Solar Energy Materials & Solar Cells 97 (2012) 109–118

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Solar Energy Materials & Solar Cells

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Molecular structure of poly(3-alkyl-thiophenes) investigated by calorimetryand grazing incidence X-ray scattering

Jose Abad a,b,n, Nieves Espinosa c, Pilar Ferrer e,f, Rafael Garcıa-Valverde c, Carmen Miguel c,Javier Padilla b, Alberto Alcolea d, German R. Castro e,f, Jaime Colchero a, Antonio Urbina c

a Departamento de Fısica, Centro de Investigacion en Optica y Nanofisica, Universidad de Murcia, Campus de Espinardo, 30100 Murcia, Spainb Departamento de Fısica Aplicada, Universidad Politecnica de Cartagena, Plaza del Hospital 1, 30202 Cartagena, Spainc Departamento de Electronica, Universidad Politecnica de Cartagena, Plaza del Hospital 1, 30202 Cartagena, Spaind Servicio de Apoyo a la Investigacion Tecnologica (SAIT), Universidad Politecnica de Cartagena, Plaza del Hospital 1, 30202 Cartagena, Spaine SpLine, Spanish CRG beamline at the ESRFacility, 6 rue Jules Horowitz B.P, 38043 Grenoble, Francef Instituto de Ciencia de Materiales de Madrid, CSIC, calle Sor Juana Ines de la Cruz, 3, 28049 Madrid, Spain

a r t i c l e i n f o

Article history:

Received 30 June 2011

Received in revised form

10 September 2011

Accepted 12 September 2011Available online 4 October 2011

Keywords:

Poly-alkyl-thiophenes

Calorimetry

Grazing incidence X-ray diffraction

Glass transition

Organic electronic devices

Organic solar cells

48/$ - see front matter & 2011 Elsevier B.V. A

016/j.solmat.2011.09.025

esponding author at: Departamento de F

ica de Cartagena, Plaza del Hospital 1, 30202

4 868 07 1096; fax: þ34 968 32 5337.

ail address: jose.abad@upct.es (J. Abad).

a b s t r a c t

A study of the molecular structure of regio-regular bulk poly-3-octyl-thiophene (P3OT) and poly-3-

hexyl-thiophene (P3HT) and the phase transitions during heating and cooling scans in a temperature

range of –158–773 1C has been performed by means of calorimetry of bulk samples and grazing

incidence X-ray diffraction from synchrotron radiation. Additional calorimetric measurements were

performed on samples in toluene solution. From the calorimetric temperature diagrams at different

scan rates, we obtain the melting and crystallization temperatures, and we identify a low temperature

calorimetric glass transition. This transition is expected because of the coexistence of amorphous and

crystalline phases, which is further supported by scanning force microscopy images where lamellar

structures have been observed. Thin films of both polymers have also been studied by grazing incidence

X-ray diffraction, and the evolution of the (1 0 0) crystalline peak monitored as a function of sample

temperature, showing different behavior in both polymers, d-spacing increases in P3HT and decreases

in P3OT for increasing temperatures. The information presented in this article will be useful to design

fabrication techniques for organic-based electronic devices, which could include high and low

temperature cycles combined with structural quenching procedures.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

The semiconducting materials composed of p-conjugated poly-mers have been studied since long time ago [1], and more recentlyhave attracted further attention due to their potential applicationsin optoelectronic devices such as organic solar cells [2–4], thinfilm transistors [5] and light-emitting diodes [6,7]. A hugeresearch effort has been devoted to understand the correlationbetween their structural properties and their performance asactive layers in organic electronic devices. More recently, thedevelopment of the bulk heterojunction approach in organic lightemitting diodes (OLEDs) and solar cells (OSCs) has created greatexpectations: a rising organic optoelectronic industry could ben-efit from highly efficient routine roll-to-roll packaging technolo-gies [8,9] that can be combined with ink-jet or printing procedures

ll rights reserved.

ısica Aplicada, Universidad

Cartagena, Spain.

[10,11]. Numerous film forming techniques have been exploredand the ideal process seems to involve solution processing of theorganic layers to provide a less energy intensive manufacturing.The application of roll-to-roll techniques using liquid-state coatingand printing methods with no vacuum, has been demonstratedrecently on a preindustrial scale [8]. These techniques could yielda huge low cost throughput provided that the structural anddynamical properties of these polymers are understood and acontrolled self-organization process at the nanoscale is developed.

