Comparative Optoelectronic Limiting Performance of Amorphous and Polycrystalline Sb ₂ S ₃ for Planar Solar Cells

Antimony sulfide (Sb 2 S 3 ) is an earth‐abundant wide‐bandgap absorber with strong visible absorption and scalable processing compatibility, yet its photovoltaic efficiency remains far below the thermodynamic limit. Here, we employ coupled optical and drift‐diffusion modeling to disentangle the effects of bandgap energetics, defect‐mediated recombination, and anisotropic carrier transport in amorphous and polycrystalline Sb 2 S 3 planar solar cells. We show that performance is recombination‐limited rather than absorption‐limited. Although amorphous Sb 2 S 3 exhibits a higher baseline open‐circuit voltage (1.33 V at 400 nm thin film), this advantage originates from isotropic transport. In polycrystalline Sb 2 S 3 , a 10:1 mobility anisotropy reduces the cross‐film diffusion length by √10, increasing transport‐limited recombination and lowering V oc to 1.27 V. When anisotropy is removed, the intrinsic voltage potential rises to 1.39 V, surpassing the amorphous phase. Thickness‐dependent analysis further reveals distinct operating regimes, establishing diffusion length, defect energetics, and absorber geometry—not crystallinity alone—as the determinants of the performance ceiling in Sb 2 S 3 photovoltaics.