Cite this: RSC Advances, 2013, 3,15664

Fast fabrication over large areas of P3HT nanostructureswith high supramolecular order

Received 4th April 2013,Accepted 19th June 2013

DOI: 10.1039/c3ra41612g

www.rsc.org/advances

Alessandro Fraleoni-Morgera,*a Giuseppina Palmaa and Jasper R. Plaisierb

The fabrication of P3HT nanopatterns (lamellae and fibres) within a few minutes, in standard laboratory

conditions (i.e., at room temperature and in air) and over areas as large as cm2, is reported. The

nanostructures are prepared using a wet-processing method. A satisfactory control over the pattern

topology (lamellae, hierarchically connected and parallel fibres, entangled but disconnected and quasi-

parallel fibres, randomly oriented fibres) is obtained by simply changing one process parameter. UV-vis

spectroscopy and X-ray diffraction analyses carried out over the so-fabricated structures evidence a very

high degree of supramolecular organization of the polymeric chains. Such a degree of order is similar or

even better than that of P3HT samples treated with thermal or solvent annealing procedures.

1. Introduction

Head-to-tail, regioregular poly(3-hexylthiophene) (P3HT) is oneof the most comprehensively studied conjugated polymers,due to its notable electric/electronic transport properties, thatmakes it a model system for polymeric photovoltaic cells (PV)1

and organic field-effect transistors (OFETs).2 These propertiesare intimately connected to the P3HT ability to self-assemblein the solid state, originating ordered, crystalline domains. Inaddition, P3HT may form fibrillar structures, with widthsranging from a few nm to tens of nm, and lengths of up toseveral microns, depending on the preparation conditions.The fibrillar morphology impacts positively on the chargetransport capability of the material, as shown by severalgroups in OFETs applications.3–6 For a single fibre, currentdensities as high as 700 A cm22 have been measured.7 Theincreased transport capability of P3HT fibres can also behelpful for organic photovoltaics,8 but the precise control overthis morphology is not straightforward.9 Therefore, thepossibility to control P3HT fibres development, orientationand dimensions in a simple way is very attractive. Such controlcan be somehow attained by dip coating,10 friction transfer,11

micro-contact printing,12 soft lithography13,14 and directionalepitaxial solidification (DES).15,16 This latter method usescrystallizable solvents (with a melting point lower than that ofP3HT) as easily disposable epitaxy-based substrates for thepolymer. Though conceptually simple, DES has a number ofdrawbacks. In particular, it suffers from a long implementa-tion time (several hours), practical complexity (it needs tomaintain the solvent/P3HT mixture liquid by heating it until

the mixture deposition is complete, to keep the mixture sealedbetween two planar substrates until the fibre growth iscomplete, and to precisely control the cooling gradient of thesubstrate/mixture/substrate sandwich), and difficulty ofachieving uniform fibre arrays over surfaces larger than afew square microns.17

Here, the fabrication of P3HT nanofibres over large areas(up to cm2) attained using a simple and fast method based ona sublimating, templating matrix, is presented. The techni-que,18 named auxiliary solvent-based sublimation-aidednanostructuring (briefly, ASB-SANS), allows to process thepolymer starting from a solution, that is liquid at roomtemperature. The desired result is obtained within minutesfrom the deposition, with simple and straightforward opera-tions. Depending on the fabrication conditions, P3HT lamellarmesostructures or nanofibres develop on Si/SiOx or on normallab glass slides. The influence of the polymer/sublimatingsubstance ratio with respect to the obtained fibre topology anddimensions is discussed. In addition, the supramolecularorganization of the obtained polymer nanostructures isinvestigated exploiting the peculiar UV-vis response of P3HT,as well as X-ray diffraction experiments. Both these techniquesallowed us to evidence a significant degree of order in the ASB-SANS-originated samples, notably obtained in absence of anypost-fabrication treatment.

2. Experimental

Materials, instruments, general procedures

P3HT, PDCB (99+%), CHCl3 (99.9+%) were purchased fromAldrich and used with no further purification.

The Si/SiOx wafers used as substrates for the nanofibredeposition were purchased from ITME (Poland), with crystal

aOrganic OptoElectronics Laboratory, Sincrotrone Trieste SCpA, SS 14.5, km 163.5,

34149 Basovizza, (TS), Italy. E-mail: alessandro.fraleoni@elettra.trieste.itbMCX Beamline, Sincrotrone Trieste SCpA, SS 14.5, km 163.5, 34149 Basovizza, (TS),

Italy

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orientation 100, p-doped (B), sheet resistivity of 0.08 Ohm persquare.

