Sunny Days for Perovskite Optoelectronics

This special issue on Perovskite Nanocrystals is guest edited by Maarten B. J. Roeffaers and Johan Hofkens. Read more about the exciting papers in the issue in this editorial. The past decade has seen the race for improved solar cell technologies accelerate, fueling new and promising research into metal halide perovskite (MHP) solar absorbers. The strong promise of MHPs for photovoltaic applications arises from their intrinsic properties, ranging from high absorption coefficients at visible wavelengths to long carrier diffusion lengths and their simple solution-based fabrication and processing. MHPs furthermore exhibit outstanding electro-optic properties that are essential for the development of next-generation solid-state lighting, such as high photoluminescence quantum yields (PLQY), narrow emission spectral linewidths, and tunable bandgaps. This special issue brings together work of well-renowned research experts working in this emerging field of materials science. In just one decade, MHP solar cell research power conversion efficiencies of about 23% have been reported. However, this empirical race to produce devices with ever-higher photo-conversion efficiencies focused far less on the understanding of their structural stability. Fu et al. give a clear summary of the challenges that current perovskite solar cells are facing. In their contribution, a systematic discussion is provided of the different degradation mechanisms in perovskite materials and solar cells. By focusing on individual perovskite nanoparticles, Kimura et al. resolve different trapping pathways of charge carriers generated after photon absorption. Here, Auger recombination and surface trap passivation by oxygen are shown to play important roles in the (de)activation mechanism. By reducing the presence of defects and grain boundaries in highly crystalline materials, perovskite single crystals should offer improved performance. In their Minireview, Zuo et al. summarize the work on MHP single crystals applied in a wide range of applications. While MHP solar cell research has undoubtedly drawn most of the attention, the near unity PLQY that can be reached for some MHPs also opens perspectives for their application in LEDs. Lattice vacancies and surface defects, however, can considerably lower the performance of MHPs in real-world devices. Zhang et al. overcome this shortcoming by focusing on nanocrystals (NCs) in combination with the application of surface passivation. In order to improve charge injection, conjugated alkylamines are used. The resulting all-inorganic CsPbBr3 NC LEDs, with green emission centered around 517 nm, show impressive electrical conductivity and luminances. Via engineering of the different layers in an LED, Lu et al. successfully generate a stable, highly flexible LED using the same all-inorganic MHP. Central in the LED architecture is a silver-photopolymer layer that replaces the traditional ITO electrode. Zhu et al. also worked on the stabilization of highly luminescent CsPbBr3 quantum dots (QDs). In their research the MHP QDs are embedded in a polymethyl methacrylate matrix during the hot-injection synthesis. The result is a relatively stable material, which is successfully applied in a remote excitation LED design. Single crystal studies have been very instrumental in generating a better understanding of the unconventional opto-electronic properties of MHP materials. In their Minireview, Chen and Orgiu provide a clear overview of current literature insights. The fast advances made by researchers in the MHP field have been made possible by the rich and versatile synthetic toolbox. Du et al. provide a clear and up-to-date overview of the different synthesis routes that can be followed to generate all-inorganic MHP nanoparticles. The compositional flexibility of MHPs enable finetuning of the opto-electronic properties. This is beautifully shown in this special issue by three contributions. In a first, Rosa-Pardo et al. report on the use of laser ablation to generate pure and mixed-halide MHP nanoparticles. Van Gompel et al. generate low dimensional MHP crystals via the incorporation of large organic cations based on pyrene. The concerns related to lead toxicity have pushed research to explore alternative lead-free MHP materials. Ghosh and Pradhan summarize recent literature on the synthesis, characterization, stability and properties up to the application of lead-free MHPs. In line with this continuing research effort Men et al. introduce a new lead-free CsGeX3 perovskite system. Before the emergence of metal-halide-based systems, perovskite oxides were well