A compatible low-bandgap donor polymer (polyN-90-heptadecanyl-2,7carbazole-alt-3,6-bis(thiophen-5-yl)-2,5-dioctyl-2,5-dihydropyrrolo 3,4 pyrrole-1,4-dione, PCBTDPP) was judicially introduced into the archetypal poly(3-hexylthiophene):6,6-phenyl-C -butyric acid methyl ester (P3HT:PC BM) photoactive system to fabricate highly efficient ternary based bulk heterojunction polymer solar cells (PSCs). The PCBTDPP ternary-based PSC with optimal loading (0.2 wt.%) displayed outstanding performance with a champion power conversion efficiency (PCE) of 5.28% as compared to the PCE (4.67%) for P3HT:PC BM-based PSC (reference). The improved PCE for PCBTDPP ternary-based PSC can be mainly attributed to the incorporation of PCBTDPP into P3HT:PC BM that beneficially improved the optical, morphological, electronic, and photovoltaic (PV) performance. This work instills a rational strategy for identifying components (donor/acceptor (D/A) molecules) with complementary beneficial properties toward fabricating efficient ternary PSCs.