Interfacial properties of free-standing poly(3-hexylthiophene) films · Interfacial properties of...

Interfacial properties of free-standing poly(3-hexylthiophene) filmsYeneneh Y. Yimer, Ali Dhinojwala, and Mesfin Tsige Citation: J. Chem. Phys. 137, 044703 (2012); doi: 10.1063/1.4736571 View online: http://dx.doi.org/10.1063/1.4736571 View Table of Contents: http://jcp.aip.org/resource/1/JCPSA6/v137/i4 Published by the American Institute of Physics. Additional information on J. Chem. Phys.Journal Homepage: http://jcp.aip.org/ Journal Information: http://jcp.aip.org/about/about_the_journal Top downloads: http://jcp.aip.org/features/most_downloaded Information for Authors: http://jcp.aip.org/authors

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THE JOURNAL OF CHEMICAL PHYSICS 137, 044703 (2012)

Interfacial properties of free-standing poly(3-hexylthiophene) filmsYeneneh Y. Yimer, Ali Dhinojwala, and Mesfin Tsigea)Department of Polymer Science, The University of Akron, Akron, Ohio 44325-3909, USA

(Received 5 May 2012; accepted 25 June 2012; published online 23 July 2012)

Using full atomistic classical molecular dynamics simulations, the interfacial properties of free-standing poly(3-hexylthiophene) (P3HT) films have been investigated. The orientations of differentparts of the P3HT chain and the surface tensions of the films were calculated in a temperature rangeof 540 K–600 K. At the liquid/vacuum interface, the P3HT chain shows ordering by exposing hexylgroups at the interface, while the chain backbone lays flat with the thiophene ring preferentially tilttoward the surface. At the interface, the terminal methyl groups of hexyl side chains are in excesscompared to the methylene groups or thiophene rings. The surface tension of P3HT in its melt stateshows similar temperature dependence to that of polymers that have long alkyl side chains. The sur-face tension values are comparable to those polymers that expose methyl or methylene groups onthe surface. The surface tension values determined for the melt state are lower than the experimen-tal reported values for crystalline P3HT films, as expected. © 2012 American Institute of Physics.[http://dx.doi.org/10.1063/1.4736571]

I. INTRODUCTION

Over the past three decades, conjugated polymers havereceived a great deal of interest mainly due to their po-tential applications in solar cells, light-emitting diodes,and field-effect transistors.1–6 Polymer-based devices, es-pecially solvent-processable ones, have numerous potentialadvantages—such as low cost, light weight, high flexibil-ity, and easy processability—compared to inorganic semi-conductor devices. Among the conjugated polymers, poly(3-hexylthiophene) (P3HT) is a particularly good candidate be-cause it offers good conductivity and solubility. In P3HT, thethiophene backbone provides good electrical conductivity andcharge carrier mobility, while the hexyl side chains providegood solubility and processability of the polymer without sig-nificantly affecting its electrical and optical properties.6–8

The performance of P3HT-based devices is strongly de-pendent on the structural packing, morphology, and interfa-cial properties of the P3HT. In order to improve the efficiencyof these devices, understanding the structural and dynamicalproperties of P3HT molecules in the bulk and at interfacesat the molecular level can be very important. Though bothbulk and interfacial properties of P3HT are equally impor-tant, most of the previous studies have focused mainly onunderstanding its bulk properties. Among them, many com-putational studies have emphasized the electronic structure,9

charge transport,10, 11 and structural properties12–14 of P3HTin bulk. Similarly, most of the previous experimental stud-ies have investigated the structure, packing, morphology,15–18

and charge carrier transport19–21 properties of P3HT in bulk.However, there are relatively few reported experimental stud-ies on the surface and interfacial properties of P3HT. Theseexperimental studies have primarily focused on the orienta-tion of the thiophene-ring backbone of the P3HT at the poly-mer/air and polymer/substrate interfaces,22–25 but less atten-

a)Electronic mail: [email protected].

tion has been given to the orientations of the hexyl side chainsat interfaces.24, 25 For device applications, it is of particularinterest to pinpoint which part of the P3HT chain contributesprimarily to the interface; this crucial piece of informationis missing from the literature. Furthermore, to the best of ourknowledge, there has been no computational study in the liter-ature that specifically addresses the structure and orientationof P3HT chains at interfaces.

In addition, to obtain better structural ordering, most thinfilms that are solution processed are thermally annealed4, 26

and the surfaces may have a tremendous impact on the finalstructural ordering of these films. For P3HT films, thermalannealing is mostly done below the bulk melting temperature.However, in a recent experimental study,27 the annealing ofP3HT on a hydrophobized substrate above its bulk meltingtemperature resulted in a pronounced surface-induced order-ing of P3HT chains. Similarly, Gurau et al.25 observed maxi-mum order in poly(3-alkylthiophene) film when it is heatedabove its melting temperature and then recrystallized justbelow its melting temperature. This approach seems to bepromising in generating highly ordered P3HT films, since an-nealing above the melting temperature removes the residualstress resulting from spin-coating processes. What is missingin these studies is how the structure of the amorphous regionof the semi-crystalline film, which significantly affects chargetransport in the system, changes due to the annealing of thefilm above its melting temperature. It is probably reasonableto assume that the amorphous region of the semi-crystallinefilm may have similar structure to the melt if the film was pre-pared by annealing it above its melting temperature. It is alsovery likely that the surface groups that are present at the sur-face in the melt state would also be preferred in the solid stateof the film after annealing. It is thus important to understandthe structure of a P3HT film in a melt state in order to elu-cidate the structure and morphology of the amorphous regionand also the surface properties of the crystalline film, whichis the main motivation for our present work.

