Investigation of Ferroelectric Perovskite Oxides for Photovoltaic Applications

Ferroelectric materials have been demonstrated to be promising in developing emerging photovoltaic technologies because of the various mechanisms that allow above-bandgap photovoltages and higher efficiencies. However, the wide bandgaps of conventional ferroelectric oxides limit their utilization of the solar spectrum. This thesis focused on the identification of chemical substituents capable of reducing the bandgap of ferroelectric perovskite oxides, while retaining a robust polarization. Building upon the discovery of (1−x)KNbO3−xBa(Ni1/2Nb1/2)O2.75 solid solutions which have bandgaps compatible with traditional semiconductors, new families of Ni- and Ni/Nb-substituted BaTiO3 were fabricated through solid state methods. The oxygen vacancies accompanying the Ni and Ni-Nb substitutions significantly lower the optical bandgap of BaTiO3 to ~1.5 eV. Although effective in reducing the bandgap, the loss of the ferroelectric polarization in KNbO3 and BaTiO3 at relatively small concentrations (≤ ~10%) of Ni and Ni/Nb prevent access to a wide range of polar solid solutions. To mitigate this issue, bandgap reduction was explored in systems with a more robust ferroelectric order, namely the tetragonally-enhanced PbTiO3-BiFeO3 system where the A site is completely occupied by ferroelectrically active Pb/Bi cations. A morphotropic-phase-boundary (MPB) additive, Bi(Ni1/2Ti1/2)O3, was found to simultaneously lower the bandgap and retain the ferroelectric order of a wide range of compositions in the PbTiO3-BiFeO3-Bi(Ni1/2Ti1/2)O3 ternary system. MPB compositions showed a switchable photovoltaic effect with an open-circuit voltage (Voc) of 6 V. Under AM1.5G illumination the short-circuit photocurrent (jsc) of these systems increased by an order of magnitude as Eg was lowered from 2.85 to 2.25 eV. The dependence of the photovoltaic response on the ferroelectric polarization, device configuration, temperature and defects were investigated in 0.5PbTiO3-0.5Bi(Ni1/2Ti1/2)O3, a tetragonal composition close to the MPB. A direct correlation between the polarization and the photovoltaic response was established. The PV properties of 0.5PbTiO3-0.5Bi(Ni1/2Ti1/2)O3 showed strong temperature dependence with Voc increasing and jsc decreasing at lower temperature; a Voc above 100 V was obtained for a 250 µm thick sample below 160 K. Temperature dependent measurements of dielectric and mechanical responses showed the photovoltaic properties are influenced by thermal depolarization and a re-entrant relaxor phase transition, and are also mediated by the polaron hopping mechanism. Post-annealing in atmospheres with different pO2’s allowed modification of the carrier concentration, which in turn was used to control the dielectric, mechanical and photovoltaic properties