Ji-Eun Jeong

Semi-crystalline photovoltaic polymers with efficiency exceeding 9% in a ∼300 nm thick conventional single-cell device

We report a series of semi-crystalline, low band gap (LBG) polymers and demonstrate the fabrication of highly efficient polymer solar cells (PSCs) in a thick single-cell architecture. The devices achieve a power conversion efficiency (PCE) of over 7% without any post-treatment (annealing, solvent additive, etc.) and outstanding long-term thermal stability for 200 h at 130 °C. These excellent characteristics are closely related to the molecular structures where intra- and/or intermolecular noncovalent hydrogen bonds and dipole–dipole interactions assure strong interchain interactions without losing solution processability. The semi-crystalline polymers form a well-distributed nano-fibrillar networked morphology with PC70BM with balanced hole and electron mobilities (a h/e mobility ratio of 1–2) and tight interchain packing (a π–π stacking distance of 3.57–3.59 A) in the blend films. Furthermore, the device optimization with a processing additive and methanol treatment improves efficiencies up to 9.39% in a ∼300 nm thick conventional single-cell device structure. The thick active layer in the PPDT2FBT:PC70BM device attenuates incident light almost completely without damage in the fill factor (0.71–0.73), showing a high short-circuit current density of 15.7–16.3 mA cm−2. Notably, PPDT2FBT showed negligible changes in the carrier mobility even at ∼1 μm film thickness.

Carbazole linked phenylquinoline-based fullerene derivatives as acceptors for bulk heterojunction polymer solar cells: effect of interfacial contacts on device performance

To understand the effect of interfacial contact between the hole transporting layer (HTL) and fullerene derivatives in the active layer of bulk heterojunction polymer solar cells (BHJ PSCs), carbazole (Cz) linked phenylquinoline (PhQ)-based fullerene derivatives, PhQHCz-C61BM and PhQEOCz-C61BM, have been successfully synthesized. They are used as acceptors with a poly(3-hexylthiophene) (P3HT) donor in the active layer, and PEDOT:PSS and MoO3 were used as the HTL. Both the derivatives are highly soluble in common organic solvents and possess high thermal stability. BHJ PSCs are fabricated with configurations of ITO/PEDOT:PSS/P3HT:PhQHCz-C61BM/LiF/Al, ITO/PEDOT:PSS/P3HT:PhQEOCz-C61BM/LiF/Al, and ITO/MoO3/P3HT:PhQHCz-C61BM/LiF/Al, ITO/MoO3/P3HT:PhQEOCz-C61BM/LiF/Al, and the device characteristics were measured under AM1.5G (100 mW cm−2). Both derivatives exhibited much lower power conversion efficiencies (PCE) of ∼0.1% when PEDOT:PSS was employed as the HTL. In contrast, the PCE increases to ∼2.2% upon replacing PEDOT:PSS with MoO3 as the HTL. This is due to the fact that protonation of the pyridyl nitrogen of the acceptor in the active layer by the –SO3H group of PEDOT:PSS in the HTL, establishes a charge injection barrier at the interfacial contact and leads to restricted charge collection at the electrodes. This was indirectly confirmed by protonation of pyridyl nitrogen in PhQHCz-C61BM by the –SO3H group in p-toluenesulphonic acid.

Ratiometric Fluorescent Ion Detection in Water with High Sensitivity via Aggregation-Mediated Fluorescence Resonance Energy Transfer Using a Conjugated Polyelectrolyte as an Optical Platform

A cationic conjugated polyelectrolyte was designed and synthesized based on poly(fluorene-co-phenylene) containing 5 mol% benzothiadiazole (BT) as a low energy trap and 15-crown-5 as a recognizing group for potassium ions. A potassium ion can form a sandwich-type 2:1 Lewis acid-based complex with 15-crown-5, to cause the intermolecular aggregation of polymers. This facilitates inter-chain fluorescence resonance energy transfer (FRET) to a low-energy BT segment, resulting in fluorescent signal amplification, even at dilute analyte concentrations. Highly sensitive and selective detection of K(+) ions was demonstrated in water. The linear response of ratiometric fluorescent signal as a function of [K(+) ] allows K(+) quantification in a range of nanomolar concentrations with a detection limit of ≈0.7 × 10(-9) M.

