Showing papers by "Sean E. Shaheen published in 2009"

Low-Bandgap Thiophene Dendrimers for Improved Light Harvesting

Abstract: This article follows our previous work on the synthesis and characterization of pi-conjugated dendrimers for use in organic solar cells. Here we discuss five new thiophene-based dendrimers that were synthesized in order to study the relationship between their chemical structures and electronic properties. Three of these dendrimers incorporate acetylene spacers, included to relieve steric strain, between the thiophene arms and phenyl cores used in previous studies. Only a small effect on the electronic properties is observed upon inclusion of the acetylene spacer in the three-arm dendrimer, 3G1-2S-Ac. In contrast, a decrease in the bandgap is observed for the four-arm dendrimer, 4G1-2S-Ac, due to a reduction of interactions between the arms in the more sterically congested 1,2,4,5-arrangement around the phenyl core, resulting in delocalization of the exciton through the phenyl core. Incorporation of electron-withdrawing cyano groups on the phenyl core of the three-arm dendrimer, 3G1-2S-CN, resulted in a very large (∼0.5 eV) decrease in the bandgap, due to stabilization of the lowest unoccupied molecular orbital, and the low energy absorption band in this material is attributed to a transition with significant intramolecular charge-transfer character. The electronic properties of three- and four-arm dendrimers with electron-donating dibutylaniline moieties attached to the end of the thiophene dendron, 3G1-2S-N and 4G1-2S-N respectively, are almost identical, indicating that they are dominated by the arms, with no through-core communication allowed, even for the para-linked arms of 4G1-2S-N. However, there is a significant increase in the molar absorptivity of these materials, concomitant with significant broadening of the absorption spectrum, which is an important attribute in light-harvesting applications.

Exciton migration in conjugated dendrimers: a joint experimental and theoretical study.

Abstract: We report a joint experimental and theoretical investigation of exciton diffusion in phenyl-cored thiophene dendrimers. Experimental exciton diffusion lengths of the dendrimers vary between 8 and 17 nm, increasing with the size of the dendrimer. A theoretical methodology is developed to estimate exciton diffusion lengths for conjugated small molecules in a simulated amorphous film. The theoretical approach exploits Fermi's Golden Rule to estimate the energy transfer rates for a large ensemble of bimolecular complexes in random relative orientations. Utilization of Poisson's equation in the evaluation of the Coulomb integral leads to very efficient calculation of excitonic couplings between the donor and the acceptor chromophores. Electronic coupling calculations with delocalized transition densities revealed efficient coupling pathways in the bulk of the material, but do not result in strong couplings between the chromophores which are calculated for more localized transition densities. The molecular structures of dendrimers seem to be playing a significant role in the magnitude of electronic coupling between chromophores. Simulated diffusion lengths correlate well with the experimental data. The chemical structure of the chromophore, the shape of the transition densities and the exciton lifetime are found to be the most important factors in determining the size of the exciton diffusion length in amorphous films of conjugated materials.

Structure-Dependent Photophysics of First-Generation Phenyl-Cored Thiophene Dendrimers

Abstract: We have prepared two series of first-generation thiophene-bridge dendrimers, with either three (3G1) or four (4G1) arms attached to a phenyl core, to elucidate their structure−property relationships. Optical properties were investigated with a combination of steady-state and time-resolved spectroscopic techniques. Steady-state spectroscopic data for the 3-arm dendrimers suggests that the exciton is delocalized over the α-conjugated thiophene segment and the phenyl core, but that the meta-linking of the dendrons prevents their electronic communication. In contrast, conjugation through the core to dendrons in the ortho and para positions is permitted in the 4-arm dendrimers, although the data suggest that the conjugation length does not extend over the full length of the α-conjugated sections of two coupled dendrons. This observation is due to steric interactions between neighboring arms, which forces the arms to twist and bend out of the plane of the phenyl core, and is particularly prevalent in disrupting...

The Effect of Ionization Potential and Film Morphology on Exciplex Formation and Charge Generation in Blends of Polyfluorene Polymers and Silole Derivatives

Abstract: Long-lived emission from an exciplex state is observed in a series of blend films of polyfluorene−triarylamine copolymers with silole derivatives. The energy of the exciplex emission is found to correlate with the energy offset between the frontier orbitals of donor and acceptor, whereas the yield of the exciplex is more strongly influenced by blend film morphology. Charge separation occurs in these films, and the external quantum efficiency of photovoltaic devices is found to depend upon both the energetics and the morphology of the blend film.

