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

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 1–3 of 21, 22, and 30 meV, respectively. Higher EA values of 40–70 meV are exhibited by 4–7‐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.

Crossover from band-like to thermally activated charge transport in organic transistors due to strain-induced traps

“…This relation between E A, μ, and L results from the fact that, with increasing L, there are more grain boundaries and possibly more disorder in the texturing of the film, as suggested by Fig. 3A, and in agreement with other reports (48,49). Our results strongly indicate now that, in addition to microstructure effects, there is a second reason for increased trapping with increasing channel length, and that is the introduction of inhomogeneous thermal strain during heating and cooling cycles.…”

Trap density of states in n-channel organic transistors: variable temperature characteristics and band transport

“…Intrinsic carrier transport, in particular, band transport, has been explored in organic single crystals, [9][10][11][12] and trap-limited charge transport has been examined in organic thin films, which have higher density of trap states due to the structural defect, grain boundaries, and chemical impurities. [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] The multiple trapping and release model for a Author to whom correspondence should be addressed. Electric mail: cho.j.ad@m.titech.ac.jp.…”

Analysing organic transistors based on interface approximation

“…[1][2][3][4][5][6][7][8][9] Historically, it dates back to Horowitz's work, in which the trap density of states (DOS) was obtained by analysing T-dependent transfer characteristics of dihexylsexithiophene transistors. [1][2][3] When we investigate organic transistors following the conventional amorphous silicon (a-Si) transistors ( Fig.…”