Improving Charge Carrier Mobility of Polymer Blend Field Effect

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Improving Charge Carrier Mobility of Polymer Blend Field Effect Transistors with Majority Insulating Polymer Phase Bin Tan, Hao Pan, Heng Li, Marilyn L. Minus, Bridgette Maria Budhlall, and Margaret J. Sobkowicz J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09775 • Publication Date (Web): 05 Jan 2018 Downloaded from http://pubs.acs.org on January 5, 2018

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The Journal of Physical Chemistry

Improving Charge Carrier Mobility of Polymer Blend Field Effect Transistors with Majority Insulating Polymer Phase Bin Tan a, Hao Pana, Heng Li b, Marilyn L. Minus b, Bridgette M. Budhlall a, Margaret J. Sobkowicz a,* a Department of Plastics Engineering, University of Massachusetts, Lowell, MA 01854, USA b Department of Mechanical and Industrial Engineering, Northeastern University, Boston, MA 02115, USA Corresponding Author: [email protected] ABSTRACT: The key approach to achieve high performance field effect transistor fabricated from semiconducting/insulating polymer blends with majority insulating polymer phase is the formation of connected fibrous structures of semiconducting polymer and good interfacial interaction of semiconducting polymer with the dielectric layer. Herein, tetrahydrofuran (THF) as a marginal solvent was used as an additive in marginal/good solvent mixtures to control the crystallite structure, phase segregation and hole transport properties of poly(3-hexylthiophene)/poly(styrene) (P3HT/PS, weight ratio: 1/4) blends, with the advantage that marginal/good solvent mixture gives P3HT sufficient time for phase segregation and relatively better solvent quality to aggregate to more stable structures, compared to other reported strategies as bad/good solvent mixtures or directly marginal solvents. Incorporation of THF reduces the P3HT solubility, forming connected fibrous structures as observed in both neat P3HT and blend films; it appears these structures are responsible for improved charge transport. Furthermore, enhanced molecular ordering, π–π stacking and conjugation length are observed with increasing THF amount. THF promotes the edge-on orientation and more stable crystal structures in P3HT, while the lattice spacing remains the same. Finally, the added THF increases hole mobility for P3HT/PS blend FETs, reaching a maximum value of 4 .0×10-3 cm2/Vs with 20 vol% THF and being comparative to neat P3HT; however, THF has insignificant influence on the hole mobility for neat P3HT FETs. Morphological characterization supports the idea that differential solubility ACS Paragon Plus Environment

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creates both enhanced chain ordering and vertical phase segregation that both improve FET performance. These results are promising for the development of environmentally stable and lower cost polymer electronics.

INTRODUCTION Semiconducting polymers are the subject of academic and industrial interest because of their tunable chemical and structural properties, solution and roll-to-roll processability, and compatibility with largearea flexible polymer substrates.1-3 However, semiconducting polymer-based field effect transistors (FETs) suffer from poor environmental stability, high costs of semiconducting polymers, large and unstable threshold voltages, and inferior charge carrier mobility compared to their inorganic counterparts, which together limit their commercialization.4-6 Over the past several years, significant strides have been made in molecular structure design, fabrication processes, morphology and aggregation control, and understanding of structure property relationships, enhancing the overall device performance.7-16 One of the most promising and intensely investigated semiconducting polymers is poly(3hexylthiophene) (P3HT), due to its relatively high hole mobility, solubility in common organic solvents, tunable microstructure and the ability to synthetically control regioregularity.17-24 It has served as a model material for exploring the correlation between microstructure and FETs performance. P3HT thin films in FETs devices are typically composed of ordered crystallites embedded in disordered amorphous matrix, and charge hopping and transport are impeded by the disordered amorphous structure. For high performance FET applications, it is critically important to control the morphology and microstructure of solution processed P3HT films by improving polymer chain order and alignment, and enhancing the π-π stacked structures. For this purpose, both intrinsic molecular parameters involving regioregularity and molecular weight,25-28 and extrinsic parameters involving solvent,12, 29 deposition method,30-32 polymer-dielectric layer interface,33-35 doping and post-thermal treatment22, 24, 36 ACS Paragon Plus Environment

