Highly conductive and low-work-function polymer electrodes for solution-processed n-type oxide thin-film transistors

We present an n-doped poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) polymer and its application in n-type oxide thin-film transistors (OxTFTs) as a source and drain electrode material. A reduced molecule of a cationic dye, methyl red (MR), was used as an effective solution-processed n-type dopant. The sequential de-doping and doping of the initially p-doped PEDOT:PSS polymer with the reduced MR (r-MR) effectively removed positive charges via cancellation by the added electrons. As a result, the electron conductivity of PEDOT:PSS increased from 3.4 S/cm to ∼51 S/cm, while its work function decreased from 4.8 eV to 3.5 eV. This is one of the lowest values of the work function reported for PEDOT:PSS. The n-doped PEDOT:PSS films were eventually applied as a suitable material to fabricate the contact electrodes of solution-processed bottom-gate top-contact amorphous indium-gallium-zinc oxide-based OxTFTs. The resultant devices exhibited electron mobility over ten times better compared to those with undoped PEDOT:PSS electrodes. Therefore, we suggest this method as a highly suitable and low-cost technique for improving electron transport in PEDOT:PSS and all solution-processed conductors. Further investigations with this method are expected to expand the application of PEDOT:PSS to other sectors of optoelectronics.


Introduction
Inexpensive and solution-processed materials have recently become a focus of interest for modern and future electronic concepts. Research institutes and industrial companies around the world are actively investigating, developing and commercializing different conductive polymers and technologies to improve their electrical properties. This interest in organic conjugated polymers mainly arises out of the need to develop mechanically robust and flexible optoelectronic devices using low-cost materials and fabrication techniques. Poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is the most commonly studied among these materials owing to its high electrical conductivity and optical semi-transparency, as well as good stability under ambient conditions [1,2]. This material is the mixture of two different ionomers, the macromolecules of PSS and PEDOT, electrostatically bound to each other via Coulomb forces [1,3]. Such mixing and repolymerization render the PSS unit negatively charged due to the deprotonation. As a result of the transfer of the π electrons from PEDOT to PSS − , the PEDOT unit becomes positively charged. This process leads to p-doping of the PEDOT core and makes the polymer the most electrically conductive among the studied organic materials [1,3,4]. However, the work function (WF) of PEDOT:PSS is too high for electron injection, therefore, it can be used as a solution-processable anode [1].
Very few cases have been reported on reducing the WF of PEDOT:PSS. Treatment of the polymer with specific solvents has been shown to lower the WF of the polymer [1,5]. Another way to reduce the WF of a material is n-doping, i. e., boosting its electron density [6]. Increasing the charge carrier density through a simple charge transfer process is among the most commonly used methods of optimizing the conductivity of organic conductors [7]. However, despite the extensive research, n-type dopants are less frequently reported in the literature compared to p-type dopants [7]. In this field, the use of reduced electron donors, generated by reducing cationic species to their neutral or anionic states for n-type doping purposes, is not a new practice. The pioneers in the field were F. Li and A. G. Werner, who reduced several cationic dyes, including pyronin B (PyB) and crystal violet, for n-doping of small molecule organic semiconductors [8,9]. The principal mechanism of their doping technique was based on a three-step, solid-state, thermally activated or photo-induced reduction of the used dyes to their leuco (colorless) forms. Further studies extended this technique to polymeric semiconductor structures, for which the precursors for n-type doping were reduced through a simple one-step solution process. The representatives among such studies include the research on the solution-processed reduction of benzyl viologen and PyB for n-doping of semiconducting polymers [7,10]. Therefore, cationic species, aside from being good p-type dopants owing to their positive charge, can also be used as n-type dopant sources, consequently making them almost universal dopants [11][12][13][14].
In this work, we applied a reduced cationic dye, methyl red (MR) as a solution-processed n-type dopant to improve conductivity and modify WF of PEDOT:PSS films. The n-doped PEDOT:PSS films were used as source and drain (S/D) electrode layers in the bottom-gate topcontact (BG/TC) oxide thin-film transistors (OxTFTs) based on amorphous indium (In) gallium (Ga) zinc (Zn) oxide (a-IGZO). The doped polymer films were investigated using various measurement methods such as ultraviolet-visible-near-infrared (UV-Vis-NIR) absorption spectroscopy and UV photoelectron spectroscopy (UPS). The WF of PEDOT:PSS decreased from 4.8 to 3.5 eV and its electron conductivity increased up to ∼ 51 S/cm after the reduced MR (r-MR) doping.

