Exposure to moisture and elevated temperatures usually results in significant degradation of organic thin film transistor (OTFT) performance. Typical observations include reduced mobility, unstable threshold voltage and the appearance of hysteresis in electrical characteristics. In this contribution we investigate the effects of environmental conditions on OTFTs based on DNTT, a high-mobility, small-molecule, organic semiconductor, with polystyrene (PS) as the gate insulator. Device characteristics were measured after consecutive 30-minute exposures to a relative humidity (RH) that was gradually increased from 20% to 80% with temperature fixed at 20ºC and for temperatures increasing from 20 o C to 90 o C with RH held at 10%. Despite significant negative shifts in turn-on and threshold voltages, only slight changes in the hole mobility were observed at the highest RH and temperature. The DNTT density of states (DoS) extracted from transfer characteristics in the linear regime using the Grünewald approach showed little change with environmental conditions. In all cases, the DoS decreased from ~1 x 10 20 down to ~1 x 10 17 cm-3 eV-1 in the 0.45 eV energy range above the hole mobility edge. Some evidence was obtained for a weak trap feature between ~0.25 and 0.35 eV above the mobility edge. These results confirm the high stability of DNTT as a semiconducting material and that OTFT instability observed here is associated almost entirely with a flatband voltage shift caused by hole trapping in the polystyrene gate dielectric or at the polystyrene/DNTT interface.

We have investigated the photoresponse of organic thin film transistors (OTFTs) based on evaporated films of dinaphtho [2,3-b:2',3'-f] thieno[3,2-b'] thiophene (DNNT) as the active semiconductor and spin-coated polystyrene as the gate insulator. Both during illumination and in subsequent measurements in the dark after long periods under illumination, transfer characteristics shift to more positive gate voltages. The greatest photoresponse was achieved at 460 nm, near the absorption maximum of DNTT. The maximum photosensitivity and photoresponsivity measured were ~10 4 and 1.6 A/W respectively. The latter is the highest reported for an organic semiconductor on a polymeric gate insulator and by suitable adjustments to device geometry could be increased to match the highest reported, ~10 5 A/W, for organic semiconductors. Weaker responses were also obtained when exposed to light from the long-wavelength tail in the absorption spectrum. At these longer wavelengths, the response arises entirely from a shift in flatband voltage caused by deep interface trapping of photo-generated electrons. At 460 nm, however, the positive shift, VON, in turn-on voltage is much greater than the shift, VT, in threshold voltage suggesting that ~3.5 x 10 11 electrons/cm 2 are trapped at the interface at the start of the gate voltage sweep, but ~60% are neutralised by holes from the channel as the device begins to turn on. While the resulting change in subthreshold slope could be interpreted as a change in the density of states (DoS) in the DNTT, this is discounted. Gate bias stress measurements made under illumination, reveal that positive bias enhances interface electron trapping while negative bias reduces the effect owing to the simultaneous trapping of holes from the accumulation channel.

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