Carboranes have been previously noted to distinctively affect the luminescent properties of semiconducting polymers when incorporated into the conjugated backbone. In this report, we use carborane-based poly(fluorene) derivatives as active materials for polymer light-emitting diodes and transistors. Optoelectronic analysis unequivocally shows that carborane does not participate in the π-conjugated network, yet their presence causes major red-shifting in device electroluminescence as well as in thin film photoluminescence. In field effect transistors, they also improve charge carrier mobility by an order of magnitude despite disrupting π-conjugation. This use of carborane-containing conjugated polymers in active devices holds promise as new responsive materials in electronic polymer applications.