High‐Capacitance Ion Gel Gate Dielectrics with Faster Polarization Response Times for Organic Thin Film Transistors

Abstract

The development of new gate dielectric materials that offer both low-temperature processability and high capacitance is an important goal for organic electronics in order to facilitate the fabrication of low-voltage circuitry on plastic substrates. [1][2][3][4] Marks, Facchetti, and colleagues, for example, have recently demonstrated that ultra-thin (∼10 nm) crosslinked polymer blend films [5,6] and self-assembled siloxane layers [7] can be used as gate insulators in organic thin-film transistors (OTFTs), providing turn-on potentials of a few volts with gate leakage current densities < 10 -8 A cm -2 . Likewise, Berggren and colleagues have demonstrated polyelectrolyte proton conductors as solution-processable, high-capacitance gate dielectrics for low-voltage OTFT operation. [8,9] In an alternative strategy, we and others have shown that a solid state polymer electrolyte consisting of a Li salt (LiClO 4 or Li bis(trifluoromethylsulfonyl)imide (LiTFSI)) dissolved in poly(ethylene oxide) (PEO) can also serve as a high-capacitance gate insulator in OTFTs. [10][11][12][13][14][15][16][17][18][19][20][21] The specific capacitance for LiClO 4 /PEO is exceptionally large (>10 lF cm -2 ), providing both low-voltage operation and very high ON-currents for OTFTs. Low voltage, LiClO 4 /PEO gated transistors based on polythiophene, [10] poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene), [11] pentacene, [12] perylene diimides, [13] and organic single crystals [14][15][16] have been demonstrated.However, the switching speed of these devices is determined by the polarization response time of the LiClO 4 /PEO, and not by the carrier mobility in the semiconductor channel, as is usually the case. [22] The slow polarization response of LiClO 4 / PEO limits transistor switching speed to a few hertz at room temperature, which is a major disadvantage for applications.In order to decrease the polarization response time while maintaining a large specific capacitance, we have investigated ion gels (Fig. 1) as novel gate dielectric materials. Ion gels comprise a polymer network swollen with an ionic liquid. [23] In this work, we have gelled ionic liquids by self-assembly of a triblock copolymer, PS-PEO-PS (Fig. 1), where the polystyrene (PS) endblocks are insoluble in the ionic liquid. [24] The ion gels contain a modest amount of polymer (as little as 4 wt %) producing sufficient mechanical integrity without appreciable change in the ionic conductivity. The room-temperature ionic conductivity is more than 10 -3 S cm -1, much larger than in the PEO/LiClO 4 or PEO/LiTFSI systems (10 -5 S cm -1 at 25°C), which dramatically decreases the polarization response time. In this Communication, we demonstrate that high-capacitance (∼ 40 lF cm -2 ) ion gel-gated OTFTs (GEL-OTFTs) can operate at frequencies up to 1 kHz. This represents a nearly 1000-fold improvement over PEO-gated transistors and a 10-fold improvement over our initial report on ion gel-gated polymer TFTs.[25] Moreover, we have surveyed the behavior of gels made from thr...

Conjugated Polymer for Voltage‐Controlled Release of Molecules

“…[31,32] There are some evidences that ions with molecular weight higher than 100 g mol −1 can diffuse into P3HT films and induce changes in their conductivity. [33] A similar property can be found in poly (3,4-ethylenedioxythiophene):poly (styrene sulfonate) (PEDOT:PSS) films, in which molecules like glutamate, aspartate, and amino butyric acid with molecular weights of above 100 g mol −1 can be transferred. [13] But the upper limit of the molecular weight for successful molecule transfer in conjugated polymers has not been systematically characterized yet.…”

Organic field-effect transistors using single crystals

“…Noting that gate electric field is confined only to electric double-layers 1 nm thick, only a fraction of a volt is necessary to accumulate the same carrier density as when 100 V is applied to gate dielectrics more than two orders of magnitude thicker. There have been reports on the use of polymer electrolytes such as LiClO 4 dissolved in poly(ethylene oxide) or ionic liquids maintained in a polymer gel [51][52][53][54][55]. However, these devices suffer from either poor mobility or slow response to the applied V G because of relatively slow ionic diffusion in the polymer platforms.…”

High performance photolithographically-patterned polymer thin-film transistors gated with an ionic liquid/poly(ionic liquid) blend ion gel

“…This has been shown to happen in OTFTs gated with hygroscopic polyelectrolytes that necessitate air moisture in order to show appreciable ionic conductivity, in which water electrolysis limits the applicable voltage range and can lead to high leakage currents. 14,15 Previous reports of ion gels used as gate insulator in OTFTs include mixtures of an ionic liquid with triblock copolymers, 16 with the fluorinated elastomer poly(vinylidenefluorideco-hexafluoropropylene), 17 as well as with poly(ethylene oxide) obtained from a UV-crosslinkable diacrylated monomer precursor. 18 Poly(ionic liquid) (PILs), which are polyelectrolytes synthesised by direct polymerisation of an ionic liquid, 19,20 are another class of materials that have not been extensively investigated as insulator materials for OTFTs.…”