Improving Charge Mobility of Polymer Transistors by Judicious Choice

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Improving Charge Mobility of Polymer Transistors by Judicious Choice of the Molecular Weight of Insulating Polymer Additive Huihuang Yang, Guocheng Zhang, Jie Zhu, Weixin He, Shuqiong Lan, Lei Liao, Huipeng Chen, and Tailiang Guo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07000 • Publication Date (Web): 21 Jul 2016 Downloaded from http://pubs.acs.org on July 24, 2016

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The Journal of Physical Chemistry

Improving Charge Mobility of Polymer Transistors by Judicious Choice of the Molecular Weight of Insulating Polymer Additive 1 Huihuang Yang, Guocheng Zhang,1,2 Jie Zhu,1Weixin He,1Shuqiong Lan,1 Lei Liao,3 Huipeng Chen,1* Tailiang Guo1* 1

Institute of Optoelectronic Display, National & Local United Engineering Lab of Flat Panel Display Technology, Fuzhou University, Fuzhou 350002, China Email: [email protected]; [email protected] Tel: 86-13107938378 2 College of Information Science and Engineering, Fujian University of Technology, Fuzhou 350108, China 3 Key Laboratory of Artificial Micro- and Nano-Structures, Ministry of Education, Department of Physics, Wuhan University, Wuhan 430072, China

Abstract In this work, we have carefully examined the morphology of semiconducting polymer: insulating polymer blends, which were deposited from inkjet printing. We attempted to study the impact of molecular weight (MW) of insulating polymer on the nano-scale morphology and function of the blends. The morphology of all the inkjet-printed samples are characterized by small angle neutron scattering (SANS), grazing incidence x-ray diffraction (GIXD), and atomic force microscopy (AFM). The SANS results shows that the domain size of the blends increases by increasing MW of insulating polymer, while the domain purity reaches the maximum with proper molecular weight of insulating polymer.

AFM images show that the connectivity of

semiconducting polymer domains is disrupted with addition of polystyrene (PS) with low molecular weight (Mw=2.5K and 20K), while well interconnected domains are observed with addition of PS with high molecular weight (Mw=182K and 2000K). GIXD results indicate that the π-π stacking distance of semiconducting polymer can be shortened with addition of PS and decreases with an increase of PS molecular 1

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weight from 2.5K to 182K. Further increasing molecular weight of PS to 2000K, results in very weak π-π stacking ordering. This work demonstrates that the domain purity, connectivity of semiconducting polymer domains, molecular packing are crucial for the charge transport.

The judicious choice of the MW of insulating

polymer could carefully control the nanoscale morphology of semiconducting polymer: insulating polymer blends which could provide blend morphology with high domain purity, well connected domains along with reduced π-π stacking distance, all of which facilitates charge transport, resulting in a significant improvement of charge mobility. Introduction Organic field effect transistor (OFET) has attracted much attention in printed thin film transistors (TFT). It has unique advantages including low-cost deposition, low-power consumption light weight, solution fabrication and large-area coverage.1,2 However, most reports in OFET are based on the single cell device from spin coating technique. For commercialization, patterned TFT arrays are usually required. Among the printing techniques, inkjet printing is an ideal solution for the fabrication of TFT arrays from solution. 3 - 5

However, the crystalline microstructure and molecular

ordering usually vary with film formation conditions.6,7 Unfortunately, little effort has been done for the understanding and optimization of the morphology of inkjet-printed TFT active layer. Meanwhile, many groups have reported that carrier mobility of organic semiconductors can be improved by blending with insulating polymer 2


8-18

The

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improved mobility has been ascribed with improved polymer ordering, crystalline orientation, vertical phase segregation, and connectivity of semiconductor phase. 15, 19 , 20

