Infrared‐to‐ultraviolet light‐absorbing BaTiO3‐based ferroelectric photovoltaic materials

Ferroelectric oxides have been demonstrated to be promising in developing emerging photovoltaic technologies because of the various mechanisms that allow above‐band‐gap photovoltages and higher efficiencies. However, the wide band gaps of conventional ferroelectric oxides (2.7‐4 eV) limit their utilization of the solar spectrum. Here the phase stability, absorption properties, ferroelectric, and photovoltaic responses of Ni‐ and Ni‐Nb‐substituted BaTiO3 were explored to evaluate their potential as visible‐light‐absorbing photovoltaic materials. Although the acceptor substitution of Ni2+ stabilized a hexagonal 6H polymorph in both systems, post‐annealing treatments allowed restoration of a tetragonal 3C phase for Ba(Ti0.99Ni0.01)O2.99 and (0.9)BaTiO3‐(0.1)Ba(Ni1/2Nb1/2)O2.75. The oxygen vacancies accompanying the Ni and Ni‐Nb substitutions significantly lower the optical band gap of BaTiO3 to ~1.5 eV and the visible light absorption can be systematically tuned between 380 and 1000 nm by varying the Ni:Nb ratio. Room temperature ferroelectricity was observed in Ba(Ti0.99Ni0.01)O2.99 with a saturation polarization = 18 μC/cm2 and remnant polarization = 1 μC/cm2. The Ni‐Nb substituted composition (0.9)BaTiO3‐(0.1)Ba(Ni1/2Nb1/2)O2.75 shows a ferroelectric response with a remnant polarization of 5 μC/cm2 at 77 K, which gradually decreases as temperature increases. Both compositions exhibit ferroelectrically switchable photoresponses under an AM 1.5 G sunlight simulator; the highest switchable steady‐state current of 8 nA/cm2 observed for (0.9)BaTiO3‐(0.1)Ba(Ni1/2Nb1/2)O2.75 exceeds those reported in previous studies of BaTiO3 ceramic samples (J Solid State Chem, 1975; 12: 193; Jpn J Appl Phys, 2013; 52: 09KF03).