Detection of protein secondary structures via the discrete wavelet transform.

Abstract: We subject the primary sequence of proteins gathered from the Structural Classification of Proteins (SCOP) database to a discrete wavelet transform (DWT) analysis to search for predictors of secondary structures. We use proteins with both alpha helices and beta sheets (the A/B , A+B databases from SCOP). The amino acids composing the protein are converted to their hydrophobicity values using three hydrophobicity scales. Results prove to be independent of the scale used. Using a DWT multiresolution decomposition, each protein is coarse grained, in effect, creating snapshots of each protein at multiple scales. For each protein, a control data set is formed by generating random realizations that remove the positional informational in the sequence but still contain the same amino acid frequencies. Regions of salient hydrophobicity in the protein sequence are identified by comparing the transforms of the original sequence with those of the control set, at each resolution. We find significant matching between regions of salient hydrophobicity and the locations of secondary structure along the amino acid chains. We calculate the sensitivity, specificity, and Matthews correlation to quantify the agreement between the wavelet detected structures and the real protein. In addition we are able to distinguish between the morphologically different subsets, A/B and A+B. We also construct a correlation function based on the DWT that correlates quasilocalized structures at lengths in wavelet space. Through a similar comparison to the control data sets, features in this space-scale correlation are identified that show correspondence to the typical lengths of the secondary structures.

Towards an all-polymer robot for search and rescue

Abstract: This paper discusses two components suitable for construction of an all-polymer robot, namely a Synthetic Neural Network and water hammer based actuation. A new data processing element, termed Synthetic Neural Network, or SNN, based on a concept of a polymer-based bistable memory device and a conventional transistor made from polymers, is proposed. A phenomenon known as the water hammer effect is described for the purposes of propulsion of the serpentine robot constructed from polymer tubing. Arresting the flow of water in the tube causes it to lurch forward. A relationship between the shape of the hose and the direction of propulsion is investigated with the goal of using the SNN to learn to control the forward progress of the robot based on polymer bend sensors.

Pi-conjugated dendrimers for solar-energy harvesting

Abstract: Organic photovoltaics (OPV) are emerging as a low-cost technology for manufacturing solar cells on large-area flexible substrates. Potential applications include essentially all aspects of solar-energy conversion, with small-scale consumer-electronic applications as the initial goal, followed by scaling up to commercial and building applications as the technology matures. Recent developments in OPV materials and processing conditions have led to efficiencies exceeding 6%.1 Although efficiencies as low as 7% may be sufficient for large-scale commercialization, reaching 10% would provide a much more compelling economic impetus. It has been estimated that a single-junction organic solar cell could reach 10% efficiency if the bandgap—the energy gap between the ground and excited states—of the polymer were∼1.5eV.2 However, new materials are required to reach this level. OPV devices are based on a blend of a fullerene-derivative acceptor and a conjugated (i.e., with electrons shared between a chain of continuous alternating single and double carbon bonds) polymer donor.3 Light absorption in the polymer generates a bound electron-hole pair (an exciton), that diffuses to the interface between the two materials, where it dissociates into a hole in the donor and an electron in the acceptor. The interpenetrating networks of the polymer and fullerene are responsible for conveying charges to the collecting electrodes that complete the device. One of the main limitations of current state-of-the-art polymers for OPV is the poorly optimized overlap between their absorption spectrum and the solar spectrum. While conjugated polymers are efficient light absorbers, their optical bandgap (which determines their absorption onset) is too high to use the red and IR parts of the solar spectrum for photocurrent generation. Aiming at a guided design of donors for OPV, we recently investigated a new class of materials, conjugated denFigure 1. (Top) OPV device structure based on a dendrimer/PCBM ([6,6]-phenyl C61 butyric acid methyl ester) active layer and (bottom) the molecular structures of dendrimers 3G1-nS and 4G1-nS (n=1–3). The red marks on the 4G dendrimer illustrate steric interactions between neighboring arms. Al: Aluminum. PEDOT:PSS: Poly(3,4)thylenedioxythiophene/polystyrenesulfore acid.