Photosensitive and Flexible Organic Field‐Effect Transistors Based on Interface Trapping Effect and Their Application in 2D Imaging Array

Flexible organic phototransistors are fabricated using polylactide (PLA), a polar biomaterial, as the dielectric material. The charge trapping effect induced by the polar groups of the PLA layer leads to a photosensitivity close to ≈104. The excellent performance of this new device design is further demonstrated by incorporating the phototransistors into a sensor array to successfully image a star pattern.

The charge trapping effect has been generally considered as a pitfall for organic electronics. However, we demonstrate that this effect can be subtly applied to enhance the photosensitivity of organic phototransistors (OPTs). The photosensitivity of traditional OPTs mainly arises from the organic semiconductor itself. Here we showed that it is possible to induce photosensitivity by engineering the organic semiconductor/dielectric interface of the device. Flexible OPTs were successfully fabricated using polylactide (PLA), a polar biomaterial, as the gate dielectric material. The polar groups of the PLA layer induce charge trapping effect at the organic semiconductor/dielectric interface, which results in reduced drain current when the device is in the dark. Under white light illumination, a significant increase in the drain current was observed, leading to a photosensitivity close to 10 4 . Even when ultralow illumination (≈0.02 mW cm −2 ) was used, the OPT based on this interface interaction still exhibited decent photosensitivity. The excellent performance of this new OPT design was further demonstrated by incorporating the devices into a 2D 10×10 array to successfully image a star pattern. The results indicate that the fl exible PLA-based OPTs with interface trapping effect could have promising applications, such as fl exible and implantable photosensors, environmentally friendly electronics, and artifi cial skin.
Organic fi eld-effect transistors (OFETs) have attracted considerable research attention due to their potential application for low-cost, fl exible, and wearable electronics. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] OPTs are an attractive category of OFETs, which can be used as photodetectors, light-induced switches, light-triggered amplifi ers, and image sensors, amongst other applications. [15][16][17][18][19][20][21][22][23] In most studies on OPTs, it was the organic semiconductor itself, which exhibited photosensitivity. Under illumination, photogenerated excitons in the OPT lead to increased charge density and enhanced drain current. In order to obtain good photosensitivity, high-quality organic semiconductor materials, (tridecafl uoro-1, 1, 2, 2-tetrahydrooctyl) trichlorosilane (FOTS) to create a self-assembled monolayer that served as a release layer. Gold gate electrodes were subsequently evaporated onto the treated silicon substrate. [ 40 ] Next, PLA and dinaphtho [2,3-b:2′,3′-f ]-thieno[3,2-b] thiophene (DNTT) were deposited onto the treated silicon wafer as the dielectric and semiconductor materials, respectively. [ 41 ] The chemical structures of PLA and DNTT are shown in Figure 1 b. A solution of PLA in chloroform was deposited onto the substrate by dip coating to provide a 3 µm thick polar dielectric layer. The roughness of this PLA dielectric layer was found to be less than 1 nm ( Figure S1a,b, Supporting Information). Organic semiconductor DNTT was then deposited onto the PLA layer by thermal evaporation. Source and drain electrodes were then deposited by thermally evaporating gold through a shadow mask. The confi guration of the fl exible PLA-based OPT array is illustrated in Figure 1 c. Using the FOTS as a release layer, the PLA-based OPT array could be easily peeled off from the template silicon substrate. The template substrate could then be reused in the same fabrication process. The total thickness of the free standing device was less than 4 µm and thus it was quite light and highly fl exible, as shown in Figure 1 d.
The photosensitivity of the PLA-based OPT was investigated under dark and white light illuminated conditions (LED, Thorlabs MCWHL5-C4). As shown in Figure 2 a, the OPT exhibited good transistor I d -V d characteristics in the dark, displaying both a linear and saturation regime. At the same applied voltage, under illuminated conditions, the device exhibited a substantial increase in the drain current, I d , as shown in Figure 2 b. Figure 2 c shows the transfer characteristics of the OPT under various light intensities, with the drain voltage V d fi xed at −60 V. The saturation source-drain current I d-sat was significantly enhanced by illumination, even when the light was as weak as 0.5 mW cm −2 . In fact, the OPT also exhibited photosensitivity to ultralow levels of light (0.02 mW cm −2 ), as shown in Figure S2e (Supporting Information), which suggests the excellent photosensitivity of the device. Figure 2 d plots the device photosensitivity, which was defi ned as the normalized ratio between I d-sat under light and I d-sat in the dark (normalized I light / I dark ), as a function of the gate voltage V g . Photosensitivity close to 10 4 was achieved at low gate voltages. The amplifi cation of the drain current under illumination was strongly dependent on the incident light intensity.
