Identifying orthogonal solvents for solution processed organictransistors

Abhinav M. Gaikwad a, *, Yasser Khan a, Aminy E. Ostfeld a, Shishir Pandya b,Sameer Abraham a, Ana Claudia Arias a, **

a Electrical Engineering and Computer Sciences Department, University of California Berkeley, Cory Hall, Berkeley, CA 94720, USAb Materials Sciences and Engineering Department, University of California Berkeley, Hearst Mining Building, Berkeley, CA 94720, USA

a r t i c l e i n f o

Article history:Received 5 October 2015Received in revised form8 December 2015Accepted 11 December 2015Available online xxx

Keywords:Organic semiconductorsOrthogonal solventsHansen solubilityTIPS-PentacenePBTTT

a b s t r a c t

Identification of solvents for dissolving polymer dielectrics and organic semiconductors is necessary forthe fabrication of solution-processed organic field effect transistors (OFETs). In addition to solubility andprintability of a solvent, orthogonality is particularly important when forming multilayer structure fromsolutions. Currently, the process of finding orthogonal solvents is empirical, and based on trial-and-errorexperimental methods. In this paper, we present a methodology for identifying orthogonal solvents forsolution-processed organic devices. We study the accuracy of Hildebrand and Hansen solubility theoriesfor building solubility boundaries for organic semiconductor (Poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene (PBTTT) and polymer dielectrics (Poly(methyl methacrylate) (PMMA), Poly-styrene (PS)). The Hansen solubility sphere for the organic semiconductor and polymer gate dielectricsare analyzed to identify solvents that dissolve PMMA and PS, but are orthogonal to PBTTT. Top gate/bottom contact PBTTT based OFETs are fabricated with PMMA gate dielectric processed with solventsthat are orthogonal and non-orthogonal to PBTTT. The non-orthogonal solvents swell the semiconductorlayer and increase their surface roughness.

Published by Elsevier B.V.

1. Introduction

Solution-processed organic semiconductors have enabled thefabrication of large-area electronics by depositing electronic ma-terials with printing methods such as gravure [1], inkjet [2],flexographic [3] and offset [4,5]. Over the past two decades, effortsin molecular engineering [6e8], understanding of interfacial effects[9], and control over processing conditions [10,11], have led tosignificant improvements in the electrical characteristics of organicfield effect transistors (OFETs) [12,13]. OFETs are ideal for applica-tions such as smart tags, sensors, and active-matrix backplanes fordisplays and imagers, where the electronic components are lessdense and distributed over large area [12,14e17].

The typical structure of OFET consists of a source and drainelectrode in contact with conjugated organic semiconductor layer,which is separated from the gate electrode by an insulating

dielectric layer. The gate voltage modulates the charge density inthe semiconductor layer and the voltage bias between the sourceand drain electrode controls the flow of current [18]. Early dem-onstrations of OFETs were hybrid devices where Si wafers withthermally grown SiO2 gate insulator were used as common gate tostudy the electronic properties of vapor deposited small molecules,and polymeric semiconductors [7,19,20]. In order to take fulladvantage of low cost fabrication processes and mechanical flexi-bility of organic semiconductor, solution processable polymer di-electrics are necessary. Insulating polymers such as Poly(methylmethacrylate) (PMMA), Polystyrene (PS), Polyvinylpyrrolidone(PVP) and CYTOP have been used extensively as gate dielectric inOFETs [21,22]. The choice of gate dielectric is important becauseperformance and stability of OFETs is sensitive to the nature of thesemiconductoredielectric interface [9,23]. Furthermore, themobility of OFETs depends on the molecular ordering of thesemiconductor layer [7,24], presence of chemical and physical traps[23,25], and alignment of the energy levels between the contactsand semiconductor [26,27].

OFETs are fabricated with four different architectures, bottomgate/bottom contact, bottom gate/top contact, top gate/bottom

* Corresponding author.** Corresponding author.

E-mail addresses: agaikwad@berkeley.edu (A.M. Gaikwad), acarias@eecs.berkeley.edu (A.C. Arias).

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Organic Electronics

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http://dx.doi.org/10.1016/j.orgel.2015.12.0081566-1199/Published by Elsevier B.V.

Organic Electronics 30 (2016) 18e29

contact and top gate/top contact [18]. Bottom gate architecture hasbeenwidely used by the research community as the semiconductorcan be deposited as the last wet processing step during the fabri-cation process, preventing the swelling of the semiconductor layerby the polymer dielectric solvent. In addition, the surface energy ofthe dielectric surface can be modified to improve the morphologyof the semiconductor layer [24]. Devices with top gate architecturesare advantageous as compared to bottom gate architectures due toeasy of fabrication, low contact resistance and environmental sta-bility [12,18]. The source and drain electrodes in top gate OFETs canbe patterned on substrates using high-resolution techniques suchas photolithography, or printed using high-speed, roll-to-rollprinting methods such as gravure, flexographic or offset, followedby the deposition of the semiconductor, dielectric layer and topelectrode. Due to the staggered configuration, OFETs with top gate/bottom contact architecture have low contact resistance in com-parison to the other architectures [28e30]. In addition, thedielectric and top electrode provide an auto-encapsulation for thesemiconductor layer, which can improve its environmental stability[21].

Even with these advantages, OFETs with top gate/bottom con-tact architecture have been difficult to implement. Finding a solventthat dissolves the polymer dielectric, but is orthogonal (solvent thatdissolves one material and does not dissolve the other) to theorganic semiconductor is a challenge and for most organic semi-conductors only a handful of suitable orthogonal solvents havebeen successfully identified [31e35]. Solubility models such asHansen and Hildebrand theory have been used extensively by theorganic solar cell community to find green, non-chlorinated sol-vents for processing the active layer [36e39]. Similar methods havenot been fully utilized for the fabrication of OFETs [31,40]. In thiswork, we present amethodology to identify orthogonal solvents forsolution processed devices. We study the accuracy of Hildebrandand Hansen solubility theories for defining solubility boundariesfor organic semiconductor (Poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene (PBTTT) and polymer dielectrics(PMMA and PS). With the goal of fabricating top gate/bottomcontact PBTTT based OFETs, the solubility data was used to identifysolvents that dissolve the polymer gate dielectrics but are orthog-onal to the semiconductor (Fig. 1). Top gate/bottom contact PBTTT

based OFETs were fabricated with polymer gate dielectric dissolvedin solvents that are orthogonal and non-orthogonal towards PBTTT.The effect of orthogonality of the polymer dielectric solution on themorphology of the PBTTT layer was studied.

