Guidelines for the use and interpretation of assays for monitoring autophagy (4th edition)1.

In 2008, we published the first set of guidelines for standardizing research in autophagy. Since then, this topic has received increasing attention, and many scientists have entered the field. Our knowledge base and relevant new technologies have also been expanding. Thus, it is important to formulate on a regular basis updated guidelines for monitoring autophagy in different organisms. Despite numerous reviews, there continues to be confusion regarding acceptable methods to evaluate autophagy, especially in multicellular eukaryotes. Here, we present a set of guidelines for investigators to select and interpret methods to examine autophagy and related processes, and for reviewers to provide realistic and reasonable critiques of reports that are focused on these processes. These guidelines are not meant to be a dogmatic set of rules, because the appropriateness of any assay largely depends on the question being asked and the system being used. Moreover, no individual assay is perfect for every situation, calling for the use of multiple techniques to properly monitor autophagy in each experimental setting. Finally, several core components of the autophagy machinery have been implicated in distinct autophagic processes (canonical and noncanonical autophagy), implying that genetic approaches to block autophagy should rely on targeting two or more autophagy-related genes that ideally participate in distinct steps of the pathway. Along similar lines, because multiple proteins involved in autophagy also regulate other cellular pathways including apoptosis, not all of them can be used as a specific marker for bona fide autophagic responses. Here, we critically discuss current methods of assessing autophagy and the information they can, or cannot, provide. Our ultimate goal is to encourage intellectual and technical innovation in the field

Solution-processed photovoltaics with advanced characterization and analysis

During the last few decades, numerous promising solar cell concepts, ranging from single-crystal silicon to thin-film technologies, have been developed and are being researched intensely by a growing number of scientific groups and companies. Thin-film kesterite Cu2ZnSn(S,Se)4 (CZTS) photovoltaic technology, in which the indium in Cu(In,Ga)(S,Se)2 (CIGS) is replaced with more abundant and less expensive zinc and tin, has emerged as a potential absorber material in next generation thin film solar cells. Despite the recent demonstration of solution-processed CZTS devices over 11% power conversion efficiency, the development of CZTS as an absorber material is still behind in terms of both fundamental understanding of the material system and in the capability to precisely control the material properties for device fabrication, as compared with those of CIGS and CdTe. This dissertation targets the three key areas in this field: (1) Defect characterization and understanding in order to recover Voc loss; (2) Phase stability and processing control to produce a purer absorber material and (3) Solution-processing with environmentally friendly solvents for large-scale production. We start by exploring various precursor systems (hydrazine, benign organic solvents and nanoparticles) and have successfully processed CZTS from a molecular solution in a benign solvent system. A single component precursor has also been developed and proved to offer more precise phase and composition control. Lastly, using electrical and optical characterization, we have conducted detailed investigations on the bulk and the interface defects that govern the carrier recombination and the resulting device characteristics. They reveal the effects of the anions in CZTS on the defect concentration and on voltage losses of the solar cells

Planar Heterojunction Perovskite Solar Cells via Vapor-Assisted Solution Process

Hybrid organic/inorganic perovskites (e.g., CH₃NH₃PbI₃) as light absorbers are promising players in the field of third-generation photovoltaics. Here we demonstrate a low-temperature vapor-assisted solution process to construct polycrystalline perovskite thin films with full surface coverage, small surface roughness, and grain size up to microscale. Solar cells based on the as-prepared films achieve high power conversion efficiency of 12.1%, so far the highest efficiency based on CH₃NH₃PbI₃ with the planar heterojunction configuration. This method provides a simple approach to perovskite film preparation and paves the way for high reproducibility of films and devices. The underlying kinetic and thermodynamic parameters regarding the perovskite film growth are discussed as well

Molecular Solution Approach To Synthesize Electronic Quality Cu₂ZnSnS₄ Thin Films

Successful implementation of molecular solution processing from a homogeneous and stable precursor would provide an alternative, robust approach to process multinary compounds compared with physical vapor deposition. Targeting deposition of chemically clear, high quality crystalline films requires specific molecular structure design and solvent selection. Hydrazine (N₂H₄) serves as a unique and powerful medium, particularly to incorporate selected metallic elements and chalcogens into a stable solution as metal chalcogenide complexes (MCC). However, not all the elements and compounds can be easily dissolved. In this manuscript, we demonstrate a paradigm to incorporate previously insoluble transitional-metal elements into molecular solution as metal–atom hydrazine/hydrazine derivative complexes (MHHD), as exemplified by dissolving of the zinc constituent as Zn­(NH₂NHCOO)₂(N₂H₄)₂. Investigation into the evolution of molecular structure reveals the hidden roadmap to significantly enrich the variety of building blocks for soluble molecule design. The new category of molecular structures not only set up a prototype to incorporate other elements of interest but also points the direction for other compatible solvent selection. As demonstrated from the molecular precursor combining Sn-/Cu-MCC and Zn-MHHD, an ultrathin film of copper zinc tin sulfide (CZTS) was deposited. Characterization of a transistor based on the CZTS channel layer shows electronic properties comparable to CuInSe₂, confirming the robustness of this molecular solution processing and the prospect of earth abundant CZTS for next generation photovoltaic materials. This paradigm potentially outlines a universal pathway, from individual molecular design using selected chelated ligands and combination of building blocks in a simple and stable solution to fundamentally change the way multinary compounds are processed

