Determination of Solubility Parameters for Organic Semiconductor Formulations

Full Paper

Determination of Solubility Parameters forOrganic Semiconductor Formulationsa

Florian Machui,* Steven Abbott, David Waller, Markus Koppe,Christoph J. Brabec

The concept of Hansen solubility parameters (HSP) is applied to organic semiconductors inorder to determine and predict their solubility behavior, which is essential for the design offunctional and environmentally friendly ink formulations for organic photovoltaics. Twodifferent conjugated polymers, one semicrystallineand one dominantly amorphous, and one fullerenederivative are selected as prototype candidates toevaluate the applicability of the HSP concept fororganic semiconductors. The method for determiningthe solubility parameters is described and the qualityof the HSP fits as well as their suitability for designingof organic electronic inks are discussed in detail.

Introduction

Organic photovoltaic devices offer a great technological

potential as an alternative source in the field of common

photovoltaics. The development of low cost photovoltaic

devices is driven by the demand for inexpensive renewable

energy sources. Furthermoreorganic solar cells have gained

great research interest due to several advantages, i.e., light

weight, the use of flexible substrates and the possibility of

F. Machui, Prof. C. J. BrabecInstitute Materials for Electronics and Energy Technology(I-MEET), University Erlangen-Nuremberg, Martensstrasse 7,91058 Erlangen, GermanyE-mail: [email protected]. S. AbbottSteven Abbott TCNF, 7 Elsmere Road, Ipswich, Suffolk IP1 3SZ, UKDr. D. WallerKonarka Technologies Inc., 100 Foot of John Street, Lowell,MA 01852, USADr. M. KoppeKonarka Austria GmbH, Altenbergerstrasse 69, A-4040 Linz,Austria

a Supporting Information this article is available from the WileyOnline Library or from the author.

Macromol. Chem. Phys. 2011, 212, 2159–2165

� 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlin

tailoring band gaps. One of themost important benefits for

a low cost processing technology is the possibility of

depositing layers from solution. The bulk heterojunction

concept, which is based on blending a polymer and a

fullerene derivative, is the most frequently used device

structures for organic solar cells. Currently poly(3-hexyl-

thiophene) (P3HT) as conjugated polymer and [6,6]-phenyl-

C61-butyric acidmethyl ester (PCBM) as fullerenederivative

are the prototypematerials for roll to roll processing. Small

bandgappolymers like thepoly[2,6-(4,4-bis-(2-ethylhexyl)-

4H-cyclopenta[2,1-b;3,4-b0]-dithiophene)-alt-4,7-(2,1,3-benzo-

thiadiazole)] (PCPDTBT) are getting more into the focus

of the solar cell research as they expand the light

absorption into the near infrared region.[1] Specifically

the last years showed continuous improvement of

the device performance due to a significant better

understanding of the device physics. Herein the

control of the solid film microstructure is one major

subject. Besides thermal or solvent annealing procedures,

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F. Machui, S. Abbott, D. Waller, M. Koppe, C. J. Brabec

the choice of the processing solvents or solvent mixtures

has an enormous influence on the resulting domain size

and thus on the cell performance.[2–12] Furthermore it

was shown that processing additives or surfactants are

a successful strategy to modify the film forming

dynamics.[13]

The production of organic solar cells is usually done via

solution processing from chlorinated solvents. However,

these solvents are strongly restricted for industrial

operation due to high safety risks and processing costs.

Therefore, environmentally friendly inks with full

functionality are essential criteria to further path the

way of mass production. In this work we suggest

the use of Hansen solubility parameters (HSP) to

determine and predict the solubility behavior of

conjugated polymers. HSPwas already successfully applied

to different systems like polymer/multi-walled carbon

nanotube composites, napthenic mineral oils, or the

negative electron beam resist hexamethylacetoxycalyx(6)-

arene .[14–16] In the field of organic semiconductors

HSP was not used until recently when Hansen and

Smith analyzed pristine C60 and Walker et al. analyzed

the conjugated polymer 3,6-bis(5-(benzofuran-2-yl)thiophen-

2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione

(DPP(TBFu)2) and [6,6]-phenyl-C71-butyric acidmethyl ester

(PC71BM).[17,18]

The term solubility parameter was first described by

Hildebrand and Scott.[19] The total Hildebrand parameter

dT is defined according to Equation 1 as the square root

of the cohesion energy density. Here V is themolar volume

and E the total cohesion energy.

dT ¼ffiffiffiffiE

V

r(1)

