Jongbeom Na

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.

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.

TiO2 nanoparticulate-wire hybrids for highly efficient solid-state dye-sensitized solar cells using SSP-PEDOTs

TiO2 photoanodes for I2-free solid-state dye-sensitized solar cells (ssDSSCs) were prepared from multifunctional new TiO2 nanostructures to enhance light harvesting and charge collection efficiency in ssDSSCs using poly(3,4-ethylenedioxythiophene)s (PEDOTs). A new type of TiO2 paste containing TiO2 nanowires (TNW) was prepared and successfully transformed to TiO2 nanoparticulate-wire hybrids (TNPW) with a large surface area of 61.4 m2 g−1 through thermal annealing. The thickness of the TNPW layer could be controlled up to 17 μm without cracks. As a as hole transport material, PEDOTs were infiltrated into the TNPW photoanode through in situ solid-state polymerization (SSP-PEDOTs) and N719 dyes were adsorbed to give ssDSSCs. The SSP-PEDOTs based ssDSSCs with TNPW photoanodes recorded a high cell efficiency (η) of 6.4% and short-circuit current (Jsc) of 14.3 mA cm−2 without scattering particles, which were 30.6 and 22.2% higher than those of traditional TiO2 nanoparticles (TNP) in the same conditions. Furthermore, liquid-state DSSCs with the TNPW photoanode attained a η of 8.4%, which was superior to that of a reference TNP cell (7.3%). The maximum η values were 7.1 and 9.9% for ssDSSCs and liquid type DSSCs, respectively, in the presence of additional scattering layers to support the importance of TNPWs. These enhanced photovoltaic performances of TNPW cells could be attributed to the unique TNPW structure that is advantageous for high charge collection with a long electron diffusion path and large surface area required for high dye adsorption efficiency.

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.

Roll type conducting polymer legs for rigid-flexible thermoelectric generator

A roll-type conducting polymer film was explored as a flexible organic p-type thermoelectric leg using poly(3,4-ethylenedioxythiophene) (PEDOT) doped with tosylate. The PEDOT films were prepared through solution casting polymerization and rolled up for a roll-type leg. Due to the high flexibility, the roll-type PEDOT leg enabled easy contact to both top and bottom electrodes. Simulation on the dynamic heat transfer and convective cooling for a vertically roosted rod- and roll-type PEDOT leg showed that the temperature difference (ΔT) between the hot and cold sides of the leg was much higher in the roll than that of the rod. The PEDOT legs were integrated with n-type Bi2Te3 blocks, to give a 36-couple rigid-flexible thermoelectric generator (RF-TEG). The maximum output voltage from the 36-couple RF-TEG under a ΔT of 7.9 K was determined as 36.7 mV along with a high output power of 115 nW. A wearable RF-TEG was prepared upon the combination of the 36-couple RF-TEG with an arm warmer, to afford an output vol...

Transparent Electrochemical Gratings from a Patterned Bistable Silver Mirror

Silver mirror patterns were formed reversibly on a polystyrene (PS)-patterned electrode to produce gratings through the electrochemical reduction of silver ions. The electrochemical gratings exhibited high transparency (T > 95%), similar to a see-through window, by matching the refractive index of the grating pattern with the surrounding medium. The gratings switch to a diffractive state upon the formation of a mirror pattern (T < 5%) with a high diffraction efficiency up to 40%, providing reversible diffractive gratings. The diffraction state was maintained in the voltage-off state (V-off) for 40 min, which demonstrated bistable reversible electrochemical grating (BREG) behavior. By carefully combining the BREGs through period matching, dual-color switching was achieved within the full color region, which exhibited three distinct optical switching states between -2.5, 0, and +2.5 V. The wide range of light tenability using the metallic BREGs developed herein enabled IR modulation, NIR light reflection, and on-demand heat transfer.

Cross Stacking of Nanopatterned PEDOT Films for Use as Soft Electrodes

Cross stacking of nanopatterned conductive polymer film was explored using a sacrificial soft template made of nanopatterned polystyrene (PS) film as a guide for nanopatterned conductive polymer film. For use as a conductive film, the PS pattern was filled with poly(3,4-ethylenedioxythiophene) (PEDOT), and then completely removed, to generate single-patterned PEDOT (SPDOT) film having a conductivity of 1079 S/cm, which was comparable to the pristine unpatterned PEDOT (UPDOT) film on a glass slide. SPDOT layers were stacked across each other to form double-layered (DPDOT) and multiple-layered patterned PEDOT film on a glass slide or polymeric substrate. The patterned PEDOT film showed enhanced optical and electrochemical activity; specifically as compared to the UPDOT film on a glass slide, the DPDOT film showed an increase in reflectance and an enhanced electrochemically active surface by 23.4% and 32.8%, respectively. The patterned PEDOT film on a polymer substrate showed high bendability up to being completely folded and maintained its conductivity for over 10 000 cycles of bending. The patterned PEDOT layers were applied to dye-sensitized solar cells (DSSCs) as a transparent conductive oxide (TCO)-free counter electrode. An N719-based DSSC with a DPDOT film recorded a photoconversion efficiency of 7.54%, which is one of the highest values among the TCO-free DSSCs based on a PEDOT counter electrode.

Protein-Engineered Large Area Adipose-derived Stem Cell Sheets for Wound Healing

Human adipose-derived stem cells (hADSCs) formed robust cell sheets by engineering the cells with soluble cell adhesive molecules (CAMs), which enabled unique approaches to harvest large area hADSC sheets. As a soluble CAM, fibronectin (FN) (100 pg/ml) enhanced the cell proliferation rate and control both cell-to-cell and cell-to-substrate interactions. Through this engineering of FN, a transferrable hADSC sheet was obtained as a free-stranding sheet (122.6 mm2) by a photothermal method. During the harvesting of hADSC sheets by the photothermal method, a collagen layer in-between cells and conductive polymer film (CP) was dissociated, to protect cells from direct exposure to a near infrared (NIR) source. The hADSC sheets were applied to chronic wound of genetically diabetic db/db mice in vivo, to accelerate 30% faster wound closure with a high closure effect (εwc) than that of control groups. These results indicated that the engineering of CAM and collagens allow hADSC sheet harvesting, which could be extended to engineer various stem cell sheets for efficient therapies.

Photothermal effect in conductive polymer layers for structural conversion into a complex 3D structure (Conference Presentation)

The conversion of photons to heat in the conductive polymer films causes local heating to increase temperature at the light exposed area. The resultant heat can be converted into other type of energy such as electrical, mechanical, or chemical energy. In particular, photothermal effect in conductive polymer layers could be used for structural changes of the 2D structures into a complex three-dimensional (3D) structure. Herein we report the preparation of photothermal conductive polymer layers (CPL) and the integration of CPLs into a 2D structured film, to optimize not only the light-to-heat but also 2D-to-3D structural conversion.