Most efforts have been made to investigate the effect of the type and

concentration of insulating polymers on the morphology and function of TFT device, while the effect of the molecular weight of insulating polymers is still not clear. Moreover, the viscosity of the ink of active layer, which is an important parameter of inkjet printing, can be well modified by the molecular weight of insulating polymers. Here we reported the impact of insulating polymer on the nanoscale morphology and TFT performance of active layer, especially the effect molecular weight of insulating polymers. The mobility can be significantly improved with addition of insulating polymer with proper molecular weight. Small angle neutron scattering (SANS) was used to provide detailed information about the morphology of the blended samples. Due to the significant contrast between protonated polymer and deuterated polymers, neutron scattering could provide unique insight about the nanoscale morphology of binary polymer blends. 21 , 22 It has been a powerful technique in the understanding of the nanoscale morphology and miscibility of conjugated polymer: fullerene blends in organic photovoltaics.23-26 The molecular packing of semiconducting polymer is examined by GIXD. The results provide the intricate relationship between molecular weight of insulating polymer, morphology, and device performance for inkjet-printed polymer blends. Experimental Section

3



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Materials:

poly[2,5-bis(alkyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-alt-5,5´-

di(thiophen-2-yl)-2,2´-(E)-2-(2-(thiophen-2-yl)vinyl)-thiophene](PDVT-8) (Mw=50K, PDI=2.4) was purchased from1-Materials,

27

Deuterated polystyrene(PS) and

protonated polystyrene with different molecular weight were purchased from Polymer Source and Fluka, respectively. The chemical structure of PDVT-8 is presented in Figure 1a. The abbreviations of samples are shown in Table 1. To fabricate the PDVT-8:PS inks, PDVT-8 (2 mg/ml) and PS (0.6 mg/ml) were co-dissolved in chlorobenzene. The semiconducting layers were deposited by inkjet printing. The obtained films were then annealed at 140 °C for 10 mins, which is above the glass transition temperature (Tg) of PS (～100°C) Inkjet Printing: The active layer was inkjet-printed with Microfab, Jetlab II inkjet printing system with a 60 µm diameter nozzle. The substrate condition and the status of in-flight drops were monitor with an optical module. Controlled lines and droplets are deposited by tuning the droplet size, driving voltage, printing speed, and substrate temperature.

A droplet size of 200pl, driving voltage of 80V, frequency of 100Hz

with 80°C substrate temperature were selected to obtain stable and uniform droplet ejections. Transistor Fabrication: A bottom contact OFET were fabricated as follows: a heavily-doped silicon wafer with an oxide thickness of 300 nm was selected as the substrate. The substrates were first washed with DI water, acetone and isopropanol, respectively. It was then dried at 80°C for 15 minutes. This was followed by treating with UV Ozone for 20 mins before treating with 10 mM solution of trichloroethylene 4


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in toluene at 60°C for 1h. After rinsing with toluene, acetone and isopropanol, it was dried with N2 flow. The semiconductor layer was formed on the substrates by inkjet printing. Finally, the Au source and drain electrodes were thermally evaporated to form bottom gate/top contact OFETs. The channel length and width are 30 µm and 500 µm, respectively. The image of the device is shown in Figure 1b. The current-voltage characteristics of the OFET were carried out using a parameter analyzer (Keithley 4200) under ambient condition. Morphology Characterization: The small angle neutron scattering (SANS) data was collected on the beamline CG2 at Oak Ridge National Lab. The scattering data were further corrected for scattering from detector dark current, the empty cell, and detector sensitivity. The final data were obtained after normalizing the corrected data using a Porasil-A standard. For the SANS experiments, deuterated polystyrene (d8-PS) was selected for a better contrast. The scattering length density (SLD) of d8-PS and PDVT-8 are 6.3 × 10-6 Å-2 and 0.7 × 10-6 Å-2, respectively.28,29 The GIXD data were obtained at BL14B1, Shanghai Synchrotron Radiation Facility. The incidence angle was fixed at 0.3° for all the samples. All the measurements were performed at room temperature. AFM (Bruker NanoScope V) was employed to examine the surface morphology. Results and Discussion The evolution of the nanoscale morphology of the PDVT-8: PS samples were examined by SANS. The SANS results of all of the inkjet-printed samples are presented in Figure 2a. The SANS curves were found to be significantly varied with 5



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PS molecular weights. To verify the miscibility of the polymer blends, the SANS curves were first fitted to Ornstein-Zernike equation, which is ascribed for miscible blends. Poor fits and negative intercepts were obtained with the Ornstein-Zernike equation, indicating that PDVT-8 and PS are not fully miscible in all the samples. The scattering data were than fitted to the Debye-Bueche equation30,31 I(Q) = Aξ3 / (1+ Q2ξ2)2

