Organic photovoltaics (OPVs) are attractive candidates for sustainable energy conversion due to their flexibility, lowcost processing, and compatibility with large-area fabrication. However, their efficiency is hindered by interfacial defects and vertical phase separation in the active layer, which induce charge imbalance and recombination losses. This work presents an interfacial engineering approach to overcome these limitations in P3HT:PC70BM-based OPVs. Two key strategies were employed: (i) reducing the post-deposition annealing time of the active layer to suppress PC70BM accumulation at the bottom electrode, and (ii) using a DCB:DCM mixed solvent system to regulate solvent evaporation, thereby promoting uniform film formation during PC70BM overlay deposition. Devices fabricated with these optimizations exhibited notable enhancements, achieving short-circuit current density up to 15.83 mA/cm2 and a 58.1% increase in power conversion efficiency compared to control devices. X-ray photoelectron spectroscopy confirmed reduced surface aggregation of PC70BM, while X-ray diffraction indicated improved P3HT crystallinity and molecular ordering. These results highlight the critical role of interfacial and morphological control in enhancing charge separation and transport, offering a practical route toward efficient, reproducible, and stable OPVs.
