Conducting polymer functionalized multi-walled carbon nanotubesnanocomposites: Optical properties and morphological characteristics

Nadia Chehata a,n, Adnen Ltaief a, Rabeb Bkakri a, Abdelaziz Bouazizi a, Emmanuel Beyou b

a Equipe Dispositifs Electroniques Organiques, Laboratoire de la Matière Condensée et des Nanosciences, Faculté des Sciences de Monastir,Boulevard de l’Environnement, 5019 Monastir, Tunisiab Ingénierie des Matériaux Polymères, 15 boulevard Latarget, Université Claude Bernard Lyon 1, 69 622 Villeurbanne Cedex, France

a r t i c l e i n f o

Article history:Received 7 December 2013Accepted 27 January 2014Available online 3 February 2014

Keywords:PolymerCarbon nanotubesCompositesFunctionalizationCharge transfer

a b s t r a c t

Donor/acceptor heterostructures formed by a donor conducting polymer and carbon nanotubes asacceptor materials are elaborated. Carbon nanotubes (CNTs) have been functionalized with polystyrene.A huge potential for functionalized CNTs for an effective charge transfer was shown. Moreover, opticalproperties have been correlated with morphological characteristics. We conclude that there is arelationship between the morphology and the performances of polymer/CNTs bulk heterojunctionnanocomposites, integrated as photoactive layer for photovoltaic application.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

In conjugated polymeric photovoltaic devices, a donor/acceptor(D/A) heterostructure is required to enhance the efficiency ofexcitons electron–hole pairs, photogeneration and their dissocia-tion at the D/A interfaces [1] into a hole and electron-producingfree charges. However, the electron charge transfer from donorconjugated polymer to acceptor material is hampered by thecompeting electron–hole recombination process, which degradesthe generation of photocurrent. In this respect, nanocomposites ofπ-conjugated polymers and carbon nanotubes are of great interestin terms of the novel electronic interaction between these twoelements and the mechanical reinforcement of the polymer film.The CNTs are used as electrodes, or blended with a polymeras acceptor material to form photoactive bulk-heterojunctiondevices. Different techniques have been used to prepare poly-mer/CNTs nanocomposites such as (i) in situ polymerization of themonomers in the presence of CNTs [2], (ii) solution drop casting,spin coating and Langmuir–Blodgett deposition; where both thematrix polymer and CNTs are dispersed in a common solvent [3],and (iii) melt processing which involves mechanical mixing of themolten polymers and CNTs [4]. However, the advantages of CNTsin photovoltaic devices are, somewhat hindered due to theirinsolubility and, hence, difficulties in homogeneous film forma-tion. Therefore, to improve the dispersion and the interfacial

adhesion of the CNTs in host polymers, chemical modification orfunctionalization of the CNTs is needed [5,6]. The functionalizationprocess will contribute to (i) yield homogeneous dispersion ofCNTs in solvents, (ii) ameliorate their compatibility with thepolymers, (iii) provide high field at the polymer/CNTs interfacesfor exciton dissociation and efficient charge transfer, and (iv)obtain homogenous morphologies for better transfer and trans-port of charges.

The aim of this work is to study the effect of functionalizedMWCNTs with polystyrene (PSMWCNTs), dispersed in MEH-PPVas electron donor, on optical properties of donor–acceptor bulkheterojunction. The dissociation of the photogenerated chargepairs upon incorporation of functionalized CNTs in the blend wasmore pronounced than unfunctionalized CNTs. Further, the rela-tion between the morphology of the nanocomposite thin films andthe efficiency of charge transfer has been highlighted.

2. Experimental details

Materials: Poly (2-methoxy-5-(2-ethyhexyl-oxy)-p-phenylene-vinylene) [MEH-PPV], used as donor polymer, was purchased fromSigma-Aldrich. MWCNTs (Graphistrength™ C100), supplied byARKEMA, have an average diameter and length of about 10 to15 nm and 0.1 to 10 μm, respectively.

