Schottky-Barrier-Dependent Electrical Characteristics in Conjugated Polymer Transistors With Various Contact Metals

Despite the intensive study on the promotion of device performance, the device physics regarding the effects of Schottky barrier on the charge injection in conjugated polymer transistors still need more discussions. Here, the indacenodithiophene–co-benzothiadiazole (IDT-BT) organic field-effect transistors (OFETs) with four different contact metals (Pt, Au, Cu, and Cr) were fabricated to explore the effects. Different Schottky barriers were achieved with the contact electrode’s work function (WF) varying. The ION/IOFF ratio of IDT-BT OFETs increases from 103 to 106 as the WF increases from 4.6 to 5.65 eV. The hole mobility also increases from 0.01 (for Cr) to 2.79 cm2V–1s–1 (for Pt) when the WF approaches the highest occupied molecular orbital (HOMO) level of the conjugated polymer. Moreover, the threshold voltage and subthreshold swing of the devices both decrease with increasing the WF. These could be ascribed to the lowered Schottky barrier with WF, which promotes charge injection. The lowest Schottky barrier (0.123 eV) and contact resistance (1.06 × 104 Ω⋅cm) can be achieved in the devices with Pt contacts, owing to the highest work function.


INTRODUCTION
Solution-processed organic field-effect transistors (OFETs) are promising for next-generation consumer electronics, because of the unique advantages of low cost, flexibility, and large-area fabrications (Liu et al., 2010;Jiang et al., 2019). In order to improve the device performance for applications, much efforts have been paid to the material synthesis, film microstructure, device fabrication, and charge transport optimization of OFETs (Chen et al., 2012;Yuan et al., 2014;Uemura et al., 2016). Among these researches, charge injection, which occurs at the interface between metal and organic semiconductor (OSC), is one of the key factors to promote the device performance. Due to the presence of the Schottky barrier, charge injection can be significantly limited. According to the Schottky-Mott limit, the Schottky barrier height can be roughly estimated from the difference between the contact electrode's work function (W F ) and the transporting energy level of the OSC (Tang et al., 2016).
On the other hand, the contact resistance (R C ) of OFETs is very high, which is due to the absence of doping as well as the heterogeneous materials at the electrode-semiconductor junction (Liu et al., 2015). With the channel length reducing, the R C impact is amplified, further degrading the charge injection (Choi et al., 2018;Wang et al., 2018). Therefore, the relationship between the charge injection and the device characteristics needs to be investigated in detail. The W F of these four contact metals is 5. 65, 5.10, 4.65, and 4.60 eV, respectively. So different heights of Schottky barriers can be formed in IDT-BT OFETs, which will help us to systematically explore the Schottky-barrier-dependent electrical characteristics.
In this work, the Schottky barrier, the RC, and charge injection in indacenodithiophene-co-benzothiadiazole (IDT-BT) OFETs with four different contact metals (Pt, Au, Cu, and Cr) were studied. The effects of the Schottky barrier on the performance parameters such as the mobility, the threshold voltage (V TH ), and subthreshold swing (SS) have also been explored.

DEVICE FABRICATION AND CHARACTERIZATIONS
The device structure of the top-gate staggered IDT-BT transistors and the molecular structure of IDT-BT polymer are shown in Figures 1A,B. The drain and source electrodes of different metals (Pt, Au, Cu, and Cr) with thickness of 30 nm were made on pre-cleaned Corning glass substrate by magnetron sputtering through shadow mask. The channel width (W) was fixed to 1200 µm while the channel length (L) varies from 60 to 350 µm. The conjugated polymer, IDT-BT, dissolved in dichlorobenzene (8 mg/mL) was spin-coated at 2000 r/min in argon-fill glove box and then annealed in the same glove box at 100 • C for 1 h. Subsequently, polystyrene dissolved in butyl acetate (80 mg/mL) was spin-coated at 2000 r/min as the gate dielectric and then annealed at 80 • C for 12 h. Finally, the top gate electrodes of Al (80 nm) were prepared by thermal evaporation through shadow mask. The thicknesses of the IDT-BT semiconductor and PS dielectric layer were estimated to about 50 and 1000 nm, respectively. The electrical properties of the fabricated IDT-BT OFETs were characterized at the temperature of 100-280 K by using probe station (Lakeshore TTPX) equipped with semiconductor parameter analyzer (Keithley 4200-SCS).

