The Construction and Operation of an Organic Electrochemical Transistor
The discovery by Heeger, et al that an electric current could be conducted by conjugated organic polymers, transpired in the year 1977. This discovery, ushered in an era of employing such substances in solid state devices, such as light emitting diodes, solar cells, photo diodes and transistors (Nilsson, Robinson, Berggren, & Forchheimer 2002: 353 – 354).
The construction of an organic electrochemical lateral transistor was effected by means of a thermal inkjet desktop printer, namely, the Canon BJC3000 printer. The principal advantage of employing this printer lies in the fact that the print head and ink tank are separated from each other. This facilitates replacement of the ink tank, via other feed mechanisms, without disturbing the inkjet head. The materials used were an aqueous solution of conjugated polymer (semiconductor) PEDOT:PSS and electrolyte (Mannerbro, Ranlof, Robinson, & Forchheimer 2008: 557).
It was ensured that the viscosity of these solutions did not exceed 10mPa. This precaution was taken, in order to prevent the clogging of the nozzles in the print head. Separate ink tanks, containing PEDOD:PSS and electrolyte, were connected to the inkjet head. Thereafter, the CMYK settings, in conjunction with the standard graphics software, were employed, in order to deposit material specific areas of the substrate. This enabled the construction of a transistor, encompassing three terminals, a drain, a source, a gate and the active channel. The drain, the source, the gate between them and the channel were simultaneously printed on the substrate, with the PEDOD: PSS (Mannerbro, Ranlof, Robinson, & Forchheimer 2008: 557).
In accordance with the conclusion reached, regarding the optimal number of layers to incorporate on the photo paper; seven layers of PEDS:PSS were deposited on the latter. Subsequently, this accretion was dried at room temperature for a few hours; thereafter, the deposit was annealed for a half hour at 600C. It was clearly seen that the resistance of the PEDOD:PSS films was inversely proportional to the increase in the number of PEDOD:PSS layers. The channel that is located between the source and the drain, is covered by electrolyte, and this extends to cover the portion from the gate area. The electrolyte connected the gate and the transistor channel (Mannerbro, Ranlof, Robinson, & Forchheimer 2008: 558).
The operation of an organic electrochemical transistor, which is fabricated from PEDOT:PSS and electrolyte, is based on a reversible electrochemical reaction between the PEDOT:PSS and the electrolyte layer. This reaction is instrumental in controlling the conductivity of the PEDOT:PSS. The process of oxidation transforms the PEDOT into PEDOT:PSS and reduces the band gap energy. In addition, its conductivity is increased by reduction with PEDOT, which has lower conductivity. Therefore, this redox reaction can be employed to effect switching in the transistor (Havener, et al.).
This relevant reaction in the electrochemical cell, can be expressed as,
When a positive voltage is applied between the gate and the source, cations from the electrolyte enter the channel and de-dope the PEDOT: PSS film. This results in an increase in the resistance of the channel. The outcome is an enhancement in the impedance between the drain and source, and a decrease in the source-drain current. The reverse oxidation reaction occurs at the gate, so as to maintain the charge balance. The application of a negative current, between the gate and the source, increases the oxidization of the PEDOT:PSS, in the channel layer. This oxidation results in a slight decrease in the channel layer resistance. However, the gate is increased in size, because of the fact that the PEDOT:PSS lends itself to easy over oxidation (Havener, et al.).
Such over – oxidation of the PEDOT: PSS results in the permanent loss of its conductivity. The transistor is rendered a resistance, when there is a zero voltage between the gate and the source. An increase in the voltage between the drain and the source causes a corresponding decrease in the voltage between the gate and the channel. Consequently, the channel layer oxidizes to the natural state (Havener, et al.).
List of References
Havener, R., Boyea, J., Malone, E., Bernards, D., DeFranco, J., Malliaras, G., et al. (n.d.). Freeform Fabrication of Organic Electrochemical Transistors. Retrieved November 21, 2009, from http://ccsl.mae.cornell.edu/papers/SFF07_Havener.pdf: http://ccsl.mae.cornell.edu/papers/SFF07_Havener.pdf
Mannerbro, R., Ranlof, M., Robinson, N., & Forchheimer, R. (2008). Inkjet printed electrochemical organic electronics. Synthetic Metals , 158, 556 – 560.
Nilsson, D., Robinson, N., Berggren, M., & Forchheimer, R. (2002). Electrochemical Logic Circuits. Advanced Materials , 17(3), 353 – 358.
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