The best devices so far have been constructed using the ‘‘bulkheterojunction’’ concept, where a blend of two materials isprepared to create a distributed p–n junction: one acts as anelectron donor, the other as an electron acceptor. The twopercolative networks created in the blend should mix in ananometer scale fitted to the mean free path of the excitonformed by light absorption. The conjugated polymers, and parti-cularly some functionalized polythiophenes, have been the mate-rials most used as electron donors, and therefore acting as holeconductors or p-type materials. It has been observed that theperformance of the devices is strongly dependent on thermal

J. Abad et al. / Solar Energy Materials & Solar Cells 97 (2012) 109–118110

post-processing and blend composition. It has been showed thatdifferent thermal processes can increase the efficiency of thedevices, but there is not a general agreement about the bestthermal process. Furthermore, reasons for this efficiency increaseare still unclear. Additionally, the choice for an optimum compo-sition of the blends has risen some controversy [12]. A deepunderstanding of the dynamics of the pure components of theblend will help to optimize and control the processing of themixtures. More recently, the synthesis of block copolymers, byadding blocks of non active structural polymers to blocks of theconventional photoactive polymers such as the poly-thiopheneshas opened a new route towards the tailoring of dynamical andstructural properties in order to control the nanostructure andtherefore the performance of the final devices [13,14].

Thin films of poly-3-hexyl-thiophene (P3HT) and poly-3-octyl-thiophene (P3OT) have been previously characterized by scanningforce microscopy and they show a layered structure wherecrystalline lamellar phases coexist with amorphous domains[15,16]. We have also performed previous X-ray diffractionexperiments on P3OT samples and all of the spectra indicate thesimultaneous presence of both crystalline and amorphous com-ponent as evidenced by sharp Bragg peaks and diffuse scattering,respectively [15]. In this article we will present a detailedcalorimetric study of bulk samples of P3OT and P3HT and theresults of grazing incidence X-ray scattering from synchrotronradiation on thin films of the same polymers spin-casted ondifferent substrates. Additional calorimetric information of thepolymers in toluene solution is also presented because the thinfilms have been processed from solution. The calorimetric scansperformed at different temperature rates have allowed us toidentify the melting and crystallization temperatures of bulksamples, giving evidence of a low temperature glass transitionand indicating a constant ratio of crystalline to amorphous phasesindependent of annealing cycles or temperature scan rates. Thecalorimetric data on solution samples present enthalpy values forthe low temperature crystallization of the solvent, which dependon the concentration of the polymers in solution, which rangesfrom 1% to 7%, covering all usual concentrations for devicepreparation from solution.

2. Experimental

Two regio-regular poly-alkyl-thiophenes, P3HT and P3OT,were studied by scanning calorimetric techniques and grazingincidence X-ray scattering. Both polymers were purchased fromSigma-Aldrich. They are regio-regular head-to-tail (more than98.5%) with an average molecular weight around 25,000 for P3OTand 45,000 for P3HT and a 99.995%. of purity, and polydispersitybetween 1.5 and 2. For scanning force microscopy (SFM) andGIXRD measurements P3OT and P3HT samples were prepared byspin-coating. A solution of 20 mg/ml P3OT (9 mg/ml, P3HT) intoluene was prepared, a 10 ml drop of this solution was depositedon a glass cover slip and then accelerated to about 3000 rpm.During this process, the drop is homogeneously spread out and athin film P3OT (P3HT) sample is formed on the substrate. Forcalorimetric measurements, the bulk polymers were weighed in amicrobalance and encapsulated in an aluminum capsule (keepingconstant pressure by means of a small hole in the capsule). For thesolution calorimetric studies, different solutions ranging from 1%to 7% where prepared for both polymers using toluene as solvent,in this case, the aluminum capsule was sealed and the scans onlyreached 25 1C avoiding pressure building up within the capsule.Differential scanning calorimetry (DSC) was performed at differ-ent temperature scan rates using a Mettler-Toledo DSC822ecalorimeter. Nitrogen was used as purge gas, and samples were

scanned at 1, 6, 10, 15, 20, 25 and 30 1C min�1 in a temperaturerange spanning from –145 1C to 250 1C. The amount of sampleused was 10.18 mg for P3HT and 10.01 mg for P3OT allowing usto keep the heat flow needed to conduct the experiments in therange of 0.5–15 mJ s�1, matching the optimum range of theinstrument. For all the solutions a total mass around 12 mg wasused. These mass values were also used to obtain an estimatedheat capacity at constant pressure from our heat flow measure-ments for both materials. Scans of an empty aluminum containerwere taken in order to obtain a data file that allowed us toperform background correction.

