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Feb 4, 2016 - Organic field-effect transistors (OFETs) are of interest because the ... polymer−insulator blends.18−23 By incorporating a high- per...
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High Mobility Organic Field-Effect Transistors from Majority Insulator Blends Michael J. Ford, Ming Wang, Shrayesh N. Patel, Hung Phan, Rachel A. Segalman, Thuc-Quyen Nguyen, and Guillermo C. Bazan Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b04774 • Publication Date (Web): 04 Feb 2016 Downloaded from http://pubs.acs.org on February 8, 2016

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High  Mobility  Organic  Field-­‐Effect  Transistors  from  Majority  Insula-­‐ tor  Blends     Michael J. Ford†‡¥, Ming Wang‡¥§, Shrayesh N. Patel¥€, Hung Phan‡¥§, Rachel A. Segalman†¥€, Thuc-Quyen Nguyen‡¥§, Guillermo C. Bazan†‡¥§ †

Materials Department, ‡Center for Polymers and Organic Solids, ¥Mitsubishi Chemical Center for Advanced Materials, §Department of Chemistry and Biochemistry, €Department of Chemical Engineering, University of California Santa Barbara, Santa Barbara, CA 93106, USA ABSTRACT: Donor-acceptor polymer semiconductors blended with commodity insulators for use in organic field-effect transistors (OFETs) could potentially provide high-mobility OFETs simultaneously with low consumption of the polymer semiconductor if phase separation is favorable for charge transport. Hole mobilities of field-effect transistors cast from a mixture of an insulating component (polystyrene) and the regioregular narrow band gap conjugated polymer PCDTPT are investigated. When deposited from solutions containing 10 wt. % PCDTPT solid concentration by weight atop nanogrooved dielectric surfaces, a mobility of 2.7±0.3 cm2 V-1 s-1 can be obtained.

Organic field-effect transistors (OFETs) are of interest since the solution processability of the semiconductor material raises the possibility of lower device fabrication costs.1-5 Substantial progress on molecular design, processing conditions, and device architecture have helped to achieve high mobility (µ).6-13 Further improvements in OFET processing are important considerations for practical implementation and widespread impact.14,15 One processing strategy for improving operational properties and lowering the cost of organic electronic devices has been to blend organic semiconductors with commodity insulating polymers.15-26 In addition to reducing materials costs, these commonly-available polymers have the potential to enhance environmental stability and improve mechanical properties when blended with the semiconductor component.15,16 As an example, mobilities of 0.4 cm2 V-1 s-1 were obtained for 5 wt. % poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2b]thiophene] blended with polystyrene (PS) and in the presence of TCNQ.20 Higher µ may still be possible. Donor-acceptor polymers, while well-studied and among the best performers in OPV and OFET applications, have not been fully explored within the context of polymer-insulator blends.18-23 By incorporating a highperforming donor-acceptor polymer into a majority-insulator blend, we expect to see high µ maintained at low polymer concentrations provided that the materials phase separate appropriately.

industrial processing demands.34 We demonstrate in this contribution that blade-coating can be used to fabricate aligned PCDTPTbased OFETs that incorporate an amorphous insulating material (PS). Of particular interest is that the resulting devices exhibit fairly constant hole µ after the addition of PS. Even for as low as 10 wt. % semiconductor one can obtain µ exceeding 1 cm2 V-1 s-1. The overall approach provides a potentially widely useful alternative route for producing high-µ, low-cost organic semiconductor devices. Figure 1. PCDTPT molecular structure PCDTPT was synthesized according to previous procedures.35 Films cast from solutions containing PCDTPT and PS mixed in varying weight ratios were deposited by bladecoating with a glass blade at two different processing conditions. The blade-coater is home-built on a hot plate so that speed and temperature can be tuned. Condition 1 is defined as a coating speed of 0.1 mm/s at 50 oC relative to Condition 2 at 1.2 mm/s and 100 oC. OFETs in the bottom-gate, bottom-contact device architecture (Si (500 µm) / DTS-treated SiO2 (300 nm)/Ni (5 nm)/Au (50 nm)/blend layer) were annealed under nitrogen at 200oC for 8 min. The µ was calculated using the saturation regime equation within voltage regimes only where saturation is observed (Figures S2 and S3). These are conservative estimates for µ in accordance with current recommended guidelines for µ determination in OFETs,5 particularly in view of the “double slope” behaviour observed in many high µ systems, including previous reports on PCDTPT.12,13,33 For further discussion on this matter refer to the Supporting Information.