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have been explored to tune the morphology and microstructure to be favorable for efficient charge transport. P3HT blends with low-cost insulating polymers have been proposed for improving the charge mobility and reducing the material costs as well as enhancing the mechanical properties of the final device.37-43 Since the insulating polymer component in the blend dilutes the bulk charge density and impedes transport, it is believed that maintaining good connectivity of the P3HT domains in the channel, especially at the polymer-dielectric layer interface will prevent charge trapping, preserve the local charge carrier density and retain the FET performance. For the ideal morphology, P3HT would segregate to the polymer-dielectric layer interface, while the insulating polymer would segregate to the polymer-air interface for encapsulation. However, the final morphology and phase separation of blend are more complicated than in the neat P3HT system, since they are not only determined by solvent evaporation rate and deposition method, but also the solubility parameters and surface tensions of both polymers, polymer-dielectric and polymer-air interfacial interactions. Although the microstructural mechanisms controlling performance of P3HT/insulating polymer blends have not been fully investigated, various methods have already been adopted to improve the blend systems and corresponding OFETs performance by processing-induced P3HT fibers inserted in insulating matrix, including using poor/good solvent mixtures,42 or poor/good solvents combining with shear coating;44 only marginal solvents;45 good solvents combining with processing control, such as “nanogrooves” on the surface of dielectric layer and ultrasonication, or combining with dopant.38, 41, 46 Qiu et al found that the mobility of FETs from P3HT/polystyrene blends was approaching that of FETs from neat P3HT when using poor/good co-solvents and aging, which promoted the formation of P3HT nanofibers resulting in a hole mobility close to that of neat P3HT.42 Lee et al found that ultrasonication assisted dissolution of P3HT/polydimethylsiloxane (P3HT/PDMS) blends improved both the mobility and on/off ratio for FETs from far worse than that for neat P3HT FETs to comparable.47 Lu et al found that FETs from P3HT/polystyrene (P3HT/PS, 5/95) blends had poor performance but could be dramatically ACS Paragon Plus Environment

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improved by both atmospheric and tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) doping.38 However, the effect of these processing factors on the properties of P3HT blends and devices and the correlation with morphology, crystallinity and phase segregation has not been fully explored. To the best of our knowledge, the strategy of using “marginal/good solvent mixtures” as a tuning approach for semiconducting/insulating polymer blends has not been investigated. The advantage of marginal/good solvent mixtures over poor/good solvent mixtures or directly marginal solvent is that marginal/good solvent mixtures provide a moderate manipulation where the relatively better solvent quality enables semiconducting polymer sufficient time for phase segregation and chain aggregation as well as more stable structures. In this work, morphology, crystallinity and phase segregation of P3HT/PS blend active layers with blend ratio at reported threshold percentage42, 45 (P3HT/PS, weight ratio: 1/4) were tuned by a mixture of good and marginal solvents (THF/chloroform) and as a consequence, the properties of FETs. Using this fundamental approach, the performance of FETs based on P3HT/PS blends could be substantially enhanced. We comprehensively report the effects of a marginal solvent additive on the microstructure, optical properties, crystal orientation and phase segregation as well as device performance of P3HT blends with majority insulating polymer.

EXPERIMENTAL SECTION Materials and Sample Preparation Regioregular poly(3-hexylthiophene-2,5 diyl) (P3HT, ≥ 96%, Mw = 71 KDa, PDI = 1.7~ 1.9) was purchased from Rieke Metals, Inc. (Lincoln, NE) and used as received. Polystyrene (PS, Mw =105 K, PDI: 2.37) was supplied by AmSty Styrenics Corporation. Chloroform and tetrahydrofuran (THF) were purchased from Sigma-Aldrich (St. Louis, MO) and used as received. P3HT and PS were dissolved separately in chloroform with concentrations of 10 mg/ml at 45 °C for 8 hours. Subsequently, P3HT solution was added to PS solutions in a volume ratio of 1:4 P3HT:PS and

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varied amounts of THF (0, 5%, 10%, 20% and 30%). These mixtures were magnetically stirred at 700 rpm for 12 hours. Films for FETs and other measurements were prepared using the same procedure. Typically, 80−150 nm thick blend films were blade coated at 1 mm/s on SiO2/Si substrates and then annealed at 150 °C for 30 minutes in nitrogen atmosphere.

Characterization FETs. Bottom-gate/bottom-contact field effect transistors with varying channel lengths (L=30 µm, 40 µm, 50 µm, 60 µm, 70 µm, 80 µm and 100 µm) and a channel width of 4 mm were used to measure the electric properties of P3HT/PS blend films. 100 nm thick source/drain (S/D) gold (Au) electrodes were deposited on 500 µm thick n-doped silicon substrates with 300 nm thick silicon dioxide as gate dielectric. The substrates were first cleaned by acetone, isopropanol, DI water and oxygen plasma, and the

substrate

surfaces

were

treated

by

grafting

a

self-assembled

monolayer

of

n-

octadecyltrichlorosilane (OTS) before coating with the P3HT/PS blend films. Current-voltage (I-V) characteristics were measured using Keithley 2612B dual SourceMeters in nitrogen atmosphere. The hole mobility is an average of at least 12 FETs. Film characterization. The morphology of P3HT/PS blend films was investigated using a Veeco atomic force microscope (AFM) with a silicon cantilever in non-contact mode, PS was removed by dissolution in dimethylformamide (not a solvent for P3HT) overnight before imaging. The ultraviolet– visible spectra of P3HT/PS blend films were collected using a Perkin Elmer Spectrometer. The sample films were prepared by blade coating on glass slides, with the same conditions as for FETs device fabrication. The surface elemental compositions of P3HT/PS blend films at both the polymer/OTS/SiO2 interface and polymer/air interface were investigated using X-ray Photoelectron Spectroscopy (XPS, VG Scientific ESCALAB MKII). Before the characterization, the Si substrate was etched away from the blend films using hydrofluoric acid. Wide angle X-ray diffraction (WAXD) of P3HT/PS blend


