A series of benzodithiophene-thiophene-based alternating copolymers were synthesized with different side-chains, and their photovoltaic characteristics were examined. The choice of solubilizing side-chains influences significantly the chain conformation, frontier orbital energy levels, intermolecular organization, and the resulting optical, morphological, and photovoltaic properties. The incorporation of an e-withdrawing carbonyl group in the side-chain decreased the highest occupied molecular orbital (HOMO, ca. −5.4 eV) level and improved the chain planarity through intrachain hydrogen bonding. The shortest π–π stacking distance (3.72 Å) was also measured for the alkylcarbonyl-substituted BDTCOT:PC₇₁BM blended film by two dimensional grazing incidence X-ray scattering. With compared to other polymers, the BDTCOT:PC₇₁BM device showed a substantially improved open-circuit voltage and short-circuit current density, leading to a 4.66% power conversion efficiency. The side-chains need to be designed to be multifunctional to induce a deep HOMO level and chain planarity (for interchain ordering) as well as good solution processability. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 854–862

A new potassium ion detection assay was developed using a dye-labeled aptamer and conjugated polyelectrolyte (CPE) as a signaling platform via 1-step and 2-step fluorescence resonance energy transfer. Guanine-rich K⁺-specific aptamers were designed as K⁺ ion recognition species with 6-carboxyfluorescein (6-FAM) and 6-carboxytetramethylrhodamine (6-TAMRA) at both termini. In the presence of K⁺ ions, the aptamers undergo a conformational change from an unfolded to folded form by forming a G-quadruplex with K⁺, bringing two dyes in proximity. FRET-induced 6-TAMRA emission was proportional to [K⁺] in a range of 22.5 μm–100 mm in water without interference by the presence of excess Na⁺ ions (100 mm). Upon the addition of CPE, a two-step FRET process from CPE to 6-TAMRA via 6-FAM was enabled, showing an intensified 6-TAMRA signal with K⁺ ions. The dynamic detection range and limit of detection (LOD) was fine-tuned from ∼millimolar to ∼nanomolar concentrations of K⁺ by modulating the signal amplification effect of CPE. The LOD was determined to be ≈3.0 nm. This detection assay also showed high selectivity against other metal ions. This sensing scheme can be extended to the detection of a wide range of target materials by simply modifying the recognition aptamer sequence.

Despite the excellent work function adjustability of conjugated polyelectrolytes (CPEs), which induce a vacuum level shift via the formation of permanent dipoles at the CPE/metal electrode interface, the exact mechanism of electron injection through the CPE electron transport layer (ETL) remains unclear. In particular, understanding the ionic motion within the CPE ETLs when overcoming the sizable injection barrier is a significant challenge. Because the ionic functionality of CPEs is a key component for such functions, a rigorous analysis using highly controlled ion density (ID) in CPEs is crucial for understanding the underlying mechanism. Here, by introducing a new series of CPEs with various numbers of ionic functionalities, energy level tuning at such an interface can be determined directly by adjusting the ID in the CPEs. More importantly, these series CPEs indicate that two different mechanisms must be invoked according to the CPE thickness. The formation of permanent interfacial dipoles is critical with respect to electron injection through CPE ETL (≤ 10 nm, quantum mechanical tunneling limit), whereas electron injection through thick CPE ETL (20–30 nm) is dominated by the reorientation of the ionic side chains under a given electric field.

A series of poly[9-(heptadecan-9-yl)-9H-carbazole-2,7-diyl-alt-(5,6-bis-(octyloxy)-4,7-di(thiophen-2-yl)benzo-[1,2,5]-thia-diazole)-5,5-diyl] compositions containing various ratios of 3,6-carbazole was synthesized for testing in a polymer solar cell. An appropriate amount of 3,6-carbazole units incorporated into the copolymer improved intermolecular charge transport, whereas excess amount of 3,6-carbazole units temporarily seized on the partial negative charge generated in the conjugation breaks. We extensively studied the effects of the incorporated 3,6-carbazole units on the intermolecular interactions, which can affect nongeminated recombination in bulk heterojunction-polymer solar cells. These properties were investigated using photocurrent- and light intensity-dependent measurements and electrochemical impedance spectroscopy. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 2047–2056

A low-band-gap alternating copolymer, poly{5,6-bis(octyloxy)-4-(thiophen-2-yl)benzo[c]-1,2,5-thiadiazole} (PTBT), was synthesized and investigated for photovoltaic applications. PTBT showed a minimized torsion angle in its main backbone owing to the introduction of solubilizing octyloxy groups on the electron-poor benzothiadiazole unit, thereby resulting in pronounced intermolecular ordering and a deep level of the HOMO (−5.41 eV). By blending PTBT with [6,6]phenyl-C61-butyric acid methyl ester (PC₆₁BM), highly promising performance was achieved with power-conversion efficiencies (PCEs) of 5.9 and 5.3 % for the conventional and inverted devices, respectively, under air mass 1.5 global (AM 1.5G, 100 mW cm⁻²) illumination. The open-circuit voltage (V_OC≈0.85–0.87 V) is one of the highest values reported thus far for thiophene-based polymers (e.g., poly(3-hexylthiophene) V_OC≈0.6 V). The inverted device also achieved a remarkable PCE compared to other devices based on low-band-gap polymers. Ideal film morphology with bicontinuous percolation pathways was expected from the atomic force microscopy (AFM) images, space-charge-limited current (SCLC) mobility, and selected-area electron-diffraction (SAED) measurements. This molecular design strategy is useful for achieving simple, processable, and planar donor–acceptor (D–A)-type low-band-gap polymers with a deep HOMO for applications in photovoltaic cells.