2. Experimental section

Poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene(PBTTT, Merck), 6,13-Bis(triisopropylsilylethynyl)Pentacene (TIPS-pentacene, Sigma Aldrich), PMMA (Sigma Aldrich, Mol.Wt. ¼ 120,000) and PS (Sigma Aldrich, Mol. Wt. ¼ 280,000) wereused as received without further purification. 2 mg/mL solutions ofthe organic semiconductors (PBTTT and TIPS-pentacene) and20 mg/mL solutions of the polymer dielectric (PMMA and PS) inthirty-three selected solvents were prepared. The solvents used inthis study are list in Table 1. In order to facilitate the dissolution ofthe polymers, the solutions were heated overnight at 70 �C.

Accurate estimation of the solubility boundaries for the organicsemiconductors and polymer dielectrics is essential to identifysolvents that solubilize the polymer dielectrics but are orthogonal(non-solvent) to the organic semiconductors. The solubilityboundary for the organic semiconductor was defined to representthe boundary between orthogonal and non-orthogonal solvents.The orthogonal solvents were defined as ‘bad solvents’ and theremaining solvents were defined as ‘good solvents’. The solubilityboundary for the polymer dielectric was defined to identify sol-vents that dissolve the polymer dielectric. Solvents that were ableto dissolve 20 mg/mL of the polymer dielectrics were classified as‘good solvents’ and the remaining solvents were classified as ‘badsolvents’.

OFETs were fabricated using in top gate/bottom contactconfiguration on a glass substrate. The substrate (Corning 1737)was cleaned in a soap solution (Decon 90) for 1 h followed byultrasonication with acetone and isopropanol for 10 min. Thesubstrate was treated in oxygen plasma for 5 min before evapo-rating gold (30 nm) with Cr (5 nm) adhesion layer. To deposit thislayer, the Cr/Au layer was patterned using standard lithographytechnique to form the source and drain contacts. A self-assembledmonolayer of PFBTwas formed on the gold electrode to increase thework function of the electrode and decrease the contact resistance.

Fig. 1. (A) Schematic representation and (B) optical micrograph of OFET with top gate/bottom contact architecture. (C) Chemical structure of polymer dielectrics (PMMA and PS) and(D) organic semiconductors (PBTTT and TIPS-pentacene).

A.M. Gaikwad et al. / Organic Electronics 30 (2016) 18e29 19

The substratewith the Cr/Au electrodes was immersed in a solutionof 10 mM solution of pentafluorobenzenethiol (PFBT) in ethanol for2 min. A solution of PBTTT was prepared by mixing 5 mg/mL ofPBTTT in dichlorobenzene. Due to poor solubility of PBTTT, thesolution turned into a gel when cooled to room temperature. Inorder to solubilize PBTTT, the solution was heated to 85 �C for20 min before spin coating. A hot solution of PBTTT was spin coatedon the glass substrate at 2000 RPM for 1 min in a glove box withlow oxygen and moisture levels (<2 ppm). The substrate washeated on a hot plate at 110 �C for 1 h to remove residual solventsfollowed by thermal annealing at high temperature to improvemolecular ordering. The substrate was placed on a hot plate at175 �C for 10min and the hot platewas cooled at 10 �C/min to roomtemperature. A polymer dielectric layer was formed by spin coatingat solution of PMMA (60 mg/mL) in various solvents at 1500 RPMfor 1 min in a glove box followed by heating the substrate on a hotplate at 115 �C for 2 h to remove residual solvent. The PMMA so-lutions were prepared by dissolving PMMA in various solvents byheating the solution overnight on a hot plate at 70 �C. The dielectricsolutions were filtered twice with a 0.1 mm filtrate before spincoating. The gate contact was formed by evaporating 30 nm gold onthe dielectric layer with a shadow mask.

The electrical characterizations of the fabricated OFETs weremeasuring using semiconductor characterization system (AgilentTechnologies, B1500A) in atmosphere under ambient condition. Allmeasurements were performed within 2 h of exposing the deviceto atmospheric conditions. A total of 8 to 10 devices weremeasuredfor each combination of polymer dielectric and organic semi-conductor. High-resolution topographic studies of the films were

carried out using atomic force microscope (MFP-3D, AsylumResearch) in the normal tapping mode under ambient conditions.The Hansen solubility sphere was fitted to the solubility data usinga custom made software. Detailed description about the softwareand operating instructions are giving in the supplementary section.In short, based on the results of the solubility study, the solventswere categorized as ‘good’ or ‘bad’ solvents. A convex hull wascreated by randomly picking three good solvents from the outerregions of the Hansen space. A sphere passing through three goodsolvents was fitted, and the radius and center of the sphere wasextracted. If all other good solvents were present inside the sphere,the center of the sphere represented the Hansen parameters for thematerial and the radius of sphere represented the radius of inter-action for the material. If any good solvents were present outsidethe sphere, a new set of three good solvents were picked and theprocess was repeated until all good solvents were enclosed insidethe sphere.

3. Results and discussion

3.1. Hildebrand and Hansen solubility theories

In Hildebrand theory, each material is given a solubility number(d), which is commonly referred as ‘Hildebrand Solubility Param-eter’ [41e43]. The solubility parameter gives a numerical value forthe total cohesive attractive interactions within a material due toVan der Waals forces. For a liquid, the solubility parameter isdefined as the square root of the total cohesive energy density(energy required in cal./cc to convert a substance from liquid to

Table 1List of solvents used for the solubility study of PBTTT and their Hansen parameters. The good solvents are marked as ‘1’ and the bad solvents are marked as ‘0’. RED number(Relative Energy Difference) gives the distance of the solvent relative to the radius of the Hansen sphere for the organic semiconductors. The grade represents the degree ofsolubility of the solvents. Grade-A solvents dissolve the organic semiconductor completely, while grade-E solvents are completely orthogonal to the semiconductor.