Functionalization of CNTs: Functionalization process of MWCNTsby polystyrene (PS) was carried out as the following: 0.1 gof MWCNTs was suspended in 10 g of styrene and sonicatedunder stirring for 2 min at ambient temperature. The obtained

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Materials Letters

http://dx.doi.org/10.1016/j.matlet.2014.01.1620167-577X & 2014 Elsevier B.V. All rights reserved.

n Corresponding author. Tel.: þ216 73 500 279; fax: þ216 73 500 278.E-mail address: nadiachehata2@gmail.com (N. Chehata).

Materials Letters 121 (2014) 227–230

suspension was added into a water solution of Sodium Dodecyl-sulfate (SDS), used as surfactant, and sonicated for 30 min tocreate droplets. After sonication, a stable mini-emulsion wasobtained and then transferred into a three necked round bottomflask connected with a condenser and inert gas inlet and outlettubes. Oxygen was removed from mini-emulsion by the applica-tion of a positive flow of nitrogen for 30 min. After, a little amountof Ammonium Persulphate (APS) was added, as an initiator, to themini-emulsion. The blend was heated up to 70 1C and the poly-merization process has been broken up after 4 h of heating.

Device preparation: One might expect MEH-PPV to bind toMWCNTs, based on the chemical similarity of the conjugatedbackbones of both conjugated materials, and given the rightsolvent environment [7]. The MEH-PPV solution was prepared inthe chloroform (10 wt%) under stirring for 1 h. Different amountsof MWCNTs and PSMWCNTs (0, 0.05, 0.10, 0.2, 0.3 and 0.4 wt%)were dispersed in chloroform and submitted to a high powerultrasonic probe (30 W/2 h). It was found that for unfunctionalizedCNTs, an unstable dispersion can be observed meaning thatit is not possible to well disperse CNTs even after sonication.In contrast, for functionalized MWCNTs, we found a stable disper-sion. Obtained solutions of each CNTs were added to the MEH-PPVinitial solution and the blend was stirred for 1 h. Layers of MEH-PPV/MWCNTs and MEH-PPV/PSMWCNTs have been obtained byspin coating on a glass substrate. Finally, an annealing step (atT¼120 1C for 20 min) was required to evaporate the residualsolvent. Post-fabrication treatment such as heating to the pointbeyond the glass transition temperature of MEH-PPV (Tg(MEH-

PPV)¼80 1C) is beneficial for manipulating the phase separationof the blend. Overall, this annealing improves charge transfer,charge transport, and charge collection throughout the device [8].As a consequence of such ordering, the hole mobility, and hencethe power efficiency of the polymer nanotubes device, increasessignificantly.

Instrumentation: Photoluminescence spectra have been per-formed with a “JOBIN YVON-SPEX Spectrum One” CCD detector,cooled at liquid nitrogen temperature. A monochromator was usedto select an excitation wavelength corresponding to the maximumof the absorption band and fixed at 500 nm. Optical microscopeimages were taken with KLA-Tencor P6 optical microscope.

3. Results and discussion

The quenching of photoluminescence (PL) intensity of a suita-ble donor polymer by an appropriate acceptor filler is a signatureof an effective donor/acceptor charge transfer from the donor tothe acceptor, as described by Sariciftci et al. [9]. Fig. 1a and b shows

the PL spectra of MEH-PPV:MWCNTs and MEH-PPV:PSMWCNTs,respectively, for different compositions of CNTs (0, 0.05, 0.10, 0.2,0.3 and 0.4 wt%). In the case of unfunctionalized CNTs (Fig. 1a), thePL intensity of MEH-PPV decreases with the increase in MWCNTsconcentration up to 0.3 wt%. The extinction of the PL intensity isconsistent with the increase of the charge transfer efficiency,which is carried out before the radiative recombination of exci-tons. When 0.4 wt% of MWCNTs was added, the PL intensity wasdecreased. The decrease of PL intensity of MEH-PPV for higherconcentration of unfunctionalized CNTs (0.4 wt%) is probablyrelated to formed CNTs aggregates. The degree of photolumines-cence quenching depends mainly on the available interface areabetween donor and acceptor moieties for charge pair dissociationand charge transfer at donor/acceptor interfaces [10]. It providesan indication of how well the CNTs are mixed with the polymer.Moreover, phase separation, which commonly occurs in nanocom-posites at high acceptor content, should lead to a photolumines-cence increase as a result of a reduced interface between the donorand acceptor phases. From Fig. 1a, we can conclude that 0.3 wt%represents the threshold concentration of MWCNTs, which shouldbe incorporated into MEH-PPV polymer to have a homogenousdispersion of CNTs. Two phenomena affect CNTs dispersion[11]: nanotubes morphology and attractive forces between thetubes. Not only the tube surfaces are attracted to each other bymolecular forces, but the extremely high aspect ratios coupledwith the flexibility also dramatically increase the possibilities forentanglements. In contrast, as shown in Fig. 1b, the PL intensity of

