Abstract
We have investigated trap density of states (trap DOS) in n-channel organic field-effect transistors based on N,N ’-bis(cyclohexyl)naphthalene diimide (Cy-NDI) and dimethyldicyanoquinonediimine (DMDCNQI). A new method is proposed to extract trap DOS from the Arrhenius plot of the temperature-dependent transconductance. Double exponential trap DOS are observed, in which Cy-NDI has considerable deep states, by contrast, DMDCNQI has substantial tail states. In addition, numerical simulation of the transistor characteristics has been conducted by assuming an exponential trap distribution and the interface approximation. Temperature dependence of transfer characteristics are well reproduced only using several parameters, and the trap DOS obtained from the simulated characteristics are in good agreement with the assumed trap DOS, indicating that our analysis is self-consistent. Although the experimentally obtained Meyer-Neldel temperature is related to the trap distribution width, the simulation satisfies the Meyer-Neldel rule only very phenomenologically. The simulation also reveals that the subthreshold swing is not always a good indicator of the total trap amount, because it also largely depends on the trap distribution width. Finally, band transport is explored from the simulation having a small number of traps. A crossing point of the transfer curves and negative activation energy above a certain gate voltage are observed in the simulated characteristics, where the critical VG above which band transport is realized is determined by the sum of the trapped and free charge states below the conduction band edge.
Highlights
INTRODUCTIONCy-NDI shows outstanding performance among the n-channel materials, with the reported mobility of 6.2 cm2/Vs due to the good thin-film morphology and the highly symmetrical brickwork packing of the planar molecules.[39] A small-molecule organic electron acceptor, DMDCNQI, forms air-stable n-channel transistors,[40,41] and recently, improved performance has been attained (μ = 0.23 cm2/Vs, on/off ratio = 2 × 106, VT = 0 V) using the low vacuum evaporation method, though the threshold voltage is more susceptible to the surface treatment due to the trap states induced during the low vacuum evaporation.[42] In order to study the trap states, we have investigated the characteristics of these transistors in the temperature range between 260 K and 200 K
Considerable research efforts have been devoted to organic field-effect transistors last decades for the possibility of low-cost alternatives of silicon electronics,[1,2,3] and there has been enormous progress in developing organic semiconductors, where the best performance exceeds amorphous silicon transistors (a-SI).[4,5,6,7,8] Along with improving device performance, underlying charge transport mechanism has been extensively studied because it is closely related to device key parameters such as mobility, threshold voltage, subthreshold swing, and electrical and environmental stability
We have investigated trap density of states in n-channel organic fieldeffect transistors based on N,N ’-bis(cyclohexyl)naphthalene diimide (Cy-NDI) and dimethyldicyanoquinonediimine (DMDCNQI)
Summary
Cy-NDI shows outstanding performance among the n-channel materials, with the reported mobility of 6.2 cm2/Vs due to the good thin-film morphology and the highly symmetrical brickwork packing of the planar molecules.[39] A small-molecule organic electron acceptor, DMDCNQI, forms air-stable n-channel transistors,[40,41] and recently, improved performance has been attained (μ = 0.23 cm2/Vs, on/off ratio = 2 × 106, VT = 0 V) using the low vacuum evaporation method, though the threshold voltage is more susceptible to the surface treatment due to the trap states induced during the low vacuum evaporation.[42] In order to study the trap states, we have investigated the characteristics of these transistors in the temperature range between 260 K and 200 K.
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