We report solution processed highly photosensitive thin film transistors (TFTs) based on poly(9,9-dioctylfluorene-cobithiophene)
(F8T2) as an active photoconducting material. Bottom gate contact coplanar device structure on Si wafer
transistors was used. The photosensitivity of the drain photocurrent was investigated for different F8T2 annealing
temperatures and illumination irradiances. Transistors annealed at 280oC show the highest drain current, approximately 8
times higher than the as-spincoated device at room temperature with a gate voltage of -40V. However, the field effect
mobilities in the saturation regime for all devices at different annealing temperatures are in the same order of ~10-4
cm2/Vs. The field effect mobilities of the transistors were not affected by illumination, but the drain photocurrent of the
transistor was significantly increased and the threshold voltage was shifted towards zero bias voltage when the polymer
absorbs photons. The measured maximum responsivity was ~18.5 A/W for an LED light source with a peak wavelength
of 465 nm and 19 nm bandwidth at 5 μW/cm2 light intensity. This is so far the highest reported for F8T2
phototransistors. The characteristics of transistors dominated by the photoconductive effect (turn-off) as well as the
photovoltaic effect (turn-on) against a wide range of illumination intensities are reported.
Semiconducting polymers can be used in light-emitting-diodes (LEDs), photovoltaics (PVs), and field-effect-transistors (FETs). In all of these devices charge carrier transport is a major issue, the mobility being directly related to device performance. In LEDs and PVs, charge transport occurs vertically through a bulk semiconducting polymer film. This bulk mobility is determined by the average interchain hopping distance a, the polaron relaxation energy λ, the level of energetic and spatial disorder σ and Σ, the presence of charge traps and different structural phases. In FETs, charge transport occurs horizontally along the interface between the semiconducting polymer film and the insulating material. The FET mobility is also determined by the above parameters but these may be different from the bulk. Also, there are additional factors such as surface features which have to be circumnavigated, specific interface trap states, and the high charge carrier densities effectively filling all the deep sites. Here we present results looking at the difference between the bulk mobility, as measured by time-of-flight (TOF) photocurrent, versus the FET mobility, as measured by the FET transfer characteristics. Three different polyfluorene copolymers are investigated. In all three materials, the room temperature hole TOF bulk mobility was found to be greater than the FET mobility. This indicates that models based on deep site filling due to the high FET carrier densities cannot be correct. Temperature measurements also show that the level of energetic disorder σ in the FETs is the same or less than that in the bulk, as is the polaron relaxation energy λ or thermal activation energy of any deep traps. The results instead indicate that it is the average interchain hopping distance which is greater at the insulator-semiconductor interface in FETs than in the bulk films, and it is this which is responsible for the difference in mobility.