Low-voltage, High-performance Organic Field-Effect Transistors Based on 2D Crystalline Molecular Semiconductors

Two dimensional (2D) molecular crystals have attracted considerable attention because of their promising potential in electrical device applications, such as high-performance field-effect transistors (FETs). However, such devices demand high voltages, thereby considerably increasing power consumption. This study demonstrates the fabrication of organic FETs based on 2D crystalline films as semiconducting channels. The application of high-κ oxide dielectrics allows the transistors run under a low operating voltage (−4 V). The devices exhibited a high electrical performance with a carrier mobility up to 9.8 cm2 V−1 s−1. Further results show that the AlOx layer is beneficial to the charge transport at the conducting channels of FETs. Thus, the device strategy presented in this work is favorable for 2D molecular crystal-based transistors that can operate under low voltages.

of the AlO x dielectric, which illustrates a particularly smooth surface with a root-mean-square (RMS) roughness of 2.23 Å. The hydrophilicity of the AlO x dielectric layer is enhanced through UV-ozone treatment. Improving the hydrophilicity is conducive to the subsequent solution-based process for molecular crystal growth. Besides, the surface roughness of UV-ozone-treated AlO x is 2.75 Å ( Supplementary Fig. S1). Given that the capacitance and gate leakage current are both critical to the gate dielectric in low-voltage OFETs, we employ an Au/AlO x / Si capacitor structure. The measured capacitance and dielectric constant are 0.37 μF/cm 2 and ~9.0, respectively (measured at the voltage frequency of 10 Hz, Fig. 1c). Moreover, the AlO x capacitance decreases from 0.37 μF/cm 2 to 0.25 μF/cm 2 when the frequency of the applied voltage increases from 10 Hz to 1 MHz. This effect is mainly due to the interfacial traps produced during the UV-ozone treatment. The dielectric capacitance of AlO x exhibits negligible change when the applied voltage increases from −6 V to 6 V ( Supplementary Fig. S2), and its leakage current is 10 −7 A/cm 2 when the applied voltage is −4 V (Fig. 1d). Therefore, the thermally deposited AlO x can be used as a gate insulating layer because of its superior performance as a dielectric material. The coffee-ring-driven method ( Supplementary Fig. S3) is utilized for the deposition of 2D films where C 8 -BTBT molecules assemble into 2D crystalline films with a large size of ~200 μm on the AlO x surface (Fig. 2a) 17 . Supplementary Fig. S4 shows a deposited bilayer C 8 -BTBT film with a large size of several millimeters. The obtained molecular films consist of different C 8 -BTBT layers with step-and-terrace structures. Figure 2b and c show the AFM images of the two steps as marked by dotted squares in Fig. 2a, where the thicknesses are 2.99 and 5.21 nm, respectively. The molecules in the third layer are nearly perpendicular to the substrates with regard to the C 8 -BTBT molecular length. The previous results show that C 8 -BTBT molecules in first layer are more tilted to the substrate than that in upper layers, because the weak van der Waals interactions among the small molecules decrease rapidly from the dielectric surface to the upper molecular layers 6,26 . The schematic illustration of C 8 -BTBT molecular packing is shown in Supplementary Fig. S5. Furthermore, the bilayer C 8 -BTBT exhibits uniform thin films with atomic smoothness (RMS roughness: 1.22 Å, Fig. 2d). The crystalline properties of the bilayer C 8 -BTBT are characterized through high-resolution AFM (Fig. 2e). More than 10 points are selected randomly for scanning by high-resolution AFM ( Supplementary Fig. S6). The AFM images show nearly identical lattice constants, namely, a = 6.21 ± 0.16 Å, b = 8.12 ± 0.12 Å, and θ = 88.1 ± 1.4°. These results indicate that the bilayer C 8 -BTBT films each contain a crystalline phase with highly morphologic uniformity over a large area. Similar crystalline characteristics are also  observed in the C 8 -BTBT trilayer ( Supplementary Fig. S7). The crystalline properties of our molecular crystals are summarized in Supplementary Table S1. Therefore, the AlO x layer formed through thermal evaporation facilitates the deposition of high-quality 2D C 8 -BTBT crystalline films.

