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Keywords:

  • conjugated polymers;
  • functionalization of polymers;
  • grafting;
  • hybrid material;
  • monolayers;
  • nanoparticles;
  • P3HT;
  • photovoltaic;
  • poly(3-hexylthiophene);
  • self-assembly;
  • silane;
  • surfaces;
  • zinc oxide

ABSTRACT

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. Acknowledgments
  8. REFERENCES AND NOTES
  9. Supporting Information

We demonstrate an efficient strategy to anchor poly(3-hexylthiophene) (P3HT) onto zinc oxide (ZnO) surfaces. Synthesis of a novel triethoxysilane-terminated regioregular P3HT is herein reported and supported by thorough characterization. Three triethoxysilane-terminated P3HTs of different molar masses were prepared via a hydrosilylation reaction from allyl-terminated P3HT. MALDI-TOF and 1H NMR were performed to characterize the polymer and show that around 80% of the chains are end-functionalized. These polymers were then grafted onto the ZnO nanorods to create a macromolecular self-assembled monolayer. This versatile technique could be subsequently applied to different metal oxide surfaces, such as silicon, titanium, or indium-tin oxide, and represents a new one-pot strategy based on triethoxysilane coupling reaction. Importantly, the influence of the molar mass on the grafting density and the polymer shell thickness was studied via thermo gravimetric analysis and transmission electron microscopy. The optical properties of the hybrid materials were determined by UV–visible absorption and photoluminescence to show a quenching effect of P3HT fluorescence by ZnO when grafted. This electronic transfer associated with an improved miscibility of the ZnO@P3HT, makes these hybrid materials suitable candidates for photovoltaic applications. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 30–38


INTRODUCTION

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. Acknowledgments
  8. REFERENCES AND NOTES
  9. Supporting Information

Conjugated polymers (CPs) have attracted considerable attention in recent years due to their wide-ranging applicability to optoelectronic device such as polymer solar cells (PSC),[1] sensors,[2] and light emitting diodes.[3] PSCs have been the topic of intensive research over the last two decades, during which time has appeared the concept of the bulk heterojunction (BHJ) active layer.[4] Based on the intimate mixing at the nanoscale of an electron donor, usually a conjugated polymer such as poly(3-hexylthiophene) (P3HT), and an electron acceptor, usually a soluble modified fullerene such as phenyl-C61-butyric acid methyl ester (PCBM), organic BHJs have shown regular advances. Nevertheless, after an enormous number of improvements, P3HT:PCBM blends level off at an efficiency of around 5%.[5] Moreover, such PSCs are not stable with aging because of PCBM aggregation in the active layer.[6, 7] In this context, the advent of inorganic materials, having original and controllable electronic and optical properties,[8, 9] has rapidly resulted in the demonstration of several hybrid devices in which the organic acceptor material is replaced by inorganic semiconducting structures. Moreover, in most cases, inorganic materials significantly outperform organic electronic materials with respect to charge carrier mobility and chemical stability.[10] Although various inorganic nanocrystals such as cadmium selenium (CdSe)[11] or silicon[12] have been studied intensively in the field of hybrid BHJ solar cells, metal oxide nanoparticles, and especially titanium oxide TiO2 and zinc oxide ZnO,[13, 14] are of particular interest due to their ease of fabrication, nontoxicity, and relatively low production costs. Hybrid solar cells using ZnO nanoparticles/polymer as the BHJ were first reported by Beek et al. in 2004. In this study, the authors mixed ZnO nanocrystals with a poly(phenylene vinylene) (PPV) and obtained an efficiency of 1.6%.[15] Later, the same group varied the shape, size, and concentration of the particles and showed that the best performances were obtained for nanocrystals of 4.9 nm in diameter. The efficiency dropped to 0.92% when combining the nanoparticles with P3HT.[16] This behavior was assigned to a coarse mixing of the blend and high film roughness generating current leakages. Therefore, the interface between the ZnO material and the polymer is essential and research has turned toward its optimization. A widely used strategy to enhance the properties of a hybrid material is to covalently attach the components; in this case, P3HT might best be anchored to ZnO particles. This serves several purposes, including alignment of energy levels at interfaces, enhancement of interfacial exciton–dissociation efficiencies, and optimization of the active layer morphology, that is, nanophase segregation is expected to be easier and more stable in time. This interfacial-engineering approach is desired to significantly improve the efficiency and stability of the active layer of PSCs and could enable large-area device manufacturing using low-cost, all-printable processes.

