Materials with bright excitons as the ground state are considered ideal platforms in the fields of light emission and light absorption. However, in systems with strong spin-orbit coupling, dark excitons that are exempt from spin selection rules become the ground state, leading to degraded optoelectronic performance. Two-dimensional Janus heterobilayers, with their stable intrinsic electric field from symmetry breaking, offer a promising tuning strategy for modifying excitonic states. In this work, the $\mathrm{Mo}\mathit{XY}{\mathrm{N}}_4$/$\mathrm{W}\mathit{XY}{\mathrm{N}}_4$ $(X, Y=\mathrm{Si}, \mathrm{Ge})$ heterobilayers are constructed, and their electronic and excitonic properties are calculated using the GW-BSE method. The results reveal that the synergistic interface effects and the Janus intrinsic electric field significantly modify the band alignment (type II→type I) and control the relative position of the first-bright excitonic state within the overall excitonic states, i.e., the energy difference between first-dark and first-bright excitonic states exhibits a significant increase $(0.218\to 0.836\mathrm{eV})$ or decrease $(0.218\to 0.017\mathrm{eV})$, rendering the bright exciton effectively quasidegenerate with the ground state. This work provides an important theoretical basis for the performance optimization of optoelectronic devices and deepens the understanding of exciton physics of materials.

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• Electronic structure
• Excitons
• Spin-orbit coupling
• 2-dimensional systems
• First-principles calculations
• GW method