Complementary Inverter Based on n‐Type and p‐Type OFETs with the Same Ambipolar Organic Semiconductor and ITO S/D Electrodes

Bottom‐gate and bottom‐contact n‐type and p‐type organic field‐effect transistors (OFETs) are simultaneously obtained by combining the ambipolar semiconductor film of diketopyrrolopyrrole‐based conjugated polymer (P4FTVT‐C32) with indium tin oxide (ITO) source/drain (S/D) electrodes. P4FTVT‐C32 thin film exhibits n‐type unipolar property with the low work functional (WF) ITO S/D electrodes modified by polyethylenimine ethoxylated (PEIE) and it exhibits p‐type unipolar property with the high WF ITO S/D electrodes modified by HCl:InCl3. Hence, complementary inverters with transition voltages near VDD/2 and the maximum gain of 138 converting “1” state input into “0” state output are achieved by two different modifications via screen printing on ITO electrodes and then, only one‐time bar coating of P4FTVT‐C32. To further improve the performance and the uniformity of the OFET devices, the modification of octadecyltrichlorosilane (OTS) is also introduced. This work provides an easy‐handling method for the fabrication of low‐cost, high performance organic electronic devices and integrated circuits.

of ITO. [22][23][24] In addition, there are many modification strategies used to reduce the ITO WF. Coated with thin metal-oxide films, such as In 2 O 3 , [25] ZnO, [26] In-doped ZnO, [27] or Al-doped ZnO, [28] ITO WF can be reduced, but not largely. The chemisorption of tetrakis(dimethylamino)ethylene (TDAE) [29] and the chemisorbed self-assembled monolayers (SAMs) of dipolar molecules [30] can reduce the ITO WF greatly, but the modification method is complex. Polymers containing simple aliphatic amine groups with large band-gap physisorbed onto the surface is a simpler modification method for reducing the WF of metal and metal oxide conductors. [31][32][33][34] After the WF is adjusted to the range of 3.50-5.60 eV, ITO can well match most organic semiconductors (OSCs). [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] Second, we hope to find one kind of organic semiconductor, instead of necessary two kinds, to construct n-type and p-type OFETs simultaneously. Organic ambipolar materials are the preferred for their characteristics of exhibiting n-type performance in OFETs constructed with low WF S/D electrodes, [34] while exhibiting p-type performance in OFETs constructed with high WF S/D electrodes. Therefore, in principle, both n-type and p-type OFETs can be obtained simultaneously by combining the WF adjustment of ITO S/D electrodes and organic ambipolar materials. Moreover, various solution based methods have been developed to prepare organic thin films, [34][35][36][37][38][39][40][41][42] such as solution shearing, inkjet printing, dip-coating, spray printing, bar-coating, screen printing, and direct-write printing. Among these methods, bar-coating is considered a simple, efficient, and low-cost method to fabricate electronic devices in industry. The coating process for one layer consumes only few seconds and the speed of the coating process can be controlled, which is efficient for the mass production of organic electronics. The thickness of the coating film often relies on the bar structure and solution properties. Moreover, the bar coating can form uniform films on a large area of rigid or flexible substrate. Therefore, it is expected to realize the preparation of the organic inverter through all-solution process.
In this work, for the above purpose, n-type and p-type bottom-gate and bottom-contact OFETs were simultaneously obtained by one-step bar coating of a diketopyrrolopyrrolebased ambipolar semiconductor (P4FTVT-C32) on the substrate with polyethylenimine ethoxylated (PEIE) and HCl:InCl 3 modified ITO electrodes by screen printing. There is no previous literature report that HCl: InCl 3 and PEIE can increase or decrease the work function of ITO, and then combined with appropriate bipolar material, p-type and n-type OFET can be obtained simultaneously. The n-type OFET has unipolar electron transport properties with on-current (I on )/off-current (I off ) ratio of 10 4 and mobility of 0.24 cm 2 V −1 s −1 and, the p-type OFET has unipolar hole transport properties with I on /I off ratio of 10 6 and mobility of 0.03 cm 2 V −1 s −1 . On the basic of these n-type and p-type OFETs, the complementary inverter performed a switching voltage near V DD /2 and a gain of 138. In addition, to further improve the OFETs performance, octadecyltrichlorosilane (OTS) modification on insulating layer SiO 2 was also carried out. P4FTVT-C32 thin-film exhibits highly efficient unipolar transport with electron mobility up to 1.00 cm 2 V −1 s −1 in n-type OFET with OTS/PEIE modified ITO as S/D electrodes and it exhibits ambipolar transport with hole and electron mobility to 1.36 and 0.39 cm 2 V −1 s −1 in OFET with OTS/HCl:InCl 3 modified ITO by as S/D electrodes. Generally, this work reported work provides a new strategy for constructing organic integrated circuits and fabricating low-cost and high-performance organic electronic devices.