0021-9606/2012/137(4)/044703/11/$30.00 © 2012 American Institute of Physics137, 044703-1


http://dx.doi.org/10.1063/1.4736571http://dx.doi.org/10.1063/1.4736571mailto: [email protected]

044703-2 Yimer, Dhinojwala, and Tsige J. Chem. Phys. 137, 044703 (2012)

In the present work, we are also interested in determiningthe surface tension of a P3HT melt as a function of tempera-ture. The surface tension is a thermodynamic parameter thatplays an important role in determining properties such as wet-ting, coating, blending, and adhesion. A polymer film usuallyachieves a thermodynamically equilibrium surface by mini-mizing the surface energy either by minimizing the surfacearea and/or by exposing group of atoms that are energeticallyfavorable to the interface. The orientation of these group ofatoms at the interface controls most of the interfacial prop-erties of the film. An understanding of the surface propertiesof P3HT at the molecular level, leading to structure-propertyrelations, is a prerequisite to the design of a suitable P3HTfilm for a given application whenever interfacial phenomenaare involved. Though the surface tension is clearly an impor-tant parameter, there exist very few experimental studies (andno computational studies, to the best of our knowledge) thathave reported surface tension values for P3HT films in thesolid state,28–30 and none that do so for the melt state.

In this paper, we report the results of molecular dynamicssimulations describing the structural and surface properties offree-standing P3HT melt thin films. We examine the orienta-tions of P3HT chains at liquid/vacuum interfaces as well asinterfacial width, surface roughness, and surface tension as afunction of temperature. In Sec. II, we will discus our sim-ulation methodology, including the details of our simulationprocedures and surface tension calculations. The results anddiscussion – the mass and number density profile, the orienta-tional ordering, interfacial width, surface roughness, and thesurface tension – are presented in Sec. III. The summary andconclusions are given in Sec. IV.

II. METHODOLOGY

A. Force field description and simulation details

In this work, we used the force field proposed by Huanget al.13 for melt P3HT and validated by the same group.13, 14, 31

In this force field, the total potential energy of the systemis represented as a sum of four types of potentials: bond-stretching, angle-bending, torsion, and non-bonded interac-tions. Huang et al.13 adopted the force field parameters fromseveral different sources. For the backbone chain, equilibriumbond lengths, angles, and partial charges were based on themodel of Marcon and Raos for tetrathiophene,32 while thedihedral potential parameters were based on the results ofab initio calculations of 3HT oligomers by Darling et al.33

The force field parameters for the hexyl group were takenfrom the optimized potential for liquid simulations-all atom(OPLS-AA) force field for n-alkanes.34 The van der Waalsinteractions are described by the Lennard-Jones potential,ULJij = 4�ij [(σij /rij )12 − (σij /rij )6], and the parameters forthe LJ diameter σ ij and the interaction strength �ij were alsoobtained from the OPLS-AA force field.34 The interaction pa-rameters (σ ij and �ij) for the heteroatomic interactions arespecified by the geometric mean of the homoatomic param-eters, as is the case with the OPLS-AA force field. The non-bonded interactions (LJ + Coulombic) are evaluated for allintermolecular interactions and for intrachain interactions be-tween atoms separated by three bonds or more. For intrachain

interactions between atoms separated by three bonds, the in-teraction potential is reduced by a factor of 2, following theOPLS-AA force field paradigm.

In this study, we consider 100% regioregular P3HTchains consisting of 100 chains with 10 monomers per chain;thus, the total number of atoms is 25 200. The initial configu-ration of the P3HT system was generated by randomly plac-ing the P3HT chains in a cubic periodic simulation box ofdimensions Lx = Ly = Lz = 67.5 Å, corresponding to a den-sity of 0.898 gm/cm3. The initial equilibration of the P3HTsystem was carried out in two steps – first the system was runin the NPT ensemble at a temperature of 600 K, far above themelting temperature of P3HT,18, 27, 35–38 and at a pressure of 1atm for 5 ns, which allowed the system to adjust to its meltdensity at this temperature. The system was further equili-brated in the NVT ensemble for 5 ns at the same temperature.Once equilibrium in the NVT ensemble was reached, a P3HTfilm with surfaces in the (x, y) plane was created by remov-ing the periodicity along the z-direction, resulting in a newbox with dimensions Lx = Ly = 68.0 Å and Lz = 102.0 Å. Toensure that periodic images of the resulting film did not inter-act in the z-direction, the simulation box dimension along thez-direction was significantly increased to Lz = 202.0 Å. TheP3HT film was then run in the NVT ensemble at temper-atures between 540 K and 600 K, in increments of 20 K,well above the melting temperature of P3HT (Refs. 27, 35,and 18, 36–38) and well below its degradation temperature(693 K).39 All simulations were carried out using theLAMMPS simulation package.40 The equations of motionwere integrated using the velocity-Verlet algorithm using atime step of 1 fs. For both NPT and NVT ensembles, a Nosé-Hoover thermostat with a damping time of 100 fs was used tomaintain the temperature of the system. For the NPT ensem-ble, an additional Nosé-Hoover barostat with a 1 ps dampingtime was used to keep the pressure of the system at 1 atm.For the non-bonded interactions, the cutoff radius was fixed at12 Å and the long-range Coulomb interactions (beyondthe cutoff radius) were calculated using the particle-particle/particle-mesh Ewald algorithm.41 The surface tensioncalculations required the P3HT film system to be equilibratedfor a long time—70 to 100 ns, depending on the temperature.The surface tension was calculated following the proceduredescribed below.