studied. Zhang et al. revisit the oxide branch and use the selective, partial substitution of cobalt and/or iron in SrCo0.8Fe0.2O3 to generated highly efficient electrocatalysts for the oxygen evolution reaction. The last contribution of this special issue explores new applications of perovskite materials. Lei et al. use the temperature-dependent photoluminescence of Cs4PbBr6 to generate a nano-thermometer for ambient conditions. To end, we wish to thank all contributors to this special issue for sharing their exciting work. We are convinced that you will enjoy reading this inspiring issue and we hope that it will have an impact on the further evolution of the field. Maarten B. J. Roeffaers graduated from the Centre for Surface Chemistry and Catalysis, KU Leuven (Belgium), in 2008 studying zeolite catalysis with fluorescence microscopy. After a postdoctoral stay (2009–2010) with Prof. Xie at Harvard University (USA) on the development and use of coherent Raman microscopy, he returned to the KU Leuven. In 2010, he started his own research group (www.roeffaers-lab.org) focusing on the development of optical microscopy tools to study heterogeneous catalysis and materials for sustainable chemistry. Amongst others, he was awarded a prestigious ERC starting grant (2012) and received the biennial ExxonMobil Chemical European Science and Engineering Award (2015). Julian A. Steele received his Ph.D. in solid-state physics at The Institute for Superconducting and Electronic Materials, University of Wollongong (Australia), under the supervision of Prof. Roger Lewis, working in the structural and optical characterization of bismuth-containing opto-electronic materials. At the start of 2016, he joined the group of Prof. Roeffaers (KU Leuven, Belgium) as postdoctoral researcher with financial support from Belgium government (FWO postdoc) where he is mainly working on nanoscale optical materials, whereby perovskites feature heavily. Elke Debroye completed her master′s degree in Chemistry in 2009 at KU Leuven. In 2013, she obtained her PhD working on developing novel bimodal MRI/optical contrast agents in the Laboratory of Bio-inorganic Chemistry. She is currently a post-doctoral Fellow of the Research Foundation Flanders (FWO) in the Laboratory of Photochemistry and Spectroscopy working on the synthesis of novel semiconductor materials and studying the structure-activity correlation of the materials at the nanoscale level by means of electron microscopy and super-resolution fluorescence microscopy. During her post-doctoral research she worked with Prof. Liz-Marzán in San Sebastian and Prof. Majima at Osaka University. Masoumeh Keshavarz did her PhD research at the High Field Magnet Laboratory and graduated from the Radboud University in Nijmegen (the Netherlands) in 2016. She investigated the physics of polymer dynamics under nanoscopic confinement using super resolution microscopy. In 2016, she started her postdoctoral research at the KULeuven working with Prof. Hofkens on the fundamental properties of metal halide perovskites, solar cell materials, under extreme conditions such as low temperatures or external magnetic fields using optical spectroscopy and electronic transport. Based on her new approaches and techniques developed during her first months of post-doctoral research, she has been awarded the FWO fellowship in 2017 for three years of research that is still ongoing. Haifeng Yuan worked on single-molecule spectroscopy in the group of Prof. Michel Orrit at Leiden University in the Netherlands and obtained his PhD in 2013. Later, he moved to KU Leuven in Belgium and started his FWO postdoctoral research on super-resolution microscopy in the group of Prof. Johan Hofkens. After a visiting stay at the group of Prof. Ted Sargent in the University of Toronto (Canada) in 2017, he continued working on microscopy and spectroscopy of perovskite materials with an extended FWO postdoctoral fellowship at KU Leuven. Johan Hofkens graduated from the Laboratory of Photochemistry and Spectroscopy, KU Leuven (Belgium), in 1993 studying TICT-states in organic molecules by time resolved fluorescence measurements. After a postdoctoral stays (1994–1995) with Prof. Masuhara at Osaka University (Japan) on the development of optical trapping and with the late Prof Barbara at University of Minneapolis (USA) on single molecule spectroscopy, he returned to the KU Leuven were he started the single molecule group, focusing on the development of optical microscopy tools to study challenging topics at the boundary of chemistry, biology and physics. Amongst others, he was awarded a prestigious ERC advanced grant (2012) and PoC grant (2017).