Polar crystalline materials enable to demonstrate fascinating photoelectric effects, including the second-order optical nonlinearity, piezoelectricity, pyroelectricity and ferroelectricity, etc. All of these functions are established on spontaneous polarization, which requires that compounds must crystallize in one of the 10 polar point groups. However, it is still challenging to rationally arrange dipole moments for the synthesis of polar compounds.

A research group led by Prof. LUO Junhua at Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, synthesized a family of polar photoelectric crystals. Based on the symmetry breaking in solids, they recently obtained a layered perovskite-type photoferroelectric crystal, bis(cyclohexylaminium) tetrabromo lead.

This material exhibits an exceptional anisotropy of bulk photovoltaic effects (PVEs). Upon photoexcitation, superior photovoltaic behaviors are created along its inorganic layers, which are composed of the corner-sharing metal-halogen octahedra. In contrast, semiconducting activities with remarkable photoconductivity are achieved in the vertical direction, showing a large on/off current ratios. The temperature-dependence of its photovoltage coincides well with that of spontaneous polarization, confirming an important role of ferroelectricity in its anisotropic PVEs.

The study enriches the understanding of bulk PVEs in ferroelectrics, and presents opportunities for innovative application of ferroelectrics in photodevices, and has been published in Angew. Chem., Int. Ed.

This work was supported by the NSF for Distinguished Young Scholars of Fujian Province, ''Chunmiao Projects'' of Haixi Institute of Chinese Academy of Sciences, and the National Natural Science Foundation of China.

Temperature-dependence of photovoltage and change of crystal structure during symmetry breaking. (Image by  Prof. LUO’s group)