Results and Discussion
As shown in Figure 1a, the bottom gate and bottom contact OFET device structure was constructed to avoid the influence of thermal radiation and solvent corrosion on the organic semiconductor layer during the electrode preparation process. The lossless organic semiconductor layer is conducive to improve the device performance, which contributes to large-scale industrial production. Here Mo is the gate electrode, SiO 2 is the gate dielectric. The molecular structure of donor-acceptor conjugated polymer P4FTVT-C32, as organic semiconductor layer, is shown in Figure 1a. The backbone of P4FTVT-C32 is highly coplanar, which enables molecular packing closely. [34,43] The P4FTVT-C32 thin film was chosen as organic semiconductor layer material of the OFETs in view of its excellent electron and hole transport properties. Although P4FTVT-C32 exhibits ambipolar transport, its HOMO (−5.4 eV) and LUMO (−3.5 eV) levels are still incompatible with the WF of ITO, which will lead to undesirable transistor behavior inevitably. [34] Therefore, the modification of ITO is very essential and significative.
In order to compare, on bare ITO electrodes, the bottomgate and bottom-contact OFETs were constructed by bar coating of P4FTVT-C32 thin films. The P4FTVT-C32 dissolved in 1,2-dichlorobenzene (3 mg mL −1 ) was used as the ink to obtain the optimized film thickness. Other OFETs fabrication and measurement procedures were in good concordance with the bar-coated OFETs. The representative transfer and output characteristics of the prepared OFETs are shown in Figure S1, Supporting Information. P4FTVT-C32 OFET demonstrates distinct ambipolar characteristics with hole mobility in negative V G region and electron mobility in positive V G region of 0.037 and 0.005 cm 2 V −1 s −1 , respectively. The other performance parameters are summarized in Table 1. The ambipolar performance of P4FTVT-C32 was far below that of previous reports, arising from the mismatching between the WF of the ITO S/D electrodes and the energy level of P4FTVT-C32. We also examined the morphology and thickness of the P4FTVT-C32 film in the channel (shown in Figure S2, Supporting Information).
As shown in the I and II parts of Figure 1b,c, we adopted screen printing process to modify ITO surface. First, the HCl:InCl 3 aqueous solution was deposited on specific position of the ITO electrodes. The cutting-off chemical bonds on the solid surface of ITO led to the unsaturated coordination of surface atoms (ions), adsorbing foreign atoms on the solid surface easily. Polarized InCl bonds were generated by adsorbing Cl − that derived from the aqueous solution of HCl:InCl 3 on the ITO surface, the solvent was fast evaporated at 100 °C and the modification of ITO was eventually realized. Likewise, ITO was modified with PEIE by screen printing. Then, as shown in the III part of Figure 1b,c, bar coating technology was chosen to prepare the P4FTVT-C32 semiconductor film in viewing of that the single orientation of the molecule microstructure resulted www.advelectronicmat.de from the shear force during scraping process was beneficial to the charge transfer.
