The article focuses on the electrochemical and optical properties of n-type conjugated polymers, containing naphthalene diimide (NDI) acceptor units in their neutral and reduced states. Cyclic voltammetry reveals significant differences between the electrochemical stability of NDI-based polymers in the first and second reduced states. Combined electrochemical, spectral and conductivity results show that the first reduced state is related to the formation of radical anions delocalised on NDI units, with formation of interchain π-stacks between adjacent reduced NDI units being probable. The second reduced state is related to the formation of spinless species, whose properties are modified to a greater extent by the donor units. These results indicate that n-doping of naphthyl diimide-based conjugated polymers is significantly different from the doping of typical p-type conjugated polymers, which entails the formation of delocalised charge carriers, such as polarons and bipolarons. The n-type (electron acceptor) performance of NDI-based polymers was practically verified by employing them, along with poly(3-hexylthiophene) electron donor, in bulk heterojunction photovoltaic devices.