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Materials Chemistry and Physics 115 (2009) 429–433

Contents lists available at ScienceDirect

Materials Chemistry and Physics

journa l homepage: www.e lsev ier .com/ locate /matchemphys

iO2:polymer bulk-heterojunction thin films made from a miscible newarbazole based TiO2 precursor with poly(N-vinylcarbazole) for enhancedharge transfer properties

incent Barliera,1, Véronique Bounor-Legaréa, Gisèle Boiteuxa, Didier Léonardb, Joel Davenasa,∗

Ingénierie des Matériaux Polymères, Université Claude Bernard Lyon 1, UMR CNRS 5223, F-69622 Villeurbanne, FranceInstitut des Sciences Analytiques, Université Claude Bernard Lyon 1, UMR CNRS 5180, F-69622 Villeurbanne, France

r t i c l e i n f o

rticle history:eceived 29 July 2008eceived in revised form6 November 2008ccepted 3 January 2009

ACS:2.35.Np

a b s t r a c t

TiO2:polymer bulk-heterojunctions (BHJ) have been elaborated from hydrolysis–condensation reactionsof a TiO2 precursor in contact to the surrounding air humidity in a polymer thin film. A new precursor:tetrakis(9H-carbazole-9-yl-ethyl-oxy) titanium [Ti(OeCarb)4], has been synthesized as a TiO2 precursorto form a blend with Poly(N-vinylcarbazole) (PVK) which is the archetype of non-conjugated photocon-ducting polymer with strong electron-donor properties. This new precursor is expected to enhance thematerials miscibility because of the chemical structure of the ligand close to the PVK repetitive unit and toinhibit premature hydrolysis by a strong steric hindrance. Commercial titanium isopropoxide [Ti(iOPr)4]

8.37.Ps3.63.−b

eywords:itanium alkoxideoly(N-vinylcarbazole)ol–gels

was used as a reference to study the influence of the chemical structure of the precursor on BHJ proper-ties. Photoluminescence studies have shown charge transfer enhancement when Ti(OeCarb)4 is used. Inorder to understand this ligand effect, photoluminescence (PL) responses were correlated with surfacechemical composition (XPS) and topography (AFM) of thin films. Results have shown that Ti(OeCarb)4

allows a better miscibility between TiO2 and PVK. The lower reactivity of Ti(OeCarb)4 to hydrolysis andits chemical structure close to the repetitive unit structure of the polymer are believed to play a main role

vem

in the BHJ property impro

. Introduction

Polymer solar cells are becoming a subject of great interestue to their low cost production and the possibility of large areand flexible device fabrication [1]. Research in this field has beenainly focused on bulk-heterojunction (BHJ) devices [2,3] involv-

ng a conjugated polymer and inorganic nanocrystals allowing theombination of the optoelectronic properties of the polymer andhe conductivity of inorganic particles [4,5]. Previous works havehown the relative efficiency of nanocrystals like CdS [6], CdSe7], ZnO [8] and TiO2 [9–11] in photovoltaic solar cell devices. Inhe specific case of conjugated TiO2:polymer composites, the pho-ocurrent is controlled by the photogeneration of excitons in the

olymer while the TiO2 semiconductor allows charge transfer andransport. The efficiency of such BHJ is however usually limitedy charge recombination phenomena and the electron transportan be hindered by surface trap states [12]. To overcome these

∗ Corresponding author. Tel.: +33 472431983; fax: +33 478892583.E-mail addresses: barlier@usc.edu (V. Barlier), Joel.Davenas@univ-lyon1.fr

J. Davenas).1 Present address: USC Chemistry Department, Los Angeles, CA 90089, USA.

254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2009.01.003

ent.© 2009 Elsevier B.V. All rights reserved.

