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Yan Zhao , Chong-an Di ,* Xike Gao,* Yunbin Hu, Yunlong Guo, Lei Zhang , Yunqi Liu,* Jizheng Wang , Wenping Hu , and Daoben Zhu*

All-Solution-Processed, High-Performance n-Channel Organic Transistors and Circuits: Toward Low-Cost Ambient Electronics

As an alternative to well-established amorphous silicon elec-tronics for low-cost, large-area, and fl exible applications, organic thin-fi lm transistors (OTFTs) have made great achievements in the past two decades. [ 1–4 ] Building on this rapid progress, organic circuits have been successfully demonstrated in various areas such as electronic papers, sensors, and radio frequency identifi cation cards (RFID). [ 5–10 ] A promising development for OTFTs, driven by the urgent demand for ultra-low-cost printed and ambient electronics, is the realization of all-solu-tion-processed complementary circuits composed of both p-channel and n-channel devices. [ 11–14 ] Compared with those exciting reports on solution-processed p-channel transistors with mobility approaching 1.0 cm 2 V − 1 s − 1 , [ 15–19 ] only a few solu-tion-processed n-channel OTFTs showing mobilities exceeding 0.1 cm 2 V − 1 s − 1 have been fabricated, despite many innovative studies of material design and device engineering. [ 13 , 20–26 ] This uneven development makes the fabrication of high-performance, solution-processed n-channel organic transistors one of the biggest remaining challenges.

Due to the intrinsic nature of electrons being trapped by oxygen, the low air-exposed performance and stability is a key issue for n-channel OTFTs, especially for solution-processed ones. Therefore, most reported solution-processed n-channel OTFTs suffer from relatively low performance (with mobilities < 0.2 cm 2 V − 1 s − 1 ) and stability for the bottom-gate devices where the active layer exposed to air, [ 20–23,25,26 ] which presents an obvious contrast with the excellent result for solution-processed top-gate devices (0.85 cm 2 V − 1 s − 1 with an encapsulated active layer). [ 13 ] In addition to the conventional stability at room tem-perature, good device stability at high temperatures under

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Y. Zhao, Dr. C.-a. Di, Y. L. Guo, L. Zhang, Prof. Y. Q. Liu, Prof. J. Z. Wang, Prof. W. P. Hu, Prof. D. B. ZhuBeijing National Laboratory for Molecular SciencesKey Laboratory of Organic SolidsInstitute of ChemistryChinese Academy of SciencesBeijing 100190, P.R. ChinaEmail: dicha@iccas.ac.cn; liuyq@iccas.ac.cn; zhudb@iccas.ac.cn Dr. X. K. Gao, Y. B. HuLaboratory of Materials ScienceShanghai Institute of Organic ChemistryChinese Academy of SciencesShanghai 200032, ChinaE-mail: gaoxk@mail.sioc.ac.cn

DOI: 10.1002/adma.201004588

ambient condition is also essential for solution-processed OTFTs. [ 14 ] If the solution-processed devices can survive high-temperature drying and/or annealing in air, ultra-low-cost fab-rication of OTFTs could be realized by avoiding vacuum and/or inert atmosphere protection. To date, this high-temperature stability of solution-processed n-channel devices in ambient air has rarely been studied. Here, we report a bottom-gate device based on a core-expanded naphthalene diimide fused with 2-(1,3-dithiol-2-ylidene)malonitrile groups [ 24 ] (NDI2OD-DTYM2, Figure 1 ) presenting a high electron mobility of up to 1.2 cm 2 V − 1 s − 1 in air. More excitingly, the outstanding sta-bility of NDI2OD-DTYM2 enables the devices to both endure the high temperature annealing treatment during fabrication and to maintain high performance even when operating at high environmental temperatures (≤120 ° C) under ambient conditions. Therefore, the construction of all-solution-processed n-channel OTFT based circuits without any necessity for vacuum and inert atmosphere protection has been realized, making n-channel OTFTs technologically relevant for low-cost ambient electronics.