PBTTT

Solvent T (MPa0.5) dD (MPa0.5) dP (MPa0.5) dH (MPa0.5) Sol. RED Grade

1-Butanol 23.20 16.0 5.7 15.8 1 2.2 DDioxane 20.47 19.0 1.8 7.4 1 0.9 D2-Methoxyethanol 24.82 16.2 9.2 16.4 0 2.5 EAcetone 19.94 15.5 10.4 7.0 0 1.4 EAcetophenone 21.72 19.6 8.6 3.7 0 1.1 EBenzene 18.51 18.4 0.0 2.0 1 0.9 CBenzyl alcohol 23.79 18.4 6.3 13.7 0 1.8 EButyl acetate 17.41 15.8 3.7 6.3 0 0.8 ECarbon tetrachloride 17.81 17.8 0.0 0.6 1 1.0 CChlorobenzene 19.58 19.0 4.3 2.0 1 0.6 AChloroform 18.95 17.8 3.1 5.7 1 0.4 ACyclohexanol 22.40 17.4 4.1 13.5 0 1.7 ECyclopentanone 17.90 16.7 3.8 5.2 1 0.4 DDichlorobenzene 19.87 18.6 6.2 3.2 1 0.5 ADietheyl carbonate 17.63 16.3 3.0 6.0 0 0.6 EDimethyl formamide 24.86 17.4 13.7 11.3 0 1.2 EDimethyl sulfoxide 26.68 18.4 16.4 10.2 0 2.1 EEthanol 26.52 15.8 8.8 19.4 0 2.5 EEthoxyethanol 22.95 15.8 9.0 14.0 0 3.0 EEthyl acetate 18.15 15.8 5.3 7.2 0 2.1 EEthylene dichloride 20.80 19.0 7.4 4.1 0 0.9 EEthylene glycol 32.95 17.0 11.0 26.0 0 4.2 EFormamide 36.65 17.2 26.2 19.0 0 4.8 EHexane 14.90 14.9 0.0 0.0 0 1.4 EIsophorone 19.94 16.6 8.2 7.4 1 1.0 DMesitylene 18.01 18.0 0.0 0.6 1 1.0 BMethanol 29.61 15.1 12.3 22.3 0 3.7 EPropyl acetate 17.62 15.3 4.3 7.6 0 1.1 EPropylene glycol 30.22 16.8 9.4 23.3 0 3.6 EPGMEA 19.26 15.6 5.6 9.8 0 1.3 ETetrahydrofuran 19.46 16.8 5.7 8.0 1 0.8 BToluene 18.16 18.0 1.4 2.0 1 0.6 BTetralin 19.01 18.7 2.0 2.8 1 1.0 A

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gaseous state).

d ¼�EV

�0:5(1)

V is the molar volume of pure solvent and E is the energy ofvaporization. The solubility parameter has units of MPa1/2.

Heat of vaporization and solubility are related to each other asthe energy required to break all the attractive bonds within asubstance while converting it from liquid to vapor state is similar tothe change in free energy required while dispersing two materialstogether. Materials with similar values of solubility parameter arelikely to be miscible with each other and a large difference in sol-ubility parameter would indicate poor miscibility. Hildebrand pa-rameters for most commonly used organic solvents have beenexperimentally measured and are readily available in the literature[43]. Solubility parameter for polymers cannot be directlymeasured as they decompose before reaching their boiling point.Instead, the solubility parameters for polymers are extracted bymeasuring the degree of swelling or dissolution in series of solventswith increasing Hildebrand value. The polymers are assigned asolubility parameter value based on solvent that swells or dissolvesthem to the highest extent. The region encompassing all the goodsolvents for a material represents the solubility window for thematerial and the bad solvents are present outside the solubilitywindow.

Even with the apparent simplicity of Hildebrand solubilitytheory, the solubility prediction can be incorrect under certainconditions. Van der Waals attractive forces are an additive combi-nation of three different forces: dispersion forces, dipole forces andhydrogen bonding forces. The Hildebrand solubility theory com-bines all the constituent forces into a single number. Solvents withsimilar Hildebrand parameter number can have large differences in

Fig. 2. Supernatant of grade A, B, C, D and E solvents used to dissolve PBTTT.

Fig. 3. (A) Solubility data of PBTTT represented using Hildebrand theory. (B) Three-dimensional representation of the solubility data of PBTTT using Hansen theory. (C) Two-dimensional Hansen plot and the Hansen parameters for PBTTT.

A.M. Gaikwad et al. / Organic Electronics 30 (2016) 18e29 21

their individual cohesive interactions, which can lead to a falseprediction of miscibility and orthogonality when the prediction isbased solely on the summation of the all the attractive forces.

Hansen solubility theory provides a more accurate estimation ofsolubility by dividing the Hildebrand solubility parameter into itsconstitutive interaction parameters. The total cohesive energy isgiven as the addition of the cohesive energy due to dispersion force,polar forces and hydrogen bonding force [44,45].

E ¼ ED þ EP þ EH (2)

Dividing equation (2) by the molar volume and using Hilde-brand formalization, we get the following equations.

E=V ¼ ED=Vþ EP=V þ EH=V (3)

d2T ¼ d2D þ d2P þ d2H (4)

The square of the Hildebrand parameter is equal to the additionof squares of the solubility parameters for the dispersion, polar andhydrogen-bonding component. The Hansen parameters for a ma-terial are represented as a point in a three-dimensional space(Hansen Space) with the dispersion, polar and hydrogen bondingcomponents plotted along the three axes. The Hansen parametersfor most organic solvents are available in the literature. The Hansenparameters for polymers are extracted by dissolving the polymer ina set of 30e40 solvents that are well dispersed in the Hansen Space[43]. Solvents are classified as good or bad solvent based on thedegree of swelling or dissolution of the polymer. The good solventsaggregate together and form a shape of an ellipsoid in the Hansenspace. Based on experimental analysis, Hansen suggested

multiplying the dispersion axis by a factor of two to transform thespace representing good solvents from an ellipsoid to a sphere,making it easy to visualize the data [43]. Computer software can beuse to fit a sphere to the region representing good solvents (soft-ware description given in the supplementary section). The center ofthe sphere represents the Hansen solubility parameters (dD,dP,dH)for the polymer and the radius is called as the ‘Radius of Interac-tion’. The region within the sphere encompasses the good solventsand the region outside the sphere represents the bad solvents. Oncethe Hansen parameters for a polymer are extracted, the solubility ofany solvent can be predicted by measuring its distance from thecenter of the sphere. If the distance is less than the radius ofinteraction, the solvent is a good solvent and if the distance isgreater than the radius, the solvent is a bad solvent. The distance ofa solvent relative to the radius of the sphere is commonly referredto as RED number (Relative Energy Difference).

