Raman imaging of carrier distribution in the channel of an ionic liquid-gated transistor fabricated with regioregular poly(3-hexylthiophene)

Raman images of carriers (positive polarons) at the channel of an ionic liquid-gated transistor (ILGT) fabricated with regioregular poly(3-hexylthiophene) (P3HT) have been measured with excitation at 785 nm. The observed spectra indicate that carriers generated are positive polarons. The intensities of the 1415 cm⁻¹ band attributed to polarons in the P3HT channel were plotted as Raman images; they showed the carrier density distribution. When the source–drain voltage V_D is lower than the source–gate voltage V_G (linear region), the carrier density was uniform. When V_D is nearly equal to V_G (saturation region), a negative carrier density gradient from the source electrode towards the drain electrode was observed. This carrier density distribution is associated with the observed current–voltage characteristics, which is not consistent with the “pinch-off” theory of inorganic semiconductor transistors.