Temperature-dependent photoconductivity as an electrical probe of polaronic and band states in ferroelectrics for energy-selective photoresponse

The origin of sub-bandgap photoresponse in ferroelectric photodetectors is nontrivial, particularly regarding the roles of dopant-induced states and intrinsic polarons. Here, we demonstrate that polarons govern wavelength-selective photoconductivity in BaTiO3-based ferroelectrics. Using temperature-dependent DC photoconductivity measurements, we disentangle contributions from small-polaron hopping, large-polaron transport, and band conduction in Ba(Ti1−xSnx)O3 ceramics. Our results show that intrinsic polaron states in BaTiO3 introduce intermediate energy levels, reducing the electrical transport gap to below 2 eV despite a fundamental bandgap of ∼3–3.1 eV. This enables visible-light-induced photoconduction without requiring dopant-induced in-gap states. Sn doping does not alter the fundamental bandgap but modifies the polaron landscape through delocalized Sn 5s orbitals, lowering electron–polaron hopping barriers while increasing dark conductivity. As a result, sub-bandgap photoconductivity becomes progressively suppressed. A crossover regime at 7–9 at. % Sn is identified, where competing polaronic and band conduction mechanisms lead to complex transport behavior. These findings establish polarons as the key factor governing energy-selective photoresponse and indicate that undoped BaTiO3, rather than Sn-doped variants, is better suited for wavelength-selective photoferroelectric applications. This work also demonstrates temperature-dependent photoconductivity measurement as an effective electrical probe for sub-gap states in ferroelectrics.