RED ¼ Ra

R¼ Distance from center

Radius of sphere(5)

RED value less than one indicates a good solvent and above oneindicates a bad solvent.

3.2. Solubility theories applied to organic semiconductors

Solutions of PBTTT with concentration of 2 mg/mL in thirty-three selected solvents were prepared (Table 1). The solutionswere stirred overnight at 70 �C and classified by visual inspection.Solvents that were completely orthogonal to the semiconductorwere classified as ‘bad solvents’ and the remaining solvents (non-

Table 2List of solvents used for the solubility study of PMMA and PS, and their Hansen parameters. The good solvents are marked as ‘1’ and the bad solvents are marked as ‘0’. REDnumber (Relative Energy Difference) gives the distance of the solvent relative to the radius of the Hansen sphere for the polymers.

PMMA PS

Solvent T (MPa0.5) dD (MPa0.5) dP (MPa0.5) dH (MPa0.5) Solubilty RED Solubilty RED

1-Butanol 23.20 16.0 5.7 15.8 0 1.0 0 1.9Dioxane 20.47 19.0 1.8 7.4 1 0.6 1 0.92-Methoxyethanol 24.82 16.2 9.2 16.4 1 1.0 0 2.1Acetone 19.94 15.5 10.4 7.0 1 0.6 0 1.2Acetophenone 21.72 19.6 8.6 3.7 1 0.6 1 1.0Benzene 18.51 18.4 0.0 2.0 1 1.0 0 1.2Benzyl alcohol 23.79 18.4 6.3 13.7 1 0.6 0 1.5Butyl acetate 17.41 15.8 3.7 6.3 1 0.6 1 0.7Carbon tetrachloride 17.81 17.8 0.0 0.6 0 1.1 0 1.3Chlorobenzene 19.58 19.0 4.3 2.0 1 0.7 1 0.9Chloroform 18.95 17.8 3.1 5.7 1 0.5 1 0.4Cyclohexanol 22.40 17.4 4.1 13.5 0 0.7 0 1.4Cyclopentanone 17.90 16.7 3.8 5.2 1 0.5 1 0.4Dichlorobenzene 19.87 18.6 6.2 3.2 1 0.5 1 0.6Dietheyl carbonate 17.63 16.3 3.0 6.0 1 0.6 1 0.6Dimethyl formamide 24.86 17.4 13.7 11.3 1 0.8 0 1.2Dimethyl sulfoxide 26.68 18.4 16.4 10.2 1 1.0 0 1.8Ethanol 26.52 15.8 8.8 19.4 0 1.3 0 2.1Ethoxyethanol 22.95 15.8 9.0 14.0 1 0.8 0 2.6Ethyl acetate 18.15 15.8 5.3 7.2 1 0.5 0 1.7Ethylene dichloride 20.80 19.0 7.4 4.1 1 0.5 1 0.7Ethylene glycol 32.95 17.0 11.0 26.0 0 2.0 0 3.8Formamide 36.65 17.2 26.2 19.0 0 2.3 0 4.4Hexane 14.90 14.9 0.0 0.0 0 1.3 0 1.7Isophorone 19.94 16.6 8.2 7.4 0 0.3 1 0.7Mesitylene 18.01 18.0 0.0 0.6 0 1.1 0 1.4Methanol 29.61 15.1 12.3 22.3 0 1.7 0 3.3Propyl acetate 17.62 15.3 4.3 7.6 1 0.6 1 0.9Propylene glycol 30.22 16.8 9.4 23.3 0 1.7 0 3.2PGMEA 19.26 15.6 5.6 9.8 1 0.6 1 1.0Tetrahydrofuran 19.46 16.8 5.7 8.0 1 0.3 1 0.5Toluene 18.16 18.0 1.4 2.0 1 0.9 1 1.0Tetralin 19.01 18.7 2.0 2.8 1 0.8 1 0.9

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orthogonal) were classified as ‘good solvents’. Fig. 2 shows theoptical image of supernatant of the solvents that showed a range ofsolubility towards PBTTT (Grade AeE). The orthogonal solventswere rated as Grade-E. The good solvents were further classifiedbased on the extent of dissolution of the semiconductor. Solvents

that dissolved the semiconductor completely were rated as Grade-A. These solvents can be used for depositing the semiconductorlayer. Solvents with small quantities of undissolved semiconductorwere rated as Grade-B. Solvents with large quantities of undis-solved semiconductor were rated as Grade-C. Solvents that had

Fig. 4. Solubility data of PMMA (A) and PS (B) represented using Hildebrand theory. Three-dimensional representation of solubility data of PMMA (C) and PS (E) using Hansentheory. (D) And (F) show the two-dimensional Hansen plot and the Hansen parameters for PMMA and PS, respectively.

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very poor solubility and swelled the semiconductor by a smallextent (observed by visual inspection) were rated as Grade-D. Re-sults of the solubility test are listed in Table 1.

Results from the solubility test for PBTTT was analyzed usingHildebrand and Hansen solubility theories. Fig. 3A show the solu-bility data for PBTTT plotted using Hildebrand methodology. Thesolvents are arranged according to their Hildebrand parameter andlabeled as ‘good solvent’ or ‘bad solvent’. Grade A to D solvents wereconsidered as good solvents and grade E solvents were consideredas bad solvents. PBTTT interacted with solvents with solubilityparameter ranging from 17.8 to 20.5 MPa0.5. Out of the thirty-threesolvents used in this study, twelve solvents interacted with PBTTTand six bad solvents were present inside the solubility window.Hence the Hildebrand theory wasn't able to provide a good esti-mate for the solubility window for PBTTT. Since the Hildebrandtheory was not sufficient to explain the solubility data, the datawasfurther analyzed using Hansen solubility theory. Fig. 3B and Cshows the solubility data of PBTTT plotted using Hansen theory. Thedispersion, polar and hydrogen bonding parameters of the solventswere plotted along the three axes. The ‘good solvents’ are plotted asfilled circles and the ‘bad solvents’ are plotted as open circles.Computer software was used to fit a sphere that encompasses allthe good solvents. Fig. 3C show the two-dimensional

representation of the Hansen plot for PBTTT. The center of sphererepresents the Hansen parameters of the material. The dispersion,polar and hydrogen parameters for PBTTT are 17.30, 4.10,4.00 MPa0.5. The radius of interaction for PBTTT was 5.51. The ac-curacy of the solubility boundary depends on the number of badsolvents present inside the sphere. Out of thirty-three solvents,only two bad solvents were present inside the sphere. Hence, theHansen solubility theory provided a better estimation of solubilityboundary for PBTTT in comparison to Hildebrand solubility theory.

