High performance and stable naphthalene diimide based n-channel organic field-effect transistors by polyethylenimine doping

Dyes and Pigments 142 (2017) 323e329

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High performance and stable naphthalene diimide based n-channel organic field-effect transistors by polyethylenimine doping Dang Xuan Long a, Eun-Young Choi b, **, Yong-Young Noh a, * a b

Department of Energy Materials Engineering, Dongguk University, 30, Pildong-ro 1-gil, Jung-gu, Seoul 04620, Republic of Korea Korea Atomic Energy Research Institute, Daedeok-daero 989-111, Yuseong-gu, Daejeon 34057, Republic of Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 23 February 2017 Received in revised form 25 March 2017 Accepted 27 March 2017 Available online 29 March 2017

In this paper, we study the effect of the structure and molecular weight of polyethylenimine (PEI) on doping efficiency of n-type organic semiconductors based on poly[[N, N 9-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,59-(2,29-bithiophene)] (P(NDI2OD-T2)). Linear and branched PEI with low (Mw ¼ 2 and 3 kDa) and high molecular weights (Mw ¼ 25 kDa) are used. It is found that the low molecular weight PEI shows higher doping efficiency than high molecular weight PEI due to better mix-ability between the dopant and the host. Doped P(NDI2OD-T2) OFETs show highly improved mobility up to 0.85 cm2/V$s (linear) at 0.1 wt% doping concentration from 0.21 cm2/V$s for pristine devices with enhanced air and operational stability. © 2017 Elsevier Ltd. All rights reserved.

Keywords: Organic field-effect transistors Molecular doping Naphthalene diimide n-type Polymer semiconductors

1. Introduction Organic field-effect transistors (OFETs) are actively studied to enable various integrated circuits to be manufactured on a rigid or flexible substrate through a solution-based printing process at low process temperatures [1e3]. Despite these beneficial features, it is challenging to apply such devices into high-end electronics of commercial products, due to their limited field-effect mobility (m) and unstable operation in the atmospheric environment, compared to inorganic semiconductors [4,5]. Several methods have been developed to improve such low mobility and environmental instability of organic semiconductors (OSCs) [6e9]. Among them, a typical method is to develop a novel OSC by organic synthesis [10,11], increase its purity, and control the crystallinity and ordering of the semiconducting layer in the OFETs to obtain high mobility [12]. However, the method of controlling the amount of charge carrier density in the semiconducting layer used in n-channel OFETs to induce high performance has been relatively less studied; even the mobility is known to be strongly dependent on charge carrier density [13e15].

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y.-Y. Noh).

(E.-Y.

http://dx.doi.org/10.1016/j.dyepig.2017.03.053 0143-7208/© 2017 Elsevier Ltd. All rights reserved.

Choi),

[email protected]

Doping is the most effective way to control the amount of charge in a semiconductor [16,17]. Different with doping mechanisms in inorganic semiconductors, the organic doping model is based on introducing small amounts of impurity material (dopant) into a semiconductor film for charge transfer between the host and dopant molecules to create additional mobile charge carriers [18,19]. For an efficient organic doping process, the lowest unoccupied molecular orbital (LUMO) level of the ptype dopant is smaller than the highest occupied molecular orbital (HOMO) level of the host molecule, facilitating hole transfer from the dopant to the host, and vice versa for the ntype doping process [16,20]. The molecular doping in OSCs has been successfully applied to the charge injecting layer in organic light emitting diode (OLEDs), contributing to lowering the driving voltage of OLEDs and leading to commercialization [18,21]. However, there are a few limitations preventing molecular doping to be freely applied in OFETs: (1) A high doping ratio strongly alters not only the conductivity, but also the morphology of the organic semiconductor film. Hence, we must consider the disruption of crystallinity or the ordering of the organic semiconductor film at high-concentration molecular doping. (2) In addition, high-concentration doping can increase the off-state current of OFETs leading to large static power consumption of circuits with OFETs. (3) Although p-type doping has been successfully demonstrated in a broad range of literature