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films was performed on multi-film bundles (for sufficient signal to noise) using a Rigaku RAPID II (operating voltage at 40 kV, current at 30 mA, Cu K, = 0.1541 nm) equipped with curved detector manufactured by Rigaku Americas Corporation. Both X-ray diffraction patterns perpendicular and parallel to the thin film surface were recorded. For X-ray parallel to the thin film surface, the beam was carefully aligned to be parallel to the thin film surface before collecting data. The melting temperature and melting enthalpy were analyzed by a differential scanning calorimeter (DSC, Model Discovery, TA Instruments, Inc.). Samples about 0.5−2.5 mg in weight scratched from thin films were heated to 300 °C at a rate of 10 °C/min under nitrogen atmosphere, and the first heating curves were recorded as a function of temperature.

RESULTS AND DISCUSSION Optical Absorption Properties The effect of THF amount on the optical absorption properties of neat P3HTfilms and P3HT/PS blend films was investigated by UV-vis spectroscopy, which provides information about the short-range molecular ordering of P3HT chains. Figure 1 presents the absorbance spectra of neat P3HT and P3HT/PS blend films coated from solutions with and without THF. The vibrational peaks around 520, 551 and 600 nm, assigned as the 0-2, 0-1 and 0-0 peak, respectively, are listed in Table 1.48-50 For neat P3HT films shown in Figure 1a, incorporation of THF significantly increases the absorption intensities at 0-0 and 0-1 peaks, indicating higher molecular ordering from planarized conformation. Furthermore, 8−10 nm red-shifts are also observed for 0-0 and 0-1 peaks, attributed to enhanced intermolecular π–π stacking. For P3HT/PS blend films (Figure 1b), 5 nm red-shift is observed for all the peaks, indicating that the PS facilitates P3HT aggregation and intermolecular π-π stacking strength even in the absence of THF. Incorporation of THF further increases the absorption intensities of P3HT/PS blend films at 0-


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0 and 0-1 peaks, and slight red-shifts (~3 nm) are observed. The effect of THF on absorption properties saturates when THF amount is higher than 20 vol% for both neat P3HT and P3HT/PS blend films. The ratio of 0-0 to 0-1 vibronic peaks (A0-0/A0-1) is an indicator of intermolecular coupling strength related to the conjugation length and intrachain ordering in P3HT aggregates.51 This ratio first increases and then levels off with the addition of THF for both neat P3HT and P3HT/PS blend films. The free exciton bandwidth (W) of the aggregates can be decoded from the A0-0/A0-1 intensity ratio by equation 1 (assuming a Huang-Rhys factor of 1),51 1 − 0.24/ = (1) 1 + 0.073/ where W is the free exciton bandwidth of the aggregates, A0-0 and A0-1 are intensities of the 0-0 and 0-1 vibrational peaks, respectively, and ω0 is the vibrational energy of the symmetric vinyl stretch dominating the coupling to the electronic transition, taken to be 0.18 eV.52 The calculated free exciton bandwidth (W) presented in Figure 2 shows that the W value dramatically drops from 86.7 meV down to 23.4 meV for neat P3HT films once 5 vol% THF is added, then only slightly decreases with further addition of THF and levels of around 15 meV. The decreased W is an indicator for increasing conjugation length, coupling and chain order inside the P3HT films, illustrating that the aggregation is affected by THF amount in agreement with other similar co-solvent systems.53-54 For P3HT/PS blend films, W drops from 86.7 meV down to 56.5 meV with PS present and further decreases with THF, leveling off around 30 meV. With the same amount of THF, neat P3HT films have lower W value, likely because of fewer topological constraints. The entanglements with PS chains likely hinder aggregation and chain ordering in the blends.

Phase Morphology The morphology and microstructure of P3HT films are affected by solvent, including solvent quality, solution concentration and boiling point of solvent. Incorporation of poor solvent has been previously ACS Paragon Plus Environment

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reported to promote chain aggregation and facilitate molecular ordering between chains through π-π stacking,50 and that result is corroborated by this work. The effect of THF volume percent on topographies of neat P3HT films is presented in Figure 3. Figure 3a shows loose, heterogeneous structure on the order of hundreds of nanometers in size in P3HT film coated from chloroform. With the incorporation of THF, smaller aggregates and fibrous structures could be observed in the films. The root mean square roughness of neat P3HT thin film surface is 1.8 (±0.24) nm, which increases up to 3.7 (±0.31), 5.6 (±0.35) and 6.0 (±0.43) nm with increasing THF to 10%, 20% and 30%, respectively. The effect of THF on the morphology of P3HT films is clearer in the AFM phase images, as shown in Figure 4. There are no obvious phase features in P3HT film coated from chloroform, while a granular texture and ordered aggregate structures is observed in films coated from chloroform/THF solutions. The grain size increases and becomes more regular and connected with higher THF content. The AFM topographies of P3HT/PS (1/4) blend films after removing the PS phase using selective dissolution with dimethylformamide are presented in Figure 5. Figure 5a shows isolated droplets that appear to be the P3HT phase, consistent with normal phase separation phenomena for polymer blends, where the major component serves as the matrix and minor component is in dispersed droplets. With addition of THF as a marginal solvent for P3HT but a good solvent for PS, P3HT chains are expected to precipitate out from P3HT/THF co-solvents, forming nanofibers suspended in a PS solution. The AFM images of Figure 5b-e corroborate this theory of structure development in the dried blend films. Some larger nodular P3HT droplets still exist up to 10% THF, but they are mostly gone by 20% THF. With the increase of THF amount, the size and density of P3HT nanofibers appear to increase, indicating that tuning the amount of marginal solvent is a powerful way to control the microstructures of P3HT/PS blend films.


