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Synthesis of TiO2-poly(3-hexylthiophene) Hybrid Particles through Surface-Initiated Kumada Catalyst-Transfer Polycondensation Florian Boon, David Moerman, Danielle Laurencin, Sébastien Richeter, Yannick Guari, Ahmad Mehdi, Philippe Dubois, Roberto Lazzaroni, and Sébastien CLEMENT Langmuir, Just Accepted Manuscript • DOI: 10.1021/la502944g • Publication Date (Web): 04 Sep 2014 Downloaded from http://pubs.acs.org on September 9, 2014
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Synthesis of TiO2-poly(3-hexylthiophene) Hybrid Particles through Surface-Initiated Kumada CatalystTransfer Polycondensation Florian Boon,†,‡ David Moerman,† Danielle Laurencin,§ Sébastien Richeter,§ Yannick Guari,§ Ahmad Mehdi,§ Philippe Dubois,† Roberto Lazzaroni‡ and Sébastien Clément§,* †
Laboratory of Polymeric and Composite Materials, Center of Innovation and Research in
Materials and Polymers (CIRMAP), University of Mons UMONS, Place du Parc 20, 7000 Mons, Belgium ‡
Laboratory for Chemistry of Novel Materials, CIRMAP, University of Mons UMONS, Place
du Parc 20, 7000 Mons, Belgium. §
Institut Charles Gerhardt Montpellier ICGM, UMR 5253, CNRS-UM2-ENSCM-UM1,
Université Montpellier 2, Bâtiment 17, CC 1701. Place E. Bataillon, 34095 Montpellier Cedex 5, France. *Corresponding author:
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Abstract TiO2/conjugated polymers are promising materials in solar energy conversion where efficient photo-induced charge transfers are required. Here, a ‘grafting-from’ approach for the synthesis of TiO2 nanoparticles supported with conjugated polymer brushes is presented. Poly(3hexylthiophene) (P3HT), a benchmark material for organic electronics, was selectively grown from TiO2 nanoparticles by Surface-Initiated Kumada Catalyst-Transfer Polycondensation (SIKCTP). The grafting of the polymer onto the surface of the TiO2 nanoparticles by this method was demonstrated by 1H and
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C solid-state NMR, XPS (X-ray photoelectron spectrometry),
thermogravimetric analysis (TGA), Transmission Electron Microscopy (TEM) and UV-Visible spectroscopy. Sedimentation tests in THF revealed improved dispersion stability for the TiO2@P3HT hybrid material. Films were produced by solvent casting and the quality of the dispersion of the modified TiO2 nanoparticles was evaluated by Atomic Force Microscopy (AFM). The dispersion of the P3HT-coated TiO2 NPs in the P3HT matrix was found to be homogeneous and the fibrillar structure of the P3HT matrix was maintained which is favourable for charge transport. Fluorescence quenching measurements on these hybrids materials in CHCl3 indicated improved photo-induced electron transfer efficiency. All in all, better physicochemical properties for P3HT/TiO2 hybrid material were reached via surface-initiated “grafted from” approach compared to the “grafting onto” approach.
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Introduction Conjugated polymers (CP) have attracted huge interest in recent years due to their applications in many optical and electronic devices such as organic light-emitting diodes (OLEDs),1-2 fieldeffect transistors (FETs),3-4 sensors5 and polymer solar cells (PSCs).6 A typical approach to fabricate PSCs is to employ the bulk-heterojunction (BHJ) concept where the photoactive layer is built upon an interpenetrating network of donor (conjugated polymer) and acceptor (mainly fullerene derivative) materials.7 While the power conversion efficiency (PCE) of PSCs has increased recently to over 8% in single BHJ PSCs,8 their instability, particularly in the presence of oxygen and moisture, as well as the poor electron mobility of fullerene derivatives are critical issues for the cell performance and lifetime.9 An alternative to fully organic BHJ consists of combining conjugated polymers with n-type inorganic semiconductors.10-11 These {conjugated polymers}/{inorganic semiconductor} hybrid materials can take benefit from the properties of both types of materials: easy solution processing of the CP on one hand, and higher thermal and ambient stabilities, and higher electron mobility of the inorganic semiconductors on the other. So far, various hybrid polymer solar cells including inorganic semiconductors such as TiO2,12-13 ZnO,14-15 CdSe,16 PbS17 have been reported. Among these, titanium dioxide is one of the most widely studied phase due to its lack of toxicity, its relatively low cost of production and its tunable morphology, as it can be fabricated in various sizes and shapes, including spheres, tubes and rods. The possibility to tailor the dimensionality and morphology of TiO2 particles offers the opportunity to optimize the charge transport in the photovoltaic device.18 For example, Chen et coll. demonstrated that the dimensionality and anisotropy of TiO2 nanocrystals in P3HT:TiO2 hybrids could lead to carrier transport behaviors in directions perpendicular or parallel to the film substrate, when switching from nanorods to
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nanoparticles.19 Another critical issue of these organic-inorganic hybrids is the inherent incompatibility between the hydrophilic surface of the metal oxide and the hydrophobic nature of the conjugated polymers, which frequently triggers phase segregation between the electron donor and acceptor components, leading to reduced carrier-dissociation efficiency.20 Thus, improving the miscibility and dispersion stability of CP:TiO2 hybrids, while maintaining good electron transfer properties is crucial. Inorganic semiconductors used in hybrid solar cells are in general prepared by colloidal synthesis, leading to the formation of nanocrystals (NCs) whose size and shape is determined by the nature of the surface organic ligands chosen (alkyl thiols, amines, phosphines and phosphine oxides) and the temperature.21-22 The organic molecules on the surface of the inorganic semiconductor NCs ensure their dispersion in solution and miscibility in the polymer matrix. However, the insulating nature of these ligands severely affects the morphology of NCs:CP films and reduces the charge transfer operating between CPs and NCs, as well as the electron transport between NCs.21-22 One of the most promising alternatives to significantly increase both miscibility and electrical performance is to directly modify the surface of the inorganic semiconductors with CPs.5a In principle, three different approaches exist for the modification of the surface of the inorganic semiconductors with CP chains. In “grafting onto” approaches, CP contains a functional group at one chain end, obtained either from the polymerization technique or by post-polymerization chemical transformations. This end-group is then reacted with a complementary functional group on the NC surface.23 This approach has the advantage of simplicity, but usually results in low grafting density of the polymer; the grafting being limited both thermodynamically and kinetically. For example, we have recently reported the grafting of a carboxylic end-
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functionalized poly(3-hexylthiophene) (P3HT) onto the surface of TiO2 nanoparticles (leading to improved photo-induced electron transfer efficiency),24 in which only 3% of this polymer was grafted to TiO2. Another straightforward technique is the “grafting through” approach, which consists in performing a solution polymerization in presence of a material whose surface is functionalized with groups which are reactive in the polymerization. Unfortunately, as in the case of the “grafting onto” approach, this strategy is intrinsically limited to low grafting densities and low layer thicknesses.25 The third approach, referred to as the “grafting from” approach is based on the immobilization of an initiator at the surface of a material, followed by the reaction of this surface-bound species with monomers in solution. Compared to “grafting onto” and “grafting through” strategies, the packing density of grafted polymers as well as the nature of the immobilized chains can be more carefully controlled.26-27 Indeed, if the initiation is efficient and if the polymerization follows a chain-growth mechanism, stretched polymer chains with high molecular weights (exceeding 106 g.mol-1) and low polydispersity (
