Effect of In Situ Annealing Treatment on the Mobility and Morphology of TIPS-Pentacene-Based Organic Field-Effect Transistors

Effect of In Situ Annealing Treatment on the Mobility and Morphology of TIPS-Pentacene-Based... In this work, organic field-effect transistors (OFETs) with a bottom gate top contact structure were fabricated by using a spray-coating method, and the influence of in situ annealing treatment on the OFET performance was investigated. Compared to the conventional post-annealing method, the field-effect mobility of OFET with 60 °C in situ annealing treatment was enhanced nearly four times from 0.056 to 0.191 cm /Vs. The surface morphologies and the crystallization of TIPS-pentacene films were characterized by optical microscope, atomic force microscope, and X-ray diffraction. We found that the increased mobility was mainly attributed to the improved crystallization and highly ordered TIPS-pentacene molecules. Keywords: Organic field-effect transistors, In situ annealing treatment, Field-effect mobility, Morphology, TIPS-pentacene Background compatibility to various substrates [18], and the different Organic field-effect transistors (OFETs) have attracted shapes of film can be patterned through shadow masks considerable attention as a promising candidate for its [19]. Additionally, compared to other methods, such as practical applications in flexible electronic papers, flat- spin casting, blade coating, and gravure printing, the panel displays, radio frequency identification (RFID) spray-coating process can realize a continuous film with- tags, and logic circus [1–7]. Up to now, several strategies out damaging the bottom layer of the device: just simply such as blade coating [6, 8, 9], ink-jet printing [10, 11], control the solvent content, droplet size, and solidifica- gravure printing [12, 13], and the recently emerged tion dynamics. spraying technologies [14–16] have been proved to be In the previous works, some novel manufacturing efficient methods for the fabrication of electronic de- methods have been applied to achieve high-performance vices. Among these methods, spray coating has been in- OFETs via spray coating. Khim et al. investigated the ef- vestigated intensively due to its unique advantage in fects of the droplet size on the performance of OFETs fab- manufacturing. Through the spray-coating method, vari- ricated using spray-printed organic semiconducting active ous materials with low solubility in less toxic solvent can layers [16]. Park et al. made an intensive study of solvent be applied due to the requirement of a low solution con- content by using a solvent-assisted post-treatment method centration [17]. Moreover, spray coating makes it pos- [20]. Meanwhile, substrate heating is demonstrated to be sible with higher speed of production and better an effective method in enhancing the crystallinity of semi- conductor films [21, 22]. For that, multiple research work has been developed. Sarcletti et al. researched the mutual * Correspondence: sdyman@uestc.edu.cn; jsyu@uestc.edu.cn State Key Laboratory of Electronic Thin Films and Integrated Devices influence of surface energy and substrate temperature on Zhongshan Branch Office, College of Electronic and Information Engineering, the mobility in organic semiconductors [23]. Also, Padma University of Electronic and Technology of China, Zhongshan Institute, et al. investigated the influence of substrate temperature on Zhongshan 528402, China State Key Laboratory of Electronic Thin Films and Integrated Devices, School the growth modes of copper phthalocyanine thin films at of Optoelectronic Information, University of Electronic Science and the dielectric/semiconductor interface [24]. Subsequently, Technology of China, Chengdu 610054, People’s Republic of China © The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Yang et al. Nanoscale Research Letters (2017) 12:503 Page 2 of 7 Mikayelyan et al. studied the effect of the substrate (PMMA) and 6,13-bis(triisopropyl-silylethynyl) pentacene temperature on the structure and morphology of the (TIPS-pentacene) are shown in Fig. 1(b) and (c), re- vacuum-evaporated films [25]. And the thermal an- spectively. The bottom gate top-contacted configur- nealing effect on the crack development also has been ation of OFETs with PMMA dielectric is illustrated in investigated [26]. Althoughalargenumberofstudies Fig. 1(d). The indium tin oxide (ITO)-coated glasses have focused on improving the intrinsic electrical were used as substrates and gate electrodes. The OFETs properties of device fabrication techniques, the influ- were fabricated in the following procedure. Firstly, the ence of in situ annealing treatment in the research ITO glasses placed on a polytetrafluoroethylene (PTFE) field of spray-coated OFETs has not received much holder were ultrasonic cleaned in detergent, acetone, attention. Meanwhile, the conventional solution deionized water, and isopropyl alcohol for 15 min each. process of OFETs usually calls for production inter- PMMA was dissolved in anisole with a concentration of ruptions and baking treatment as well as the process 100 mg/mL. Then, a 520-nm PMMA film, functioning being time consuming. Therefore, the development of as the gate dielectric, was spin coated on the substrates a novel annealing processing