Characterization of bias stress induced electrical instability in liquid-crystalline semiconducting polymer thin-film transistors

Abstract: We have investigated the bias stress effect on organic thin- film transistors. The transistors utilize a novel donor- acceptor type liquid-crystalline semiconducting co polymer, poly(didodecylquaterthiophene-didodecylbithiazole), PQTBTz- C12, which contains both electron-donating quaterthiophene and electron-accepting 5,5…-bithiazole units. The threshold voltage (VT) shift induced by DC bias stress has been analyzed for different gate-source and drain-source voltages. By fitting ？T versus the time to a stretched exponential function, characteristic charge trapping time constant and dispersion parameter (＆#65533; of the VT shift were determined for each condition. The time constants decrease with increasing gate-drain voltages. We also observed the VT shift due to charge trapping can recovered by halting the device for several hours. The recovery rate from DC OFF bias stress is slightly slower than the recovery from DC ON bias stress. The difference in the recovery rates between ON and OFF can be attributed to the different charge releasing time of the deep trap state for hole and electron. The compensation of different charge and the spontaneous recovery from the bias stress have influence on VT shift of the OTFTs subjected on AC bias stress, resulting in more and less moderate VT shift of the OTFT under AC bias stress compared to that under DC bias.

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