Materials
Commercial MR (ACS reagent, crystalline, C 15 H 15 N 3 O 2 , MW = 269.30 g/mol) and sodium borohydride (powder, ≥ 98.0%, NaBH 4 , MW = 37.83 g/mol, hereinafter referred to as 'SBH') were purchased from Sigma Aldrich and used as received, without further purification or treatment. 1.0-1.3 wt% aqueous solution of PEDOT:PSS (PH1000) polymer was purchased from Heraeus Clevios GmbH and was used as it was, too. Figure 1(a-c) shows the molecular structures of these chemicals.
a-IGZO was synthesized through a commonly reported sol-gel method [15]. For this purpose, three precursor materials namely, indium nitrate hydrate, gallium nitrate hydrate, and zinc nitrate hydrate powders, were mixed in 2-methoxyethanol with the molarity of 0.1 M. The molar ratio of the precursor solution was maintained at 4:3:2 for the In:Ga:Zn content. The solution was stirred using a magnetic stirrer at 69°C overnight before use.

MR reduction
20 mg of MR and 7 g of SBH were mixed in 20 ml deionized water (DIW). The reaction started immediately. Chloroform (CF) measuring 10 ml was added into the solution to extract the reduced MR. Since DIW is lighter than CF and is immiscible with it, it floats on the surface of CF, forming an interface. During the reduction reaction, the leuco (colorless) r-MR was transferred from water to CF, and the solution gradually lost its intense red color to become completely colorless. The chemical reaction was carried out under ambient conditions for more than a week. No catalyst was used to facilitate or speed up the reaction. The r-MR was extracted using a syringe when the reduction reaction was finally finished. The reduction is schematized in Figure 2(a). Figure 2(b-d) shows the comparison of the colors of MR solutions before and after the reduction.

PEDOT:PSS doping
The extracted r-MR solution was used as an n-type dopant in a solution-processed host-dopant blend system for S/D electrode applications. For this purpose, r-MR solution was added directly into the aqueous PEDOT:PSS (PH1000) solution at different volume ratios (v/r). The r-MR-doped PEDOT:PSS (PH1000) solutions were stirred using a magnetic stirrer at 80 o C for more than 24 h until fully dissolved. The PEDOT:PSS (PH1000) solutions were intentionally not treated further with any other optimization techniques in order to avoid observation of any other phenomenon beyond the sole effects of the n-dopant.