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Vertical Phase Segregation The phase separation in semiconducting polymer/insulating polymer blend thin films, including both thermodynamic and kinetic factors, is critically important for determining the final device performance. The phase separation in this system is controlled by polymer solubility in solvent, polymer/polymer interaction, comparative surface energies of polymer, solvent and substrates, and evaporation rate of solvent. In this P3HT/PS blend, chloroform and THF have similar surface energy and boiling point,55-56 so the most important parameters that determine the final phase separation and morphology are relative polymer solubility in the cosolvents, and substrate surface energy. The Flory-Huggins interaction parameter () was reported to be 1.04, 0.99, 0.41, 0.39 and 0.48 for P3HT/THF, P3HT/chloroform, PS/THF, PS/chloroform and P3HT/PS, respectively, indicating P3HT is less soluble than PS in both solvents.23 The maximum concentration of P3HT that could be dissolved in THF is around 1 mg/ml at 45 °C.20 Thus, once THF is added to the P3HT/PS solution, P3HT chains start to aggregate to minimize unfavorable interactions with THF, forming ordered fibrous structures. The color of P3HT/PS solutions changes from orange to purple and continues to darken with further addition of THF— another indicator for P3HT fibrous structure formation in solution. During the coating process, P3HT (surface energy, ≈ 21.0 mJ/m2) is likely to collapse out of the liquid phase first and migrate to the interfaces as the solvent evaporates, resulting in a P3HT enriched top surface where solvents evaporate first.57 This phenomenon was reported in Lu et al.’s work where P3HT/PS(5/95) blends were coated from odichlorobenzene on SiO2 substrate;38 on the other hand, P3HT also tends to migrate down to OTS/SiO2 ( ≈ 20.5 mJ/m2) surface to minimize surface tension.58 Thus, there is a competition between upward and downward migration of P3HT molecular chains, which are also constrained by the still-welldissolved PS molecular chains. Additionally, PS ( ≈ 40.2 mJ/m2) tends to remain in the solvents longer and becomes confined in the center of the film. These predicted results also depend on the solvent evaporation rate and P3HT/PS interactions.59


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To further examine the phase separation in P3HT/PS thin films, the dielectric layer (silicon oxide) of FETs substrates was etched using hydrofluoric acid to remove the blend film intact, and the elemental composition of both top and bottom surfaces of the blend films were measured using XPS. The repeat unit of P3HT molecules contains one sulfur and ten carbon atoms, while the repeat unit of PS molecules only contains eight carbon atoms. The weight percentage of P3HT on both bottom surface (interface with dielectric layer) and top surface can be estimated from the sulfur to carbon ratio from XPS measurement and the weight average molecular weight (detailed calculations are listed in Supporting Information), which could give a reasonable estimation of P3HT percentage.

The

experimentally determined sulfur and carbon atomic compositions and the calculated weight percentage of P3HT on both surfaces are presented in Figure 6. The results indicate that P3HT represents 60.7 and 53.4 wt% of the polymer at the bottom and top surfaces of thin films coated from chloroform, respectively, higher than the designed 20 wt% for the bulk blend. This indicates that P3HT has preferentially migrated to either the bottom or top surface during film formation. Although it is not directly measured here, it can be inferred that the P3HT content is depleted and PS enriched in the center of the film. Moreover, the amount of P3HT at the bottom surface steadily increases with the increase of THF amount from 60.7 to 79.1 wt%. This indicates that the incorporation of THF promoted P3HT downward migration, which may be attributed to denser fibrous structures, and less entanglement between PS chains and ordered P3HT chains. For bottom gate/bottom contact OFETs, the buried interface enriched P3HT morphology is beneficial for enhancing charge carrier density and mobility. Additionally, the amount of P3HT at the top surface first increases from 53.4 to 65.3 wt% upon addition of 5 vol% THF, but then gradually decreases to 50.9 wt% with further addition of THF. This top surface decrease is likely due to the stronger attraction of the OTS surface for the aggregated P3HT compared to the air interface.


