Recent angle-resolved photoelectron spectroscopy (ARPES) measurements of the hole effective mass in ${\mathrm{CsPbBr}}_3$ revealed an enhancement of $\sim 50\%$ compared to the bare mass computed from first principles for ${\mathrm{CsPbBr}}_3$ at $T=0\mathrm{K}$. This large enhancement was interpreted as evidence of polaron formation. Employing accurate finite-temperature first-principles calculations, we show that the calculated hole effective mass of ${\mathrm{CsPbBr}}_3$ at $T=300\mathrm{K}$ can explain experimental results without invoking polarons. Thermal fluctuations are particularly strong in halide perovskites compared to conventional semiconductors such as Si and GaAs, and cannot be ignored when comparing with experiment. We not only resolve the debate on polaron formation in halide perovskites, but also demonstrate the general importance of including thermal fluctuations in first-principles calculations for strongly anharmonic materials.

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• Electron-phonon coupling
• Electronic structure
• Phonons
• Polarons
• Ab initio molecular dynamics
• Effective mass theory
• First-principles calculations