Solution Processing with a Good Solvent Additive for Highly Reliable

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Solution Processing with a Good Solvent Additive for Highly Reliable Organic Thin-Film Transistors Jin Yeong Na, Min Kim, and Yeong Don Park J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b03833 • Publication Date (Web): 06 Jun 2017 Downloaded from http://pubs.acs.org on June 13, 2017

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The Journal of Physical Chemistry

Solution Processing with a Good Solvent Additive for Highly Reliable Organic Thin-Film Transistors Jin Yeong Na,† Min Kim,‡,* Yeong Don Park†,* †

Department of Energy and Chemical Engineering, Innovation Center for Chemical

Engineering, Incheon National University, Incheon 22012, South Korea ‡

Center for Nano Science and Technology @Polimi, Istituto Italiano di Tecnologia, Via

Giovanni Pascoli 70/3, 20133 Milan, Italy

*Corresponding authors: [email protected] (mk), [email protected] (ydp)

Abstract High performance transistors require homogeneous crystalline thin films with high charge carrier mobilities on large substrates. We report a versatile solution process that uses a good solvent additive with a high boiling point. This method enables the fabrication of uniform, large-area, polymer thin films with improved thin film crystallinity and electrical characteristics, and high reliability organic transistors. We demonstrate here that the addition of a small amount of a good solvent additive with a high boiling point to the processing solution is highly effective in not only inducing the strong selfassembly of the conjugated polymer during film solidification by increasing the solvent evaporation time but also in ensuring the formation of a uniform film morphology. These characteristics were found to enhance the device performance of a poly(3-hexylthiophene) film processed from a mixed solvent containing chlorobenzene and 1,2,4-trichlorobenzene, which was found to exhibit a highest average field-effect mobility of 0.012 cm2 V-1s-1 with excellent reliability over a 16 device array.

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1. Introduction Organic field-effect transistors (FETs) are an attractive alternative to conventional inorganic-based technologies because of their potential applications in flexible electronic devices including displays, radio-frequency identification tags, integrated circuits, memory devices, and sensors because they can be processed with cost-effective and large-area solution-printing techniques.1-3 As a result of the extensive research in recent years into organic semiconductor materials, the charge carrier mobilities of organic FETs now exceed the benchmark value (1 cm2V-1s-1) set by hydrogenated amorphous silicon transistors.4-8 Moreover, organic FETs are useful systems for the investigation of charge transport phenomena in organic materials.9 Charge transport in an organic semiconductor film is known to have a significant dependence on its crystalline structure, particularly the molecular orientation and the film morphology.10-12 The molecular structures arising during thin film formation can be controlled by using various processing techniques such as the optimization of the film deposition method, the use of various additives, and post-treatment techniques, all of which can influence the supramolecular assembly of conjugated polymers and the charge-carrier transport properties of polymer FETs.13-17 One approach to improving device performance that has been extensively studied is the solution crystallization method with a solvent additive.17-21 The use of a binary solvent mixture consisting of a good host solvent and a small amount of a non-solvent additive has also recently been demonstrated to enable the formation of well-ordered crystalline domains and film morphologies without any additional post-treatment process. In such binary-solvent strategies, non-solvents with higher boiling points than those of the good host solvents are commonly used because they induce the formation of ordered aggregates of pi-conjugated polymers in the solution state.22-23 In such systems, the lessvolatile non-solvent resides within the evolving film for a long period of time and therefore promotes supramolecular aggregation during solvent evaporation. However, it is important to carefully control and optimize the processing conditions such as the composition of the solvent mixture and the aging 2


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time after solvent mixing, because the presence of too much non-solvent additive or a long aging time can cause severe polymer precipitation in the solution state, which eventually results in a non-uniform film morphology with many defects.10,