(1)

where A is scale factor and ξ is the correlation length , which is the average domain size of the PDVT-8-rich phase and the PS-rich phase. The Debye-Bueche describes a two-phase system with randomly distributed domains. As shown in Figure 2, the scattering curves were found to be well fitted with Debye-Bueche equation, and correlation lengths ξ are listed in Table 2. ξ increases from 108 Å to 815 Å with an increase of molecular weight of PS. The correlation length is dominated by the phase with small volume fraction, which is PS-rich phase in this case. An increase of PS molecular weight (chain length) results in a larger PS-rich domain size and subsequent correlation length. Moreover, the detailed information about the phase separation in the PDVT-8:PS blends can be obtained with a further analysis of the SANS results. The scale factor A can be expressed by30,32 A= 8πΦ1Φ2(b1-b2)2

(2)

where Φ1 and Φ2 are the volume fraction of phase 1 and phase 2, while b1 and b2 are the scattering length density (SLD) of phase 1 and phase 2. The scale factor of all the 6


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PDVT-8:PS samples those were determined from eq.1 are listed in Table 2. The scale factor varies from 0.0034 to 0.0076 cm-1Å-3. Moreover, according to eq. 2, the scale factor should equal to 0.017 cm-1Å-3 if PDVT-8 and PS are fully phase separated. The scale factor of all the PDVT-8:PS blends are much lower than this value, which indicates partial miscible blends of PDVT-8:PS. As the scale factor is proportional to the square of SLD contrast, a higher scale factor value implies a larger phase-separated system (higher domain purity). The highest scale factor value is found in PDVT-8:PS3 blend, which indicates that the largest phase separation occurred in this blend. More details about the nanoscale morphology can be obtained by fitting the scattering data to the Teubner-Strey (TS) model.33 It is commonly used to describe the two-phase systems. 34-36 The Teubner-Strey model was derived from Landau theory and the dependence of scattering intensity on q is given by: I(q)=ୟ

ଵ

మ ర మ ାୡభ ୯ ାୡమ ୯

+ Iୠ

(3)

where Ib is the incoherent scattering. The fitting parameters a2, c1, and c2 are related to correlation length (ξTS) and the repeating distance of two domains (d): ଵ ୟ

ଵ/ଶ

ξTS= ൤ଶ ቀୡమ ቁ మ

ଵ ୟమ ଵ/ଶ

d= 2π ൤ଶ ቀୡ ቁ మ

ୡ

+ ସୡభ ൨

ିଵ/ଶ

మ

ୡభ

− ସୡ ൨

ିଵ/ଶ

మ

(4)

(5)

The correlation length is associated with average domain size of the PDVT-8-rich phase and the PS-rich phase, whereas the repeating distance between two domains is associated with the length scale of PDVT-8 rich phase. The fitting to the TS model are 7



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presented in Figure 2b, and correlation lengths ξTS and the repeating distance of two domains d are listed in Table 2. ξTS remains consistent from both Debye-Bueche model and Teubner-Strey model. An increase of the length scale of PDVT-8 rich phase is found with an increase with molecular weight of PS. The qualitative and quantitative analysis of the SANS results offers unique insight into the domain purity and structure of these PDVT-8:PS mixtures. An increase of domain size and the repeating distance of two domains have been found with an increase of MW of PS. The domain purity first increases and then decreases with an increasing of molecular weight of PS. The highest degree of domain purity is found in PDVT-8:PS3 blend. According to Flory-Huggins theory, mixing PDVT-8 with a higher MW PS results in a lower entropy of mixing, which increases the free energy of mixing. This should result in a decrease in PDVT-8 miscibility with an increase in PS MW.

