Polarization is one of the unique properties of ferroelectric materials; yet the polarization mechanism for enhancing ferroelectric photovoltaic performance is rarely been investigated, particularly in terms of bandgap variation. In this work, the effect of high-field polarization on the enhanced photovoltaic performance of a ferroelectric ceramic, 0.98KNbO 3 -0.02SrCo 0.5 Hf 0.5 O 3−δ (KNSCH2), was explored in terms of bandgap variation. The bandgap of the KNSCH2 sample shrank after polarization because of the increase in potential energy band overlap and the upward shift of the valence band due to increased oxygen-vacancy defects. The polarization optimized the energy band structure of KNSCH2, promoting the separation and transport of photoinduced carriers and thus further enhancing its photovoltaic performance. The KNSCH2 sample shows a twofold enhancement in J sc after 60 kV/cm polarization. The degree of the lattice distortion of KNSNH2 increased following polarization, causing a minute increase in its cell asymmetry. The reasons for the bandgap narrowing and the creation of sub-bandgaps in the KNSCH samples were also investigated. This work opened new doors to understanding the mechanisms underlying the polarization-enhanced photovoltaic performance of ferroelectric materials.
The inversion asymmetry of polar crystals enables ferroelectric ceramics to possess unique physical properties, among which the anomalous photovoltaic effect drives their potential application in photovoltaic conversion. Semiconducting (1−x) KNbO3−xBaNi.5Hf.5O3−δ (KNBNH, x = 2, 4, 6, 8%) ferroelectric ceramics with enhanced photovoltaic performance were prepared by utilizing conventional solid‐state sintering strategies. In these ceramics, the defect‐induced bandgap intermediate state is derived from the 3d split state of Ni and plays a dominant role in lowering the bandgap of KN. The defective bandgap state induced by Ni in KNBNH promotes its absorption of light and the separation of photogenerated carriers, thus enhancing its photovoltaic response. The KNBNH6 shows a maximum value of 138.7 nA/cm2 for short‐circuit photocurrent density (Jsc) among these ceramics, which is further enhanced to 702.9 nA/cm2 after 30 kV/cm polarization. Structural investigations after polarization indicate that polarization induced lattice distortion in KNBNH6, leading to an increase in the polarity of its cells. This work provides an understanding of defect‐induced bandgap states and high‐field polarization to enhance ferroelectric photovoltaic properties.
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