To set the threshold voltage of organic thin-film transistors (and thereby, for example, the switching voltage of digital integrated circuits) to a specific, well-defined value during the device manufacturing process, we have investigated the use of ultra-thin gate dielectrics incorporating functional self-assembled monolayers based on aliphatic organic molecules with more or less electronegative substituents, such as alkyl and fluoroalkylphosphonic acids.

As an example, we have synthesized 1H,1H,2H,2H-perfluorododecylphosphonic acid in a two-step reaction. In the first step, the corresponding fluoroalkyl diethyl ester was obtained by a Michaelis-Arbuzov reaction of 1-iodo-1H,1H,2H,2H-perfluorododecane and triethylphosphite. In the second step, the diethyl fluoroalkyl phosphonate was hydrolyzed in hydrochloric acid, yielding the desired 1H,1H,2H,2H-perfluorododecylphosphonic acid.

With single-component self-assembled monolayers, i.e., with monolayers composed entirely of either an alkyl or a fluoroalkylphosphonic acid, the threshold voltage of the transistors can be set to one of two distinct and well-defined values that are separated by about 2 Volts. To reveal the mechanism by which the self-assembling molecules affect the threshold voltage of the transistors, we have employed Kelvin probe force microscopy (KPFM).

In addition to single-component self-assembled monolayers we have also investigated the use of mixed self-assembled monolayers that allow continuous tuning of the threshold voltage to any value within the voltage range determined by the single-component monolayers.

Below-one-volt organic thin-film transistors with large on/off current ratios
U. Zschieschang, V. P. Bader, H. Klauk
Organic Electronics, vol. 49, pp. 179-186, October 2017

Threshold-Voltage Shifts in Organic Transistors Due to Self-Assembled Monolayers at the Dielectric: Evidence for Electronic Coupling and Dipolar Effects
M. Aghamohammadi, R. Rödel, U. Zschieschang, C. Ocal, H. Boschker, R. T. Weitz, E. Barrena, H. Klauk
ACS Applied Materials & Interfaces, vol. 7, pp. 22775-22785, October 2015

Mixed Self-Assembled Monolayer Gate Dielectrics for Continuous Threshold Voltage Control in Organic Transistors and Circuits
U. Zschieschang, F. Ante, M. Schlörholz, M. Schmidt, K. Kern, H. Klauk
Advanced Materials, vol. 22, no. 40, pp. 4489-4493, October 2010

Fluoroalkylphosphonic acid self-assembled monolayer gate dielectrics for threshold-voltage control in low-voltage organic thin-film transistors
U. Kraft, U. Zschieschang, F. Ante, D. Kälblein, C. Kamella, K. Amsharov, M. Jansen, K. Kern, E. Weber, H. Klauk
Journal of Materials Chemistry, vol. 20, no. 31, pp. 6416-6418, July 2010

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