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 ( µ 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 ( µ 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 µ 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 ( µ 0 ) are in the range of ∼0.2–0.8 cm 2  V −1 s −1 in this materials set, largely independent of µ 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 µ eff . The observation that E A is independent of conduction state energy, and that changes in trapped charge with temperature correlate with room temperature µ eff , support the applicability of trap‐limited mobility models such as a MTR mechanism to this materials set.