To investigate the modification effect of HCl:InCl 3 and PEIE on ITO, the ultraviolet photoemission spectroscopy (UPS) was used to measure the WF of modified ITO. The WF of modified ITO decreases from 4.70 to 3.83 eV after PEIE modification, while it increases from 4.70 to 5.06 eV after HCl:InCl 3 modification ( Figure S3, Supporting Information). The results of UPS certified the regulating effect of HCl:InCl 3 , PEIE, and combined screen printing method on the WF of ITO. We also examined the contact angles of the bare and the modified SiO 2 , ITO surfaces to illustrate the interfacial properties of ITO/OSCs and SiO 2 /OSCs ( Figure S4, Supporting Information).
The transfer and output characteristics of OFETs prepared with ITO modifications are exhibited in Figure 2. The device performance parameters extracted from transfer characteristics in the linear region are summarized in Table 1. HCl:InCl 3 modified OFET exhibits unipolar p-type transport behavior with a hole mobility of 0.03 cm 2 V −1 s −1 , high I on /I off ratio of ≈10 6 , small threshold voltage of ≈0 V, and negligible   Comparing the properties of OFETs with and without ITO modification, we find that after HCl:InCl 3 and PEIE modification, the threshold voltages of both p-type and n-type OFETs decrease remarkably, which can be attributed to the reduction of hole and electron injection barriers, respectively. The surface roughness, RMS 4.5 nm, of the 66 nm ITO electrode is very small, so the contact resistance is mainly due to the thickness of ITO and the mismatch between the energy levels of the electrode and the organic semiconductor material. We will optimize the two factors in the further research.
As shown in Figure 3a, a CMOS inverter was constructed by connecting one n-type OFET (PEIE-modified ITO, P4FTVT-C32 as OSC) and one p-type OFET (HCl:InCl 3 -modified ITO, P4FTVT-C32 as OSC). At low operation voltage, the transfer characteristic curves of the OFETs used to construct the inverter are shown in Figure S5, Supporting Information. OFETs fabricated with HCl:InCl 3 and PEIE convey unipolar p-type (V G = 10 to −10 V) and n-type (V G = −10 to 10 V) transport behaviors, respectively. Moreover, their I on and the I off were almost identical in the order of magnitude. The voltage switching characteristic of CMOS inverter is shown in Figure 3b. The output HIGH and LOW voltages were less than V DD and V SS , which was mainly due to the low mobility of the OFETs. The maximum voltage gain (absolute value of dV Out /dV In ) calculated from the voltage transfer curve was 138 at V DD = 80 V (Figure 3c).
An input pulse signal with peak-to-peak amplitude (V pp , 10 V) was connected to the gate input. Just as expected, an amplified inverted signal was observed at the center output terminal with an output/input voltage gain of ≈10 (Figure 3d). Here, our work successfully proves that the organic CMOS inverter with only one kind of semiconductor and one kind of S/D electrodes material is feasible. This kind of CMOS inverter demonstrates its potential application in low-voltage electronics. However, the inverter is still limited by the low mobility of the OFETs, the operating frequency is only 1 Hz. Increasing the mobility of OFETs at low voltage and improving the operating frequency still need to continue probing into.
The electrical properties of modified ITO devices are greatly improved compared to that of bare ITO. However, we found the nonlinear phenomena in small drain-source voltage regions of the output curves (Figure 2c,d), which can be attributed to the injection barrier at the electrodes/OSCs contacts. In addition, the morphologies (OM images and AFM images) of P4FTVT-C32 films on HCl:InCl 3 or PEIE-modified substrate ( Figure S6, Supporting Information) were also examined. The P4FTVT-C32 thin films in the channel of the ITO S/D electrodes displayed uniform but disordered fiber-like structure, which might be related to the modified substrate or bar coating speed. There is still space for the improvement of device characteristics.
OTS is always used in SiO 2 surface modification to change the hydrophobic state and improve the quality of semiconductor films. [44,45] Therefore, the substrate was modified by OTS in this study. The substrate (with ITO S/D electrodes and SiO 2 dielectric) was pre-modified by OTS with chemical vapor method before ITO modification and semiconductor deposition. Figure  S7  www.advelectronicmat.de shows the morphologies of P4FTVT-C32 film between the OTSmodified ITO S/D electrodes.
