributed equally to this work.

Energy saving electrochromic windows from bistable low-HOMO level conjugated polymers

Energy saving electrochromic windows were established by controlling the interfacial charge transport using low-HOMO level ( 90 min) at the voltage-off state with a high color contrast (879 cm2 C−1).

Photothermally Activated Pyroelectric Polymer Films for Harvesting of Solar Heat with a Hybrid Energy Cell Structure

Photothermal effects in poly(3,4-ethylenedioxythiophene)s (PEDOTs) were explored for pyroelectric conversion. A poled ferroelectric film was coated on both sides with PEDOT via solution casting polymerization of EDOT, to give highly conductive and effective photothermal thin films of PEDOT. The PEDOT films not only provided heat source upon light exposure but worked as electrodes for the output energy from the pyroelectric layer in an energy harvester hybridized with a thermoelectric layer. Compared to a bare thermoelectric system under NIR irradiation, the photothermal-pyro-thermoelectric device showed more than 6 times higher thermoelectric output with the additional pyroelectric output. The photothermally driven pyroelectric harvesting film provided a very fast electric output with a high voltage output (Vout) of 15 V. The pyroelectric effect was significant due to the transparent and high photothermal PEDOT film, which could also work as an electrode. A hybrid energy harvester was assembled to enhance photoconversion efficiency (PCE) of a solar cell with a thermoelectric device operated by the photothermally generated heat. The PCE was increased more than 20% under sunlight irradiation (AM 1.5G) utilizing the transmitted light through the photovoltaic cell as a heat source that was converted into pyroelectric and thermoelectric output simultaneously from the high photothermal PEDOT electrodes. Overall, this work provides a dynamic and static hybrid energy cell to harvest solar energy in full spectral range and thermal energy, to allow solar powered switching of an electrochromic display.

Electrofluorescence switching of fluorescent polymer film

AbstractElectrofluorescence switching has attracted much research attention recently due to its potential applications in the areas of ion sensing, bio analysis, fluorescence imaging, and signaling recognition events. This review describes the chemical structure of electrofluorescent polymers their synthetic strategies, and their patterning characteristics. The designs of these polymers are based on fluorescence quenching through an energy transfer between a fluorophore and an acceptor, which is similar to various types of fluorescence-based sensors. Electrofluorescence devices were designed to demonstrate stable switching processes with different durations while measuring the photophysical properties. The patterning process has shown promise if used to create fluorescent displays, sensors, or for security document applications, but an investigation of patternable polymers and related mechanisms remains for future work.

PEDOT as a Flexible Organic Electrode for a Thin Film Acoustic Energy Harvester

An efficient thin film acoustic energy harvester was explored using flexible poly(3,4-ethylene dioxythiophene) (PEDOT) films as electrodes in an all-organic triboelectric generator (AO-TEG). A thin film AO-TEG structured as PEDOT/Kapton//PET/PEDOT was prepared by the solution casting polymerization(SCP) on the dielectric polymer films. As-prepared AO-TEG showed high flexibility and durability due to the strong adhesion between the electrodes and the dielectric polymer. The short-circuit current density (Jsc), open-circuit voltage (Voc), and maximum power density (Pw) reached 50 mA/m(2), 700 V, and 12.9 W/m(2) respectively. The output current density decreased with the increase in the electrode resistance (Re), but the energy loss in the organic electrodes was negligible. The AO-TEG could light up 180 LEDs instantaneously upon touching of the AO-TEG with a palm (∼120 N). With the flexible structure, the AO-TEG was worn as clothes and generated electricity to light LEDs upon regular human movement. Furthermore, the AO-TEG was applicable as a thin film acoustic energy harvester, which used music to generate electricity enough for powering of 5 LEDs. An AO-TEG with a PEDOT electrode (Re = 200 Ω) showed instantaneous peak-to-peak voltage generation of 11 V under a sound pressure level (SPL) of 90-100 dB. The harvested acoustic energy through the AO-TEG was 350 μJ from the 4 min playing of the same single song. This is the first demonstration of a flexible triboelectric generator (TEG) using an organic electrode for harvesting acoustic energy from ambient environment.