The Hildebrand and Hansen solubility theorywas also applied toa small molecule semiconductor (TIPS-pentacene). The solubilitydata for TIPS-pentacene is give in Table S1. Fig. S1AeFig. S1C showsthe solubility data represented using Hildebrand and Hansen sol-ubility theory. The Hansen theory was again able to provide anexcellent fit for the solubility data. No bad solvents were presentinside the solubility sphere of TIPS-pentacene.

3.3. Solubility theories applied to polymer dielectrics

The solubility of PMMA and PS in thirty-three selected solventswas studied. Solutions with concentration of 20 mg/mL were pre-pared. The solutions were heated overnight at 70 �C to assist withthe dissolution of the polymers. Solvents that dissolved the

Fig. 5. Schematic summarizing the solvents orthogonal to PBTTT, the solubility of PMMA and PS in the orthogonal solvents, and their printabiltiy on PBTTT.

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polymers completely were classified as ‘good solvents’ and theremaining solvents were classified as ‘bad solvents’.

Table 2 shows the results from the solubility study for PMMAand PS. Fig. 4A and 4B show the solubility data for PMMA and PS,respectively, represented using Hildebrand methodology. Solventsdissolving 20 mg/mL of PMMA and PS completely had a Hildebrandparameter value ranging from 17.4 to 26.7 and 17.4e21.7 MPa0.5,respectively. The ester groups along the backbone of PMMA im-proves its solubility in organic solvents, leading to a larger solubilitywindow as compared to PS. Similar to PBTTT, both PMMA and PShad many (six for PMMA and three for PS) non-solvents presentinside their solubility window. In comparison, Hansen theoryprovided a better fit to the solubility data for PMMA and PS(Fig. 4CeF). PS had one bad solvent present inside the Hansensphere while PMMA had two bad solvents present inside theHansen sphere. The dispersion, polar and hydrogen bonding pa-rameters for PMMA and PS are 17.92, 7.34, 7.83 and 17.53, 5.44,5.75 MPa0.5, respectively. The radius of interaction for PMMA andPS was 9.42 and 5.56, respectively.

3.4. Identification of solvents orthogonal to organic semiconductors

The solubility study of the organic semiconductors and polymerdielectrics demonstrate that the Hansen solubility theory has ahigher accuracy in defining solubility boundaries for a material incomparison to the Hildebrand theory. Once the Hansen solubilitysphere for a material is built, the solubility or orthogonality of anyorganic solvent can be predicted to a good accuracy based on thelocation of the solvent with respect to the Hansen sphere. For thefabrication of top gate/bottom contact OFETs, we need to identifysolvents that will dissolve the polymer gate dielectric but areorthogonal to the semiconductor. The Hansen solubility sphere forPBTTT, PMMA and PS were analyzed to identify solvents thatdissolve the polymer dielectric but are orthogonal to the semi-conductor. Out of thirty-three solvents, twenty-one solvents wereorthogonal to PBTTT. Out of the twenty-one orthogonal solvents, sixsolvents dissolved PS and thirteen dissolved PMMA (Fig. 5). Apartfrom the solubility of the polymer dielectric, printability of thedielectric solution on the semiconductor layer plays an important

Fig. 6. (A) Location of PGMEA, EA, PA, BA, dioxane and benzene in the Hansen space. The green sphere represents the Hansen solubiltiy sphere for PBTTT. (B, C, D) Two dimensionalHansen plot for PBTTT. The gray solid and open circles represents good and bad solvents for PBTTT, respectively. The solid and dash circles represent the Hansen solubility sphere forPBTTT and PMMA, respectively.

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role. Printability is the ability of the solution to deposit a uniformlayer of the material during the printing and drying process. Theprintability of a solvent depends on the surface energy of thesubstrate, and the surface tension and vapor pressure of the sol-vent. While considering printability of a solvent, the number ofusable orthogonal solvents dropped to three and six for PS andPMMA, respectively (Fig. 5).

In case of TIPS-pentacene, out of thirty-three solvents only fivewere orthogonal. Out of the five orthogonal solvents none dissolvedPS, and only DMSO andmethanol dissolved PMMA. Both DMSO andmethanol based PMMA solutions had very poor printability, andthe solutions dewet from the TIPS-pentacene layer during the spincoating process. The Hansen solubility model predicts the interac-tion between the material and solvent but the printability of thesolution cannot be predicted.

3.5. Top gate/bottom contact PBTTT based OFETs

Based on the analysis of the solubility data, we fabricated top

gate/bottom contact PBTTT based OFETs with PMMA gate dielectricdissolved in solvents that are completely orthogonal to PBTTT andsolvents that interact with PBTTT so some extent. PGMEA, propylacetate, butyl acetate, ethyl acetate, dioxane and benzene werechosen as the solvent to dissolve PMMA. PGMEA, propyl acetate,butyl acetate and ethyl acetate were completely orthogonal to-wards PBTTT. Benzene (Grade C) and dioxane (Grade D) were non-orthogonal solvents and lay inside the Hansen sphere of PBTTT.Fig. 6A e 6D show the location of the selected solvents in com-parison to the Hansen sphere for PBTTT. The OFETs were fabricatedusing spin coating method on glass substrates with gold contactstreated with pentafluorobenzenethiol (PFBT) to increase their workfunction [46]. The PBTTT layer was annealed at high temperature(175 �C) for 10 min to improve its molecular ordering. The outputand transfer curves for the OFETs are presented in Figs. 7 and 8. Themobility m and threshold voltage VT were extracted from thestandard equation in the saturation region,

Fig. 7. Transfer characteristics of top gate/bottom contact PBTTT based OFETs with PMMA gate dielectric using (a) PGMEA, (b) Propyl acetate, (c) Butyl acetate, (d) Ethyl acetate, (e)Dioxane and (f) Benzene as the solvent, respectively.

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ID ¼�W2L

�mCiðVG � VT Þ2 (6)

Where ID is the drain current, W and L are the channel width andlength, respectively, Ci is the capacitance per unit area of the gatedielectric, and VG is the gate voltage. The mobility of the devices

under saturation regime ranged between 0.018 and 0.052 cm2/V(Table 3).