The UV-vis spectra were recorded on a Perkin Elmer Lambda35 spectrophotometer (1 nm resolution, 2 nm slit).

The SEM photos were taken with a Carl Zeiss Supra 40Scanning Electron Microscope. The microscope photos weretaken with an Olympus BH-2 microscope equipped with anOlympus camera.

X-ray powder diffraction patterns at room temperature wereobtained on the XRD1 beamline at the synchrotron lightsource Elettra, Trieste (Italy). The measurements were per-formed in transmission geometry using a MarCCD detector,and the data were integrated using the program Fit2D. Beforedata collection, detector distance, orientation and beam centerwere obtained by measuring the diffraction pattern of LaB6.The data have been collected using a wavelength of 1.2 Å. Unitcell parameters were refined using the method described byNovak and Colville.19

All the chemical manipulations have been carried out undera fume hood. All the experimental procedures were carried outat room temperature and in air.

Preparation of the P3HT spin-coated thin films

The samples TF-A and TF-B (where TF stands for thin film)were prepared from a solution composed by 12 mg P3HT in 1mL CHCl3. The polymer was completely dissolved by gentlyheating the solution. The solution was then spin-coated onto aglass slide at 800 rpm for 3099. Sample TF-B has been annealedat 80 uC for 59 prior to recording its UV-vis spectrum.

Sample TF-C has been prepared using the same spin-coatingparameters of TF-A and -B, starting from a solution composedby 12 mg P3HT and 1 mL CHCl3. After complete polymerdissolution, PDCB (180 mg) was added to the solution. Thevolume of this amount of PDCB correspond to about 0.13 mLand did not alter significantly the overall P3HT concentrationin the solution with respect to TF-A and TF-B.

Preparation of the Nano Fibrillar (NF)-A/E samples

P3HT ternary solutions according to the composition given inTable 1 have been prepared into separate vials.

The vials were closed and gently warmed until reaching abrilliant orange colour of the solution. The resulting solutionshave been left cooling for a few minutes, then one drop fromeach solution was deposed onto the substrate (glass/Si/SiOx)via a Pasteur pipette.

3. Results and discussion

It is known that the ASB-SANS technique is effective ingenerating large, ordered nanofibres arrays of PMMA, anorganic polymer.18 The method was hence applied to P3HT, bymixing it with CHCl3 (auxiliary solvent, AS) and para-dichlorobenzene (PDCB, the sublimating substance, SS). Inthis system PDCB, which is a solvent for P3HT, acts acrystalline template for P3HT. After the templating phase,PDCB leaves easily and quickly the system by sublimation (seealso supplementary material given in ref. 18 for details on therole of PDCB in determining the final fibrillar morphology ofthe polymer). In a first experiment, a P3HT-PDCB-CHCl3

ternary mixture was spin-coated onto a glass slide (sample TF-C). As a control experiment, two more samples, TF-A and TF-B,were spun from a normal CHCl3/P3HT solution, using thesame spin-coating parameters of TF-C. The sample TF-B wasthermally annealed at about 80 uC for 59.

The visual aspect of TF-C was strikingly different from thatof TF-A and TF-B (Fig. 1). In particular, the sample TF-Cshowed P3HT domains with spherulite-like zones evidencingthe classic maltese cross, as large as several millimeters(Fig. 1c), while in the TF-A and TF-B samples only the normalappearance of plain, amorphous P3HT films was noticed(Fig. 1a, b).

Optical microscopy of TF-C revealed that the crystal-likedomains are organized as 2-dimensional analogs of spher-ulites (Fig. 1d). Domain sizes range from a few mm to almost 1cm. A further magnification showed that the 2D spherulitesare constituted of fibrillar structures, with well defined andlowly dispersed widths, and lengths ranging from several unitsto several hundreds of microns (Fig. 1e). Bao et al.20 andLudwigs et al.17 already obtained similar 2D spheruliticstructures, with dimensions from a few units to a couple ofhundreds microns in diameter, by means of slow solvent-aidedannealing and solvent swelling techniques, respectively.

An analogous of the TF-C sample (same starting solutionand spin-coating parameters) was realized onto a Si/SiOx slidefor SEM observations. A low SEM magnification revealed thatthe macroscopic morphology of this sample is the same of TF-C (Fig. 1f). A higher magnification showed that the structuresconstituting the spherulites possess a non-negligible height(about 1–2 microns), hence they are lamellar-like, rather thantruly fibrillar. The lamellae width ranges between 40 and 200

Table 1 Composition parameters for the solutions originating the P3HT samples discussed in the article, and main morphological features of the developednanostructures

Sample ID

Composition

Nanofiber organization type

Nanofiber average diameter (nm)

P3HT (mg) PDCB (mg) CHCl3 (mL) Main Branch (where applicable)

NF-A 5 100 1 Hierarchical 500–1500 100–600NF-B 5 200 1 Hierarchical 500–1500 100–600NF-C 5 400 1 Aligned but entangled single fibers 200–500 —NF-D 5 1000 1 Aligned single fibers 70–200 —NF-E 5 1500 1 Apparently disorganized single fibers 30–100 —

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nm. In many instances, several lamellae are fused together(Fig. 1f, g).

