Light: A new handle to control the structure of cesium lead iodide

Abstract

CsPbI3 is of high interest for photovoltaic applications owing to its low bandgap, provided it could be stabilized in its γ perovskite phase. Instead, it energetically prefers to adopt a so-called yellow, δ phase, with much larger bandgap and reduced photovoltaic properties. Here, using an original constrained Density Functional Theory method, we mimic the effect of thermalized photoexcited carriers, and show that larger concentrations in photo-excited carriers (i.e. larger optical pump fluences) effectively reduce the energy difference between the δ non-perovskite ground state and the γ perovskite phase. Even further, the stability of the phases could be potentially reversed and therefore the γ phase stabilized under strong illumination. We also report large photostriction, i.e., large photo-induced strain for all phases in this material, making CsPbI3 suitable for other applications such as photo-driven relays and photoactuators.

Ultrastrong Coupling between Polar Distortion and Optical Properties in Ferroelectric MoBr2O2

“…This behavior is understandable since the polar distortion (polarization) primarily occurs along the c direction, while the reduction in volume under light illumination is alleviated by the expansion of a and b . Remarkably, the magnitude of the relative lattice strain observed in MoBr 2 O 2 is much greater than that observed in BaTiO 3 and PbTiO 3 , , due to the large coupling between polar distortion and lattice strain; see Figure S10. It is noteworthy that all three lattice constants of MoCl 2 O 2 undergo a contraction upon light illumination, which starkly contrasts with the behavior observed in other compounds examined in this study and reported in previous works. , This unusual phenomenon is presumably because of a negative thermal expansion with increasing temperature in MoCl 2 O 2 .…”