Triethylene glycol, a common side chain, is attached to two different dye moieties, diketopyrrolopyrrole (DPP) and isoindigo (II), and their bromo derivative monomers are copolymerized, respectively, with common bisstannyl alkylated bithiophene via Stille coupling. The resulting donor–acceptor low‐bandgap copolymers, namely, PTDPP‐DT and PTII‐DT, are rationally deigned and synthesized conjugated polymeric systems suitable for doping. Both polymers are successfully investigated as single‐component and composite‐system‐based electrochemical transistors and light‐emitting electrochemical cells. A PTDPP‐DT thin film exhibits relatively high electrical conductivity of up to 80 S cm−1 in the electrochemically doped state, whereas PTII‐DT thin film prevents the macroscopic charge transport due to a large‐scale crystalline disorientation. Upon evaluating both polymers as active conjugated materials in light‐emitting electrochemical cells, they both exhibit emission under efficient electron/hole doping conditions.