The morphology of the thin films was studied at roomtemperature and ambient conditions using SFM. A NanotecElectronica SFM system with a phase locked loop (PLL)/dynamicmeasurement board was used with Olympus OMCL-AC-typecantilevers (nominal force constant: 2 N/m; resonance frequency:70 kHz). Unless specified otherwise, imaging was performed innon-contact dynamic SFM (NC-DSFM) using the oscillation ampli-tude as feedback parameter, for more information see [16].

GIXRD is a very suitable technique to track the eventualstructural changes with an information depth selected at willfrom few unit cells to several hundreds of microns by changingthe incidence angle. We therefore used GIXRD to characterize thepolymer layers orientations. The GIXRD experiments were carriedout at fixed wavelength of 0.826 A on a six-circle diffractometer atSpLine beamline (BM25B), European Synchrotron Radiation Facil-ity (ESRF), Grenoble, France [17]. A point scintillation detector,which allows performing high-resolution diffraction experiments,was used. The sample was placed on a heating stage covered withan air-tight Kapton housing filled with 1 bar of nitrogen in orderto avoid the degradation of the sample. Two incidence angles,a¼0.151 and 2.001, have been used and the temperature rangewas from 21 1C to 120 1C.

3. Results and discussion

It is generally accepted that polyalkythiophenes are semi-crystalline with crystalline domains surrounded by amorphousmaterial. The crystalline domains have a orthorhombic unit cell(see Fig. 1) with lattice parameters in case of P3OT a¼2.07 nm,b¼0.76 nm and c¼0.77 nm and for P3HT a¼1.68 nm, b¼0.76 nmand c¼0.77 nm [18,19]; where – following the usual convention –the lattice parameter a is along the direction of the alkyl tails ofthe molecule, the parameter b along the stacking direction of themolecules (perpendicular to the projection shown in Fig. 1) andthe direction c is along the backbone of the molecule.

We present the measurement results in different subsections,the first one devoted to the morphology of the films, the secondone will present calorimetric measurements performed on bothbulk polymers (P3OT and P3HT) and its toluene solutions, and thethird one will show the GIXRD results.

3.1. Morphology

Fig. 2 shows topography images (a) and (b) of a P3OT thin filmand (c) and (d) of a P3HT thin film prepared using an equivalentprotocol. In the morphology of the P3OT thin films (a) and (b) twodistinct regions are clearly distinguished: a lower region, whichusually covers most of the sample, and a second higher regionwith a characteristic layered structure. A detailed study about themorphology of the P3OT thin films is found in reference [20]. Thestructures corresponding to the higher regions have one or twolayers, even though occasionally up to 5 layers have beenobserved and each layer has a height around 4–5 nm. Thiscorresponds to the double of a lattice parameter of crystalline

Fig. 1. Schematic representation of the molecular arrangement of regioregular head to tail P3HT and P3OT. The parameters are taken from reference [19,20].

Fig. 2. A series of SFM images showing (a and b) the morphology of a P3OT thin film and (c and d) the morphology of a P3HT thin film. (a) Dz¼100 nm. (b) Dz¼60 nm,

(c) Dz¼100 nm and (d) Dz¼60 nm.

J. Abad et al. / Solar Energy Materials & Solar Cells 97 (2012) 109–118 111

P3OT structure (see Fig. 1). The P3OT films show a root meansquare (rms) roughness of about 3 nm for image areas of 900 mm2,Fig. 2(a), and 25 mm2, Fig. 2(b). The morphology of the P3HT filmsis rougher than the P3OT films, Fig. 2(c) and (d), with a surfacerms roughness of about 24 nm and 10 nm for image areas of900 mm2 and 25 mm2, respectively. Even though layered struc-tures are also observed in these films, marked with arrows inFig. 2(c), the number and size of these structures present on thesurface are lower than in the P3OT films. In addition due to the

much higher roughness, these structures are barely recognized inthe image, Fig. 2(c). In Fig. 2(d) a characteristic rod-like structureis observed. This kind of morphology has been already reported inP3HT films by some groups [21–23].