Metal
W F (eV) All the electrical parameters are the average values obtained from at least 20 devices for each contact metal. The W F values are obtained from the handbook on metal work function.
voltage V D = -100 V), the I ON /I OFF ratio constantly increases from 10 3 to 10 4 , 10 5 , and 10 6 for the transistors with Cr, Cu, Au, and Pt contacted, respectively, mainly due to the rising on-state drain current (I D ) that augments from 10 −6 A (Cr contact) to 10 −4 A (Pt contact). The result is consistent with the lower Schottky barrier for higher work-function metal adopted in the OFETs, as predicted by the Schottky-Mott limit where the Schottky barrier equals to the energy difference between the metal's work function (W F ) and the highest occupied molecular orbital (HOMO) level of the OSC.

Key Electrical Parameters of Devices
The key electrical parameters are summarized in Table 1. The rising on-state current suggests the promotion of the mobility. The saturation field-effect mobility (µ SAT ) is thus calculated from the saturation regime by using the following equation: where L, W, I D , V G , and C INS are the channel length, channel width, drain current, gate voltage, and unit-area gate capacitance of the gate dielectric, respectively (Uemura et al., 2016). As listed in Table 1, the µ SAT constantly increases from 0.01 to 0.03, 1.20, and 2.79 cm 2 V −1 s −1 for the OFETs with Cr, Cu, Au, and Pt contacts, respectively. The IDT-BT OFETs with Pt electrodes show the highest mobility (2.79 cm 2 V −1 s −1 ). It is two times higher than the mobility of IDT-BT OFETs with Au electrodes (Shin et al., 2017;Wang et al., 2017). This value is even higher than the devices with Cu and Cr electrodes by two orders of magnitude. This result indicates that the contact metals with different W F can indeed significantly affect the device performance by altering charge injection.
The examination of the literature shows that the W F of Pt is 5.65 eV, which is nearest to the HOMO level of IDT-BT (5.4 eV) (Xu et al., 2017). Hence, the Schottky barrier for Pt contact OFETs is 0.25 eV, which is beneficial for the hole injection from the electrode to the channel. For the other three metals, the value is at least 0.3 eV. The energy barriers are really high, so the hole injection from the source electrodes into channel is difficult. Therefore, the charge transport and the electrical performance of the OFETs are deteriorated. The sub-threshold swing (SS) and the threshold voltage (V TH ) reflect the influence of the Schottky barrier (Jiang et al., 2019). This is because the charge injection can also be limited by high Schottky barrier that is not directly or not efficiently modulated by the gate voltage. Thus, it leads to slow and late charge accumulation, i.e., large SS and high V TH . Note that small SS and V TH are desirable for practical applications especially for high-speed operation and low-power consumption (Lai et al., 2018;Wei et al., 2018). From Table 1, we can find that the SS and V TH values monotonically decrease with increasing the contact's W F , indicating the improved charge injection. Furthermore, the electrical parameters can be modulated with different heights of Schottky barrier. High Schottky barrier was formed, when the W F difference between the contact metal and the HOMO level of IDT-BT is large. It will further lead to poor charge injection and non-ohmic contacts. On the other hand, a nearly ohmic contact is expected, when the W F of the contact metal approaches the HOMO level of IDT-BT. It will promote the charge injection and accumulation (Ishii et al., 1999).