The OPT devices can also operate at a fi xed V g mode, with the output current modulated by light intensity. Figure 2 e,f show the I d -V d curves of the PLA-based OPT under different intensities of incident light at a fi xed V g of 0 and −60 V, respectively (also see Figure S2a,b in the Supporting Information for results with V g = −40 V and −20 V, respectively). For each applied gate voltage, output I d -V d characteristics similar to that of typical p-type OFETs were observed. The I d -V d curves show both a linear and a saturation regime, but the light intensity, instead of the gate voltage, modulated the output I d . Figure S2c,d (Supporting Information) shows the log scale plot of Figure 2 e,f. The OPT exhibited good performance even at low light intensity. At just 0.5 mW cm −2 , the photosensitivity ( I light / I dark ) was shown to be over 100 and 10 at a fi xed V g of 0 V and −60 V, respectively. These combined results suggest that the output drain current can be controlled both by the gate voltage and the intensity of the incident light. [ 42,43 ] By changing the applied V g ,  the photosensitivity of the device can be controlled in a wide range, which provides additional modulation methods for practical applications.
It was assumed that the photosensitivity behavior of OPTs could be driven by capacitance change of dielectric, or photoinduced charge transfer, which is effectively affected by morphology and grain boundary within the active layer, or charge trapping process between semiconductor and dielectric interface. [44][45][46] To further understand the photoresponse of our devices, capacitance variation of the PLA dielectric along with light intensity was measured and shown negligible response to light, as shown in Figure S3 (Supporting Information). In addition, the morphology of semiconductor fi lm of the DNTT OFET with PLA dielectric layer and that of a DNTT OFET with octadecyltrichlorosilane (OTS)-treated SiO 2 dielectric layer were characterized by atomic force microscopy (AFM) as shown in Figure S1c,d (Supporting Information). The AFM images of DNTT deposited respectively on PLA dielectric and OTS-treated SiO 2 dielectric show that they have similar textures of small islands and similar size of grains. In contrast, the photosensitivity of the DNTT OFET with different dielectric exhibited obvious difference. Figure 3 a shows the OFET with the OTS-SiO 2 dielectric layer exhibits almost no photosensitivity, while the I d-sat of the PLAbased OPT was enhanced by two orders of magnitudes between dark and illuminated environments at V g = −60 V though they have very similar morphologies. At a lower gate bias, a more signifi cant difference was observed for the two OPTs, in which case the I d-sat of the PLA-based OPT was enhanced by more than 10 3 times ( V g = −20 V, V d = −60 V) under light, as shown in Figure 3 b. Thus we believe that the enhanced photosensitivity of our PLA-based OPT is induced by the interfacial charge trapping effect.
More specifi cally, the enhanced photosensitivity behavior can be explained based on the multiple trap and release model. [ 39,[47][48][49] Polar groups in the PLA fi lm can induce high density charge traps at different energy levels at the organic semiconductor/dielectric interface, where the majority of charge carriers are concentrated. [ 14 ] The deep traps can capture charge carriers and reduce the carrier density, while the shallow traps reduce the carrier transportation rate by temporarily trapping the carriers. This charge trapping effect results in an ultralow drain current when the device is in the dark, which is highly desired for OPTs. After the device exposed upon illumination, photoinduced excitons would be generated and supplement the conducting channel with charge carriers, acting as detrapping of the charge carriers on the interface. The higher  the light intensity, the more the photoexcitons, which leads to a signifi cant increase of I d-sat for the OPTs. In contrast, such polar groups are not present on the interface of the device fabricated with an OTS-modifi ed SiO 2 dielectric layer, and thus the device shows almost no photosensitivity (Figure 3 a,b). The effective charge carrier mobility and threshold voltage of the PLA-based OPT was estimated from the device's transfer characteristics (see Figure S2f, Supporting Information), and plotted as a function of light intensity as shown in Figure 3 c. Due to the interface charge trapping effect, the OPT showed low charge mobility and a high initial threshold voltage when the device was in the dark. Under weak light illumination (<10 mW cm −2 ), the mobility increases signifi cantly and the threshold voltage shifts dramatically because of the photogenerated charge carriers. When the intensity of the incident light was further increased, the conducting channel tends to be saturated with charge carriers, thus there was less change of mobility and threshold voltage.
The above behavior was further studied by evaluating the device photosensitivity upon with different wavelength of incident light. Spectrum of UV-Vis ( Figure S4a, Supporting Information) shows that DNTT possesses an absorption peak around 450 nm, which means in our device, photoexcitons can only be generated when the wavelength of the incident light is less than 450 nm. Correspondingly, a "cut-off" change of the OPT I on / I off ratio along with light wavelength was found also around 450 nm. In the range lower than that, the device presents high I on / I off ratio, while in the range higher than 450 nm, the OPT shows very limited photosensitivity. This fi nding strongly supports our explanation as mentioned above.
In order to further substantiate the potential of the PLAbased OPT in optoelectronic applications, we investigated our device's photoresponse upon on-and-off switching of light illumination at various V g and V d confi gurations. Figure 3 d shows the reproducible and reversible drain current response to the switching of light. The response time of the device is about 50 ms ( Figure S5, Supporting Information). In addition, the OPT devices also exhibited obvious photosensitivity when they were operated under low applied voltage ( Figure S6a,b, Supporting Information). These results indicate that the PLA-based OFET has reliably high photosensitivity and is therefore of interest from a practical application point of view.
