Strong Bulk Photovoltaic Effect in Planar Barium Titanate Thin Films

The bulk photovoltaic effect (BPE) leads to the generation of a photocurrent from an asymmetric material. Despite drawing much attention due to its ability to generate photovoltages above the band gap ($E_g$), it is considered a weak effect due to the low generated photocurrents. Here, we show that a remarkably high photoresponse can be achieved by exploiting the BPE in simple planar BaTiO$_3$ (BTO) films, solely by tuning their fundamental ferroelectric properties via strain and growth orientation induced by epitaxial growth on different substrates. We find a non-monotonic dependence of the responsivity ($R_{\rm SC}$) on the ferroelectric polarization ($P$) and obtain a remarkably high BPE coefficient ($\beta$) of $\approx$10$^{-2}$ 1/V, which to the best of our knowledge is the highest reported to date for standard planar BTO thin films. We show that the standard first-principles-based descriptions of BPE in bulk materials cannot account for the photocurrent trends observed for our films and therefore propose a novel mechanism that elucidates the fundamental relationship between $P$ and responsivity in ferroelectric thin films. Our results suggest that practical applications of ferroelectric photovoltaics in standard planar film geometries can be achieved through careful joint optimization of the bulk structure, light absorption, and electrode-absorber interface properties.Comment: 12 pages, 8 figure

Making cheaper labor: Domestic outsourcing and development in the Galilee

Middle Eastern Studie

Visible-light-absorbing potassium Niobate-Titanate-Molybdate ferroelectrics

Abstract The interactions of ferroelectric (FE) perovskite oxides (ABO₃) with light are increasingly being studied for different applications, such as photovoltaics and optoelectronics. The combination of different cations at the A and B sites to form solid solutions allows tuning of the material’s properties and, most importantly, the band gap (Eg), which sets the wavelength range of light absorption. Classic FE perovskite oxides, such as BaTiO₃, KNbO₃, and PbTiO₃, exhibit Eg > 3 eV, which limits their implementation in visible-light-absorbing devices. Furthermore, the tuning of their Eg via a solid solution strategy to a lower Eg range is limited by the requirement for the presence of a d⁰ metal at the B site, which is necessary for the FE distortion, but leads to a larger Eg. This gives rise to the challenge of decreasing Eg, while maintaining FE distortion. Here, we use first-principles calculations to explore the FE and optical properties of the (KNbO3)x(KTi1/2Mo1/2O3)1−x(KNTM) perovskite oxide solid solution. The introduction of Ti⁴⁺ and Mo⁶⁺ into the parent KNbO₃ decreases the Eg to about 2.2 eV for x = 0.9, while preserving or enhancing polarization. Experimental fabrication and characterization show that the obtained KNTM material at x = 0.9 has an orthorhombic structure at room temperature and a direct gap of <2.2 eV, confirming first-principles-based predictions. These properties make KNTM a promising candidate for further studies and applications as a visible-light-absorbing FE material