Molecular order and alignment can mediate OFET charge carrier µ.24-29 Introducing “nanogrooves” (Figure S1), by scratching substrates with a diamond lapping film, was recently used to promote alignment and yield high-µ devices with regioregular poly[4-(4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b’]dithiophen2yl)-alt-[1,2,5]thiadiazolo[3,4-c]pyridine] (PCDTPT, Figure 1).12,13,33 These results required a slow-drying tunnel configuration. A more scalable technique like blade-coating would be amenable to Table 1. Device performance parameters for various processing conditions and blend compositions. Conditions Condition 1

µ without nanogrooves [cm2 V-1 s-1]

10 wt. %

25 wt. %

50 wt. %

75 wt. %

90 wt. %

100 wt. % PCDTPT

0.25±0.08

0.23±0.03

0.33±0.03

0.28±0.06

0.25±0.05

0.25±0.02

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µ with nanogrooves [cm2 V-1 s-1]

1.9±0.3

2.8±0.8

2.4±0.3

2.6±0.1

2.6±0.2

2.5±0.2

nanogrooved µ anisotropy [µpara/µperp]

9.5

14.7

11.4

11.3

11.3

11.4

µ without nanogrooves [cm2 V-1 s-1]

0.57±0.09

0.89±0.07

0.69±0.09

0.68±0.13

0.45±0.07

0.59±0.08

µ with nanogrooves [cm2 V-1 s-1]

2.7±0.4

4.2±0.6

4.2±0.4

5.5±0.3

5.3±0.3

6.1±0.6

nanogrooved µ anisotropy [µpara/µperp]

11.9

11.1

8.4

11.0

9.0

11.4

Figure 2a and Figure 2b provide µ as a function of PCDTPT:PS ratio for the two different coating conditions with and without nanogrooves. We first focus on devices cast onto substrates without nanogrooves. The average µ and standard deviation calculated from at least 3 devices for each blend composition cast under Condition 1 can be found in Table 1. For these data, the µ values from 10 wt. % PCDTPT devices (0.24±0.08 cm2 V-1 s-1) do not vary significantly from those obtained from neat PCDTPT devices (0.25±0.02 cm2 V-1 s-1). Processing with PS thus does not impact greatly the µ of PCDTPT. For Condition 2, again without nanogrooved substrates, a similar trend occurs. The µ observed from 10 wt. % PCDTPT devices (0.57±0.09 cm2 V-1 s-1) does not vary significantly from the µ obtained from pristine PCDTPT devices (0.59±0.08 cm2 V-1 s-1).35 Overall, a two-fold increase in µ was observed when modifying casting conditions from Condition 1 to Condition 2. That the higher blade-coating speed yields better performance is in agreement with previous studies that demonstrated how modifying solution shearing conditions can affect ordering.37 Specifically, the order parameter for PCDTPT improved as temperature and coating speed were increased, as determined by near edge X-ray absorption fine structure (NEXAFS).34 Moreover, that µ is about 0.6 cm2 V-1 s-1 for both neat PCDTPT devices and 10 wt. % PCDTPT devices indicates good interconnectivity of semiconductor domains within the blend film. We now focus the discussion on the impact of nanogrooved substrates, which promotes alignment of PCDTPT.12,13,33 Average µ and 95% confidence limits from at least 10 devices on multiple substrates for each blend composition and deposition condition are provided in Figure 2 and summarized in Table 1. The µ for each device is provided in Tables S1 and S2. From Table 1, examination of the performance of devices with and without nanogrooves highlights that the nanogrooves improve µ by ~4.5-11-fold for both deposition methods and all blend compositions. This improvement is noteworthy relative to previous reports that involve more time-consuming protocols.12,13,33 Blade-coating onto nanogrooved substrates was already demonstrated to be a viable method for aligning PCDTPT, as determined by NEXAFS studies; however, µ from these films has not been reported.34 When casting under Condition 1, µ was 1.9±0.2 cm2 V-1 s-1 for 10 wt. % PCDTPT devices and 2.5±0.2 cm2 V-1 s-1 for 100 wt. % PCDTPT devices. This corresponds to a ~1.3-fold decrease in µ upon dilution with PS. For Condition 2, µ was 2.7±0.3 cm2 V-1 s-1 for 10% PCDTPT devices and 6.1±0.4 cm2 V-1 s-1 for 100 wt. % PCDTPT devices. Thus, there appears to be a steeper µ dependence on composition for these nanogrooved, faster-casting-speed devices than for any other condition used in the study. However, with 25 wt. % PCDTPT devices, only a ~1.5-fold decrease was observed when compared to the 100 wt. % PCDTPT devices. Steeper declines have been observed in poly(3-hexylthiophene) upon dilution with PS.17 Although we do not believe it to be the case, if PS segregated to the SiO2 surface, its capacitance would need to be considered. For the sake of argument, if we assume even a

