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 develop novel ferroelectric materials that exhibit enhanced FPV response under visible light irradiation. Besides, the dynamics of photocarriers in the ferroelectric single crystals are investigated by the transient absorption spectroscopy method.