In this study, the concentration of the polymer dielectric solu-tions (60 mg/mL) and the spin speed (1500 RPM) was kept constantto remove the influence of viscosity and drying rate on morphologyof the gate dielectric. Due to a large difference in the vapor pres-sure, boiling point, viscosity of the polymer dielectric solutions, and

Fig. 8. Output characteristics of top gate/bottom contact PBTTT based OFETs with PMMA gate dielectric using (a) PGMEA, (b) Propyl acetate, (c) Butyl acetate, (d) Ethyl acetate, (e)Dioxane and (f) Benzene as the solvent, respectively.

Table 3Electrical parameters of PBTTT based OFETs with top gate/bottom contact architecture (W/L¼ 1mm/50 mm). The field-effect mobilities were calculated in the saturated regimeusing standard device formalism.

Solvent Thickness (nm) Mobilitya (cm2/Vs) Threshold (V) ON/OFF SS (V/dec) Solubiltiy rating Ra (nm) Mobility at 0.25 MV/cmc (cm2/Vs)

PGMEA 300 0.052(±0.002) �5.56(±0.16) ~103.8 3.11(±0.03) E 1.014 0.026(±0.001)PA 490 0.041(±0.003) �7.72(±0.63) ~103.7 5.29(±0.94) E 0.998 0.025(±0.002)BAb 180 0.036(±0.002) �3.93(±0.21) ~103.6 2.79(±0.43) E 1.079 0.023(±0.002)EA 760 0.039(±0.002) �7.37(±0.95) ~103.2 7.14(±0.34) E 1.066 0.027(±0.001)Dioxane 683 0.030(±0.002) �6.94(±0.55) ~103.0 6.70(±0.28) D 1.114 0.024(±0.002)Benzene 1524 0.018(±0.002) �13.04(±1.95) ~103.0 10.73(±2.66) C 1.357 0.018(±0.002)

a The mobility was measured in the saturation regime at VGS ¼ VDS ¼ 30 V.b The mobility of devices with BA as the solvent for PMMA was measured at VGS ¼ VDS ¼ 20 V.c The mobility was measured at gate bias of 0.25 MV/cm.

A.M. Gaikwad et al. / Organic Electronics 30 (2016) 18e29 27

wettability of the dielectric solutions on PBTTT, the thickness of thedielectric layers after drying was in the range of 300e1500 nm[1,7,47]. The variation in dielectric thickness between the deviceslead to differences in charge accumulation when the gate bias iskept at a constant voltage. To make a fair comparison betweenOFETs with different gate dielectric thicknesses, the mobilities at aconstant gate bias of 0.25 MV/cm was calculated. By normalizingthe data we observe that the mobility of devices processed withorthogonal solvents had a narrow distribution (0.023e0.027 cm2/V,Table 3). The mobility of devices processed with dioxane (Grade-Dsolvent) was 0.024 cm2/V, showing that grade-D solvent had verylittle influence on the morphology of PBTTT and they can beconsidered as orthogonal solvents due to their very poor interac-tion with the semiconductor at room temperature. The mobility ofdevices process with benzene (Grade-C solvent) was 0.018 cm2/Vshowing a slight influence due to swelling of the semiconductor.The electrical measurements show that PMMA polymer gatedielectric processed with orthogonal solvents has negligible effecton the electrical characteristics of the OFETs.

To investigate the effects of the polymer dielectric solution onthe morphology of the semiconductor, the annealed PBTTT layerswere immersed in PGMEA, propyl acetate, butyl acetate, ethyl ac-etate, dioxane and benzene for 2 min followed by spinning at 2000RPM. The immersion process simulates the deposition of thedielectric solution on top of the semiconductor. Fig. 9A and B showsthe topographical AFM images of PBTTT layer, as spun and afterannealing at 175 �C, respectively. The topographical AFM imagesshow an increase in the domain size for the annealed sample ascompared to the as spun sample. In general, the mobility and ON/OFF ratio of the OFETs improved after annealing the sample [7,48].Figs. S2 and S3 show the amplitude and three-dimensional topo-graphical AFM images of the samples. The amplitude images rep-resented the slope of change in height. A rougher amplitude imageindicates smaller domain size in comparison to a sample withsmoother amplitude image. The amplitude image for the annealedsample (Fig. S2B) is smoother as compared to the as spun sample(Fig. S2A), confirming the increase in the domain size after theannealing process. The increase in roughness in the amplitudeimages (Fig. S2C e S2H) for all samples after solvent treatmentindicates that all solvents, both orthogonal and non-orthogonal,interact with PBTTT layer to some extent. In case of orthogonal

solutions, the solvent reduces the average roughness (Ra) of thePBTTT (0.998e1.079 nm) in comparison to the sample with nosolvent treatment (1.097 nm) (Fig. 9). The samples treated withdioxane (Grade D) had a slightly higher surface roughness (Ra),1.114 nm but there was no significant difference in the mobility ofdevices as compared to devices processed with orthogonal sol-vents. Benzene (Grade C) increased the surface roughness (Ra) ofPBTTT to 1.357 nm and a drop in the electrical mobility (0.018 cm2/V) of the OFETs was observed. The non-orthogonal solvents interactwith the semiconductor layer during the spin coating process,which swell the semiconductor layer and increase its surfaceroughness [49]. Hence, non-orthogonal solvents are necessary toprevent interaction between the dielectric solution and the semi-conductor layer.

4. Conclusion

Hansen theory had a higher accuracy in defining solubilityboundaries for organic semiconductors and polymer dielectrics incomparison to Hildebrand theory. The Hansen solubility spheres forthe organic semiconductors and polymer dielectrics can be used toidentify solvents that will dissolve the polymer dielectric but haveno interaction towards the semiconductor (orthogonal solvent) andvise-versa. A number of orthogonal dielectric solutions wereidentified for PBTTT by analyzing the solubility boundaries ofPMMA and PBTTT. There was a negligible difference in mobility ofOFETs processed with orthogonal dielectric solution whereas; thenon-orthogonal dielectric solution swelled the semiconductorlayer, which reduced the mobility of the OFETs. The methodologypresented in this paper can be used for the fabrication of othersolution processed multilayer structures where identification ofgood and orthogonal solvents is necessary.

Author contributions

A.M.G. and A.C.A. conceived the project and designed the ex-periments. A.M.G, Y.K., A.E.O, S.P and S.A carried out the experi-ments. A.M.G and A.C.A wrote the paper and Y.K., A.E.O, S.P and S.Acommented on the paper.

