Mixed-halide perovskites are ideal materials for the demanding applications of tandem solar cells and emission-tunable light-emitting diodes (LEDs) because of their high compositional flexibility and optoelectronic performance. However, one major obstacle to their use is the compositional instability some mixed-halide perovskites experience under illumination or charge-carrier injection, during which the perovskite material demixes into regions of differing halide content. Such segregation of halide ions adversely affects the electronic properties of the material and severely limits the prospects of mixed-halide perovskite technology. Accordingly, a considerable amount of research has been performed aiming to uncover the underlying mechanisms and mitigating factors of the halide segregation process. Here we present a perspective of strategies designed to reduce the effects of halide segregation in working mixed-halide perovskite devices, based on recent literature reports. We discuss a multitude of mitigating techniques, and conclude that a combination of stoichiometric engineering, crystallinity control and trap state passivation is clearly imperative for abating halide segregation. In addition, the reduction of halide vacancies and control over illumination and temperature can, to a certain extent, mitigate halide segregation. Less direct approaches, such as a change in atmospheric environment, perovskite incorporation into a nanocrystalline composition, or direct control over the crystallographic structure of the perovskite, may however prove too cumbersome to be of practical use. This perspective paves the way for the design and creation of phase-stable, mixed-halide perovskite materials for photovoltaic and LED applications.

/pdf/preventing-phase-segregation-in-mixed-halide-perovskites-a-59r1kcl2py.pdf

Preventing phase segregation in mixed-halide perovskites: a perspective

In situ studies of the degradation mechanisms of perovskite solar cells

The rapid development of additive manufacturing has fueled a revolution in various research fields and industrial applications. Among the myriad of advanced 3D printing techniques, two-photon polymerization lithography (TPL) uniquely offers a significant advantage in nanoscale print resolution, and has been widely employed in diverse fields, for example, life sciences, materials sciences, mechanics, and microfluidics. More recently, by virtue of the optical transparency of most of the resins used, TPL is finding new applications in optics and photonics, with nanometer to millimeter feature dimensions. It enables the minimization of optical elements and systems, and exploration of light-matter interactions with new degrees of freedom, never possible before. To review the recent progress in the TPL related optical research, it starts with the fundamentals of TPL and material formulation, then discusses novel fabrication methods, and a wide range of optical applications. These applications notably include diffractive, topological, quantum, and color optics. With a panoramic view of the development, it is concluded with insights and perspectives of the future development of TPL and related potential optical applications. 

https://zenodo.org/record/8006624/files/2301%20Adv%20Funct%20Materials%20-%202023%20-%20FORTH%20-%20Two%E2%80%90Photon%20Polymerization%20Lithography%20for%20Optics%20and%20Photonics%20%20Fundamentals%20%20Materials%20%281%29.pdf

Two‐Photon Polymerization Lithography for Optics and Photonics: Fundamentals, Materials, Technologies, and Applications

https://onlinelibrary.wiley.com/doi/pdf/10.1002/aenm.202000310

Ion Migration-Induced Amorphization and Phase Segregation as a Degradation Mechanism in Planar Perovskite Solar Cells

Halide ion mobility in metal halide perovskites remains an intriguing phenomenon, influencing their optical and photovoltaic properties. Selective injection of holes through electrochemical anodic ...

Electrochemical Hole Injection Selectively Expels Iodide from Mixed Halide Perovskite Films.

Light-sheet 3D microprinting via two-colour two-step absorption

Multiple-cation mixed-halide perovskites show high power-conversion efficiencies and recently improved stability. But, even most advanced absorber materials still suffer from instabilities of the bandgap under illumination and applied bias. Here, we employ modulation spectroscopy as a highly sensitive electro-optical measurement technique to directly reveal such instabilities. We find a reversible decrease of the absorber bandgap of up to 70 meV in complete solar cells. In situ X-ray diffraction measurements under illumination and bias confirm structural changes of the perovskite and their reversibility, which are attributed to a segregation of the halides. These processes are most strongly activated when illumination and bias are combined, which leads to a 5 times increased shift compared to that with illumination only. Since this scenario is intrinsic to the solar cell’s operation conditions, it can never be avoided completely. Furthermore, the bandgap decrease is strongly enhanced, but still reversible...

/pdf/the-bandgap-as-a-moving-target-reversible-bandgap-1smma429lc.pdf

The Bandgap as a Moving Target: Reversible Bandgap Instabilities in Multiple-Cation Mixed-Halide Perovskite Solar Cells

Photonic molecules (PMs) are of great interest for, e.g., optical filters/sensors or topological and exceptional-point photonics. A key requirement for their versatile application is the tunability of the PM’s coupling strength. This important feature is realized in the here introduced widely and precisely tunable PM on an all-polymeric chip-scale platform. The PM consists of two disk-shaped whispering gallery mode cavities on a liquid crystal elastomer (LCE) substrate. The coupling strength of the PM is controlled via the contraction of the LCE under an external stimulus like local heating. We reveal the reversible (de)coupling via the analysis of laser supermodes emitted from a dye-doped PM. The tunability of the PM’s coupling strength is apparent from the pronounced mode splittings observed in single-fiber transmission spectra and is consistent with coupled-mode theory. Finally, we demonstrate the applicability of the PM as an add-drop filter with a highly controllable intensity transfer. In this light, our PM on an LCE substrate represents a novel platform system for tunably coupled photonic resonators.

Tunable coupling of chip-scale photonic molecules via thermal actuation

Reversible bandgap instabilities in multiple-cation mixed-halide perovskite solar cells

Liquid-Crystal-Elastomers were integrated into building blocks containing Whispering-Gallery-Mode resonators. Exploiting their directional mechanical actuation, full tunability of single cavities and flexible coupling of resonator pairs were realized and verified by fiber-transmission spectroscopy.

Flexible Photonics Based on Whispering-Gallery-Mode Resonators and Liquid-Crystal-Elastomers

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