Fortunately, the OTS modification achieves both enhanced hydrophobicity of SiO 2 surface ( Figure S9, Supporting Information) and WF tunable ITO ( Figure S3, Supporting Information). As expected, the WF of ITO decreased from 4.70 to 3.82 eV after OTS/PEIE modification while increased from 4.70 to 5.12 eV after OTS/HCl:InCl 3 modification. Meanwhile, a direct writing method was proved to be the most optimum selection for the ITO modification after OTS process in this work, and the schematic diagram of the method is shown in Figure S10, Supporting Information.

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OTS/PEIE modified OFET possesses unipolar n-type transport behavior with an electron mobility of 1.00 cm 2 V −1 s −1 . The threshold voltage decreases significantly with the electron injection barrier decreases after PEIE modification (Figure 4b,d). Therefore, the addition of OTS modification improved the properties of OFETs greatly. We examined the morphology and thickness of the P4FTVT-C32 film in the channel (shown in Figure S11, Supporting Information). The P4FTVT-C32 thin film in the channel of the OTS/HCl:InCl 3 -modified ITO and PEIE-modified ITO S/D electrodes displayed uniform but no obvious fiber-like structure, which might be related to the modified substrate. But the surface of OTS-, HCl:InCl 3 -, or PEIE-modified substrate is coated with P4FTVT-C32, whose morphology is more orderly than that of bare substrate. Therefore, in terms of the morphology of organic semiconductors alone, the device having modified substrate performance will be better. Because the morphology we tested and needed here is on the insulating layer SiO 2 , we have not explored too much the impact of OTS modified ITO on the charge transport of the device. Moreover, it should be noted that the noise has not been completely eliminated. More works will be done to improve the contact problem and then further eliminate the noise by seeking suitable modification materials to match the organic semiconductor material.
The CMOS inverter consisted of one OFET with OTS-/ HCl:InCl 3 -modified ITO and one OFET with OTS-/PEIEmodified ITO was constructed. As shown in Figure S12a, Supporting Information, the CMOS inverter exhibits voltage switching characteristic. The voltage gain was calculated from the voltage transfer curve and yields a maximum gain of 4 at V DD = 17 V ( Figure S12b, Supporting Information). The main reason for the poor performance of the inverter is that ITO/ OTS/HCl:InCl 3 /P4FTVT-C32 constructed device does not fully display p-type performance, which leads to the simultaneous conduction of two devices when the constructed inverter is at low level, thus greatly reducing the inverter gain. We can seek new modification methods or replace bipolar organic semiconductor materials to obtain complete p-type OFET.

Conclusion
A simple solution-based technology was developed to fabricate p-type and n-type OFETs simultaneously based on ITO S/D electrodes and the same ambipolar conjugated polymer. Such p-type and n-type OFETs used for CMOS inverters delivered good performance. First, the WF adjustment of ITO and the precise modification of the ITO electrodes were realized by screen printing or direct writing. Subsequently, barcoating method was used for producing large-scale organic ambipolar semiconductor layers with uniform thickness. The reported method played a crucial role in fabricating p-type OFETs, n-type OFETs, and CMOS inverters on modified ITO electrodes and expanding the application of ITO in terms of organic electronics simultaneously. This solution modification method has tremendous application potential in organic solar cells, organic light-emitting diodes, photodetectors, etc. Moreover, the technology developed in this work is provided with processability, scalability, and material compatibility and shows wide application perspective for next-generation OSCintegrated circuits.
OFETs Fabrication and the Modification of SiO 2 and ITO: The devices were fabricated on ITO/glass substrate. The substrate preparation comprised the following steps. First, 250 nm Mo was sputtered on a transparent glass substrate by magnetron sputtering as gate electrode. Next, 300 nm SiO 2 was deposited on the Mo surface by chemical deposition as an insulating layer. Finally, 66 nm ITO was sputtered on the SiO 2 insulating layer as S/D electrodes. The channel width and length are 5 mm and 50 µm, respectively (W/L = 100). The substrate was cleaned in detergent, deionized water, acetone, and isopropyl alcohol in ultrasonic bath for 10 min separately, and then the ITO/glass substrate was dried by purity nitrogen flow. Eventually, the organic semiconductor layer fabricated on the prepared substrate was used to construct OFETs.