To achieve the goal of producing fibres, rather thanlamellar structures, five CHCl3/P3HT/PDCB solutions, withprogressively higher PDCB : P3HT ratios (see Table 1), wereprepared and deposed on Si/SiOx slides by drop casting. Theresulting P3HT patterns were imaged by SEM (Fig. 2).

Excluding the drop edges, where accumulation of materialled to fascinating but ill-defined structures, the patterns left atthe center of the drops resembled those obtained with asimilar procedure for PMMA.18 In particular, higher concen-trations of P3HT delivered hierarchically organized nanos-tructures (NF-A and NF-B, see Table 1). The structures arecharacterized by main fibres developed along a lineardirection, with secondary fibres departing at about 90u fromthe main ones (Fig. 2a–d). In these samples the main fibreshave widths ranging between 500–1500 nm (Fig. 2b, d). Thesecondary fibres have widths of about 100–600 nm in bothcases. In general, for NF-B both main and secondary fibreshave dimensions in the lower part of the mentioned ranges,whereas for NF-A the dimensions are in the higher ranges,though the dimensional differences between the two sampleswere found to be minimal (compare Fig. 2b and d).

Straight and very long (from hundreds of microns for NF-Cto 20–30 microns for NF-D, see fig. 2e–h) fibres, with no trace

of secondary structures, were obtained when a lower concen-tration of P3HT was kept in the starting ternary solution. Thediameter of these fibres is in the range 200–500 nm for NF-Cand 70–200 nm for NF-D. The minimum fibre width, about 30–100 nm with a length of few microns, was reached with thestarting solution NF-E. However, the fibres in the latter sampleappear to be very disorganized (Fig. 2i–j). From these examplesit is evident that a reasonable control over the fibres width canbe exerted simply by increasing the PDCB/P3HT ratio(compare Table 1 and Fig. 2). The same holds for the topology(connected/disconnected fibres) of the general pattern result-ing from the ASB-SANS procedure, confirming previous resultsobtained for PMMA.18 This suggests that ASB-SANS can beused to produce well defined patterns from a rather broadvariety of polymers. Moreover, it looks like it is possible toexploit this method also for producing isolated polymer fibreshaving much reduced lateral dimensions, in the range of a fewtens of nanometers (see Fig. 2g–j). This can hold someimportance in applications requiring quantum confinementlike, for example, lasing.21 With respect to the homogeneity ofthe samples, rather uniform patterns were found over areas as

Fig. 1 Visual aspects of P3HT-based thin films, obtained by spin-coating onto aglass slide (2.5 6 2.5 cm): (a) a simple P3HT/chloroform solution (sample TF-A);(b) the same solution used for (a), but with a further thermal annealing step(sample TF-B); (c) an ASB-SANS-designed ternary solution (CHCl3/PDCB/P3HT,sample TF-C), with no further treatment. For sample TF-C it is possible to notice acrystalline appearance. (d, e) optical microscope photos (image sizes 1170 6866 mm and 116.6 6 86.6 mm, respectively) of sample TF-C. (f, g) SEM images ofa P3HT thin film spin coated onto Si/SiOx wafer starting from the same ASB-SANS solution used for the sample TF-C, at different magnifications. It is possibleto appreciate the very similar morphology of this sample with TF-C at similarmagnification (f), and that the fibrils are indeed nanosized lamellae having aheight of about 1.5–2 mm (g).

Fig. 2 SEM photos at low and high magnifications of the samples NF-A (a,b), B(c,d), C (e,f), D (g,h) and E (i,j).

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large as several tens of mm2, as is visible from the SEM photosof Fig. 2a, c, e, g. Due to the peculiar nanofabricationtechnique, which relies on the formation of PDCB crystals astemplates,18 a more macroscopic (i.e. at scales going from cm2

up) homogeneity is difficult to be obtained with the presented,unoptimized ternary mixture deposition methods. In fact, thehere described drop-casting approach delivers differentlyoriented PDCB crystalline domains within the same sample,and consequently in each domain the fibres are differentlyoriented. Nonetheless, no dramatic morphological differenceamong P3HT patterns developed in different PDCB domainswas noticed. This suggests that the realization of extended,uniform PDCB crystals domains should result in similarlyuniform and extended P3HT patterns.

