Bulk Photovoltaic effects and strain-related photo-effects in non-centrosymmetric materials

Two years after the invention of modern prototype solar cells, it was found that a ferroelectric material, BaTiO₃, exhibits a photovoltaic effect distinct from that of p-n junctions, later called the bulk photovoltaic (BPV) effect. Under uniform illumination, a homogeneous ferroelectric material gives rise to a current under zero bias, i.e. short-circuit current (I_SC), that depends on the polarization state of the incident light, and produces an anomalously large photo-voltage well exceeding the bandgap energy. The microscopic origins of this effect are still under debate. It supposed to originate from the asymmetric distribution of photoexcited non-equilibrium carriers in k-space, caused by absence of centrosymmetry in the material. In the recent past, the entire field of photo-ferroelectrics has been revitalized by the reports of photovoltaic effect in BiFeO₃ (BFO), which is a ferroelectric/multiferroic material with one of the lowest band gap and significant semiconducting properties.

The present talk will firstly present the basics of the bulk photovoltaic effect and address the electronic origin of the anomalous BPV in BiFeO₃. We will show that the existence of the abnormal PVE in BFO is directly correlated to the presence of photoelectric active sub-band gap levels and moreover, the PV effect can be largely tuned by modifying the occupancy (pumping) of these levels.

We will also show light-induced reversible switching of ferroelectric polarization at room temperature in the same BiFeO₃ thin films by the mediation of the tip-enhanced photovoltaic effect. The enhanced short-circuit photocurrent density at an AFM tip contact area generates a local electric field well exceeding the coercive field, enabling full optical control of ferroelectric polarization. Finally, we will discuss a new photovoltaic effect which turns the BPV effect into a universal effect allowed in all semiconductors by mediation of the flexoelectric effect.

Yang, M., Bhatnagar, A. & Alexe, M. Electronic Origin and Tailoring of Photovoltaic Effect in BiFeO₃ Single Crystals. Adv. Electron. Mater. 1, 1500139 (2015). Alexe, M. & Hesse, D. Tip-enhanced photovoltaic effects in bismuth ferrite. Nature Communications 2, 256 (2011). Yang, M., Kim, D.J, & Alexe, M., Flexo-photovoltaic effect, Science (submitted).
Yang, M., Bhatnagar, A. & Alexe, M. Electronic Origin and Tailoring of Photovoltaic Effect in BiFeO₃ Single Crystals. Adv. Electron. Mater. 1, 1500139 (2015).
Alexe, M. & Hesse, D. Tip-enhanced photovoltaic effects in bismuth ferrite. Nature Communications 2, 256 (2011).
Yang, M., Kim, D.J, & Alexe, M., Flexo-photovoltaic effect, Science (submitted).

Bio: Marin Alexe studied Physics at the University of Bucharest and obtained his PhD in 1995 from the institute of atomic physics in Bucharest. He subsequently joined the Max Planck Institute of Microstructure Physics as a postdoctoral fellow where he got promoted to a senior scientist (permanent position). In 2012, he moved to Sung Kyun Kwan University in Korea where he was appointed as an adjunct professor before he moved back to Europe in 2013 where he works since as a chair of the functional material laboratory at the University of Warwick, UK. Professor Marin Alexe's research interests include nanoscience and nanotechology, ferroelectric nanostructures, ultra-thin films, as well as physics and engineering of complex oxide thin films.