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with different hosts and dopant combinations [22,23], the prospect of stable n-doped doping still remains scarce due to the intrinsic instability of n-dopants [18,24]. Finding an efficient and air-stable n-type dopant is much more challenging than for a ptype counterpart [20,21]. For a direct electron transfer from a dopant to a host for n-type doping, the HOMO level of the dopant must lie above the LUMO level of the host material, making ndoping fundamentally more difficult because such a high HOMO reduces the stability of the dopant against oxidation. For air-stable n-dopant molecules, o-MeO-DMBI, pyronin B chloride and rhodocene dimer have been proposed [25]. However, in particular, the air-stable n-dopant showed less doping efficiency compared with the unstable dopants at a low concentration [26]. Recently, PEI polymers (Fig. 1(a)) with simple aliphatic amine groups have been reported for fabrication of air-stable low work function electrodes by modifying the electrode surface [27]. In addition, PEI has been successfully used as an n-dopant for graphene and small molecular OSCs due to its high density of electrondonating amine groups [28]. Recently, PEI was also successfully applied as an n-type dopant for donor-acceptor type conjugated polymers in OFETs [29]. Different from small molecular dopants, PEI is a polymer material that can manifest various properties and species based on its wide range of molecular weight, branch and linear structure, polydispersity, and solubility properties. Even branched PEIs were successfully applied in doping systems; investigation of different PEI species would be necessary, not only for understanding the functions of the PEI in the devices, but also for the rational design of OFETs with high performance and stability. In this study, we report the effect of the molecular structure and the molecular weight of PEI on doping efficiency for a naphthalene diimide based n-type conjugated polymer. Linear (l-PEI) and branched PEI (b-PEI) with two different molecular weights have been mixed with the ntype polymer poly[[N, N 9-bis(2-octyldodecyl)- naphthalene1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,59-(2,29-bithiophene)] (P(NDI2OD-T2)), at various dopant concentrations to obtain high performance and stable OFETs.

2. Methods 2.1. Device fabrication Source and drain (S/D) electrodes of Au/Ni (12 nm/3 nm) were patterned on glass substrates (Corning Eagle 2000) using a conventional lift-off photolithography technique. After cleaning in an ultrasonic bath with sequentially de-ionized water, acetone and isopropanol, the S/D patterned substrates were treated with oxygen plasma. The semiconductor P(NDI2OD-T2) (Fig. 1(c)) was dissolved in chlorobenzene (CB) to obtain a 5 mg/mL solution. Branched high Mw-, low Mw-PEI and linear PEI (Sigma Aldrich) were used as the n-dopant and dissolved in 2-ethoxyethanol or CB. To make a doped P(NDI2OD-T2) base solution, blending of P(NDI2OD-T2) and dopant solutions (0.05, 0.1, 0.2, 0.4 and 0.8 wt %) were prepared by mixing the two separated solutions, and then subsequent shaking was performed to obtain a homogenously mixed blend system, which was maintained at 80  C overnight. The undoped and doped semiconductor solutions were spincoated onto a patterned glass substrate as active layers and then annealed at 110  C for 20 min. For the polymer gate dielectric, poly(methyl methacrylate) (PMMA) was spin-coated (z480 nm), and baked at 80  C for 2 h to remove any residual solvent. The TG/ BC OFETs were completed by forming gate electrodes on the channel region by evaporation of the thin Al film (50 nm) using metal shadow masks show in Fig. 1(b). 2.2. Thin film and device characterization The electrical characteristics of the OFETs and the inverters were measured using a Keithley 4200-SCS in a nitrogen-filled glove box. The mFET and VTh were calculated in the saturation regime using a gradual channel approximation equation. The UPS measurements were performed using a PHI 5000 VersaProbe II (ULVAC-PHI, Inc) with a base pressure of 4.2  109 Torr. Contact resistance (RC) in OFETs, which was extracted from the Y-function method (YFM). The detail of method was explain in Supporting Information part.

Fig. 1. (a) Chemical structure of linear (left) and branched (right) polyethylenimine molecule. (b) Top gate/bottom contact (TG/BC) OFET device structure. (c) Chemical structure of the P(NDI2OD-T2) semiconductor.

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3. Results and discussion Fig. 1(a) shows the chemical structures of linear-PEI (Mw ¼ 3 kDa) and branch-PEI (Mw ¼ 2 kDa, 25 kDa). Besides the topological difference between these two types of PEI, the contents of the amino groups are different. The former contains mainly a secondary amine, while the latter bears primary, secondary, and tertiary amines. The linear-PEI and two branch-PEIs (with different molecule weights) were dissolved in 2-methoxyethanone solvent at 0.1 wt% and directly mixed with P(NDI2OD-T2) at various dopant concentrations to check the doping effect. To obtain n-type doping, the dopants must donate electrons to the LUMO of the host molecules. In our system, the Fermi level (EF) of PEI (1.2 ~ 2.4 eV) is well above the LUMO levels of P(NDI2OD-T2) (3.9 eV). Ultraviolet photoelectron spectroscopy (UPS) measurements were conducted on pristine and three types of PEI-doped P(NDI2OD-T2) films to check the shift of EF and the HOMO level by doping (Fig. 2). With the addition of 0.2 wt% PEI dopant, the EF of P(NDI2OD-T2) shifted from 4.15 eV to 3.75 eV (branched high Mw-PEI), 3.65 eV (branched low Mw-PEI) and 3.68 eV (linear PEI), corresponding to DEF ¼ 0.4 eV, 0.5 eV and 0.47 eV, respectively. The large shift of EF toward the conduction band edge of P(NDI2ODT2) clearly indicates an increase of electron density by 0.2 wt% doping of all used PEI. The PEI with low Mw showed better doping efficiency than the PEI with high Mw because it is difficult to achieve a well-mixed film with efficient charge transfer from the dopant to the host at high Mw-PEI due to its high mixing entropy. If Mw is similar, the branched PEI demonstrated better doping efficiency since the polymer contains more amine groups (primary, secondary and tertiary amines) than the linear PEI. Fig. 2 (b) show

