Fine-tuning the photophysical properties and nanoscale morphology of the photoactive layer by incorporating an additional donor or acceptor is a promising strategy for improving the performance of organic solar cells (OSCs). Here, we strategically incorporated a wide-bandgap donor (PBDB-T) into the PTB7-Th:COi8DFIC-based host binary to attain a relatively high power conversion efficiency (PCE). The complementary absorption spectra of these materials enabled the harvesting of solar spectrum in a wide wavelength range of 400 nm to 1000 nm. An efficient energy transfer from PBDB-T to PTB7-Th was confirmed by steady-state and time-resolved photoluminescence measurements. The introduction of PBDB-T resulted in an optimized active layer morphology, thereby markedly improving the exciton-dissociation and charge-collection efficiencies, leading to the improvement of the short-circuit current density ( J SC ) and fill factor (FF). As a result, ternary OSCs fabricated with 20 wt% PBDB-T exhibited an average efficiency of 9.74%, compared to 8.87% in the host binary. Impedance spectroscopy analysis at various bias voltages within the operating regime revealed a reduction in the bulk resistance and an increase in the recombination resistance for the optimized ternary OSC, validating the observed enhancements in J SC and FF. Furthermore, transient photovoltage and photocurrent measurements revealed a long carrier lifetime of 22.30 µs and a short extraction time of 904 ns in the ternary system, which were beneficial for the OSC performance. In addition, the incorporation of PBDB-T resulted in reduced non-radiative recombination in the device through efficient energy transfer. The role of PBDB-T in improving the performance of the PTB7-Th:COi8DFIC-based host binary was investigated through systematic photophysical, morphological and electrical characterizations.