Exposure to moisture and elevated temperatures in organic thin film transistor (OTFT) usually results in significant degradation in the electrical performance. In this thesis the effects of temperature, T, and relative humidity, RH, on PS-DNTT OTFTs are investigated. Device characteristics were measured after 30-min exposure to RH that was gradually increased from 20% to 80% with T fixed at 20 ºC and also for T increasing from 20 ºC to 90 ºC with RH held at 10%. The turn-on and threshold voltages show a negative shift with minimal change in mobility upon exposure to higher RH and T. A very minimal change was observed in the deeper states in the density of states (DoS) that was extracted from transfer characteristics in the linear regime using the Grünewald approach. These results suggest that OTFT instability is due to the flatband voltage shift caused by hole trapping/detrapping in the polystyrene gate dielectric or at the polystyrene/DNTT interface. Understanding the origin of electrical instability in OTFTs over long periods of time is also essential to realize high performance circuits. In this thesis, the effects of bias stress on PS-DNTT OTFTs is investigated over a range of temperature and relative humidity. It was found that the threshold voltage, VT, always shifted in the direction of the applied gate voltage. It was also observed that the threshold voltage shift, ΔVT, reduced as the drain voltage increased. The time-dependences of ΔVT in both linear and saturation regimes are well described by the stretched exponential function. Contrary to most previous reports, the threshold voltage at long times, VT(∞), asymptotes to a value well below the applied gate voltage. The VT change is minimal with increasing humidity under saturation bias and temperature. In all cases, the DoS exhibits similar behavior with weak features appearing at the deeper states. This is unlikely to be related to DNTT as there is no change in the gate-voltage dependence of mobility, but rather due to a changing flat-band voltage when electron/hole occupancy of interface states changes as the device turns on. Effect of illumination on the electrical performance as well the underlying physics of these effects are important for the development of several applications such as the backplane for display technology and photosensors. In this thesis the effect of illumination on PS-DNTT OTFTs has been investigated with light of different wavelengths and intensities. The greatest effect was observed at 460 nm with significant changes occuring in the subthreshold slope. Interestingly the profile of the deeper states in the DoS spectrum did apparently change. It was also found that changes in threshold voltage, turn-on voltage, subthreshold slope and responsivity all appear to saturate at higher light intensity due to a trap limited effect. Since there is no significant change observed in mobility, the DoS changes can be explained by (i) trapping of photogenerated electrons, (ii) an unstable shift in the light-induced flat-band ΔVFB, caused by detrapping or neutralization of electrons as the transistor is turned on so that ΔVT(∞) < ΔVON(∞), (iii) accumulation of electrons in DNTT near to source contact or (iv) the effect of the electron quasi Fermi level, QFL. The effect of bias stress and illumination on PS-DNTT has also been investigated with different wavelengths, time and intensity. It was observed that the transfer characteristic shows a parallel shift toward more positive voltages under positive bias stress (PBS) regardless of the wavelength. However under negative bias stress (NBS) at λ ≥ 520 mm, the transfer characteristic shifts negatively due to the dominant effect of hole trapping at the PS-DNTT interface.