The Reichmanis Research Group works at the interface of chemical engineering, chemistry, materials science, optics, and electronics spanning the range from fundamental concept to technology development and implementation. Research interests include the chemistry, properties and applications of materials technologies for electronic and photonic applications, with particular focus on polymeric and nanostructured materials for advanced technologies.
Whether oligomeric or polymeric in nature, organic materials have been shown to be attractive candidates for both passive and active roles in electronic devices because of their compatibility with high through-put, low cost processing techniques; and their capability to be precisely functionalized through the techniques of organic synthesis to afford desired performance attributes. Structure at both the molecular and nano-scales will impact attributes such as morphology (surface roughness, grain size), adhesion, mechanical integrity, solubility and chemical and environmental stability. These factors in turn will affect device performance, notably electrical performance (mobility, conductivity, on/off ratio, threshold voltage).
In the area of active materials, research has focused on the design and development of organic semiconductor materials and processes for printable, plastic electronic applications. The viability of mass printing technologies such as gravure and offset printing for organic device manufacturing was demonstrated with commercial materials such as poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS), dielectrics such as Luxprint®, and semiconductors such as poly(9,9-dioctylfluorene-co-bithiophene) (F8T2) in a collaborative program including BASF and the University of Chemnitz. (see A.C. Huebler, F. Doetz, H. Kempa, H. E. Katz, M. Bartzsch, N. Brandt, I Hennig, U. Fuegmann, S. Vaiyanathan, J. Granstrom, S. Liu, A. Sydorenko, T. Zilliger, G. Schmidt, K. Preissler, E. Reichmanis, P. Eckerle, F. Richter, T. Fischer, U. Hahn, Org Electronics, 8, 480-486 (2007).
The Reichmanis Group is currently exploring polymeric and hybrid organic/inorganic materials chemistries for electronic and photonic applications, plastic optoelectronics in particular. To take full advantage of organic semiconductor technology, solution processed materials are required for conventional mass printing applications. The development of viable active polymer materials for such applications requires not only the development of relevant chemistries, but also the development of compatible device processes. Key to understanding the issues leading to the design of new materials and processes engineered to afford desired characteristics is an understanding of materials morphology in both thin films and single crystals. In particular, the former depends not only on inherent materials characteristics, but is also highly dependent upon parameters such as the deposition process; vacuum vs solution, temperature (of deposition and anneals), molecular environment surrounding the films.
We have identified a critical relationship between crystallanity and charge transport. Ultrasonication of π-conjugated polymer solutions, poly(3-hexylthiophene) (P3HT) in particular, provides a unique and facile approach to achieve "tunable" crystallanity in a thin film of the rigid-rod polymer. Using AFM, X-ray diffraction, and UV-Vis, we showed that charge transport in conjugated polymer thin films is distributed between disordered, quasi-ordered and ordered phases. Further experiments, to achieve not only controllable crystallanity, but also a tunable domain size will provide vital information to enhance our understanding of the complex structure-property relations in organic semiconductor thin films. Moreover, such a solution phase control of solid state properties could lead to facile methods (changing solvent polarity, polymer regioregularity etc.) of manipulating charge transport.
Subtle structural changes in conjugated polymers close to the gate dielectric in field effect transistors (FETs) dominate their electrical properties. The most critical step in fabricating solution-processed FETs is the solidification process of polymer solution at the polymer/gate dielectric interface, in which the charge transport properties of the conjugated polymers are determined by nanostructure. We have found that conducting channel formation in P3HT FETs can be monitored via in situ sheet conductance measurements using four-contact geometry field effect devices. We suggest that the initial variations of the electrical properties result from the competition between formation of the P3HT micro- and nanostructure associated with self-organization of the polymer chains. Understanding of these factors will lead to control and optimization of processes associated with Ļ-conjugated systems. In a collaborative program with the group of Professor Mohan Srinivasarao (School of Materials Science and Engineering), we are exploring significant molecular interactions occurring during the solvent evaporation process. (see Min S. Park, Avishek Aiyar, Byoungnam Park, Jung O. Park, Elsa Reichmanis, and Mohan Srinivasarao, “Study of Conformational Change of P3HT Chains Using In-Situ Polarized Raman Spectrosocpy”, JACS, 2011)
Efforts are underway to develop facile methods for the preparation of composite materials based on conducting organic polymers (COPs) that avoid use of toxic stabilizers and surfactants. The multifunctionality, including optical, electrical, and dielectric properties as well as low-cost and easy processability arising from hybrid composite materials based on COPs, empowers their application in a variety of energy-related devices producing, converting or saving energy. Two related materials sets are currently being explored: metal-COP core-shell, and inorganic semiconductor-COP composite materials. For instance, silver-PPy core-shell and ZnO-P3HT composites are under development as representative examples. Research is aimed at both the optimization of the synthetic approaches and characterization of hybrid nanocomposite materials. A complementary component focuses on potential applications of the nanocomposites which include high-k polymer composites for embedded capacitors, and photovoltaic cells.
Although significant progress has been made, organic semiconducting polymers typically have low charge carrier mobility, low oxidation stability and a relatively large bandgap relative to their inorganic counterparts. From a molecular perspective, intra- and inter-molecular π-orbital overlap (or π – π stacking) determines the charge transport performance. We are engaged in studying the effects of molecular coplanarity, intramolecular charge transport and electron-withdrawing substitution on the optical and electronic properties of candidate polymers with the aim of facilitating their field-effect charge transport and photovoltaic performance. For instance, we are evaluating the effect of the incorporation of linkages to improve molecular coplanarity, thereby extending the π - conjugated system. Further, tuning of the HOMO/LUMO energy levels will be studied through use of different donors and acceptors within a D-π-A copolymer backbone. The utility of coplanar fused aryl structures will also be examined. The materials will be fully characterized and they will be incorporated into device architectures such as OFETs, diodes and OPVs.
Nanoporous low-dielectric constant materials exhibiting a dielectric constant as low as 1.4 were explored for applications in advanced silicon technology. Specifically, triblock polymers, poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) (PEO-b-PPO-b-PEO), were used as molecular templates in poly(methyl silsesquioxane) (MSQ) matrices to fabricate nanoporous organosilicates. The results demonstrated that aggregation of block copolymers in the MSQ matrix can be prevented with the fast solvent evaporation which accompanies spin casting. The "closed-pore" materials attained ultralow dielectric constants (k ~1.5) with high electrical breakdown field and good mechanical strength. (see S. Yang, P. A. Mirau, C. Pai, O. Nalamasu, E. Reichmanis, E. Lin, H-J. Lee, D. Gidley, J. Sun, Chemistry of Materials, 13(9), 2762 (2001)).
In past research efforts, Elsa Reichmanis has had impact on the field of microlithography, which is central to the manufacture of electronic devices. Her work has contributed to the development of a molecular level understanding of how chemical structure affects materials function leading to new families of lithographic materials and processes enabling advanced VLSI manufacturing. Notably, she was responsible for the design of new imaging chemistries for 193 nm lithography that were the first, readily accessible and manufacturable materials for this technology. (see "Organic Materials Challenges for 193 nm Imaging", E. Reichmanis, O. Nalamasu, F. M. Houlihan, Accounts of Chemical Research, 32(8), 659 (1999)).