Robert J. Spry

Theoretical Analysis of the Crystalline Colloidal Array Filter

The major parameters of a crystalline colloidal array optical filter have been investigated theoretically, and the results compared to published experimental data. This filter consists of an aqueous suspension of polystyrene spheres in a lattice which produces Bragg diffraction of incident light. The development of expressions for the filter bandwidth and attenuation utilized both dynamical x-ray diffraction theory and light scattering theory. The theoretical attenuation function indicates that extremely high absorbance values are achievable with relatively thin filters.

Synthesis and characterization of fluorinated benzoxazole polymers with high Tg and low dielectric constant

Next generation microelectronic packaging requirements are driving the need to produce increasingly lower dielectric constant materials while maintaining high thermal stability and ease of processing. Efforts have focused on the synthesis and analysis of new polymers with the goals of high thermal stability [degradation temperature (T d ) > 400 °C, low glass-transition temperature (T g ) > 350 °C], low water uptake (< 1%), solubility in selected organic solvents, dielectric constant less than 2.5, and low thermal expansion coefficient. These stringent combined goals have been largely achieved with flexible aromatic benzoxazole polymers. Intramolecular hydrogen bonding between pendant hydroxyl groups and the double-bond nitrogen of the benzoxazole has been exploited to increase the polymer T g , whereas the incorporation of perfluoroisopropyl units effectively decreases the dielectric constant. Out-of-plane impedance measurements on films of materials in this family (38-134 μm thick) have resulted in typical dielectric values of 2.1-2.5 at 1 MHz, depending on copolymer ratios and functionalizations. Results have been correlated with optical waveguide measurements of films 4-μm thick to determine film anisotropy and the high-frequency dielectric constant, and have been corroborated by in-plane interdigitated electrode dielectric measurements on samples 0.75 μm thick. Candidate materials exhibited extremely low water uptake (0.2%) even after submersion in boiling water for several days. Dynamic mechanical analysis of the polymers enabled the determination of the influence of intermolecular hydrogen bonding on the T g and loss tangent magnitude. Finally, the coefficient of thermal expansion has been examined and correlated with copolymer constitution.

Investigation of electrostatic self-assembly as a means to fabricate and interfacially modify polymer-based photovoltaic devices

This work focuses on studying a water-based processing method for fabricating and modifying polymer-based photovoltaic devices based on donor–acceptor type complexes. Electrostatic self-assembly is a simple technique that involves immersion of a substrate into dilute aqueous solutions of positively and negatively charged polymers. Extremely thin layers of these polymers are adsorbed onto the surface and their structure can be tailored by manipulating deposition conditions such as the concentration, pH, and salt content. Poly(p-phenylene vinylene) (PPV) containing bilayers were examined as the donor block and water soluble, functionalized C60 molecules were investigated for the acceptor block. By varying the number of bilayers deposited in each individual block (i.e., the block thickness), we have been able to demonstrate a peak in device performance. By controlling the thickness of both the donor and acceptor blocks, we have determined the optimal device architecture for this system. Additionally, we have...

Electrostatic self-assembly as a means to create organic photovoltaic devices

Abstract Recently, there has been a significant amount of work done on making photovoltaic devices (solar cells) from thin films of conjugated polymers and other organic systems. The advantages over conventional inorganic systems include the potential to create lightweight, flexible, and inexpensive structures. The challenge, however, has been to create more highly efficient devices. To date, the primary photovoltaic device mechanism that has been utilized is that of photoinduced charge transfer between an electron donor and acceptor. In this study, similar photovoltaic devices are fabricated using a water-based electrostatic self-assembly procedure, as opposed to the more conventional spin-coating and/or vacuum evaporation techniques. In this process, layers of oppositely charged species are sequentially adsorbed onto a substrate from an aqueous solution and a film is built up due to the electrostatic attraction between the layers. The technique affords molecular level control over the architecture and gives bilayer thickness values of the order of tens of angstroms. By repeating this process a desired number of times and utilizing different cations and anions, complex architectures can be created with very accurate control over the thickness and the interfaces. We have examined a number of systems built from a variety of components including a cationic PPV precursor, functionalized C 60 , and numerous other polyelectrolytes. We report on the device characteristics of these films and on the overall applicability of this technique to the fabrication of photovoltaic devices.

Photovoltaic interface modification via electrostatic self-assembly

The device efficiency of PPV-C 60 based photovoltaic devices has been substantially increased by increasing the interfacial area between the electron donor and acceptor layers. Electrostatic Self-Assembly (ESA) provides a means to deposit thin films of electroactive materials with a very controlled thickness and has shown usefulness in modifying physical and electrical interfaces. In this study, we attempt to control the effective interfacial area by modifying the interface between the PPV electron donor and C 60 -based electron acceptor with molecularly blended ESA bilayers of PPV and derivatized C 60 . It is observed that with only 2 bilayers of (PPV/C 60 - ) a 3-fold increase in device efficiency is obtained. Thus, ESA films offer promise for the nanoscaled modification of interfaces in organic-based photocells.

Theoretical intensity-dependent response of nonlinear periodic structures

We have modeled the response of a nonlinear periodic structure by means of the Abeles 2×2 matrix method. Our structure differs from the usual rejection‐band filter designs, in that we have chosen the filter elements to be index matched in the absence of radiation, providing a rejection band that both grows and shifts as a function of incident intensity. The intensity output function of the model not only directly demonstrates optical bistability, but also limiting, switching, self‐pulsing, and chaos.

New aromatic benzazole polymers II: Synthesis and conductivity of benzobisthiazole-co-polymers incorporated with 4-N,N-dimethylaminotriphenylamine groups

New aromatic benzobisthiazole copolymers containing 10-70 mol % of 4-N,N-dimethylamino-triphenylamine functionality were prepared from the respective dinitrile or dicarboxylic acid monomers, terephthalic acid, and 2,5-diamino-1,4-benzene-dithiol dihydrochloride in polyphosphoric acid. At the first approximation, the copolymers containing 10 mol % or less of the triarylamino moieties in the polymer chains still preserve the capability to form anisotropic (nematic) solutions at 10 wt % polymer concentration. This is an important requirement for processing the copolymers into fibers and films with good to excellent mechanical properties. Films with good mechanical integrity were cast from the dilute methanesulfonic acid solutions of the copolymers under reduced pressure. They showed electrical conductivity values of the order of 10 -11 -10 -10 S/cm in pristine state, with four to seven orders of magnitude increase upon exposure to mild oxidizing agents such as iodine vapor. On the contrary, the parent polymer, poly(p-phenylene benzobisthiazole) is an insulator with conductivity of less than 10 -12 S/cm, and its conductivity does not improve at all with exposure to iodine vapor.