Light-Induced Color Change and Negative Photoconductivity in Bandgap Engineered Lead Titanate Ceramics

Motivated by the principle of above-bandgap photovoltage and thus the theoretical possibility of breaking the Shockley-Queisser limit, research interest in bandgap engineering of conventional ferroelectric materials is on the rise. Due to the reduced bandgaps and the resultant interaction between the incident photon energy and the unit cells and domains, many fundamentally interesting phenomena have been observed in ferroelectrics under visible light, including photo-excited domain switching and photoferroelectric cumulative effect. These phenomena could result in optical and optoelectrical applications far beyond the bulk photovoltaic effect. This talk will report a newly synthesized PbTiO₃-based ceramic that is modified to achieve a narrow band gap, which has been predicted via calculations by the density functional theory (DFT). This new material changes its color from maroon to brown when exposed to visible light, accompanying with negative photoconductivity. A hypothesis to explain the phenomena could be that the photo-excited electrons might reduce Ti⁴⁺ to Ti³⁺, causing the color change. Meanwhile, the leftover photo-excited holes might contradict the pristine n-type conduction and thus decreasing conductivity under illumination. This interesting phenomenon may be useful for optical or optoelectronic components requiring property change under visible light, such as photo-sensitive membrane, photosensor, and photo-resistive switch.