Professor Paolo G. Radaelli OSI

Abstract:

We present a crystallographic and optoelectronic study of the double perovskite Cs2AgBi1–xInxBr6. From structural characterization we determine that the indium cation shrinks the lattice and shifts the cubic-to-tetragonal phase transition point to lower temperatures. The absorption onset is shifted to shorter wavelengths upon increasing the indium content, leading to wider band gaps, which we rationalize through first-principles band structure calculations. Despite the unfavorable band gap shift, we observe an enhancement in the steady-state photoluminescence intensity, and n-i-p photovoltaic devices present short-circuit current greater than that of neat Cs2AgBiBr6 devices. In order to evaluate the prospects of this material as a solar absorber, we combine accurate absorption measurements with thermodynamic modeling and identify the fundamental limitations of this system. Provided radiative efficiency can be increased and the choice of charge extraction layers are specifically improved, this material could prove to be a useful wide band gap solar absorber.

Abstract:

Mixed-halide perovskites offer bandgap tunability essential for multijunction solar cells; however, a detrimental halide segregation under light is often observed. Here we combine simultaneous in situ photoluminescence and X-ray diffraction measurements to demonstrate clear differences in compositional and optoelectronic changes associated with halide segregation in MAPb(Br_0.5I_0.5)₃ and FA_0.83Cs_0.17Pb(Br_0.4I_0.6)₃ films. We report evidence for low-barrier ionic pathways in MAPb(Br_0.5I_0.5)₃, which allow for the rearrangement of halide ions in localized volumes of perovskite without significant compositional changes to the bulk material. In contrast, FA_0.83Cs_0.17Pb(Br_0.4I_0.6)₃ lacks such low-barrier ionic pathways and is, consequently, more stable against halide segregation. However, under prolonged illumination, it exhibits a considerable ionic rearrangement throughout the bulk material, which may be triggered by an initial demixing of A-site cations, altering the composition of the bulk perovskite and reducing its stability against halide segregation. Our work elucidates links between composition, ionic pathways, and halide segregation, and it facilitates the future engineering of phase-stable mixed-halide perovskites.

Abstract:

The datasets included herein contain experimental results (X-ray absorption, dichroic photoemission electron microscopy, diffraction, magnetometry etc.) and related analysis for the investigation of antiferromagnetic topological textures. The processes used in the obtaining, reducing and analysing the datasets can be found in the Methods and Supplementary Information sections of the published manuscript.

Abstract:

The interconversion of charge and spin currents via spin-Hall effect is essential for spintronics. Energy-efficient and deterministic switching of magnetization can be achieved when spin polarizations of these spin currents are collinear with the magnetization. However, symmetry conditions generally restrict spin polarizations to be orthogonal to both the charge and spin flows. Spin polarizations can deviate from such direction in nonmagnetic materials only when the crystalline symmetry is reduced. Here, we show control of the spin polarization direction by using a non-collinear antiferromagnet Mn₃GaN, in which the triangular spin structure creates a low magnetic symmetry while maintaining a high crystalline symmetry. We demonstrate that epitaxial Mn₃GaN/permalloy heterostructures can generate unconventional spin-orbit torques at room temperature corresponding to out-of-plane and Dresselhaus-like spin polarizations which are forbidden in any sample with two-fold rotational symmetry. Our results demonstrate an approach based on spin-structure design for controlling spin-orbit torque, enabling high-efficient antiferromagnetic spintronics.