technique is thus a key and baked at 150 °C for 1 h in air to remove the solvent step towards utilizing the full potential of the spray residue. Thirdly, the 30-nm TIPS-pentacene active layer process. was deposited onto substrates placed on a hot plate via In this study, we introduced a simple in situ anneal- a spray-coating process with in situ annealing treat- ing treatment in fabricating high-performance OFETs, ment, and the concentration of the TIPS-pentacene so- and various substrate temperatures were applied in the lution was 3 mg/mL in dichlorobenzene. During our in situ annealing treatment. With the 60 °C in situ an- experiments, the speed of spray coating was 20 μL/s nealing treatment, the mobility of the OFET device sig- and the height (from the airbrush to the substrate) was nificantly enhanced from 0.056 to 0.191 cm /Vs, which 12 cm, and all the experiments were done at room was mainly attributed to the improved crystallization temperature (20 °C). Finally, a 50-nm-thick gold (Au) and ordered 6,13-bis(triisopropyl-silylethynyl) penta- was thermally deposited as the source and drain elec- cene (TIPS-pentacene) molecules. To elucidate the trodes on the TIPS-pentacene film by a shadow mask. mechanism of this performance enhancement, optical The thickness of the TIPS-pentacene film was charac- microscope, atomic force microscope (AFM), and X- terized by a step profiler. The pure PMMA layer and ray diffraction (XRD) were used to analyze the morph- the PMMA/TIPS-pentacene layer were measured sep- ology and crystallization of the TIPS-pentacene films. arately, and the thickness of the TIPS-pentacene film Our work demonstrates that with simple in situ anneal- can be calculated by subtraction. The device channel ing treatment, high-performance OFETs with an effi- width/length ratios are 100 (L = 100 μm, W =1cm). cient manufacturing process can be realized by The electrical characteristics of all devices were mea- carefully controlling the conditions of the in situ an- sured with a Keithley 4200 source meter (Cleveland, nealing method. OH, USA) in air atmosphere. The field-effect mobility (μ) was extracted in the saturation regime from the 1/2 highest slope of |I | vs. V plotsbyusing thefol- Methods DS GS lowing equation: The device fabrication apparatus is shown in Fig. 1(a). The chemical structures of poly(methyl methacrylate) I ¼ðÞ W =2L μCðÞ V −V DS i GS TH where I is the drain-source current, and L DS (100 μm) and W (1 cm) are the channel length and width, respectively. C is the capacitance per unit of the dielectric layer, and V and V are the gate GS TH voltage and the threshold voltage, respectively. The surface morphologies of the TIPS-pentacene were characterized with an optical microscope (U-MSSP4, OLYMPUS) and atomic force microscope (AFM) (MFP-3D-BIO, Asylum Research) in a tapping mode, and the structure characterization was taken by X-ray Fig. 1 a Schematic representation of OFET fabrication by spray powder diffraction (XRD, TD-3500, Dandong, China) coating. b, c Molecular structures of PMMA and TIPS-pentacene and with an accelerating voltage of 30 kV and an applied d device architecture of the OFET used in this study current of 20 mA. Yang et al. Nanoscale Research Letters (2017) 12:503 Page 3 of 7 Results and Discussion when the in situ annealing temperature increased to The OFETs based on 120 °C post-annealing treatment 120 °C, things get even worse, and an obvious decrease for 20 min were fabricated as device A, and those based of I from 12.1 to 0.17 μA and μ from 0.04 to on on in situ annealing treatment with the temperatures of 0.0005 cm /Vs was obtained. As a result, the perform- 60, 90, and 120 °C were fabricated as devices B, C, and ance of devices C and D was much worse than the post- D, respectively. The typical transfer characteristic, tested annealed device A. at a source-drain voltage (V )of −40 V and the gate The representative transfer and output plots of the DS voltage (V )of20to −40 V, was tested and presented OFETs prepared by spray-coating method with different GS in Fig. 2a. The output characteristics were tested under a annealing treatment are depicted in Fig. 2. It can be V of −40 V and a V of 0 to −40 V at a step of −10 V, clearly seen that device B demonstrates the highest elec- DS GS as shown in Fig. 2b–e. Several fundamental parameters, trical performance, including near zero threshold volt- including saturation current (I ), field-effect mobility ages and a narrow subthreshold swing. However, with on (μ), threshold voltage (V ), subthreshold swing (SS), and the increase of substrate temperature in the in situ an- on/off ratio (I /I ), which could be used to evaluate nealing treatment, an attenuation of electrical perform- on off the performance of OFET are summarized in Table 1. ance was revealed. The subthreshold swing exhibited an Not unexpectedly, all devices demonstrated typical p- obvious trend of increment along with the in situ an- type transistor characteristics. It can be clearly found nealing temperature, which implies a relatively high trap that the in situ annealing treatment has tremendous in- density at the interface between the dielectric and semi- fluence on the electronic properties of OFETs. Espe- conductor layer [27]. cially, with the 60 °C in situ annealing treatment, the To scrutinize