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), sensors, and light emitting diodes. 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. 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%. 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. Although various inorganic nanocrystals such as cadmium selenium (CdSe) or silicon 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%. 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. 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 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, ZnO wafers, and CdSe nanorods, each of them bearing azide moieties. Heck coupling reactions were also used to attach vinyl terminated P3HT onto aryl bromide functionalized CdSe nanorods or quantum dots. Finally, esterification was a method of choice for preoxidized graphene oxide or carbon nanotube 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).
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.