John D. Berrigan

Solvent and polymer matrix effects on TIPS-pentacene/polymer blend organic field-effect transistors

We report on a systematic study of solvent and polymer matrix effects on the phase segregation behavior of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) blends incorporated into two different amorphous polymer matrices, poly (α-methyl styrene) and poly (triarylamine), and using two solvents, chlorobenzene and tetralin. Optical microscopy, X-ray diffraction analyses, and optical absorption measurements are used to evaluate the film morphology, crystallinity, and optical density, respectively. These analyses are correlated with the extent of vertical segregation of TIPS-pentacene, as observed for the blended films by depth-profile XPS analyses. The microstructure and vertical phase segregation of TIPS-pentacene in blend films are found to be strongly influenced by the choice of solvent. Tetralin, a solvent with a high boiling temperature, was found to be more desirable for achieving distinct phase segregation/crystallization of TIPS-pentacene in blend films and best performance in OFETs with a dual-gate geometry. The electrical properties of top and bottom channels were consistent with the morphological characterization and OFETs processed from tetralin showed higher mobility values than those from chlorobenzene. Further modification of the annealing conditions in the TIPS-pentacene/PTAA/tetralin ternary system led to top-gate OFETs with mobility values up to 2.82 cm2/Vs.

Hybrid 3D Printing of Soft Electronics

Hybrid 3D printing is a new method for producing soft electronics that combines direct ink writing of conductive and dielectric elastomeric materials with automated pick-and-place of surface mount electronic components within an integrated additive manufacturing platform. Using this approach, insulating matrix and conductive electrode inks are directly printed in specific layouts. Passive and active electrical components are then integrated to produce the desired electronic circuitry by using an empty nozzle (in vacuum-on mode) to pick up individual components, place them onto the substrate, and then deposit them (in vacuum-off mode) in the desired location. The components are then interconnected via printed conductive traces to yield soft electronic devices that may find potential application in wearable electronics, soft robotics, and biomedical devices.

Syntheses of nanostructured Cu- and Ni-based micro-assemblies with selectable 3-D hierarchical biogenic morphologies

A combined layer-by-layer (LbL) surface amine amplification and electroless deposition process has been developed to convert biologically replicable three-dimensional (3-D) nanostructured micro-assemblies (such as siliceous diatom frustules) into freestanding Cu-bearing or Ni-bearing structures that retain the starting biogenic microscale 3-D shapes and nanoscale patterns. After reacting the hydroxyl-bearing surfaces of these biotemplates with an aminosilane, a LbL polyacrylate/polyamine deposition process was used to dendritically amplify the surface amine concentration. Subsequent binding of metal chloride catalysts to these amine-enriched surfaces enabled the rapid electroless deposition of thin, conformal, continuous, and nanocrystalline or amorphous metallic coatings on the 3-D biotemplates. Selective removal of the underlying templates then yielded freestanding Cu-bearing or Ni-bearing structures. The conformality and continuity of the thin coatings, and the fidelity with which the biogenic shape and fine features were preserved in the freestanding structures, were significantly enhanced by the amplification of surface amines (and the associated enrichment of catalytic sites) resulting from the LbL polyacrylate/polyamine treatment. Monolithic and multicomponent structures (e.g., Cu, multilayer Au/Cu, CuO, and Ni–P alloy) with bio-derived morphologies have been synthesized utilizing this approach. This readily-scalable process may be used to convert self-assembled rigid templates (of biological or synthetic origin) into nanostructured transition metal-based micro-assemblies with a wide variety of selectable 3-D hierarchical morphologies for use in numerous functional and structural applications.

Roles of thermally-induced vertical phase segregation and crystallization on the photovoltaic performance of bulk heterojunction inverted polymer solar cells

Brief 160 °C annealing treatments dramatically enhanced the performance of bulk heterojunction inverted polymer solar cells with an ITO/ZnO/P3HT:PCBM/MoO3/Ag structure. The influence of such treatments on cell performance has been correlated to vertical phase segregation and crystallization within the photoactive layer of such cells. The photoactive layer, comprised of a mixture of P3HT and PCBM deposited on ZnO, was annealed for 10–30 min at 160 °C. Depth profiling with X-ray photoelectron spectroscopy (XPS) revealed that such annealing resulted in enrichment of the P3HT concentration near the ZnO layer, particularly after 20 and 30 min of annealing. Crystallization of P3HT was detected by X-ray diffraction (XRD) analyses after 10 to 30 min of such annealing, with little difference in the extent of crystallization detected over this time frame. It was found that vertical segregation does not seem to play a role as significant as that of crystallization on cell performance.

Conversion of porous anodic Al2O3 into freestanding, uniformly aligned, multi-wall TiO2 nanotube arrays for electrode applications

A combined conformal coating and gas/solid reaction process has been used for the first time to convert porous anodic alumina templates into robust, freestanding, multi-wall titania nanotube (MWTNT) arrays that resist appreciable nanotube bundling upon drying. A sol–gel infiltration process was first used to apply a thin conformal titania coating to the alumina templates. After selective etching to generate a gap between the template and the titania coating, exposed alumina was converted into nanocrystalline titania via reaction with titanium tetrafluoride gas and then with humid oxygen. After selective dissolution of residual aluminum-bearing phases and drying, freestanding, non-agglomerated, well-aligned MWTNT arrays were generated (i.e., with coating-derived inner titania tubes resting inside reaction-derived outer titania tubes). When incorporated as aligned electrodes in dye-sensitized solar cells, the dye loading and power conversion efficiencies were higher by factors of 2.2 and 1.8, respectively, than for solar cells with single-wall titania nanotube array electrodes. By controlling the conditions used for alumina template formation, sol–gel coating, and gas/solid reaction, this hybrid process may be used to generate robust, uniformly aligned MWTNT arrays with dimensions and functional chemistries tailored for a variety of electrical, optical, chemical, or biochemical applications.