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Keywords:

  • Transistors;
  • Organic field-effect transistors;
  • Charge transport

Abstract

The temperature dependence of field-effect transistor (FET) mobility is analyzed for a series of n-channel, p-channel, and ambipolar organic semiconductor-based FETs selected for varied semiconductor structural and device characteristics. The materials (and dominant carrier type) studied are 5,5′′′-bis(perfluorophenacyl)-2,2′:5′,2″:5″,2′′′-quaterthiophene (1, n-channel), 5,5′′′-bis(perfluorohexyl carbonyl)-2,2′:5′,2″:5″,2′′′-quaterthiophene (2, n-channel), pentacene (3, p-channel); 5,5′′′-bis(hexylcarbonyl)-2,2′:5′,2″:5″,2′′′-quaterthiophene (4, ambipolar), 5,5′′′-bis-(phenacyl)-2,2′: 5′,2″:5″,2′′′-quaterthiophene (5, p-channel), 2,7-bis((5-perfluorophenacyl)thiophen-2-yl)-9,10-phenanthrenequinone (6, n-channel), and poly(N-(2-octyldodecyl)-2,2′-bithiophene-3,3′-dicarboximide) (7, n-channel). Fits of the effective field-effect mobility (µeff) data assuming a discrete trap energy within a multiple trapping and release (MTR) model reveal low activation energies (EAs) for high-mobility semiconductors 13 of 21, 22, and 30 meV, respectively. Higher EA values of 40–70 meV are exhibited by 47-derived FETs having lower mobilities (µeff). Analysis of these data reveals little correlation between the conduction state energy level and EA, while there is an inverse relationship between EA and µeff. The first variable-temperature study of an ambipolar organic FET reveals that although n-channel behavior exhibits EA = 27 meV, the p-channel regime exhibits significantly more trapping with EA = 250 meV. Interestingly, calculated free carrier mobilities (µ0) are in the range of ∼0.2–0.8 cm2 V−1 s−1 in this materials set, largely independent of µeff. This indicates that in the absence of charge traps, the inherent magnitude of carrier mobility is comparable for each of these materials. Finally, the effect of temperature on threshold voltage (VT) reveals two distinct trapping regimes, with the change in trapped charge exhibiting a striking correlation with room temperature µeff. The observation that EA is independent of conduction state energy, and that changes in trapped charge with temperature correlate with room temperature µeff, support the applicability of trap-limited mobility models such as a MTR mechanism to this materials set.