Remote
Defect-mediated photovoltaic effect and low-frequency relaxation in ferroelectric ceramics: a comprehensive temperature dependent study

Ferroelectric materials exhibit the bulk photovoltaic effect which originates from the lack of inversion symmetry of the polar structure. The mechanism, which fundamentally differs from the conventional junction-based photovoltaic effect, gives rise to unique photoresponses such as above-band-gap open circuit voltages (V_oc) and the linear IV characteristic. In this study, we systematically characterize the temperature dependence of photovoltaic responses for a narrow bandgap ferroelectric ceramic (0.5PbTiO₃-0.5BiNi_1/2Ti_1/2O₃) from 80 K to 500 K. The results reveal a series of transitions for V_oc and short-circuit current (j_sc). Dielectric and dynamic mechanical measurements suggest that the transitions in the temperature dependence of V_oc and j_sc are related to the change of relaxation behaviors of the ferroelectric ceramic. Below ~160 K, polar nanoregions start to form in the ferroelectric matrix where polarization inhomogeneity leads to the plateau of V_oc. Above 300 K, polaron hopping is attributed to the relaxation behavior and dictates the photoconduction process. The latter is dominated by the defect chemistry of the sample and can be altered by the redox reaction. We demonstrate that temperature dependence of the photovoltaic performance of 0.5PbTiO₃-0.5BiNi_1/2Ti_1/2O₃ can be precisely controlled by defect concentration, leading to a photovoltaic material that can be used in a wide range of temperatures.

The authors acknowledge support at Drexel University from the NSF under grant nos. 1705440 and 1608887 and also by the U.S. Army Research Office under W911NF-19-2-0119 and W911NF-21-1-0126.