10‐Year Anniversary Special Issue: Photophysics of Halide Perovskites – from Bulk to Nano

Within the last decade, halide perovskites have seen a marvelous rise in scientific interest as highly efficient, cheap, and solution-processable semiconductors. From optoelectronics to spintronics, from solar cells to light-emitting diodes (LEDs), lasers and transistors – halide perovskites have by now permeated almost every domain that seemed traditionally reserved only for the purest of inorganic and covalently bound semiconductors, like silicon or gallium arsenide. Yet, these materials offer so much more than just being a copy-cat; they could be evaporated or roll-to-roll inkjet-printed like newspaper on flexible substrates, making them easy to implement in tomorrow's wearables and the "internet of things". They could be magnetically doped or made to be structurally chiral, sustain their properties as optically interacting semiconductors with added functionalities like in sensing or quantum information technologies. And they can be fabricated across various dimensionalities, from zero-dimensional quantum dots to 1D rods and 2D layered materials, all the way to large bulk crystals on the kilogram scale. At the heart of this ongoing search for the perfect composition for the desired application is the photophysics underlying this family of semiconductors. What makes for a perfect absorber in a solar cell, how can charges be efficiently generated and extracted – or how can they recombine most effectively under light emission in an LED? Photophysical probes based on static and transient optical spectroscopy across various energy- and time-scales can offer valuable insights into the nature of excitations, the losses of charges, or the emergence of entirely underexplored phenomena arising, for example, from polariton formation or twisted Moiré quantum matter. With this Special Issue we celebrate the wide range of phenomena to explore and mechanistic insights to be gained from studying the photophysics of halide perovskites, and also commemorate ten years of Advanced Optical Materials as a place for the community to report and read about the latest progress in the field. This issue includes contributions on the exciton fine structure of layered 2D halide perovskites based on magneto-optical measurements (202300877), as well as based on computational results (202202801). Excitation relaxation pathways in mixed-phase 2D perovskites are discussed (202301331), as is the impact of the organic spacer cations on Auger recombination in those layered materials (202301230). Cation migration is investigated in physically paired 2D and 3D halide perovskites (202300957), and the origin of the energetic disorder in mixed-halide perovskites is being addressed from a theory perspective (202301105). Anion exchange is considered for perovskite nanowires (202300435), and, changing the dimensionality further, the effect of B-site doping in nanoplatelets is reviewed (202301001). Also reviewed is the multichromism in halide perovskites (202301342), and their use as photocatalysts (202300626). Fine-tuning of the most confined, quasi-zero-dimensional perovskite nanocrystals is reported (202301009), as is the effect of ligand passivation in red-emitting nanocrystals (202300396). Significant charge separation in perovskites mediated by Rhodamine was found (202300941), and insights were gained from photoluminescence mapping under varied laser pulse fluence and repetition rate (202300996). It was also reported on the technique of nanoscale surface photovoltage spectroscopy (202301318), and on the relevance of photoluminescence measurements compared to JV curves for furthering the understanding of perovskite solar cells (202301019). In keeping with the device focus on solar cells, layer-by-layer printed nanocrystal solar cells were demonstrated (202301008), as was the liquid-phase transfer of perovskite films for planar heterojunction fabrication (202301255). The size-dependent exciton-phonon coupling in perovskite quantum dots was reviewed (202301534). Moving beyond lead-based halide perovskites, it was reported on self-trapped excitons in cesium bismuth halides (202300199), and on the remarkable thermochromism in cesium sodium iron chloride (202301102), while a data-driven synthesis screening approach of various lead-free double-perovskite compositions was also explored (202301245). Finally, I would like to express my greatest gratitude to the whole AOM Family, in particular Drs. Richard Murray and Anja Wecker, who have been just fantastic in handling this project. It was a pleasure to be able to contribute to this remarkable journal on the occasion of its tenth anniversary. And what better way to celebrate is there than with great and diverse science, made possible by all the colleagues and reviewers who enabled this project to bear fruit? With this, what remains to be said is: Happy Birthday AOM, and to the next ten years of advanced optical materials research in halide perovskites and beyond! Sascha Feldmann is a Research Group Leader & Rowland Fellow at Harvard University (US) and has recently been appointed Tenure-Track Assistant Professor & Head of the Laboratory for Energy Materials at École Polytechnique Fédérale de Lausanne (Switzerland). Prior to starting his group in 2022, Sascha studied Chemistry at Heidelberg University (Germany) from 2012 to 2017 and completed his Ph.D. in Physics at the University of Cambridge (UK) in 2020, where he continued to work as an independent EPSRC Doctoral Prize Fellow. His group develops and employs ultrafast magneto-chiroptical spectroscopy to understand the next generation of soft semiconductors with the overarching goal of maximizing efficiency for a sustainable energy future, unlocking applications ranging from flexible light-weight solar cells & displays all the way to entirely new applications in quantum information processing.