Improving the long-term stability of PBDTTPD polymer solar cells through material purification aimed at removing organic impurities

While bulk heterojunction (BHJ) solar cells fabricated from high Mn PBDTTPD achieve power conversion efficiencies (PCE) as high as 7.3%, the short-circuit current density (JSC) of these devices can drop by 20% after seven days of storage in the dark and under inert conditions. This degradation is characterized by the appearance of S-shape features in the reverse bias region of current–voltage (J–V) curves that increase in amplitude over time. Conversely, BHJ solar cells fabricated from low Mn PBDTTPD do not develop S-shaped J–V curves. However, S-shapes identical to those observed in high Mn PBDTTPD solar cells can be induced in low Mn devices through intentional contamination with the TPD monomer. Furthermore, when high Mn PBDTTPD is purified via size exclusion chromatography (SEC) to reduce the content of low molecular weight species, the JSC of polymer devices is significantly more stable over time. After 111 days of storage in the dark under inert conditions, the J–V curves do not develop S-shapes and the JSC degrades by only 6%. The S-shape degradation feature, symptomatic of low device lifetimes, appears to be linked to the presence of low molecular weight contaminants, which may be trapped within samples of high Mn polymer that have not been purified by SEC. Although these impurities do not affect initial device PCE, they significantly reduce device lifetime, and solar cell stability is improved by increasing the purity of the polymer materials

Linear Side Chains in Benzo[1,2-b:4,5-b′]dithiophene–Thieno[3,4-c]pyrrole-4,6-dione Polymers Direct Self-Assembly and Solar Cell Performance

While varying the size and branching of solubilizing side chains in π-conjugated polymers impacts their self-assembling properties in thin-film devices, these structural changes remain difficult to anticipate. This report emphasizes the determining role that linear side-chain substituents play in poly(benzo[1,2-b:4,5-b′]dithiophene–thieno[3,4-c]pyrrole-4,6-dione) (PBDTTPD) polymers for bulk heterojunction (BHJ) solar cell applications. We show that replacing branched side chains by linear ones in the BDT motifs induces a critical change in polymer self-assembly and backbone orientation in thin films that correlates with a dramatic drop in solar cell efficiency. In contrast, we show that for polymers with branched alkyl-substituted BDT motifs, controlling the number of aliphatic carbons in the linear N-alkyl-substituted TPD motifs is a major contributor to improved material performance. With this approach, PBDTTPD polymers were found to reach power conversion efficiencies of 8.5% and open-circuit voltages of 0.97 V in BHJ devices with PC71BM, making PBDTTPD one of the best polymer donors for use in the high-band-gap cell of tandem solar cells

Dendrimers as anti-inflammatory agents

Dendrimers constitute an intriguing class of macromolecules which find applications in a variety of areas including biology. These hyperbranched macromolecules with tailored backbone and surface groups have been extensively investigated as nanocarriers for gene and drug delivery, by molecular encapsulation or covalent conjugation. Dendrimers have provided an excellent platform to develop multivalent and multifunctional nanoconjugates incorporating a variety of functional groups including drugs which are known to be anti-inflammatory agents. Recently, dendrimers have been shown to possess anti-inflammatory properties themselves. This unexpected and intriguing discovery has provided an additional impetus in designing novel active pharmaceutical agents. In this review, we highlight some of the recent developments in the field of dendrimers as nanoscale anti-inflammatory agents

Concentration-dependent thermochromism and supramolecular aggregation in solution of triblock copolymers based on lengthy oligothiophene cores and poly(benzyl ether) dendrons

Self-complexation of triblock copolymers based on undeca- and heptadecathiophene cores attached to generation three Fréchet-type poly(benzyl ether) dendrons in solution at low temperature leads to the formation of relatively small but well-defined supramolecular assemblies. The formation of these aggregates is associated with a strong thermochromic effect. Upon cooling dilute solutions, minor concentration-independent changes in the optical absorption due to intrachain planarization precede significant concentration-dependent thermochromic changes that are shown to originate from intermolecular p-p stacking interactions. The structurally well-defined nature of the substrates enables the determination of distinct temperature regimes for both intra- and intermolecular thermochromic changes. Furthermore, combining thermodynamic expressions for both the concentration and temperature dependence of aggregation allows a quantitative analysis of the self-assembly phenomenon. The results show that the supramolecular aggregates are relatively small, yet well-defined involving from 5 to 6 molecules in an average aggregate. The apparent size limitation of the aggregates is ascribed to steric constraints imparted by the dendritic wedges. Relatively high exothermicities accompany the aggregation process, increasing with conjugation length from -73.0 kJ/mol with the undecathiophene triblock (G3-T11-G3) to -85.8 kJ/mol for the heptadecathiophene analogue (G3-T17-G3), suggesting that p-p stacking is the driving force for complexatio