The domain purity follows the Flory-Huggins theory and is

driven by the thermodynamic force with PS MW from 2.5K to 183K, which results in an increase of domain purity with an increase of MW. However, with further increase of PS molecular weight from 183K to 2000K, a decrease of domain purity is observed. It should be noted here the morphology of PDVT-8:PS blend from inkjet printing is not at equilibrium status. The morphology of blends is also affected by the chain conformation of PDVT-8 and PS in solution and the drying process. An increase of PS molecular weight would cause enhanced chain entanglement between PS and PDVT-8 in solution, which would inhibit the phase separation of PS and PDVT-8 during drying process, resulting in a decrease of domain purity. It thus indicates that 8


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the resultant domain purity of active layer is dominated by the chain conformation PDVT-8 and PS in solution for PDVT-8: PS4 blend. A closer view of the surface morphology for all the samples was obtained by tapping mode atomic force microscopy (AFM) for PDVT-8: PS blends with different PS molecular weight, which is shown in Figure 3. An increase of domain sizes can be observed with an increase of PS MW, which is consistent with SANS results. The root-mean-square roughness increased with PS MW as well, with values of 3.56, 3.74, 6.10 and 12.4, respectively. Moreover, the interconnectivity of domains is disrupted with addition of PS with low MW (PS1 and PS2), while well interconnected domains were observed with addition of PS with high MW PS (PS3 and PS4). Therefore, the AFM images further verify the SANS results that the domain size increases with increasing molecular weight of PS. Moreover, the connectivity of domains is found to be disrupted with addition of PS1 (2.5K) and PS2 (20K), which is associated with the less phase-separation and larger interfacial area between the two phases. The larger interfacial area in PDVT-8:PS1 and PDVT-8:PS2 blends, is associated with smaller domain sizes in these two blends. The molecular packing of semiconducting polymer was examined by grazing incidence x-ray diffraction (GIXD). Figure 4a shows the out-of-plane x-ray diffraction profiles extracted from GIXD. As shown in Figure 4a, a (100) peak at q ≈ 2.7 nm-1 was presented in all the curves, which is ascribed to the lamellar distance of PDVT-8 with two alkyl side chains.27 Figure 4b shows the in-plane x-ray diffraction profiles, where the (100) peak was not observed. It implies that the PDVT-8 9



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molecules would favor having on edge-on structure on the substrate for all the samples, which facilitates charge transport. Moreover, the (010) peak at q around 16.2 nm-1 that is associated with π-π stacking of PDVT-8 molecules was observed for all the samples except PDVT-8:PS4 sample, in which this peak is very weak. The π-π stacking distance calculated from (010) peak was 3.87, 3.85, 3.82, and 3.80 Å for PDVT-8, PDVT-8:PS1, PDVT-8:PS2, and PDVT-8:PS3, respectively. The π-π stacking distance became shorter with addition of PS and decreased with an increase of PS molecular from 2.5K to 200K. The decrease of π-π stacking distance with addition of insulating polymer has been reported by several groups which is associated with an extended intra-chain conjugation and better planarity by blending with insulating polymer.37-39 The GIXD results therefore show that the π-π stacking distance of PDVT-8 molecule was shorten with addition of PS and decreased with an increase of PS molecular weight from 2.5K to 183K, which is associated with domain purity. Lower domain purity indicates more PS presented in PDVT-8 rich phase, which would enhance the molecular disordering of PDVT-8. Moreover, the π-π stacking peak vanishes with addition of PS with molecular weight of 2000K. It indicates that long PS molecular chain enhanced the entanglement between PS and PDVT-8 chains, which disrupted the PDVT-8 molecular ordering, resulting in very weak π-π stacking ordering. Hence, it clearly demonstrate that PS molecular weight significantly impacted the morphology of PDVT-8:PS blends. To directly correlate molecular weight to 10


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OFET properties, bottom gate/top contact OFETs were fabricated. Figure 5 presents typical transfer curves for the inkjet-printed devices. The filed effect mobility (µ) in the saturation region was calculated by ID =

µC iW 2L

(VG − Vth ) 2

(6)