24-25

Therefore, the effects of the chemical and physical

characteristics of solvent additives on film morphology, crystalline structure, and device characteristics must be thoroughly understood. In this study, we investigated the effects of the properties of various solvent additives, such as their solubility and boiling point, on the morphology and molecular order of poly(3-hexylthiophene) (P3HT) films used as active layers in p-channel organic FETs. P3HT is often used in investigations of the fundamental electrical properties of processed films.26 By using a Hansen solubility parameter (HSP) program, we selected chlorobenzene (CB) as the main good solvent, and chloroform (CF), 1,2dichlorobenzene (DCB), and 1,2,4-trichlorobenzene (TCB) as the good-solvent additives, and systematically compared their effects on P3HT films with those of the non-solvent additives acetonitrile (ACN) and N,N-Dimethylformamide (DMF) (Table 1). We found that P3HT films with uniform morphologies and high crystallinity are produced when the additive is a good solvent with a high boiling point. It was demonstrated that the high boiling point of the good solvent additive means that the resulting P3HT film has a uniform morphology and high crystallinity because it provides sufficient solvent evaporation time for self-assembly even during large-area film formation. In particular, the maximum mobility of the organic FET device processed in the presence of a good solvent additive with a high boiling point was 0.012 cm2V-1s-1; this result was achieved by using a solution containing 5 vol% TCB, which produced a film with not only a higher charge carrier mobility but also a higher reliability than the organic FET device processed in the presence of non-solvent additives or without any additive from a high boiling point solvent.

2. Experimental 3



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Preparation of Polythiophene Thin Films and FET Devices: Regioregular poly(3-hexylthiophene) (P3HT) was obtained from Rieke Metals, Inc. (regioregularity ~95%, Mw = 37 kDa) and used without further purification. Chloroform, chlorobenzene, 1,2-dichlorobenzene, and 1,2,4-trichlorobenzene were used as received from Aldrich. Highly doped Si was used as a gate electrode as well as a substrate. A thermally grown silicon dioxide (SiO2) layer with a thickness of 300 nm was employed as the gate dielectric (capacitance = 10.8 nF cm−2). Substrates were cleaned in acetone and in ethanol for 30 min by using an ultrasonicator and dried with N2 blowing before use. Hexamethyldisilazane (HMDS) (Aldrich) was used as an organic interlayer material between the organic active material and the dielectric layer and was applied to the SiO2 substrate via spin-casting. Each P3HT chlorobenzene solution (10 mg/mL) was prepared at 50°C in a sealed vial to prevent evaporation. The solvent additives chloroform, 1,2-dichlorobenzene, and 1,2,4-trichlorobenzene were gradually added at various volume ratios (1, 2, 5, 10, and 20%) with respect to chlorobenzene into P3HT solutions with concentrations of 10 mg/mL, and then stirred overnight at 50°C. Each warm P3HT solution was cooled at room temperature for 5 minutes. P3HT thin films were spin coated at 1500 rpm (Spin1200D, MIDAS). P3HT-based FETs with bottom-gate top-contact structures were fabricated by evaporating gold through a shadow mask (channel length 100 µm, channel width 2000 µm). Identical P3HT films were fabricated on transparent glass substrates instead of Si substrates in preparation for the UV-Vis absorption measurements. Characterization: UV-Vis absorption spectra were recorded by using a UV-Vis spectrophotometer (Thermo Scientific, Genesys 10S). The film morphologies were characterized with an atomic force microscope (Multimode 8, Digital Instruments) operated in tapping mode with a silicon cantilever and an optical microscope (Olympus BX51). X-ray diffraction was performed at the 3C and 9A beamlines of the Pohang Accelerator Laboratory, Korea. The films were aligned with an incidence angle of 0.12°. The electrical performances of the organic FETs were characterized by using a semiconductor analyzer (Keithley 4200) at room temperature. The field-effect mobility (µFET) and the threshold 4


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voltage (VT) of each organic FET were estimated in the saturation regime (VD = –80 V) by using the equation:

ID =

W 2 µFETCg (VG − VT ) , 2L

where ID is the drain current, Cg is the capacitance of the gate dielectric, and VG is the gate–source voltage. The thicknesses of the P3HT films were determined by using an ellipsometer (J. A. Woollam Co. Inc.).