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the UPS of pristine and P(NDI2OD-T2) film with doping concentration of 0.2 wt% and 0.8 wt%, respectively. The EF of the P(NDI2OD-T2) film was gradually shifted by increasing the doping concentration. At 0.8 wt% doping concentration, the EF of linear PEI and branched PEI with low Mw doping was almost identical, due to a high carrier concentration already filling the trap site located in the tail states, which leads to saturation of the EF shift at a high doping ratio [24]. To check the chemical composition at the doped film surface, Xray photoelectron spectroscopy (XPS) was measured for bare and 0.2 wt% doped P(NDI2OD-T2) films with three different types of PEI. From Fig. 2 (c), the full XPS spectra of the pristine and doped P(NDI2OD-T2) films showed clear carbon 1s peaks (C1s) and nitrogen 1s peaks (N1s). The C1s region in the XPS spectra is depicted more clearly in Fig. S3 (a) and all films showed clear C1s peaks near 283.5 eV, corresponding to the strong CeC bond from P(NDI2ODT2). The intensity of the C1s peaks was slightly increased in the doped films due to a high concentration of carbon present in the PEI. The N1s region in the XPS spectra is depicted in Fig. 2 (d). The N1s photo-electron line was shifted from 398.1 eV for bare P(NDI2OD-T2) film to 398.6, 398.7 and 398.6 eV for branched high Mw-PEI, low Mw-PEI and linear PEI doped film, respectively. The slightly increased binding energy in the doped film indicates the appearance of the amine group (eNH) (400 eV) at the surface. The intensity ratio of the N1s and C1s peak ([N]/[C]) for the 0.2 wt% doped P(NDI2OD-T2) film was 0.032, 0.046 and 0.043 for branched high Mw-PEI, low Mw-PEI and linear PEI dopants, respectively. The slightly larger intensity of the N1s peak for low Mw-PEI dopant means that the PEI polymer located at the film surface was the larger portion than other PEI doped films due to low Mw. “Fig. S2

Fig. 2. UPS spectra of bare and doped P(NDI2OD-T2) films with (a) 0.2 wt% and (b) 0.8 wt% doping concentration with linear PEI (Mw ¼ 3 kDa) and branched PEI (Mw ¼ 2 kDa, 25 kDa) on indium thin oxide film. (c) Fully scanned XPS and (d) N1s XPS spectra of bare and 0.2 wt% doped P(NDI2OD-T2) films with three PEI (linear PEI (red line), low Mw-PEI (blue line) and high Mw-PEI (orange line)) dopants. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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shows the contact angle of the pristine and doped P(NDI2OD-T2) films. The contact angle slightly decreased from 102.4 for pristine to 101.1, 98.6 and 99.3 for branched high Mw-PEI, low MwPEI and linear PEI, respectively.” These XPS and contact angle results are well correlated with the largest shift of EF in the UPS data for the low Mw-PEI doped P(NDI2OD-T2) film. This morphology at the branched low Mw-PEI doped P(NDI2OD-T2) film enables highly efficient doping even at relatively low doping concentrations in the OFETs with a TG/BC geometry where the transistor channels are formed above the semiconductor film. We studied the effect of doping on the currentevoltage characteristics of P(NDI2OD-T2) OFETs. The transfer characteristics of pristine and PEI doped P(NDI2OD-T2) OFETs with various concentrations (0.05, 0.1, 0.2 and 0.4 wt%) are shown in Fig. 3. By PEI doping, the on-state current for the negative gate voltage (VG) decreased significantly. In addition, the onset voltage (Von) in the pristine device decreased significantly from 27.2 V to 6.3, 2.4 and 1.7 V by doping of branched high Mw-PEI, low Mw-PEI and linear PEI, respectively. The results confirm that a small amount of doping effectively increases the concentration of electrons in the transistor channel so that the transistor was turned on earlier. However, the off-state current (Ioff) of the doped devices displayed a different trend. For doped OFETs with high Mw-PEI, Ioff was reduced after doping but remained constant at all doping concentrations (Fig. 3 (a)). This result implies that doping with a branched high Mw-PEI does not result in effective doping even if the doping concentration is increased, which is consistent with the above-mentioned

