A novel polyelectrolyte containing triazine (TAZ) and benzodithiophene (BDT) scaffolds with polar phosphine oxide (P═O) and quaternary ammonium ions as pendant groups, respectively, in the polymer backbone (PBTAZPOBr) was synthesized to use it as a cathode interfacial layer (CIL) for polymer solar cell (PSC) application. Owing to the high electron affinity of the TAZ unit and P═O group, PBTAZPOBr could behave as an effective electron transport material. Due to the polar quaternary ammonium and P═O groups, the interfacial dipole moment created by PBTAZPOBr substantially reduced the work function of the metal cathode to afford better energy alignment in the device, thus enabling electron extraction and reducing recombination of excitons at the photoactive layer/cathode interface. Consequently, the PSC devices based on the poly[4,8-bis(2-ethylhexyloxyl)benzo[1,2-b:4,5-b']dithiophene-2,6-diyl-alt-ethylhexyl-3-fluorothithieno[3,4-b]thiophene-2-carboxylate-4,6-diyl]:[6,6]-phenyl-C71-butyric acid methyl ester (PTB7:PC71BM) system with PBTAZPOBr as CIL displayed simultaneously enhanced open-circuit voltage, short-circuit current density, and fill factor, whereas the power conversion efficiency increased from 5.42% to 8.04% compared to that of the pristine Al device. The outstanding performance of PBTAZPOBr is attributed not only to the polar pendant groups of BDT unit but also to the TAZ unit linked with the P═O group of PBTAZPOBr, demonstrating that functionalized TAZ building blocks are very promising cathode interfacial materials (CIMs). The design strategy proposed in this work will be helpful to develop more efficient CIMs for high performance PSCs in the future.