RESEARCH

Research Overview

Organic electronic materials exhibit certain properties that make them very attractive for applications in electronics. First, their electronic properties can be fine tuned via chemical synthesis. For example, the color of emission of a polythiophene can be tuned to the blue, the green, or the red part of the spectrum by the introduction of appropriate side groups to the polymer's main chain. Second, organic thin films can be deposited on a variety of substrates, including mechanically flexible ones such as plastic and paper. This might allow the fabrication of devices in a roll-to-roll fashion, which will dramatically decrease cost. Our research effort covers many aspects of the field of organic electronics. We study the growth of organic films, develop techniques to pattern organic materials and fabricate devices such as organic light emitting diodes, organic thin film transistors and organic photovoltaics. We also investigate the mechanisms of charge injection and transport in organic materials and their connection to device physics. In addition, we explore the use of organic thin film transistors as biosensors. Some of the current projects in our laboratory are listed below.

 

Organic Film Growth (with Clancy, Engstrom, Headrick, Blakely, Smilgies)

We are interested in understanding the growth physics of films of organic semiconductors such as pentacene. These molecules are used in organic thin film transistors, and the film structure and morphology determines the transistor characteristics. Our work horse is a vacuum sublimation chamber which we place in the Cornell High Energy Synchrotron Source. We carry out a variety of experiments to probe the growth of films in situ and in real time, and combine these studies with atomic force microscopy and with electrical measurements. Some of our results have been summarized in this review paper: R. Ruiz, D. Choudhary, B. Nickel, T. Toccoli, K.-C. Chang, A.C. Mayer, P. Clancy, J.M. Blakely, R.L. Headrick, S. Iannotta and G.G. Malliaras, “Pentacene film growth”, Chem. Mater. 16, 4497 (2004). The picture shows an atomic force micrograph near the edge of a pentacene film, which shows layered growth.

Patterning and Processing of Organic Electronic Materials (with Ober, Holmes)

The vast majority of organic electronic materials are not amenable to standard photolithographic patterning, due to their interaction with solvents used in this process. We have developed a generic approach that overcomes this problem, and allows the patterning of both small molecules and polymers. At the Cornell NanoScale Facility, we are developing new procedures for the fabrication of organic electronic devices, such as the transistor shown in the picture, where three patterning steps were used to define a 2 micron channel length transistor with conducting polymer source and drain electrodes and a pentacene channel.

Charge Injection and Transport in Organic Semiconductors (with Kahn, Brédas, Dunlap, Novikov, Marohn, Ralph)

Some important aspects of charge injection and transport in organic semiconductors are not well understood. This hinders improvements in the performance of organic electronic devices. We have developed techniques to isolate and characterize injection and transport properties, as well as developed approaches to optimize injection. Some of our results have been summarized in this review paper: Y. Shen, A. Hosseini, M.H. Wong and  G.G. Malliaras, “How to make ohmic contacts to organic semiconductors”, ChemPhysChem. 5, 16 (2004). We are also interested in developing a molecular picture of injection and transport and use state-of-the-art electron beam lithography at the Cornell Nanoscale Science and Technology Facility to fabricate organic thin film transistors with very small dimensions, such as the one shown in the picture, where the channel length is only 30 nm.

Organic Semiconductor Device Physics (with Abruña, Bernhard, Anthony, Wise)

Organic electronic materials enable the fabrication of a variety of devices, including light emitting diodes, thin film transistors, and solar cells. We are interested in understanding the operation mechanism of these devices, in optimizing their performance, and in using these devices in novel applications. One example is the electroluminescent device shown in the picture. The active layer in this device is a mixed conductor - a material that is a semiconductor and an ion conductor at the same time. As a result of the mixed conductivity, the device efficiency is not critically dependent on the metals used for electrodes, as is the case with organic light emitting diodes. This allows the fabrication of novel device architectures, such as large-area, fault-tolerant illumination panels. Here is a recent review on these devices: J.D. Slinker, J. Rivnay, J.S. Moskowitz, J.B. Parker, S. Bernhard, H.D. Abruña, and G.G. Malliaras, “Electroluminescent devices from ionic transition metal complexes”, J. Mater Chem. 17, 2976 (2007).

Applications of Organic Thin Film Transistors to Biosensors  (with Batt, and Agave BioSystems)

Organic electronic materials have properties that make them ideal for applications in sensing. For example, one can achieve specificity to a particular analyte by introducing certain chemical or biological groups to the organic material. Our research in this area focuses on organic thin film transistors, particularly ones made from the conducting polymer PEDOT:PSS. These devices convert a recognition event into an easily measurable electrical current and amplify it at the same time. The figure shows data obtained from a glucose sensor - a large modulation is observed only when both glucose and glucose oxidase are present in the solution. We are working on the integration of these devices with microfluidic channels, aiming towards the "system on a chip" concept. Here is a recent review on these devices: J.T. Mabeck, and G.G. Malliaras, “Chemical and biological sensors based on organic thin-film transistors”, Anal. Bioanal. Chem.  384, 343 (2006).