Amorphous Thieno[3,2-b]thiophene and Benzothiadiazole Based Copolymers for Organic Photovoltaics

Three types of amorphous thienothiophene (TT)-benzothiadiazole (BT) based copolymers (PFTTBT) were synthesized by incorporating alkyl-substituted fluorene moieties as a third component in the polymer backbone. Their optical, electrochemical, morphological, and photovoltaic properties were examined by a comparison with those of a crystalline TT-BT derivative (PTTBT14). PTTBT14 was reported to have a high hole mobility (0.26 cm(2)/(V s)) due to the pronounced interchain ordering but poor photovoltaic power conversion efficiency (PCE) of 2.4-2.6% was reported due to excessively strong self-interactions with poor miscibility with fullerene structures. By incorporating fluorene units, the UV-vis spectra showed an increased bandgap (∼1.9 eV) with the disappearance of the packing-originated shoulder peak, and the valence band decreased compared to crystalline PTTBT14. The amorphous PFTTBT polymers showed substantially improved photovoltaic properties compared to PTTBT14, even though they showed poor hole mobility (∼10(-6) cm2/(V s)) and fill factor. The optimal devices were achieved by blending with excess PC71BM (polymer:PC71BM=1:4 by weight), showing little improvement in the thermal and additive treatments. Under simulated solar illumination of AM 1.5 G, the best PCE of 6.6% was achieved for a PFehTTBT:PC71BM device with an open-circuit voltage of 0.92 V, a short-circuit current of 15.1 mA/cm2, and a fill factor of 0.48. These results suggest that it is useful to disrupt partially the interchain organizations of excessively crystalline polymers, enabling fine-control of intermolecular ordering and the morphological properties (i.e., miscibility with fullerene derivatives, etc.) to utilize the advantages of both crystalline and amorphous materials for further improving PCE of polymer solar cells.

Combination of conjugated polyelectrolytes and biomolecules: A new optical platform for highly sensitive and selective chemo- and biosensors

AbstractConjugated polyelectrolytes (CPEs) have been studied extensively as the optical platform for chemo- and biosensor applications with high sensitivity by virtue of their light harvesting effect. A range of fluorescent CPEsbased sensing schemes have been developed by utilizing fluorescence quenching via photo-induced electron transfer and fluorescence resonance energy transfer, etc. Highly sensitive and selective detection assays can be realized by combining the tunable optical properties of synthetic CPEs with specific binding characteristics of biomolecules (such as aptamer, antibody, protein, etc.). This article reviews the range of fluorescence-based optical assays with the CPEs-biomolecule complex as a signaling platform with in situ detection, remarkable sensitivity and selectivity, etc.

Enhanced Polarization Ratio of Electrospun Nanofibers with Increased Intrachain Order by Postsolvent Treatments

Polarized emission that is beneficial to lighting and display applications can be demonstrated by aligning emissive chromophores, which can be achieved using an electrospinning technique. We investigate the photophysical properties of nanofibers based on poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]/poly(ethylene oxide) blends both with and without postsolvent treatments. Two different solvents were sequentially used in an attempt to extract the insulating electrospinnable polymer and increase the polarization ratio of the nanofiber meshes by molecular reorganization. The polarization ratio of emission from the nanofiber meshes treated with N,N-dimethylformamide (DMF) following treatment with acetonitrile solvents was found to be increased. An increase in the 0-0 emission vibronic intensity relative to that of the 0-1 peak and a reduction in the photoluminescence (PL) bandwidth were found. In addition, the PL decays faster and the parallel component along the nanofiber axis increases after the DMF treatment, indicating that the radiative recombination process becomes faster. Our results consistently show that postsolvent treatment promotes stronger J-aggregate character, with longer coherence lengths of the exciton along the long axis of the nanofibers, due to enhanced intrachain order.

