IRG 2: Crystalline Organic Semiconductors

The last decade has witnessed considerable progress in the development of organic semiconductors as an alternative to amorphous silicon for low cost, thin film electronics. The attractive properties of organic semiconductors- low temperature processability, efficient electroluminescence, and reasonable charge carrier mobilities- have led to expectations of a new ‘plastic’ electronics with applications ranging from flexible flat panel displays and smart cards to low cost solar cells. Organic light emitting diodes (OLEDs), in particular, have attained performance suitable for display technologies and are being commercialized. The ultimate penetration of plastic electronics into the marketplace, however, depends upon creation of additional devices, e.g. field effect transistors (FETs), which require further improvements in the transport properties of organic semiconductors. The achievement of better transport properties (e.g., higher carrier mobilities) necessitates a comprehensive understanding of the connection between molecular structure, intermolecular interactions, and electronic delocalization. The primary goals of this IRG are to elucidate structure-property relationships and to apply that knowledge to the synthesis of new organic semiconductors, particularly π-stacking organic semiconductors, with enhanced performance in FETs.

Our program focuses on the synthesis, structure, electronic properties and transport behavior of small conjugated oligomers that readily form polycrystalline thin films and single crystals. In Synthesis of New Organic Semiconductor Materials, we use our expertise in molecular synthesis and crystal engineering to design new crystalline semiconductors with systematic variations in molecular structures and crystal packing for structure-property studies and for improved performance in FETs. The new FET materials are optimized via an iterative design loop in which characterization of structure, electronic properties, and transport by IRG members provides feedback for new or revised synthesis targets. In Electronic Structure and Carrier Dynamics, we employ computation and photoelectron spectroscopy to determine the electronic density of states and electron-phonon coupling in the new materials. The connection between structure and electronic properties established in these studies guides further synthesis and provides a basis for interpreting Structure-Transport Relationships, wherein we aim to map the full spectrum of electrical transport behavior in organic crystals and films as a function of temperature, pressure, and charge carrier density. Transport measurements rely on an arsenal of methods, with an emphasis on FETs; properties such as carrier mobility provide a benchmark for the success of our organic semiconductor design efforts.

IRG 2 research areas
Synthesis of New Organic Semiconductor Materials
Electronic Structure and Carrier Dynamics
Structure-Transport Relationships

The principal investigators and their primary areas of expertise are:

• C. Daniel Frisbie, Program Leader - organic electronics, AFM
• Jean-Luc Brédas - computational chemistry for organic materials
• Christopher Douglas - organic synthesis
• Allen Goldman - electron tunneling/transport, MBE
• Russell Holmes - organic optoelectronics, thin film growth
• Kent Mann - organic synthesis, electrochemistry
• Paul Ruden - transport theory and device modeling
• Michael Ward - organic crystal growth and design, AFM
• Xiaoyang Zhu - ultrafast photoemission spectroscopy

The collaborators are:

• Zhenan Bao (Bell Labs)
• Howard E. Katz (Bell Labs)
• Dennis E. Vogel (3M)
• Mamoun Bader (Asst. Prof., CHEM, PSU-Hazleton)