The bulk photovoltaic effect is not bound by the same limits as the standard photovoltaic effect of semiconductor junctions, and in particular it can yield photovoltages be bigger than the band gap. While the bulk photovoltaic effect only exists in materials lacking a center of symmetry, it can be mobilized in nominally centrosymmetric materials polarized by a strain gradient via flexoelectricity. The so-called flexophotovoltaic effect was first evidenced by Yang et al (Science 2018) in dielectric materials, but it is also theoretically possible in semiconductors. As I will show, semiconductor halide perovskites indeed display large flexophotovoltaic effects, reaching bigger-than-bandagap photovoltages.

The interplay between light and polarization also affects the spontaneous strain in polar materials, causing photostriction (non-thermal deformation induced by light). With the advent of free-standing films that are unclamped to rigid substrates and are moreover optically accessible from either side of the film, functionalities that tap into this photo-mechanical degree of freedom become increasingly attractive. In the second part of my talk, I will present the photostrictive deformation of free-standing ferroelectric thin films and provide evidence for the mechanism behind their large photo-actuation.

Photoexcitation of a crystal induces electronic and structural changes. In ferroelectrics, electronic excitations can modify the spontaneous polarization and thereby the lattice structure. At the same time, photo-induced heating may lead to structural phase transitions. In this study, we investigate the time evolution of the out-of-plane lattice parameter and the polarization in a ferroelectric thin film BaTiO3 (001), grown on a GdScO3 substrate with a bottom electrode SrRuO3 in between. To monitor the photo-induced lattice strain propagation and polarization dynamics, we employ time-resolved X-ray diffraction at the European XFEL, optical second harmonic generation, and optical reflectivity. An above-bandgap photoexcitation, using 266 nm femtosecond (fs) optical laser, drives a transient ultrafast drop of the polarization and a reversible structural phase transition in the picosecond (ps) timescale. In particular, the ferroelectric polarization is reduced by 15% within 200 fs, followed by a fast (τ = 7 ps) and slower (τ = 90 ps) exponential recovery. At the same time, we show that, upon photo-induced transient heating, part of the thin film transforms from the tetragonal to the quasi-cubic phase. This leads to an initial compression of the out-of-plane lattice parameter of 0.03% after few ps, followed by an expansion of maximum 0.3% within 20 ps. To interpret our observation, we employ a two temperature model combined with strain wave simulations, with the aim to distinguish electronic and lattice contributions to the strain and polarization dynamics. Our findings offer a way to reversibly control the phase and the polarization of a ferroelectric thin film at ultrafast timescales using light.

In recent years, multimodal perovskite materials, covering a broad range of temperatures, are becoming more relevant for application in energy conversion systems. Lead-free polar perovskite oxides form one group of possible candidates to fulfill the requirements due to their structural flexibility and chemical tunability. Especially, light active polar perovskite oxides, i.e., photoferroelectrics, are promising for coupling multifunctional properties. These convert, for example, mechanical energy or light illumination to electrical energy. Moreover, transforming light to strain or chemical reactions in the context of photocatalysis becomes possible for photoferroelectric materials. One way to achieve these coupled functionalities is by introducing additional band gap states via compositional tuning combined with switchable spontaneous polarization. Substituting B-site cations with non-d0 metal cations, like in Sn-doped BaTiO3, has shown excellent photoferroelectric properties.
Besides combining functional properties, maintaining the electro-mechanical performance over a wide range of temperatures is crucial to expand the applicability. Here, Sn-doped BaTiO3 was investigated to understand the effect of field-induced phase transformation and the coexistence of multiple crystallographic phases on the temperature-dependent electro-mechanical properties.