NIR Electrofluorochromic Properties of Aza-Boron-dipyrromethene Dyes

The photophysical properties of near-infrared (NIR) emissive aza-boron-dipyrromethene (aza-BDP) dyes incorporating nitrofluorene and alkoxy decorations were intensively investigated. Their highly reversible one-electron reduction process showed characteristic electrofluorochromic (EF) properties in the NIR range, depending on the substituents. The nitrofluorene ethynyl-substituted (Type I) dyes showed smaller EF effects than the alkoxy-containing (Type II) dyes because of the difference in their intrinsic fluorescence contrast between the neutral and reduced states (radical anion). In addition, the Type II chromophores showed a larger diffusion coefficient for ion transport, which enhanced the EF contrast and the response time for the fluorescence change at a given step potential. With an optimized condition, the NIR EF ON/OFF ratio reached a value of 6.1 and a long cyclability over 1000 EF cycles between −0.4 V and +0.4 V switching potentials, with approximately 20% loss of the initial ON/OFF switching ratio. The NIR EF switching was visually observed through a visible light cut-off filter, featuring high fluorescence contrast.

Multi-color fluorescence switching with electrofluorochromic polymers

The multi-color switching of electrofluorochromism was examined from the thin electrofluorochromic (EF) films of polyfluorene (PFO) and poly(propylenedioxythiophene–phenylene) (P(ProDOT-Ph)). The PFO and P(ProDOT-Ph) films showed vivid blue and yellow fluorescence, respectively, at neutral state but their emission was quenched upon application of oxidation potential leading to ion radical states. The fluorescence from the polymer films was reversibly switched to vivid color when the films were returned to their neutral states. The EF color switching ratio (rc1/c2) between the fluorescent state (c1) and dark (c2) state for the yellow color EF device with P(ProDOT-Ph) film was about four times higher than that of the PFO. Because the two polymer films have different colors and working potentials, a multi-color switching device was fabricated by coating the P(ProDOT-Ph) and PFO films onto working and counter electrodes, respectively. The multi-color EF device showed fluorescence switching from blue (B) to white (W) to yellow (Y), and vice versa, depending on the applied potential. The rc1/c2 for yellow (c1) and blue (c2) switching (Y/B) was larger (9.71) than those for Y/W and W/B. Moreover, the EF switching for Y/B in the multi-EF device was also very effective and showed the largest EF efficiency (EEF = 3.82 × 106) among the EF color switching.

Programmable dual electrochromism in azine linked conjugated polymer

A conjugated polymer consisting of tri-EDOT and azine groups was synthesized and explored for a programmable dual electrochromic device. Electrochemical doping (oxidation) and de-doping (reduction) resulted in the two distinct redox pairs, depending on the applied potential range. In situ spectroelectrochemical analyses revealed that the stepwise redox process of two units was associated with dual electrochromic responses: tri-EDOT unit for purple electrochromism followed by azine unit for blue electrochromism. Dual electrochromism of purple and blue was attainable at below 1.0 V which could induce the color transition upon the potential application of around ± 1 V. Maximum coloration efficiency of 393 cm2/C with a response time of 1 s was obtained.

Photothermal effect of conducting polymer electrodes for hybrid energy harvester

Solar energy harvesting research has largely concentrated on photovoltaic conversion, that is, generating electricity from light. To increase electricity outputs, researchers have experimented with hybridized energy conversion methods, such as solar cells that convert mechanical vibration into electricity.1, 2 Sunlight also contains heat, a potential source of additional energy. Using photothermal conversion to generate electricity from superfluous heat through pyroelectric or thermoelectric conversion could provide an effective path to harvesting abundant solar heat. Other efforts to harvest solar heat, such as hybrid solar cells with thermoelectric modules or solar selective absorbers, have shown an increase in energy collection, but energy conversion and heat collection were not optimized. Furthermore, the hybrid harvesters have been bulky. In this work, we use a hybrid energy harvester with a photothermal layer—a thin polymer film combined with a pyroelectric polymer film—to collect solar heat. The hybrid energy harvester (see Figure 1) in this work collects solar heat using photothermal,3 pyroelectric,4 and thermoelectric5 methods combined with a solar cell.6 It consists of a dye-sensitized solar cell (DSSC), a pyroelectric film device, and a thermoelectric device. We also built circuitry composed of capacitors, switches, and a diode bridge rectifier to operate an LED lamp and electrochromic display (ECD) device. The rectifier converted the pyroelectric film device output (AC to DC), which was then stored in a capacitor (47mF, 25V) used to turn on the LED lamp. A second capacitor (10F, 5.4V) stored output energy generated by the series-connected photovoltaic and thermoelectric modules and drove the ECD. Figure 1. Energy harvesting and storage circuitry diagram consisting of an LED lamp, capacitors, switches, and an electrochromic display (ECD) device (insets show photographic images of ECD switching operation and an LED lit up). DSSC: Dye-sensitized solar cell. PVDF-TrFE: Poly(vinylidenefluoride-co-trifluoroethylene) film. PEDOT: Poly(3,4-ethylenedioxythiophene) electrode.