B. Surface tension and tail correction calculation

At a given temperature, the surface tension, γ p, of theP3HT melt system was calculated during the simulation usingthe following Kirkwood-Buff equation:42

γp = Lz2

[〈pz〉 − 〈px〉 + 〈py〉

2

], (1)

where 〈pz〉 and 〈px 〉 +〈py 〉2 are normal and tangential pressurecomponents, respectively. The angle brackets denote time av-erage and the factor of 1/2 exists to account for the symme-try in the simulation box. The surface tension calculated us-ing Eq. (1) does not take into account the contribution of theLennard-Jones interaction beyond the cutoff radius. The “tail



correction,” γ tail, to the surface tension can be calculated fromthe mass density profile ρ(z) (Ref. 43) as

γtail = π2

∫ ∞−∞

∫ 1−1

∫ ∞rc

r3dU (r)

drg(r)(1 − 3s2)(ρ(z)ρ(z − sr)

− (ρG(z))2)drdsdz, (2)where U(r) is the pairwise potential, g(r) is the radial distri-bution function, rc is the cutoff for the dispersion terms in thenon-bonded interactions, s = cos(θ ) in cylindrical polar co-ordinates, and ρG(z) is a Gibbs dividing surface.44 The Gibbsdividing surface is defined by

ρG(z) = ρl + ρv2

+ ρl − ρv2

sgn(z), (3)

where ρ l and ρv are the averaged densities of the liquid andthe vacuum phases, respectively. In Eq. (2), for our case, theradial distribution function is assumed to be g(r) = 1 forr ≥ rc. Based on previous simulation studies,45, 46 we fit ρ(z)to an error function

ρ(z) = ρl − ρv2

(erf

((|z0| − |z|)√

2�

)+ 1

), (4)

where zo is the position of the Gibbs dividing surface and � isthe interfacial width. The overall surface tension of the systemis given as the sum of the surface tension of the simulationresult and the tail correction: γ = γ p + γ tail.

III. RESULTS AND DISCUSSION

A. Density profiles

1. Mass density profiles

One of the simplest ways to characterize the interface isby calculating the mass density profile, which is easily ac-cessible to simulations and can also be determined in experi-ments. Here, we calculated the mass density profile of the filmat four different temperatures from 540 K to 600 K in inter-vals of 20 K. The mass density at a given value of z, ρ(z), iscalculated by partitioning the simulation cell into bins alongthe z-direction, with bin thickness �z = 1 Å, and summingthe masses of atoms in each bin per partitioned volume.

Figure 1 shows time-averaged mass density profiles ofP3HT film at four different temperatures. The mass densityprofiles show two symmetric and smooth liquid/vacuum in-terfaces. The observed stable and smooth interfaces are goodindications of the applicability of the force field parameterswe used in describing the P3HT film. Furthermore, at a giventemperature the middle region of the film shows an approx-imately constant mass density (cf. region IV of Figure 6),which is expected to correspond to the bulk liquid density ofP3HT at that given temperature. To compute the bulk liquiddensity, ρb, at each temperature, we took the region in whichthe density is essentially constant and calculated the averagedensity within this region. The calculated bulk densities atdifferent temperatures are tabulated in Table I and are alsoshown in the inset of Figure 1. As can be seen in the inset,the predicted bulk density of P3HT decreases linearly from0.852 g/cm3 at 540 K to 0.787 g/cm3 at 600 K.

-40 -20 0 20 40

Z (Å)

0

0.2

0.4

0.6

0.8

ρ (g

/cm

3 )

540 560 580 600T (K)

0.78

0.8

0.82

0.84

0.86

FIG. 1. Average mass density profiles of P3HT along the z axis at 540 K(circles), 560 K (pluses), 580 K (triangles), and 600 K (stars). The solid linesare the fit to the density profiles using Eq. (4). Inset: variation of bulk liquiddensity, ρb, with temperature and the broken line is included as a guide to theeye.

In the mass density profile, we also observe two promi-nent features with decreasing temperature: the liquid/vacuuminterface becomes sharper, and the mass density profile showsa peak at the boundary of the bulk and interface regions. Toelucidate the former observed behavior, we characterized theinterface by determining the dependence of interfacial widthon temperature. The interfacial width, �, and the position ofthe Gibbs dividing surface, zo, at each temperature were ob-tained by fitting Eq. (4) to the density profiles. As depictedin Figure 1 by the solid lines, Eq. (4) fits the density profilesvery well, especially at the interface. The interfacial width andthe film thickness, l, defined as the distance between the twoGibbs dividing surfaces,47 are given in Table I. As shown inTable I and in line with the general expectation for polymerfilms both interfacial width and film thickness increase withincreasing temperature. We also would like to add that the liq-uid densities, ρ l, extracted from the fit to the density profilesusing Eq. (4) are in excellent agreement with the ρb reportedin Table I.