UPS results. As indicated from the results of the XPS, it is believed that the branched high Mw-PEI is mainly located at the bottom of the film and does not lead to effective doping of the transistor channel placed at the top of the semiconducting film, even if the doping amount is increased. Conversely, branched low Mw-PEI doped OFETs showed rapid decrease of Ioff at negative VG region even at very low doping concentration. (Fig. 3(b) and (c)). This indicate that branched low Mw-PEI performs as a more effective dopant than high Mw-PEI. This is consistent with the abovementioned UPS and XPS results. Alternatively, if the doping amount is too high, the charge amount is greatly increased and both the on current (Ion) and Ioff are simultaneously increased. The change of m in accordance with the doping concentration depends on the Ioff dependence on the doping concentration. The OFET mobility as a function of concentration is shown in Fig. 4 (a). For OFETs doped with branched high Mw-PEI, m slowly increased from 0.21 to 0.85 cm2/V$s at 0.2 wt% doping and then gradually decreased to 0.5 cm2/V$s at 0.8 wt% doping. However, when doping with branched low Mw-PEI, m increased to 0.8 (branched) and 0.85 cm2/V$s (linear) at 0.1 wt% and then decreased more rapidly to 0.08 and 0.25 cm2/V$s at 0.8 wt%, respectively. In summary, m increased from about 0.1 wt% to the maximum and began to decrease further for OFETs doped with low Mw-PEI, whereas branched high Mw-PEI showed a maximum m at 0.2 wt% which then slowly decreased. The basic transistor properties are summarized in Table 1. The increase in m of the OFETs with increased carrier concentration is achieved by the increased carrier effectively filling in the traps of the tail states and inducing more mobile

Fig. 3. Transfer characteristics of pristine and doped P(NDI2OD-T2) OFETs with (a) linear, (b) branched low Mw- and (c) branched high Mw-PEI dopants with different doping concentrations (0.05, 0.1, 0.2 and 0.4 wt%). (d) Transfer characteristics of pristine and doped P(NDI2OD-T2) OFETs with different types of PEI dopants (at 0.2 wt% concentration).

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Fig. 4. (a) Field-effect mobility and (b) turn-on voltages of pristine and doped P(NDI2OD-T2) OFETs as a function of doping molecular ratio for linear (red line), low Mw (blue line) and high Mw (orange line) PEI dopants. Dashed green line is the expected trend. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

carriers in the transport band. However, increasing the doping concentration continuously does not lead to increased m of the transistor, which is understood to be due to the collapse of crystallinity or ordering of the semiconducting film when a large amount of dopant is included in the organic semiconductor film [24]. Another benefit of doping is a significant reduction of Rc. We also investigate Rc of our doped devices by Y-function method. The Rc of all PEI doped OFETs are highly reduced to 41.2 (branched high Mw-PEI 0.2 wt%), 38.3 (branched low Mw-PEI 0.1 wt%) and 50.9 kU cm (linear PEI 0.1 wt%) from pristine devices 240.1 kU cm (P(NDI2OD-T2). Output characteristics of pristine and doped device also show a high improvement of Rc clearly (Fig. S1). Generally, the n-channel OFETs suffered from poor ambient and bias stress stability originating from a number of trap sites concentrated around 3.8 eV owing to hydrated O2 in air [30]. The P(NDI2OD-T2) OFETs also rapidly degraded after exposure in air due to their low LUMO energetic level (3.9 eV). We checked the change of the ambient and operational stability of the P(NDI2OD-

T2) OFETs by PEI doping. All doped OFETs showed improved operational stability as illustrated in Fig. 5. Basic OFET parameters, such as Von, Ioff and m of all PEI doped P(NDI2OD-T2) OFETs were mostly unchanged after 50-cycle scanning, while pristine devices showed a 26% decrease of m and a large shift for Von (Fig. 5 (a)). Recently, highly air-stable p-type OFETs have been demonstrated by addition of specific additives to remove residual water molecules in the OSC film [31]. In this paper, we demonstrate highly improved air stability of n-type OFETs by addition of a PEI dopant. After 12 days stored in air with a humidity of 60%, the device m of pristine P(NDI2OD-T2) OFETs was reduced 80% from the fresh devices, as shown in Fig. 5(e). However, improved ambient stability was clearly observed in all PEI doped devices. The improved air stability in the doped devices may be explained by removing the water residue in the OSC film by formation of hydrogen-bonds between the NH group and water. Among the three PEI dopants, doped devices with branched low Mw-PEI molecules showed better ambient stability than high Mw-PEI dopants due to better mix-ability and a larger amount of amine group. The air stability of