3. Results and Discussion The morphologies of thin films prepared with spin coating depend strongly on the processing conditions such as the solvent evaporation rate, the surface characteristics of the substrate, and the aggregation of the polymer material in the solvent.27 The spreading coefficient, S = γlg(cosθ–1) = γsg – γlg – γsl, can be used to characterize the formation of dewetted films, where θ is the contact angle at the three-phase line of the rim, and γlg and γsl are the surface tension of the substrate and the interfacial tension between the liquid and the solid substrate respectively. Negative S values are associated with hole growth, and indicate that the solid surface prefers to remain exposed.28 In particular, when an organic semiconductor solution is coated onto a hydrophobic substrate with low γsg, the film coverage and morphology are significantly affected by the intermolecular interactions between the solution and substrate, which are related to the surface tension and boiling point of the solvent as well as to the aggregation and crystallization of the organic semiconductor molecules.29 Therefore, solution processing requires sophisticated control if it is to deposit uniform films with high crystallinity of polymer semiconductor materials on hydrophobic substrates. In this study, we chose six different solvents, CF, CB, DCB, TCB, ACN, and DMF to investigate the effects of the solvent evaporation rate and polymer aggregation on film formation on HMDS-treated 5



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silicon substrates. First we compared four good solvents for P3HT, namely CF, CB, DCB, and TCB, in which P3HT has similarly high solubility based on calculations using Hanssen solubility parameter differences (Table 1).30 The boiling points of the four solvents, CF, CB, DCB, and TCB range from 61°C to 214°C and their surface tensions range from 26.7 mN/m to 39.1 mN/m. The images in Figures 1a and S1a of the spin-coated P3HT films show that the films processed from high boiling point solvents such as DCB and TCB have severely dewetted morphologies with a rim structure. This result can be explained in terms of the solvent boiling point and surface tension. Solvents such as DCB and TCB exhibit high surface tension, which means that the spreading coefficient is negative and is likely to induce dewetting of the film, especially on hydrophobic substrates due to the agglomeration of liquid droplets during the spin coating process. Furthermore, the high boiling points of DCB and TCB prolong solvent evaporation and solidification, affording solutes longer times to reach thermodynamically stable states. This mobility induces the dewetting of films of polymer semiconductor materials and the formation of a rim structured morphology, which is detrimental to establishing charge carrier transport in organic transistor devices. To overcome this dewetting issue, we used the high boiling point solvents DCB and TCB as additives. When DCB and TCB are used as additives with volume ratios in the range 5 to 20%, the additive-processed films are uniform without any defects or dewetting (Figures 1b, 1c, S1b, and S1c). This result is explained by the hypothesis that the solution containing the solvent additive enters a metastable state that produces stable polymer thin films and no pin-holes, and thus effectively circumvents the dewetting problems.28 Even when the amount of the added good solvent was as high as 20 vol%, the processed films contained no defects, and had homogeneous morphologies and very similar film thicknesses, as required for reliable thin-film transistor devices (Figure S2). To observe the effects of varying the solvent additive, we compared the films obtained with the nonsolvent additives ACN and DMF with those obtained with the good solvent additives DCB and TCB. ACN and DMF were added into the P3HT CB solutions at various volume ratios, which were spin6