The roughness of the film surface can be characterized byusing the following root-mean-square roughness W :

W =〈√

1

N2

∑i

(Hi(t) − H (t))2〉

, (5)

where N2 is the number of grids on the surface (in the presentcase, we used N = 15) and Hi(t) and H (t) are the height ofgrid i at time t and the average height of the grids at time t, re-spectively. The time average, 〈〉, is taken over the last 2 ns. Ata given temperature, roughness for both the top and bottom

TABLE I. Simulation results for bulk density (ρb), interfacial width (�),film thickness (l), and surface roughness (W ).

T (K) ρb (g/cm3) � (Å) l (Å) W (Å)

540 0.852 3.576 70.018 4.2560 0.833 4.121 71.539 4.7580 0.807 4.131 73.762 4.9600 0.787 4.451 75.618 5.2



FIG. 2. Isochore plots of surface height variation (the difference between Hiand H ) of the top surface of the film at the end of the simulation at 540 K(left) and 600 K (right).

sides of the film surface were calculated, and the averagevalue of the two surfaces is tabulated in Table I. As shownin Table I, the roughness increases with increasing temper-ature due to thermal fluctuations. At each temperature, theroughness, W , is found to be slightly greater than the interfa-cial width, �. Though the two parameters are interchangeablyused to characterize surface roughness, the variation is due tothe fact that � does not take the lateral structure of the roughsurface into account while W does.

To better characterize the topology of the film at liq-uid/vacuum interfaces, an isochore profile was constructed bytaking the height of each grid (Hi) with respect to the averageheight (H (t)). In Figure 2, isochore plots of the top surface ofthe film at 540 K and 600 K are shown. As can be seen, forboth temperatures, the fluctuation in surface height with re-spect to the average height is in the range of ±10 Å. However,more pronounced peaks and valleys are observed at 600 Kthan at 540 K, as expected from the roughness results.

The observed density peak at the boundary of the in-terface in Figure 1 can be explained better by investigatingthe number density of the different components of the P3HTchain and is presented below.

2. Segment number density profiles

Compared to linear polymer chains, the P3HT chain hasa relatively complex structure due to rigid thiophene ringson the backbone and long flexible hexyl side chains thatare terminated with methyl groups. To determine how the

different components of the P3HT chain are distributedthrough out the film, the number density of thiophene rings,methyl groups, and hexyl side chains have been calculated.The segment number density at a given value of z is calcu-lated by partitioning the simulation cell into bins along thez-direction (with slab thickness �z = 3.0 Å) and summingover the number of target groups in each bin per partitionedvolume. This slab thickness is chosen in order to obtain goodstatistics within the film profile. For each group of atoms(thiophene ring, methyl group, and hexyl group), we took thegeometric center as representative of the groups’ respectivepositions.

The number density profile of thiophene rings and methyland hexyl groups at two representative temperatures, 540 Kand 600 K, are shown in Figure 3. On the vacuum side, methylgroups show higher number density than hexyl groups andthiophene rings. The snapshot shown in Figure 4 also con-firms the methyl groups are in excess at the liquid/vacuuminterface. The three number densities meet around the Gibbsdividing surface, and the trend is reversed on the liquid (bulk)side, where the thiophene rings dominate over methyl andhexyl groups. In addition, there is a peak in the ring num-ber density profile at the boundary of the interface, similar tothe peak we observed in the mass density profile in Figure 1.This means there is an enhanced thiophene ring density at theboundary of the interface, indicating thiophene ring orderingat the interface that is more pronounced at lower temperatures.

From the above observations, we can conclude that P3HTforms a stable structure at the liquid/vacuum interface by ex-posing the hydrophobic part of the chain (methyl groups) tothe vacuum side and the hydrophilic part (thiophene rings) tothe liquid side. Recent molecular dynamics simulation studiesof the small molecule 4-methylpyridine at the liquid/vacuuminterface49 showed similar number density profiles. The ob-served enhanced number density profile of the nitrogen atomson pyridine was attributed to the segregation of the nonpo-lar part (the methyl groups) from the polar part (pyridine)at the interface, which can be thought of as a result of thepresence of the hexyl group. Furthermore, in normal alka-nes, the methyl groups are excess at the vacuum (air) side,of liquid/vacuum interfaces, as opposed to the liquid (bulk)

-40 -20 0 20 40

Z (Å)

0

0.001

0.002

0.003

0.004

Num

ber

Den

sity

(Å

-3)

-40 -20 0 20 40

Z (Å)

0

0.001

0.002

0.003

0.004

Num

ber

Den

sity

(Å

-3)

(a) (b)

FIG. 3. Number density profile of thiophene rings (circles), hexyl groups (stars), and methyl groups (triangles) at 540 K (left) and 600 K (right). Solid lines areincluded as a guide to the eye.