Table 1 Fundamental parameters of pristine and doped P(NDI2OD-T2) OFETs with PEI doping at different doping concentrations. The characteristics of the FETs were measured with VD ¼ þ80 V. The average mobility of the OFETs were measured with channel length, L ¼ 20 mm and channel width, W ¼ 1000 mm. Semiconductor/dopant P(NDI2OD-T2) P(NDI2OD-T2)/Branched High MW PEI

P(NDI2OD-T2)/Branched low Mw PEI

P(NDI2OD-T2)/linear low Mw PEI

Dopant (doping ratio) pristine 0.05 wt%PEI 0.1 wt%PEI 0.2 wt%PEI 0.4 wt%PEI 0.8 wt%PEI 0.05 wt%PEI 0.1 wt%PEI 0.2 wt%PEI 0.4 wt%PEI 0.8 wt%PEI 0.05 wt%PEI 0.1 wt%PEI 0.2 wt%PEI 0.4 wt%PEI 0.8 wt%PEI

Electron mobility (cm2/vs) 0.21 (±0.02) 0.4 (±0.05) 0.65 (±0.05) 0.85 (±0.07) 0.6 (±0.05) 0.5 (±0.05) 0.38 (±0.05) 0.8 (±0.06) 0.4 (±0.07) 0.15 (±0.07) 0.08 (±0.03) 0.35 (±0.03) 0.85 (±0.06) 0.65 (±0.05) 0.45 (±0.04) 0.25 (±0.05)

VTh [V] 27.2 (±2.2) 12.8 (±2.1) 9.5 (±1.8) 7.7 (±1.5) 6.3 (±1.3) 7.2 (±1.6) 10.4 (±2.0) 4.5 (±1.7) 3.3 (±0.8) 2.4 (±0.4) 3.7 (±0.7) 9.8 (±2.0) 4.5 (±1.7) 3.2 (±0.4) 2.1 (±0.4) 1.7 (±0.6)

Ion/off 2

10 104 104 104 104 104 104 105 103 102 102 103 104 106 103 102

Contact resistance [kU.cm] 240 (±49.6) 126.4 (±37.4) 86.7 (±28.5) 41.2 (±16.2) 67.2 (±13.7) 91.2 (±23.1) 135.8 (±27.2) 50.9 (±16.1) 131.6 (±32.5) 256.6 (±33.1) 331.6 (±36.5) 135.8 (±39.2) 38.3 (±16.1) 61.7 (±22.8) 99.6 (±21.1) 156.6 (±25.5)

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Fig. 5. Fifty-cycle scanned transfer curves of: (a) pristine and doped P(NDI2OD-T2) OFETs with (b) high Mw-PEI, (c) linear PEI and (d) low Mw-PEI for the bias stress test. Normalized electron mobility of pristine and 0.2 wt% doped P(NDI2OD-T2) devices with three PEI stored in ambient air (e) and a chamber with silica gel (humidity <15%) (f) for 12 days.

the n-type OFETs was greatly improved when the device was stored in very low-moisture silica gel chambers (humidity <15%). After 12 days of storage, the m of P(NDI2OD-T2) OFETs doped with low MwPEI was maintained about 90% of the original device. These results indicate that water in air has a very important effect on the air stability of n-type OFETs and the doping can not only increase device performance but also improve the ambient stability of n-type OFETs. 4. Conclusion In conclusion, we have investigated the effect of the molecular weight and the structure of the PEI polymer on the doping effect and the properties of n-type P(NDI2OD-T2) OFETs. Low Mw-PEI exhibit better doping efficiency than high Mw-PEI due to better intermixing. The doped OFETs with branched low Mw-PEI exhibited significantly improved m, on/off ratio, air, and operational stability. The doping efficiency strongly depends not only on the dopant concentration, but also on the molecular weight and the structure of the dopant molecule. These results provide a simple and effective method to achieve high performance and stability of n-type polymer OFETs. Acknowledgements The authors gratefully acknowledge the financial support of the project from the Center for Advanced Soft-Electronics (2013M3A6A5073183) funded by the Ministry of Science, ICT & Future Planning.

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