Tuning the threshold voltage in electrolyte-gated organic field-effect transistors

Abstract

Low-voltage organic field-effect transistors (OFETs) promise for low power consumption logic circuits. To enhance the efficiency of the logic circuits, the control of the threshold voltage of the transistors are based on is crucial. We report the systematic control of the threshold voltage of electrolyte-gated OFETs by using various gate metals. The influence of the work function of the metal is investigated in metal-electrolyte-organic semiconductor diodes and electrolyte-gated OFETs. A good correlation is found between the flat-band potential and the threshold voltage. The possibility to tune the threshold voltage over half the potential range applied and to obtain depletion-like (positive threshold voltage) and enhancement (negative threshold voltage) transistors is of great interest when integrating these transistors in logic circuits. The combination of a depletion-like and enhancement transistor leads to a clear improvement of the noise margins in depleted-load unipolar inverters.organic electronics | polyelectrolytes | thin-film transistors | gate electrode material T he possibility to process organic electronic materials from solution allows for printing electronic systems on a wide variety of large area and flexible substrates thus enabling new applications and ultralow cost electronics (1-3). The transistor is a corner stone in modern electronics. The OFETs are promising devices for applications that require medium-speed operation such as driving circuits for displays (4) and sensors (5, 6); however, to enable their integration in portable devices or electronic labels, i.e., distributed systems typically driven by printed batteries (7) or solar cells (8), the OFETs should operate at low voltage and power. A low driving voltage can be achieved by employing a gate dielectric composed of a ultrathin, cross-linked polymer (9), a self-assembled monolayer in combination with a thin metal oxide layer (10), or high-permittivity dielectrics (11); however, high-quality inorganic dielectrics or ultrathin organic dielectrics are normally difficult to combine with common printing technologies. Recently, various electrolytes have successfully been explored as the gate insulator in low-voltage operating OFETs (12-17). In this case, the formation of the gate capacitance includes polarization of electronic and ionic charges. Indeed, if we consider a p-channel OFET, applying a negative voltage to the gate attracts cations from the solution towards the electrolyte/gate interface while anions are repelled towards the electrolyte/semiconductor interface. Such ion redistribution results in the formation of electrical double layers (EDLs) at the two interfaces. These two EDLs can be assimilated to two capacitors in series in which the redistributed ions in the electrolyte are balanced by oppositely charged electronic charge carriers at the gate electrode and in the semiconductor, respectively (Fig. 1A). The ionic and electronic charges within these EDLs are separated by only a few Å resulting in a very high capacitance (typi...

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“…The shifting of threshold voltages has been demonstrated in other flexible materials (e.g., organic transistors) using selfassembled monolayers (55) or gate electrode metals (56). However, there were relatively large SDs of threshold voltages.…”

“…However, there were relatively large SDs of threshold voltages. Additionally, because fully continuous tuning of threshold voltages over a wide range could not be realized, the inverters had low noise margins (56). In comparison, our SWNT TFTs have excellent uniformity (very small SD in threshold voltages) at different doping concentrations on both rigid and flexible substrates; therefore, they become promising flexible materials for largescale CMOS circuits.…”

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