FIG. 4. Snapshot of the top 30 Å of the P3HT film at 540 K. To show P3HTchain conformations at liquid/vacuum interface, representative P3HT chainsare shown in van der Waals representation where sulfur, carbon, and hydro-gen are colored yellow, green, and gray, respectively. For clarity, the rest ofthe P3HT chains in the film are shown as pink lines. The solid vertical linesare the edges of the simulation box and the snapshot was generated using theVMD software.48

side.50 Since P3HT has a rigid backbone which would con-strain the orientation of the hexyl groups, in order to havehigher number density of methyl groups at the liquid/vacuuminterface, the plane of the thiophene rings close to the in-terface should be tilted towards the normal to the surface,while the chain preferentially lays parallel to the surface. Thisshould induce better packing of the thiophene rings close tothe interface, which will in turn result in increasing the thio-phene ring density considerably at the boundary of the inter-face, as is evidenced in both Figures 1 and 3. This warrants aquantitative description of the orientation of the different seg-ments of the P3HT chain at the interface, presented in detail inSubsection III B.

B. Molecular orientation

The differences observed in segment number density pro-files for different parts of the P3HT chain and the enhancedthiophene ring density at the interfaces of the film have bothstructural and thermodynamic origins. The thermodynamicorigin has been already briefly discussed above in terms ofthe difference in polarity between the different parts of theP3HT chain. To qualitatively elucidate the structural origin ofthese differences, we have calculated the orientational orderparameter of the thiophene rings, backbone segments, hexylside chains, and methyl groups of the chain. This will allow usto compare our simulation results with experiment, as the ori-entational ordering of different parts of P3HT at the interfacecan be measured using different experiential techniques. Forexample, using sum-frequency generation vibrational spec-troscopy, the methylene and methyl groups at the interface canbe easily probed.51 Near-edge x-ray absorption fine structurespectroscopy (NEXAFS) (Refs. 22–24) can also be used todetermine the orientation of thiophene ring and hexyl groupsat the interface. It is therefore possible to directly compareour simulation results with experiment. The orientational or-der parameter can be calculated using the second-order Leg-endre polynomial, which is given by

P2(z) = 12〈3 cos2(θ ) − 1〉, (6)

FIG. 5. Definition of the orientational angle θ of different parts of P3HTchains: (a) backbone segment, (b) thiophene ring, (c) hexyl group, and(d) methyl group.

where θ is the angle between the z axis, i.e., normal to the in-terface, and a vector used to describe the orientation of each ofthe different parts of the P3HT chain. The orientational vectorfor the four different parts of the P3HT chain is defined as fol-lows. (1) Backbone segment: the vector originating from thefirst carbon atom on the thiophene ring and ending at the firstcarbon atom on the neighboring thiophene ring, as shown inFigure 5(a). (2) Thiophene ring: the vector originating fromthe thiophene sulfur atom and ending at the carbon atom onthe thiophene ring that connects the thiophene ring to a hexylgroup, as shown in Figure 5(b). (3) Hexyl group: the vectororiginating from carbon atom on the thiophene ring (whichbonds thiophene ring to its hexyl group) and ending at thecarbon atom on the methyl group, as shown in Figure 5(c). Inorder to compare with experiment, the orientation of the C–Cbonds in the hexyl group is also determined. (4) Methyl group:the vector originating from the carbon atom of the methylgroup and ending at the center of the hydrogen atoms of themethyl group, as shown in Figure 5(d). The average, 〈〉, istaken over all possible orientations and time steps within aspecified slice of width, �z = 3.0 Å , over a given region inthe z-direction. The value of the order parameter P2(z) is thus1 when the orientational vector is perpendicular to the surface,while 0 may imply random orientation and −0.5 implies thatthe orientational vector is parallel to the surface.

In order to better quantify the orientational ordering ofthe different components of the P3HT chain within the film,we have divided the film into four regions, and calculated thenormalized probability distribution of the angle θ , P(θ ), ineach region. We calculated P(θ ) in each region by partition-ing θ values from 0 to π into bins of width �θ = π /60 andsumming over the number of angles that lie between θ andθ + �θ in each bin and normalizing it with respect to thetotal number of possible orientations. We would like to pointout that since both the mass and number density profiles ex-hibit symmetry, the P(θ ) reported for the interfacial part of thefilm in the present work was calculated using the top side ofthe film.



-40 -20 0 20 40

Z (Å)

-0.2

-0.1

0

0.1

0.2

0.3

0.4

P2(

Z)

0

0.001

0.002

0.003

0.004

Num

ber

Den

sity

(Å

-3)

I II IVIII III II I

><

-40 -20 0 20 40

Z (Å)

-0.2

-0.1

0

0.1

0.2

0.3

0.4

P2(

Z)

0

0.001

0.002

0.003

0.004

Num

ber

Den

sity

(Å

-3)

IVIIIII III II II

><

(a) (b)

FIG. 6. Orientational order parameter profile, P2(z) (circles), and the number density profile (stars) of the backbone segments at 540 K (left) and 600 K (right).The broken vertical lines divide the film into different regions.

