Ferroelectric materials exhibit a characteristic photovoltaic effect that can generate photovoltage far exceeding their bandgap energy. The absence of limitation in the photovoltage by the bandgap is a great advantage of the ferroelectric photovoltaic (FPV) effect over the conventional PV effect in p-n junctions of semiconductors. Since typical ferroelectric oxides have the wide bandgap, a vanishingly small photoresponse under visible light has been an issue to be overcome. To activate the photoresponse under visible-light irradiation, Matsuo and Noguchi have proposed a materials design strategy ‘gap-state’ engineering, in which defect states within the bandgap derived from intentionally introduced impurities act as a stepping stone for photocarrier generation. The aim of this research unit is to clarify the photovoltaic properties and photocarrier dynamics in transition metal-doped ferroelectrics with the gap states. In this collaboration research, Ceracomp Co., Ltd. supplies ferroelectric single crystals synthesized by a solid-state single crystal growth (SSCG) method and their FPV properties are evaluated in Kumamoto university. The SSCG method is a cost-effective crystal growth method that enables the fabrication of single crystals with complex chemical compositions which are difficult to obtain in conventional melt-growth techniques. By combining the gap-state engineering and the SSCG method, we will develop novel ferroelectric materials that exhibit enhanced FPV response under visible light irradiation.