Molecular-Scale Understanding of Cohesion and Fracture in P3HT:Fullerene Blends.

Quantifying cohesion and understanding fracture phenomena in thin-film electronic devices are necessary for improved materials design and processing criteria. For organic photovoltaics (OPVs), the cohesion of the photoactive layer portends its mechanical flexibility, reliability, and lifetime. Here, the molecular mechanism for the initiation of cohesive failure in bulk heterojunction (BHJ) OPV active layers derived from the semiconducting polymer poly(3-hexylthiophene) [P3HT] and two monosubstituted fullerenes is examined experimentally and through molecular-dynamics simulations. The results detail how, under identical conditions, cohesion significantly changes due to minor variations in the fullerene adduct functionality, an important materials consideration that needs to be taken into account across fields where soluble fullerene derivatives are used.

Decohesion kinetics in polymer organic solar cells.

We investigate the role of molecular weight (MW) of the photoactive polymer poly(3-hexylthiophene) (P3HT) on the temperature-dependent decohesion kinetics of bulk heterojunction (BHJ) organic solar cells (OSCs). The MW of P3HT has been directly correlated to its carrier field effect mobilities and the ambient temperature also affects OSC in-service performance and P3HT arrangement within the BHJ layer. Under inert conditions, time-dependent decohesion readily occurs within the BHJ layer at loads well below its fracture resistance. We observe that by increasing the MW of P3HT, greater resistance to decohesion is achieved. However, failure consistently occurs within the BHJ layer representing the weakest layer within the device stack. Additionally, it was found that at temperatures below the glass transition temperature (∼41-45 °C), decohesion was characterized by brittle failure via molecular bond rupture. Above the glass transition temperature, decohesion growth occurred by a viscoelastic process in the BHJ layer, leading to a significant degree of viscoelastic deformation. We develop a viscoelastic model based on molecular relaxation to describe the resulting behavior. The study has implications for OSC long-term reliability and device performance, which are important for OSC production and implementation.

An analysis of nicotine conditioned place conditioning in early postweanling and adolescent rats neonatally treated with quinpirole.

This study investigated nicotine place conditioning in early postweanling and adolescent male and female rats neonatally treated with quinpirole, a dopamine D(2)/D(3) agonist. Previous research has shown that neonatal quinpirole treatment results in an increase of dopamine D(2)-like receptor sensitivity that persists throughout the animal's lifetime, relevant to psychosis. Rats were neonatally treated with quinpirole or saline from postnatal day (P)1-21, and animals were conditioned with nicotine or saline daily from P23-30 as early postweanlings or P32-39 as adolescents in a two- or three-chambered place conditioning apparatus. A drug free test was given on P31 for early postweanlings, and P40 for adolescents. Results on the two chamber apparatus revealed that nicotine increased time spent in the drug-paired context at both ages tested. Neonatal quinpirole treatment resulted in less time spent in the drug-paired context in early postweanling males and increased time spent in the drug-paired context in adolescent females conditioned with nicotine. Adolescent females neonatally treated with saline and conditioned with nicotine on the two chamber apparatus did not differ from controls. On the three-chambered apparatus, nicotine increased time spent in the drug-paired context in both ages tested, which was blocked by neonatal quinpirole in early postweanling males, but enhanced by neonatal quinpirole treatment in adolescents. These results demonstrate both age and sex differences in the effects of nicotine and point to significant differences in performance depending on the apparatus used. Additionally, neonatal quinpirole enhanced the effects of nicotine, but this is true only in adolescents and task-dependent.

Templated chemistry for sequence-specific fluorogenic detection of duplex DNA.

We describe the development of templated fluorogenic chemistry for detection of specific sequences of duplex DNA in solution. In this approach, two modified homopyrimidine oligodeoxynucleotide probes are designed to bind by triple-helix formation at adjacent positions on a specific purine-rich target sequence of duplex DNA. One fluorescein-labeled probe contains an α-azidoether linker to a fluorescence quencher; the second (trigger) probe carries a triarylphosphine group that is designed to reduce the azide and cleave the linker. The data showed that at pH 5.6 these probes yielded a strong fluorescence signal within minutes on addition to a complementary homopurine duplex DNA target. The signal increased by a factor of about 60, and was completely dependent on the presence of the target DNA. Replacement of cytosine in the probes with pseudoisocytosine allowed the templated chemistry to proceed readily at pH 7. Single nucleotide mismatches in the target oligonucleotide slowed the templated reaction considerably; this demonstrated high sequence selectivity. The use of templated fluorogenic chemistry for detection of duplex DNAs has not been previously reported and could allow detection of double-stranded DNA, at least for homopurine-homopyrimidine target sites, under native and nondenaturing conditions.