In this study, two low bandgap copolymers composed of fluorene (Fl), cyclopentadithiophene (CDT), and 4,7-bis(2-thienyl)-2,1,3-benzothiadiazole (DBT) were synthesized, and their optical, electrochemical, and photovoltaic (PV) characteristics were investigated for applications in PV devices. The feed ratio of the Fl and CDT moieties was modulated to tune the electronic structures and resulting optical properties of the polymers. In the copolymeric structures, the Fl-CDT unit absorbs the short-wavelength UV/vis regions, and the CDT-DBT (or Fl-DBT) unit with strong intramolecular charge transfer characteristics covers the long-wavelength visible regions. P1 exhibited a wide UV absorption spectrum covering the UV and entire visible region in the range of 300–800 nm, and P2 showed absorption covering from 300 to 700 nm. UV/vis and electrochemical studies confirmed the desirable highest occupied molecular orbital/lowest unoccupied molecular orbital levels of the copolymers with bandgaps of 1.62–1.86 eV, enabling efficient electron transfer and a high open-circuit voltage when blending them with fullerene derivatives. When the polymers were blended with [6,6]phenyl-C₆₁-butyric acid methyl ester, P1 exhibited the best device performance with an open-circuit voltage of 0.66 V, short-circuit current of 4.92 mA cm⁻², and power conversion efficiency of 1.13% under Air Mass 1.5 Global (AM 1.5G, 100 mW cm⁻²) illumination. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

We have synthesized four types of cyclopentadithiophene (CDT)-based low-bandgap copolymers, poly[{4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-2,6-diyl}-alt-(2,2′-bithiazole-5,5′-diyl)] (PehCDT-BT), poly[(4,4-dioctyl-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-2,6-diyl)-alt-(2,2′-bithiazole-5,5′-diyl)] (PocCDT-BT), poly[{4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-2,6-diyl}-alt-{2,5-di(thiophen-2-yl)thiazolo[5,4-d]thiazole-5,5′-diyl}] (PehCDT-TZ), and poly[(4,4-dioctyl-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-2,6-diyl)-alt-{2,5-di(thiophen-2-yl)thiazolo[5,4-d]thiazole-5,5′-diyl}] (PocCDT-TZ), for use in photovoltaic applications. The intramolecular charge-transfer interaction between the electron-sufficient CDT unit and electron-deficient bithiazole (BT) or thiazolothiazole (TZ) units in the polymeric backbone induced a low bandgap and broad absorption that covered 300 nm to 700–800 nm. The optical bandgap was measured to be around 1.9 eV for PehCDT-BT and PocCDT-BT, and around 1.8 eV for PehCDT-TZ and PocCDT-TZ. Gel permeation chromatography showed that number-average molecular weights ranged from 8000 to 14 000 g mol⁻¹. Field-effect mobility measurements showed hole mobility of 10⁻⁶–10⁻⁴ cm² V⁻¹ s⁻¹ for the copolymers. The film morphology of the bulk heterojunction mixtures with [6,6]phenyl-C₆₁-butyric acid methyl ester (PCBM) was also examined by atomic force microscopy before and after heat treatment. When the polymers were blended with PCBM, PehCDT-TZ exhibited the best performance with an open circuit voltage of 0.69 V, short-circuit current of 7.14 mA cm⁻², and power conversion efficiency of 2.23 % under air mass (AM) 1.5 global (1.5 G) illumination conditions (100 mW cm⁻²).

An indenofluorene-based copolymer containing blue-, green-, and red light-emitting moieties was synthesized by Suzuki polymerization and examined for application in white organic light-emitting diodes (WOLEDs). Tetraoctylindenofluorene (IF), 2,1,3-benzothiadiazole (BT), and 4,7-bis(2-thienyl)-2,1,3-benzothiadiazole (DBT) derivatives were used as the blue-, green-, and red-light emitting structures, respectively. The number-average molecular weight of the polymer was determined to be 25,900 g/mol with a polydispersity index of 2.02. The polymer was thermally stable (T_d = ∼398 °C) and quite soluble in common organic solvents, forming an optical-quality film by spin casting. The EL characteristics were fine-tuned from the single copolymer through incomplete fluorescence energy transfer by adjusting the composition of the red/green/blue units in the copolymer. The EL device using the indenofluorene-based copolymer containing 0.01 mol % BT and 0.02 mol % DBT units (PIF-BT01-DBT02) showed a maximum brightness of 4088 cd/m² at 8 V and a maximum current efficiency of 0.36 cd/A with Commission Internationale de L'Eclairage (CIE) coordinates of (0.34, 0.32). The EL emission of PIF-BT01-DBT02 was stable with respect to changes in voltage. The color emitted was dependent on the thickness of the active polymer layer; layer (∼60 nm) too thin was unsuitable for realizing WOLED via energy transfer. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3467–3479, 2009