Recent insights for achieving mixed halide perovskites without halide segregation

The incorporation of halide perovskites in optoelectronics has provided a fast advance in the fabrication of new sensitizers with a balanced light-harvesting, free carrier transportation, and a progressive overcoming of the low tolerance to the moisture. Within these emerging materials, mixed halide perovskites as APbX3−xYx, (A = MA+, Cs+, FA+; X, Y=Cl−, Br−, I−) have been highlighted due to their facile band gap tunability in the entire visible region by varying the halide composition, which making these systems enormously appealing for the design of optoelectronic devices. Nonetheless, their performance in real devices is strongly limited as mixed halide perovskites exhibit photoinduced and current-induced phase segregation, losing their original photophysical properties and effective band gap tunability to generate halide-rich domains. The phase segregation has been the key factor to decrease the photovoltaic parameters in solar cells as open-circuit voltage and photoconversion efficiency, also limiting the performance of tandem devices and the potentiality of color design in perovskite LEDs. This review summarizes recent trends to hinder the phase segregation

Ligand & band gap engineering: tailoring the protocol synthesis for achieving high-quality CsPbI3 quantum dots

Hot-injection has become the most widespread method used for the synthesis of perovskite quantum dots (QDs) with enormous interest for application in optoelectronic devices. However, there are some aspects of the chemistry involved in this synthesis that have not been completely investigated. In this work, we synthesized ultra-high stable CsPbI3 QDs for more than 15 months by controlling two main parameters: synthesis temperature and the concentration of capping ligands. By increasing the capping ligand concentration during the QD synthesis, we were able to grow CsPbI3 in a broad range of temperatures, improving the photophysical properties of QDs by increasing the synthesis temperature. We achieved the maximum photoluminescence quantum yield (PLQY) of 93% for a synthesis conducted at 185 °C, establishing an efficient surface passivation to decrease the density of non-radiative recombination sites. Under these optimized synthesis conditions, deep red LEDs with an External Quantum Efficiency (EQE) higher than 6% were achieved. The performance of these LEDs is higher than that of the reported CsPbI3 QD-LEDs containing standard capping agents, without additional elements or further element exchange. We show that it is possible to produce stable CsPbI3 QDs with high PLQY and red emission beyond the requirement of the Rec. 2020 standards for red color

Controlling the Phase Segregation in Mixed Halide Perovskites through Nanocrystal Size

Mixed halide perovskites are one of the promising candidates in developing solar cells and light-emitting diodes (LEDs), among other applications, because of their tunable optical properties. Nonetheless, photoinduced phase segregation, by formation of segregated Br-rich and I-rich domains, limits the overall applicability. We tracked the phase segregation with increasing crystalline size of CsPbBr3–xIx and their photoluminescence under continuous-wave laser irradiation (405 nm, 10 mW cm–2) and observed the occurrence of the phase segregation from the threshold size of 46 ± 7 nm. These results have an outstanding agreement with the diffusion length (45.8 nm) calculated also experimentally from the emission lifetime and segregation rates. Furthermore, through Kelvin probe force microscopy, we confirmed the correlation between the phase segregation and the reversible halide ion migration among grain centers and boundaries. These results open a way to achieve segregation-free mixed halide perovskites and improve their performances in optoelectronic devices

Photocatalytic and Photoelectrochemical Degradation of Organic Compounds with All-Inorganic Metal Halide Perovskite Quantum Dots

Inspired by the outstanding optoelectronic properties reported for all-inorganic halide perovskite quantum dots (QDs), we have evaluated the potential of these materials toward the photocatalytic and photoelectrochemical degradation of organic compounds, taking the oxidation of 2-mercaptobenzothiazole (MBT) as a proof-of-concept. First, we determined electrochemically the energy levels of dispersions of perovskite QDs with different band gaps induced by the different ratios between halides (Br and I) and metallic cations (Pb and Sn). Then, we selected CsPbBr3 QDs to demonstrate the photocatalytic and photoelectrochemical oxidation of MBT, confirming that hole injection takes place from CsPbBr3 QDs to MBT, resulting in the total degradation of MBT as evidenced by electrospray mass spectrometry analyses. Although the stability and toxicity of these QDs are major issues to address in the near future, the results obtained in the present study open promising perspectives for the implementation of solar-driven catalytic strategies based on these fascinating materials

Efficient and Stable Blue- and Red-Emitting Perovskite Nanocrystals through Defect Engineering: PbX2 Purification