Due to the high light sensitivity, the PLA-based OPT is very suitable for optoelectronic applications such as photosensitive imaging. We demonstrated this application by incorporating the PLA-based OPTs into a 10 × 10 array to sense an object. As shown in Figure 4 a, a star pattern was imaged using the PLAbased OPT array. The I d-sat of each OPT in the array was measured and normalized to the background I d-sat that was measured without the presence of the star pattern (light intensity = 4 mW cm −2 ). The OPTs in the array that were blocked by the star pattern displayed lower values of normalized I d-sat , while  the rest OFETs displayed the same values as the background I d-sat . The output results were exhibited in a matrix form that displayed similar pattern as the star object (Figure 4 b), which indicates the PLA-based OPT array is reliable for photosensitive imaging. The blurred edges of the image are due to the partially blocked OPTs.
In conclusion, by taking advantage of the interface charge trapping effect, fl exible and biocompatible OPTs have been fabricated based on OFETs with a polar PLA layer acting as the dielectric material. The resulting OPTs showed good photosensitivity upon illumination as weak as 0.02 mW cm −2 and high photosensitivity (close to 10 4 ) using high light intensity. In contrast with traditional OPTs, whose photosensitivity mainly arises from the organic semiconductor itself, the photosensitivity of our PLA-based OPTs was mainly realized by engineering the organic semiconductor/dielectric interface of the device. The interface charge trapping effect has been generally considered a negative attribute in organic electronics, but here we subtly use the same effect to enhance the photosensitivity of OFETs. In order to demonstrate the reliable photosensitivity of our PLA-based OPTs, the devices were incorporated into a 10 × 10 photosensing array and used to successfully image a star-shaped object. Our results demonstrate a new strategy to develop high performance OPTs, which are applicable to many other organic electronics for sensing applications.

Experimental Section
Materials : Polylactide was purchased from Natureworks Company and then recrystallized by ethyl acetate. FOTS were purchased from Sigma-Aldrich. Organic semiconductor DNTT was synthesized according to literature reported previously, followed by vacuum sublimation purifi cation. [ 41 ] OFET Devices Fabrication : OFET devices were made using Si wafers as template substrate. FOTS chloroform solution (1, v/v%) was used to modifi ed the substrates by spin coating at 3000 rpm for 20 s, followed by sonication cleaning in chloroform for 30 min. With the FOTS release layer, the transistor could be peeled off from silicon substrates easily after the fabrication process was completed. 80 nm gold gate electrodes were thermally evaporated onto the substrate after the deposition of the release layer. PLA dielectric fi lms were made by dip coating 50 g L −1 PLA chloroform solution at a speed of 20 µm s −1 (Dip Coater, Shanghai SANYAN SYDC-100H). The resulted fi lms were dried at 60 °C in air overnight. The average thickness of PLA dielectric is 3 µm. After that, 60 nm DNTT fi lm was deposited at the rate of 0.3 Å s −1 by vacuum thermal evaporation ( T substrate = 60 °C, P ≈ 5 × 10 −4 Pa). 80 nm gold source-drain electrodes were thermally evaporated through a shadow mask to form top-contact OFET devices. The channel length ( L ) and width ( W ) are 0.1 mm and 3.8 mm, respectively. OFETs with silica dielectric layer were fabricated for experiment control. Si wafers with 300 nm of thermally grown SiO 2 layers were immersed in OTS toluene solution (1.2, v/v%) at room temperature for 3 h, followed by washed with toluene and ethanol. Subsequently, the substrate were annealed at 120 °C for 20 min and cleaned by sonication in toluene for 30 min. The OTS-treated substrates were washed with ethanol and water and dried in fl owing pure nitrogen. The deposition process of semiconductor and gold electrodes was the same as the fabrication of PLA-based OFETs.
Device Characterization : The surface morphologies of PLA fi lm and DNTT thin fi lm are investigated by atomic force microscope (AFM, SEIKO SPA-300HV) operated in tapping mode. PLA fi lm and DNTT fi lm (on quartz substrate) UV-vis spectra were recorded on a Cary-60 UVvis spectrophotometer (Agilent Technologies). For the characterizations of the devices under various illumination intensities, a LED (white light, Thorlabs MCWHL5-C4) was employed as a light source and the illumination intensities were measured by an optical power meter (Thorlabs PM100D). Nine wavelengths of monochromatic light were obtained by the UV lamp (Uvata, Shanghai) and the LED irradiating through visible band-pass fi lter kit (Thorlabs FKB-VIS-10), which were used to characterize the change of I d-sat along with wavelength of light. The light was illuminated from the top side of the devices. The measurement of PLA fi lm capacitance was taken by a LRC bridge (TH2827C, Changzhou Tonghui Electronics Co., Ltd.). All electrical characterizations of OFETs were carried out under vacuum with a Keithley 4200-SCS.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.