Figure 2. PCDTPT:PS blend µ for Condition 1 (a) and Condition 2 (b). thick 50 nm PS layer then µ would be 3.8 cm2 V-1 s-1 for 10 wt. % PCDTPT cast at Condition 2 on nanogrooved substrates. This would still prove composition-independent µ; however, our morphological data does not support this. The high µ maintained across all PCDTPT:PS composition once again indicates that PCDTPT maintains good interconnectivity to facilitate charge transport. The effect of the nanogrooved dielectric surface suggests vertical phase separation whereby PCDTPT preferentially selfassembles near the dielectric. From a practical perspective, these results demonstrate blade-coating as a facile method of alignment and coating donor-acceptor polymer blend films. The morphology of the blends was investigated by optical and atomic force microscopies (Figure 3). Optical microscopy was used to visualize the macroscopic features after annealing at 200oC. Micrographs for blend films deposited under Condition 1 atop nanogrooves are shown in Figure 3a-c. In these images, the bare substrate appears dark purple, while PCDTPT thin films are blue-

green (Figure S4.) Attempts to cast films of PS on the DTStreated SiO2 for comparison show discontinuous films with spheri-

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cal globules that are characteristic of poor substrate interactions and dewetting (Figure S4). For 10 wt. % PCDTPT films, semitransparent globules are visible, which we attribute to PS. Smaller PS domains are visible even in 50 wt. % films. PS dewetting is wellknown to occur upon annealing at high temperatures on silicon substrates38, and these films were annealed at 200oC. For comparison, a micrograph of a 10 wt. % PCDTPT film cast at Condition 2 is shown in Figure S5 along with micrographs of the 10 wt. % PCDTPT film before and after annealing, which further demonstrate PS dewetting. The dewetting of PS serendipitously enables AFM investigation of aligned substructures that we assign to PCDTPT. We now examine the impact of nanogrooves. Details for the scratching procedure and AFM of the resulting scratched dielectric can be found in the SI. Previous work on neat PCDTPT by NEXAFS analysis has shown polymer alignment adjacent to the nanogrooves. For complete details see Ref. 34. Figure 3d-f shows the topographic AFM images for 90 wt. %, 50 wt. % and 10 wt. % PCDTPT in PS prepared under Condition 1 onto nanogrooved substrates. AFM imaging locations were chosen to avoid PS globules, as determined from features observed via optical microscopy. Aligned and interconnected fibers are observed in the case of the 10 wt% PCDTPT film, where apparently the majority of the PS has dewetted from the surface. The situation for the 50 wt. % and 90 wt. % is much less obvious, and it is less clear what can be assigned to either of the two components. AFM of 10 wt. % and pristine PCDTPT, annealed and unannealed, are consistent with PS segregating at the top surface (Figure S5.) Since fiber alignment was

dicular to the nanogrooves due to its relevance to alignment characterization.32 Mobility anisotropy is not observed for devices cast onto unmodified substrates (Figure S6). Tables S1 and S2 list the µ for all parallel and perpendicular devices. The µ anisotropy was found to range from ~8-14 in favor of the nanogroove direction (Table 1). These data suggest that the nanogrooves give rise to aligned polymer domains in which the polymer backbones are oriented parallel to their direction. Indeed, aligned polymer fibers can be observed in Figure 3f for the 10 wt. % PCDTPT film. These morphologies are akin to previously observed films prepared on nanogrooved substrates by slower casting techniques.12,13,33 Such a significant difference between parallel and perpendicular µ suggests that PCDTPT interacts strongly with the nanogrooves, indicating a preference of the semiconductor to the dielectric when compared to PS. If PS was preferentially accumulated near the grooved dielectric, alignment would be obstructed and µ would be dependent on the composition of the blend. Since our annealing temperature of 200 oC is above the glass transition temperature of PS (Figure S7) and microscopy images (Figure 3a-c) indicate PS dewetting, it was considered relevant to investigate whether annealing above these thermal transitions was necessary for favourable phase separation. Comparative µ values between a 50 wt. % blend device and a pristine PCDTPT device cast at Condition 1 were used in this inquiry. Both the 50 wt. % and pristine PCDTPT devices exhibited a similar upward trend in µ through the different annealing temperatures (Figure S8). The average µ, as measured for 3 as-cast devices, was Figure 3. Optical (a-c) and atomic force (d-f) microscopy images of 90 wt. % PCDTPT (a, d), 50 wt. % PCDTPT (b, e), and 10 wt. % PCDTPT (c, f). Optical micrographs are 280 µm wide. The scale bar on AFM images represents 500 nm. 0.24 cm2 V-1 s-1 (100 wt. %) and 0.30 cm2 V-1 s-1 (50 wt. %). Upon annealing to 250oC in steps of 50 oC, the performance improved to 3.2 cm2 V-1 s-1 (100 wt. %) and 2.8 cm2 V-1 s-1 (50 wt. %). This trend suggests that annealing may not be critical to phase separation in this case. The upward trend is consistent with the conclusion that annealing can improve the order within PCDTPT12 thereby improving µ, which occurs similarly in both cases.

