Anomalous photovoltaic (APV) effect was discovered in ferroelectric materials, which is essentially different with conventional p-n junction-based photovoltaics. Above-bandgap photovoltage as the most intriguing feature of APV effect enables a possible power conversion efficiency beyond the Shockley-Queisser limit of p-n junction based solar cells.

The APV effect has been studied for over fifty years, and its origin is commonly acknowledged as the breaking of inversion symmetry. In a recent study published in Advanced Materials, the APV effect in a centrosymmetric vanadate BiVO₄ was observed by Prof. YI Zhiguo's team at Fujian Institute of Research on the Structure of Matter of Chinese Academy of Sciences (CAS) and Prof. LI Yongxiang at Shanghai Institute of ceramics of CAS.

In this study, the BiVO₄ ceramic was prepared by spark plasma sintering. By X-ray diffraction analysis, the phase structure of BiVO₄ was identified as I-centred monoclinic scheelite-type structure.

Through UV-visible diffuse reflectance spectrum study, researchers found that the bandgap BiVO₄-was indicated to be E_g = 2.43 eV, which can absorb light with a wavelength shorter than 530 nm.

Photovoltaic characterization conducted on polished BiVO₄ ceramic surface shows an open-circuit voltage of 41 V and a short-circuit current ~0.67 nA under 400 nm monochromatic light, and the APV response extended to the visible light range ~530 nm.

Through the studies on the microstructure and the strain-field analysis, researchers revealed localized asymmetries caused by strain fluctuation in bulk centrosymmetric BiVO₄ ceramic. The strain-induced local polarization due to the flexoelectric effect was suggested to account for the APV effect observed in BiVO₄ ceramic.

This study suggested potential photelectric applications for BiVO₄, and provided insights into mechanisms underlying the APV effect.

Strain-induced APV effect in centrosymmetric BiVO₄ ceramic (Image by Prof. YI’s group)