Toward an Enhanced Room-Temperature Photovoltaic Effect in Ferroelectric Bismuth and Iron Codoped BaTiO₃

Abstract

Ferroelectric (FE) materials usually possess very high band gap (∼3–4 eV) and extremely poor electrical conductivity, which renders them unsuitable for photovoltaic applications. Here, we demonstrate that a carefully designed Bi–Fe codoped BaTiO3 (BTO) system (Ba1–xBixTi0.9Fe0.1O3−δ, 0 ≤ x ≤ 0.10) provides a unique platform with the simultaneous optimization of low band gap, high FE polarization, and reasonable conductivity. We, thereby, find that the Jahn–Teller distortion associated with the doped transition metal ions, tetragonality (c/a), and oxygen vacancy content lead to such a controlled tuning of optical band gap, FE polarization, and electrical conductivity, respectively, over a wide range. While x = 0.00 (only Fe-doped) stabilizes in the undesirable paraelectric-hexagonal phase, x = 0.02 (Bi–Fe codoped) is engineered to possess a low band gap (∼1.55 eV), high FE polarization (∼5.2 μC/cm2) due to significant recovery of the FE tetragonal phase by more than 60%, and reasonably high electrical conductivity compared to BaTiO3, which cause it to exhibit the largest photovoltaic response within the series. Such an approach of optimizing the desired physical properties in a closely related mixed phase material where the ferroelectricity is engineered in the majority tetragonal BTO phase, while the minority hexagonal BTO phase aids in the reasonable conductivity (a combination that is not realizable in usual single phase FE materials), along with an optimum band gap, is promising in the realization of many more potential FE-based photovoltaic materials.