Bis(trifluoromethane)sulfonimide lithium salt (Li-TFSI) has been popularly employed as an efficient p-dopant that increases the conductivity of a hole transport layer (HTL) in perovskite solar cells and dye-sensitized solar cells. However, the working mechanism of the Li-TFSI dopant is a long-standing question. The hygroscopicity of Li-TFSI makes it difficult to isolate the exact doping mechanism. In this study, we unveil the role of Li-TFSI in the p-doping to the N,N'-di(1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine (NPB) HTL. A series of systematic in situ measurements using ultraviolet and inverse photoelectron spectroscopy reveal that electron transfer from NPB to Li-TFSI occurs due to the lower-lying negative polaronic level of Li-TFSI rather than the positive polaronic level of NPB. The hole injection barrier between NPB and indium tin oxide is significantly reduced with Li-TFSI doping, enhancing the device performance of hole-only devices and organic light-emitting diodes dramatically. With excessive dopants, however, the agglomerative property of Li-TFSI became dominant, decreasing the doping efficiency. These results provide robust guidelines for developing an efficient doping method for a molecular system with high conductivity.