Microstructures and photovoltaic effect of KNbO₃–SrCo_0.5Hf_0.5O_3−δ ferroelectric semiconductors

Abstract

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.

Effect of defect‐induced bandgap intermediate states on the photovoltaic performance of KN‐based ferroelectric ceramics

“…The diffuse‐reflectance spectrum of samples was collected by using ultaviolet‐visible‐near infrared (UV‐Vis‐NIR) spectrophotometer (Japan Jasco, V−570). The equations Kubelka‐Munk (1) and Tauc (2) 25,26 were used to calculate the bandgap values of samples. $$$$\begin{equation}\alpha \approx F\ \left( R \right) = \frac{{{{\left( {1 - R} \right)}}^2}}{{2R}}\end{equation}$$$$ $$$$\begin{equation}{\left( {\alpha h\upsilon } \right)}^n = \ A\ \left( {h\upsilon - {E}_g} \right) = {\left( {h\upsilon F\left( R \right)} \right)}^n,\end{equation}$$$$…”