3.2. Differential scanning calorimetry

DSC was performed at different temperature scan rates, namely1, 6, 10, 15, 20, 25 and 30 1C min�1 in a temperature range

Fig. 4. Melting (Tm) and crystallization (Tc) temperatures for P3HT and P3OT

obtained from the peak temperatures of the heating and cooling calorimetric scans

as a function of the temperature change rate (dT/dt).

J. Abad et al. / Solar Energy Materials & Solar Cells 97 (2012) 109–118112

spanning from –145 1C to 250 1C. One initial scan of heating to250 1C followed by a rapid cooling at rates higher than70 1C min�1 (using liquid nitrogen flow in order to quench thedynamics of crystallization) was performed and discarded beforestarting our monitored temperature scans. From the heating andcooling thermograms of bulk P3HT and P3OT samples we canobtain the melting temperature, Tm (using heating scans). Thecrystallization temperature Tc (using cooling scans) can beobtained from the maximum of the peaks shown in the respectiveplots. Fig. 3 shows the heat flow during heating and cooling scansat the above mentioned rates of temperature scan. The obtainedvalues for both transition temperatures for the P3HT and P3OTsamples at different scan rates are plotted in Fig. 4. A slight slopecan be observed in the crystallization temperatures, whilst themelting temperatures are more stable.

Some structural reorganizations at lower temperatures wereclearly seen for both samples, in both cases being stronglydependent on the scan rates, therefore giving an indication thatthey are not true structural phase transitions but partial reorga-nizations of the non-crystallized amount of the sample. Thecrystallization of the poly-thiophenes would not be completeunless very slow cooling rates and enough time for full crystal-lization are guaranteed. In most cases, a coexistence of crystallinephases and amorphous phases are obtained. This hypothesis isfurther reinforced by the onset of a glass transition at lowtemperatures observed in the calorimetric experiments. Alsosome evidence of the cooperative nature of the dynamical processis an indication of the presence of a glass transition in this system.

In most calorimetric experiments it is usual to obtain a specificheat capacity (at constant pressure) C¼(dE/dt)/(dT/dt) plottedversus T, where E is the energy added to the system (measuredas a heat flow) and dT/dt is the temperature scan rate. It canbe given per unit of mass (cp in J K�1 g�1), or per number of moles

Fig. 3. (a) Cooling and (b) heating DSC temperature scans at different scan rates for a bu

plots (a). The melting peak is observed at the right hand side of the plots (b). (c) Cooling

P3OT. The crystallization peak is observed at the right hand side of the plots (c). The sig

the left hand side of the plots (d). The different scan rates (dT/dt) are indicated by the

(Cp in J K�1 mol�1) [24]. In our case we have calculated a cp pergram of polymer. Nevertheless, it should be mentioned that only ifthe system is in thermal equilibrium, the quantity calculated in thisway is the heat capacity, and therefore, in the presence of

lk sample of P3HT. The crystallization peak is observed at the right hand side of the

and (d) heating DSC temperature scans at different scan rates for a bulk sample of

nature of a glass transition at low temperature is evident from the slope change in

numbers of each plot.

J. Abad et al. / Solar Energy Materials & Solar Cells 97 (2012) 109–118 113

reorganizations, the meaning of cp is lost or should be reinterpretedas a quantity related with the enthalpy of the reorganization.

In Fig. 5(A), we show the strong evidence of a calorimetric glasstransition for P3HT, a cooling branch describing the supercooledliquid state of the sample shows a final knee indicating the onset ofthe glass transition (indicated by an arrow labeled with ‘‘a’’ in thefigure), which for P3HT at the scanning rate of 6 1C min�1 isTg¼�113 1C (160 K). Small reorganization peaks can be observedin the supercooled liquid branch of the P3OT and P3HT scans. Theydo not appear to follow a reproducible pattern. The enthalpies of thetransformation obtained by integration of the peaks are all of thesame order (0.21–0.62 J g�1, very small compared to the values forthe enthalpy of fusion), as can be illustrated by the peak seriesobtained from the P3HT cooling scan at 6 1C min�1, shown in thesupercooled liquid branch of Fig. 5(A). Also a clear indication ofa glass transition is the dependence of the onset of the transition(the Tg) with the scan rate at which the experiment wasperformed. It can be clearly seen in Fig. 5(B) where differentbranches of the supercooled liquid are superimposed, givingdifferent glass transition temperatures for the P3OT, indicatedby the a,b,c and d arrows. The transitions are more evident in theP3OT sample than in the P3HT sample, where only three valuescan be obtained. The Tg values as a function of the temperaturescan rate for both polymers are summarized in Fig. 6(A).

