Journal of the American Ceramic Society, Vol.102, No.7, 4188-4199, 2019
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 (2.7-4eV) 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)O-2.99 and (0.9)BaTiO3-(0.1)Ba(Ni1/2Nb1/2)O-2.75. The oxygen vacancies accompanying the Ni and Ni-Nb substitutions significantly lower the optical band gap of BaTiO3 to similar to 1.5eV and the visible light absorption can be systematically tuned between 380 and 1000nm by varying the Ni:Nb ratio. Room temperature ferroelectricity was observed in Ba(Ti0.99Ni0.01)O-2.99 with a saturation polarization=18C/cm(2) and remnant polarization=1C/cm(2). The Ni-Nb substituted composition (0.9)BaTiO3-(0.1)Ba(Ni1/2Nb1/2)O-2.75 shows a ferroelectric response with a remnant polarization of 5C/cm(2) at 77K, which gradually decreases as temperature increases. Both compositions exhibit ferroelectrically switchable photoresponses under an AM 1.5G sunlight simulator; the highest switchable steady-state current of 8nA/cm(2) observed for (0.9)BaTiO3-(0.1)Ba(Ni1/2Nb1/2)O-2.75 exceeds those reported in previous studies of BaTiO3 ceramic samples (J Solid State Chem, 1975; 12: 193; Jpn J Appl Phys, 2013; 52: 09KF03).