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Crystallinity The crystalline structure of neat P3HT and P3HT/PS films was measured using 2-dimensional transmission wide angle X-ray diffraction (WAXD) with either X-ray beam parallel or perpendicular to film surface, as represented by the schematic presented in Figure 7a (Figure S1 and S2, Supporting Information). This technique provides an overall orientation along the bulk film instead of only the interface with dielectric layer. With X-ray parallel to film surface as indicated in Figures 7b and 7e, strong diffraction arcs of (100), (200) and (300) planes representing lamellar packing direction, and weak diffraction arcs of (020) plane representing π-π stacking direction are observed. With the X-ray beam perpendicular to film surface (Figure 7c and 7f), strong diffraction rings of (020) planes are observed. Combining the diffractions patterns from both X-ray parallel and perpendicular to the films, a predominant edge-on orientation can be concluded because the (100) direction is normal to and (020) direction is in-plane with the films.11, 60-62 Figure 7d presents the full width at half maximum (FWHM) of (100) diffraction peaks through azimuthal scan, which decreases with the increase of THF amount for both neat P3HT and P3HT/PS blend films, indicating that the incorporation of THF promotes alkyl stacking in lattice. The integrated 1-dimensional crystalline profiles of films are presented in Figure 8. Diffraction peaks at 2θ = 5.3°, 10.6°, 16.9° and 23.6° represent the (100), (200), (300) and (020) planes (plane spacing: d(100)= 16.5, d(010)= 3.8), respectively.11, 60-61 It can be seen from Figure 8a and 8c that dominant (100) diffraction peaks are observed for all the films with X-ray parallel to film surface. With X-ray perpendicular to film surface, however, both strong (100) and (020) peaks are observed for neat P3HT and P3HT/PS blend films coated from chloroform, as shown in Figure 8b and 8d. With increasing THF amount, the (100) peak diminishes, while the (020) peak intensifies (Figure S2, Supporting Information), indicating promotion of the edge-on orientation. Furthermore, the incorporation of THF did not change the peak locations, i.e., lattice spacing. In summary, WAXD


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results show slightly more organized crystallites in pure P3HT films, but a preferential edge-on orientation (considered to be better for electronic transport) when THF is added. The crystallization behaviors of neat P3HT and P3HT/PS blend films were further examined using differential scanning calorimetry (DSC). The first heating curves are presented in Figure 9, the melting temperatures and enthalpies are summarized in Table 2. The melting points for both neat P3HT and P3HT/PS blend films increase with increasing THF amount up to 20 vol%, then decrease with THF content of 30 vol%. The increased melting point and enthalpy are indicators of more stable crystal domains and higher crystallinity. Whereas, P3HT/insulating polymer blends from only marginal solvent (dichloromethane) did not show any changes in melting points and enthalpy.45 This indicates that marginal/good solvent mixtures can promote more stable crystallite than only marginal solvent. Yuan et al reported that microstructural differences in P3HT result in distinct crystallization behavior, where J-aggregates showed higher melting point (256 ºC) and larger enthalpy (28.92 J/g) than those for H-aggregates (242 ºC and 22.52 J/g, respectively).62 Thus, the increasing melting points and enthalpy in this work may be also indicators of more pronounced J-aggregates in both P3HT and P3HT/PS blend films, which is also supported by the A0-0/A0-1 values from UV-visible spectra (Figure 1). Both the neat P3HT and P3HT/PS blend films coated from chloroform/THF show A0-0/A0-1 values in the region of 0.84−0.95, higher than the typical H-aggregates region (~ 0.5−0.8) and lower than the J-aggregates region (>1.0), indicating a coexistence of H- and J- aggregates, where J- aggregates are favored with increasing THF amount.63

Field Effect Transistors The charge carrier mobility of P3HT field effect transistors (FETs) ranges from about 10-5 cm2/Vs up to 10-1 cm2/Vs, depending on the solid-state packing, chain ordering, regioregularity, and molecular weight of the polymer, polymer/dielectric layer interaction, and device design.11,49,64-65The FET performance was measured using a bottom-gate/bottom-contact FET architecture. The representative ACS Paragon Plus Environment

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output characteristics and transfer curves of FETs are presented in Figure 10. Typical I-V curves are shown for neat P3HT FETs (Figure 10a), but P3HT/PS blend (20 vol% THF) FETs show much lower drain-source current at positive and zero gate volts (Figure 10c, inset). Neat P3HT FETs show similar saturated drain-source current at high gate voltage (Figure 10b) as THF content varies, while P3HT/PS blend FETs are more sensitive to THF amounts (Figure 10d). The mobility in the saturation region was calculated according to equation 2 at = −50 and ! = −70.66 "! =

#$ ( − &' ) (2) 2%

where # is the hole mobility, W is the FETs channel width, L is the FETs channel length, C is the gate dielectric capacitance per unit area (1.15 × 10) F/m2), is the gate voltage, &' is the threshold voltage and ! is the drain-source voltage. In this work varied channel lengths from 30 to 100 µm were used and the results showed that the hole transport properties were not dependent on channel length. The calculated mobility along with threshold voltage and on-off ratio are presented in Figure 11 and summarized in Table 3. The hole mobility of neat P3HT FETs just slightly decreases with the increase of THF amount, while similar threshold voltage is observed with varied THF amounts, indicating that there is no noticeable impact of THF on hole transport properties for neat P3HT FETs. It is interesting to note that even though incorporation of marginal solvent enhanced the degree of molecular ordering (red-shift and promoted edge-on orientation) and crystallinity (larger area under melting peak) of neat P3HT films as observed in UV-Vis spectra, WAXD and DSC data, the FETs performance did not improve. The probable reason is the increased surface roughness as observed in AFM and the micronscale agglomeration resulting in less efficiency in long-distance charge transport.67 Most recently, a study suggested that morphology and structure of grain boundaries is an essential factor that determines macroscale charge transport performance instead of only localized aggregates.68 Similar phenomena were reported by Change et al, where they showed that good/poor solvent mixtures (chloroform/2,3dimethylbutane and chlorobenzene/acetone) did not obviously change the charge transport properties