Effect of Contact Metals on Contact Resistance
Due to the absence of doping in OFETs, the contacts between metal and OSC are basically Schottky contacts with great barrier height. The contact resistance (R C ) is usually very high, which is a critical issue to OFETs (Liu et al., 2015). The transfer line method (TLM) can be adopted to extract the Rc of the IDT-BT OFETs. In  the linear regime, the total resistance (R TOT ) can be written as the sum of the R C and channel contact (R CH ) according to: where V D is the drain voltage. The R C is independent on the channel length (L), while the R CH proportionally declines with reducing L. Therefore, for a fixed V G , when R TOT is plotted against L, a straight line can be obtained with whose y-axis intercept is R C (Natali and Caironi, 2012). For our devices, the contact resistance normalized by the channel width (R C × W) is shown in Figure 3. Note that the R C is clearly lower for the Au and Pt contacted OFETs, compared to Cu and Cr contacted devices. A lowest value of 1.06 × 10 4 ·cm is obtained for Pt-based devices at the V G of −80 V. The R C increases to 1.03 × 10 5 ·cm for the OFETs with Au electrodes, and then markedly increases to 3.86 × 10 6 and 4.37 × 10 6 ·cm for that with Cu and Cr electrodes, respectively (Huang et al., 2019). Such high R C may dominate the R TOT in particular when L is short because the R CH proportionally declines with reducing L while the R C does not. So, the overall device properties can be governed by the contact properties (Xu et al., 2017). As a result, largely overestimated mobility may be caused. Only the channel is sufficiently long to ensure the insignificant contact impact so as to extract accurate mobility (Liu et al., 2017). The charge injection is difficult, when the R C is large. So the drain current is suppressed and the electrical characteristics of the device are deteriorated. When the R C is low, the charges, only need a small energy, can pass through the barrier at the interface of electrode and semiconductor. Furthermore, charge accumulation will be quickly and easily occurred for the OFETs with lower R C . Therefore, the charge injection is strong, resulting the high performance of IDT-BT OFETs.
Intrinsic Schottky Barrier of Pt-and Au-Based OFETs Figures 4A,B show the output characteristics measured at different temperatures (T). According to the thermionic emission, the saturation current of a reverse-biased Schottky junction can be calculated from Eq. (3): where A is the diode area, A * is the Richardson constant, T is the absolute temperature, q is the elementary charge, B is the height of the Schottky barrier, and k is the Boltzmann constant (Rhoderick and Williams, 1988). Therefore, through the output characteristics at variable temperatures, B can be extracted from the plot of ln(I 0 /T 2 ) versus (q/kT). The Pt-based and Au-based devices (measured at V D = -1 V and V G = -50 V) clearly show such a dependency (Figures 4C,D). The B value can be extracted form the slope of the linear fitting of ln(I 0 /T 2 ) versus (q/kT) curve (Xu et al., 2017). Because the imageforce barrier lowering is significant in OFETs, which makes B highly dependent on the external biases (Bartolomeo et al., 2018), linear fitting of B is made against V G (at small V D = -1 V) to eliminate the associated effect (Brondijk et al., 2012), as seen in Figures 4E,F. The intrinsic Schottky barriers without biasing influence ( B,p0 ) of 0.123 and 0.487 eV are obtained for Pt-based and Au-based devices, respectively. These values are different from the theoretical prediction given by the Schottky-Mott limit, due to the different interface chemistry and physics (Xu et al., 2017). The interface of electrode and semiconductor exhibits an additional dipole barrier that tends to change the W F and hence the interface barrier height (Wan et al., 2005). Furthermore, gap states, which can be easily generated in OFETs, are also major influences on Schottky barrier. The hopping ladder and Fermi-level pinning occur and thus the charge injection through the Schottky barrier is limited (Tung, 2000). Despite that, one can still draw conclusion. The minor B,p0 = 0.123 eV means nearly ohmic contacts formed in Pt-based devices, where the charge injection is efficient. By contrast, the high Schottky B,p0 = 0.487 eV means that the charge injection in Au-based devices is by low-efficiency thermionic emission over high energy barrier. Therefore, the different Schottky barriers lead to different charge injection mechanisms and thus giving rise to very different electrical characteristics.

CONCLUSION
Schottky-barrier-dependent electrical properties of the transistors using conjugated polymer of IDT-BT and four contact metals have been presented. It is found that the contact's work function changes the Schottky barrier height, alternating the mechanism of charge injection. A highest mobility of 2.79 cm 2 V −1 s −1 and lowest R C of 1.06 × 10 4 ·cm are observed from Pt-based devices, attributable to the nearly ohmic contacts with a small Schottky barrier B,p0 = 0.123 eV. This well explains the greatly improved I ON /I OFF ratio, V TH , and SS. Therefore, our results demonstrate that Schottky barrier is crucial for performance optimization to achieve practical organic devices and circuits.

DATA AVAILABILITY STATEMENT
The datasets generated for this study are available via the corresponding author or other researchers following publication.