A further indication of the existence of the glass transition isthe fact that even after slow cooling, keeping the temperature

Fig. 5. (A) Specific heat capacity at constant pressure for P3HT measured for the heati

supercooled liquid branch correspond to the scan for which we show the values of the r

for P3OT measured for the cooling temperature scans at different temperature change

Fig. 6. (A) Glass transition temperatures (Tg) obtained from the specific heat capacity pl

to extrapolate a Tg at infinitely slow temperature scan rates, which for both polymers w

capacity at different temperature scan rates. The divergence of the branches indicates

below the Tg (more than 12 h in our experiments) does not allowfull crystallization of the samples. In all the crystalline branches ofthe plots, it can be observed that some peaks are still presentindicating the coexistence of amorphous phases, which explainsthe dynamics observed in these branches. From the melting of thesamples we can obtain a fusion enthalpy by integration of the DSCendotherms normalized to the weight of the samples.

The enthalpies of fusion of the 100% crystalline materials canbe found in the literature, and are DHo(P3HT)¼99 Jg�1 andDHo(P3OT)¼74 Jg�1 [13]; then we can obtain a percentage ofcrystallinity, X(%), using the approximation given by X¼DH/DHo

[14]. For our measured values for the enthalpy of fusion, weobtain the crystallinity ratio after each one of the heating scans(each scan is followed by a cooling scan at the same rate, startingwith an initial cooling scan of 10 1C min�1 before the first heatingscan of 6 1C min�1). The results of the calculation are summarizedin Table 1. There exists a trend for which faster scans induce alower degree of crystallinity, but the effect is very small, and thevalues are all of the same order. The numbers obtained clearlyindicates that the samples are mostly amorphous, which iscoherent with the picture of glass transition and intermediatereorganizations in the supercooled liquid branch and also somereorganization features in the ‘‘crystalline branch’’.

Finally, we would like to raise a final discussion about oneissue that is still not fully understood but which poses someinteresting questions about the characteristics of the dynamics

ng and cooling temperature scans at a rate of 6 1C min�1. The small peaks in the

eorganization enthalpies in Table 1. (B) Specific heat capacity at constant pressure

rates (6, 10, 15, 20 1C min�1) and a heating branch.

ots at different temperature scan rates. The linear trend is evident and can be used

ill be around –133 1C. (B) Detail of the P3HT heating branches for the specific heat

a further strong collective reorganization at intermediate temperatures.

J. Abad et al. / Solar Energy Materials & Solar Cells 97 (2012) 109–118114

involved during the observed glass transition in these poly-alkyl-thiophenes. In most of the heating scans shown in the heat flowplots, Fig. 3(c) and (d), as well as in the specific heat capacity (forexample, shown in detail by arrow ‘‘b’’ in Fig. 5(B) for P3HT, butcan also be seen in Fig. 5(B) for P3OT), evidence for the onset of asignificant reorganization is shown. This shape in the heat flowplots probably arise from the onset of a collective reorganizationof the side-chains which start moving at a temperature aroundT¼–10 1C. To support the idea of the existence of this criticaltemperature, intermediate between the low temperature glasstransition and the higher melting and crystallization tempera-tures, we present in Fig. 6(B) (detail of the P3HT heating branch) aclear indication of the dynamical nature of this reorganization.The different plots, each one measured at a different temperaturescan rate give different evolutions of the specific heat capacity,which in this case will depend of such rate. This is an insight ofthe nature of the reorganization, which cannot be accounted as atrue phase transition; therefore, it must be a phenomena relatedwith the dynamics of the low temperature glass transitionreported in this article.

Fig. 7. Heating DSC temperature scans of (a) P3HT and (b) P3OT in toluene solution

(dashed line).