Where the Ci is the capacitance of the dielectric layer per unit area, VG is the gate-source voltage, and Vth is the threshold voltage. The results were summarized in Table 3. The charge mobility of devices decreases with addition of PS with low molecular weight (PS1 and PS2) while a higher hole mobility was found with addition of high molecular weight PS (PS3 and PS4). The maximum charge mobility (0.58 cm2V-1s-1) was obtained in PDVT-8:PS3 device, which is about five times of that of PDVT-8 device (0.12 cm2V-1s-1). The properties of FETs can be further improved by blending with an insulated polymer and morphology of the active layer has been found to dramatically impact the performance of the resultant devices. Unfortunately, less effort has been made on the impact of the MW of insulating polymer on the morphology and device performance of semiconducting polymer: insulating polymer blends. The results presented above clearly show that the molecular weight of PS dramatically impacts the nanoscale morphology, domain purity, connectivity of domains, molecular packing and FET properties of PDVT-8:PS blends. These morphological alterations are correlated to the charge transport and then device performance. PDVT-8:PS1 and PDVT-8:PS2 blends have smaller domains along with less domain purity, which disrupts the 11



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connectivity of domains, inhibiting charge transport. Meanwhile, the π-π stacking distance decreases with addition of PS1 and PS2, which improved charge transport. The resultant hole mobility is found to decrease with addition of PS1 and PS2, which indicates that the charge mobility is dominated by the connectivity of domains. PDVT-8:PS3 blend has better phase separation, good connectivity between domains, shorter π-π stacking distance than that of neat PDVT-8, all of which facilitates charge transport, providing a hole mobility that is about five times of that of neat PDVT-8. Further increasing the PS molecular weight from 183K to 2000K, causes a decrease of domain purity and an increase of domain sizes. The domains remains connected in PDVT-8:PS4 blends. The decrease of domain purity is associated with the enhanced entanglement between PDVT-8 and PS chains in solution due to the long PS chain length. The enhanced entanglement also affects the molecular packing PDVT-8, which results in the absence of π-π stacking peak and then a decrease of hole mobility with an increase of PS molecular weight from 183K to 2000K. Due to the weak semiconducting-insulating domain interaction, the presence of insulating polymer could efficiently facilitate the intra-phase carrier transport in the organic semiconducting domain, resulting in improved charge mobility.40,41 However, the presence of insulating polymer would also affect the molecular packing, purity and connectivity of semiconducting polymer domains, which might inhibit the charge transport. Therefore, a judicious choice of the MW of insulating polymer is required to carefully control the morphology of semiconducting polymer: insulating polymer blends which would further improve the charge mobility. 12


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Conclusion The results presented in this manuscript provide information about the effect of the molecular weight of PS on the domain size, domain purity, connectivity of domains, molecular packing of PDVT-8 and FET properties in PDVT-8:PS mixtures from inkjet printing. The charge mobility of PDVT-8:PS1 (Mw=2.5K) and PDVT-8:PS2 (Mw=20K) blends is found to be dominated by the connectivity of PDVT-8 domains. Higher domain purity, good connectivity between domains, and smaller π-π stacking distance than that of neat PDVT-8 are observed in PDVT-8:PS3 (Mw=183K) blends, all of which facilitates charge transport, giving a hole mobility that is five times of that of neat PDVT-8. Further increasing PS molecular weight from 183K to 2000K, results in very weak π-π stacking ordering, which decreases the hole mobility from 0.58 to 0.16 cm2V-1s-1, although the domains are well connected. This work demonstrates that the judicious choice of the molecular weight of insulating polymer could carefully control the morphology of semiconducting polymer: insulating polymer mixtures which would further improve the charge mobility. Author Information Corresponding Authors *Email: [email protected]; [email protected] Note The authors declare no competing financial interest. Acknowledgements 13



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The authors acknowledge financial support from National Natural Science Foundation of China (51503039). The research at ORNL’s HFIR was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, US Department of Energy. The GIXD was granted by BL14B1 station of Shanghai Synchrotron Radiation Facility. We gratefully thanked the staff members of BL14B1 for their help with experiments and data reduction. Reference

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(20) Kwon, S.; Yu, K.; Kweon, K.; Kim, G.; Kim, J.; Kim, H.; Jo, Y. R.; Kim, B. J.; Kim, J.; Lee, S. H.; et al. Template-Mediated Nano-Crystallite Networks in Semiconducting Polymers Nat. Commun. 2014, 5, 4183. (21) Russell, T. P.; ITO, H.; Wignall, G. D. Neutron And X-Ray-Scatering Studies on Semicrystalling Polymer Blends Macromolecules, 1988, 21, 1703-1709. (22) Hahn, K.; Schmitt, B. J.; Kirschey, M.; Kirste, R. G.; Sailie, H.; Schmittstrecker, S.