limitations and to allow a fast charge transfer rate, polymer andnanocrystal phases should be mixed at a scale similar to the exci-ton diffusion length (typically about 5–15 nm) [13–15]. In thiscontext, the formation of such bulk-heterojunction TiO2:polymerthin films is limited by the difficulty to homogeneously mix thehydrophilic TiO2 particles with hydrophobic polymer in solution.Moreover, during the formation of the film, solvent evapora-tion often causes the aggregation of TiO2 particles in the film.Recently, van Hal et al. [16] proposed a new procedure for prepar-ing BHJ in which a continuous interpenetrating network of TiO2was created via hydrolysis–condensation reactions of Ti(iOPr)4in poly[2-methoxy-5-(3′,7′-dimethyl-octyloxy)-p-phenylene viny-lene] (MDMO-PPV) thin films. The influence of the relative humidity(RH) on the morphology during spin-coating has led to the conclu-sion that the TiO2 precursor should not be too reactive to avoidhydrolysis before the end of film formation [17]. In the more gen-eral context of TiO2 particle synthesis, it is known that the reactivityof titanium alkoxides to hydrolysis can be controlled by parame-

ters such as pH [18], H2O/Ti ratio [19] or size of ligands [20,21].The addition of acid and water during the process will, however,severely hamper the polymer solubility or even be harmful for thepolymer. Consequently, the increase of ligand size is the parame-ter of choice, together with the relative humidity control during

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compared using UV–vis absorption and photoluminescencespectroscopy.

UV–vis absorption spectra of [Ti(iOPr)4]BHJ and [Ti(OeCarb)4]BHJare displayed in Fig. 2. The characteristic absorption peaks of PVK(235, 265, 300 and 345 nm) and particularly 1Lb (345 nm) and 1La

30 V. Barlier et al. / Materials Chem

pincoating, to avoid premature hydrolysis of the TiO2 precur-or.

In this study, we propose to use a bulky precursor to reduce theydrolysis rate. Moreover, the precursor chemical structure shoulde similar to that of the selected polymer in order to increase theiriscibility. A new precursor: tetrakis (9H-carbazole-9-yl-ethyl-

xy) titanium [Ti(OeCarb)4], has been synthesized to produce moreomogeneous blends with poly(N-vinylcarbazole) (PVK), which

s a non-conjugated photoconducting polymer exhibiting stronglectron-donor properties [22–26]. Combining the steric hindrancend the specific chemical structure of the ligands, TiO2:PVK BHJhotovoltaic efficiency enhancement is expected compared to BHJomposites prepared from usual precursors. For that purpose, theommercial titanium isopropoxide [Ti(iOPr)4], which exhibits smallteric hindrance, has been used as a reference. The charge trans-er properties (characterized by PL quenching) of TiO2:polymerhin films made from each precursor are discussed in relation withurface elemental composition [X-ray photoelectron spectroscopyXPS)] and morphology [Atomic force microscopy (AFM)].

. Experimental

Molecular structures of poly(N-vinylcarbazole) [PVK], titanium isopropoxideTi(iOPr)4] and Tetrakis (9H-carbazole-9-yl-ethyl-oxy) titanium [Ti(OeCarb)4] areresented in Fig. 1.

Poly (N-vinylcarbazole) (PVK) (M.W 90,000 g mol−1, Acros Organics) wasurified by extraction in a standard Soxhlet apparatus with ethyl acetateCHROMASOLV® Plus, for HPLC, 99.9%,Aldrich) at 70 ◦C before its solubilisation inHF (CHROMASOLV® Plus, for HPLC,≥99.9%, inhibitor-free, Sigma-Aldrich). The solu-ion was precipitated in methanol (CHROMASOLV® Plus, for HPLC, ≥99.9%, Aldrich)nder vigorous stirring. The precipitate was then filtrated on Buchner funnel andried under vacuum for 12 h at room temperature.

Titanium(IV) isopropoxide, Ti(iOPr)4 (98+%, Acros Organics) was used withoutny further purification.

Titanium tetrakis (9H-carbazole-9-yl-ethyl-oxy), Ti(OeCarb)4, was synthesizeds described earlier [21].

