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Organic and Polymer Semiconductor Electronics
Organic and polymer semiconductor thin films of pi-conjugated systems have the ability to transport charge and are used as high-performance functional active layers in field-effect transistors, solar cells, and light-emitting diodes. Our research group is interested on the dependence of electron (n-type) and hole (p-type) mobility on electronic/molecular structure, crystal packing, photo excitation, and defects in organic crystals and polymer semiconductor thin films. Below are a series of “themes” my research program is actively investigating.
1. Oligomers are Model Systems for Understanding/Designing Polymer Semiconductors. Polymer semiconductors offer the possibility of low-cost, solution-processable electronics for solar energy conversion, flexible displays, and basic computational devices. However, their performance can be unreliable due to batch-to-batch variations, including differences in crystallinity, regioregularity, and processing conditions. Well-defined, monodispersed oligomers in controlled systems do not suffer from these effects. However, for oligomers to be used as representative models for their polymer analogues, the oligomers must mirror the electronic structure of the polymers. Therefore, a key issue in polymer electronics is to determine at which chain length does an oligomer “electronically” behave like the corresponding polymer? On a molecular scale, the optoelectronic properties in semiconductor systems are largely a result of the degree of conjugation; the oligomer under study must exhibit a degree of conjugation equal to the polymer. Furthermore, understanding the evolution of chain packing and conjugation length with regard to chain length is imperative to creating accurate models. Our approach has been to synthesize well-defined series of oligomers of representative polythiophenes as shown below.
Synthetic Steps to the Oligothiophenes (DDQT 1-6). Reagents and Conditions: (i) NBS, AcOH/chloroform (1:1), 0 °C to room temperature; (ii) BuLi, trimethyltin chloride, THF, −78 °C to room temperature; (iii) bis(tri-n-butyltin), Pd(PPh3)4, toluene, 115 °C; (iv) 2, Pd(PPh3)4, toluene, 115 °C; (v) 1, Pd(PPh3)4, toluene, 115 °C.
- L. Zhang et. al. J. Am. Chem. Soc. 2013, 135, 844-854.
2. Graphene-Like Fragments from Polycyclic Aromatic Hydrocarbon Semiconductors. It is well recognized that the appropriate arrangement of organic molecules in the solid state is decisive for efficient charge-carrier transport. There are two common packing motifs adopted by oligoacenes in the solid state that yield strong intermolecular interactions. One is the ‘‘herringbone’’ packing arrangement, which provides edge-to-face interactions with minimal π-π stacking, yielding two-dimensional electronic interactions in the solid state (e.g. pentacene). The other is face-to-face packing, typically with some degree of displacement along the short and long axes of the molecules to decrease electrostatic repulsion. However, the still-unsolved challenge in crystal engineering is to avoid the most common herringbone π-stacking motif by rational design. This research embarks on the synthesis, self assembly, molecular packing, and charge-transport of graphene-like molecular fragments. Understanding basic transport in these materials will pave the way for producing the appropriate materials in organic electronic devices such as transistors and solar cells.
- H. Lee, Y. Zhang, L. Zhang, T. Mirabito, E. Burnett, S. Trahan, A. Mohebbi, J. J. Watkins, S. C. B. Mannsfeld, F. Wudl, A. L. Briseno. "Rubicene: A Molecular Fragment of C70 for use in Organic Transistors" J. Mat. Chem. C. 2014,DOI: 10.1039/C3TC32117G.
- L. Zhang, A. Fonari, V. Coropceanu, J.-L. Bredas, A. L. Briseno, "Graphene-Like Fragments from Triisopropylsilylethynyl-Functionalized Polycyclic Aromatic Hydrocarbon Semiconductors: Electronic Structure, Molecular Packing, and DFT Calculations," Chem. Eur. J. 2013, 19, 17907–17916.
3. Organic Single-Crystalline Nanostructured Interfaces. Exciting chemistry occurs at organic interfaces. Consider an organic solar cell: exciton creation, diffusion, charge separation, charge transport, and collection all occur at the organic interface. From a fundamental viewpoint, the role of the interface must have optimal electronic and physical communication in order to yield highly efficient devices. From a technological viewpoint, one must understand, control, and have a rational design of the desired electronic and optical properties at the organic interface for the development of solar cells, integrated circuits, light-emitting transistors, and a host of potential new device concepts that have not yet been developed. The use of organic single-crystalline interfaces will have a major impact in accelerating the emerging area of organic electronics, as these highly ordered systems will enable one to extract intrinsic charge carrier transport phenomena that cannot be accurately determined from disordered systems common to amorphous and/or polycrystalline films used in mainstream devices.
One theme of our experimental investigations is to understand structure-property relationships by exploring solid-state molecular packing, charge transport, electronic/molecular structure, and fundamental excitonic interactions at interfaces of low-dimensional organic single-crystalline nanostructures. We perform experiments at single-crystalline organic-organic interfaces with discrete nanowire devices (i.e. transistors, solar-cells) that will enable detailed information to be obtained on intrinsic charge-transport as well as the optical properties. Single nanowire-level measurements will define fundamental limits of performance in these “one-dimensional systems.” These classes of nanostructures will enable one to bridge fundamental science with applied research that will be central to the discovery of potentially new device concepts in areas of nanoscale electronics and organic photovoltaics.