Byeonggwan Kim

Highly efficient, iodine-free dye-sensitized solar cells with solid-state synthesis of conducting polymers.

In ssDSSCs, hole transporting materials (HTMs) and the control of interfacial properties between the nanoporous TiO 2 layer and the HTM are critical to the photoconversion effi ciency of the cells. Recently, HTMs have been extensively investigated as potential replacements for conventional I 3 − /I − redox electrolyte systems using iodine (I 2 ) in DSSCs. [ 12–18 ] As an organic HTM, spiro-OMeTAD was used for an iodine free ssDSSC and showed higher effi ciency of 5.1%. [ 17 , 18 ] Inorganic HTMs such as CuI showed a good effi ciency of 4.7%, [ 12 ] but the crystal formation of metal oxide gradually reduced cell performance. [ 19 ]

Flexible PEDOT electrodes with large thermoelectric power factors to generate electricity by the touch of fingertips

Highly conductive PEDOT films were prepared by solution casting polymerization using finely tuned oxidation solution and used as electrodes for the precise control of the oxidation level of the polymer electrochemically. They exhibited a large power factor of 1,270 μW m−1 K−2 and could be processed as flexible and cuttable thermoelectric films to generate electricity by fingertips.

Nanopatterning of Mesoporous Inorganic Oxide Films for Efficient Light Harvesting of Dye‐Sensitized Solar Cells

Nanopatterning provides facile process to well-arrayed mesoporous inorganic oxide films at low cost by using readily available pastes and elastomeric nanostamps. The fabricated nanopattern boosted the light-harvesting efficiency of dye-sensitized solar cells (DSSCs) by a light-trapping technique. The iodine-free solid-state DSSCs showed a 40 % increase in the current density and high efficiency (7.03 %).

Solution processable and patternable poly(3,4-alkylenedioxythiophene)s for large-area electrochromic films.

Interest in electrochromic π -conjugated polymers (ECPs) has recently increased considerably for their tunability of colors, low cost of preparation, and low-voltage operation. [ 1–11 ] Among the ECPs, polymers from heterocyclic monomers containing pyrrole, [ 12 ] thiophene, [ 13 ] and selenophene [ 14–16 ] derivatives have been applied to electrochromic devices for simple fi lm processing and high electrochromic contrast. In particular, recent advances in the synthesis of poly(3,4-alkylenedioxythiophene)s (PXDOTs) and their derivatives have offered new opportunities for low energy consumption and electrochemically stable transmissive/absorptive electrochromic devices (ECDs) with fast switching times and full color. [ 17–21 ]

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.

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.

Noninvasive photodetachment of stem cells on tunable conductive polymer nano thin films: selective harvesting and preserved differentiation capacity.

Viable mesenchymal stem cells (MSCs) were efficiently and selectively harvested by near-infrared (NIR) light using the photothermal effect of a conductive polymer nano thin film. The poly(3,4-ethylenedioxy thiophene) (PEDOT)-coated cell culture surfaces were prepared via a simple and fast solution-casting polymerization (SCP) technique. The absorption of PEDOT thin films in the NIR region was effectively triggered cell harvesting upon exposure to an NIR source. By controlling the NIR absorption of the PEDOT film through electrochemical doping or growing PEDOT with different thin film thickness from 70 to 300 nm, the proliferation and harvesting of MSCs on the PEDOT surface were controlled quantitatively. This light-induced cell detachment method based on PEDOT films provides the temporal and spatial control of cell harvesting, as well as cell patterning. The harvested stem cells were found to be alive and well proliferated despite the use of temperature increase by NIR. More importantly, the harvested MSCs by this method preserved their intrinsic characteristics as well as multilineage differentiation capacities. This PEDOT surfaces could be used for repetitive culture and detachment of MSCs or for efficient selection or depletion of a specific subset from heterogeneous population during culture of various tissue-derived cells because there were no photodegradation and photobreakage in the PEDOT films by NIR exposure.

Molecular Engineering of Organic Sensitizers with Planar Bridging Units for Efficient Dye‐Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) have received a great deal of attention as low-cost alternatives to conventional p– n junction solar cells. In these cells, the sensitizer is the key component. Although several Ru polypyridyl complexes exhibited high efficiencies above 10 % and long-term stability, they are quite expensive and hard to purify. Recently, the performance of solar cells based on organic sensitizers has been remarkably improved, resulting in impressive efficiencies in the range of 8–10 %. However, one of the drawbacks of organic sensitizers is the sharp and narrow absorption bands of their UV spectra in the blue region, impairing their light-absorption capabilities. Therefore, molecular engineering of organic sensitizers is required in order to broaden and redshift their absorption spectra. A successful approach was achieved through structural modification of the bridged unit. The introduction of a planar p-conjugated unit in the bridged framework is presumed to be the reason for the increase in the spectral response in the red region of the solar spectrum. Although organic-dye-based cells using an I /I3 electrolyte have afforded high power conversion efficiencies, recent studies on replacing the conventional I /I3 electrolyte with a Co/Co electrolyte have received renewed attention. Recently, Yella et al. reported an efficiency of 12.3 % by using a Co/Co electrolyte in conjunction with a porphyrin sensitizer. While high conversion efficiency of 9–10 % has been reached with organic sensitizers and porphyrin dyes using a liquid electrolyte, such as I /I3 or Co/Co redox couple, the stability issue still remains a major challenge due to leakage and evaporation. Accordingly, extensive studies have been conducted to substitute liquid electrolytes with quasi-solid-state or solid-state electrolytes. Herein, we report meticulously designed organic sensitizers incorporating a planar indenoACHTUNGTRENNUNG[1,2-b]thiophene or indenoACHTUNGTRENNUNG[1,2-b]thienoACHTUNGTRENNUNG[2,3-d]thiophene bridging unit to understand the structure–property relationship (Scheme 1). We also investigate the photovoltaic performance of dyes using I /I3 , Co/Co, polymer gel, and solid-state electrolytes.

Highly conductive PEDOT electrodes for harvesting dynamic energy through piezoelectric conversion

A highly conductive PEDOT material was explored as a transparent electrode for a piezoelectric PVDF film. The potential of a piezoelectric film device consisting of a PEDOT/PVDF/PEDOT layer was realized in fabricating a transparent and flexible energy harvester that generates electricity from repetitive physical displacements (stretching and pressing). The output voltage and current density of the energy generator were 16.4 V (peak-to-peak) and 0.2 μA cm−2 (peak-to-peak), respectively, when applying 800 mN of stretching power. These values are much higher than the PEDOT:PSS–CNT system and comparable to those reported earlier for inorganic-based nanogenerators. Finally, a self-lighting system consisting of an LED bulb, a capacitor, and the PEDOT/PVDF/PEDOT device was demonstrated under a dynamic stretching condition, allowing for the large-scale, low-cost production of a miniaturized active thin-film energy harvester.