The surface grafting of conjugated polymer is a recent field that started in 2004 when Emrick et al. showed that CdSe nanocrystals could be integrated into PPV thin films without aggregation.[17, 18] Several techniques have been used via the “grafting through” method to create conjugated brushes, such as Yamamoto,[19, 20] Sonogashira,[21, 22] or Suzuki[23] coupling polycondensations. “Grafting from” has also been developed to graft P3HT via surface-initiated Kumada catalyst transfer polycondensation. As the polymerization proceeds like a chain polymerization, grafting from was adapted for CP brushes elaboration.[24-32] Last, the “grafting onto” methodology was used and consists in the reaction of end-functionalized polymers with reactive groups on a solid substrate. Huisgen 1,3-dipolar cycloaddition, so-called click chemistry has been applied to graft ethynyl-terminated P3HT to prefunctionalized graphene oxide sheets,[33] ZnO wafers,[34] and CdSe nanorods,[35] each of them bearing azide moieties. Heck coupling reactions were also used to attach vinyl terminated P3HT onto aryl bromide functionalized CdSe nanorods[36] or quantum dots.[37] Finally, esterification was a method of choice for preoxidized graphene oxide[38] or carbon nanotube[39] with either acid or hydroxyl chain ends. In all these studies, two steps are required; first functionalize the surface and then make this group polymerize or react with the end-chain-modified polymer.

In this article, we report the original synthesis of triethoxysilane end functionalized P3HTs via the controlled Grignard Metathesis polymerization (GRIM). These polymers were used to create in one step a macromolecular self-assembled monolayer on raw ZnO nanorods (Scheme 1).

image

Scheme 1. Synthetic procedure for ZnO@P3HT hybrids nanorods (NRs).

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The versatile functional triethoxysilane moiety has been largely used by our group for many substrates with different shapes and chemical composition in order to control the grafting density values of coil polymers.[40, 41] It could be applied to different metal oxide surfaces such as titanium or indium-tin oxide, very useful substrates for photovoltaic applications. To the best of our knowledge, this is the first time that this strategy has been used to graft a conjugated polymer onto a surface. For the first time also, the influence of the conjugated polymer molar mass on the grafting density was studied. The polymers and the hybrid materials have been thoroughly characterized to evidence the covalent attachment of the polymer to the metal oxide surface. Finally, some preliminary properties of the hybrid materials have been studied such as the dispersion stability and the electronic properties to anticipate their potential applications in active layer of photovoltaic cells.

EXPERIMENTAL

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. Acknowledgments
  8. REFERENCES AND NOTES
  9. Supporting Information

Materials

All reactions were performed under predried nitrogen using flame-dried glassware and conventional Schlenk techniques. Syringes used to transfer reagents or solvents were purged with nitrogen before use. Chemicals and reagents were used as received from Aldrich (France) and ABCR (Germany) and stored in the glove box. Solvents (Baker, France) were used as received; THF was distilled over sodium and benzophenone under nitrogen.

Instrumentations

1H and 29Si nuclear magnetic resonance (NMR) spectra were recorded using a Bruker 400 MHz instrument in CDCl3 at ambient temperature. Gel permeation chromatography (GPC) was performed using a bank of 4 columns (Shodex KF801, 802.5, 804, and 806) each 300 × 8 mm2 at 30 °C with THF eluent at a flow rate of 1.0 mL min−1 controlled by a Malvern pump (Viskotek, VE1122) and connected to Malvern VE3580 refractive index and Malvern VE3210 UV–visible detectors. Calibration was against polystyrene standards. Thermal gravimetric analysis (TGA) was performed on a TGA Q50, TA Instruments at a heating rate of 10 °C min−1 under nitrogen. UV–visible absorption spectra were recorded on a Shimadzu UV-2450PC spectrophotometer. MALDI-MS spectra were performed by the CESAMO (Bordeaux, France) on a Voyager mass spectrometer (Applied Biosystems). The instrument was equipped with a pulsed N2 laser (337 nm) and a time-delayed extracted ion source. Spectra were recorded in the positive-ion mode using the reflectron and with an accelerating voltage of 20 kV. Samples were dissolved in THF at 10 mg mL−1. The DCTB matrix {T-2-[3-(4-t-butylphenyl)−2-methyl-2-propenylidene] malononitrile} solution was prepared at a concentration of 10 mg mL−1 in THF. The solutions were combined in a 10:1 volume ratio of matrix to sample. One to two microliters of the obtained solution were deposited to the sample target and vacuum-dried.

