Polarization-gradient KNbO3 film with a large photovoltaic current

Abstract

Potassium niobate (KNbO3, KNO) has been intensively investigated for photovoltaic performance because of its non-toxicity and excellent nonlinear optical properties. The corresponding photovoltaic current density, however, remains very low due to a wide bandgap. Chemical doping and strain engineering strategies have been employed to tailor the band structure to enhance photovoltaic current density. Nevertheless, the original current density is still at a level of several tens of nA/cm2, significantly limiting device applications. In this work, we report a lattice-gradient KNO film on (100) single-crystal 0.7 wt. % Nb doped SrTiO3 (NSTO) substrate processed by annealing, generating a polarization-gradient that allows us to generate a large current density via a built-in field. The film exhibits a remarkable short-circuit current density (Jsc) of 58.63 µA/cm2 under the 375 nm irradiation of 500 mW/cm2 light intensity, where the corresponding responsivity (117.26 µA/W) is ∼3.82 times higher than those of reported KNO-based materials. It was revealed that the annealing process driven interfacial structure evolution from disorder to atomic-scale smoothness, accompanied by the transformation of the polarization shielding mechanism. After this process, an intriguing lattice-gradient throughout the film was established to have a uniform polarization direction, possibly accounting for the improved photovoltaic current density of KNO film. These findings may trigger interest in developing KNO as a potential key material for lead-free optoelectronic or photodetector devices.