1. Backbone segment orientation

Figure 6 shows the average orientational order parameterP2(z) of the backbone segments at two representative temper-atures, 540 K and 600 K. The average ring number density isalso shown in the same figure as a reference for identifyingthe different regions of the film. In order to correlate the be-havior of P2(z) with the ring number density, we have dividedthe film into four regions separated by broken vertical lines inthe figure: (I) the vacuum region, where the ring number den-sity is below 10%; (II) the 90–10 region, where the numberdensity decreases from 90% to 10% of the bulk density; (III)the dense region, which extends from the end of the 90–10 re-gion to the first minimum of the ring density oscillation; and(IV) the bulk region, which is everything else.

In Figure 6, in the bulk region for both temperatures,P2(z) ≈ 0, implying that the backbone segments are closeto a random orientation in this region. In regions II and III,P2(z) < 0 indicates that the backbone lies preferentially par-allel to the surface. Note that the order parameter P2(z) showsa deep minimum at the boundary of the two interfaces. In re-gion I, P2(z) > 0 suggests that the backbone segments on theouter side of the interface are oriented preferentially normalto the surface. A more detailed analysis of the data shows thatthe backbone segments at chain ends are the ones that domi-

nate region I, since they want to expose the hydrogen atomsat the chain ends and experience less steric hindrance to doso. The orientational order parameter calculation for the hy-drogen atoms at the chain ends (not shown here) shows thehydrogen atoms to be sticking out of the surface and domi-nating the vacuum side of the interface.

Clearly, the degree of orientational order at the interfaceis higher at lower temperatures and correlates very closelywith the amplitude of the peak in the ring number densityin region II. Because the ordering in regions II and III is notvery strong, the amplitude of the peak in the ring number den-sity is small. However, the amplitude in both the orientationand ring number density increases with decreasing tempera-ture and may significantly increase as the crystallization tem-perature is approached; at this point, this is beyond the timescale of standard molecular dynamics simulations for poly-mers such as P3HT with very slow dynamics.

Figure 7 shows the angular probability distribution, P(θ ),for the backbone segments in regions II, III, and IV at 540K and 600 K. In the bulk region at both temperatures, P(θ )shows a broad normal (Gaussian) distribution. Therefore, theorientation of the backbone in the bulk region is random. Thepeak position at 540 K is slightly off from π /2, which maybe due to the fact that some chains are participating in both

0 0.4 0.8 1.2 1.6 2 2.4 2.8 3.2θ (rad)

0

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P(θ

)

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0

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P(θ

)

(a) (b)

FIG. 7. Probability distribution of the tilt-angle θ of the backbone chain in region II (stars), region III (triangles), bulk region (circles) at 540 K (left), and600 K (right).



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IV III IIIIIII II><

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ber

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sity

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-3)

IVIIIIII III II I><

(a) (b)

FIG. 8. Orientational order parameter profile, P2(z) (circles), and number density profile (stars) of thiophene rings at 540 K (left) and 600 K (right).

the interfacial and bulk regions of the film, with the effectbecoming observable as the temperature is lowered. In re-gions II and III for both temperatures, P(θ ) shows a narrowerGaussian distribution with a higher peak than the bulk region.The narrower distribution and higher peak in the middle ofthe distribution indicate that the backbone segments prefer toorient more parallel to the surface, in agreement with whatwe concluded above based on the orientational order param-eter. Furthermore, at 540 K, region III serves as a transientregion by displaying a distribution that is between regions IIand IV. At 600 K, region III (which is not part of the interfa-cial region at 540 K) has become part of region II (same P(θ )),which agrees with the observed increase in interfacial widthwith increasing temperature.

2. Thiophene ring orientation

Figure 8 shows the average orientational order parame-ter P2(z) and the respective average number densities of thethiophene rings at 540 K and 600 K. In the bulk region, forboth temperatures, P2(z) ≈ 0 implies that the orientation ofthe thiophene rings is essentially random. In regions II andIII, P2(z) > 0 indicates a preference of the thiophene rings tobe oriented normal to the surface. However, the value of theorder parameter is small, hence the ordering is weak. In re-gion I, the implication of P2(z) < 0 may not be immediatelyobvious. As pointed out above, region I is dominated by chainends, with the hydrogen atom at a chain end sticking out of thesurface. Since the thiophene ring vector defined in Figure 5(b)is almost perpendicular to the orientation of the hydrogenatom at a chain end, the ring vector should orient parallel tothe surface, as confirmed by the negative P2(z) value, for thehydrogen atom to stick out of the surface. This is also consis-tent with the orientation of the backbone with respect to thering vector at the vacuum region.

We also calculated the percentage of thiophene rings thatpreferentially expose sulfur atoms towards the surface normalin different regions of the film. When the angle between thethiophene ring vector and the surface normal (i.e. the z-axis)is less than π /2, the ring is considered to be preferentiallyexposing its sulfur atom towards the surface normal. At 540K, the percentage of thiophene rings that expose sulfur atoms

towards the surface normal are 45%, 49%, and 50% for the90–10 region, dense region, and bulk region, respectively. Thecorresponding values at 600 K is 45%, 48%, and 50%. There-fore, at the interface, most of the thiophene rings expose tothe surface the carbon atom that connects the thiophene ringto the hexyl group so that the hexyl group can easily stick outto the surface.