Table 2Crystallization and melting temperatures and enthalpies from DSC measurements in p

Polymer in toluene P3HT/toluene

Crystallization Melting

%wt Tc (1C) DH (J g�1) Tm (1C) DH

0% (pure toluene) �139.3 83.21 �94.7 104

1% �129.5 68.84 �94.8 102

2% �146.3 60.43 �95.1 87

3% �145.6 74.81 �95.2 94

5% �129.3 71.13 �95.3 87

7% �122.2 57.19 �95.4 74

Table 1Crystallinity of P3HT and P3OT obtained from the enthalpies of fusion at different

temperature scan rates of the bulk sample DSC measurements.

dT/dt

(1C min�1)

P3HT P3OT

DH

(J g�1)

Crystallinity

X(%)

DH

(J g�1)

Crystallinity

X(%)

6 21.7 21.9 6.3 8.5

10 15.1 15.2 5.3 7.2

15 14.4 14.5 4.7 6.4

20 15.7 15.9 5.1 6.9

25 14.8 14.9 3.9 5.3

30 14.7 14.8 3.8 5.1

A calorimetric study has also been performed for both poly-mers in toluene solution. This solvent is one of the most commonused for polymer solutions, is chlorine free, and could be acandidate for a solution processed large scale production ofdevices. The solutions were prepared at 1%, 2%, 3%, 5% and 7%weight concentration; higher concentrations are not stable andthe polymer tends to precipitate. In this case, the samples areliquid and the calorimetric study was performed in sealedaluminum capsules, and the temperature scans were performedfrom room temperature to –115 1C (at a scanning rate of 0.6 1C/min,both for cooling and heating ramps). A clear trend of modifica-tion of the solutions’ melting temperature can be observed.Fig. 7(a) and (b) show a detail between –103 and –93 1C, wherethe pure toluene melting is superimposed to the plots of differentpolymer concentration, the enthalpies of fusion can be calculatedand are presented in Table 2 and plotted in Fig. 8. As expectedfor a glass forming liquid (toluene), the melting behavior followsa clear pattern, but the crystallization temperatures are stronglymodified by the addition of a solute, and in this case, the weightconcentration dependence does not allow us to obtain a clearcrystallization behavior as a function of weight concentra-tion, although all enthalpy values are within the same order ofmagnitude.

In order to obtain further information about the dynamics ofthe movements of the polymers in solution, as well as the bulkpolymers in the intermediate temperature regime, other experi-mental techniques better suited for a full spectrographic char-acterization should be taken into account. Probably the best onesare neutron scattering experiments, in particular quasielasticneutron spectroscopy, but also elastic neutron scattering atdifferent temperatures; in both cases, coherent scattering willgive us information of the collective motions and incoherentscattering will allow us to calculate diffusion coefficients of themacromolecules. A correlation of the calorimetric study presented

for different weight concentrations. The pure toluene behavior is superimposed

olymer solutions at different weight concentrations in toluene.

P3OT/toluene

Crystallization Melting

(J g�1) Tc (1C) DH (J g�1) Tm (1C) DH (Jg�1)

.35 �139.3 83.21 �94.7 104.35

.34 �128.5 73.53 �94.8 99.15

.61 �123.2 76.64 �95.0 97.61

.51 �124.8 78.01 �95.0 95.51

.63 �137.9 59.02 �95.1 88.09

.58 �125.1 72.32 �95.4 86.88

Fig. 8. Melting and enthalpy of fusion for both polymers in solution (left: P3OT, right: P3HT) as deduced from the empirical data presented in Fig. 7 and summarized in

Table 2.

Fig. 9. X-ray diffraction profiles of P3OT and P3HT. The spectra are taken for a

beam incidence angle of 0.151.

J. Abad et al. / Solar Energy Materials & Solar Cells 97 (2012) 109–118 115

in this article and neutron scattering measurements will clarifythe origin of the observed transitions and reorganizations.

3.3. Grazing incidence X-ray measurements

Examples of diffraction spectra for P3OT and P3HT thin films at25 1C are shown in Fig. 9. These spectra are in general similar tothose reported previously [18,19,25–27]. Both spectra correspondto an incidence angle between the surface film and incident beamof 0.151. We estimate a pentration depth for this incidence angleof about 260 nm [28], therefore, at this angle the X-ray probesmainly the polymer thin film (polymer thickness of about200 nm). The P3OT spectrum shows peaks at 2y¼2.31, as wellas a broad shallow peak around 12.81. The peak at 111 is believeddue to come either from the substrate or the sample holder. Asobserved in Fig. 10, this feature does not change with theannealing, in contrast to the peak at 2.31 and the broad featurecentered around 12.81, both move to slightly higher angles athigher temperatures. These peaks correspond to lattice constantsd values of 2.06 and 0.43 nm for the sharp peaks, while for thebroad features we obtain 0.37 nm. The peak at 2.06 nm isindicative of the lamellar interlayer spacing for P3OT, while thebroad feature corresponds to an amorphous halo, which has beenattributed to side chain disorder.