Current efforts to reduce the density of structural defects such as surface passivation, doping, and modified synthetic protocols have allowed us to grow high-quality perovskite nanocrystals (PNCs). However, the role of the purity of the precursors involved during the PNC synthesis to hinder the emergence of defects has not been widely explored. In this work, we analyzed the use of different crystallization processes of PbX2 (X = Cl– or I–) to purify the chemicals and produce highly luminescent and stable CsPbCl3–xBrx and CsPbI3 PNCs. The use of a hydrothermal (Hyd) process to improve the quality of the as-prepared PbCl2 provides blue-emitting PNCs with efficient ligand surface passivation, a maximum photoluminescence quantum yield (PLQY) of ∼ 88%, and improved photocatalytic activity to oxidize benzyl alcohol, yielding 40%. Then, the hot recrystallization of PbI2 prior to Hyd treatment led to the formation of red-emissive PNCs with a PLQY of up to 100%, long-term stability around 4 months under ambient air, and a relative humidity of 50–60%. Thus, CsPbI3 light-emitting diodes were fabricated to provide a maximum external quantum efficiency of up to 13.6%. We claim that the improvement of the PbX2 crystallinity offers a suitable stoichiometry in the PNC structure, reducing nonradiative carrier traps and so maximizing the radiative recombination dynamics. This contribution gives an insight into how the manipulation of the PbX2 precursor is a profitable and potential alternative to synthesize PNCs with improved photophysical features by making use of defect engineering

Preparation of nanoscale inorganic CsPbIxBr3-x perovskite photosensitizers on the surface of mesoporous TiO2 film for solid-state sensitized solar cells

Metal chalcogenide quantum dot (QD)-like all-inorganic nanoscale perovskite photosensitizers of CsPbIxBr3-x were prepared on the surface of mesoscopic TiO2 film by a direct two-step spin-coating of lead and cesium halide precursors for application into solid-state dye-sensitized solar cells (DSSCs), as confirmed by impedance frequency response analysis. A few nanometer-sized hemisphere-shaped dots of CsPbIxBr3-x perovskites were deposited and distributed separately onto TiO2, which were checked by scanning and transmission electron microscopic (SEM and TEM) techniques. The as-deposited CsPbIxBr3-x perovskites were stable only in the case of including about 20% or more bromide in the composition of halides. When the bromide content increased in the ratio of halides of CsPbIxBr3-x, gradual decrease in lattice spacing and blue-shift of emission peaks were observed in X-ray diffraction (XRD) and photoluminescence (PL) measurements, respectively. These well-defined nano-particulate CsPbIxBr3-x perovskites were incorporated into solid-state DSSCs and tested as a new type of photosensitizers. The initial power conversion efficiency (PCE) of ca. 1.0–3.5% based on relatively thin mesoporous TiO2 film (~1 μm) looks promising with many parameters remaining for possibly more optimization. The best result, 3.79%, was obtained from CsPbI2.2Br0.80 25 days after initial measurement. These CsPbIxBr3-x-sensitized cells displayed a stable record of PCE over ~2 month and no hysteresis behavior in current-voltage traces

Unravelling the Photocatalytic Behavior of All-Inorganic Mixed Halide Perovskites: The Role of Surface Chemical States

Within the most mesmerizing materials in the world of optoelectronics, mixed halide perovskites (MHPs) have been distinguished because of the tunability of their optoelectronic properties, balancing both the light-harvesting efficiency and the charge extraction into highly efficient solar devices. This feature has drawn the attention of analogous hot topics as photocatalysis for carrying out more efficiently the degradation of organic compounds. However, the photo-oxidation ability of perovskite depends not only on its excellent light-harvesting properties but also on the surface chemical environment provided during its synthesis. Accordingly, we studied the role of surface chemical states of MHP-based nanocrystals (NCs) synthesized by hot-injection (H-I) and anion-exchange (A-E) approaches on their photocatalytic (PC) activity for the oxidation of β-naphthol as a model system. We concluded that iodide vacancies are the main surface chemical states that facilitate the formation of superoxide ions, O2●–, which are responsible for the PC activity in A-E-MHP. Conversely, the PC performance of H-I-MHP is related to the appropriate balance between band gap and a highly oxidizing valence band. This work offers new insights on the surface properties of MHP related to their catalytic activity in photochemical applications

Etiquette for medical students email communication with faculty members: a single-institution study

This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.Abstract Background Email is widely used as a means of communication between faculty members and students in medical education because of its practical and educational advantages. However, because of the distinctive nature of medical education, students inappropriate email etiquette may adversely affect their learning as well as faculty members perception of them. Little data on medical students competency in professional email writing is available; therefore, this study explored the strengths and weaknesses of medical students email etiquette and factors that contribute to professional email writing. Methods A total of 210 emails from four faculty members at Seoul National University College of Medicine were collected. An evaluation criteria and a scoring rubric were developed based on the various email-writing guidelines. The rubric comprised 10 items, including nine items for evaluation related to the email components and one item for the assessment of global impression of politeness. Three evaluators independently assessed all emails according to the criteria. Results Students were identified as being 61.0 % male and 52.8 % were in the undergraduate-entry program. The sum of each component score was 62.21 out of 100 and the mean value for global impression was 2.6 out of 4. The results demonstrated that students email etiquettes remained low-to-mediocre for most criteria, except for readability and honorifics. Three criteria, salutation (r=0.668), closing (r=0.653), and sign-off (r=0.646), showed a strong positive correlation with the global impression of politeness. Whether a student entered a graduate-entry program or an undergraduate-entry program significantly contributed to professional email writing after other variables were controlled. Conclusions Although students in the graduate-entry program demonstrated a relatively superior level of email etiquette, the majority of medical students did not write emails professionally. Educating all medical students in email etiquette may well contribute to the improvement of student–faculty relationships as well as their email writing