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coated onto Si wafers. All the films fabricated from P3HT solutions with 5 vol% of the four different solvent additives were uniform, regardless of the additive solubility. The good solvent additives DCB and TCB enabled uniform film formation even when large additive amounts, i.e. 20%, were used. However, large amounts (20 vol%) of the non-solvent additives ACN or DMF resulted in the formation of strong aggregates of P3HT in solution, and thus in non-uniform films (Figures 1c and S1c). These results indicate that the addition of a good solvent into the P3HT solution is a more reliable and reproducible approach to the production of uniform films with solution processing. Light absorption spectra were collected to investigate the effects of the solvent additives on polymer aggregation in the solution state (Figure 2). The spectrum of the diluted P3HT solution in CB contains only one peak at λ=455 nm associated with the intrachain pi–pi transition of P3HT; the CF, DCB, and TCB additives do not produce significant intermolecular interaction peaks in the absorption spectra because they are good solvents for P3HT, as expected from the Hanssen parameters (Figure 2d). However, when the non-solvent additives ACN and DMF were added into the solutions, intermolecular vibronic peaks at lower energies appear for additive amounts above 10 vol% (Figure S3). These results demonstrate that the good solvents completely dissolve the polymer molecules in the solution state without any polymer pre-aggregation, whereas the poor solvents induce the polymer to aggregate. To assess the crystallinities of the processed films, the UV-Vis absorption spectra of the P3HT films processed from solutions with various additives were compared (Figure 3). When the P3HT polymer molecules aggregate, forming crystalline structures, vibronic bands appear at 558 and 610 nm that originate in the intermolecular electronic structure.31-32 The absorption spectra of the films processed with the high boiling point solvent additives DCB and TCB contain a strongly pronounced (0-0) transition peak at 610 nm, which is one of the intermolecular vibronic bands and indicates the formation of well-ordered aggregates of P3HT chains. A good solvent additive with a high boiling point facilitates crystallization of the polymer chains by prolonging the solvent evaporation time, 7



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which means that the polymer molecules crystallize even though there was no pre-aggregation in the solution state. However, the presence of the CF solvent additive does not affect the vibronic bands of the cast film because it has a low boiling point and undergoes rapid evaporation. The lower energy bands in the absorption spectra provide information about the polymer chain conjugation length, which is associated with intramolecular and intermolecular ordering. Using Spano’s model with the assumption that the crystalline regions are composed of weakly interacting Haggregates, the A0-0 and A0-1 vibronic bands can be fitted to calculate the free exciton bandwidth (W), which is related to the conjugation length of an individual polymer chain.33 ‫ܣ‬଴ି଴ ݊଴ିଵ 1 − 0.24ܹ/߱଴ ଶ = ൬ ൰ ‫ܣ‬଴ିଵ ݊଴ି଴ 1 + 0.073ܹ/߱଴ A0−0 and A0−1 are the intensities of the (0−0) and (0−1) transitions respectively, and ω0 is the vibrational energy of the symmetric vinyl stretch (assumed to be 180 meV). The W values calculated for the films processed from solutions with a solvent additive are compared in Figure 3d. The films processed with the CF additive have constant W values (0.063 eV) regardless of the amount of the solvent additive. In contrast, the films processed from DCB have W values that depend strongly on the amount of added DCB, decreasing from 0.064 eV to 0.044 eV. Furthermore, the trend in the results for the TCB additive is similar to that obtained for the DCB additive. Even for small amounts of added TCB, W decreases more strongly from 0.064 eV to 0.038 eV than for the two other solvent additives. These results indicate that the addition of a good solvent with a higher boiling point induces the formation of P3HT with a longer conjugation length and stronger pi-pi interactions than that produced from a solution with a solvent additive with a lower boiling point. The intermolecular packing and molecular orientation of the P3HT thin films were investigated by performing grazing incidence X-ray diffraction measurements. Figure 4 compares the XRD images obtained from grazing incidence (GIXRD) measurements for the P3HT films processed from 8