E¼DH� RT (2)

The contributions to E in Equation 2 are the difference in

enthalpy of evaporation DH, the absolute temperature T

and the global ideal gas constant R. The total cohesion

energy E of a solvent can be determined by measuring the

evaporation energy for the liquid, i.e. the energy required

for breaking all cohesive bonds. The approach of Hansen

is an extension of the Hildebrand theory, where the

Hildebrand parameter is split into at least three different

contributions. These originate from the atomic dispersive

interactions (ED), the permanent dipole-permanent dipole

molecular interactions (EP), and the molecular hydrogen

bonding interactions (EH).[17] Overall, Hansen suggests

rewriting the Hildebrand parameter E in the following

notation:

E ¼ ED þ EP þ EH (3)

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Dividing Equation 3 by themolar volumeV, theHSP d are

received:

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E

V

� �¼ ED

V

� �þ EP

V

� �þ EH

V

� �(4)

ThesecanbegeneralizedaccordingtoEquation1and4to:

d2T ¼ d2D þ d2P þ d2H (5)

The three Hansen parameters are frequently used in a

simplifiednotationasD,P,H. Theunitof theseparameters is

MPa1/2. The solubility properties can be visualized in a

three-dimensional coordinate system with the axis dD, dP,

and dH. TheHSP coordinates of the solute are determined by

analyzing the solubility of this solute in a series of solvents

with known Hansen parameters. The solubility space is

then determined by fitting a spheroid into the solubility

space, with the solvents inside the spheroid and the non-

solvents outside. Following this procedure, the solubility

parameters of the solute under analysis are the center

coordinates of the sphere. The radius of the sphere, R0,

indicates themaximumdifference for solubility. The center

of the sphere depends on enthalpy and the radius partially

captures entropic effects. Good solvents are within the

sphere, bad ones are outside. Furthermore, an equation for

the solubility ‘‘distance’’ parameter, Ra, between two

materials based on their respective partial solubility

parameter can be defined via Equation 6.

R2a ¼ aðdD2�dD1Þ2 þ bðdP2�dP1Þ2 þ cðdH2�dH1Þ2 (6)

where Ra is the distance between one solvent and the

solute, dD2 the dispersive component for the solvent, dD1the dispersive component of the solute, and a, b, and c are

weighting factors. Hansen suggested a setting of a¼ 4 and

b¼ c¼ 1 based on empirical testing. Different ratios of

weighting factors convert the Hansen spheroid into an

ellipsoid, when the scale for the dispersion parameter is

doubled the spheroidal shaped volume is converted into a

spherical body.[20] However, the use of fixed variables for

complex mixtures has been questioned.[21] Other experi-

ments have shown that, in general, solubility regions are

unsymmetrical.[22] However, the overwhelming practical

evidence is that the value of 4 is the most generally

applicable value and has been used in this paper.[20]

A simple composite affinity parameter, the relative

energy difference (RED) number is defined according to

Equation 7. RED is a measure for the distance of a solvent

from the center of the volume in Hansen space.

RED ¼ Ra

R0(7)

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Determination of Solubility Parameters for Organic Semiconductor . . .

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A solvent with identical solubility parameters as the

solute will have a RED number equal to 0. Solvents with an

RED equal to 1.0 are located on the surface of the solubility

sphere. Good solvents have a REDnumber smaller than one

and are inside of the solubility sphere. Non-solvents have a

value larger than one and are outside the sphere. The larger

the RED number, the worse the solvent. In solubility tables

the RED number is indirectly given as 0 for a non-solvent

with a RED number higher than 1.0 and as 1 for a solvent

with a RED number equal or lower than 1.0.

Until now only very few attempts have been made to

calculate the solubility parameters at higher tempera-

tures.[23] Higher temperature lead to an increase in

solubility as well as a larger solubility parameter sphere,

whereas dD, dP, and dHdecreasewith increased temperature.

The hydrogen bonding parameter, in particular, is themost

sensitive to temperature. As the temperature is increased,

more andmorehydrogenbonds areprogressively brokenor

weakened, and this parameter will decrease more rapidly

than the others. Increasing the temperature can cause a

non-solvent to become a solvent. For liquids, the change of

the dD, dP, and dH with temperature can be estimated by

Equation 8–10, where g is the coefficient of thermal

expansion:[20]

dd

d

dd

d

dd

d

Fig

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D

T¼ �1:25gdD (8)

P

T¼ �0:5gdD (9)

H

T¼ �dHð1:22� 10�3 þ 0:5gÞ (10)

Figure 1. Chemical structures of analyzed materials.