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even though the degree of molecular ordering and crystallinity were enhanced, due to the deteriorated morphology which impeded efficient carrier transportation; improvement was only observed in good/poor solvent mixtures with a strong hydrogen bond between the good and bad solvents (chloroform/acetone).54 In contrast, THF has a significant effect on transport in P3HT/PS blend FETs, as shown in Figure 11 and Table 3. The average hole mobility increases with the increase of THF amount from 2.6×10-4 cm2/Vs without THF to be maximized at 4.0×10-3 cm2/Vs with 20 vol% THF, which is equivalent to neat P3HT FETs. Moreover, THF also beneficially reduces the threshold voltage from 23.4 V for P3HT/PS blend without THF to −4.7 V with 20 vol% THF, indicating enhanced FETs response to gate voltage. One possible reason is the formation of nanofiber structures of P3HT, another possible reason is the passivation effect of polystyrene induced by THF. Similar phenomenon was also observed in diketopyrrolopyrrole-dithienylthieno[3,2-b]thiophene (I)/PS blend films, where PS helped passivate the surface with formation of (I) fibrous network (maximized effect was observed in film with a 40/60 (I)/PS ratio), which resulted in a smaller hysteresis effect and decreased threshold voltage.69 The on-off ratio for P3HT/PS blend FETs also increases with incorporation of THF, by simultaneously enhancing the on-current and reducing the off-current. The effect of THF on transport in the P3HT/PS blend FETs compared with neat P3HT FETs may be due to the nanofiber structures of P3HT, which may have less barrier for carrier transport.70 To further test the theory that P3HT has preferentially migrated to the polymer/SiO2 interface, the performance of FETs from P3HT/PS blends with 0 vol% and 20 vol% THF after etching PS phase were also analyzed. All these FETs were prepared using the same procedures as non-etched ones, and then were immersed into DMF solvent for 8 hours to remove PS, followed by gentle dipping into fresh DMF to further remove any residual PS. After that, the FETs were annealed at 150 ºC for 30 minutes before testing. As a control group, neat P3HT FETs with 0 vol% THF were prepared following the same procedures. Twelve samples were tested and the average transistor performance metrics were ACS Paragon Plus Environment

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calculated. For FETs from P3HT/PS blends with 0 vol% THF, 50% failure rate was observed, while for FETs from P3HT/PS blends with 20 vol% THF, no failure was observed. The representative output characteristics of FETs are presented in Figure 12. The FETs from P3HT/PS blends show good performance similar to FETs from neat P3HT. The average hole mobility, threshold voltage and on-off ratio are summarized in Table 4. Remarkably, the DMF-etched FETs from both neat P3HT and P3HT/PS blends with 20 vol% THF show similar mobility as non-etched FETs, while etched FETs from P3HT/PS blends with 0 vol% THF show 6 times lower mobility. Additionally, all the etched FETs show similar on-off ratio as non-etched FETs, while the threshold voltage of etched FETs shows scatter. This comparison study verifies that addition of THF improves the P3HT network quality, especially near the dielectric substrate. It also indirectly confirms that DMF preferentially removes the PS phase but leaves P3HT networks intact.

CONCLUSIONS In this work, P3HT/PS (1/4) blends were prepared for FETs application, and THF as a marginal solvent was used to tune the morphology and hole mobility of P3HT/PS blend thin films as compared to neat P3HT. The incorporation of THF induced fibrous structures in neat P3HT films with improved crystalline order as THF amount was increased. Incorporation of THF eliminated droplet morphology in favor of the P3HT nanofibers in P3HT/PS blend films. Furthermore, vertical segregation was observed in the blend films, where P3HT enriched both bottom and top surfaces, and the amount of P3HT at bottom surface increased with the increase of THF percentage. Red-shifts were observed for both neat P3HT and P3HT/PS blend films when THF was incorporated, and the optical absorption intensities at 0-0 and 0-1 peaks were significantly increased, indicating higher electronic ordering, planarized conformation, and enhanced intermolecular π–π stacking. The incorporation of THF promoted the edge-on orientation in P3HT crystal and increased the melting point, while did not ACS Paragon Plus Environment

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change the lattice spacing. Finally, THF did not obviously change the hole transport properties of neat P3HT FETs but it enhanced the hole mobility and reduced the threshold voltage of P3HT/PS blend FETs resulting in similar performance to neat P3HT films. The best performance was observed in P3HT/PS blend FET with 20 vol% THF which was equivalent to neat P3HT FETs. All these results demonstrate that good/marginal solvent mixture is a powerful approach to improve the optoelectronic properties of semiconducting/insulating polymer blends with insulating polymer as a major component. These findings could have important implications for efficient hole transport for FETs with reduced material cost and increased mechanical as well as air stability.

