Analysis of the persistent photoresponse of C8BTBT transistors in the near-bandgap spectral region

Abstract The variations in physical parameters of an organic field-effect transistor having dioctylbenzothieno[2,3-b]benzothiophene (C8BTBT) as the channel semiconductor were investigated under different light irradiation conditions at wavelengths of 350 nm, 370 nm, 400 nm and by increasing exposure doses. The progress of the electro-optical history of the transistor was evaluated by repeating I–V scan cycles both in the dark and under light exposure. The information recorded upon different exposure times was used to detect the photoactivated charge-trapping effects. The device showed a stable I–V response in the dark bias ( V DS = −10 V, −10 V ≤ V GS ≤ +10 V) conditions and a persistent threshold voltage ( V T ) shift under illumination at all irradiation wavelengths. We suggested that the observed dose-dependent V T drifts were due to charge retention in trap sites within the organic semiconductor. The threshold voltage was recognized as the main parameter affected by charge retention. V T variations were modelled versus time through a single exponential revealing a maximum in charge relaxation times for irradiations at wavelengths of 370 nm, in proximity of the C8BTBT bandgap energy. Furthermore, bias-stress effects and persistent photoinduced V T drifts were found to depend on comparable characteristic times. Therefore, a common nature for both the bias-stress decay and relaxation from photoexcitation mechanisms is likely.