OxTFT device fabrication
P-type doped silicon (Si) wafers with 200 nm thermally grown Si dioxide (SiO 2 ) were used as substrates for the OxTFTs, where Si and SiO 2 served as bottom gate and gate insulator, respectively. The Si/SiO 2 substrates were cleaned in de-ionized water, acetone, and 2-propanol for 10 min each using ultrasonic bath and then dried at 150 o C for 15 min to remove the residual cleaning solvent. The prepared a-IGZO solution was then spin-coated on the substrates at 3000 rpm for 30 sec and pre-annealed  at 110 o C for 10 min on a hot plate to remove the solvent. The n-type oxide semiconductor, a-IGZO, was chosen as an ideal semiconductor to test the water-based n-doped polymer electrodes owing to its robust nature against water, good semiconducting properties, solutionprocessability and widely used in emerging display industries. In order to avoid high leakage currents, the unnecessary parts of the a-IGZO films were removed with acetone using cotton tipped swabs to form isolated active layer (AL) islands prior to annealing. The film was then baked at 500 o C in a furnace for 3 h to form the AL for the TFTs. The thickness of the ready AL was ∼ 30 nm. Due to the good wettability of the PEDOT:PSS with a contact angle less than 33.5°dropped in close proximity for use as source and drain electrodes, and hydrophobic lines were patterned on the a-IGZO islands using transfer printing [16] in order to define the channel dimensions.
For this purpose, a polydimethylsiloxane (PDMS) stamp was shortly immersed in 6 wt% fluoropolymer solution 3 M Novec TM EGC-1700. It was then carefully contacted onto the a-IGZO-coated substrates and the line patterns (banks) were printed on the AL. The Si/SiO 2 /a-IGZO with EGC lines were dried on a hotplate at 50 o C for 1 min. Details of this process are detailed in Ref. 16. Finally, among the various direct printing techniques, the pristine or doped PEDOT:PSS solutions were simply drop-cast through a syringe on the line-patterned a-IGZO islands and annealed at 100 o C for 20 min. Here, the n-doped PEDOT:PSS (PH1000) with the lowest r-MR dopant concentration of 20:1 v/r was used as S/D electrodes in a-IGZO-based TFTs due to its highest electrical conductivity. The device structure is given in Figure 3(a). The width of the line pattern between two isolated polymer drops determines the channel length. The channel width and length of the OxTFTs with the polymer S/D were approximately 1650 and 600 μm, respectively.

Device and film characterization
Conductivity of the films was assessed using the current-voltage (I-V) measurements from hole-and electron-only devices with indium-tin oxide (ITO) electrodes. The structure of these devices was glass substrate/150 nm ITO electrodes/pristine or n-doped PEDOT:PSS (PH1000) film ( ∼ 92 nm) and is shown in Figure 3(b). The channel width and length were 4.5 and 1.5 mm, respectively. Keithley 2400 source meter was used to conduct the I-V measurements. Conductivity (σ ) was calculated according to the equation (1): where, L and W are the channel length and width, respectively; d is the film thickness determined by an Alpha step profilometer and R is the film resistance calculated by Ohm's law. The OxTFTs were electrically characterized using an HP4155C parameter analyzer at room temperature under ambient conditions. The field-effect mobility (μ) and threshold voltage (V th ) values were extracted in the saturation regime by the conventional equation (2): where, I d is the drain current; W and L are the channel width and length, respectively; C i is the gate insulator capacitance; V g is the gate voltage. UV-vis-NIR absorption spectroscopy (PerkinElmer Lambda 950 UV-vis spectrometer) and UPS measurements were conducted under ambient air conditions. The thin-films for these measurements and I-V characterization of the hole-and electron-only devices were prepared on bare (UV-vis-NIR absorption measurement) and ITO coated (for I-V and UPS measurements) glass substrates by three-step spin coating at 700 rpm for 5 s, 2000 rpm for 10 s, and 3000 rpm for 30 s. Prior to the spin coating, the substrates were UV-ozone treated for 10 min. The spincoated films were annealed at 100 o C for 20 min. The final films had thickness around ∼ 92 nm. All the fabrication and measurement procedures, except annealing of the polymer films, were performed in ambient atmosphere. The polymer films were annealed in a glove box.

