Luminescence Behavior of Self-Assembled Multilayer Heterostructures of Poly(Phenylene-Vinylene)
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ABSTRACT Multilayer thin film heterostrutuctures comprised of alternating layers of a poly(phenylene vinylene) (PPV) precursor and various polyanions were fabricated using a new self-assembly technique. Thermal treatment of these films produced multilayers of the fully conjugated form of PPV. Alternating multilayers fabricated with inert polyanions such as sulfonated polystyrene exhibited a very strong photoluminescence whereas alternating multilayers fabricated with electron acceptor polyanions such as sulfonated C60 and poly(thiophene acetic acid) displayed a highly quenched luminescence. This latter behavior suggests that self-assembled multilayers can be used to examine photoinduced charge transfer interactions between PPV and various electron acceptors.
INTRODUCTION The recent discovery that certain conjugated polymers such as poly(phenylene vinylene) (PPV) exhibit electroluminescence behavior has stimulated an intense research effort to identify new materials and new thin film structures with enhanced light emitting properties [1,2]. Recent developments in this field have also shown that the luminescence behavior of PPV and related derivatives can be completely quenched when the conjugated polymer is molecularly mixed with strong photo-activated electron acceptors such as C60. In this latter case, it is believed that quenching occurs by a mechanism involving the photoinduced charge transfer from the excited conjugated polymer to the acceptor [3,4]. This effect, in turn, can be utilized to create thin film photo-rectifiers and photovoltaic heterojunctions. In order to take full advantage of the interesting luminescent properties of PPV, it is necessary to develop processing techniques that can be used to fabricate multilayer heterostructures with well defined layer thicknesses and controllable layer functionalities. Of particular interest are multilayer thin films in which layers of functionally different materials are used to control the injection, transport and recombination of electrons and holes in Schottky barrier based light emitting diodes [5,6]. Alternatively, to exploit the luminescence quenching behavior of this material, the fabrication of multilayer thin films with molecular layers of PPV alternating with molecular layers of an acceptor are highly desirable. In both cases, supramolecular control over the number and positioning of the heterojunctions created between the various electroactive layers is required. Unfortunately, traditional thin film processing techniques such as spin coating do not provide this level of control. We have recently demonstrated a new and very versatile mean of fabricating multilayer thin films of conjugated polymers [7-9]. This technique is based on the spontaneous adsorption of a charged polymer onto a surface from a dilute polymer solution. By alternately dipping a substrate
into a dilute solution of a positively charged polymer and a negatively charged polymer, it is possible to fabricate multilayer thin films in a layer-by-layer manner. This process can be carried
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