Experimental Section

P3HT (with Mw ¼65 600 g �mol�1, polydisper-

sity index (PD)¼2.04, and regioregularity

ure 2. HSP diagrams for solutes, 34 solvents at 2.5 g � L�1 and 60 8

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(RR)¼96.6 from Merck), PCPDTBT (with Mw ¼34 400g �mol�1

and PD¼ 2.06 from Konarka Technologies Inc.), and PCBM (with

purity of >99% from Solenne BV) were used as received. The

structures of the analyzed materials are shown in Figure 1. For

the determination of the HSP a selection of 34 solvents, broadly

distributed in the Hansen space, was used. All solvents were

common laboratory solvents of high purity obtained from

commercial chemical suppliers. The solvents with their tempera-

ture-dependent HSP values are listed in Table S1 in the Supporting

Information.

Solutionswithconcentrationsof2.5and10g � L�1wereprepared

for the solubility parameter determination. The fluids were stirred

for at least 12h at room temperature. The solubility of the samples

was analyzed by the experienced eye and graded into categories.

Most consistent results were gained with two categories, i.e. no

solvent (0) and good solvent (1). A categorization into three to

four categories did not improve the fit accuracy in our case.

Subsequently the solubility was analyzed at different tempera-

tures varying from 25 to 140 8C after stirring for at least 4 h each.

The solubility behavior of eachmaterial in each solvent at different

temperatures is listed in Table S2 in the Supporting Information.

The influence of the different concentrations seemed to be

negligible for concentrations between 2.5 and 10g � L�1. Solvents

with a boiling point lower than the measurement temperature

were rated with their lower temperature value. The HSP

coordinates were calculated by the HSPiP software.[24]

C a) P3HT, b) PCPDTBT, c) PCBM.

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Table 1. Temperature dependency of analyzed materials (2.5 g � L�1, 34 solvents).

Solute T[-C]

dD[MPa1/2]

dP[MPa1/2]

dH[MPa1/2]

dT[MPa1/2]

R0

[MPa1/2]

Ina) Outb) Wrong

inc)

Wrong

outd)Fite)

P3HT 25 18.5 5.3 5.3 20.0 2.7 5 29 0 1 0.971

60 18.7 1.4 4.5 19.3 4.3 15 19 2 3 0.853

80 18.2 1.8 4.5 18.8 4.3 16 18 1 3 0.882

100 18.1 1.7 3.6 18.5 4.4 16 18 1 4 0.853

120 15.8 2.4 3.4 16.3 5.3 19 15 3 3 0.824

140 16.9 3.4 3.5 17.6 5.2 22 12 2 2 0.882

PCPDTBT 25 19.6 3.6 8.8 21.8 7.8 23 11 3 0 0.912

60 17.3 3.6 8.7 19.7 8.2 29 5 0 0 1.000

80 16.8 3.6 8.3 19.1 7.5 29 5 0 0 1.000

100 16.2 3.8 7.8 18.4 7.3 29 5 0 0 1.000

120 16.0 3.7 7.7 18.1 7.2 29 5 0 0 1.000

140 15.5 3.5 7.4 17.5 7.0 29 5 0 0 1.000

PCBM 25 20.4 3.5 7.2 21.9 7.5 23 11 0 0 1.000

60 19.3 3.6 6.7 20.7 7.0 24 10 1 0 0.971

80 18.7 4.0 6.1 20.1 7.0 25 9 1 0 0.971

100 18.6 5.5 4.8 20.0 7.8 25 9 1 0 0.971

120 17.1 4.1 5.7 18.5 6.2 26 8 1 0 0.971

140 16.7 3.2 5.1 17.8 6.7 26 8 1 0 0.971

a)Number of solvents; b)Number of non-solvents; c)Solvents that do not dissolve the solute but are inside of the solubility volumes;d)Solvents that do dissolve the solute but are outside the solubility volume; e)Fit accuracy according Equation 11 and 12.

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Results and Discussion

Figure 2 shows the 3D and 2D contour plots of the HSP for

P3HT (Figure 2a), PCPDTBT (Figure 2b), andPCBM (Figure 2c)

at 60 8C and at a concentration of 2.5 g � L�1. A temperature

Figure 3. a) Dependence of number of solvents for a solute on temperatemperature.



of 60 8Cwas chosen as reference as this is a frequently used

processing temperature for the semiconductor printing.

