Ferroelectric oxides with high piezoelectricity and low band gap for solar energy harvesting applications

Abstract

Ferroelectric materials have attracted considerable interest for photovoltaic applications due to its spontaneous electric polarization that can promote the separation of photo-excited charge carriers and lead to an above-band gap voltage. However, due to the wide optical band gap (2.7-4 eV), the ferroelectric oxides absorb only 8-20 percent of the solar spectrum and exhibit a poor solar energy conversion efficiency which limits their use in photovoltaic applications. In general, the common ferroelectric oxides with a perovskite structure ABO₃ exhibit a wide optical band gap due to the large difference of electronegativity between the oxygen ions and the B-site transition-metal ions. However, the B-site ion also plays a crucial role in the ferroelectric properties of perovskite materials. Any attempt of replacing the B-site ion with the aim of reducing the optical band gap may result in the degradation of the ferroelectric and dielectric properties. In this study, we aim at developing perovskite ferroelectric oxides with large polarization and low optical band gap for solar energy harvesting applications. Barium titanate-based ferroelectric ceramic Ba₀.₉Ca₀.₁Ti₀.₉Sn₀.₁O₃ (abbreviated as BCTSn-0.1-0.1) has been chosen as the base material due to its large piezoelectric coefficient d₃₃ (520 pC/N). After replacing Sn⁴⁺ with Fe³⁺, the optical band gap of the ceramic, i.e., Ba₀.₉Ca₀.₁Ti1-xFexO3-δ or BCTFe-0.1-x, decreases with increasing the doping level of Fe³⁺. The BCTFe-0.1-0.025 ceramic which exhibits the optimized properties for photovoltaic responses, i.e., a low optical band gap (2.78 eV) and good piezoelectric properties (190 pC/N), has been chosen to demonstrate the ferroelectric photovoltaic effect. The ferroelectric photovoltaic effect of BCTSn-0.1-0.1 and BCTFe-0.1-0.025 has been investigated using interdigitated electrode (IDE) pattern and vertical ITO electrode configuration. For the samples with IDE pattern, the photocurrent of BCTFe-0.1-0.025 is lightly larger than that of BCTSn-0.1-0.1 (0.22 nA vs 0.07 nA). On the other hand, the steady photocurrent of the BCTFe-0.1-0.025 ceramic with vertical electrode configuration is 5.5 nA or 11 nA/cm², which is 3 times larger than that of the BCTSn-0.1-0.1 ceramic (1.5 nA or 3 nA/cm²). With the aim of studying the effect of the reduction in optical band gap, the photovoltaic response of the non-ferroelectric BCTFe-0.1-0.1 ceramic deposited with asymmetric electrodes of ITO-Cr/Au has been investigated. It is expected that the resulting asymmetric Schottky barriers formed at the top and bottom interfaces will induce the photovoltaic response. The short-circuit current and open-circuit voltage of the BCTFe-0.1-0.1 ceramic measured under 1 sun illumination are 10.9 nA/cm² and 0.16 V, respectively. Because of the reduced optical band gap (i.e., 2.24 eV), the photovoltaic response is observed under the illumination of green lights (e.g. 580 nm). Therefore, the enhancement of the photovoltaic current observed in the BCTFe-0.1-0.025 ceramic should be partly attributed to its lower optical band gap. It demonstrates that it is possible to improve the ferroelectric photovoltaic response by reducing the optical band gap and retaining the piezoelectricity at the same time. On the other hand, the photovoltaic response has also been observed in the BCTFe-0.1-0.1 ceramic deposited with symmetric ITO electrode configuration. This should be attributed to the accumulation of oxygen vacancies in one side of the samples. The distribution of oxygen vacancies can be modulated by an external field and thus the photocurrent is switchable. The switchable photocurrent is observed on both ferroelectric and non-ferroelectric ceramics.