Jose Alonzo

PS‐b‐P3HT Copolymers as P3HT/PCBM Interfacial Compatibilizers for High Efficiency Photovoltaics

A conducting diblock copolymer of PS-b-P3HT was added to serve as a compatibilizer in a P3HT/PCBM blend, which improved the power-conversion efficiency from 3.3% to 4.1% due to the enhanced crystallinity, morphology, interface interaction, and depth profile of PCBM.

Assembly and Organization of Poly(3-hexylthiophene) Brushes and Their Potential Use as Novel Anode Buffer Layers for Organic Photovoltaics

Buffer layers that control electrochemical reactions and physical interactions at electrode/film interfaces are key components of an organic photovoltaic cell. Here the structure and properties of layers of semi-rigid poly(3-hexylthiophene) (P3HT) chains tethered at a surface are investigated, and these functional systems are applied in an organic photovoltaic device. Areal density of P3HT chains is readily tuned through the choice of polymer molecular weight and annealing conditions, and insights from optical absorption spectroscopy and semiempirical quantum calculation methods suggest that tethering causes intrachain defects that affect co-facial π-stacking of brush chains. Because of their ability to modify oxide surfaces, P3HT brushes are utilized as an anode buffer layer in a P3HT-PCBM (phenyl-C₆₁-butyric acid methyl ester) bulk heterojunction device. Current-voltage characterization shows a significant enhancement in short circuit current, suggesting the potential of these novel nanostructured buffer layers to replace the PEDOT:PSS buffer layer typically applied in traditional P3HT-PCBM solar cells.

Forces of interaction between surfaces bearing looped polymer brushes in good solvent

In a previous publication we suggested [Huang et al., Macromolecules, 2008, 41, 1745–1752] that looped polymer brushes formed by tethering chains by both ends to a surface may exhibit a polydispersity-like effect due to a distribution of distances between tethering points, leading to segment density profiles dominated by a long and diffuse exponentially-decaying tail. To study this issue in more detail, the force profiles (forces of interaction as a function of separation distance) of a series of looped polymer brushes made by preferential adsorption of poly(2-vinylpyridine)–polystyrene–poly(2-vinylpyridine) (PVP-b-PS-b-PVP) triblock copolymers of varying molecular weight and asymmetry ratio are measured using the surface forces apparatus. The force profiles are analyzed using an equivalent diblock model, which considers the triblock copolymer brushes as being comprised of two diblock copolymers of half the PS molecular weight. While scaling the dependencies of the interaction energy and distance on molecular weight, the tethering density and segment size coalesce the measured force profiles to the “universal profile”, it is necessary to include polydispersity in the description of the equilibrium structure. This is done using the self-consistent field model of Milner et al. [Macromolecules, 1988, 21, 2610–2619]. For looped brushes formed from the symmetric and moderately symmetric triblock copolymers we find that the polydispersity due to molecular weight distribution effectively accounts for the observed force profiles. On the other hand, agreement between the measured and predicted force profiles of looped brushes formed from highly asymmetric copolymers at low degrees-of-compression is achieved only if a much smaller value of the polydispersity index is used in the fitting. The implication of these results is that the shape of the segment density profiles is not due to the previously proposed anchor-induced polydispersity arising due to loop formation; however in the case of highly asymmetric copolymers, loop formation may constrain the stretching of the chains relative to what is expected for brushes formed from the equivalent diblock copolymer.

Grafting density effects, optoelectrical properties and nano-patterning of poly(para-phenylene) brushes

Well-defined conjugated polymers in confined geometries are challenging to synthesize and characterize, yet they are potentially useful in a broad range of organic optoelectronic devices such as transistors, light emitting diodes, solar cells, sensors, and nanocircuits. Herein we report a systematic study of optoelectrical properties, grafting density effects, and nanopatterning of a model, end-tethered conjugated polymer system. Specifically, poly(para-phenylene) (PPP) brushes of various grafting density are created in situ by aromatizing well-defined, end-tethered poly(1,3-cyclohexadiene) (PCHD) “precursor brushes”. This novel precursor brush approach provides a convenient way to make and systematically control the grafting density of high molecular weight conjugated polymer brushes that would otherwise be insoluble. This allows us to examine how grafting density impacts the effective conjugation length of the conjugated PPP brushes and to adapt the fabrication method to develop spatially patterned conjugated brush systems, which is important for practical applications of conjugated polymer brushes.