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

Bias stress effects in organic thin-film transistors were investigated. A donor-acceptor type liquid-crystalline semiconducting copolymer, poly(didodecylquaterthiophene-alt-didodecylbithiazole), PQTBTz-C12, was used as the active channel material. This substance contains both electron-donating quaterthiophene and electron-accepting 5,5'-bithiazole units. The threshold voltage (V-T) shifts induced by direct current ( DC) bias stress were studied under different gate-source and drain-source voltages. By fitting Delta V-T versus stress time in compliance with a stretched exponential relationship, characteristic charge trapping time constants (tau) and dispersion parameters (beta) for the V-T shifts were determined for each stress condition. The time constants decrease with increasing gate-drain voltages. It was also observed that the V-T shift due to charge trapping can be recovered by releasing the device from bias stress for several hours. The recovery rate from DC OFF bias stress is slightly slower than the recovery from DC ON bias stress. Such a difference can be attributed to the different charge releasing time from the deep trap states for holes and electrons. The immediate compensation of opposite charges by applying an alternating current (AC) bias stress provides spontaneous charge detrapping at each cycle and thus results in relatively moderate total VT shifts compared to those under DC bias only. (C) 2011 American Institute of Physics. [doi:10.1063/1.3656442]