the surface morphology of TIPS- electrical performance of OFET was successfully en- pentacene films, an optical microscope was employed. hanced, including a positive shift of V (from −1.7 to As depicted in Fig. 3, the diverse shapes and morpholo- TH −0.9 V), and an increasing μ (from 0.056 to 0.191 cm / gies of TIPS-pentacene films were obtained, and differ- Vs); the mobility of device B is almost fourfolds higher ent crystal grain sizes can be obviously seen from the than that of the post-annealed device A. However, when optical microscope. Large crystal grains are presented in applying with 90 °C in situ annealing treatment, an ex- Fig. 3a, b, and the TIPS-pentacene film with the 60 °C in tensive degradation of device performance appears along situ annealing treatment is much more uniform, and with the increasing substrate temperature, including a slender and longish grains are found to grow along the forward drift of V from −0.9 to 2.0 V, and a decreas- direction of the channel. It indicates a better organization TH ing μ ranged from 0.191 to 0.04 cm /Vs. Furthermore, of TIPS-pentacene molecules, resulting in the better Fig. 2 a Transfer curves of devices A–D. b–e Output curves of devices A, B, C, and D, respectively Yang et al. Nanoscale Research Letters (2017) 12:503 Page 4 of 7 Table 1 Electrical characteristics of the OFETs spray coated with post-annealing of 120 °C for 20 min and in situ annealing treatment of 60, 90, and 120 °C, respectively Annealing temperature I μ V I /I SS on TH on off −6 −2 2 (10 A) (10 cm /Vs) (V) (V/decade) Post-annealing 20.31 5.56 −1.7 1.62 × 10 1.40 60 °C 76.87 19.05 −0.9 3.47 × 10 1.25 (in situ annealing) 90 °C 12.10 4.00 2.0 3.00 × 10 3.81 (in situ annealing) 120 °C 0.17 0.05 −3.5 71 4.52 (in situ annealing) electrical performance of the OFET device. However, greatest morphology of device B can be ascribed not when the template temperature rises to 90 or 120 °C, cir- only to the proper annealing temperature but also to the cular morphology with small grains start to appear in de- favored condition for molecular self-organization. When vices C and D, as shown in Fig. 3c, d. According to the the OFETs are prepared at a relatively low substrate previous study, the alteration of TIPS-pentacene film temperature, gentle evaporation of the solvent can be morphologies would lead to the variation of the electrical maintained, leading to a reduced solvent evaporation properties of OFET devices [28–30]. rate, and the consecutive droplets kept the film wet. Ac- Furthermore, AFM was employed to characterize the tually, this modulation of substrate temperature directly morphologies of spray-coated TIPS-pentacene films. As influences the solvent evaporation rate. Lower annealing depicted in Fig. 4b, well-ordered TIPS-pentacene grains temperature allows TIPS-pentacene crystals to grow are formed on PMMA dielectric, whereas irregular crys- slowly with ordered molecules [33], while higher sub- tal grains with different shapes are shown in Fig. 4a, strate temperature contributes to quickly solidification, which corresponds well with the optical microscope im- without a relatively slow drying process of solvent [34]. ages in Fig. 3a and b. Interestingly, when the substrate Thus, a longer time was obtained for molecular self- temperature exceeded 60 °C, significant changes in the organization during the spray process, which is respon- TIPS-pentacene film morphology can be observed. Fig- sible for a higher degree of phase separation and a larger ure 4c, d show typical sprayed rounded morphology with domain size [33, 35, 36]. As a consequence, slender and a large density of small TIPS-pentacene grains, and these longish grains are formed, and the bridges for carrier grains exhibit microcrystalline morphology comprising transportation in the channel region can be built of many island clusters with different sizes as shown in through these long grains which are longer than the inserts. Additionally, with further increasing anneal- 110.8 μm [37]. ing temperature to 120 °C, a much smaller grain array is To further investigate the molecule orientation and formed resulting in sparse distribution with plentiful packing in the spray-coated TIPS-pentacene films, XRD grain boundaries to have a negative effect on carrier was introduced. As shown in Fig. 5, the individual transport [16, 31, 32]. Such results indicate that the an- traces exhibit a series of narrow Bragg peaks assignable nealing temperature can greatly affect the film-forming to the reflections (00l) of TIPS-pentacene [38], and the properties, leading to a significant difference in film density indicates that the substrate temperature will morphologies. dramatically affect the crystallinity of the TIPS- As we can see, the changes in substrate temperature pentacene molecules [39]. Compared to device A with lead to different morphologies and grain size. And the post-annealing treatment, device B has the strongest Fig. 3 Optical microscope images of spray-coated TIPS-pentacene layer. a Substrate temperature of room temperature followed by post-annealing at 120 °C for 20 min, b–d In situ annealing temperature of 60, 90, and 120 °C, respectively Yang et al. Nanoscale Research Letters (2017) 12:503 Page 5 of 7 Fig. 4 AFM height and 3D images of spray-coated TIPS-pentacene