Lithium Niobate-Type Oxides as Visible Light Photovoltaic Materials

Photovoltaic (PV) materials, which directly convert solar radiation into electrical energy, are one of the most important renewable energy materials platforms. The power conversion efficiency of many candidate PV compounds, however, is often too low for broad commercial use. In general, there are three main features to optimize for efficient power conversion: (i) absorption of solar light, (ii) separation of photon-excited electron and hole pairs, and (iii) transportation of electron and hole carriers from bulk to surface. First, the band gap should be smaller than 2 eV to match the energy distribution of the solar spectrum and large enough to generate sufficient open circle voltages, which makes 1 to 2 eV the optimal band gap range. Second, a built-in electric field can improve charge separation and is usually achieved via a p–n junction. Finally, the charge carrier diffusion length should exceed the absorption depth, and this can be supported by selecting chemistries that give large band dispersions (small electron and hole effective masses) near the Fermi level.