Phase-Engineered Non-Degenerate Sliding Ferroelectricity Enables Tunable Photovoltaics in Monolayer Janus In₂S₂Se

2D sliding ferroelectrics, with their enhanced efficiency of charge separation and tunability, provide promising platforms for next-generation photovoltaic devices. However, recent systems predominantly exhibit dual degenerate polarization states with weak intensity, limiting the optimal manipulations of photovoltaic effects through sliding ferroelectricity. Here, this issue is addressed by introducing two strengthened and distinct non-degenerate sliding ferroelectric states (WZ' and ZB') in Janus In₂S₂Se, which can be achieved by Se-to-S substitution in monolayer In₂Se₃. First-principles calculations demonstrate the experimental feasibility and reversible transition between these states triggered by atomic layer sliding. Remarkably, the WZ'-to-ZB' switch enhances carrier mobility, reduces photogenerated carrier lifetimes, narrows the bandgap with an indirect-to-direct transition, and induces a pronounced redshift and photocurrent enhancement in the infrared region. Conversely, the WZ' state, with stronger polarization, achieves higher photoelectric conversion efficiency under visible light. This work establishes a state-engineered framework of how non-degenerate sliding ferroelectricity orchestrates distinct photovoltaic behaviors, and the intrinsic physical correlations may offer novel perspectives for next designing and regulating innovative photovoltaic devices.