The colour of the ASB-SANS-generated structures was foundto be darker than that of their plain film counterparts(although from Fig. 1c this is not evident, due to photographictechnical limitations). Indeed, it is known that in fibrillarstructures P3HT chains are better self-organized than innormal thin films, and that this self-organization results in adarker sample colour, i.e. a red-shift of the UV-vis absorptionspectrum.22 In particular, the UV-visible spectra of supramo-lecularly organized structures of P3HT in the solid state show amaximum located around 515–520 nm (depending on thecharacteristics of the polymer, i.e. regioregularity degree,molecular weight) and two well distinguishable vibronicshoulders.23 The first shoulder is located around 545 nm,and is associated, like the maximum absorption peak, to p–p*intrachain interactions. The second shoulder is insteadlocated around 600 nm and has been associated to p–p*interchain interactions.24 These features become more evidentwhen the polymer film is thermally annealed, or when thesolvent used to fabricate the film is allowed to evaporate veryslowly.25,26 Both these treatments are able to promote a betterp-stacking of the chains, therefore the 545 and 600 nmfeatures are considered signatures of effective self-organiza-tion of the polymer. In view of these characteristics of P3HT,the UV-vis spectra of TF-A, TF-B, TF-C and NF-A have beenrecorded, and reported in Fig. 3 (spectra of NF-B/E were nottaken because the very small amount of P3HT left on the glassslides after the PDCB sublimation delivered very poor qualityspectra).

The spectrum of the sample TF-B shows a small butappreciable increase of the intensity of the vibronic shouldersat 543 and 600 nm with respect to TF-A, as expected after theannealing step.

In the sample TF-C a very marked spectral shape change isvisible. In particular, the vibronic signal corresponding to theintrachain interaction (about 547 nm) is the most intensesignal of the spectrum, representing the maximum ofabsorption. Also the interchain vibronic feature at 603 nm isremarkably strong. In addition, a careful look at the wholespectrum reveals that the same is slightly red-shifted (5–6 nm)with respect to the ones of TF-A and -B. All these featuresclearly point to a high degree of self-organization of thepolymer. The observed consistent absorption from 660 to 1100

nm can be attributable to effective light scattering. A similarspectral shape is observed for NF-A, although in this case thelower intensity of the interchain vibronic signal at 603 nmsuggests a lower amount of self-organization than in the caseof TF-C (Fig. 3).

These features indicate that the P3HT in the lamellar andfibrillar samples experience a degree of self-organization(under both the intra- and the inter-molecular points of view)much higher than that of plain films. Interestingly, this wasachieved in just a few minutes from the deposition of thestarting ternary solution, with no need for lengthy andsometimes complex procedures like thermal or solventannealing. Neither any special care for the depositionconditions, substrate type, temperature or ambient pressurecontrol, etc., was necessary to obtain the described, highlyorganized fibres/lamellae.

X-ray diffraction was carried out on the powders obtainedremoving gently the P3HT lamellae (sample TF-C) and fibres(sample NF-A, as representative of the nanofibre specimens; asfor the UV-vis spectra, sample NF-B/E presented a too lowamount of material for allowing any meaningful XRD analysis)from their respective substrates. Fig. 4b, red curve, shows thediffraction pattern of the lamellar structures (TF-C) afterintegration and background subtraction. Similarly to thatreported by Wu et al.,27 sharp diffraction peaks visible up tohigh angles of 2h indicate a high degree of long rangeordering. Using the indexing as reported by Wu et al. it waspossible to refine the unit cell parameters. The calculated andmeasured d and Q values of the indexed reflections for TF-Care given in Table 2.

The refinement gave a monoclinic unit cell, with para-meters: a = 15.91(3) Å, b = 7.688 Å, c = 7.728(7) Å, beta =94.88(15)u (see Fig. 4a for a visual reference of the cellparameters). It should be noted that the b-axis is determinedonly using the 020 reflection, that is assumed to be at the sameposition as the 002 reflection, as has been shown by Tashiro.28

Clearly, in the direction of the b-axis, the degree of ordering is

Fig. 3 UV-Vis spectra of samples TF-A, TF-B, TF-C and NF-A (respectively, purple,blue, red and green curves).

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lower than in direction of the other two main axes. Manydiffraction peaks have mixed indices (h ? 0 and l ? 0),pointing to a high degree of order of the side chains and to aregular p-stacking, and confirming the remarkable level of self-organization already evident from the UV-vis spectroscopy.