layer. a Substrate temperature of RT (followed by post-annealing at 120 °C 20 min). b–d In situ annealing temperatures of 60, 90, and 120 °C, respectively. Insets: high magnification AFM; the scan size bar of inserts is 1 μm peak intensity, which is consistent with the micro- with the processing condition of the active layer. With graphs of the TIPS-pentacene films, indicating that the template temperature of 60 °C, the mobility of the TIPS-pentacene deposited with 60 °C in situ an- OFETs fabricated by in situ annealing method increases nealing treatment yields the best crystallinity of TIPS- from 0.056 to 0.191 cm /Vs. The performance enhance- pentacene. When the substrate temperature increases ment was attributed to the higher crystallization and or- to 90 and 120 °C, an inferior order of TIPS- dered grains. This in situ annealing treatment of the pentacene was formed, which was responsible for the spray-coating method is expected to be an effective way decline in the device performance [40]. for the fabrication of high-performance OFETs as well as a high potential for low-cost manufacturing and ap- plication versatility. Conclusions In summary, we have fabricated and tested OFETs by Acknowledgements The authors gratefully acknowledge the financial support from the spray coating TIPS-pentacene with in situ annealing Foundation for Innovation Research Groups of National Natural Science treatment, and the surface morphologies and the Foundation of China (NSFC) (Grant No. 61421002), the Foundation of NSFC crystallization of the obtained film were investigated. (Grant No. 61675041), and Science & Technology Department of Sichuan Province (Grant Nos. 2016HH0027 and 2016FZ0100). The results show that the electrical performance of TIPS-pentacene-based OFETs has strong correlation Authors’ Contributions FY and XW did the most experiments in this work. FY and HF made the research plan, and they contributed to writing the manuscript. YT carried out the data analysis. All the authors read and approved the final manuscript. Authors’ Information Fuqiang Yang received his B.A. degree from Wuhan Textile University in 2015. He has been studying for his Master degree at State Key Laboratory of Electronic Thin Films and Integrated Devices (SKLETFID) and UESTC since 2015, where his main research interests are in OFETs and OFET-based sensors. Xiaolin Wang has been studying for his B.A. degree at the School of Optoelectronic Information at UESTC since 2016. He has been working as a student at SKLETFID since 2017, where his main research interests are in OFETs and OFET-based sensors. Huidong Fan received his B.A. degree from the Yingcai Experimental School at UESTC in 2013. He has been studying for his Ph.D. degree at the State Key Laboratory of Electronic Thin Films and Integrated Devices (SKLETFID) at UESTC since 2013, where his main research interests are in OFETs and OFET- based sensors. Ying Tang got her Ph.D. degree from the Graduate School of Physical Science and Technology at Lanzhou University in 2007. She is a postdoctor of SKLETFID at UESTC majoring in organic electronic materials and devices. Jianjun Yang got his Ph.D. degree from UESTC in 2002. Now, he is the Fig. 5 Normalized XRD spectra of spray-coated TIPS-pentacene films professor of the State Key Laboratory of Electronic Thin Films and Integrated with both post-annealing and in situ annealing treatment Devices Zhongshan Branch Office, College of Electronic and Information Yang et al. Nanoscale Research Letters (2017) 12:503 Page 6 of 7 Engineering, University of Electronic and Technology of China, Zhongshan based on electrospray-printed small-molecule/polymer semiconducting Institute, majoring in optoelectronic materials and devices. blends. J Mater Chem C 4(16):3499–3507 Junsheng Yu got his Ph.D. degree from the Graduate School of Bio- 16. Khim D, Baeg KJ, Yu BK, Kang SJ, Kang M, Chen Z, Facchetti A, Kim D, Noh Applications and System Engineering at Tokyo University of Agriculture and YY (2013) Spray-printed organic field-effect transistors and complementary Technology in 2001. He is the Professor of SKLETFID and UESTC majoring in inverters. J Mater Chem C 1(7):1500–1506 organic photoelectronic and electronic materials and devices. 17. Vak D, Kim SS, Jo J, Oh SH, Na SI, Kim J, Kim DY (2007) Fabrication of organic bulk heterojunction solar cells by a spray deposition method for low-cost power generation. Appl Phys Lett 91(8):081102 Competing Interests 18. Susanna G, Salamandra L, Brown TM, Di Carlo A, Brunetti F, Reale A (2011) The authors declare that they have no competing interests. Airbrush spray-coating of polymer bulk-heterojunction solar cells. Sol Energ Mat Sol C 95(7):1775–1778 19. Ma W, Yang C, Gong X, Lee K, Heeger AJ (2005) Thermally stable, efficient Publisher’sNote polymer solar cells with nanoscale control of the interpenetrating network Springer Nature remains neutral with regard to jurisdictional claims in morphology. Adv Funct Mater 15(10):1617–1622 published maps and institutional affiliations. 20. Park HY, Yang H, Choi SK, Jang SY (2011) Efficient solvent-assisted post- treatment for molecular rearrangement of sprayed polymer field-effect Received: 13 June 2017 Accepted: 16 July 2017 transistors. ACS Appl Mater Interfaces 4(1):214–221 21. 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Effect of In Situ Annealing Treatment on the Mobility and Morphology of TIPS-Pentacene-Based Organic Field-Effect Transistors