Photoluminescence of supramolecular oligothiophene assemblies

The photoluminescence of supramolecular assemblies of two lengthy oligothiophenes having eleven and seventeen repeat units that are end-substituted with third generation poly(benzyl ether) dendritic wedges, G3-T11-G3 and G3-T17-G3 respectively, is presented. Recent thermochromism studies revealed that these novel materials form nanoscale aggregates in solution. Variable temperature photoluminescence measurements confirm the complete transformation of a molecularly dissolved phase into an aggregated phase within a rather temperature regime

Concentration-dependent thermochromism and supramolecular aggregation in solution of triblock copolymers based on lengthy oligothiophene cores and poly(benzyl ether) dendrons

Self-complexation of triblock copolymers based on undeca- and heptadecathiophene cores attached to generation three Fréchet-type poly(benzyl ether) dendrons in solution at low temperature leads to the formation of relatively small but well-defined supramolecular assemblies. The formation of these aggregates is associated with a strong thermochromic effect. Upon cooling dilute solutions, minor concentration-independent changes in the optical absorption due to intrachain planarization precede significant concentration-dependent thermochromic changes that are shown to originate from intermolecular p-p stacking interactions. The structurally well-defined nature of the substrates enables the determination of distinct temperature regimes for both intra- and intermolecular thermochromic changes. Furthermore, combining thermodynamic expressions for both the concentration and temperature dependence of aggregation allows a quantitative analysis of the self-assembly phenomenon. The results show that the supramolecular aggregates are relatively small, yet well-defined involving from 5 to 6 molecules in an average aggregate. The apparent size limitation of the aggregates is ascribed to steric constraints imparted by the dendritic wedges. Relatively high exothermicities accompany the aggregation process, increasing with conjugation length from -73.0 kJ/mol with the undecathiophene triblock (G3-T11-G3) to -85.8 kJ/mol for the heptadecathiophene analogue (G3-T17-G3), suggesting that p-p stacking is the driving force for complexatio

Interchain delocalization of photoinduced neutral and charged states in nanoaggregates of lengthy oligothiophenes

Photoluminescence (PL) and photoinduced absorption (PIA) spectroscopy measurements were performed on two lengthy oligothiophenes, G3-T11-G3 and G3-T17-G3, doubly end-substituted with third generation poly-benzyl ether dendrons. These oligothiophenes form well-defined nanoaggregates in dichloromethane solution upon cooling. The molecularly dissolved and aggregated phases interconvert reversibly in a narrow temperature range. PL and PIA spectroscopy were used to investigate the optical signatures of photoexcited singlet, triplet, and charged states as a function of aggregation. The extent of aggregation could be controlled by varying the temperature. Both the fluorescence and the triplet absorption spectra of the aggregated phase were significantly bathochromically shifted when compared to the spectra of the isolated molecules in solution. These bathochromic shifts indicate that interchain delocalization of the singlet and triplet photoexcitations occurs within the dendritic nanoassemblies. Charged states of G3-T11-G3 and G3-T17-G3 were selectively created by photoexcitation in the presence of an external electron acceptor (tetracyanoethylene). The principal absorption bands of the charged states shift to lower energy upon aggregation. Surprisingly, new high-energy bands are observed in the PIA spectrum of the aggregated phase. These transitions are clear signatures of two-dimensionally delocalized polaronic charge carriers within the nanoaggregates (i.e. intermolecular delocalization over the constituent molecules within the aggregate)

Micro-HPLC of proteins using a novel monolithic stationary phase with a gradient of functionalities prepared via photoinitiated grafting

The use of photografting for the functionalization of porous poly(butyl methacrylate-co-ethylene dimethacrylate) monolithic columns enabled the preparation of columns with a homogeneously grafted poly(2-acrylamido-2-methyl-1-propanesulfonic acid, AMPS) chains. Recently, novel monolithic columns with a longitudinal gradient of concentration of functional groups were prepared and characterized using electron probe microanalysis and the separation performance evaluated in CEC. We demonstrated that these monolithic columns afforded higher separation efficiency and peak capacity compared to their homogeneously grafted counterparts. Consequently, the application of monolithic columns with this new type of chemistry enabled rapid and efficient separations. In this study, we now demonstrate the applicability of the new type of monolithic columns in µ-HPLC. For example, the separation of proteins in ion-exchange mode was studied. Optimization of the chromatographic conditions such as the shape of the mobile phase gradient and the flow rate allowed a rapid separation of four proteins in a short period of time. We also found that the stationary phase involving the gradient of functionalities exhibited high selectivity. These separation methods can be useful in the proteomic research