Results and discussion
A cationic azo dye, MR, was chosen for the n-doping purpose as the cationic species are easily reduced to their neutral or anionic states, where they are very unstable and reactive and tend to be oxidized to reform their stable original forms. The reactions reduce the azo N = N bridge responsible for the bright red coloration of this dye and produce the colorless aromatic amines (see the chemical reaction given in Figure 2(a)), that in turn render the reduced form of the dye colorless (leuco) [14]. Various reducing agents, such as nicotinamide adenine dinucleotide phosphate (NADPH) and SBH, have been successfully utilized to reduce MR in previous studies [17]. As a cationic species, MR demonstrates good pdoping properties. Although several cationic species have already been reported as good n-type dopants in their reduced forms, the n-doping properties of MR have yet to be known [7][8][9][10][12][13][14].
We chose SBH as an electron donor for the reduction of MR as it is a more available and common hydride (H − , a proton with 2 electrons) source. NADPH may also be easily replaced by SBH in chemical reduction reactions. Although the chemical reduction of MR using SBH is kinetically slow without a catalyst, high concentrations of SBH (7 g in our work) eventually reduces the dye [14]. The electron donating state of MR, r-MR, was obtained following most commonly reported reduction protocols. The experimental details are given in Section 2.2 and Figure 2.
N-dopants are commonly used to improve conductivity and to lower the WF for electron transport (n-type) materials. In our work, a well-known hole transport (ptype) material, PEDOT:PSS, was chosen as the material of interest owing to its outstanding optoelectronic properties. This polymer is also very stable and structurally less sensitive to the deteriorative effects of dopants among other solution-processed organic conductors. However, n-doping progresses differently in nand p-type materials. As such, n-doping of an n-type material is relatively easier because the current flow is already carried by electrons in such hosts and the addition of n-dopants increases the concentration of these electrons. The p-type hosts, on the other hand, possess a large number of holes that have to be eliminated before n-doping of the material can be initiated to provide the material with free electrons to carry the electrical current. Therefore, n-doping of a p-type material (compensation doping) needs relatively more dopant concentration and time. Theoretically, this process can be divided into two stages. First, the electrons added by the dopant recombine with the holes of the host. In this stage, the dopant concentration has to be sufficient to cancel all the holes in the host lattice. After all holes are recombined with the added electrons, the second stage of the doping starts, and the dopant provides the host with electron carriers. As a result, the conductivity of the host starts to increase. PEDOT:PSS is primarily a p-doped conjugated polymer and de-doping it for complete removal of the positive charges is a prerequisite for the secondary n-doping to take place. The possible n-doping schemes for electron and hole transport materials are proposed in Figure 4. Figure 5(a) shows the UV-vis absorption spectra of the cationic MR and r-MR films. The MR film displayed a strong UV and visible light absorption with a UV-vis peak centering at ∼ 441 nm, whereas the r-MR film has low absorption with no visible peaks at the wavelengths above 350 nm, indicating the decolorization of the characteristic red color of MR after reduction and full transparency of the r-MR film. Figure 5(b) shows the UV-vis-NIR spectra of the undoped and doped PEDOT:PSS films. As seen, from Figure 5(a,b), both the host PEDOT:PSS and the dopant r-MR demonstrate high transparency within the visible and NIR region. However, the doped films display strong absorption close to the NIR region with new absorbance peaks at ∼ 650 and ∼ 850 nm, which are not inherent to either the host or the dopant, indicating charge transfer complex formation between the two materials [18]. This is also confirmed by the increased current and conductivity of the films (see Figure 6).   and led to the aggregation of the dopant and increased disorder of the molecular structure. This is because once the saturation point of the reduction is reached, the increased concentration of the dopant will only add impurity species that do not participate in the charge transfer process and act as extra charge trapping sites. Such impurity aggregation may also change the favorable conformational transitions between the quinoid and benzoid structures of the polymer [1,7,19]. Figure 6(b) shows the evolution of the conductivity of the doped films according to the n-dopant concentration.
Since the conductivity of PEDOT:PSS can be modified by solvent treatment, we conducted additional experiments to eliminate any doubts about where the improvement in conductivity stems from [1]. For these experiments, we added small amounts of pure CF into the PEDOT:PSS (PH1000) solution and compared the electrical properties of the films deposited from these solutions with that of the undoped polymer film. The results are presented in Figure S1(a). As seen, adding only pure CF does not change the conductivity of the polymer, which indicates that the conductivity enhancement is solely induced by the r-MR addition.
The variation in energy levels after the r-MR doping was investigated using UPS measurements to determine the WF of the films. Figure 7 shows the secondary cutoff and the highest occupied molecular orbital (HOMO) region spectra of the undoped and r-MR-doped films. The WF difference between the two films was estimated by calculating the difference between their secondary electron edges (SEE). The SEE of the doped polymer clearly shifted towards higher kinetic energies as compared to the undoped film, indicating lowered WF (see Figure 7(a)) [20]. As a result, the WF of the polymer decreased from 4.8 to 3.5 eV. Figure 7(b) shows the HOMO edge of the doped film moved away from the Fermi level towards higher energies, which is also a sign of n-doping and increased electron density in the film [7].  When compared with the undoped PEDOT:PSS, the n-doped polymer displayed more promising potential to be employed as S/D electrodes for n-type transistors owing to its remarkably reduced WF and Schottky barrier height for electrons. Hence, we fabricated n-type a-IGZO-based TFTs with PEDOT:PSS (PH1000) S/D doped with 20:1 v/r of r-MR to confirm this possibility. a-IGZO was used as the AL material owing to its excellent features including high electron mobility and better robustness against water degradation compared with organic semiconductors [21,22]. The latter is especially important since PEDOT:PSS is a water-based conductive polymer [1]. The conduction band minimum (CBM) and valence band maximum (VBM) levels of a-IGZO is −4.2 and −7.5 eV, respectively [23]. The energetic levels of the polymer contact electrodes and a-IGZO are given in Figure 8(a). A reference device with the undoped polymer S/D electrodes was also fabricated. Figure 8(b) displays the transfer curves of the fabricated OxTFTs with the polymer contact electrodes. As seen, the TFT with The output characteristics of the a-IGZO TFTs with the undoped and doped PEDOT:PSS S/D are shown in Figure 8(c,d), respectively. The doped polymer S/D clearly displays better electrical characteristics for the operation voltages ranging from 0 to 50 V. The TFT with the doped S/D shows over six-fold increase in the current compared to the TFT with the undoped S/D. This increase is attributed to the lower contact resistance and better injection properties. Additionally, a simplified comparison is made between the effects of the doped and undoped polymer S/D electrodes on the transistor parameters (threshold voltage and mobility) for a number of measured OxTFTs devices and is shown in Figure 8(e).

