Light-induced effects in highly stable organic thin film transistors on flexible plastic substrate

Instability induced by gate bias stress and light exposure have been investigated in organic thin film transistors made on flexible plastic substrate. P-channel OTFTs, with staggered top-gate configuration were fabricated on PEN, 125?m thick, using: evaporated gold for source-drain contacts; solution-processed pentacene derivative, (SmartKem? p-FLEX(TM) by SmartKem-Ltd), 30nm thick, as organic semiconductor; fluoro-polymer (CYTOP) film, 500nm thick as gate dielectric; aluminium as gate electrode. The active material and the dielectric were spin-coated and lithographically patterned. OTFTs channel lengths ranged from 2 to 100?m. Devices showed high field-effect mobility (up to 3 cm2/Vs), low threshold voltage (~-5V) and sub-threshold slope (~1V/dec). In order to test electrical stability of the devices, gate bias-stress at high |Vgs| (-40 V) has been performed, both at RT (vacuum and air) and at substrate temperature up to 330K (vacuum only), for long bias stress times (up to 80ks), monitoring the transfer characteristics of the devices at selected times. Excellent electrical stability was found: the characteristics showed no degradation during gate bias stress performed at RT in vacuum (both in linear and sub-threshold regime), while small positive threshold voltage shift (~0.5V) was observed during experiments performed in air and at high T. The very high stability of the devices under gate polarization allows us to investigate the effects of illumination on the electrical characteristics. Illumination produced right shift of the characteristics, mainly in the sub-threshold regime (several volts), while the on-regime was less affected (small VT variation). When devices were stored in dark without polarization, characteristics fully recovered, with relaxation time of the order of 100ks, depending on substrate temperature. In order to analyse this phenomenon, we monitored the relaxation dynamics during dark relaxation changing the light intensity and duration, light wavelength, gate polarization during illumination and substrate temperature during dark relaxation. To explain the experimental data, different mechanisms, including charge trapping and metastable defect creation, are considered and a model that describes the observed phenomena is presented.