To investigate the electrical performance of NDI2OD-DTYM2 based devices, bottom-gate bottom-contact (BGBC) OTFTs (Si/SiO 2 substrates) were fabricated. The organic active layers were deposited by different solution processing techniques including spin-coating, inkjet printing, and brush painting. The TFT fabri-cation and measurement details are reported in the Supporting Information. Comparable device performances for devices fabricated under inert and ambient conditions were obtained. Therefore, the deposition, drying, and annealing processes of active layer were performed under ambient atmosphere in this report.

Table 1 shows the device performances of NDI2OD-DTYM2-based devices. For the bottom-gate device structure of NDI2OD-DTYM2, the bottom-contact OTFTs (annealed in air) showed electron mobilities ( μ e ) of 0.25–0.45 cm 2 V − 1 s − 1 , which are comparable to those of the top-contact devices (0.26–0.42 cm 2 V − 1 s − 1 , annealed in vacuum). [ 24 ] This indicates that NDI2OD-DTYM2 is applicable to both bottom-contact and top-contact device geometries. Moreover, the device performance was dependent on the fi lm deposition approach. Inkjet-printed devices with the same substrate, dielectric, and source–drain electrodes exhibited lower mobilities (0.08–0.30 cm 2 V − 1 s − 1 ), due to the rough surface of the inkjet-printed fi lm (Figure S1, Sup-porting Information). Interestingly, we obtained excellent active fi lms with large grains by using the brush painting methods (Figure S1, Supporting Information), affording very high device

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Figure 1 . a) Illustration of the chemical structure of NDI2OD-DTYM2, fabrication processes, and device structure of n-channel OTFTs. b) Optical images of an all-solution-processed fl exible organic oscillator fabricated under all-ambient environment.

performance with μ e > 0.30 cm 2 V − 1 s − 1 (up to 0.54 cm 2 V − 1 s − 1 ), on/off ratio ( I on / I off ) > 10 5 , threshold voltage ( V T ) < 10 V, sub-threshold slope ( S) < 2.5 V per decade ( Figure 2 a,b). As these processing techniques are similar to spray-painting and roll-to-roll techniques, NDI2OD-DTYM2-based OTFTs are promising for low-cost large-area applications.

Electrode modifi cation is a widely applied approach to improve carrier injection and device performance. [ 2 , 3 ] When the bottom Au source-drain electrodes (Si/SiO 2 substrate) were modifi ed by pentafl uorobenzene thiol (PFBT), a well-known self-assembled monolayer (SAM) modifi cation material, [ 2 ] the device performance improved dramatically, exhibiting a high mobility that exceeded 0.55 cm 2 V − 1 s − 1 . Figure 2 c,d show the device characteristics of NDI2OD-DTYM2-based OTFTs. The devices exhibited a maximum electron mobility of up to 1.2 cm 2 V − 1 s − 1 , I on / I off = 10 7 , V T = − 4.5 V, and S = 0.85 V per decade. This mobility, which is a new record for the air-stable and solution-processed n-channel OTFTs, approaches that of many excellent n-channel devices based on vacuum-deposited molecular semiconductors. [ 27 , 28 ] As indicated in the atomic force microscopy (AFM) images (Figure S2, Supporting Information), PFBT-Au-modifi ed electrodes induced the formation of large

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Table 1. Performance parameters for NDI2OD-DTYM2-based BGBC OTFTs (measured under am

Substrate Electrodes (S-D)

Dielectric [d,nm] (OTS modifi ed)

Active Layer Deposition

μ [cm 2 V − 1

Silicon Au SiO 2 (300) Spin-coating 0.25–0.4

Silicon Au SiO 2 (300) Inkjet-printing 0.08–0.3

Silicon Au SiO 2 (300) Brush-painting 0.30–0.5

Silicon Au(PFBT) SiO 2 (300) Spin-coating 0.55–1.2

Silicon Inkjet-printed Ag SiO 2 (300) Spin-coating 0.20–0.5

Silicon inkjet-printed Ag SiO 2 (300) Inkjet-printing 0.10–0.2

Silicon Inkjet-printed Ag SiO 2 /PAN/PMSQ

(300/500/50)