All solution manipulations were carried out using Schlenk techniques.All substrates were cleaned beforehand in an acetone (CHROMASOLV® Plus,

or HPLC, ≥99.9%, Aldrich) and then ethanol (ACS reagent, ≥99.5%, Sigma-Aldrich)ath at 100 ◦C, rinsed with isopropyl alcohol (CHROMASOLV® Plus, for HPLC, 99.9%,ldrich), and dried under nitrogen flow. Spin-coating was carried out under argont relative humidity RH = 0 for all solutions containing the TiO2 precursor. The tem-erature during spin-coating was around 20 ◦C.

The TiO2:PVK BHJ devices were prepared using the following route. A solution ofVK (5 mg mL−1) in anhydrous toluene was beforehand prepared, filtered (0.45 �m,TFE, Roth) and stored under argon. The TiO2 precursors were solubilized in anhy-rous toluene at a concentration consistent with the expected TiO2 volume ratioTi(iOPr)4: 30 �L mL−1and Ti(OeCarb)4: 90 mg mL−1 for a 1:1 (v/v) ratio). An equalolume of solution of TiO2 precursor was added drop by drop to the solution ofVK under argon and vigorous stirring. Then, the final solution was spin-coated at000 rpm for 60 s. Quartz substrates were used for optical absorption and photolu-inescence properties. Silicon wafers were used for surface elemental composition,

hickness and topography measurements.To convert the TiO2 precursor in the TiO2 phase, thin films were kept in a con-

rolled environment (20 ◦C, RH = 50% and obscurity) for 24 h. The TiO2 concentrationalues given in the text are calculated from the amount of materials in solution.otice that amorphous rather than crystalline TiO2 is formed because no annealing

reatment is used during the process.

ig. 1. Molecular structures of poly(N-vinylcarbazole) [PVK], titanium isopropoxideTi(iOPr)4] and Tetrakis (9H-carbazole-9-yl-ethyl-oxy) titanium [Ti(OeCarb)4].

nd Physics 115 (2009) 429–433

X-ray photoelectron spectroscopy (XPS) measurements were carried out usinga Riber SIA 200 spectrometer. A non-monochromatized AlK� source generated at10 kV, 20 mA was used to irradiate the sample surfaces and XPS spectra were col-lected at a 65◦ take-off angle between the sample and the analyzer. The pressurein the instrumental chamber was less than 1.0 × 10−4 Pa. No radiation damage wasobserved during the data collection. All spectra were referenced to the C1s peak (C–Cand C–H bonds) whose binding energy was fixed at 285.0 eV. The standard deviationof binding energy measurements was estimated to be about 0.15%. Atomic concen-trations, as determined from XPS peak areas, are considered to be accurate withinaround 10%.

The surface morphology and thickness of the films were studied by atomic forcemicroscopy (AFM) with a PicoSPM Molecular Imaging apparatus operated underambient condition in tapping mode. Microfabricated silicon cantilevers (Nanoworld)with a spring constant of 42 N m−1 were used.

The UV–vis spectra were collected over the spectral range 200–500 nm using aPERKIN ELMER Lambda 35 spectrophotometer.

Photoluminescence spectra were collected under photoexcitation at 300 nm andrecorded at 360 nm, to follow the PL dependence with the TiO2 content, using a JOBINYVON-SPEX Spectrum one CCD detector cooled at liquid nitrogen temperature.