Emission Spectroscopy (Photoluminescence)

Corrected steady-state emission and excitation spectra were recorded at 1-nm resolution using a photon counting Edinburgh FLS920 fluorescence spectrometer with a xenon lamp. The concentrations in CHCl3 were adjusted to an absorbance around 0.1 at 450 nm (excitation wavelength) in a 1-cm quartz fluorescence cell (Hellma). The absolute fluorescence quantum yields were measured using Rhodamine B as a standard (0.43 in chloroform) for an excitation wavelength of 500 nm.[42]

Transmission Electronic Microscopy

Analysis of the core@shell nanoparticles shape and the thickness of the P3HT monolayer were obtained by transmission electron microscopy (TEM) with a JEOL JEM-2100 FX transmission electron microscope, using an accelerating voltage of 200 kV at room temperature.

Synthesis of Allyl-Terminated P3HT

Allyl-terminated P3HTs of high regioregularities were synthesized using literature procedures.[43] The GRIM method was applied to synthesize the desired polymer in a flamed-dried 100-mL round flask bottom under inert atmosphere at room temperature. Initially, 2,5-dibromo-3-hexylthiophene (1) (3.06 mmol) and freshly distilled THF 10 mL were added into the flask. After mixing for several minutes, isopropyl magnesium chloride (3.06 mmol) was then added via a syringe and stirred for 2 h at room temperature. The reaction mixture was diluted to 50 mL with dried THF, and 1,3-bis(diphenylphosphino)propane nickel-(II) chloride Ni(dppp)Cl2 (0.087 mmol for P1, 0.078 for P2, and 0.065 for P3) was added. The polymerization proceeded for 10 min before adding allyl magnesium bromide (1.53 mmol) and then the reaction continued for another 30 min to ensure high end-group functionalization before quenching with methanol. The resulting solid polymer was washed by Soxhlet extraction using ethanol and acetone, and recovered with chloroform. The three allyl-terminated P3HT with number average molar masses (Mn according to GPC) are P3HT (P1) [5600 g mol−1, Ð = 1.14], P3HT (P2) [8000 g mol−1, Ð = 1.16], P3HT (P3) [11,000 g mol−1, Ð = 1.1] were synthesized using the same procedure and varying the amount of catalyst.

Yield = 50%. 1H NMR (400 MHz, CDCl3, δ (ppm): 6.98 (s, 1H), 6.0 (m, 1H), 5.15 (m, 2 H), 3.52 (d, 2H), 2.8 (t, 2H), 1.7 (q, 2H), 1.3–1.5 (m, 6H), 0.92 (t, 3H).

Synthesis of Triethoxysilane-Terminated P3HT

In a flame-dried 50-mL flask, 100 mg of allyl-terminated P3HT (2 equiv) was mixed with 4 mg of H2PtCl6 (catalyst, 1 equiv) and 15 mL of THF. The solution mixture was degased for 15 min to avoid air. Under stirring, 0.3 mL (0.26 g, 100 eq) of triethoxysilane was added dropwise. The mixture was stirred for 30 min at room temperature before its temperature was raised to 55 °C for 5 h. Finally, the polymer was precipitated twice in dry ethanol, filtered under nitrogen, and stored in the glove box to avoid hydrolysis/condensation of the polymer end chain.

Yield >90%. 1H NMR (400 MHz, CDCl3), δ (ppm): 6.98 (s, 1H), 3.87 (q, 6H), 2.8 (t, 2H), 1.7 (q, 2H), 1.3–1.5 (m, 6H), 1.25 (t, 9H), 0.92 (t, 3H). 29Si NMR (δ, CDCl3): −45.4 ((EtO)3SiC) ppm.