Spin-coating 0.15–0.5

Glass Inkjet-printed Ag PAN/PMSQ (800/50) Spin-coating 0.15–0.5

Glass Inkjet-printed Ag PAN/PMSQ (800/50) Inkjet-printing 0.08–0.4

PET Inkjet-printed Ag PAN/PMSQ (800/50) Spin-coating 0.11–0.4

PET Inkjet-printed Ag PAN/PMSQ (800/50) Inkjet-printing 0.07–0.4

NDI2OD-DTYM2 grains near the electrodes, compared with those in devices with bare Au electrodes. Additionally, PFBT-Au electrodes possess a lower workfunction (4.77 eV) than bare Au electrodes (5.1 eV). [ 29 ] Therefore, the dramatically enhanced mobility should be mainly ascribed to the improved contact at the interface between PFBT-Au and NDI2OD-DTYM2 and the decreased electron injection barrier.

As mentioned above, the NDI2OD-DTYM2 fi lm can endure high-temperature treatment under ambient conditions. To examine its prominent thermal stability, X-ray photoelec-tron spectroscopy (XPS), UV-vis absorption spectra, and AFM measurements were made on NDI2OD-DTYM2 thin fi lms that had

been annealed in air. The XPS measurement was performed to explore the effect of air exposure at high temperatures on the chemical bonding in NDI2OD-DTYM2. Figure S3 (Supporting Information) shows the evolution of the N 1s, C 1s, S 1s, and O 1s core-level XPS spectra with increasing annealing tem-perature. All the peaks remained unchanged, even at annealing temperatures up to 220 ° C, demonstrating the excellent thermal stability of NDI2OD-DTYM2 fi lms. The UV-vis spectra (Figure S4, Supporting Information) of fi lms annealed at 140–180 ° C in air showed obvious red shifts of approximately 6 nm compared to the as-fabricated spin-coated fi lms. This suggests that in the fi lms annealed at 140–180 ° C, intermolecular π – π stacking of NDI2OD-DTYM2 becomes more effi cient resulting in a more ordered lamellar microstructure. This hypothesis was confi rmed by AFM and optical characterization ( Figure 3 a,b). NDI2OD-DTYM2 fi lms annealed at 140–180 ° C displayed a much enhanced grain size (from as-deposited 200 nm to about 1000 nm). These results demonstrate that thermal annealing, even at high temperatures (up to 180 ° C), can be performed in air to achieve highly ordered NDI2OD-DTYM2 fi lms.

To probe the stability of NDI2OD-DTYM2 as a semi-conductor for OTFT applications, we evaluated the variation

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bient conditions).

s − 1 ] I on / I off [log 10 ]

V T [V]

S [V per decade]

5 5–8 –4.0–6.2 1.4–2.1

0 5–7 –0.5–5.5 1.5–2.4

4 5–8 –3.8–6.0 1.2–2.2

0 6–8 –4.8–6.2 0.8–1.5

0 5–7 2.5–8.0 1.5–2.8

5 5–7 1.0–8.5 1.5–3.0

1 6–7 5.0–9.8 1.6–2.8

7 5–7 –0.5–7.6 1.5–3.1

2 4–6 3.5–11.0 1.5–3.0

5 5–6 –2.0–6.5 1.8–2.8

5 4–6 –0.6–10.5 1.1–2.8

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Figure 2 . a) Output and b) transfer characteristics of a NDI2OD-DTYM2-based device with a brush-painted organic active layer. c) Output and d) transfer characteristics of a NDI2OD-DTYM2-based device with a PFBT-modifi ed Au electrode.

of the electron mobility during the annealing process in air for devices with bare Au electrodes. Figure 3c shows the mobility measured at room temperature as a function of the annealing temperature under ambient conditions. The as-prepared devices with different channel lengths exhibited a low mobility of about 0.02 cm 2 V − 1 s − 1 . The mobility increased dramatically to max-imum values of approximately 0.4 cm 2 V − 1 s − 1 after annealing at elevated temperatures of up to 160–170 ° C. After the opti-mization of the annealing in air, the NDI2OD-DTYM2-based devices were able to maintain high device performance during the second round of thermal treatments in air (Figure 3 d).