3. Results and discussion

3.1. Charge transfer in TiO2:poly(N-vinylcarbazole)bulk-heterojunctions

The charge transfer efficiency in TiO2:PVK bulk-heterojunctionselaborated from titanium isopropoxide ([Ti(iOPr)4]BHJ) and tetrakis(9H-carbazole-9-yl-ethyl-oxy) titanium ([Ti(OeCarb)4]BHJ) are

Fig. 2. Absorption spectra (UV–vis spectroscopy) of TiO2:PVK (different vol. ratios)bulk-heterojunctions elaborated from Ti(iOPr)4 (a) and Ti(OeCarb)4 (b). Layer thick-ness was evaluated from AFM characterization.

istry and Physics 115 (2009) 429–433 431

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V. Barlier et al. / Materials Chem

300 nm) transitions, assigned to delocalised electron states alonghe polymer chain[27], do not show any shift. The spectral posi-ion of these transitions and their relative absorption intensities areot appreciably affected when the TiO2 precursor is added, indicat-

ng that no charge-transfer or electronic interactions occur in theround state. Fig. 2a shows the absorption increase of [Ti(iOPr)4]BHJt short wavelengths for an increasing TiO2 content. Moreover, AFMeasurements still show an enhancement of the film thicknesshen the TiO2 content is increased. The film thickness increases

rom 33 to 44 nm for 2–18 vol.% TiO2. This enhancement is assignedo TiO2 but may be due also to the residual alcohol moieties (iPrOH)emaining after hydrolysis. In Fig. 2b, the [Ti(OeCarb)4]BHJ absorp-ion spectra show a strong absorption enhancement assigned to theH-carbazole-9-yl-ethyl-oxy groups arising from 9H-carbazole-9-thanol (produced from the hydrolysis–condensation reactions) orrom the TiO2 precursor ligands. The absorption enhancement at00 nm is attributed to the usual carbazole absorption from the pre-ursor. The AFM measurements show an enhancement of the filmhickness when the TiO2 content is increased. More accurately, thelm thickness is found to be 45 nm for a 2 vol.% TiO2 but increasesbove 72 nm for the 5 vol.% ratio. When the TiO2 content is furtherncreased (until 18 vol. %), the thickness is almost unchanged. Thistrong enhancement in thickness confirms the presence in the filmf residual alcohol moieties (9H-carbazole-9-ethanol).

The Fig. 3 shows the PL spectra of [Ti(iOPr)4]BHJ (Fig. 3a) andTi(OeCarb)4]BHJ (Fig. 3b) thin films and the Fig. 4 shows the PLfficiency, excited at a wavelength corresponding to a conduction

and of the polymer (� = 300 nm absorption band) for increasingiO2 contents. A quenching effect of the polymer fluorescence isbserved for at a relatively low concentration (2 vol.%) for both pre-ursors. At higher concentrations, the quenching is not completeut the PL intensity steadily decreases. Considering that Ti(iOPr)4

ig. 3. Normalized photoluminescence intensity (�ex = 300 nm) of TiO2:PVK (differ-nt vol. ratios) bulk-heterojunctions elaborated from Ti(iOPr)4 (a) and Ti(OeCarb)4

b).

Fig. 4. Normalized photoluminescence intensity (�ex = 300 nm and �em = 360 nm)of TiO2:PVK (different vol. ratios) bulk-heterojunctions elaborated from Ti(iOPr)4

(square) and Ti(OeCarb)4 (circle).

and Ti(OeCarb)4 contribute also to the photoluminescence (PL spec-tra of TiO2 precursors not shown here), it can be concluded that thequenching effect is due to the presence of amorphous TiO2 domainsin the polymer matrix.

A better charge transfer is highlighted in thin films derived fromTi(OeCarb)4 than from titanium isopropoxide. To understand theeffect of 9H-carbazole-9-yl-ethyl-oxy ligands on the photolumines-cence quenching, the chemical composition and the morphology of[Ti(iOPr)4]BHJ and [Ti(OeCarb)4]BHJ were compared.

3.2. TiO2:poly(N-vinylcarbazole) bulk-heterojunctions chemicalcomposition and morphology

To evaluate the surface chemical composition and the morphol-ogy of TiO2:PVK thin films, X-ray photoelectron spectroscopy (XPS)and atomic force microscopy (AFM) were respectively used.