Synthesis of ZnO Nanorods

ZnO nanoparticles were prepared according to a published method.[44, 45] Zinc acetate (99.99%) potassium hydroxide (99.99%, 15% water) and methanol (anhydrous, 99.8%) were purchased from Sigma/Aldrich and used as-received without further purification. All the glassware was washed with a concentrated solution of sodium hydroxide and rinsed with deionized water before use. In a typical synthesis, potassium hydroxide (0.047 mol in 88.75 mL of methanol) and zinc acetate (0.017 mol in 162.18 mL of methanol and 965 µL of deionized water) solutions were prepared separately by sonication at room temperature until complete dissolution. Both solutions were transparent and colorless. Zinc acetate solution was heated in a glass flask to 60 °C under nitrogen atmosphere and potassium hydroxide solution was then added dropwise under magnetic stirring. The solution rapidly became white and turned to transparent again with further addition of potassium hydroxide. After a few hours aging at 60 °C, the solution finally became white indicating formation of 4–5 nm ZnO nanoparticles. The solution was condensed and heated under magnetic stirring at 60 °C for 24 h giving the nanorods. Finally, as-obtained ZnO nanorods were washed for several times with methanol by decantation.

Grafting Triethoxysilane-Terminated P3HT onto ZnO Nanorods

ZnO nanorods (NRs) were dispersed in THF (2 mg mL−1, 5 mL) by ultrasonication for 1 h. About 2 mL solution of triethoxysilane-P3HT (20 mg mL−1) in THF was added to the mixture. From the ZnO nanorods specific surface area (SSA, determined by BET), we calculated that the P3HT was introduced at an excess of 2 chains nm−2 of ZnO surface in order to saturate the surface. The ungrafted or free chains were removed at the end of the reaction by solubilizations and washings. The reaction then proceeded at 60 °C for 12 h under inert atmosphere. The medium was cooled to RT and ZnO@P3HT was purified by centrifugation (10,000 rpm, 10 min) with removal of the supernatant containing excess of organic component. The purification was repeated several times until the UV–visible spectra of the THF supernatant became featureless (no P3HT absorption around 450 nm). The precipitated particles were collected, dried, and stored under nitrogen. A change in the color of the ZnO NRs was clearly observable from white to violet after grafting of P3HT (dry state).

RESULTS AND DISCUSSION

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. Acknowledgments
  8. REFERENCES AND NOTES
  9. Supporting Information

Synthesis of Triethoxysilane-Terminated P3HT

3-Hexylthiophene was polymerized via the GRIM method,[46] with the introduction of allyl magnesium bromide to yield three allyl end-functional P3HTs (P3HT-allyl) with high end chain functionalization, high regioregularities, and low dispersities (Ð) (Table 1). A further postfunctionalization via the hydrosilylation method[47] was performed under dry conditions to transform the alkene into triethoxysilane end-groups in quantitative yields. However, due to the high sensitivity of the Si-OEt moiety to hydrolysis, the silane end-functional polymers (P3HT-Si) were stored in a glove box under inert atmosphere. Figure 1 shows a superposition of the 1H NMR spectra of P3HT-allyl and P3HT-Si (other 1H NMR spectra can be found in Supporting Information Figs. SI-1–SI-6). End groups were identified by their chemical shifts and splitting patterns. The allyl-terminated polymer shows three peaks: h (CH2, 3.49 ppm, d), i (CH, 5.98 ppm, m), and j (CH2, 5.12 ppm, t) pertaining to the end chain. Triethoxysilane-terminated P3HT spectrum in Figure 1 shows a complete disappearance of allylic protons and appearance of two peaks k (CH2, 3.87 ppm, q) and l (CH3, 1.25 ppm, t). Comparing the integrations of proton d with proton d′ and d″, a regioregularity of more than 96% was calculated. For the molar mass obtained (11,000 g mol−1 for the highest), the proton d′ and d″ would correspond to two units, that is, 4 protons. Based on this we can calculate by 1H NMR the Mn by comparing protons d with d′ and d″, and also chain-end functionalization, by comparing of i, j, or h to d′ and d″. Table 1 gives an overview of the results; more than 70% of the macromolecules were successfully end functionalized. 29Si NMR performed on the polymers (see Supporting Information Fig. SI-7) shows the presence of a signal at −45.4 ppm pertaining to (EtO)3SiC group, confirming the functionalization and the absence of hydrolysed alkoxysilane functions.[48]

Table 1. Macromolecular Characteristics of Synthesized P3HTs
PolymerMna (g mol−1)Mnb (g mol−1)Mnc (g mol−1)= % Enda= % EndbSi % Enda% RRaÐc
  1. a

    Calculated from NMR.

