A soluble high molecular weight copolymer of benzo[1,2-b:4,5-b']dithiophene and benzoxadiazole for efficient organic photovoltaics.

The synthesis and characterization of a soluble high molecular weight copolymer based on 4,8-bis(1-pentylhexyloxy)benzo[1,2-b:4,5-b']dithiophene and 2,1,3-benzoxadiazole is presented. High efficiency organic photovoltaic (OPV) devices comprised of this polymer and phenyl-C(71) -butyric acid methyl ester (PC(71) BM) were fabricated by additive processing with 1-chloronapthalene (CN). When the active layer is cast from pristine chlorobenzene (CB), power conversion efficiencies (PCEs) average 1.41%. Our best condition-using 2% chloronapthalene as a solvent additive in CB-results in an average PCE of 5.65%, with a champion efficiency of 6.05%.

Effect of processing additive on the nanomorphology of a bulk heterojunction material.

The bulk heterojunction (BHJ) material Si-PDTBT:PC(70)BM is sensitive to the use of a small amount of 1-chloronaphthalene (CN) as a processing additive; CN as a cosolvent (e.g., 4% in chlorobenzene) causes in a factor of 2 increase in the power conversion efficiency of BHJ solar cells. The morphology of the BHJ material, prepared with and without the CN additive is studied with top-down transmission electron microscopy, cross-sectional transmission electron microscopy, and atomic force microscopy. The improved performance is the result of changes in the nanoscale morphology. Field-effect transistor measurements are consistent with the observed changes in morphology.

Binuclear initiators for the telechelic synthesis of elastomeric polyolefins.

Novel binuclear complexes, 4,4'-bis{[N-(2,6-diisopropylphenyl)-2-(2,6-diisopropylphenylimino)propanamidato-κ(2)-N,O-(trimethylphosphine)nickel(II)]methyl}-1,1'-biphenyl (2a) and 4,4'-bis{[N-(2,6-diisopropylphenyl)-2-(2,6-diisopropylphenylimino)-4-methylpentamidato-κ(2)-N,O-(trimethylphosphine)nickel(II)]methyl}-1,1'-biphenyl (2b), were synthesized by linking two nickel centers through a bis(benzyl) fragment. When activated with nickel bis(1,5-cyclooctadiene) (Ni(COD)(2)), 2a and 2b are capable of polymerizing ethylene in a quasi-living fashion, producing polymers with approximately twice the molecular weights relative to those obtained by using a structurally related mononuclear system. In addition, 2b/Ni(COD)(2) was utilized to synthesize a series of pseudo-triblock polyethylene (PE) macroinitiating copolymers, bearing atom-transfer radical polymerization (ATRP) initiators. Pseudo-pentablock copolymers were also prepared by taking advantage of a pressure-pulsing technique, wherein the ethylene pressure was increased from 100 to 500 psi in order to produce semicrystalline ethylene-rich end-blocks. Copolymers with elastomeric properties were synthesized by grafting n-butyl acrylate from the PE macroinitiators via ATRP. Examination using monotonic and step-cyclic stress-strain tests demonstrates that the materials exhibit large strains at break (1600-2000%) and excellent elastic recoveries at large strains (∼80%). That materials with such desirable properties could not be attained using a mononuclear initiator demonstrates the clear advantage of growing the polymer via a telechelic mechanism.

New polyethylene macroinitiators and their subsequent grafting by atom transfer radical polymerization.

A semicrystalline polyethylene (PE) macroinitiator was prepared by copolymerizing ethylene and an inititating monomer (5-norbornen-2-yl-2'-bromo-2'-methyl propanoate) (3) using [N-(2,6-diisopropylphenyl)-2-(2,6-diisopropylphenylimino)isobutanamidato]-Ni(eta1-CH2Ph)(PMe3) (1) and Ni(COD)2 (bis(1,5-cyclooctadiene)-nickel) (2). Although 3 decomposes Ni(COD)2, if the initiating species (1/2) are exposed to ethylene for a period of time, t1, and then 3 is introduced for another period of time, t2, the polymerization takes place in a controlled manner. Variations in temperature, pressure, and [3] afford a great deal of control over the composition and architecture of the PE macroinitiator. Subsequent grafting with methyl methacrylate (MMA) yields PE-graft-PMMA with narrow polydispersities and increasing PMMA content at longer reaction times. Because the products are semicrystalline materials with high melting points, they are anticipated to function as blend compatibilizers for linear polyethylene.

Pseudo-tetrablock copolymers with ethylene and a functionalized comonomer.

Pseudo-tetrablock copolymers comprised of ethylene and 5-norbomen-2-yl acetate (1), were synthesized using the initiator system (L(i)Pr2)Ni(eta1-CH2Ph)(PMe3)(2)[(L(i)Pr2) = N-(2,6-diisopropylphenyl)-2-(2,6-diisopropylphenylimino)propanamide] and 2.5 equivalents of Ni(COD)2 [bis(1,5-cyclooctadiene) nickel.

Low band gap donor-acceptor conjugated polymer nanoparticles and their NIR-mediated thermal ablation of cancer cells.

Low band gap D-A conjugated PNs consisting of 2-ethylhexyl cyclopentadithiophene co-polymerized with 2,1,3-benzothiadiazole (for nano-PCPDTBT) or 2,1,3-benzoselenadiazole (for nano-PCPDTBSe) have been developed. The PNs are stable in aqueous media and showed no significant toxicity up to 1 mg · mL(-1) . Upon exposure to 808 nm light, the PNs generated temperatures above 50 °C. Photothermal ablation studies of the PNs with RKO and HCT116 colorectal cancer cells were performed. At concentrations above 100 µg · mL(-1) for nano-PCPDTBSe, cell viability was less than 20%, while at concentrations above 62 µg · mL(-1) for nano-PCPDTBT, cell viability was less than 10%. The results of this work demonstrate that low band gap D-A conjugated polymers 1) can be formed into nanoparticles that are stable in aqueous media; 2) are non-toxic until stimulated by IR light and 3) have a high photothermal efficiency.

Streamlined microwave-assisted preparation of narrow-bandgap conjugated polymers for high-performance bulk heterojunction solar cells.

The most efficient plastic solar cells comprise a blend of conjugated polymer and a suitable electron acceptor, typically a fullerene derivative. Therefore narrow-bandgap conjugated polymers are currently sought for the fabrication of such devices. A significant challenge is being able to predict device function and performance from consideration of the molecular connectivity and dimensions of the partners within the active layer. Improved chemical syntheses are therefore required to make structurally varied polymers and enable the delineation of structure-function relationships with the aim of improving power conversion efficiencies. Here, we demonstrate that microwave heating in combination with the screening of comonomer reactant ratios can be used to obtain donor-acceptor copolymers with high average molecular weights and properties that make them suitable for solar cell incorporation. Furthermore, we highlight the importance of high molecular weight and the contribution of solubilizing side groups in determining the final device properties.