Though our results are for a P3HT film in a melt state,they contain implications for a P3HT film in a solid state, andcomparing our results to experimental results for solid P3HTfilm is fully justified. Using NEXAFS for regioregular P3HTthin films at air/film interfaces, several average tilt-anglemeasurements of thiophene ring have been reported.22–24 Hoet al.24 found tilt angles for spin-casted thin film, using differ-ent solvents, of 57◦–60◦ and drop-cast thin-film P3HT of 59◦.Hao et al.22, 23 investigated the orientation of the thiophenering for different thin-film samples. The measured average tiltangles are 62◦, 72◦, and 70◦ for samples spin coated at highspeed, low speed, and dip coated, respectively. Moreover, in-vestigations of the effect of annealing on the orientation fordifferent solution concentrations show average tilt angles inthe ranges 55◦–58◦ and 59◦–62◦ for the thin films before andafter annealing, respectively.

In spin-coating processes, the rate of solvent evapora-tion is proportional to the film thickness. The difference inspin coating may lead to different microstructures of the pro-cessed thin films because the final microstructure is mainlya result of the competition between rate of solvent evapora-tion and interfacial tension.52 For regioregular P3HT filmsprepared with spin coating the thiophene ring orientation canvary depending on the spin-coating parameters.53 More face-on conformations are obtained using fast spin speeds and highsolution concentration. Spin-coated thin films of P3HT pre-pared with slow spin-coating rates, slow solvent evaporationrates, and annealed above the melting temperature result in themost thermodynamically stable orientations. At the polymer-air interface, the thiophene rings adopt a highly ordered edge-on orientation.20 Our simulation were conducted in the meltstate; thus, the thin films can easily attain a thermodynami-cally stable state. The orientations of the thiophene rings atthe interfaces we obtained are consistent with the experimen-tally observed thiophene ring orientations for P3HT thin filmsthat are thermodynamically stable.



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><<

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<

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FIG. 9. Orientational order parameter profile, P2(z) (circles), and number density profile (stars) of hexyl groups at 540 K (left) and 600 K (right). Also shownin the figure is the order parameter for the C–C bonds in the hexyl group (rectangles).

3. Hexyl group orientation

Figure 9 shows the hexyl group orientational order pa-rameter and the corresponding number density for 540 K and600 K. The orientational order parameter of the C–C bondsin the hexyl group is also shown in the figure and displaysthe same orientation as the the hexyl group. The orientationof the hexyl groups is consistent with our proposed picture ofthe orientation of a P3HT chain in a thin film—random ori-entation in the bulk region and in the surface region (regionsII and III), backbone segments preferentially orient parallelto the surface, with thiophene rings slightly tilted toward thesurface normal, and the hexyl groups preferentially orient par-allel to the surface normal (P2(z) > 0).

The angular distributions, P(θ ), for hexyl groups in re-gions II, III, and IV at 540 K and 600 K are shown inFigure 10. In the bulk region, for both temperatures, θ hasa broad Gaussian distribution with peak value at π /2, whichimplies that the hexyl groups have random orientations. In re-gion II, θ shows Gaussian distributions skewed to small val-ues of θ (θ


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(a) (b)

FIG. 11. Orientational order parameter profile, P2(z), (circles) and number density profile (stars) of methyl groups at temperatures (a) 540 K and (b) 600 K.

From the density and orientation calculations, we canconfidently draw the following conclusions: at the interface,the P3HT chain lies preferentially parallel to the surface withthe thiophene rings tilted towards the surface normal and thehexyl groups sticking out toward the surface so that the methylgroups are preferentially exposed to the surface (cf. Figure 4).In the bulk region, the P3HT chain shows almost perfectlyrandom orientation. The transition from random orientationin the bulk to orientational ordering at the interface seems tohave resulted in an enhancement of the number of thiophenerings at the boundary of the interface, which in turn inducesa denser region than the bulk. Based on this observation, weshould expect the surface tension properties of a P3HT film tobe dominated by the surface properties of the hexyl part of thechain and less on the thiophene backbone, which is confirmedby our surface tension calculations presented below.

C. Surface tension

Figure 13 shows the running time average of the surfacetension calculated using Eq. (1), i.e., without the tail correc-tion, for four different temperatures. As can be seen in thefigure, to get reliable surface tension data takes at least 50 nsof simulation time even at the highest temperature studied.This clearly demonstrates the slow dynamics of the P3HT

chains in the film, which is common among polymer chainswith rings and side chains. We ran the simulations until thetime variation of the surface tension did not seem to changeany more and calculated the average over the last 10 ns. Thecorresponding tail correction to the surface tension was calcu-lated using Eq. (2). For comparison, the surface tension withand without the tail correction are shown in Figure 14. Thecontribution of the tail correction to the surface tension is sig-nificant but does not change the qualitative trend of the surfacetension as a function of temperature.