  2. b

    Calculated from MALDI-TOF.

  3. c

    Calculated from SEC (polystyrene conventional calibration). “=” means allyl, Si triethoxysilane, and RR regioregularity.

P13,8002,7005,60070697096%1.14
P25,3003,9008,00084728497%1.16
P37,8005,50011,0001007510098%1.1
image

Figure 1. 1H NMR (400 MHz, CDCl3) of allyl-terminated and triethoxysilane-terminated P3HT P3.

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To further confirm this, MALDI-TOF investigation was carried out (spectra, Supporting Information Figs. SI-8–SI-10) and reviewed in Table 1. The molar masses determined by MALDI-TOF are lower than those obtained by 1H NMR, in agreement with previous studies.[49] Similarly, polystyrene-calibrated GPCs overestimate molar masses by a factor from around 1.5 to 25.0,51 Two populations were identified by MALDI-TOF. As a representative example, P3HT-allyl P1, consisted of a mixture of 70% of H-P1-allyl and 30% of H-P1-H. The percentage of the allyl population is given in Table 1 for the three polymers. P3HT-Si were unsuccessfully analyzed by MALDI-TOF as only small chains were extracted from the matrix and detected in the spectra; a not untypical result given the low polarity of the polymer.[52] It is noted that the dispersities were below 1.2 (see Supporting Information Fig. SI-11).

Grafting P3HT onto ZnO Nanorods

The bare ZnO particles were dispersed in THF and mixed with an excess of silane-terminated polymer. The grafted particles were characterized after an extensive cleaning procedure (see Experimental section). FTIR characterization was used to verify the grafting of P3HT onto ZnO NRs. Figure 2 shows the IR spectra of P3HT P1, bare ZnO particles, and hybrid ZnO@P1. ZnO@P1 spectrum shows the characteristic frequencies of both ZnO, that is, a broad absorption band between 3000 and 3500 cm−1 revealing the presence of the surface hydroxyl groups, and P3HT with a strong absorption peaks at 2960, 2923, and 2852 cm−1, attributable to the asymmetrical C[BOND]H stretching mode of methyl and methylene protons of the hexyl side chain group.

image

Figure 2. Infra-red spectra of P3HT (P1), bare ZnO, and grafted particles ZnO@P1.

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image

Figure 3. Thermo gravimetric analysis of bare and grafted ZnO NRs under nitrogen at a heating rate of 10 °C min−1.

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TGAs were performed under nitrogen with a heating rate 10 °C min−1 in order to examine the degradation of ZnO@P3HT NRs and to quantify the amount of P3HT covalently linked to the NRs due to the thermal degradation of the organic phase (Figure 3). The crude ZnO NRs showed a weight loss of 2.4% between 100 and 600 °C, while for pure P3HT a weight loss of about 70% was observed between 400 and 600 °C (Supporting Information Fig. SI-12). The calculated weight losses for P3HT in the hybrids ZnO@P1, ZnO@P2, and ZnO@P3 were, respectively, 2.7, 3.7, and 1.9% (Table 2), calculated via the following formula (1):

  • display math(1)
Table 2. Hybrids Materials Characteristics
Hybrid MaterialP3HT (wt %)aAbsorbance 450 nmbσa (chains nm−2)hc (nm)
  1. a

    Calculated from TGA.

  2. b

    Calculated from UV spectroscopy at λ = 450 nm.

  3. c

    Determined from TEM images. + is a qualitative information of the P3HT absorbance onto ZnO.