SUPPORTING INFORMATION Method and equations for calculating weight percentage of P3HT on film surfaces using XPS data. 2dmensional WAXD patterns of all the neat P3HT and P3HT/PS blend films.

AUTHOR INFORMATION Corresponding Authors E-mail: [email protected]; Phone: +1 978 934 3433; Fax: +1 978 934 3089.

ACKNOWLEDGEMENTS The authors gratefully acknowledge support of the National Science Foundation under CMMI1538108.

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Captions for Figures and Tables Figure 1. Effect of THF amount on UV-vis spectra of a) neat P3HT and b) P3HT/PS blend films. Figure 2. Effect of THF amount on exciton bandwidth of a) neat P3HT and b) P3HT/PS blend films. Figure 3. AFM topographic images of neat P3HT films coated from chloroform/THF solutions with a) 0%, b) 10%, c) 20% and d) 30% THF (2 #m × 2 #m). Figure 4. AFM phase images of neat P3HT films coated from chloroform/THF solutions with a) 0%, b) 10%, c) 20% and d) 30% THF (2 #m × 2 #m). Figure 5. AFM topographic images of P3HT/PS blend films coated from chloroform/THF solutions, PS etched out with DMF dissolution. a) 0%, b) 5%, c) 10%, d) 20%, and e) 30% THF with 5 #m × 5 #m scale, and f) 5% THF with 2 #m × 2 #m scale. Figure 6. a) Atom compositions and b) P3HT percentage at top and bottom surfaces of P3HT/PS blend films versus THF amount. Figure 7. WAXD patterns of films: a) schematic of WAXD measurement, b) and e) neat P3HT and P3HT/PS blend films with X-ray parallel to film surface, respectively; c) and f) neat P3HT and P3HT/PS blend films with X-ray perpendicular to film surface, respectively; d) full width at half maximum of (100) diffraction peaks through azimuthal scan. Figure 8. 1-dimensional WAXD profiles: a) and c) are neat P3HT and P3HT/PS blend films with Xray parallel to film surface, respectively; b) and d) are neat P3HT and P3HT/PS blend films with X-ray perpendicular to film surface, respectively. c) and d) are normalized to PS amorphous peak at 2θ = 19.4°. Figure 9. DSC first heating curves of a) neat P3HT and b) P3HT/PS blend films. Figure 10. Output characteristics of FETs from a) neat P3HT with 0% THF and c) P3HT/PS blend with 20% THF (insert plots show I-V curves for VG= 0 and 10 V), and transfer curves for b) neat P3HT and d) P3HT/PS blend FETs with a range of added THF (VD=−50 V).


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Figure 11. Effect of THF amount on hole mobility and threshold voltage for a) neat P3HT and b) P3HT/PS blend FETs. Figure 12. Representative output characteristics of FETs from a) neat P3HT with 0 vol% THF, b) P3HT/PS blend with 0 vol% THF, c) P3HT/PS blend with 20 vol% THF, and d) transfer curves after etching PS. Table 1. UV-vis spectra peak positions, and A0-0/A0-1 peak intensity ratios of thin films. Table 2. Melting points and enthalpy for neat P3HT and P3HT/PS blend films. Table 3. Average hole mobility (µ), threshold voltage (Vth) and on-off ratio (Ion/Ioff) of FETs (Ion/Ioff was calculated in the term of ID (VG=−70 V)/ID (VG=−20 V)). Average of at least 12 FETs. Table 4. Average hole Mobility (µ), threshold voltage (Vth) and on-off ratio (Ion/Ioff) of FETs after etching PS ((Ion/Ioff was calculated in the term of ID (VG=−70 V)/ID (VG=−20 V)). Average of at least 12 FETs.


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Table 1. UV-vis spectra peak positions, and A0-0/A0-1 peak intensity ratios of thin films. THF (vol%) 0 5 10 20 30

amount

Neat P3HT films λ0-0 λ0-1 (nm) (nm) 600 553 609 561 610 561 610 561 609 561

λ0-2 (nm) 521 526 526 526 526

A0-0/A0-1 0.73 0.92 0.95 0.96 0.95

P3HT/PS λ0-0 (nm) 604 603 605 608 605

blend films λ0-1 λ0-2 (nm) (nm) 558 526 560 526 561 526 560 526 560 526

A0-0/A0-1 0.82 0.84 0.89 0.90 0.91

Table 2. Melting points and enthalpy for neat P3HT and P3HT/PS blend films. P3HT Films

P3HT/PS Blend Films

THF amount 0 5 10 20 30 0 5 10 20 30 (vol%) Melting point 245.4 249.3 248.9 253.4 248.5 243.2 245.8 247.3 251.1 248.6 (ºC) Enthalpy 22.12 21.97 24.36 22.4 22.78 22.62 24.265 24.81 24.745 27.6 (J/g)

Table 3. Average hole mobility (µ), threshold voltage (Vth) and on-off ratio (Ion/Ioff) of FETs (Ion/Ioff was calculated in the term of ID (VG=−70 V)/ID (VG=−20 V)). Average of at least 12 FETs. Neat P3HT FETs THF amount µ Vth (V) (vol%) (×10-3 cm2/Vs) 0 3.6 ± 1.2 12 ± 3.7 5 2.7 ± 0.9 14 ± 2.1 10 3.7 ± 0.6 8 ± 3.4 20 2.4 ± 0.4 15 ± 3.7 30 2.9 ± 0.5 14 ± 3.6 a Average of 24 FETs.

