Finding the optimal condition from a wide range of cell fabrication conditions and design parameters is typically a time-consuming and cumbersome task. In this study, the combination of the Taguchi approach and Grey relational analysis was employed for optimization of the conversion efficiency of hydrogenated amorphous silicon/crystalline silicon heterojunction (a-Si:H/c-Si HJ) solar cells. With the help of the Taguchi method via an orthogonal array, the reconstruction of the impact of input parameters on single performance characteristics is still ensured while reducing the number of simulations by 99.8%. The simulated results suggested that the density of interfacial defects (Dit) plays a key role in obtaining a high open-circuit voltage (Voc) and fill factor (FF), respectively. Meanwhile, the emitter thickness is the dominant factor in achieving a high short-circuit current density (Jsc). As a result, these two factors dominate the conversion efficiency. Furthermore, the overall optimal condition is also obtained by the Grey relational analysis. The simplified HJ cell configuration using this optimal condition displayed the highest conversion efficiency of 25.86%, yielding a 2.25% absolute increase in efficiency compared to the initial condition. The results highlight the effectiveness of our proposed approach in reducing the number of experiments needed for cell optimization.