X-ray diffraction of powders obtained from NF-A (nano-fibres, Fig. 4b, green curve) shows a lower degree of long rangeorder than that present in the lamellar structures. Diffractionpeaks are present only at low angles, they are less in numberthan in TF-C, and are significantly broader, indicative of a

lower crystallinity of this sample with respect to TF-C. Inaddition, in several cases their exact positions are difficult toidentify due to their low intensities. The unit cell of NF-A canbe anyway estimated using the method described above,delivering the cell parameters a = 16.97(1) Å, b = 7.688 Å, c =7.785(7) Å, beta = 94.6(1)u.

As no reflection with k ? 0 was found, the b-axis given is thesame as that of the lamellar structure. The structure seems tobe elongated in the direction of the a-axis, which indicates alower interaction between the side chains. Furthermore, thepeaks are shifted with respect to those of the lamellarstructure.

This different behaviour between TF-C and NF-A is in linewith that already evidenced by the UV-vis data (Fig. 3).Nonetheless, the differences between the UV-vis spectra ofthe two samples are visible but not dramatic, while thosefound in the XRD patterns appear to be rather marked(although in both cases a good supramolecular order degree isinferable from the XRD pattern, even with respect to similarreports over highly organized P3HT samples4,17,22,28). As X-raydiffraction requires long range order to get sharp diffractionpeaks, the large difference between the diffraction patternscan be explained by a lower degree of long range order in theNF-A sample. Both samples, however show a large degree oforganization, as is evident from the UV spectra. Anotherpossible (and likely) cause of this difference is that the XRDspectra of both samples were collected in the powderconfiguration. To prepare the samples, the material wasremoved from the growth substrate (a Si/SiOx chip) using aspatula. Since the lamellae constituting the sample TF-C arewell packed also along the vertical direction (see Fig. 1g) andwere less adherent to the substrate, their removal wasstraightforward. On the other hand, the NF-A sample waspretty much adherent to the substrate, and its removalrequired to exert a non-negligible mechanical action, causinga visible crushing of the material to be examined. This could atleast partly explain the observed marked difference betweenthe X-rays diffraction patterns of the two samples, and theconcurrent small difference between the corresponding UV-visspectra, for recording which the P3HT patterns were notmechanically altered. In any case, both the TF-C and NF-Asample present a diffraction pattern underlining a definitehigh self-organization of the P3HT chains, that overallconfirms the UV-vis data.

4. Conclusions

In conclusion, we have reported about a novel technique forrapidly producing highly organized nanolamellar and nanofi-brillar P3HT structures, over large areas (i.e., cm2) and ondifferent substrates (Si/SiOx and glass). The method, namedauxiliay solvent-based sublimation-aided nanostructuring(ASB-SANS), uses a sublimable organic crystal (in this casepara-dichlorobenzene, PDCB) to produce within minutesP3HT patterns (lamellar or fibrillar ones), starting from a

Fig. 4 (a) Sketch of the polymer chains arrangement and reference for the inter-and intra-chain distances reported in Table 2. (b) Powder diffraction pattern ofP3HT samples TF-C (green curve) and NF-A (red curve). In the inset: further detailof the 30–50u zone of the plot, evidencing sharp diffraction peaks.

Table 2 Indexed Bragg reflections of the TF-C P3HT samples

2-theta (u) (hkl) dobs (Å) dcalc (Å) Qobs Qcalc

4.28 100 16.068 15.853 0.0039 0.00408.61 200 7.993 7.927 0.0157 0.0159

13.05 300 5.280 5.284 0.0359 0.035817.96 020 3.844 3.844 0.0677 0.067717.96 002 3.844 3.850 0.0677 0.067518.78 102 3.678 3.670 0.0739 0.074220.61 202 3.354 3.353 0.0889 0.089023.15 302 2.990 2.993 0.1118 0.111726.16 402 2.651 2.651 0.1423 0.1423

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liquid (hence handy) ternary solution. The control over thepattern topology acting on the PDCB/P3HT ratio has beendemonstrated, showing that a decrease of this parameter leadsto the shift from hierarchical to random organization of theresulting nanostructures. When fibres are produced, the sameparameter has also a strong influence over the fibres width,with higher ratios delivering smaller widths, and vice versa.UV-vis spectroscopy showed that both the ASB-SANS-generatedlamellae and fibres possess a remarkable supramolecularorder, higher than that achievable with physical treatments ofthe P3HT films like heating, rubbing, solvent annealing or thelike. This high degree of supramolecular order of thepolymeric chains was confirmed by X-ray diffraction analyses.Possible applications of the method in organic optoelectronics(like for example organic photovoltaics or organic field effecttransistors) are under investigation.

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