The dried substrate was treated with UV-ozone for 10 min, and then transferred into a vacuum drying oven with small amounts of OTS in it. The OTS modified layer on substrate surface was formed under vacuum at 120 °C for 2 h. The OTS monolayer was formed through cleaning the substrate with chloroform and cyclohexane for 3 min, respectively.
The pre-patterned substrate was coated with P4FTVT-C32 film layer directly, or modified with HCl:InCl 3 or PEIE prior to the deposition of P4FTVT-C32 film. The solution of P4FTVTC32 was prepared by dissolving the polymer in 1,2-dichlorobenzene at a concentration of 3.0 mg mL −1 , and deposited on selected substrate by bar coating method. The bar coating process was performed in air. P4FTVT-C32 solution was applied on to substrate in front of the bar. Then a wet film was formed by moving the bar at coating speed of 20 mm s −1 . The substrate temperature for the coating process is 100 °C. The obtained polymer thin film was then thermally annealed at 150 °C for 10 min under an inert atmosphere.
The HCl solution was diluted with deionized water to a weight concentration of 0.24% and the InCl 3 was dissolved in deionized water to a weight concentration of 0.2%. Next, the HCl:InCl 3 modification solution was prepared by mixing the aqueous solutions of HCl and InCl 3 according to the same weight. Then modified the ITO with HCl:InCl 3 by screen printing or direct writing method. The substrate needed to be heated at 100 °C for 2 min in ambient. Then the HCl:InCl 3 solution was printed on the hot substrate by a 420 mesh screen and the obtained ultrathin layer was annealed at 150 °C for 10 min under an inert atmosphere. In the process of direct writing using a 10 µL pipette, the volume of the droplet of HCl:InCl 3 solution was 2 µL, and the substrate was heated at 100 °C. The bottom surface of liquid film moved at ≈10 mm s −1 to obtain ultrathin HCl:InCl 3 layer.
The PEIE solution was diluted with 2-methoxyethanol to a weight concentration of 0.08%. The solution was then stirred at room temperature for 30 min for thorough mixing. In the process of screen printing, the substrate needed to be heated at 100 °C for 2 min in ambient. Then the PEIE solution was printed on the hot substrate by a 420 mesh screen and the obtained ultrathin layer was annealed at 100 °C for 10 min under an inert atmosphere. In the process of direct writing using a 10 µL pipette, the volume of the droplet of PEIE solution was 2 µL, and the substrate was heated at 100 °C. The bottom surface of liquid film moved at ≈10 mm s −1 to obtain ultrathin PEIE layer.
Characterizations: The work function measurements of ITO modified by HCl:InCl 3 were obtained by photoelectron spectroscopy with RIKEN KEIKI AC-2 spectrometer in the air. The work function measurements of ITO modified by PEIE were obtained by KRATOS Axis Ultra DLD spectrometer with a base pressure >2 × 10 −9 Torr and He 1 (h = 21.22 eV) as the excitation source. AFM images were captured by tapping mode www.advelectronicmat.de using an Oxford Cypher VRS in air. The morphology images were captured using an optical microscope (OLYMPUS BX51M). Contact angle measurements were performed with an OCA15 pro instrument which is equipped with a photo camera CCD. The transfer and output I-V curves of the field-effect transistors were measured using a standard JANIS probe station coupled to the Agilent B1500A semiconductor device analyzer at a nitrogen atmosphere. The field-effect mobility of OFET at linear region was calculated from the transfer I-V based on the following equation, I SD = WµC i V SD (V G − V th )/L was used, where I SD and V SD represent the source-drain current and voltage, W, and L represent the width and length of the channel, µ stands for the mobility, V G and V th refer to the gate and threshold voltages, and C i is the dielectric capacitance per unit area. The I-V curves were obtained after the device been continuously tested for 50 times. When testing the characteristics of the inverter, the input square wave signal was loaded by a RIGOL DG6142 Waveform Generator and the output signal was collected by a Tektronix DPO 4104 Digital Phosphor Oscilloscope.

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