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Solution Processable Organic Field Effect Transistors
Figure 1: Schematic of a BC-BG FET highlighting the metal-semiconductor and dielectric-semiconductor interfaces that play a role in charge injection and transport processes, respectively.
The low-cost, low-temperature fabrication of organic field effect transistors (OFETs) makes them attractive candidates for applications in displays, radio frequency identification (RFID) tags, and sensors, as well as opens up new opportunities for flexible electronic applications. Recent demonstrations of ambipolar transport in OFETs have broadened the range of applications to multi-functional devices such as CMOS-like inverters, light-emitting FETs, and photosensing FETs. But reports of ambipolar transport in FETs based on high mobility semiconductors such as pentacene and rubrene are still very few. The main challenges are attributed to overcoming the extrinsic factors of high injection barriers for one of the charge carrier types (electrons/holes) at the metal-semiconductor interface, carrier trapping at the dielectric-semiconductor interface, and trap generation upon exposure to air. In our group, we tailor device interfaces to fabricate n- and p-type unipolar and ambipolar OFETs, enabling our exploration of the fundamental physics of electron and hole transport in organic semiconductors and of electron and hole injection across the interfaces of organic devices. We evaluate the role of source-drain metal and gate dielectric materials properties, device architecture, and scaling in their performance. We apply our advances in engineering OFETs and develop processes to integrate these devices in a range of applications.
Figure 2: Optical micrograph of BC-BG FETs on a Kapton® substrate (b) Transfer characteristics for L=20 µm and W=300 µm unmodified (-) and benzenethiolate modified (-) FETs and for L=150 µm and W=2250 µm unmodified (----) and benzenethiolate modified (----) FETs (c) Transfer characteristics for FETs with different channel lengths between 5 and 200 µm (d) Transfer characteristics and gain curves of CMOS-like inverters constructed from ambipolar FETs at negative drive voltages.
One route we have investigated to manipulate charge transport in OFETs is to chemically engineer the metal-semiconductor and semiconductor-dielectric device interfaces (Figure 1). We have demonstrated improved electron and hole injection in solution-processed, bottom contact-bottom-gate (BC-BG) pentacene FETs by modifying the semiconductor-dielectric interface, by spin-coating benzocyclobutene (BCB) on top of thermally oxidized (SiO2) n+ Si wafers to bury the SiO2 surface believed to trap electrons, and the metal-semiconductor interface, by assembling aromatic and aliphatic thiolates to lower the device contact resistance [1]. We applied this approach to build BC-BG OFETs on flexible, plastic substrates and have further implemented parylene as a low-leakage, gate dielectric layer [2] (Figure 2). In the BC-BG geometry, the Au source-drain electrodes are derivatized with aromatic or aliphatic thiolates [2] and, we have also demonstrated the application of carbodithiolate [3] self-assembled monolayers to improve electron and hole injection (Figure 2b). Hole and electron mobilities of 0.05–0.1 and 0.01–0.05 cm2/V-s are achieved in these devices. In the same aspect ratio (width/length of channel) devices, we observed an increase in electron and hole currents with increasing channel length due to a lowering of the contact resistance (Figure 2c). As compared to devices in BC-BG geometry, those in top contact-top gate (TC-BG) geometry show better balanced hole and electron currents and lower contact resistances for both types of carriers (Figure 3), which is anticipated to arise from the increased electrode-pentacene contact area. CMOS-like inverters with gains of up to 110 were demonstrated, which are the highest gain single organic semiconductor-based flexible inverters reported till date (Figure 2d).
Figure 3: (a) Schematic of a TC-BG device and its (b) transfer and output characteristics in (c) hole and (d) electron acumulation regimes.
Other areas of our focus include developing charge injection and transport models using temperature-dependent transport measurements, applications of unipolar/ambipolar FETs in light emitting FETs and various circuit topologies, and the investigation of other solution-processable organic semiconductors in FETs. Recently we have developed substrate compatible multi-step flexible processes for building complex circuits (ring oscillators, inverters, common source amplifier).
Published Papers
1. Saudari S.R., Frail P.R., Kagan C.R. “Ambipolar transport in solution-deposited pentacene transistors enhanced by molecular engineering of device contacts”, Appl. Phys. Lett., 95, 023301, (2009). [Appeared in July 27, 2009 issue of Virtual Journal of Nanoscale Science and Technology]
2. Saudari S.R., Lin Y.-J., Lai Y., Kagan C.R. “Device configurations for ambipolar transport in flexible, pentacene transistors”, Adv. Mater., 44, 5063-5068, (2010).
3. Schulz P., Zangmeister C.D., Zhao Y.L., Frail P.R., Saudari S.R., Gonzalez C.A., Kagan C.R., Wuttig M, van Zee R.D. “Influence of the Bonding Chemistry of Thiolated and Carbodithiolated Self-Assembled Monolayers on the Valence Band Structure of Gold”, Journal of Physical Chemistry C, 114 (48), 20843–20851, (2010).
Publications
“Device Configurations for Ambipolar Transport in Flexible, Pentacene Transistors,” Sangameshwar Rao Saudari, Yu Jen Lin, Yuming Lai and Cherie R. Kagan, Advanced Materials, 44, 5063-5068, (2010)“Ambipolar transport in solution-deposited pentacene transistors enhanced by molecular engineering of device contacts,” Sangameshwar Rao Saudari, Paul R. Frail, Cherie R. Kagan , Appl. Phys. Lett, 95, 023301 (2009)
“Organic and Organic-Inorganic Hybrid Molecular Devices,” Proceedings of the 12th International Micromachine/Nanotech Symposium, 31 (2006).
Group Members Involved
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