A previous study has been focused on the steric hindrance effecton the hydrolysis–condensation of various titanium alkoxides inthin films [21]. It was shown by XPS that the reaction rates forTi(OeCarb)4 thin films is slower than for Ti(iOPr)4. The same kind ofanalysis is not possible in the composite film because the XPS C1speak is dominated by the polymer contribution to the C1s spec-trum. A high alkoxide content should be used for a convenientcharacterization. However, the difficulty to remove residual 9H-carbazole-9-ethanol in polymer thin films arising from the bulkystructure of this alcohol makes it difficult to precisely identify thefinal chemical structure of TiO2 made from Ti(OeCarb)4. A lowalkoxide content should then be favoured for a more convenientcharacterization. Here, a comparative study of chemical compo-sition (XPS) combined with topography (AFM) was performed onTiO2:PVK (5:95 vol. ratio) made from each precursor (Table 1 andFig. 5). A weak TiO2 concentration was chosen to minimize the con-tribution from residual alcohol groups which would complicate

data analysis. The results reveal similar elemental compositions(Table 1) for both composites but the film morphology is clearly dif-ferent as seen from AFM height images (Fig. 5). The [Ti(OeCarb)4]BHJ(top) shows a structure homogeneously mixed at a nanometer scale

Table 1XPS atomic chemical surface composition of TiO2:PVK (5:95 vol. Ratio) bulk-heterojunctions elaborated from Ti(iOPr)4 (left) and Ti(OeCarb)4 (right).

Titanium precursor Surface elemental composition of TiO2:PVK BHJ (%)

C O N Ti

Ti(iOPr)4 85.1 7.5 5.0 2.2Ti(OeCarb)4 87.5 5.0 5.9 1.5

432 V. Barlier et al. / Materials Chemistry and Physics 115 (2009) 429–433

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Fig. 6. Variation of roughness mean square (RMS) as a function of the TiO2 con-tent for TiO2:PVK bulk-heterojunctions elaborated from Ti(iOPr)4 (square) andTi(OeCarb)4 (circle).

ig. 5. Tapping mode AFM height images (2 �m × 2 �m) of TiO2:PVK (5:95 vol. Ratio)ulk-heterojunctions elaborated from Ti(iOPr)4 (top) and Ti(OeCarb)4 (bottom).

ith a very low roughness (<1 nm) while [Ti(iOPr)4]BHJ (bottom)xhibits a significant heterogeneity with higher roughness (>3 nm),ighlighting that the enhancement of the film homogeneity islearly due to the chemical influence of the 9H-carbazole-9-yl-thyl-oxy ligands.

The Fig. 6 shows the variation of RMS roughness for [Ti(iOPr)4]BHJnd [Ti(OeCarb)4]BHJ at different TiO2 concentrations. The topogra-hy of [Ti(iOPr)4]BHJ shows a roughness increase: the roughnessf the film increases with the TiO2 content, until more than

nm for the highest concentration. This result highlights the dif-culty to obtain a homogeneously nanostructured film surfaceith the Ti(iOPr)4 precursor. The heterogeneities result from the

ecurrent problem of phase separation that can be explained byhe modification of hydrophilic nature of the alkoxide during the

Fig. 7. Tapping mode AFM phase images (2 �m × 2 �m) of TiO2:PVK (18:82 vol. ratio)bulk-heterojunctions elaborated from Ti(iOPr)4 (top) and Ti(OeCarb)4 (bottom).

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rocess. In fact, in a water free solution, the more hydropho-ic nature of Ti(iOPr)4 is compatible with the polymer. However,hen the solution is deposited on the substrate by spin-coating,

i(iOPr)4 can partially react with atmospheric moisture to formydrophilic species,–Ti–OH. If Ti(iOPr)4 is hydrolyzed before filmormation, an heterogeneous structure is obtained. The topographyf [Ti(OeCarb)4]BHJ is relatively smooth (less than 1 nm) regard-ess of the TiO2 content. This result can be explained by differentypotheses. First, 9H-carbazole-9-yl-ethyl-oxy ligands close to theepetitive unit structure of the desired polymer (PVK) allow a betteriscibility between both materials. Secondly, as stated earlier [21],

