We report on solution processed, highly light sensitive thin film transistors (TFTs) based on poly(9,9-dioctylfluorene-co-bithiophene) (F8T2). Transistors without heat treatment showed the highest saturation mobility, while devices annealed at 280°C showed the highest drain current. The latter annealed transistors were found to give highly stable and reproducible performance over many light cycles. Measurements were carried out using an inorganic light emitting diode (LED) light source with a peak wavelength of 465nm and 19nm bandwidth from 0to400μW∕cm2 light intensity on TFTs with an F8T2 film thickness of 30nm. The TFT OFF current was found to increase both with light intensity and gate bias. The bulk photogenerated carrier density was calculated to change from 5×1011to1×1013cm−3 over the measured light intensity range. The TFT saturation mobility did not change with light intensity, remaining constant at 1.2×10−4cm2∕Vs. The TFT ON current instead increased due to a shift in the turn-on voltage VT. This changed from −27to−20V over the measured light intensity range, initially changing rapidly but then saturating at higher intensity values. Contact resistance RC measurements showed large values in the dark. RC rapidly decreases with increasing light intensity, again saturating at higher values. From these results, we propose a phototransistor model in which illumination varies the device performance by effecting injection. By considering this shift in RC as photoassisted barrier lowering which additionally varies the width of the region depleted of carriers between the injecting interface and the channel, it is possible to explain the observed shift in VT as a change in the fraction of the gate bias dropped across the contact capacitance CC. By operating the phototransistor at a value of Vg=−5V (below VT), it was possible to achieve a highly linear response of the photocurrent with light intensity. Alternatively, by operating at a value of Vg=−40V (above VT), it was possible to maximize the photoresponsivity within the measured range. A photoresponsivity of 18.5A∕W at 5μW∕cm2 light intensity was achieved.

Organic thin film transistors based on poly(3,3‴-didodecylquarter-thiophene) were characterized under illumination with a fixed wavelength but various intensities from dark to 1100 μW cm−2. Typically the illumination process should increase the drain current through the increase in the number of charge carriers in the channel in the form of polarons, as a result of generation and dissociation of excitons or electron-hole pairs. However, the rate of the current increase was found to decrease as the light intensity was increased, and eventually the level of drain current reached a maximum before declining. We suggest that the physics behind this oversaturation behavior is related to the increasing number of electron-hole recombination events associated with the increase in polaron density in the channel. When the polaron density goes above a threshold value at high light intensity, the number of polarons cannot increase further as they are already closely packed and the recombination overtakes generation, resulting in a decrease in the drain current from its peak value. We show that quantitative analysis agreed well with our model, and in our device the polaron diameter and mean free path are 19 and 2 nm, respectively.

The apparent shift of threshold voltage of organic thin-film transistors under light illumination has been explained as a result of the superposition of a photo-generated current on the dark current overall biases. Our model has been confirmed by demonstrating that the apparent threshold voltages calculated under different illumination intensities matched perfectly with the experimental values, for two devices with different channel widths. Our model indicates that (1) there is a photo-current associated with the photo-excitation process in organic thin-film transistors and (2) the apparent threshold voltage under illumination is not the intrinsic threshold voltage of a device as measured in the dark; instead, it is monotonically shifted from the intrinsic value due to the increase in photo-current under normal laboratory conditions.

Off‐center spin coating is a method to fabricate thin film on a substrate where the substrate is located at an off‐center distance away from the rotating center of the spin coater. Here, a mathematical model to calculate the thickness of a film fabricated by an off‐center spin‐coating technique was developed and proposed. The model showed that the off‐center film thickness was calculable by using four factors—the on‐center film thickness, mass fraction of solid in the wet film, length of the substrate in the radial direction, and off‐center distance. Simply, the off‐center film thickness was inversely proportional to the off‐center distance to the exponent of one‐third, that is, the further the off‐center distance, the thinner the film. The model was verified where the thicknesses of the films calculated by using the model were compared with the experimental values obtained from the off‐center spin‐coated films of poly(vinylidene fluoride) at various off‐center distances. Both the modeled and the experimental data were of the same trend and in a good agreement with each other, indicating the validity of the model. The limitations of the model were also discussed. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48356.

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 280 o C 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 cm 2 /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 ȝW/cm 2 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.

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<p class="Text">The measured total luminous flux of a linearly-shaped lamp by using the integrating sphere substitution method against a standard spherical lamp can be deviated from accurate by the presence of a baffle in the sphere. The baffle introduced two main effects on the sphere response: the baffle reflection, or high-signal region, and the baffle shadow, or low-signal region. Once the baffle condition changed, the two effects changed, causing the measured value to change differently regarding the lamp alignment. In the perpendicular alignment, increasing the baffle length increased the measured flux value. This was due to the dramatic increase in the magnitude of the signal in the baffle reflection behind the baffle. In a coaxial alignment, on the contrary, increasing the baffle length resulted in the decrease of the measured flux value. This was due to the increase in the area of the baffle shadow on the opposite hemisphere from the baffle, which increased at the higher ratio than the area of the baffle reflection. In both alignments, the measurement uncertainty increased with the baffle length due to the increasing magnitude of the signal fluctuations. The trends were similar for all linearly-shaped test lamps with different lengths and diameters.</p>

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