ZnO@P12.7++0.253 ± 1
ZnO@P23.7+++0.243 ± 1
ZnO@P31.9+0.094 ± 1

Interestingly, the highest value was found when P2 was used as macromolecular grafting agent. This result is further confirmed by UV–visible spectroscopy by qualitatively dosing the P3HT content of the hybrids materials. Normalizing the spectra to the maximum absorption wavelength of ZnO at 371 nm (Supporting Information Fig. SI-13) the absorbance at λ = 450 nm was qualitatively compared for the three hybrid materials. Relative absorbance is reviewed in Table 2. Within the three macromolecular grafting agents, P2 was also found to be the most efficient grafting agent, followed by P1 and finally P3, meaning that molar mass has an important role within the grafting onto methodology.[53, 54]

With the NRs specific surface area (SSA) (24 m2 g−1 determined by BET), the polymer molar mass and the weight fraction of P3HT in the hybrids materials (fwP3HT) determined by TGA, it is possible to calculate the surface grafting density (σ) of the polymer monolayer (calculation developed in Supporting Information) via the following formula (2):

  • display math(2)

where Na is Avogadro constant.

ZnO@P1 and ZnO@P2 present the same grafting density values with 0.25 and 0.24 chains nm−2, respectively. These values are typically associated to the “polymer brush” regime. ZnO@P3 has a lower grafting density value of 0.09 chains nm−2, where the monolayer is in a “semidilute” regime. The chains must have more room to fold while covering the entire surface. This conformation variation could be attributed to the molar mass of the macromolecular grafting agent, P1 and P2 have a lower degree of polymerization than P3; therefore, the steric hindrance induced by a grafted P3 chain is more important than for a P1 one. In the grafting onto methodology, once a few initial chains have been grafted a steric hindrance prevents the chains in solution from reaching the surface; they must first diffuse through the existing polymer film. This “excluded volume” barrier becomes more pronounced as the thickness of the tethered polymer layer increases.[55]

TEM was used to determine the thickness of the grafted P3HT layer onto the ZnO NRs surface. Figure 4 shows both bare particles and ZnO@P1, a clear complete and homogeneous polymer shell is identified around ZnO NRs leading to core@shell hybrid material. TEM images for others grafted particles (Supporting Information Figs. SI-14–SI-18) confirm that the polymer shell forms a continuous and homogeneous monolayer. The average polymer shell thicknesses (h) as measured from the TEM images for the three hybrids materials are given in Table 2. ZnO@P1 and ZnO@P2 have a polymer shell of 3 ± 1 nm and ZnO@P3 has a shell of 4 ± 1 nm. These very close values are not only linked to the polymer molar mass but also to the grafting density. Because the grafting density of P3 is lower than the one of P1 and P2, the P3-grafted chains could be more folded, reducing the thickness.

image

Figure 4. TEM images for (a) bare ZnO nanorods and (b) ZnO@P1 (scale bar = 20 nm).

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Using a phosphonic acid end-functionalized P3HT to react with Zn-OH surface moieties of nanowires, Fréchet et al. have observed lamellar chain packing oriented parallel to the surface, when P3HT (7000 g.mol−1 by MALDI-TOF) is grafted on the ZnO surface, and explained this by a chain folding.[56] From the estimated unit cell parameter of the P3HT and the lamellar fold length (5–10 nm),[57, 58] the authors calculated a shell thickness of 6–11 nm. If we follow Fréchet's calculation, the thickness of the P3HT brushes would be between 4 and 9 nm which is in good agreement with the TEM measurement.

Hybrid Materials Properties

To study the influence of the polymer shell on the particles stability, the bare and functionalized particles were dispersed in THF by ultrasonication during 30 min. A first concentration of 4 mg mL−1 was prepared and the sedimentation was followed visually. After 1 h, the bare ZnO solution started to be transparent as the particles aggregated at the bottom of the container. On the contrary, grafted particles stay dispersed even after 24 h (Supporting Information Fig. SI-19). UV–visible spectroscopy was used to quantify this phenomenon. Transmission for particles solutions in THF (C = 0.08 mg mL−1) was recorded at λ = 370 nm. After 800 min, the transmission of grafted particles solution was at 5% when the one for the neat particles was at 20% (transmission started at 0%, Supporting Information Fig. SI19). This variation shows the important role of the P3HT monolayer as a stabilizer in the solution (a similar effect is expected in a P3HT matrix, work in progress to be exposed in the forthcoming article).