observed by AFM for 10 wt. % PCDTPT, the anisotropy of the µ was explored by calculating the ratio of the µ parallel and perpen-

Grazing incident wide-angle x-ray scattering (GIWAXS) was used to probe the crystallinity of the films.39 It is important to note that at the incident angle chosen, GIWAXS probes the bulk of the sample, whereas charge transport in OFETs is typically only relevant in the first few monolayers nearest to the gate dielectric. Figure 4 shows the in-plane GIWAXS data for various PCDTPT:PS wt. %. PS does not disrupt the π-stacking peak, which was measured at ~0.35 nm for all compositions.34 The out-of-plane images (Figure S9) show third-order alkyl stacking peaks even in the 25 wt. % blend. High order peaks in blend systems have been previously associated with semiconductor solidification favorable for charge transport.16 X-ray scattering results indicate that PCDTPT retains a similar crystalline state in the blend film as in the pristine film, and is consistent with a strong driving force for phase separation.

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Figure 4. In-plane GIWAXS of PCDTPT:PS blends. To conclude, we have successfully fabricated OFETs that contain mostly an insulator, namely PS, and a donor-acceptor semiconductor. Composition-independent µ   was attributed to preferred interactions between PCDTPT and the dielectric layer, as evidenced by 1) dewetting of PS after annealing, 2) improved µ upon addition of nanogrooves on the dielectric surface, and 3) composition-independent µ anisotropy. All of these factors suggest that PCDTPT interacts more strongly with the dielectric surface relative to PS. Mobilities as high as 2.7 cm2 V-1 s-1 for blends that contained 90 wt. % PS were observed. This value is among the highest reported mobilities for majority-insulator blend systems, comparable to blends that are used to fabricate single crystal-like OFETs.40 Film deposition involves the blade-coating technique, which is operationally simple to use, wastes less material than spin coating, and is faster than capillary-driven film formation in a constrained environment.33 The thin film morphology probed using AFM and GIWAXS, in conjunction with the mobilities attained, also suggests that the semiconducting polymer phase separates to form connected domains that support charge carrier transport. These results have the potential to enable a novel route for the fabrication of solution-processable high-µ OFETs that is economically viable.

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Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515.  

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ASSOCIATED  CONTENT     Supporting Information Contains detailed experimental procedures, current-voltage characteristics, additional microscopy data, profilometery data, differential scanning calorimetry data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR  INFORMATION  

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Corresponding  Author   G.  C.  Bazan.  E-­‐mail:  [email protected]   Funding  Sources   This work is supported by Mitsubishi Chemical Center for Advanced Materials.

Notes  

The authors declare no competing financial interest

ACKNOWLEDGMENT     Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of

(12) Tseng, H.-R.; Ying, L.; Hsu, B. B. Y.; Perez, L. A.; Takacs, C. J.; Bazan, G. C.; Heeger, A. J. High Mobility Field Effect Transistors Based On Macroscopically Oriented Regioregular Copolymers. Nano Lett. 2012, 12, 6353–6357. (13) Tseng, H.-R.; Phan, H.; Luo, C.; Wang, M.; Perez, L. A.; Patel, S. N.; Ying, L.; Kramer, E. J.; Nguyen, T.-Q.; Bazan, G. C.; Heeger, A. J. High-Mobility Field-Effect Transistors Fabricated With Macroscopic Aligned Semiconducting Polymers. Adv. Mater. 2014, 26, 2993–2998.

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