Fig. 10(a) shows the P3OT spectra at different annealingtemperatures, the main changes are observed on the peak atabout 2y¼2.31, in which an intensity decrease is appreciated withthe annealing temperature. These changes are clearly observed inFig. 10(b) for four selected temperatures. A peak shift at higher 2yangles is also seen in addition to the decrease in the peakintensity, which indicates a decrease of the interlayer d-spacingwith the temperature. Furthermore, the estimated coherencelength obtained using the Debye–Scherrer relation increases withtemperature. All these results are summarized in Fig. 11. Thedecrease of the (1 0 0) peak d-spacing with the temperatureshown in Fig. 11(a) can be understood as an increase of theinterdigitation between the main backbone chains of the polymerwith the annealing. This could be due to a mobility increase withthe temperature of the alkyl side chains leading to this inter-digitation increase. The (1 0 0) peak area decreases with theannealing temperature shown in Figs. 10(b) and 11(b) can bequalitatively related to the degree of crystallinity. Therefore adecrease of the polymer crystallinity with the temperatureincrease is observed. Also an increase of the (1 0 0) peak coher-ence length occurs upon annealing, Fig. 11(b).

The thermal behavior of the P3HT thin films and the spectrafor different annealing temperatures are shown in Fig. 12(a). Forthe P3HT spectrum at a¼0.151 a very sharp peak at 2.981 isobserved, which corresponds to a d¼1.59 nm of the well orga-nized lamellar structure (Figs. 9 and 12). The second (2 0 0) and

the third (3 0 0) order of the sharp peak are also observed. Wehave to note that this lamellar d-spacing is lower than the valuereported in the literature, 1.68 nm, and could indicate somedegree of interdigitation of the side chains of the polymer ascommented below in more detail [18,19].

The main changes as compared to the results obtained for theP3OT sample are the sharpness of the main diffraction peak, anintensity decrease of the (1 0 0) peak for annealing temperaturesabove 75 1C, as it is observed in detail in Fig. 12(b) and theintensity decrease of the broader feature at about 121. Also inFig. 12(b) is important to note a shift of the (1 0 0) peak to lowerangles with the temperature, this result is summarized inFig. 13(a) where the distance between the backbone P3HT chainsincreases from 1.59 to 1.71 nm when the temperature is increased.The thermal behavior of the P3HT films, Fig. 13(a), contrasts withthe thermal evolution observed for the P3OT films, Fig. 11(a).Fig. 13(b) shows the changes in the area (1 0 0) peak as function ofthe annealing temperature. The area of this peak decreases veryslightly until 75 1C and above this temperature decreses quickly.By contrast with the results obtained for the P3OT films where thearea (1 0 0) peak decreases markedly with the annealing tempera-ture. Again there is an increase of the (1 0 0) peak coherencelength with annealing, Fig. 12(b). In an indication that the crystalsize perpendicular to the substrate is increased with the thermaltreatment. This effect has been also reported elsewhere [29].

The P3HT thin films have been also measured at higher beamincidence angle (2.001). We estimate a pentration depth for thisincidence angle of about 3 mm [28]; therefore, at this angle the

Fig. 10. (a) X-ray diffraction profiles of P3OT at different annealing temperatures. (b) A zoom of the 1–51 region. The spectra are taken for a beam incidence angle of 0.151.

Fig. 11. P3OT d-spacing (a), area and coherence length (b) of the (1 0 0) peak as function of the annealing temperature. The spectra are taken for a beam incidence angle of

0.151.

Fig. 12. X-ray diffraction profiles of P3HT at different annealing temperatures (b) is a zoom of the 1–51 region. The spectra are taken for a beam incidence angle of 0.151.

Fig. 13. P3HT d-spacing (a), area and coherence length (b) of the (1 0 0) peak as function of the annealing temperature. The spectra are taken for a beam incidence angle of

0.151.

J. Abad et al. / Solar Energy Materials & Solar Cells 97 (2012) 109–118116

Fig. 14. (a) X-ray diffraction profiles of P3HT at different annealing temperatures. (b) A zoom of the 2–41 region. The spectra are taken for a beam incidence angle of 2.001.

