(Invited) Ionic Liquid-Gated PCBM Transistors: Pushing the Limit of the Doping in Organic Transistors

Electrolyte-gated (EG) organic transistors are interesting for their low voltage operation and versatile cost-effective fabrication and printability.¹ Room temperature ionic liquids (RTILs) are attractive as gating media for their chemico-physical properties, such as ionic conductivity up to 10 mS·cm^-1, negligible volatility, and electrochemical stability windows up to ca 5 V.² The availability of RTILs with different molecular structures gives the possibility to control specifically the interactions between the ions constituting the RTIL and the channel material in view of an optimized doping. Clear guidelines to establish an effective doping as a function of the ions constituting the electrolyte, are yet to be established. In this work we report on EG transistors with phenyl-C61-butyric acid methyl ester (PCBM), as the channel material, and room temperature ionic liquids based on bis(trifluoromethylsulfonyl)imide ([TFSI]) anion, as the gating medium. The cations constituting the ionic liquids were 1-ethyl-3-methylimidazolium ([EMIM]), 1-butyl-3-methylimidazolium ([BMIM]) or 1-butyl-1-methylpyrrolidinium ([PYR₁₄]). Electrical measurements show that the cation strongly affects the behavior of n-type EG PCBM transistors. The higher charge carrier mobility, lower on/off ratio and faster response time obtained with [PYR₁₄][TFSI]-gated transistors, suggests lower cation incorporation into the PCBM channel material. [EMIM][TFSI]- and [BMIM][TFSI]-gated PCBM transistors featured similar electric behavior, coherent with the similar molecular structure of the cations.

1. S. H. Kim et al., Adv. Mater. Weinheim, 25, 1822–1846 (2013).

2. M. Galiński, A. Lewandowski, and I. Stępniak, Electrochim. Acta, 51, 5567 (2006).