Variable Temperature Mobility Analysis of n-Channel, p-Channel, and Ambipolar Organic Field-Effect Transistors

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 (mu(eff)) data assuming a discrete trap energy within a multiple trapping and release (MTR) model reveal low activation energies (E(A)s) for high-mobility semiconductors 1-3 of 21, 22, and 30 meV, respectively. Higher E(A) values of 40-70 meV are exhibited by 4-7-derived FETs having lower mobilities (mu(eff)). Analysis of these data reveals little correlation between the conduction state energy level and E(A), while there is an inverse relationship between E(A) and mu(eff). The first variable-temperature study of an ambipolar organic FET reveals that although n-channel behavior exhibits E(A) = 27 meV, the p-channel regime exhibits significantly more trapping with E(A) = 250 meV. Interestingly, calculated free carrier mobilities (mu(0)) are in the range of similar to 0.2-0.8 cm(2) V(-1) s(-1) in this materials set, largely independent of mu(eff). 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 (V(T)) reveals two distinct trapping regimes, with the change in trapped charge exhibiting a striking correlation with room temperature mu(eff). The observation that E(A) is independent of conduction state energy, and that changes in trapped charge with temperature correlate with room temperature mu(eff), support the applicability of trap-limited mobility models such as a MTR mechanism to this materials set.