In this study, we aim to improve our understanding of the doping mechanism involved in the polymer poly[(4,8-bis-(2-ethylhexyloxy)-benzo(1,2-b:4,5-b')dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiophene-)-2-6-diyl)] (PBDTTT-c) doped with tris[1-(trifluoroethanoyl)-2-(trifluoromethyl)ethane-1,2-dithiolene] [Mo(tfd-COCF3)(3)]. We follow the evolution of the hole density with dopant concentration to highlight the limits of organic semiconductor doping. To enable the use of doping to enhance the performance of organic electronic devices, doping efficiency must be understood and improved. We report here a study using complementary optical and electrical characterization techniques, which sheds some light on the origin of this limited doping efficiency at a high dopant concentration. Two doping mechanisms are considered, the direct charge transfer and the charge transfer complex. We discuss the validity of the model involved as well as its impact on the doping efficiency. Published by AIP Publishing.