Discussion
The bilayer crystalline films and AlO x are used as conducting channels and gate dielectrics, respectively, to fabricate the planar transistors. Figure 3a and b show the typical transfer and output characteristics of a bilayer C 8 -BTBT-based FET, respectively. These properties indicate that a low operating voltage (−4 V) is sufficient to operate the device properly with an evident field effect. The drain current in the output curves reaches the saturation region even at a drain voltage of −4 V. A nearly linear increase in the drain current is also observed in the small range of the drain voltage, indicating a nearly ohmic contact with an efficient charge injection from the metal contact to the conducting channel (inset of Fig. 3b). Moreover, the device exhibits a high electrical performance and yields a carrier mobility of up to 6.5 cm 2 V −1 s −1 , near-zero threshold voltage of −0.7 V, small subthreshold swing of 160 mV dec −1 , and large on/off ratio of >10 5 . The carrier mobility value was calculated in the saturation region using the equation: where W and L are the channel width and length, respectively, C i is the gate dielectric capacitance, V T is the threshold voltage. Besides, 2D crystalline films with only several layers can greatly enhance the charge injection process by significantly decreasing the access resistance related to the charge injection from the metal/semiconductor interface to the active channel 27,28 . And the width-normalized contact resistance in our device is estimated to be ~360 Ω cm by using the Y-function method; the value is among the lowest ones for the contact resistance of organic transistors (Supplementary Figs S8 and S9). For comparison, we also fabricated BGTC FETs based on a five-molecular-layer C 8 -BTBT crystal. The threshold voltage and mobility are −2.7 V and 0.98 cm 2 V −1 s −1 , respectively ( Supplementary Fig. S10). Besides, the width-normalized contact resistance is 7600 Ω cm, which is much larger than that in FETs based on bilayer crystal ( Supplementary Fig. S11). We observed a negligible hysteresis from the transfer curve as the applied gate voltage sweeps backwards. We also prepared 20 devices with AlO x dielectrics, obtaining an average mobility of 4.7 ± 1.9 cm 2 V −1 s −1 (Fig. 3c). Furthermore, the highest mobility obtained is 9.8 cm 2 V −1 s −1 (Supplementary Fig. S12). To the best of our knowledge, our device exhibits a record-high value of the carrier mobility for low-voltage OFETs (Supplementary Table S2). Typical OFETs based on C 8 -BTBT reported in literature are summarized in Supplementary Table S3. Apart from low-voltage operation and high performance, suitable bias-stress stability is also significant for practical applications. Figure 4a shows the bias-stress characteristics of a bilayer C 8 -BTBT-based FET. Even when tested in the ambient condition, the drain current of the device shows nearly negligible change under a prolonged operation of 10 4 s. Apart from the high-quality bilayer C 8 -BTBT crystalline films, low-voltage operation generates limited heat during the electrical measurements, hence also contributes to the stability of our transistor device. Figure 4b shows that the shapes of the transfer curves before and after the bias-stress test exhibit a small shift to a more negative gate voltage. There is a negligible change in the threshold voltage during the test (~0.1 V) (Supplementary Fig. S13). The estimated carrier mobility slightly decreases from 6.3 cm 2 V −1 s −1 to 5.7 cm 2 V −1 s −1 . We also evaluated the stability by testing a device maintained in ambient condition for up to 30 days. Figure 4c shows that both the drain current and carrier mobility only decrease slightly.
Consequently, the presented results prove the promising features of thermally evaporated AlO x as a gate dielectric for low-voltage OFETs with bilayer molecular crystals as conducting channels. For comparison, we prepared the FET samples that utilized SiO 2 and HfO 2 (Fig. 5). A large operating voltage of −20 V is necessary to operate the SiO 2 -based device, and the estimated carrier mobility is 4.8 ± 2.1 cm 2 V −1 s −1 . The operating voltage can be properly lowered to −4 V, whereas the carrier mobility can be as low as 0.4 ± 0.3 cm 2 V −1 s −1 when applying HfO 2 as the gate insulator. The decreased mobility in the device with HfO 2 is mainly due to the strong interaction at the interface between the conducting channel and high-κ dielectric 29,30 . This interaction results in the increased localization of the charge carriers 6, 31, 32 , which is consistent with the result that OFETs based SiO 2 exhibit a higher carrier mobility than that based on AlO x . Further studies on the charge carrier properties in our ultrathin molecular crystals is of great interest. Thus, to develop a technique that allows for the fabrication of 2D-crystalline-film-based transistors that employ vacuum as the dielectric layer is necessary 29 .  