Organic Semiconductor Processing
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 π-conjugated systems.
Enhanced Mobility and Effective Control of Threshold Voltage in P3HT Based Field-Effect Transistors via Inclusion of Oligothiophenes
Improved organic field-effect transistor (OFET) performance through a polymer-oligomer semiconductor blend approach is demonstrated. Incorporation of 2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene (BTTT) into poly(3-hexylthiophene) (P3HT) thin films leads to approximately a 5-fold increase in charge carrier mobility, a 10-fold increase in current on-off ratio, and concomitantly, a decreased threshold voltage to as low as 1.7 V in comparison to single component thin films. The blend approach required no pre- and/or post treatments, and processing was conducted under ambient conditions. The correlation of crystallinity, surface morphology and photophysical properties of the blend thin films was systematically investigated via X-ray diffraction, atomic force microscopy and optical absorption measurements respectively, as a function of blend composition. The dependence of thin-film morphology on the blend composition is illustrated for the P3HT:BTTT system. The blend approach provides an alternative avenue to combine the advantageous properties of conjugated polymers and oligomers for optimized semiconductor performance.
Students: Ping-Hsun Chu
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).
If organic semiconductors are to be commercialized, it will be necessary to produce thousands, even millions of devices with very tightly controlled electrical and mechanical properties. These properties are a sensitive function of the semiconductor’s structure, so effective control of the material properties stems from effective control of the microstructure formed during processing. Identifying a microstructural target requires a simple quantitative representation of the structure; in other words, a set of microstructural features that most affect the desired properties. Extracting these features from the large variety of microstructural data can be accomplished through image analysis, peak fitting, and other techniques from the rapidly growing field of data science.
Once relevant microstructural features are identified, traditional experimental design methodologies can be applied to enable the rational and controllable fabrication of nanostructured organic electronics.
Students: Nils Persson, Michael McBride
For the creation of practical and aesthetic electronic devices, flexible batteries are considered as a promising solution, owing to their potential to adapt to mechanical stress and thereby shape transformation. Furthermore, to keep pace with the recent trends, considerable efforts have been made to develop the facile, robust flexible lithium-ion batteries based on not only developing the advanced materials, but also constructing the newly flexible, systematical platforms. For example, there has been focused on the development of flexible batteries adopting soft materials such as polymer electrolytes, nano-sized active materials, and highly patterned, flexible current collectors, as well as possessing the flexible, elaborate frames like a cable-type, serpentine interconnect, or origami structure. Despite of these intensive efforts, P3HT, one of the representative conjugated conducting polymers, has not yet been investigated for the battery materials. Here, we believe, therefore, our role is to reveal the physiochemical phenomena of the P3HT in the battery electrode systems and on the basis of this research results, our scientific interests/concepts will be enlarged to building the robust, flexible electrode frames.
Students: Yo Han Kwon, Krysten Minnici
Synthesis of Semiconducting Polymers
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.
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.
Students: Zhibo Yuan
Biomaterials such as PSLG and cellulose nanocrystals are being explored for use in "green" electronics. Cellulose nanocrystals are rigid, rod-like particles that form a lyotropic liquid crystal in water. If these particles can enforce long-range order in semiconducting polymers such as P3HT, we expect that the charge carrier mobility will increase due to improved pi-pi stacking. Other cellulose derivatives such as cellulose nanofibrils show promise in paper-based battery applications as well.
Students: Bailey Risteen, Brian Khau
Polarized optical microscopy image of chromonics in cylindrical capillary with planar anchoring under crossed polarizers.
Liquid crystalline phases have properties between solid crystal and isotropic liquid. The molecules of liquid crystals have partial ordering.
Lyotropic chromonic liquid crystals is a group of liquid crystals composed of plank-like molecules with poly-aromatic core and hydrophilic side chains. With the presence of a solvent, usually water, chromonic molecules stack face to face into aggregates. Liquid crystalline phases (nematic and columnar phases) form with relatively high concentration.
Curved confinement of liquid crystals results in interesting alignment since the elastic free energy is affected by curvature. We are currently interested in the director configuration of nematic chromonics confined in micron-size capillaries of various shapes. The extremely low twist elastic constant of chromonics may provide phenomena different from ordinary thermotropic liquid crystals. We have discovered the ground states of chromonics confined in cylinder with significant twist deformation.
Students: Rui Chang