The open-circuit voltage ($V_{\mathrm{oc}}$) of $\mathrm{Cd}(\mathrm{Se},\mathrm{Te})$ solar cells has stalled near 900 mV, limiting further efficiency gains despite state-of-the-art devices achieving 23.1%. As a critical step in overcoming this barrier, we report here a physics-based device model that explicitly integrates defect interactions with potential fluctuation effects to explain this long-standing limitation. By coupling first-principles defect calculations with device-level simulations, we show that voltage deficits in $\mathrm{As}$-doped $\mathrm{Cd}(\mathrm{Se},\mathrm{Te})$ are governed primarily by two mechanisms: bandgap fluctuations and electrostatic disorder induced by low dopant activation (2–4%). The combined effects of bandgap and electrostatic fluctuations contributes 90–130 mV $V_{\mathrm{oc}}$ loss for associated Urbach energy of 27–30 meV, whereas the effect of spatial variations in defect transition energies is found to be small. These results identify viable pathways for breaking the 900 mV barrier, namely enhancing dopant activation and reducing alloy disorder. More broadly, the modeling framework established here provides the transferable approach to quantitatively connect defect interactions and real-space disorder with device performance, offering guidance for other polycrystalline thin-film semiconductors with band tails.

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• Optoelectronics
• Point defects
• Solar energy harvesting devices
• Doped semiconductors
• II-VI semiconductors
• Solar cells
• Density functional calculations
• Multiscale modeling