Correlation of on-state conductance with referenced electrochemical potential in ion gel gated polymer transistors

We report direct measurement of the electrochemical potential at organic semiconductor/gate dielectric interfaces in printed polymer transistors employing a gel electrolyte as the gate insulator. An oxidized silver wire reference electrode was embedded into the gel electrolyte, and its potential relative to the grounded source contact was measured simultaneously with the transistor transfer characteristics. The referenced turn-on voltages of transistors based on three common polymer semiconductors [(poly-3- hexylthiophene, poly(3, 3 -didodecylquaterthiophene), and poly(9, 9′ -dioctylfluorene-co-bithiophene)] were found to correlate with the reported highest occupied molecular orbital levels (ionization potentials) for these materials. Further, analysis of the transfer characteristics revealed a negative differential transconductance regime at high gate-induced carrier densities (∼ 1015 cm-2), which we attribute to a combination of band filling and a mobility lowering effect.