Structure

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Thermodynamics

in

Polymer

Blends-neutron-Scattering

Measurements on Blends of Polymer Blends of Poly(Methyl Methacylate) and Poly(Styrene-Co-Acrylonitrile) Polymer, 1992, 33, 5150-5166. (23)

Chen, H. P.; Hu, S.; Zang, H. D.; Hu, B.; Dadmun, M. Precise Structural

Development and its Correlation to Function in Conjugated Polymer: Fullerene Thin Films by Controlled Solvent Annealing Adv. Funct. Mater., 2013, 23, 1701-1710. (24) Chen, H.; Hsiao, Y. C.; Hu, B.; Dadmun, M. Tuning the Morphology and Performance of Low Bandgap Polymer: Fullerene Heterojunctions via Solvent Annealing in Selective Solvents Adv. Funct. Mater. 2014, 24, 5129-5136. (25) Chen, H.; Hsiao, Y. C.; Chen, J. H.; Hu, B.; Dadmun, M. Distinguishing the Importance of Fullerene Phase Separation from Polymer Ordering in the Performance of Low Band Gap Polymer:Bis-Fullerene Heterojunctions Adv. Funct. Mater. 2014, 24, 7284-7290.

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(26) Chen, H. P.; Peet, J.; Hu, S.; Azoulay, J.; Bazan, G.; Dadmun, M. D. The Role of Fullerene Mixing Behavior in the Performance of Organic Photovoltaics: PCBM in Low-Bandgap Polymers Adv. Funct. Mater., 2014, 24,140-150. (27) Chen, H. J.; Guo, Y. J.; Yu, G.; Zhao, Y.; Zhang, J.; Gao, D.; Liu, H. T.; Liu, Y. Q. Highly p-Extended Copolymers with Diketopyrrolopyrrole Moieties for High-Performance Field-Effect Transistors Adv. Mater. 2012, 24, 4618-4622. (28) Burford, R. P.; Markotsis, M. G.; Knott, R. B. Small Angle Neutron Scattering and Transmission Electron Microscopy Studies of Interpenetrating Polymer Networks from Thermoplastic Elastomers Nucl. Instr. Meth. Phys. B, 2003, 208, 58-65. (29) Zhang, G. C.; Yang, H. H.; He, L. L.; Hu, L. Q.; Lan, S. Q.; Li, F. S.; Chen, H. P.; Guo, T. L. Importance of Domain Purity in Semi-Conducting Polymer:Insulating Polymer Blends Transistors J. Polym. Sci. Polym. Phys. 2016, 10.1002/polb.24080. (30) Debye, P.; Bueche, A. M. Scattering by an Inhomogeneous Solid. II. The Correlation Function and Its Application J. Appl. Phys. 1949, 20, 518–525. (31) Debye, P.; Anderson, R.; Brumberger, H. Scattering by an Inhomogeneous Solid J. Appl. Phys. 1965, 28, 679. (32) Chen, H. P.; Peet, J.; Hsiao, Y. C.; Hu, B.; Dadmun, M. The Impact of Fullerene Structure on Its Miscibility with P3HT and Its Correlation of Performance in Organic Photovoltaics Chem. Mater. 2014, 26, 3993-4003. (33) Teubner M.; Strey, R. Origin of the Scattering Peak in Microemulsions J. Chem. Phys. 1987, 87, 3195-3200. 18