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Abstract

In this work, organic field-effect transistors (OFETs) with a bottom gate top contact structure were fabricated by using a spray-coating method, and the influence of in situ annealing treatment on the OFET performance was investigated. Compared to the conventional post-annealing method, the field-effect mobility of OFET with 60 °C in situ annealing treatment was enhanced nearly four times from 0.056 to 0.191 cm /Vs. The surface morphologies and the crystallization of TIPS-pentacene films were characterized by optical microscope, atomic force microscope, and X-ray diffraction. We found that the increased mobility was mainly attributed to the improved crystallization and highly ordered TIPS-pentacene molecules. Keywords: Organic field-effect transistors, In situ annealing treatment, Field-effect mobility, Morphology, TIPS-pentacene Background compatibility to various substrates [18], and the different Organic field-effect transistors (OFETs) have attracted shapes of film can be patterned through shadow masks considerable attention as a promising candidate for its [19]. Additionally, compared to other methods, such as practical applications in flexible electronic papers, flat- spin casting, blade coating, and gravure printing, the panel displays, radio frequency identification (RFID) spray-coating process can realize a continuous film with- tags, and logic circus [1–7]. Up to now, several strategies out damaging the bottom layer of the device: just simply such as blade coating [6, 8, 9], ink-jet printing [10, 11], control the solvent content, droplet size, and solidifica- gravure printing [12, 13], and the recently emerged tion dynamics. spraying technologies [14–16] have been proved to be In the previous works, some novel manufacturing efficient methods for the fabrication of electronic de- methods have been applied to achieve high-performance vices. Among these methods, spray coating has been in- OFETs via spray coating. Khim et al. investigated the ef- vestigated intensively due to its unique advantage in fects of the droplet size on the performance of OFETs fab- manufacturing. Through the spray-coating method, vari- ricated using spray-printed organic semiconducting active ous materials with low solubility in less toxic solvent can layers [16]. Park et al. made an intensive study of solvent be applied due to the requirement of a low solution con- content by using a solvent-assisted post-treatment method centration [17]. Moreover, spray coating makes it pos- [20]. Meanwhile, substrate heating is demonstrated to be sible with higher speed of production and better an effective method in enhancing the crystallinity of semi- conductor films [21, 22]. For that, multiple research work has been developed. Sarcletti et al. researched the mutual * Correspondence: sdyman@uestc.edu.cn; jsyu@uestc.edu.cn State Key Laboratory of Electronic Thin Films and Integrated Devices influence of surface energy and substrate temperature on Zhongshan Branch Office, College of Electronic and Information Engineering, the mobility in organic semiconductors [23]. Also, Padma University of Electronic and Technology of China, Zhongshan Institute, et al. investigated the influence of substrate temperature on Zhongshan 528402, China State Key Laboratory of Electronic Thin Films and Integrated Devices, School the growth modes of copper phthalocyanine thin films at of Optoelectronic Information, University of Electronic Science and the dielectric/semiconductor interface [24]. Subsequently, Technology of China, Chengdu 610054, People’s Republic of China © The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Yang et al. Nanoscale Research Letters (2017) 12:503 Page 2 of 7 Mikayelyan et al. studied the effect of the substrate (PMMA) and 6,13-bis(triisopropyl-silylethynyl) pentacene temperature on the structure and morphology of the (TIPS-pentacene) are shown in Fig. 1(b) and (c), re- vacuum-evaporated films [25]. And the thermal an- spectively. The bottom gate top-contacted configur- nealing effect on the crack development also has been ation of OFETs with PMMA dielectric is illustrated in investigated [26]. Althoughalargenumberofstudies Fig. 1(d). The indium tin oxide (ITO)-coated glasses have focused on improving the intrinsic electrical were used as substrates and gate electrodes. The OFETs properties of device fabrication techniques, the influ- were fabricated in the following procedure. Firstly, the ence of in situ annealing treatment in the research ITO glasses placed on a polytetrafluoroethylene (PTFE) field of spray-coated OFETs has not received much holder were ultrasonic cleaned in detergent, acetone, attention. Meanwhile, the conventional solution deionized water, and isopropyl alcohol for 15 min each. process of OFETs usually calls for production inter- PMMA was dissolved in anisole with a concentration of ruptions and baking treatment as well as the process 100 mg/mL. Then, a 520-nm PMMA film, functioning being time consuming. Therefore, the development of as the gate dielectric, was spin coated on the substrates a novel annealing processing technique is thus