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\bibitem{Yang_etal1990} {\textquotedblleft}\textit{Conductivity and electron density of undoped model compounds of poly(phenylene vinylene)}{\textquotedblright}. Yang Z, Geise HJ, Mehbod M, Debrue G, Visser JW, Sonneveld EJ, Van {\textquoteright}t dack L, Gijbels R, Synthetic metals \textbf{39}, 137 (1990).   http://doi.org/10.1016/0379-6779(90)90179-O

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