Fe³⁺‐ and Nb⁵⁺‐Substituted Na_0.5Bi_0.5TiO₃: A Route toward Enhanced Ferroelectric Photovoltaic Response in Al/NBFNT/Ag Solar Cell through Reduced Bandgap and Controlled Oxygen Vacancy

Abstract

Bulk ferroelectrics hold tremendous potential for the rapidly growing field of photovoltaic applications due to the separation of charge carriers caused by polarization, resulting in a high photovoltage. However, the wider bandgap in ferroelectrics limits their photoresponse. Attempts to tune the bandgap degrade ferroelectric polarization, while increased oxygen vacancy (V) in oxide perovskite hinders electron conduction and undermines ferroelectricity. Therefore, controlling V and maintaining polarization in reduced bandgap ferroelectric photovoltaics remains challenging. To address this, aliovalent Fe3+ and Nb5+ are co‐substituted on the B‐site of Na0.5Bi0.5TiO3 ceramics. Various techniques confirm the controlled V concentration, and UV‐absorption data reveal a reduction in bandgap energy from 3.01 to 2.38 eV through metal substitution. Density functional theory calculations identify a localized intermediate gap state within the bandgap, created by Fe–O hybridization, acting as a charge carrier scaffold and further reducing the bandgap energy to 2.47 eV. Remarkably, the Al/NBFNT75/Ag configuration achieves an augmented short‐circuit current density of 18.92 nA cm−2, while the open‐circuit voltage of the Al/NBFNT50/Ag device reaches 4.13 V. The resulting Al/NBFNT75/Ag device exhibits a maximum photovoltaic power density of 0.725 mW m−2 which is 9.5 times greater than Al/Na0.5Bi0.5TiO3/Ag. This approach successfully reduces V and achieves narrow bandgap conditions through suitable aliovalent substitution, leading to enhanced photovoltaic power, photosensing response, photoresponsivity, and specific detectivity.