The Hansen spheroid is calculated by a least mean square

fitting routine. All the relevant HSP parameters like the dD,

dP, and dH values, the solubility radius, the fit accuracies as

ture; b) dependence of of solubility radius of solubility spheres on the

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well as the total solubility parameter dT are directly

obtained from the fit. Good solvents within the solubility

sphere are marked with filled spheres while non-solvents

aremarkedwith full cubes. Solvents thatdonotdissolve the

solute but are inside of the solubility volumes, are

abbreviated as ‘‘wrong in’’, and marked with open cubes.

Solvents that do dissolve the solute but are outside the

solubility volume are abbreviated as ‘‘wrong out’’ and

marked with open spheres. P3HT was certainly the most

difficult component to dissolve. Only 16 out of the 34

solventswere able to dissolve P3HT at 60 8C. In comparison,

29 out of the 34 solvents dissolved PCPDTBT (Figure 2b), and

24outof 34 solvents dissolvedPCBM(Figure2c)under these

conditions. The significantly better solubility of PCPDTBT

andPCBMis immediately recognizedby thebigger radiusof

their spheres. Table 1 summarizes the HSP values for the

three different semiconductors. T represents the analyzing

temperature in 8C,R0 thesolubility radius inMPa1/2, ‘‘in’’ the

number of solvents and ‘‘out’’ the number of non-solvents

that were not able to dissolve the solute, ‘‘wrong in’’

represents solvents that do not dissolve the solute but

are inside of the solubility volumes, and ‘‘wrong out’’

solvents that do dissolve the solute but are outside the

solubility volume. The fit parameter specifies the fitting

accuracy, whereas 1.0 represents the best fit. The program

evaluates the input data using a quality-of-fit function

with the form:

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DATAFIT ¼ ðA1A2 . . .AnÞ1=n (11)

with the number of solvents n and

Ai ¼ e�ðerror distanceÞi (12)

Figure 4.HSP solubility limits and regimes for P3HT, PCPDTBT, andPCBM at various temperatures for a) the disperse parts, b) polarparts, and c) hydrogen-bonding parts.

where the error distance is the distance of the solvent in

error to the sphere boundary. Aiwill be 1.0 for a given good

solvent within the sphere and a bad solvent outside the

sphere.[20]

Several tests were run to verify the quality and the

tolerance of the fitting routine. For instance, we used two

different solvent sets, onewith34andonewith47 solvents,

to separately determine the solubility parameters. In the

case of PCBM at 25 8C, we found that for the two different

solvent sets thesolubilityparametersonlyvariedby5%and

the solubility radius by only 7%. With increasing tempera-

ture the number of ‘‘good’’ solvents is also increasing. The

temperaturebehaviorof the three components isvisualized

in Figure3. P3HThasavery strongly expressed temperature

effect – thenumber of solvents is increasingnearly fourfold

between roomand 140 8C (Figure 3a). As a consequence, the

solubility radius of P3HT is also increasing with higher

temperatures (Figure 3b). For PCPDTBTandPCBMthis effect

is significantly less expressed or even absent. At this point

it is important to note that P3HT is a semi-crystalline

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semiconductor with a strong tendency to aggregate.

Typically rigorous stirring at elevated temperatures is

necessary to break the aggregates and dissolve P3HT. HSP

cannot handle aggregates or crystallites. This is consistent

with the general lower quality of the spheroid fit (see Table

1 Parameter ‘‘fit’’) for P3HT. Additionally, the number of

‘‘wrong solvents in or out’’ was significant for P3HT. We

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Figure 5. HSP diagram for solutes at 60 8C with 34 solvents, 2.5 g � L�1 for P3HT, PCPDTBTand PCBM.

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haveanalyzed theHSPsofP3HTwitheven

larger solvent sets of up to 47 solvents and

did not get better consistency. One of the

problems with P3HT is the dissolution

of the aggregates. Depending on the

synthesis route, raw P3HT is obtained

in various crystallinity grades. Higher

temperatures are required to break

these aggregates. A number of solvents,

categorized as a ‘‘non-solvent’’ at lower

temperatures, became a ‘‘solvent’’ at

elevated temperatures. Once the aggre-

gates were broken, these solvents

remained solvents even when the

solutions were cooled down to lower

temperatures, until they started to form

gels and became non-solvents again. A

second problem in the HSP analysis of

P3HT is thepartial solutionability of lower

molecular weight fractions, which can be

solubilized even in non-solvents like ethyl

acetate or hexane.[25] Overall, the lower

quality of the fit together with the quite

large number of wrong solvents marks a

question mark behind the validity of the

HSP analysis for P3HT, at least for the low

temperature regimes.Attempts to identify

twosolubilitydomainsusingatwo-sphere

approach gave satisfactory fits to the data.