The optical properties of ZnO@P3HT materials were under investigation using UV–visible absorption and photoluminescence. Figure 5(a) shows the absorption spectra of bare ZnO, pure P3HT P2, grafted ZnO@P2 and mixed ZnO/P2 in chloroform solution. The absorption peak of P2 was observed at 450 nm in agreement with literature value for P3HT.[59, 60] The grafted polymer absorbed at around the same wavelength but the presence of ZnO particles in solution induced diffusion artifact on the spectra [Fig. 5(a)] that made difficult to see and estimate the variation of the wavelength maximum. The bare ZnO nanorods presented a maximum at 373 nm in pure CHCl3 and showed no discernible change after mixing with P3HT. But this characteristic peak was clearly blue shifted by 3 nm in ZnO@P2 spectrum which may be attributed to the change in dielectric environment, revealing the intimate contact between ZnO particles and P3HT and energy perturbation of the quantum-confined excitation.[37]

image

Figure 5. (a) UV–visible absorption and (b) photoluminescence (λex = 450 nm) spectra of chloroform solutions of P3HT, bare, and grafted ZnO particles, and a mixture composed of ZnO and P3HT (weight ratio = 96/4).

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Photoluminescence (PL) spectra of the polymer P2, P2 and bare ZnO particles blend and ZnO@P2 hybrid material, under an incident light with a wavelength of 450 nm are presented in Figure 5(b) (the excitation spectra can be found in Supporting Information Fig. SI-20). The dominant peak of P2 at 580 nm is an emission characteristic of the P3HT backbone[59, 60] that arises from the relaxation of excited π-electron to the ground state while the shoulder around 640 nm is related to interchain states. The addition of ZnO nanoparticles to the polymer solution, in a concentration calculated with respect to the mass composition of ZnO@P2 (i.e., ZnO/P3HT = 96.3/3.7) did not change the photoluminescence properties of P2. It was supposed, that under these conditions, the concentration of ZnO was too low to quench significantly the emission signal. On the contrary, the emission spectrum of ZnO@P2 showed a strong decrease in the PL intensity, resulting from an efficient charge transfer from the polymer to the ZnO particles.[61] The decrease of the absolute fluorescence quantum yields from P2 to ZnO@P2, measured to be 0.12 and 0.03, respectively, confirms these observations. The intimate contact helps the quenching that occurs between ZnO and P3HT and this property is crucial for photovoltaic devices.

CONCLUSIONS

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. Acknowledgments
  8. REFERENCES AND NOTES
  9. Supporting Information

This work demonstrates the efficient grafting procedure of triethoxysilane terminated P3HT onto zinc oxide nanorods. Three (C2H5O)3Si-terminated regioregular P3HTs with different molar masses were synthesized via a hydrosilylation reaction from allyl-terminated P3HT. MALDI-TOF and 1H NMR were performed to characterize the polymer and show that around 80% of the chains were end-functionalized. The raw ZnO nanorods were then grafted with triethoxysilane terminated P3HT by a one-step procedure of condensation. IR spectroscopy and TGA analysis confirm the efficiency of the simple procedure. TEM images for the three hybrids materials show a continuous and homogeneous polymer shell 4 ± 1 nm, not only linked to the polymer molar mass but also to the grafting density. Finally, UV-visible absorbance and photoluminescence show the electron transfer from irradiated P3HT to the ZnO-grafted particles. This result suggests that these hybrid core@shell materials could be suitable for the elaboration of photovoltaic active layers by mixing ZnO@P3HT hybrids with a P3HT matrix. Also interesting, this chain-end functionalized P3HT and this simple technique of grafting are currently applied to different metal oxide surfaces with various shapes in order to develop more stable hybrid photovoltaic devices. These studies are currently underway and will be the topic of forthcoming reports.

Acknowledgments

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. Acknowledgments
  8. REFERENCES AND NOTES
  9. Supporting Information

This work was supported by the French Ministry of Research and COST Action MP1202 “Rational design of hybrid organic-inorganic interfaces: the next step towards advanced functional materials”. The authors thank C. Absalon from CESAMO (Université de Bordeaux) for the MALDI-TOF analyses.

REFERENCES AND NOTES

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. Acknowledgments
  8. REFERENCES AND NOTES
  9. Supporting Information

Supporting Information

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. Acknowledgments
  8. REFERENCES AND NOTES
  9. Supporting Information

Additional Supporting Information may be found in the online version of this article.

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pola26964-sup-0001-suppinfo01.pdf2388KSupplementary Information

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