Achieving Ultrahigh Photocurrent Density of Mg/Mn-Modified KNbO₃Ferroelectric Semiconductors by Bandgap Engineering and Polarization Maintenance

Most traditional oxide ferroelectric ceramics produced a low photocurrent density in the order of ∼nA/cm2, which greatly limits their application in photovoltaic devices. In this study, a novel solid solution of Mg/Mn-modified KNbO3 (i.e., (1 - x)KNbO3-xMgMnO3-δ, x = 0.00-0.08) ferroelectric semiconductors was synthesized using the solid-phase method. An ultrahigh short-circuit current density (Jsc) of 15.8 μA/cm2 was achieved in the 0.94KN-0.06MM ceramic under 1 sun illumination (AM1.5G, 100 mW/cm2), which is two orders of magnitude higher than that of typical (KNbO3)1-x(BaNi1/2Nb1/2O3-δ)x ferroelectric semiconductors. After high-field poling, a further enhanced Jsc (40 μA/cm2) was obtained in the 0.94KNbO3-0.06MgMnO3-δ ceramic, which is 2.5 times higher than that of the unpoled sample. Moreover, a large open-circuit voltage (Voc) of 16.5 V and a high Jsc of 13.8 μA/cm2 were successfully achieved in the 0.94KN-0.06MM ceramic under visible light (λ > 420 nm) illumination. The high Jsc in the modified system was induced by a combination of low optical bandgaps of 2.60-1.00 eV and a perfectly ferroelectric internal built-in field. On the basis of density functional theory calculations, the bandgap reduction in ceramics was mainly attributed to the introduction of the spin-up Mn 3dxz state in the valence band maximum and spin-up Mn 3dz2 state in the conduction band minimum. The conduction species from the grains and grain boundaries of the 0.94KN-0.06MM ceramic were mainly composed of doubly ionized oxygen vacancies induced by the introduction of Mn3+. These findings provide a new view for developing novel oxide ferroelectric photovoltaic materials with high photocurrent density.