Bulk Photovoltaic Effect in Ferroelectric Vertically Aligned Nanocomposites

Exploiting physical and electrical properties of polar materials is currently an area of substantial research interest, with the scope to create multifunctional materials that aim to solve the challenges of the next decades in the fields of renewable energy, transportation, and quantum computing to name a few. Among these, ferroelectric materials whose spontaneous polarisation can be switched by an applied electric field, are known to generate currents without the need of a heterojunction as required for traditional semiconductors. The phenomenon is known as the bulk photovoltaic effect (BPVE), and it results in large photovoltages well above the bandgap which have the potential to surpass the Shockley-Quessier (S-Q) limit for single junction devices^1,2. However, the high photovoltage is limited by the conductivity which combined with the intrinsic wide band gap results in unremarkable efficiencies below 1%^3,4. Different approaches have been adopted to overcome this limitation like ionic doping, heterostructures or localized surface plasmons resonances (LSPR)⁵. Here we report the use of a two-phase nanocomposite approach coupling a ferroelectric with a narrow band gap material that allows us to exploit the built-in voltage of the former with the high absorption and conductivity of the latter. Epitaxial nanocomposites thin films of BaTiO₃:Sm₂O₃ and BaTiO₃:MgO were fabricated via pulsed laser deposition (PLD) and the secondary phase was etched away and replaced with α-Fe₂O₃ phase. The combination of atomic force microscopy allowed us to assess the electrical properties of the films, the presence of BPVE and how these are affected by strain in vertically aligned nanocomposites (VANs).<br/><b>References</b><br/>1. Lopez-Varo, P. <i>et al.</i> Physical aspects of ferroelectric semiconductors for photovoltaic solar energy conversion. <i>Phys Rep</i> <b>653</b>, 1–40 (2016).<br/>2. Spanier, J. E. <i>et al.</i> Power conversion efficiency exceeding the Shockley-Queisser limit in a ferroelectric insulator. <i>Nature Photonics</i> vol. 10 (2016).<br/>3. Wu, L., Podpirka, A., Spanier, J. E. & Davies, P. K. Ferroelectric, Optical, and Photovoltaic Properties of Morphotropic Phase Boundary Compositions in the PbTiO3-BiFeO3-Bi(Ni1/2Ti1/2)O3 System. <i>Chemistry of Materials</i> <b>31</b>, 4184–4194 (2019).<br/>4. Li, C. <i>et al.</i> Enhanced photovoltaic response of lead-free ferroelectric solar cells based on (K,Bi)(Nb,Yb)O3 films. <i>Physical Chemistry Chemical Physics</i> <b>22</b>, 3691–3701 (2020).<br/>5. Han, X., Ji, Y. & Yang, Y. Ferroelectric Photovoltaic Materials and Devices. <i>Advanced Functional Materials</i> vol. 32 Preprint at https://doi.org/10.1002/adfm.202109625 (2022).