Organic thin film transistors based on N-alkyl perylene diimides: Charge transport kinetics as a function of gate voltage and temperature

We report structural and electrical transport properties of a family of π-stacking soluble organic semiconductors, N,N′-dialkyl-3,4,9,10-perylene tetracarboxylic diimides (alkyl - pentyl [1], octyl [2], and dodecyl [3]). The structures of evaporated polycrystalline films of 1-3 were studied using X-ray diffraction and atomic force microscopy. Films of 1-3 pack similarly with the direction of π-π overlap in the substrate plane. Organic thin film transistors (OTFTs) based on 1-3 deposited on SiO ₂ gate dielectric showed linear regime electron mobilities of 0.1, 0.6, and 0.2 cm ²/(V s), respectively, corrected for contact resistance. OTFTs of 2 had saturation electron mobilities as high as 1.7 cm ²/(V s) with on-to-off current ratios of 10 ⁷. Variable temperature measurements were used to examine the charge transport kinetics in the range 80-300 K and revealed (1) thermally activated electron mobilities with activation energies dependent on gate voltage and (2) the presence of well-defined isokinetic points, i.e., temperatures at which Arrhenius plots at different gate voltages intersect for a given film. Isokinetic points indicate a common charge transport mechanism and can be explained in terms of the multiple trapping and release (MTR) transport model. MTR assumes trap-limited band transport, and quantum chemical calculations were used to verify that delocalized transport is likely in 1-3; a conduction bandwidth of 0.58 eV was calculated for 1. Using MTR, the trap concentrations were estimated to be ∼10 ¹² cm ^-2 for deep traps, and ∼6 × 10 ¹³ cm ^-2 for shallow traps. However, a nonmonotonic dependence of the electron mobility on gate voltage was also observed, which is not predicted by MTR and suggests that the transport mechanism is more complicated, perhaps due to the discrete layered structure of these materials. The high values for the electron mobility and on-to-off current ratio suggest that substituted perylene diimides represent a promising class of n-channel conductors for OTFTs.