he sensitivity to the hydrolysis–condensation reactions is loweror Ti(OeCarb)4 than for Ti(iOPr)4. When limiting the reactivity ofhe precursor, the hydrolysis–condensation reaction during depo-ition is reduced. Finally, 9H-carbazole-9-ethanol resulting fromhe hydrolysis–condensation reactions of Ti(OeCarb)4, would acts a plasticizer for PVK, allowing miscibility and film-forming prop-rty improvements. This plasticizing effect has been validated by aelt approach where 9H-carbazole-9-ethanol enhanced the pro-

essability of PVK (data not shown here). The Fig. 7 illustrates theiscibility improvement of both materials at a nanometer scale

hrough the AFM phase images thin film of [Ti(iOPr)4]BHJ (top) andTi(OeCarb)4]BHJ (bottom) for 18 vol.% TiO2. At this high TiO2 con-ent, [Ti(OeCarb)4]BHJ shows a very homogeneous structure withonor and acceptor materials mixed at a nanometric scale.

These results indicate that the development of a specific tita-ium alkoxide (with bulky ligand exhibiting a chemical structureimilar to the polymer) is a powerful tool for the elaborationf TiO2:polymer BHJ thin films exhibiting improved propertiesowards exciton dissociation (shown by improved PL quenching)wing to improved miscibility and limited premature hydrolysis.

. Conclusions

An efficient donor–acceptor material based on TiO2:Polymerulk-heterojunction (BHJ) has been obtained by the in-situ produc-ion of the TiO2 phase through an hydrolysis–condensation processf a TiO2 precursor in polymer thin films. A new tetrakis(9H-arbazole-9-yl-ethyl-oxy) titanium [Ti(OeCarb)4] precursor wasynthesized to form an homogeneous blend with Poly(N-inylcarbazole) (PVK) which is considered as the model of non-onjugated photoconducting polymers with strong electron–donorroperties. This new precursor exhibits a strong steric hindrancend a chemical structure of ligand close to the PVK repetitive unit.he influence of the chemical structure of the precursor on BHJroperties was highlighted by the comparison with commercialitanium isopropoxide [Ti(iOPr)4] used as a reference. Photolumi-escence studies have shown an efficient charge transfer at theiO2/PVK interface. This behaviour could be interpreted in rela-ion with the film surface morphology (AFM) and surface chemicalomposition (XPS). The AFM measurements revealed a more homo-

eneous structure and very low roughness when Ti(OeCarb)4 wassed as the precursor. The composite morphology improvementan be attributed to the chemical structure of this newly synthe-ized TiO2 precursor. In fact, 9H-carbazole-9-yl-ethyl-oxy ligandsear the PVK repetitive unit structure increase the miscibility of

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nd Physics 115 (2009) 429–433 433

both materials. Moreover, the rate of Ti(OeCarb)4 hydrolysis wasreduced due to the bulky chemical structure of the ligands and wasthe origin of a plasticizing effect of the residual alcohol moieties[9H-carbazole-9-ethanol].

Thanks to a similar molecular structure between the precursorand the polymer and a strong steric hindrance, Ti(OeCarb)4 allowedthe production of homogeneous TiO2:PVK nanocomposites withhigher efficiency charge transfer properties. This result triggers anew approach based on modified titanium alkoxides/polymer thinfilms combining good miscibility and steric hindrance leading tothe improvement of charge transfer properties which is a key issuefor next generation photovoltaic materials and also for the devel-opment of new information storage media.

Acknowledgements

This study was supported by REGION RHONES ALPES (Prospec-tive Program).

This project was included in the European Network of ExcellenceNanofun-Poly.

We thank Pierre Alcouffe for AFM measurements and the NMRdepartment of the fédération des polyméristes Lyonnais (FR 2151,CNRS) for NMR characterization of the new precursors.

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