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(34) Shin, T. G.; Muter, D.; Meissner, J.; Paris, O.; Findenegg, G. H. Structural Characterization of Surfactant Aggregates Adsorbed in Cylindrical Silica Nanopores Langmuir 2011, 27, 5252-5263. (35) Yoonessi, M.; Heinz, H.; Dang, T. D.; Bai, Z. Morphology of Sulfonated Polyarylenethioethersulfone Random Copolymer Series as Proton Exchange Fuel Cells Membranes by Small Angle Neutron Scattering Polymer 2011, 52, 5615-5621. (36) Lannuzzi, M. A.; Reber, R.; Lentz, D. M.; Zhao, J.; Ma, L.; Hedden, R. C. USANS Study of Porosity and Water Content in Sponge-Like Hydrogels Polymer 2010, 51, 2049-2056 (37) Wang, H. Y.; Chen, L.; Xing, R. B.; Liu, J. G.; Han, Y. C. Simultaneous Control over both Molecular Order and Long-Range Alignment in Films of the Donor−Acceptor Copolymer Langmuir 2015, 31, 469-479. (38) Li, A.; Bilby, D.; Dong, B. X.; Amonoo, J.; Kim, J.; Green, F. P. Macroscopic Alignment of Poly(3-hexylthiophene) for Enhanced Long-Range Collection of Photogenerated Carriers J. Polym. Sci. Polym. Phys. 2016, 54, 180-188. (39) Chen, L.; Wang, H. Y.; Liu, J. G.; Xing, R. B.; Yu, X. H.; Han, Y. C. Tuning the pi-pi Stacking Distance and J-Aggregation of DPP-Based Conjugated Polymer via Introducing Insulating Polymer J. Polym. Sci. Polym. Phys. 2016, 54, 838-847. (40) Lu, G.; Tang, H.; Huan, Y.; Li, S.; Li, L.; Wang, Y.; Yang, X. Enhanced Charge Transportation in Semiconducting Polymer:insulating Polymer Composites: The Role of an Interpenetrating Bulk Interface Adv. Funct. Mater. 2010, 20, 1714-1720. 19



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Table 1. Abbreviation and description of polystyrenes with different molecular weights.

Protonated polystyrene

Deuterated polystyrene (d8-PS)

Abbreviation

Mw(kDa)

Mw/Mn

Mw(kDa)

Mw/Mn

PS1

2.5

1.03

2.4

1.07

PS2

20

1.02

24.5

1.07

PS3

183

1.03

228

1.17

PS4

2000

1.05

2000

1.3

Table 2. Structure parameters obtained from Debye-Bueche model: correlation length (ξ ) and scale factor (A); Structure parameters obtained from Teubner-Strey model: correlation length (ξTS) and the repeating distance from two domains (d).

ξ (Å) A(cm-1 Å-3) ξ TS(Å) d (Å)

PDVT-8:PS1 108 0.0034 112 2779

PDVT-8:PS2 312 0.0038 311 4443

PDVT-8:PS3 726 0.0076 634 6342

PDVT-8:PS4 815 0.006 754 10316

Table 3. Summary of field effects measured for all the samples: hole mobility, on/off ratio, and threshold voltage (Vth). Sample

Mobility (cm2V-1s-1)

Pristine

0.12±0.03

PDVT-8:PS1

0.02±0.005

PDVT-8:PS2

0.07±0.01

PDVT-8:PS3

0.58±0.07

PDVT-8:PS4

0.16±0.03

On/off

Vth (V) 3

-7±2

2

-5±2

3

-6±2

4

-9±2

3

11±3

(2±1)×10 (5±2) ×10 (2±1)×10 (5±2)×10 (8±1)×10

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a)

b)

Figure 1. a) chemical Structure of PDVT-8; b) image of the device from Inkjet printing. 22


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a)

b)

Figure 2. The absolute small angle neutron scattering intensity with a) Debye-Anderson-Brumberger fitting and b) Teubner-Strey fitting.. 23



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a)

c)

b)

d)

Figure 3. AFM tapping mode topographies of a) PDVT-8:PS1, b) PDVT-8:PS2, c) PDVT-8:PS3, d) PDVT-8:PS4. Scale bar is 600nm.

24


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a)

b)

Figure 4. a) out-of plane and b) in-plane x-ray profiles extracted from GIXD for all the samples.

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a)

b)

Figure 5. Device performance for inkjet-printed FETs: a) transfer characteristics of and b) the square root of IDS vs.VGS for all the FET devices at VDS=-40V.

26


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Table of Content

27


Improving Charge Mobility of Polymer Transistors by Judicious Choice

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