a key and baked at 150 °C for 1 h in air to remove the solvent step towards utilizing the full potential of the spray residue. Thirdly, the 30-nm TIPS-pentacene active layer process. was deposited onto substrates placed on a hot plate via In this study, we introduced a simple in situ anneal- a spray-coating process with in situ annealing treat- ing treatment in fabricating high-performance OFETs, ment, and the concentration of the TIPS-pentacene so- and various substrate temperatures were applied in the lution was 3 mg/mL in dichlorobenzene. During our in situ annealing treatment. With the 60 °C in situ an- experiments, the speed of spray coating was 20 μL/s nealing treatment, the mobility of the OFET device sig- and the height (from the airbrush to the substrate) was nificantly enhanced from 0.056 to 0.191 cm /Vs, which 12 cm, and all the experiments were done at room was mainly attributed to the improved crystallization temperature (20 °C). Finally, a 50-nm-thick gold (Au) and ordered 6,13-bis(triisopropyl-silylethynyl) penta- was thermally deposited as the source and drain elec- cene (TIPS-pentacene) molecules. To elucidate the trodes on the TIPS-pentacene film by a shadow mask. mechanism of this performance enhancement, optical The thickness of the TIPS-pentacene film was charac- microscope, atomic force microscope (AFM), and X- terized by a step profiler. The pure PMMA layer and ray diffraction (XRD) were used to analyze the morph- the PMMA/TIPS-pentacene layer were measured sep- ology and crystallization of the TIPS-pentacene films. arately, and the thickness of the TIPS-pentacene film Our work demonstrates that with simple in situ anneal- can be calculated by subtraction. The device channel ing treatment, high-performance OFETs with an effi- width/length ratios are 100 (L = 100 μm, W =1cm). cient manufacturing process can be realized by The electrical characteristics of all devices were mea- carefully controlling the conditions of the in situ an- sured with a Keithley 4200 source meter (Cleveland, nealing method. OH, USA) in air atmosphere. The field-effect mobility (μ) was extracted in the saturation regime from the 1/2 highest slope of |I | vs. V plotsbyusing thefol- Methods DS GS lowing equation: The device fabrication apparatus is shown in Fig. 1(a). The chemical structures of poly(methyl methacrylate) I ¼ðÞ W =2L μCðÞ V −V DS i GS TH where I is the drain-source current, and L DS (100 μm) and W (1 cm) are the channel length and width, respectively. C is the capacitance per unit of the dielectric layer, and V and V are the gate GS TH voltage and the threshold voltage, respectively. The surface morphologies of the TIPS-pentacene were characterized with an optical microscope (U-MSSP4, OLYMPUS) and atomic force microscope (AFM) (MFP-3D-BIO, Asylum Research) in a tapping mode, and the structure characterization was taken by X-ray Fig. 1 a Schematic representation of OFET fabrication by spray powder diffraction (XRD, TD-3500, Dandong, China) coating. b, c Molecular structures of PMMA and TIPS-pentacene and with an accelerating voltage of 30 kV and an applied d device architecture of the OFET used in this study current of 20 mA. Yang et al. Nanoscale Research Letters (2017) 12:503 Page 3 of 7 Results and Discussion when the in situ annealing temperature increased to The OFETs based on 120 °C post-annealing treatment 120 °C, things get even worse, and an obvious decrease for 20 min were fabricated as device A, and those based of I from 12.1 to 0.17 μA and μ from 0.04 to on on in situ annealing treatment with the temperatures of 0.0005 cm /Vs was obtained. As a result, the perform- 60, 90, and 120 °C were fabricated as devices B, C, and ance of devices C and D was much worse than the post- D, respectively. The typical transfer characteristic, tested annealed device A. at a source-drain voltage (V )of −40 V and the gate The representative transfer and output plots of the DS voltage (V )of20to −40 V, was tested and presented OFETs prepared by spray-coating method with different GS in Fig. 2a. The output characteristics were tested under a annealing treatment are depicted in Fig. 2. It can be V of −40 V and a V of 0 to −40 V at a step of −10 V, clearly seen that device B demonstrates the highest elec- DS GS as shown in Fig. 2b–e. Several fundamental parameters, trical performance, including near zero threshold volt- including saturation current (I ), field-effect mobility ages and a narrow subthreshold swing. However, with on (μ), threshold voltage (V ), subthreshold swing (SS), and the increase of substrate temperature in the in situ an- on/off ratio (I /I ), which could be used to evaluate nealing treatment, an attenuation of electrical perform- on off the performance of OFET are summarized in Table 1. ance was revealed. The subthreshold swing exhibited an Not unexpectedly, all devices demonstrated typical p- obvious trend of increment along with the in situ an- type transistor characteristics. It can be clearly found nealing temperature, which implies a relatively high trap that the in situ annealing treatment has tremendous in- density at the interface between the dielectric and semi- fluence on the electronic properties of OFETs. Espe- conductor layer [27]. cially, with the 60 °C in situ annealing treatment, the To scrutinize the surface