The ability to utilize low-temperature processing technique is of vital importance for practical applications of OTFTs. We therefore investigated the dependence of the mobilities of NDI2OD-DTYM2-based devices on the annealing temperature and annealing time. As shown in Figure 3 e, when the annealing temperature was higher than 140 ° C, the electron mobility was found to be strongly related to the annealing time. The mobility increased from ≈ 0.02 cm 2 V − 1 s − 1 to 0.45 cm 2 V − 1 s − 1 after thermal annealing for 2 min at 170 ° C in air. It should be noted that mobilities of well over 0.2 cm 2 V − 1 s − 1 and 0.3 cm 2 V − 1 s − 1 can be easily obtained within 5 min even for annealing tem-peratures of 140 ° C and 150 ° C, respectively. These results indi-cate that the devices were compatible with fl exible electronic applications that use low-temperature (≤150 ° C) processing techniques.

The temperature-dependent mobility (i.e., device operating mobility measured at various environmental temperatures) of a device in air has rarely been reported for n-channel OTFTs. This is probably due to the poor stability of electron transport under ambient conditions. Since the NDI2OD-DTYM2-based devices showed excellent environmental stability, we were able to carry

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out this measurement. Two different devices with similar mobil-ities were measured in air and in vacuum (Figure 3 f). In both cases, the device mobilities were maintained when the substrate temperature was increased to 80 ° C. Although higher tempera-tures (over 100 ° C) led to a rapid loss in mobility, the device performance recovered when the temperature was decreased to room temperature. The decreased mobility at high temperature ( > 100 ° C) is mainly due to the increased level of carrier trap-ping and scattering. [ 30 , 31 ] The excellent high-temperature opera-tion constitutes another advantage of NDI2OD-DTYM2-based n-channel devices.

The in-plane structure of the annealed NDI2OD-DTYM2 fi lm was investigated using grazing incidence X-ray scattering to probe the crystalline planes, which were oriented perpendic-ular to the dielectric layer (Figure S5, Supporting Information). The annealed fi lm at 170 ° C showed a short π -stacking dis-tance of 3.47 Å. This π -stacking distance is comparable to the values reported for the excellent naphthalene tetracarboxylic-diimide-based semiconductors. [ 28 ] We suggest the close packing of the π -planes, together with the protection afforded by the long-branched N -alkyl chains, inhibits the ingress of oxygen, which effectively reduces the occurrence of electron trapping. This, combined with the low energy of the lowest unoccupied molecular orbital (LUMO), is responsible for the excellent device performance and outstanding device stability.

As NDI2OD-DTYM2 exhibits a unique combination of good solubility, high electron mobility, and excellent environment stability, it allows all-solution-processed n-channel devices to be fabricated in ambient air if the electrodes and dielec-tric layer can also be solution processed in air. Fortunately, inkjet-printed silver can serve as source–drain electrodes for NDI2OD-DTYM2-based devices (Table 1 ). Double dielectric

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Figure 3 . a) AFM images and b) optical microscopy images of a NDI2OD-DTYM2 fi lm after thermal annealing at different temperature (on hot plate) for 3 min in air. c) Mobility with different channel lengths as a function of annealing temperature. d) Mobility of devices after optimized annealing treatment at 160 ° C for 3 min as function of further thermal treatment temperature (heated on hot plate for 3 min). e) Mobility with different annealing temperature as a function of annealing time. f) Temperature dependence of measured mobility with different dielectric layers.

layers composed of polyacrylonitrile (PAN) and polymethyl-silsesquioxane (PMSQ) were used in order to resist any sol-vent corrosion that could otherwise occur during deposition of the silver source–drain electrodes and the NDI2OD-DTYM2 active layer. After octadecyltrichlorosilane (OTS) modifi cation on a PAN/PMSQ dielectric layer, the spin-coated NDI2OD-DTYM2-based devices on Si/SiO 2 substrates afforded mobili-ties of 0.15–0.51 cm 2 V − 1 s − 1 , while the devices on glass and PET substrates displayed maximum mobilities of up to 0.57 and 0.45 cm 2 V − 1 s − 1 , respectively. High-performance devices can also be obtained by inkjet-printing. Figure 4 a provides an example of an all-solution-processed fl exible n-channel tran-sistors array with an inkjet-printed active layer. The devices

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showed good saturation, sharp turn-on characteristics, and a high mobility of up to 0.45 cm 2 V − 1 s − 1 (Figure 4 b,c and Figure S6, Supporting Information), confi rming the good quality of the active fi lms and the excellent interface between the organic fi lm and the PAN/PMSQ dielectric layer.