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solutions with various solvent additives. The 2D XRD patterns for the processed films contain distinct (100) diffraction peaks only in the out of plane direction; the (010) diffraction peak in the in-plane direction indicates that the orientation of the P3HT polymer in the films is predominantly edge-on, which can be beneficial in transistor devices with parallel charge transport. The (100) peak associated with lamellar packing appears more intense for the film processed with TCB additive than for those processed with DCB and CF additive, which indicates a higher degree of intermolecular packing (Figure 4(b)). The crystal coherence lengths of the (100) and (010) diffraction peaks are compared in Table S2. The TCB additive-processed film has a longer crystal coherence length in the out of plane direction, 8.4 nm, than the films processed with DCB or CF additive, 7.8 nm. The intensity of the (010) diffraction peak in the in-plane direction due to pi-pi stacking is higher, and the crystal coherence length of the TCB additive-processed film is also longer than those of the CF- and DCB additive-processed films (Figure 4(c)). These results are in good agreement with the UV-vis absorption measurements. The crystallization of the P3HT film obtained by spin-casting from a solution in a high boiling point solvent without any additives is relatively strong, as shown in Figure S4, but could not be reliably reproduced because of the dewetting problem. Atomic force microscopy (AFM) was used to determine the surface morphologies of the processed films (Figures 5 and S5). When we added CF into the P3HT CB solution, it had little effect on the surface roughness and film morphology of the processed film when compared to the processing of the P3HT film without a solvent additive. The boiling point of CF is lower than that of the main solvent CB, so it will evaporate prior to the main solvent and thus have only weak effects on film formation. In contrast, a solvent additive with a higher boiling point than the main solvent changes the surface roughness and surface morphology of the cast P3HT film. As the volume ratio in the mixed solvent of the high boiling point solvent increases, the density of the P3HT nanowires increases, as shown in the phase images in Figures 5b and c. In particular, the mixed solvents containing 5 vol% and 10 vol% TCB produce clearly well-ordered nanowires without post treatment. The main solvent CB evaporates 9



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first, and the remaining DCB or TCB enable the P3HT solution cast onto the substrate to fully selfassemble during solidification. As the volume ratio of the high boiling point solvent additive increases, the surface roughness of the cast film increases, which is attributed to the prolonged evaporation time for self-assembly that enhances the crystallinity of the π-stacked P3HT nanowires.34 From these results, we conclude that adding a high boiling point solvent to the P3HT solution effectively assists the formation of uniform films with a highly crystalline structure due to the low evaporation rate of the high boiling point solvent. The addition of the higher boiling point solvent TCB results in strong P3HT crystallization, and is more effective than using the high boiling point solvent as a solvent without additives or using the high boiling point solvent as a solvent without additives or using a nonsolvent as the additive because of the dewetting problems shown in Figure 1. To investigate the electrical characteristics of the P3HT films processed in mixed solvents, the fieldeffect mobilities of the P3HT films in FETs with a top-contact transistor geometry were measured. Typical drain current (ID) versus gate voltage (VG) plots operating in accumulation mode are shown in Figure 6. When a high boiling point good solvent is added, the on-current and field-effect mobility increase gradually as the amount of additive increases and reach saturation at nearly 5 vol%. A maximum mobility of 1.2×10-2 cm2V-1s-1 was found for the FET based on a P3HT film processed from a mixed solvent containing 5 vol% TCB. This remarkable increase in the field-effect mobility can be explained by the enhanced crystallinity and favorable molecular orientation of the film, as confirmed by the UV-Vis and XRD results. Further, the good solvent additive TCB provides a wide range of processability; a high loading of the solvent additive, 20 vol%, still provides a high field-effect mobility of 0.98×10-2 cm2V-1s-1. The DCB-processed devices also exhibit enhanced charge transport properties for a wide processing window: field-effect mobilities of 0.70×10-2 cm2V-1s-1 and 0.80×10-2 cm2V-1s-1 were obtained with additions of 5 vol% and 20 vol% respectively. However, the field-effect mobility of the FET containing a P3HT film processed from the low boiling point solvent CF has constant value, 0.10×10-2 cm2V-1s-1, for additions in the range 5 vol% to 20 vol%. These results 10


