*

In this work, using first principles density functional theory and density perturbation functional theory with Hubbard U correction, we calculated the energy of polaronic states and computed the total dielectric constant in bismuth vanadate (BiVO₄). The results reveal the formation of electron polarons at vanadium sites as well as the hole polaron at oxygen sites and within BiO₈ polyhedra. The calculated energy of polaronic states, in agreement with experimental results, identified the effective polaronic energy gap is lower than the calculated optical gap (2.10 eV), and even smaller than the least required potential energy (1.23 eV) to split water to produce hydrogen. Additionally, the computed total dielectric constant increases significantly upon the formation of the hole-polaron in BiO₈ cluster, enhancing the polarizability. This enhancement correlates closely with the material stiffness constant that modulates the effective potential well and, consequently, impacts the polaron mobility. We will present the atomistic mechanism underlying the reduction solar-to-hydrogen (STH) efficiency due to the small polaron formation in BiVO₄.