The all-solution-processed devices fabricated under ambient conditions showed outstanding operating and environmental stabilities. For example, they were able to withstand 10 5 on/off switch cycles, 10 4 cycling test of transfer characteristics, storage in air for more than 2 months, strong UV irradiation (300 W Xe-Hg arc lamp) for more than 30 min, dipping in many sol-vents (e.g., water, ethanol, isopropyl alcohol, and hexane) for 1 min, and long-time operation testing (Figure S7,S8,

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Figure 4 . a) Optical images, b) output characteristics, and c) transfer characteristics of all-solution-processed fl exible OTFTs. d) Optical images of a fl exible fi ve-stage organic oscillator. Electrical characteristics of e) an organic inverter and f) an organic oscillator.

Supporting Information). More excitingly, the devices showed a stepwise improvement in mobility during operating tests at increasingly high temperatures in air, from 0.3 cm 2 V − 1 s − 1 at 40 ° C to a maximum of 0.6 cm 2 V − 1 s − 1 at 120 ° C (Figure 3 f). This peak-mobility temperature (i.e., the operating temperature with maximum mobility) was 20–40 ° C higher than those of devices based Si/SiO 2 substrates. Given the lower hysteresis of the all-solution-processed devices (with the PAN/PMSQ dielec-tric layer) compared to that of devices based on Si/SiO 2 sub-strates (Figure S6,S9, Supporting Information), the excellent interface between PMSQ-OTS and NDI2OD-DTYM2 might be responsible for the high peak-mobility temperature.

By virtue of the excellent device performance and remark-able stability of all-solution-processed n-channel devices, all-solution-processed organic circuits based on a single n-type semiconductor with all fabrication processes carried out in air is possible. As a feasibility test, organic inverters with single NDI2OD-DTYM2-based transistors were built on both glass and PET substrates. The inverter structure is shown in the inset of Figure 4 e. All fabrication steps and device measurements, including the deposition, drying, and annealing of each func-tional layer, were performed in air. An inverter switch response was clearly observed from logic “1” (16 V) to logic “0” (0 V). For all the fl exible devices, voltage gains higher than 10 were rou-tinely obtained, implying potential applications in more com-plex logic circuits. Further integration was successful, yielding a fi ve-stage fl exible ring oscillator with a switching frequency > 1.2 KHz at a rail voltage of 30 V (Figure 1 b and Figure 4 f; the oscillator structure is shown in the inset of Figure 4 f).

In summary, the realization of all-solution-processed n-channel OTFTs under ambient conditions was demonstrated. The NDI2OD-DTYM2-based BGBC transistors exhibited high electron mobility (up to 1.2 cm 2 V − 1 s − 1 ), easy solution process-ability, and excellent ambient stability. These prominent prop-erties allowed the successful construction of air-stable fl exible

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n-channel transistors and oscillators at low fabrication tempera-ture without vacuum or inert atmosphere protection. Following the example of organic circuits based on high-performance, all-solution-processed n-channel transistors with all fabrication procedures performed in air, we believe that this work will open unprecedented opportunities for forthcoming achievements in ultra-low-cost ambient electronics.

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

Acknowledgements The authors acknowledge Zhen Dai, Haifan Xiang, Prof. Jian Xu (Polymer Physics Laboratory, Institute of Chemistry), and Prof. Haisheng Xu (Department of Physics, East China University of Science and Technology) for providing PMSQ as the dielectric layer and silver printable ink. Beijing Synchrotron Radiation Facility (BSRF) is acknowledged for the grazing incidence X-ray diffraction measurements. This research was fi nancially supported by the National Natural Science Foundation (60901050, 60911130231 and 20902105, 60736004), the Major State Basic Research Development Program (2011CB932303), the Chinese Academy of Sciences, and Beijing Municipal Education Commission (YB20098000104).

Received: December 15, 2010 Revised: February 7, 2011

Published online: March 11, 2011

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