This work concerns the physical mechanisms of metal n-doping in charge transport layers for optoelectronic devices, for which the doping level is constrained by transparency requirements so as to avoid parasitic absorption. Comparing various metal dopants, we claim that enhanced conductivity at low doping is initiated by the electrical doping effect, namely, metal-semiconductor charge donation. Electrical measurements show that doping effects at low concentration strongly depend on the work function of the introduced metal, and not every metal works as an efficient dopant. Practical applicability is demonstrated by introducing doped transport layers in prototypical bilayer solar cells in conventional and inverted architectures.