Fig. 9. Topographical atomic force microscopy (AFM) images (2 mm � 2 mm) of spin coated PBTTT film as spun (A) and after annealing at 175 �C for 10 min (B), and annealed PBTTTfilm after immersing in PGMEA (C), propyl acetate (D), butyl acetate (E), ethyl acetate (F), dioxane (G) and benzene (H) for 2 min followed by spinning at 2000 RPM and annealing at110 �C for 1 h.

A.M. Gaikwad et al. / Organic Electronics 30 (2016) 18e2928

Acknowledgment

The authors would like to thank Dr. Igal Deckman, Dr. AvinashBhadani and Adrien Pierre for many helpful discussions and Prof.LaneMartin for access to the AFM instrument. This work is partiallysupported by the National Science Foundation under Grant No. EFRI1240380. This work is also supported in part by Systems onNanoscale Information fabriCs (SONIC), one of the six SRC STARnetCenters, sponsored by MARCO and DARPA. A.O. wishes toacknowledge financial support from the NSF Graduate FellowshipResearch Program under Grant No. DGE-1106400.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.orgel.2015.12.008.

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A.M. Gaikwad et al. / Organic Electronics 30 (2016) 18e29 29

1

Supporting Information

Identifying Orthogonal Solvents for Solution Processed Organic

Transistors

Abhinav M. Gaikwad,* Yasser Khan, Aminy E. Ostfeld, Shishir Pandya, Sameer Abraham and

Ana Claudia Arias*

A. M. Gaikwad, Y. Khan, A. E. Ostfeld, S. Abraham, Prof. A. C. Arias

Electrical Engineering and Computer Sciences Department, University of California Berkeley,

Cory Hall, Berkeley, California 94720, USA.

*E-mail: agaikwad@berkeley.edu, acarias@eecs.berkeley.edu

S. Pandya

Materials Sciences and Engineering Department, University of California Berkeley,

Hearst Mining Building, Berkeley California 94720, USA.

2

TIPS P

Solvent T

(MPa0.5) δD

(MPa0.5)

δP

(MPa0.5)

δH

(MPa0.5) Sol. RED Grade

1-Butanol 23.20 16.0 5.7 15.8 1 0.7 C

Dioxane 20.47 19.0 1.8 7.4 1 0.7 B

2-Methoxyethanol 24.82 16.2 9.2 16.4 1 0.7 C

Acetone 19.94 15.5 10.4 7.0 1 0.3 C

Acetophenone 21.72 19.6 8.6 3.7 1 0.6 A

Benzene 18.51 18.4 0.0 2.0 1 0.9 A

Benzyl alcohol 23.79 18.4 6.3 13.7 1 0.6 C

Butyl acetate 17.41 15.8 3.7 6.3 1 0.4 A

Carbon tetrachloride 17.81 17.8 0.0 0.6 1 0.9 A

Chlorobenzene 19.58 19.0 4.3 2.0 1 0.7 A

Chloroform 18.95 17.8 3.1 5.7 1 0.5 A

Cyclohexanol 22.40 17.4 4.1 13.5 1 0.6 B

Cyclopentanone 17.90 16.7 3.8 5.2 1 0.4 A

Dichlorobenzene 19.87 18.6 6.2 3.2 1 0.5 A

Dietheyl carbonate 17.63 16.3 3.0 6.0 1 0.5 A

Dimethyl formamide 24.86 17.4 13.7 11.3 1 0.6 C

Dimethyl sulfoxide 26.68 18.4 16.4 10.2 0 0.5 E

Ethanol 26.52 15.8 8.8 19.4 1 0.7 C

Ethoxyethanol 22.95 15.8 9.0 14.0 1 1.0 A

Ethyl acetate 18.15 15.8 5.3 7.2 1 0.6 A

Ethylene dichloride 20.80 19.0 7.4 4.1 1 0.3 A

Ethylene glycol 32.95 17.0 11.0 26.0 0 1.6 E

Formamide 36.65 17.2 26.2 19.0 0 1.8 E

Hexane 14.90 14.9 0.0 0.0 1 1.0 B

Isophorone 19.94 16.6 8.2 7.4 1 0.0 B

Mesitylene 18.01 18.0 0.0 0.6 1 0.9 A

Methanol 29.61 15.1 12.3 22.3 0 1.3 E

Propyl acetate 17.62 15.3 4.3 7.6 1 0.4 A

Propylene glycol 30.22 16.8 9.4 23.3 0 1.3 E

PGMEA 19.26 15.6 5.6 9.8 1 0.4 A

Tetrahydrofuran 19.46 16.8 5.7 8.0 1 0.2 A

Toluene 18.16 18.0 1.4 2.0 1 0.7 A

Tetralin 19.01 18.7 2.0 2.8 1 0.9 A

Table S1. List of solvents used for the solubility study of TIPS-pentacene, and their Hansen

parameters. The good solvents are marked as ‘1’ and the bad solvents are marked as ‘0’. RED

number (Relative Energy Difference) gives the distance of the solvent relative to the radius of

the Hansen sphere for the organic semiconductors. The grade represents the degree of

solubility of the solvents. Grade-A solvents dissolve the organic semiconductor completely,

while grade-E solvents are completely orthogonal to the semiconductor.

3

Figure S1. (A) Solubility data of TIPS-pentacene represented using Hildebrand theory. (B)

Three-dimensional representation of the solubility data of TIPS-pentacene using Hansen

theory and (C) show the two-dimensional Hansen plot and the Hansen parameters for TIPS-

pentacene.

Figure S1A-S1C shows the solubility data represented using Hildebrand and Hansen

solubility theory. In contrast to PBTTT, TIPS-pentacene had a much larger solubility window.

Due to its small molecular weight and alkyl side groups, TIPS-pentacene was soluble in most

organic solvents used in this study. TIPS-pentacene was soluble in solvents with solubility

parameter ranging from 14.9 – 26.5 MPa0.5. The dispersion, polar and hydrogen parameters

for TIPS-pentacene are 16.79, 8.49, 7.58 MPa0.5, respectively. The radius of interaction for

TIPS-pentacene was 11.99 and no bad solvent was present inside the Hansen sphere for TIPS-

pentacene.