The concept behind a two sphere fit is to

match the high dD value of the thiophene

unit with one sphere while using a second sphere with

a low dD value for the hexyl group. However, in this

manuscript we will focus on the one sphere concept to

describe conjugated polymers.

PCPDTBT and PCBM show amuch higher quality fit at all

temperatures and amuch lower number of ‘‘wrong in’’ and

‘‘wrong out’’ solvents. The design of printing formulations

requires the knowledge of themutual solubility regime for

all components. This information is easily extracted from

analyzing each solubility parameter separately. The lower

and upper limit of the solubility regime is the diameter of

the solubility sphere. These parameters are plotted for

each material in Figure 4 and listed in Table S3 in the

Supporting Information. Not surprisingly, P3HT is limiting

thesolubility forcomposite inks. Forall threeparameters dD,

dP, and dH, the solubility range of P3HT lies within the

parameter regime of PCPDTBT and PCBM.

The regime of joint solubility for all three components at

60 8C is indicated by a dashed line in Figure 4. The solubility

regime for every analyzed temperature is listed in Table S3

in the Supporting Information. All solvents within this

regime are expected to dissolve all three semiconductors

and this is visualized in Figure 5.



All solventswithin the cross-sectionof the threevolumes

are marked with full symbols; solvents outside the cross-

section are marked with open symbols. Hansen and Smith

had analyzed pristine C60 with the following solubility

parameters at 25 8C: dD¼ 19.7MPa1/2, dP¼ 2.9MPa1/2, and

dH¼ 2.7MPa1/2,witha radiusR0of3.9MPa1/2. [17] Compared

to pristine C60, we analyzed PCBM with the following

solubilityparameters: dD¼ 20.4MPa1/2, dP¼ 3.5MPa1/2, and

dH¼ 7.2MPa1/2 and a solubility radius R0 of 7.5MPa1/2 at

25 8C. The larger solubility radius of PCBMreflects themuch

higher solubility of PCBM versus pristine C60.

Conclusion

Hansen solubility parameters were used to determine the

solubility of different organic semiconductors in various

solvents. Three different organic semiconductors were

chosen: a semi-crystalline polymer (P3HT), a dominantly

amorphous polymer (PCPDTBT), anda substituted fullerene

(PCBM). The polymers were chosen to benchmark the

validity, since HSP does not include an entropy part. Our

analysis found a very consistent fit for PCPDTBT over a

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broad temperature regime. The quality of the HSP analysis

was significantly lower for the semi-crystalline polymer

P3HT.No set of solubility parameterswas found to describe

P3HT without ‘‘wrong in’’ or ‘‘wrong out’’ solvents in a

temperature regime of 25–140 8C. The lower quality of the

fitting parameter for P3HT suggests that this particular

polymermightbebetter describedwithanellipsoid instead

of a spheroid.

A wide temperature regime was found to be essential

for a consistent determination of the HSP. HSP values for

organic semiconductors should not be extrapolated to

higher temperatures but be determined at the temperature

of interest. Both, PCPDTBT and PCBM, are described best

with a parameter setwhere the solubility radius has no or a

weak negative temperature dependence. This is in good

agreement with the expectations for a theory which does

not take into account entropy. Some polymers can become

less soluble in a given solvent at higher temperatures

because their expansion coefficients are much lower than

the solvent so the HSPmatch becomesworse. The evidence

from this work is that expansion coefficients are in the

samerangeas thesolventsusedandtherefore the reduction

in their HSP matches those of the solvents. Overall,

HSP is found to be representative for describing and

predicting the solubility of dominantly amorphous organic

semiconductors. Crystalline or dominantly aggregating

semiconductors require further in-depth investigations.

This opens the opportunity to use HSP for the design of

organic semiconductor inks.

Acknowledgements: The authors are grateful for financial sup-port from the Cluster of Excellence Engineering of AdvancedMaterials, Erlangen, and the Deutsche Forschungsgemeinschaft inthe framework of the SPP1355 project.

Received: May 16, 2011; Published online: August 19, 2011; DOI:10.1002/macp.201100284

Keywords: conjugated polymers; fullerenes; Hansen solubilityparameters (HSP); ink formulations; organic photovoltaics

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Determination of Solubility Parameters for Organic Semiconductor Formulations