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indicate that the good solvent additive must have a higher boiling point than the main solvent to enhance the field-effect mobility. When non-solvents with respect to P3HT such as ACN and DMF are used as additives, the P3HT molecules form ordered aggregates in the solution state and then nanofibrils in the thin film as the solvent evaporates during solidification. However, when the amount of the non-solvent additive in the solution is significantly increased, the pre-aggregation of P3HT in solution can inhibit uniform film formation, which means that the processing conditions for enhancing device performance are narrow. In this study, it was observed that the optimal levels of the non-solvent additives with respect to the field-effect mobility were in the range 5–10 vol%. Maximum mobilities of 1.1×10-2 and 0.56×10-2 cm2V-1s-1 were observed in the FETs prepared from solutions with 10 vol% ACN and 5 vol% DMF respectively (Figure S6). When more than 20 vol% of the non-solvent additives are added, the processed films are significantly dewetted or defective, and are not appropriate for use in FET devices. These properties of non-solvent additives are detrimental to the reliability and reproducibility of largearea solution processes. We demonstrated the reliability of the good solvent additive approach by analyzing the field-effect mobilities of devices based on 16 transistor units on a 1 square inch wafer (Figure 7). First, we tested the solvents CB, DCB, and TCB without additives as reference systems. Using solvents with high boiling points such as DCB and TCB but no solvent additives resulted in serious dewetting problems, so those films did not completely cover their substrates, which resulted in the malfunctioning of several device arrays. However, the devices processed from high boiling point solvent mixtures exhibit reliable performances with no malfunctioning. The average field-effect mobilities and standard deviations of the fabricated FET devices are compared in Figure 7g. The devices fabricated from solvents with no additives were found to exhibit variable device performances with a lower average field-effect mobility and a higher standard deviation than devices processed from high boiling point solvent mixtures. The device processed from CB without additives was found to exhibit the lowest 11



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average field-effect mobility, 0.12×10-2 cm2 V-1 s-1. The devices processed from DCB and TCB without additives had slightly increased average field-effect mobilities, 0.47×10-2 and 0.30×10-2 cm2 V-1 s-1 respectively, but significantly increased standard deviations of ±0.0039 (83%) and 0.0024 (80%) cm2V-1s-1 respectively due to their nonhomogeneous film morphologies. In contrast, the devices processed from mixtures containing good solvents, CB:DCB and CB:TCB, were found to exhibit increased average field-effect mobilities with significantly lower standard deviations than the devices processed from solutions without additives. In particular, the device processed from CB:TCB had the highest average field-effect mobility, 1.2×10-2cm2V-1s-1, and a low standard deviation, ± 0.0014 (11%) cm2V-1s-1. The device processed from the solution containing the non-solvent additive CB:ACN was found to exhibit an increased field-effect mobility, 0.8×10-2 cm2 V1

s-1 and a higher standard deviation, ± 0.0019 (24%) cm2V-1s-1 than the devices processed with good

solvent additives. These results demonstrate that processing films from solutions containing good solvent additives can produce not only high charge carrier mobilities but also reduce device performance variation in an array of devices.

4. Conclusion In conclusion, we investigated the effects of the presence in the spin-casting solution of additives that are a good solvent for P3HT molecules on the morphology, crystallinity, and electrical properties of the resulting thin film. By examining the microstructures of the prepared thin films with light absorption and GIWAXS, it was demonstrated that the good solvent additives with high boiling points induce strong pi-pi stacking during film formation and enable the preparation of uniform thin films without dewetting problems. This enhancement in the thin film crystallinity is attributed to the extended solvent drying time, which provides sufficient time for the P3HT molecules to self-assemble. These characteristics are consistent with the device performances of the thin films, which are reliably 12


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high in large-area devices. The device processed from a CB:TCB solvent mixture was found to exhibit the highest average field-effect mobility, 0.012 cm2V-1s-1, with a small standard deviation, ± 0.0014, over an array of 16 devices. Finally, we note that this method can be used to promote the long-range ordering of P3HT crystallinity and to prepare uniform films even on large substrates, which cannot be achieved by using a high boiling point solvent on its own or non-solvent additives such as ACN and DMF, because these approaches do not enable the reproducible preparation of uniform smooth films. This strategy can be combined with various solution-printing techniques (e.g. ink-jet, micro contact, and spray printing) for the deposition of organic semiconductor layer on a flexible substrate over a large area. It is expected that this preliminary study will provide a foundation for the development of low-cost large-area flexible electronics. This solution-based processing method is clearly appropriate to polymer electronics applications.