4

Figure S2. Amplitude atomic force microscopy (AFM) images (2μm × 2μm) of spin coated

PBTTT film - as spun (A) and after annealing at 175°C for 10 min (B), and annealed PBTTT

film after immersing in PGMEA (C), propyl acetate (D), butyl acetate (E), ethyl acetate (F),

dioxane (G) and benzene (H) for 2 minutes followed by spinning at 2000 RPM and annealing

at 110°C for 1 hr.

Figure S3. Three-dimensional topographical atomic force microscopy (AFM) images (2μm ×

2μm) of spin coated PBTTT film - as spun (A) and after annealing at 175°C for 10 min (B),

and annealed PBTTT film after immersing in PGMEA (C), propyl acetate (D), butyl acetate

(E), ethyl acetate (F), dioxane (G) and benzene (H) for 2 minutes followed by spinning at

2000 RPM and annealing at 110°C for 1 hr.

5

Hansen Sphere Fitting Algorithm

Solvents from the solubility test are plotted in the Hansen space. The solvents are

classified either as a ‘good solvent’ or ‘bad solvent’ based on the degree of dissolution of the

material. The Hansen parameters for a material are extracted by implementing an iterative

scheme. In the first step, a convex hull is created, where only the outermost good solvents in

the Hansen space are considered (Figure S4A). Three solvents are randomly chosen from the

outermost solvent set, and their corresponding 2𝛿𝐷, 𝛿𝑃, and 𝛿𝐻 values are taken into account

for creating an encapsulating sphere of radius, R and center at (2𝑥𝐷 , 𝑥𝑃, 𝑥𝐻). If the three

solvents are placed on the surface of the sphere, Equation (S1), (S2), and (S3) will be satisfied,

where the center of the three solvents are located at (2𝛿𝐷1, 𝛿𝑃1, 𝛿𝐻1), (2𝛿𝐷2, 𝛿𝑃2, 𝛿𝐻2), and

(2𝛿𝐷3, 𝛿𝑃3, 𝛿𝐻3).

(2𝛿𝐷1 − 2𝑥𝐷)2 + (𝛿𝑃1 − 𝑥𝑃)2 + (𝛿𝐻1 − 𝑥𝐻)2 = 𝑅2 (𝑆1)

(2𝛿𝐷2 − 2𝑥𝐷)2 + (𝛿𝑃2 − 𝑥𝑃)2 + (𝛿𝐻2 − 𝑥𝐻)2 = 𝑅2 (𝑆2)

(2𝛿𝐷3 − 2𝑥𝐷)2 + (𝛿𝑃3 − 𝑥𝑃)2 + (𝛿𝐻3 − 𝑥𝐻)2 = 𝑅2 (𝑆3)

Equations (S2) and (S3) is subtracted from equation (S1), to give equations (S4) and (S5) with

linear terms of the unknowns (𝑥𝐷 , 𝑥𝑃, 𝑥𝐻).

8(𝛿𝐷1 − 𝛿𝐷2)𝑥𝐷 + 2(𝛿𝑃1 − 𝛿𝑃2)𝑥𝑃 + 2(𝛿𝐻1 − 𝛿𝐻2)𝑥𝐻

= 4𝛿𝐷12 − 4𝛿𝐷2

2 + 𝛿𝑃12 − 𝛿𝑃2

2 + 𝛿𝐻12 − 𝛿𝐻2

2 (𝑆4)

8(𝛿𝐷1 − 𝛿𝐷3)𝑥𝐷 + 2(𝛿𝑃1 − 𝛿𝑃3)𝑥𝑃 + 2(𝛿𝐻1 − 𝛿𝐻3)𝑥𝐻

= 4𝛿𝐷12 − 4𝛿𝐷3

2 + 𝛿𝑃12 − 𝛿𝑃3

2 + 𝛿𝐻12 − 𝛿𝐻3

2 (𝑆5)

The linear system of equations (S4) and (S5) can be solved for (𝑥𝐷 , 𝑥𝑃, 𝑥𝐻). Using the

values obtained, the radius of Hansen sphere, R can be calculated. If the remaining good

solvents are present outside the Hansen sphere, new set of three solvents are picked from the

outer most solvent set (Figure S3B). This process is repeated until all the good solvents are

present inside the sphere.

6

Figure S4. Algorithm for constructing Hansen sphere (a) a convex hull is created using the

good solvents (green filled circles). The connected green circles (at the outer edge) are used

for calculation. The bad solvents (red filled circles) and the good solvents inside the hull are

not considered in the algorithm. (b) After creating the convex hull, three good solvents from

the hull are used to create an encapsulating sphere, if all the good solvent are inside Hansen

sphere parameters are calculated, if a good solvent is found outside sphere, the code uses new

set of solvents and runs iteratively to find encapsulating sphere until all the good solvents are

encapsulated.

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Using the Hansen Sphere Fitting Program

STEP 1

Extract the Hansen.zip file. The directory will have the files listed below.

STEP 2

For using the program, first edit the database file for the solvents based the solubility

test results. A template file (template.txt) is provided in the Hansen folder. The .txt file can be

edited in excel as shown below

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‘T’ represents the total Hildebrand parameter, and ‘dd’, ‘dp’ and ‘dh’ represents the

dispersion, polar and hydrogen bonding parameters of the solvents, respectively, in MPa0.5. In

the ‘Used’ column, enter ‘y’ if the solvent was used and enter ‘n’ if the solvent wasn’t used.

In the ‘Worked’ column, enter ‘y’ if the solvent dissolved the material and enter ‘n’ if the

solvent did not dissolve the material.

STEP 3

To start the program, run the hansen.m file.

STEP 4

The program will ask to select the database file. Select the database file and click open.

!

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STEP 5

After running the script, a dialog window will appear denoting that the script ran successfully

as shown below.

Three pdf files and a text file will be generated

a. <filename>_solvents.pdf - shows all the solvents in the Hansen space

b. <filename>_hansen.pdf - Hansen plot with all the good solvents encapsulated within

the sphere

c. <filename>_2d.pdf - solvents in two-dimensional space

d. <filename>_report.txt – list of all solvents used and their RED value

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Program Output

<filename>_solvents.pdf - shows all the solvents in the Hansen space

<filename>_hansen.pdf - Hansen plot with all the good solvents encapsulated within the

sphere

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<filename>_2d.pdf - solvents in two-dimensional space and the Hansen parameters and

radius of interaction of the material.

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<filename>_report.txt – list of all solvents used and their RED value