Marin Alexe

University of Warwick
Ferroelectrics & Photovoltaics
Two years after the invention of modern 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 (ISC), that depends on the polarization state of the incident light, and produces an anomalously large photo-voltage well exceeding the bandgap energy. The effect has been largely forgotten , the entire field of photo-ferroelectrics has been revitalized by the reports of abnormal photovoltaic effect in BiFeO3 (BFO)[i], which is a ferroelectric/multiferroic material with one of the lowest band gap and significant semiconducting properties. The present tutorial will discuss the basics of the photovoltaic effect in both semiconductors and ferroelectrics along with the electronic origin and phenomenological theory of the BPV. We will present the associated effects such as the role of ferroelectric domain walls and photoelectric active sub-band gap levels in  the abnormal PVE in BFO as well as persistent photoconductivity.[ii] We will also address the tip-enhanced photovoltaic effect and how light can induce reversible switching of ferroelectric polarization at room temperature in BiFeO3 thin films.[iii] Other effects based on light interaction with the ferroelectrics such as photostriction, photorefractive effects or light-enhanced piezoelectric coefficient will also be presented. Finally, we will discuss the interaction of strain field and gradients with matter how the BPV effect into a universal photovoltaic effect. namely flexo-photovoltaic effect, allowed in all semiconductors.[iv] References [i] Yang, S. Y. et al. Above-bandgap voltages from ferroelectric photovoltaic devices. Nature Nanotechnology 5, 143–147 (2010) [ii] Yang, M., Bhatnagar, A. & Alexe, M. Electronic Origin and Tailoring of Photovoltaic Effect in BiFeO3 Single Crystals. Adv. Electron. Mater. 1, 1500139 (2015). [iii] Alexe, M. & Hesse, D. Tip-enhanced photovoltaic effects in bismuth ferrite. Nature Communications 2, 256 (2011). [iv] Yang, M., Kim, D.J, & Alexe, M., Flexo-Photovoltaic Effect, Science 360, 904 (2018).
Presenter Bio

Professor Marin Alexe has been appointed as Chair of Functional Materials at the University of Warwick after spending about 18 years at the Max Planck Institute of Microstructure Physics-Halle (1996-2013). He has received the PhD degree from the Institute of Atomic Physics Bucharest in 1995. His research interest is physics and engineering of ferroelectrics, complex oxide thin films for information technology and integration of functional materials for oxide electronics. He has published more than 300 papers and 2 books and has an h-index of 61. In the last five years he has given more than 30 invited talks and lectures at international meeting and conferences. He was recently awarded Wolfson Research Merit and Theo Murphy “Blue Sky” Awards of the Royal Society as well as Alexander von Humboldt research Award.

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