Charge carrier densities in chemically doped organic semiconductors verified by two independent techniques

The charge carrier density of the p-type doped organic semiconductor 2,7-bis(9-carbazolyl)-9,9-spirobifluorene is determined for varied doping concentrations. As p-type dopant molybdenum trioxide is used. We determine the carrier density by measuring the polaron induced optical absorption and by a capacitance-voltage analysis. We show that both results are in excellent agreement. An almost linear dependence of the charge carrier density on the doping concentration is observed. Carrier densities on the order of 10(18) cm(-3) at a dopant concentration of 1 mol % can be achieved. Overall, a low doping efficiency on the order of 2%-4.5% is evidenced. (C) 2010 American Institute of Physics. [doi:10.1063/1.3427416]