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Fig. 1. (a) PL spectra of pure MEH-PPV and MEH-PPV:MWCNTs nanocomposites, and (b) PL spectra of pure MEH-PPV and MEH-PPV:PSMWCNTs nanocomposites, fordifferent amounts of MWCNTs and PSMWCNTs.

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Fig. 2. Variation of the quenching efficiency parameter η as function of CNTsconcentration into donor polymer. I0 and I are the integrated PL intensities of thedonor in the absence and presence of the acceptor, respectively.

N. Chehata et al. / Materials Letters 121 (2014) 227–230228

MEH-PPV still decreases with the progressive increase of functio-nalized MWCNTs concentration up to 0.4 wt%. This reductionof PL emission is associated to an efficient electron transfer fromMEH-PPV to PSMWCNTs along MEH-PPV:PSMWCNTs interfaces.The functionalization process helps to ameliorate the dispersion ofCNTs in solution and polymer matrix and thus to avoid theiraggregation. Moreover, the diminution of the polymer domainsfree of electron acceptors having the same size than the diffusionlength of the exciton (�10 nm), will participate to a rapid diffu-sion and dissociation of exciton. It is worthy to note that therelative position of donor LUMO and acceptor LUMO is funda-mental to ensure efficient photoinduced charge transfer, whereelectrons will preferentially be transferred through the nanotubesand leaving holes to get transferred through the polymer chains.Some studies have reported that the work function of MWCNTsranges from 4.6 to 4.9 eV [12]. It was also reported that afterfunctionalization process, the obtained functionalized CNTs willhave higher work function, reaching a value of 5.1 eV [13].The high electron affinity of functionalized nanotubes will favoran efficient charge transfer. The load of charge transfer canbe evaluated by calculating the parameter η, which define thetransfer efficiency, by using the following known expression [14]:

η¼ 1� II0

ð1Þ

where I0 and I are the integrated PL intensities of the donor(MEH-PPV) in the absence and the presence of the acceptor(CNTs),respectively. From Fig. 2, it is clear that the charge transfer is moreefficient for MEH-PPV:PSMWCNTs than MEH-PPV:MWCNTs. Thehigh length to diameter aspect ratio of MWCNTs greatly supportscharge tunneling between the donor/acceptor systems throughpolystyrene (PS) shielding nanotubes and the charge transfer ismore efficient.

To obtain an insight in the role of functionalization, we furthercompared the surface morphology of the MEH-PPV:MWCNTs andMEH-PPV:PSMWCNTs nanocomposites. Fig. 3a and b shows theoptical microscope images of the MEH-PPV:MWCNTs and MEH-PPV:PSMWCNTs nanocomposites with different concentrations ofCNTs. From Fig. 3a, we can observe homogenous distribution ofunfunctionalized CNTs within polymer matrix up 0.3 wt% of CNTs.For MEH-PPV:MWCNTs (0.4 wt%), we can observe the formationof CNTs aggregations. We can conclude that the reason of thediminution of PL intensity and the domination of recombinationphenomena is related to the non-uniform distribution of CNTs.From Fig. 3b, we can observe homogenous distribution of functio-nalized CNTs within polymer matrix up 0.4 wt% of functionalizedCNTs. Despite the aggregation formation of PSMWCNTs at 0.4 wt%,their sizes are still less important that those obtained at the sameconcentration of unfonctionalized CNTs.

4. Conclusion

The obtained results prove that the functionalization processhelps to ameliorate the properties of dispersion of CNTs intoMEH-PPV conjugated polymer, and hence improve the electrontransfer from donor to acceptor. Besides, there is a relationshipbetween the morphology and performances of polymer/CNTs bulkheterojunctions as donor/acceptor system, integrated as photo-active layer for photovoltaic application.

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