Block copolymers containing charge transport groups and semiconductor nanoclusters : synthesis, characterization, and optoelectronic applications

Abstract

Block copolymers containing pendant charge transport groups were synthesized for use in optoelectronic devices. The polymers were synthesized by the living ring-opening metathesis polymerization (ROMP) of functionalized norbomene derivatives, using a well-defined ruthenium carbene complex, RuC]z(=CHPh)(PCy3) 2, as the initiator. A monomer with pendant carbazole groups, 2-([methyldi(N-carbazolyl)]silyl)norbomene, was used as the hole-transporting species. A monomer with pendant dinitrobenzene groups, 2-[(3',5'-dinitrobenzoyl)methyl]norbomene, was used as the electrontransporting species. Both homopolymers and diblock copolymers were synthesized. Spincoated films of the diblock copolymer were prepared, which exhibited eithe; microphase separated or nonmicrophase separated morphologies depending on the choice of casting solvent. Charge transfer complex formation was observed in the diblocks, and the extent of complex formation was found to be greater in the nonmicrophase separated samples. Block copolymers containing the charge transport groups and different types of coordinating ligands were also prepared. Microphase separated diblock copolymers that contained a carbazole block and an alcohol block were synthesized. Cadmium sulfide nanoclusters were selectively synthesized within the spherical alcohol-functionalized microdomains, by treating the films with dimethylcadmium and hydrogen sulfide gas. The clusters were extremely small (less than 2 nm in diameter) and exhibited interesting optoelectronic properties, including a highly blue-shifted absorption edge and a broad, red-shifted photoluminescence peak indicative of poor surface passivation. Triblock copolymers containing a dinitrobenzene block, a carbazole block, and a phosphine block were synthesized, and the phosphine groups were used to load the polymer with monodisperse solution-synthesized cadmium selenide nanoclusters that were better passivated. Photovoltaic devices were fabricated from the CdSe-loaded triblocks. Microphase separated samples that possessed a morphology of cylinders oriented perpendicular to the substrate developed a larger open circuit voltage than did nonmicrophase separated samples, when the clusters were photoexcited. The improved performance is attributed to a decreased carrier recombination rate, which results from the rapid separation of the oppositely charged carriers into separate domains or channels. The effect of morphology on device performance, using samples of the exact same chemical composition, is thus clearly demonstrated for the first time.