Synthesis and characterization of hybrid CdS/MEH-PPV nanocomposites for photovoltaic applications
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Synthesis and characterization of hybrid CdS/MEH-PPV nanocomposites for photovoltaic applications Anna M. Laera, Vincenzo Resta, Emanuela Piscopiello, Monica Schioppa, and Leander Tapfer ENEA, Unità Tecnica Tecnologia dei Materiali Brindisi (UTTMATB), S.S. 7 “Appia” km 706, 72100 Brindisi, Italy ABSTRACT Inorganic-organic nanocomposites, with II-VI or III-V semiconductor nanocrystals (NCs) embedded in semiconducting polymer matrix, are very promising materials for photovoltaic applications. Here, we present an effective and easy synthesis procedure to obtain a hybrid nanocomposite with CdS NCs dispersed in poly[2-methoxy-5-(2-(2’-ethyl-hexyloxy)-1,4phenylene vinylene] (MEH-PPV) conjugated polymer. CdS NCs are synthesized directly within the matrix through the decomposition of a suitable unimolecular precursor dispersed homogeneously in the polymer. We show that CdS NCs are formed at low annealing temperature avoiding structural damages and without affecting the functional properties of the MEH-PPV polymer. The NCs diameter ranges between 1.5nm and 4nm depending on the annealing temperature. In addition, no coalescence phenomena of CdS NCs were noticed in TEM observations even at very high particle density (40 wt %). INTRODUCTION Nanocomposite structures based on II-VI or III-V semiconductor NCs dispersed in conjugated organic polymers are promising candidates for application in the field of large-area, flexible, low-cost photovoltaic devices due to their peculiar mechanical [1,2], optical [3], and electrical properties [4]. In the active layer of photovoltaic devices, called “bulk heterojunction”, inorganic NCs act as efficient electron acceptors from the organic polymers that have a suitable low ionization potential. In order to assure an efficient collection and transport of the photogenerated carriers, it is necessary to achieve a dense interpenetrating network of NCs in the matrix, avoiding agglomeration and particle clustering [5]. Bulk heterojunctions made of the same constituents display different efficiency depending on the grade of dispersion of NCs [5,6]. The most common synthetic strategies to obtain inorganic-organic nanocomposites are ex situ processes in which NCs are dispersed in polymer solutions, and afterwards deposed by casting or spin-coating [7]. During these procedures, the NCs may form micro-size aggregate compromising optoelectronic properties of the resulting nanocomposite. In the present work, we propose a convenient in situ route, to synthesize CdS NCs in the conjugated polymer MEH-PPV, that allows to obtain homogeneous hybrid blends. The presented strategy is based on the thermal decomposition of the cadmium complex [Cd(SBz)2]2.MI (MI = 1-methyl imidazole), that is highly soluble in most of the common organic solvents, and can be well dispersed in polymers. Solution of [Cd(SBz)2]2.MI and MEH-PPV in chloroform have been used to obtain composite films by spin coating or casting. The annealing of these films, by using a controlled and reproducible temperature ramp, induces the nucleation and
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