Note that AlO x also has a high dielectric constant, whereas the obtained carrier mobility is similar to that of the device with SiO 2 , and much higher than those of the HfO 2 -based devices. The maximum density of interfacial traps (N trap ) is estimated from the values of the subthreshold swing (SS) to examine the performance, especially the carrier mobility, exhibited in the device that uses AlO x : where q is the electronic charge, SS is the subthreshold swing, e is Euler's number, k is the Boltzmann's constant, T is the absolute temperature, and C i is the gate dielectric capacitance. The trap density of AlO x is ~3.9 × 10 12 cm −2 , which is in the same range as those of SiO 2 and HfO 2 . The intrinsic charge transport behavior is determined by performing the temperature-dependent measurement on the electrical performance of the FETs with different oxide dielectrics (Fig. 6a). The carrier mobilities calculated from the transfer curves can all be fitted to linear lines in the plots of ln(μ FET ) versus 1/T (Fig. 6b), which indicates that the hopping transport dominates in all devices. Furthermore, the activation energy (E a ) can be estimated through the Arrhenius equation: where μ 0 is the trap-free mobility. Temperature-dependent measurements for the devices with different dielectric layers are summarized in Table 1. The device that utilizes AlO x exhibits the lowest E a value of 30.8 meV and a high μ 0 of 12 cm 2 V −1 s −1 . And E a is considered to be related to the width of the distribution of trap states [33][34][35] . Therefore, the high carrier mobility obtained in the AlO x -based device is attributed to a low energetic disorder, a narrow width for the density of trap states in the dielectric interface, and a close packing among the C 8 -BTBT molecules [36][37][38] . The results reveal that AlO x can provide a beneficial interface for the transport of charge carriers. Besides, despite a good structural quality of our bilayer C 8 -BTBT crystals and high electrical performance obtained from the AlO x -based transistors, the charge transport exhibits as a hopping-like rather than a band-like behavior. Similar results were also reported in literature, which imply that the property of the dielectric layer can affect the charge transport in the conducting channel 29,[38][39][40][41] . Besides, our recent results also reveal that the charge transport behavior can be greatly influenced by the contact resistance [42][43][44] .
In conclusion, we fabricated low-voltage and high-performance OFETs that employ solution-processed bilayer molecular crystals and high-κ material of AlO x as the conducting channels and the gate dielectrics, respectively. The devices can operate under a low applied voltage of −4 V and exhibit excellent electrical performance with the highest carrier mobility of up to 9.8 cm 2 V −1 s −1 . Moreover, further studies indicated that the AlO x application in FET devices is favorable to the interfaces among the 2D molecular crystals, in which the charge carrier transport has small activation energy. The results demonstrated the advantages of the proposed strategy to attain low-voltage and high-performance OFETs.

Methods
Fabrication of the AlO x layer: The Si substrate was sequentially cleaned by sonication in acetone and isopropanol for 10 min each. The oxide dielectric of AlO x with a thickness of ~18 nm was thermally evaporated under a deposition speed of 0.1 Å s −1 with a base pressure of 10 −5 Torr. The AlO x was then treated by UV-ozone for 15 min. Deposition of the 2D C 8 -BTBT Crystals: The p-type organic semiconductor C 8 -BTBT was supplied by Nippon Kayaku Co. and was adopted without further purification. C 8 -BTBT (1.0 wt%) was dissolved in a mixture of anisole and p-anisaldehyde (0.5 wt%) which were the good solvent and the antisolvent, respectively. The UV-ozone-treated AlO x was sequentially cleaned in acetone and isopropanol. Before the droplet was casted onto the AlO x substrate, the solution was shaken for ~30 s to deposit from a homogeneous solution. A mechanical pump was then employed to vent the air through a pipe positioned ~1 mm from the droplet ( Supplementary  Fig. S3).
Characterizations of the C 8 -BTBT Crystals: An Olympus BX51 was used to obtain the optical microscopy images. Two AFM types were performed in this work. The characterizations were performed on a Veeco Multimode 8 under the ambient conditions for the regular AFM. The experiments were then performed on an Asylum Cypher under ambient conditions utilizing Asylum ARROW UHF AFM tips for the high-resolution AFM.
Fabrication and Electrical Measurements of FETs: Few-layered C 8 -BTBT was deposited onto the AlO x substrates for the OFET fabrication, as shown in Fig. 1. Patterned Au films with a thickness of 100 nm and Au pads with dimensions of 30 μm × 100 μm were thermally evaporated under a deposition speed of 0.2 Å s −1 . The two Au pads were subsequently transferred to the top of the C 8 -BTBT crystal to form the source and drain electrodes ( Supplementary Fig. S14). Electrical measurements were performed utilizing an Agilent B1500 semiconductor parameter analyzer in a closed-cycle cryogenic probe station with a base pressure of 10 −5 Torr.