P3HT/PS blend FETs Ion/Ioff 103~ 104 103~ 104 103~ 104 103~ 104 103~ 104

µ (×10-3 cm2/Vs) 0.26 ± 0.01 0.6 ± 0.2 2.1 ± 1.1 a 4.0 ± 1.5 a 1.4 ± 0.8

Vth (V)

Ion/Ioff

23.4 ± 4 10.4 ± 3.9 6.8 ± 2.8 −4.7 ± 3.5 3.7 ± 2.4

< 102 ~ 103 103~ 104 104~ 105 103~ 104

23 ACS Paragon Plus Environment


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Table 4. Average hole Mobility (µ), threshold voltage (Vth) and on-off ratio (Ion/Ioff) of FETs after etching PS ((Ion/Ioff was calculated in the term of ID (VG=−70 V)/ID (VG=−20 V)). Average of at least 12 FETs. FETs

µ (×10-3 cm2/Vs)

Vth (V)

Ion/Ioff

Neat P3HT-0 vol% THF P3HT/PS-0 vol% THF P3HT/PS-20 vol% THF

3.44 ± 0.77 0.04 ± 0.04 3.31 ± 0.90

3.2 ± 1.8 27.7± 11.7 7.7 ± 4.9

103~ 104 ~ 102 103~ 104


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TOC Graphic



Figure 1. Effect of THF amount on UV-vis spectra of a) neat P3HT and b) P3HT/PS blend films. 35x14mm (600 x 600 DPI)


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Figure 2. Effect of THF amount on exciton bandwidth of a) neat P3HT and b) P3HT/PS blend films. 35x29mm (600 x 600 DPI)



Figure 3. AFM topographic images of neat P3HT films coated from chloroform/THF solutions with a) 0%, b) 10%, c) 20% and d) 30% THF (2 µm × 2 µm). 72x63mm (600 x 600 DPI)


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Figure 4. AFM phase images of neat P3HT films coated from chloroform/THF solutions with a) 0%, b) 10%, c) 20% and d) 30% THF (2 µm × 2 µm). 72x63mm (600 x 600 DPI)



Figure 5. AFM topographic images of P3HT/PS blend films coated from chloroform/THF solutions, PS etched out with DMF dissolution. a) 0%, b) 5%, c) 10%, d) 20%, and e) 30% THF with 5 µm × 5 µm scale, and f) 5% THF with 2 µm × 2 µm scale. 108x140mm (600 x 600 DPI)


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Figure 6. a) Atom compositions and b) P3HT percentage at top and bottom surfaces of P3HT/PS blend films versus THF amount. 32x12mm (600 x 600 DPI)



Figure 7. WAXD patterns of films: a) schematic of WAXD measurement, b) and e) neat P3HT and P3HT/PS blend films with X-ray parallel to film surface, respectively; c) and f) neat P3HT and P3HT/PS blend films with X-ray perpendicular to film surface, respectively; d) full width at half maximum of (100) diffraction peaks through azimuthal scan. 49x31mm (300 x 300 DPI)


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Figure 8. 1-dimensional WAXD profiles: a) and c) are neat P3HT and P3HT/PS blend films with X-ray parallel to film surface, respectively; b) and d) are neat P3HT and P3HT/PS blend films with X-ray perpendicular to film surface, respectively. c) and d) are normalized to PS amorphous peak at 2θ = 19.4°. 70x60mm (600 x 600 DPI)



Figure 9. DSC first heating curves of a) neat P3HT and b) P3HT/PS blend films. 34x14mm (600 x 600 DPI)


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Figure 10. Output characteristics of FETs from a) neat P3HT with 0% THF and c) P3HT/PS blend with 20% THF (insert plots show I-V curves for VG= 0 and 10 V), and transfer curves for b) neat P3HT and d) P3HT/PS blend FETs with a range of added THF (VD=50 V). 66x51mm (600 x 600 DPI)



Figure 11. Effect of THF amount on hole mobility and threshold voltage for a) neat P3HT and b) P3HT/PS blend FETs. 33x14mm (600 x 600 DPI)


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Figure 12. Representative output characteristics of FETs from a) neat P3HT with 0 vol% THF, b) P3HT/PS blend with 0 vol% THF, c) P3HT/PS blend with 20 vol% THF, and d) transfer curves after etching PS. 67x53mm (600 x 600 DPI)


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