Fig. 15. P3HT d-spacing (a), area and coherence length (b) of the (1 0 0) peak as function of the annealing temperature. The spectra are taken for a beam incidence angle of 2.001.

J. Abad et al. / Solar Energy Materials & Solar Cells 97 (2012) 109–118 117

X-ray probes not only the thin film but also the glass substrate.Fig. 14 shows the X-ray diffraction profiles at different annealingtemperatures. The main differences in the line shape of thespectra as compared to the one shown in Fig. 12 is the broadfeature that appears at about 131, similar to the value obtained forP3OT. Even though for this higher angle of incidence the X-rayprobes also the glass substrate, we attribute this broad peak to thepolymer film, since on the one hand the peaks vary withtemperature, and on the other the position of the peaks coincidewell with the inter and intramolecular spacing within of the P3HTpolymer chains. These values are similar for P3OT and P3HT, as isindeed observed in the corresponding spectra. We note that thisbroad peak corresponds to the two distances (see Fig. 1) c/2¼0.38between the ‘‘arms’’ on a chain (intramolecular) as well as to thedistance b/2¼0.38 between adjacent chains (intermolecular).

Analyzing in detail the evolution of the (1 0 0) peak with theannealing temperature, Fig. 14(b), we find some differences withthe behavior of the film at lower angle of incidence, Fig. 12(b).These changes are summarized in Fig. 15. The distance betweenthe backbone P3HT chains increases from 1.60 to 1.71 nm whenthe temperature is increased in a similar way to lower angle ofincidence (0.151). While the area of the (1 0 0) peak increaseswith the temperature, qualitatively this area increase indicates anincrease of the crystallinity with the thermal annealing. Thiscrystallinity increase of the P3HT films with the temperaturehave been also reported elsewhere [29].

4. Conclusions

Practical uses of p-conjugated polymers, and in particular poly-alkyl-thiophenes, include the fabrication of organic electronic

devices. To exploit the full benefit of roll-to-roll techniques formassive production, a further understanding of the structural anddynamical processes of the polymers during the fabrication stagewill be needed. In these processes temperature is one of the mostsignificant parameters. In this article we present a calorimetricstudy of two of the most widely used poly-alkyl-thiophenes, i.e.P3OT and P3HT. We have measured the melting and crystal-lization temperatures, which are, respectively, Tm¼165 1C andTc¼113 1C for P3OT, and Tm¼224 1C and Tc¼195 1C for P3HT. Alsoevidence of a low temperature calorimetric glass transition hasbeen demonstrated for both polymers, obtaining in this case avalue for Tg¼–133 1C very similar for P3OT and P3HT. Thedependence of Tg with the temperature scan rate is much strongerthan typical melting and crystallization temperatures. The crystal-linity of the polymers has been obtained after temperature scansat different rates; the ratio of crystallinity is low in both cases(around X¼7% for P3OT and X¼15% for P3HT), and seems to beindependent of the temperature scan rate. The GIXRD results showan increase of the crystallinity for the P3HT and not in the P3OTwith the annealing temperature. The monitorization of the unitcell d-spacing with temperature indicates that interdigitation ofthe side chain is produced for the P3HT and not in the P3OT. Thereported improvement in efficiency of organic solar cells uponannealing can be related to this onset of side-chain interdigitation,which could be accompanied by the interpenetration of PCBM inthe blends of the active layer.

This information could open new routes to the processing ofdevices which contain an active layer with blends of poly-alkyl-thiophenes, such as organic solar cells and organic light emittingdiodes. Temperature cycles and control of low temperaturepreparation procedures, which include quenching of the struc-tures before rising to room temperature, could deliver a better

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control of the structure at the nanoscale, and not only the increaseof crystalline domains within the polymeric phase of the blends,but also the interdigitation of side-chains opening and closingspaces for the diffusion of PCBM within the polymeric matrix areimportant parameters for the control of the morphology at thenanoscale.

Acknowledgments

The authors acknowledge financial support from Spanish Ministryof Science and Innovation (Grants Consolider-HOPE CSD2007-00007and MAT2010-21267-C01/02) and from Comunidad Autonoma de laRegion de Murcia (Grant CARM-D429-2008). We also thank theSpLine staff for their assistance in using beamline BM25B-SpLineand the financial support of the Spanish Ministerio de Ciencia eInnovacion (PI201060E013) is acknowledged.

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