Conclusion
In this work, we demonstrated a highly conductive n-doped PEDOT:PSS polymer with a low WF suitable for electron injecting electrode applications. The reduced molecule of a cationic azo dye, r-MR, was used as the n-dopant. As a result, the WF of the polymer was significantly reduced from 4.8 to 3.5 eV, while its conductivity increased up to ∼ 51 from 3.4 S/cm. The n-doped PEDOT:PSS films were used as the S/D electrodes for BG/TC a-IGZO TFTs, demonstrating more than ten times enhanced electron mobility compared to the undoped PEDOT:PSS. This is a simple and low-cost method to increase electron density and reduce the WF of PEDOT:PSS polymer and other solution-processable conductors.
In addition, our results show that cationic MR can also be used as an n-type dopant when it is reduced to its leuco (colorless) state and can highly improve the electron transport properties (electrical conductivity, sheet resistance) of its host. Therefore, considering our previous results on p-doping properties of MR [12][13][14], we propose a new class of universal dopants for simple solution-processed nand p-type doping for organic materials. These reducible cationic dopants are solutionprocessed and can be applied to a wide range of deposition systems such as roll-to roll and inkjet printing among others. The host material's electrical performance can be easily controlled through the dopant concentration. We believe n-doping of PEDOT:PSS can significantly widen the practical applications of this polymer. Our results will also encourage further investigations for better progress in the field.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Funding
This research was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2018R1A6A1A03026005, 2020R1I1A3073634) and the Ministry of Science and ICT (2021R1A2C2011560).
Ye-seul Lee received her master's degree from Hanbat National University in Daejeon, South Korea, in 2022. Her current work includes polymeric insulating coating films and highly heat-resistant substrates for application of flexible electronic devices such as organic light-emitting diodes (OLEDs) and organic thin film transistors (OTFTs).