Supporting Information : (Field effect transistor mobilities and standard deviation, d-spacings and crystal coherence lengths, OM images, Film thicknesses, UV-vis absorption spectra, 2D GIXD, AFM phase images, transfer curve ) This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgement This research was supported by the Incheon National University Research Grant in 2015 and by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2016R1D1A1B03931906).

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Figure captions Figure 1. Photographs of P3HT films spin-cast on HMDS-coated Si wafers from various solvents with various additive volume ratios. (a) CF, CB, DCB, and TCB from the left, (b) 5 vol% solvent additives (DCB, TCB, ACN, and DMF from the left) in chlorobenzene, (c) 20 vol% solvent additives (DCB, TCB, ACN, and DMF from the left) in chlorobenzene. Figure 2. UV-vis absorption spectra of diluted P3HT solutions containing various good solvents at various volume ratios with respect to chlorobenzene: (a) chloroform, (b) 1,2-dichlorobenzene, and (c) 1,2,4-trichlorobenzene. The insets show photographs of the diluted P3HT solutions with various solvent additives at various volume ratios. (d) Solubility parameters (δT) and boiling points of the solvents. The dotted line is the solubility parameter of P3HT. Figure 3. UV-Vis absorption spectra of the thin films spin-cast from P3HT solutions in CB containing various volume ratio of additives, showing the normalized absorption bands at the 0–1 transition (λ = 558 nm: (a) CF/CB, (b) DCB/CB, and (c) TCB/CB. (d) The ratios of the intensities of the first (λ = 603 nm) and second (λ = 558 nm) vibronic transitions. The inset shows the variation in the interchain coupling energy W with the volume ratio of the solvent additive with respect to chlorobenzene for the various solvent additives. Figure 4. (a) 2D GIXD patterns obtained from P3HT thin films spin-cast from solutions containing 10 vol% of various additives in chlorobenzene. Extracted X-ray intensity 1D profiles along the (b) out-of-plane and (c) in-plane directions. Figure 5. Tapping mode AFM phase images of P3HT films processed from mixed solvents: (a) CF/CB, (b) DCB/CB, (c) TCB/CB (1, 5, and 10 vol% from the left). The insets show a height image of each film. (d) The root-mean-square (RMS) roughnesses of the surfaces of these films. Figure 6. Plot of the drain current versus the gate voltage at a fixed drain voltage of -80 V on both 18


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linear (left axis) and log (right axis) scales for the following devices containing P3HT thin films processed from mixed solvents: (a) CF/CB, (b) DCB/CB, and (c) TCB/CB. (d) The variations with the additive volume ratio with respect to chlorobenzene in the field-effect mobilities for the saturation regime of the P3HT FETs for the various additives. Figure 7. The average field-effect mobilities of P3HT films spin-cast from various P3HT solutions: (a) CB, (b) DCB, (c) TCB, (d) CB/DCB 5%, (e) CB/TCB 5%, and (f) CB/ACN 10%. (g) Device performance statistics of P3HT films processed from various solutions. These results confirm the reliability of the method. The gradation of colors revealed the field-effect mobility of a unit cell within 1 square inch wafer for each sample.

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Table 1. Physical properties and solubility parameters the (δT) of P3HT and solvents.

Surface Tb

Viscosity

δD

δP

δH

δT

δT

(cP)

(cal/cm3)1/2

(cal/cm3)1/2

(cal/cm3)1/2

(cal/cm3)1/2

=δP3HT-δT

9.02

1.94

2.05

9.73

tension (°C) (mN/m) P3HT CF

61

26.7

0.563

8.65

1.5

2.8

9.21

0.52

CB

131

33

0.753

9.28

2.1

1.0

9.57

0.16

oDCB

180

35.7

1.32

9.35

3.1

1.6

9.98

0.25

TCB

214

39.1

1.611

9.88

2.05

1.57

10.21

0.48

ACN

82

28.7

0.37

7.5

8.8

3

11.9

2.17

DMF

153

34.4

0.79

8.51

6.70

5.53

12.16

2.42

20


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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 6

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TOC Graphics

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Solution Processing with a Good Solvent Additive for Highly Reliable

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