Controllable Self-Induced Passivation of Hybrid Lead Iodide Perovskites toward High Performance Solar Cells

To improve the performance of the polycrystalline thin film devices, it requires a delicate control of its grain structures. As one of the most promising candidates among current thin film photovoltaic techniques, the organic/inorganic hybrid perovskites generally inherit polycrystalline nature and exhibit compositional/structural dependence in regard to their optoelectronic properties. Here, we demonstrate a controllable passivation technique for perovskite films, which enables their compositional change, and allows substantial enhancement in corresponding device performance. By releasing the organic species during annealing, PbI₂ phase is presented in perovskite grain boundaries and at the relevant interfaces. The consequent passivation effects and underlying mechanisms are investigated with complementary characterizations, including scanning electron microscopy (SEM), X-ray diffraction (XRD), time-resolved photoluminescence decay (TRPL), scanning Kelvin probe microscopy (SKPM), and ultraviolet photoemission spectroscopy (UPS). This controllable self-induced passivation technique represents an important step to understand the polycrystalline nature of hybrid perovskite thin films and contributes to the development of perovskite solar cells judiciously

Rational Defect Passivation of Cu₂ZnSn(S,Se)₄ Photovoltaics with Solution-Processed Cu₂ZnSnS₄:Na Nanocrystals

An effective defect passivation route has been demonstrated in the rapidly growing Cu₂ZnSn­(S,Se)₄ (CZTSSe) solar cell device system by using Cu₂ZnSnS₄:Na (CZTS:Na) nanocrystals precursors. CZTS:Na nanocrystals are obtained by sequentially preparing CZTS nanocrystals and surface decorating of Na species, while retaining the kesterite CZTS phase. The exclusive surface presence of amorphous Na species is proved by X-ray photoluminescence spectrum and transmission electron microscopy. With Na-free glasses as the substrate, CZTS:Na nanocrystal-based solar cell device shows 50% enhancement of device performance (∼6%) than that of unpassivated CZTS nanocrystal-based device (∼4%). The enhanced electrical performance is closely related to the increased carrier concentration and elongated minority carrier lifetime, induced by defect passivation. Solution incorporation of extrinsic additives into the nanocrystals and the corresponding film enables a facile, quantitative, and versatile approach to tune the defect property of materials for future optoelectronic applications

Nanoscale Joule Heating and Electromigration Enhanced Ripening of Silver Nanowire Contacts

Solution-processed metallic nanowire thin film is a promising candidate to replace traditional indium tin oxide as the next-generation transparent and flexible electrode. To date however, the performance of these electrodes is limited by the high contact resistance between contacting nanowires; so improving the point contacts between these nanowires remains a major challenge. Existing methods for reducing the contact resistance require either a high processing power, long treatment time, or the addition of chemical reagents, which could lead to increased manufacturing cost and damage the underlying substrate or device. Here, a nanoscale point reaction process is introduced as a fast and low-power-consumption way to improve the electrical contact properties between metallic nanowires. This is achieved <i>via</i> current-assisted localized joule heating accompanied by electromigration. Localized joule heating effectively targets the high-resistance contact points between nanowires, leading to the automatic removal of surface ligands, welding of contacting nanowires, and the reshaping of the contact pathway between the nanowires to form a more desirable geometry of low resistance for interwire conduction. This result shows the interplay between thermal and electrical interactions at the highly reactive nanocontacts and highlights the control of the nanoscale reaction as a simple and effective way of turning individual metallic nanowires into a highly conductive interconnected nanowire network. The temperature of the adjacent device layers can be kept close to room temperature during the process, making this method especially suitable for use in devices containing thermally sensitive materials such as polymer solar cells

Thin-Film Deposition and Characterization of a Sn-Deficient Perovskite Derivative Cs₂SnI₆

In this work, we describe details of a two-step deposition approach that enables the preparation of continuous and well-structured thin films of Cs₂SnI₆, which is a one-half Sn-deficient 0-D perovskite derivative (i.e., the compound can also be written as CsSn_0.5I₃, with a structure consisting of isolated SnI₆^4– octahedra). The films were characterized using powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), UV–vis spectroscopy, photoluminescence (PL), photoelectron spectroscopy (UPS, IPES, XPS), and Hall effect measurements. UV–vis and PL measurements indicate that the obtained Cs₂SnI₆ film is a semiconductor with a band gap of 1.6 eV. This band gap was further confirmed by the UPS and IPES spectra, which were well reproduced by the calculated density of states with the HSE hybrid functional. The Cs₂SnI₆ films exhibited <i>n</i>-type conduction with a carrier density of 6(1) × 10¹⁶ cm^–3 and mobility of 2.9(3) cm²/V·s. While the computationally derived band structure for Cs₂SnI₆ shows significant dispersion along several directions in the Brillouin zone near the band edges, the valence band is relatively flat along the Γ–X direction, indicative of a more limited hole minority carrier mobility compared to analogous values for the electrons. The ionization potential (<i>IP</i>) and electron affinity (<i>EA</i>) were determined to be 6.4 and 4.8 eV, respectively. The Cs₂SnI₆ films show some enhanced stability under ambient air, compared to methylammonium lead­(II) iodide perovskite films stored under similar conditions; however, the films do decompose slowly, yielding a CsI impurity. These findings are discussed in the context of suitability of Cs₂SnI₆ for photovoltaic and related optoelectronic applications