Band gap narrowing and enhanced visible light absorption in some lead free ferroelectrics

Ferroelectric materials, due to their improved dielectric, ferroelectric and piezoelectric properties, are being used long time ago as sensors, actuators in energy harvester applications. Although the photovoltaic effect in ferroelectrics has been reported around six decades ago, only in the last two decades has it aroused greater interest from researchers. A wide bandgap of classical ferroelectric oxides limiting light absorption, in the visible spectrum, could be pointed out as one of the main constraints for photovoltaic applications. Recent findings that photovoltage can be altered by manipulating the domain structure, by applying an electric field, indicate that reducing bandgap energy in ferroelectric materials has become an important commitment to improve the performance for solar light harvesters. Typical approaches to reduce their bandgap in ferroelectrics, as well as to tune other physical properties (dielectric, piezoelectric …), is to use specific processing procedures and dopants.

In this work the influence of sintering and heterovalent doping in the structural, dielectric, ferroelectric and optical absorption of some lead free ferroelectric materials, with perovskite (KNN and barium titanate-BT) or Aurivillius (bismuth titanate-BIT) structures, have been investigated. The investigated single phase bulk ceramics were prepared through the conventional oxide mixture process. The observed decrease in the band gap (red shift) are correlated with changes in the electronic structure, tilting angles, bond lengths, octahedral distortions, and oxygen vacancies formation, still maintaining a ferroelectric spontaneous polarization.