morphology of TIPS- electrical performance of OFET was successfully en- pentacene films, an optical microscope was employed. hanced, including a positive shift of V (from −1.7 to As depicted in Fig. 3, the diverse shapes and morpholo- TH −0.9 V), and an increasing μ (from 0.056 to 0.191 cm / gies of TIPS-pentacene films were obtained, and differ- Vs); the mobility of device B is almost fourfolds higher ent crystal grain sizes can be obviously seen from the than that of the post-annealed device A. However, when optical microscope. Large crystal grains are presented in applying with 90 °C in situ annealing treatment, an ex- Fig. 3a, b, and the TIPS-pentacene film with the 60 °C in tensive degradation of device performance appears along situ annealing treatment is much more uniform, and with the increasing substrate temperature, including a slender and longish grains are found to grow along the forward drift of V from −0.9 to 2.0 V, and a decreas- direction of the channel. It indicates a better organization TH ing μ ranged from 0.191 to 0.04 cm /Vs. Furthermore, of TIPS-pentacene molecules, resulting in the better Fig. 2 a Transfer curves of devices A–D. b–e Output curves of devices A, B, C, and D, respectively Yang et al. Nanoscale Research Letters (2017) 12:503 Page 4 of 7 Table 1 Electrical characteristics of the OFETs spray coated with post-annealing of 120 °C for 20 min and in situ annealing treatment of 60, 90, and 120 °C, respectively Annealing temperature I μ V I /I SS on TH on off −6 −2 2 (10 A) (10 cm /Vs) (V) (V/decade) Post-annealing 20.31 5.56 −1.7 1.62 × 10 1.40 60 °C 76.87 19.05 −0.9 3.47 × 10 1.25 (in situ annealing) 90 °C 12.10 4.00 2.0 3.00 × 10 3.81 (in situ annealing) 120 °C 0.17 0.05 −3.5 71 4.52 (in situ annealing) electrical performance of the OFET device. However, greatest morphology of device B can be ascribed not when the template temperature rises to 90 or 120 °C, cir- only to the proper annealing temperature but also to the cular morphology with small grains start to appear in de- favored condition for molecular self-organization. When vices C and D, as shown in Fig. 3c, d. According to the the OFETs are prepared at a relatively low substrate previous study, the alteration of TIPS-pentacene film temperature, gentle evaporation of the solvent can be morphologies would lead to the variation of the electrical maintained, leading to a reduced solvent evaporation properties of OFET devices [28–30]. rate, and the consecutive droplets kept the film wet. Ac- Furthermore, AFM was employed to characterize the tually, this modulation of substrate temperature directly morphologies of spray-coated TIPS-pentacene films. As influences the solvent evaporation rate. Lower annealing depicted in Fig. 4b, well-ordered TIPS-pentacene grains temperature allows TIPS-pentacene crystals to grow are formed on PMMA dielectric, whereas irregular crys- slowly with ordered molecules [33], while higher sub- tal grains with different shapes are shown in Fig. 4a, strate temperature contributes to quickly solidification, which corresponds well with the optical microscope im- without a relatively slow drying process of solvent [34]. ages in Fig. 3a and b. Interestingly, when the substrate Thus, a longer time was obtained for molecular self- temperature exceeded 60 °C, significant changes in the organization during the spray process, which is respon- TIPS-pentacene film morphology can be observed. Fig- sible for a higher degree of phase separation and a larger ure 4c, d show typical sprayed rounded morphology with domain size [33, 35, 36]. As a consequence, slender and a large density of small TIPS-pentacene grains, and these longish grains are formed, and the bridges for carrier grains exhibit microcrystalline morphology comprising transportation in the channel region can be built of many island clusters with different sizes as shown in through these long grains which are longer than the inserts. Additionally, with further increasing anneal- 110.8 μm [37]. ing temperature to 120 °C, a much smaller grain array is To further investigate the molecule orientation and formed resulting in sparse distribution with plentiful packing in the spray-coated TIPS-pentacene films, XRD grain boundaries to have a negative effect on carrier was introduced. As shown in Fig. 5, the individual transport [16, 31, 32]. Such results indicate that the an- traces exhibit a series of narrow Bragg peaks assignable nealing temperature can greatly affect the film-forming to the reflections (00l) of TIPS-pentacene [38], and the properties, leading to a significant difference in film density indicates that the substrate temperature will morphologies. dramatically affect the crystallinity of the TIPS- As we can see, the changes in substrate temperature pentacene molecules [39]. Compared to device A with lead to different morphologies and grain size. And the post-annealing treatment, device B has the strongest Fig. 3 Optical microscope images of spray-coated TIPS-pentacene layer. a Substrate temperature of room temperature followed by post-annealing at 120 °C for 20 min, b–d In situ annealing temperature of 60, 90, and 120 °C, respectively Yang et al. Nanoscale Research Letters (2017) 12:503 Page 5 of 7 Fig. 4 AFM height and 3D images of spray-coated TIPS-pentacene layer. a Substrate