Synthesis and optical properties of pH-responsive conjugated polyampholytes

Conjugated polyampholytes (CPAs) containing both positive and negative ionic groups in their side-chains were designed and synthesized. Two types of random copolymers were prepared by the incorporation of fluorene, phenylene and 2,1,3-benzothiadiazole (BT) moieties in the main chain. Both quaternary ammonium bromide and carboxylic acid functionalities were introduced successfully in the side-chain through sequential protection, quaternization and deprotection reactions. The resulting ionic polymers were soluble in water and their optical characteristics were examined by changing pH. The fluorescence resonance energy transfer (FRET)-sensitized BT emission of the polymers increased with increasing solution pH. Under basic conditions, deprotonation of the carboxylic acid groups induced intra- and/or interchain aggregation via electrostatic complexation between the cationic ammonium and anionic carboxylate groups. The FRET ratio between the green and blue emissions showed a linear relationship with solution pH. This new type of water-soluble fluorescent bioassays and bioimaging applications through bioconjugation with a targeting moiety.

An Ionic 1,4-Bis(styryl)benzene-Based Fluorescent Probe for Mercury(II) Detection in Water via Deprotection of the Thioacetal Group

Highly sensitive and selective mercury detection in aqueous media is urgently needed because mercury poisoning usually results from exposure to water-soluble forms of mercury by inhalation and/or ingesting. An ionic conjugated oligoelectrolye (M1Q) based on 1,4-bis(styryl)benzene was synthesized as a fluorescent mercury(II) probe. The thioacetal moiety and quaternized ammonium group were incorporated for Hg2+ recognition and water solubility. A neutral Hg2+ probe (M1) was also prepared based on the same molecular backbone, and their sensor characteristics were investigated in a mixture of acetonitrile/water and in water. In the presence of Hg2+, the thioacetal group was converted to aldehyde functionality, and the resulting photoluminescence intensity decreased. In water, M1Q successfully demonstrated highly sensitive detection, showing a binding toward Hg2+ that was ~15 times stronger and a signal on/off ratio twice as high, compared to M1 in acetonitrile/water. The thioacetal deprotection by Hg2+ ions was substantially facilitated in water without an organic cosolvent. The limit of detection was measured to be 7 nM with a detection range of 10–180 nM in 100% aqueous medium.

Cell imaging and DNA delivery in fibroblastic cells by conjugated polyelectrolytes

This study concentrates on the potential application of conjugated polyelectrolytes (CPEs) to cell imaging and DNA delivery. Four different types of polyfluorene copolymers, namely, PAHFP‐Br, PAEFP‐Br, PAHFbT‐Br, and PSBFP‐Na, which have the same π‐conjugated backbone but different side chains, were synthesized. For cytotoxicity testing, L‐929 fibroblastic cells were treated with increasing concentrations (0–50 µM) of each CPE and then cell viability was determined by WST‐8 assay. Cellular uptake of CPEs into cultured L‐929 cells was observed by fluorescence microscopy. To examine DNA delivery by CPEs, the cells were incubated for 1 H with PAHFP‐Br/fluorescein (Fl)‐labeled single‐stranded DNA (ssDNA‐Fl) complex and then visualized by fluorescence microscopy. Cytotoxicity of CPEs was increased in a dose‐dependent manner but at lower than 10 µM, PAHFP‐Br, PAEFP‐Br, and PSBFP‐Na did not show any cytotoxic effects on the cells. When added to cell cultures at 1 µM, PAHFP‐Br/ssDNA‐Fl complex was delivered and then dissociated into PAHFP‐Br and ssDNA‐Fl within the cells. This result implies that PAHFP‐Br can enable cell imaging and DNA delivery into fibroblastic cells. Therefore, it is suggested that PAHFP‐Br with various advantages such as low cytotoxicity and high fluorescence efficiency can be extensively used as a potential agent for cell imaging and gene delivery.

Enhanced Electron Transfer Mediated by Conjugated Polyelectrolyte and Its Application to Washing-Free DNA Detection

Direct electron transfer between a redox label and an electrode requires a short working distance (<1-2 nm), and in general an affinity biosensor based on direct electron transfer requires a finely smoothed Au electrode to support efficient target binding. Here we report that direct electron transfer over a longer working distance is possible between (i) an anionic π-conjugated polyelectrolyte (CPE) label having many redox-active sites and (ii) a readily prepared, thin polymeric monolayer-modified indium-tin oxide electrode. In addition, the long CPE label (∼18 nm for 10 kDa) can approach the electrode within the working distance after sandwich-type target-specific binding, and fast CPE-mediated oxidation of ammonia borane along the entire CPE backbone affords high signal amplification.