The simulation result shows that the surface tensionmonotonically decreases with increasing temperature. Asshown in Figure 3, the hydrophobic hexyl side chains aredominant at the surface and contribute to the observed lowsurface tension values of the P3HT melt. This indicatesthat water, which has a very high surface tension of around72 m Nm−1 will not wet a P3HT surface, as confirmed byexperiment.54

To the best of our knowledge, there are no theoretical orexperimental surface tension values available for melt P3HTin the literature. However, for solid-state P3HT, experimentalreported values from contact-angle measurements are in therange of 21–36 m Nm−1.28–30 The differences in the reportedexperimental surface tension values may be due to the dif-ferences in molecular weight, morphology (crystallinity) of

0 0.4 0.8 1.2 1.6 2 2.4 2.8 3.2θ (rad)

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P(θ

)

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)

(a) (b)

FIG. 12. Probability distributions of the tilt-angle θ of the methyl groups region II (stars), region III (triangles), and bulk region (circles) at 540 K (left) and600 K (right).



10 20 30 40 50 60 70Time (ns)

8

12

16

γ p (

mN

/m)

FIG. 13. Surface tension as function of equilibration time at 540 K (circles),560 K (pluses), 580 K (triangles), and 600 K (stars).

sample, and the temperature at which measurements weredone. Surface tension calculated from contact-angle mea-surement is an indirect way of determining the surface ten-sion, and this might cause large discrepancies in the reportedexperimental surface tension values. Moreover, as we dis-cussed earlier, depending on the thin-film processing condi-tions, the thiophene rings can have face-on or edge-on orien-tations which will determine whether the thiophene rings orthe methyl groups are exposed to the surface. Hence, this cancontribute to the large differences observed in reported sur-face tension values of P3HT films.

In an attempt to compare the simulation surface tensionvalues with experiment, the surface tension data were fit bya linear regression, as shown by the solid line in Figure 14.The line fits the data very well, with a very high linear cor-relation coefficient of 0.998. The temperature coefficient of−0.072 m Nm−1 K−1 found in this study is similar to thevalues found for polymers that have side chains similar toP3HT (Ref. 55) and also to other polymers.56, 57 While weare well aware that we should not extrapolate the fit below thecrystallization temperature of P3HT, we can not resist mak-

540 560 580 600T (K)

8

10

12

14

16

18

Sur

face

Ten

sion

(m

N/m

) γp

γp + γ

tail

a - b*T

FIG. 14. Surface tension of P3HT as a function of temperature. The simula-tion surface tension γ p (triangles) and surface tension with tail correction,γ = γ p + γ tail (circles), have been plotted. The solid line is a lin-ear fitting to the total surface tension, with fitting parameters a = 55.982± 2.0237 m Nm−1 and b = 0.0716 ± 0.00355 m Nm−1K−1.

ing the following compelling argument to have a crude esti-mate of the surface tension of P3HT at room temperature. Al-though the bulk crystallization temperature of P3HT is around400 K,58 the hexyl side chains, which dominate the sur-face morphology of the film, do not crystallize until theyare cooled far below their thermodynamic melting point of333 K.59 We can thus safely assume that the hexyl sidegroups of a P3HT film should maintain their melt propertiesat least up to this temperature and that the error we incur byextrapolating the fit to this point should not be significant. Tobe on the safe side, we can assume the extrapolated value ofthe surface tension to room temperature (i.e., 34.5 m Nm−1)to be an upper bound to the true value at room tempera-ture since the surface tension below the crystallization tem-perature of the side chains may be monotonically decreasingwith temperature.60, 61 Though this hypothesis is open to ex-perimental verification, the predicted surface tension value of34.5 m Nm−1 is as argued at the upper end of the experimentalrange.

IV. SUMMARY AND CONCLUSIONS

The structural and interfacial properties of melt P3HTfree-standing films were investigated using all-atomisticmolecular dynamics simulations. Mass density and numberdensity profiles of the films and orientational ordering for dif-ferent parts of the P3HT chain segments in the films were cal-culated. The number density profile revealed that the surfacewas mainly covered by the methyl groups. At the boundaryof the bulk and interfacial regions, a peak in the mass densityprofile was observed, which was more pronounced at lowertemperatures than higher temperatures. At this boundary thethiophene ring number density was found to be higher thanin the bulk region due to the interface effect which inducedmore ordering to the thiophene rings that are at and closeto the interface. At the interface, the thiophene rings prefer-entially expose the carbon atoms that connect the thiophenerings to the hexyl groups rather than the sulfur atoms. The ori-entational ordering calculations show both methyl and hexylgroups sticking out to the surface with strong orientationalordering observed at vacuum side of the interface. The thio-phene ring segments orient parallel to the surface with thethiophene ring plane preferentially tilted toward normal to theinterface.

The equilibrated surface tension value at a given tem-perature was obtained by running the simulation for at least50 ns. The surface tension shows similar temperature de-pendence as that of polymers with alkyl side chains. Thesimulation-measured surface tension of melt P3HT is smallerthan all experimentally reported values for crystalline P3HTfilms.

Though computationally very expensive, our future workwill focus on the effect of chain length on the reported in-terfacial properties and also on the structure and dynamics ofsupported P3HT films on different substrates. To circumventthe computational cost, we plan to represent the alkyl sidechains with a united atom model that will significantly reducethe computational cost.



ACKNOWLEDGMENTS

The authors thank Gary M. Leuty for helpful commentson the draft of this paper and also Wondwosen Mekonnen forhelp with the isochore plots. Part of the simulation work wasperformed at the Southern Illinois University HPC Infrastruc-ture (SIHPCI). This work was supported by the National Sci-entific Foundation (NSF) (Grant No. DMR0847580).

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