temperature of RT (followed by post-annealing at 120 °C 20 min). b–d In situ annealing temperatures of 60, 90, and 120 °C, respectively. Insets: high magnification AFM; the scan size bar of inserts is 1 μm peak intensity, which is consistent with the micro- with the processing condition of the active layer. With graphs of the TIPS-pentacene films, indicating that the template temperature of 60 °C, the mobility of the TIPS-pentacene deposited with 60 °C in situ an- OFETs fabricated by in situ annealing method increases nealing treatment yields the best crystallinity of TIPS- from 0.056 to 0.191 cm /Vs. The performance enhance- pentacene. When the substrate temperature increases ment was attributed to the higher crystallization and or- to 90 and 120 °C, an inferior order of TIPS- dered grains. This in situ annealing treatment of the pentacene was formed, which was responsible for the spray-coating method is expected to be an effective way decline in the device performance [40]. for the fabrication of high-performance OFETs as well as a high potential for low-cost manufacturing and ap- plication versatility. Conclusions In summary, we have fabricated and tested OFETs by Acknowledgements The authors gratefully acknowledge the financial support from the spray coating TIPS-pentacene with in situ annealing Foundation for Innovation Research Groups of National Natural Science treatment, and the surface morphologies and the Foundation of China (NSFC) (Grant No. 61421002), the Foundation of NSFC crystallization of the obtained film were investigated. (Grant No. 61675041), and Science & Technology Department of Sichuan Province (Grant Nos. 2016HH0027 and 2016FZ0100). The results show that the electrical performance of TIPS-pentacene-based OFETs has strong correlation Authors’ Contributions FY and XW did the most experiments in this work. FY and HF made the research plan, and they contributed to writing the manuscript. YT carried out the data analysis. All the authors read and approved the final manuscript. Authors’ Information Fuqiang Yang received his B.A. degree from Wuhan Textile University in 2015. He has been studying for his Master degree at State Key Laboratory of Electronic Thin Films and Integrated Devices (SKLETFID) and UESTC since 2015, where his main research interests are in OFETs and OFET-based sensors. Xiaolin Wang has been studying for his B.A. degree at the School of Optoelectronic Information at UESTC since 2016. He has been working as a student at SKLETFID since 2017, where his main research interests are in OFETs and OFET-based sensors. Huidong Fan received his B.A. degree from the Yingcai Experimental School at UESTC in 2013. He has been studying for his Ph.D. degree at the State Key Laboratory of Electronic Thin Films and Integrated Devices (SKLETFID) at UESTC since 2013, where his main research interests are in OFETs and OFET- based sensors. Ying Tang got her Ph.D. degree from the Graduate School of Physical Science and Technology at Lanzhou University in 2007. She is a postdoctor of SKLETFID at UESTC majoring in organic electronic materials and devices. Jianjun Yang got his Ph.D. degree from UESTC in 2002. Now, he is the Fig. 5 Normalized XRD spectra of spray-coated TIPS-pentacene films professor of the State Key Laboratory of Electronic Thin Films and Integrated with both post-annealing and in situ annealing treatment Devices Zhongshan Branch Office, College of Electronic and Information Yang et al. Nanoscale Research Letters (2017) 12:503 Page 6 of 7 Engineering, University of Electronic and Technology of China, Zhongshan based on electrospray-printed small-molecule/polymer semiconducting Institute, majoring in optoelectronic materials and devices. blends. J Mater Chem C 4(16):3499–3507 Junsheng Yu got his Ph.D. degree from the Graduate School of Bio- 16. Khim D, Baeg KJ, Yu BK, Kang SJ, Kang M, Chen Z, Facchetti A, Kim D, Noh Applications and System Engineering at Tokyo University of Agriculture and YY (2013) Spray-printed organic field-effect transistors and complementary Technology in 2001. He is the Professor of SKLETFID and UESTC majoring in inverters. J Mater Chem C 1(7):1500–1506 organic photoelectronic and electronic materials and devices. 17. Vak D, Kim SS, Jo J, Oh SH, Na SI, Kim J, Kim DY (2007) Fabrication of organic bulk heterojunction solar cells by a spray deposition method for low-cost power generation. Appl Phys Lett 91(8):081102 Competing Interests 18. Susanna G, Salamandra L, Brown TM, Di Carlo A, Brunetti F, Reale A (2011) The authors declare that they have no competing interests. Airbrush spray-coating of polymer bulk-heterojunction solar cells. Sol Energ Mat Sol C 95(7):1775–1778 19. Ma W, Yang C, Gong X, Lee K, Heeger AJ (2005) Thermally stable, efficient Publisher’sNote polymer solar cells with nanoscale control of the interpenetrating network Springer Nature remains neutral with regard to jurisdictional claims in morphology. Adv Funct Mater 15(10):1617–1622 published maps and institutional affiliations. 20. Park HY, Yang H, Choi SK, Jang SY (2011) Efficient solvent-assisted post- treatment for molecular rearrangement of sprayed polymer field-effect Received: 13 June 2017 Accepted: 16 July 2017 transistors. ACS Appl Mater Interfaces 4(1):214–221 21. 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