29 research outputs found
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., CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>) 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 CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> 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 Cu<sub>2</sub>ZnSnS<sub>4</sub> 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 (N<sub>2</sub>H<sub>4</sub>) 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(NH<sub>2</sub>NHCOO)<sub>2</sub>(N<sub>2</sub>H<sub>4</sub>)<sub>2</sub>. 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 CuInSe<sub>2</sub>, 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, PbI<sub>2</sub> 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 Cu<sub>2</sub>ZnSn(S,Se)<sub>4</sub> Photovoltaics with Solution-Processed Cu<sub>2</sub>ZnSnS<sub>4</sub>:Na Nanocrystals
An effective defect passivation route
has been demonstrated in
the rapidly growing Cu<sub>2</sub>ZnSn(S,Se)<sub>4</sub> (CZTSSe)
solar cell device system by using Cu<sub>2</sub>ZnSnS<sub>4</sub>: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 Cs<sub>2</sub>SnI<sub>6</sub>
In this work, we describe details
of a two-step deposition approach
that enables the preparation of continuous and well-structured thin
films of Cs<sub>2</sub>SnI<sub>6</sub>, which is a one-half Sn-deficient
0-D perovskite derivative (i.e., the compound can also be written
as CsSn<sub>0.5</sub>I<sub>3</sub>, with a structure consisting of
isolated SnI<sub>6</sub><sup>4–</sup> 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 Cs<sub>2</sub>SnI<sub>6</sub> 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 Cs<sub>2</sub>SnI<sub>6</sub> films exhibited <i>n</i>-type conduction with a carrier density of 6(1) × 10<sup>16</sup> cm<sup>–3</sup> and mobility of 2.9(3) cm<sup>2</sup>/V·s.
While the computationally derived band structure for Cs<sub>2</sub>SnI<sub>6</sub> 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 Cs<sub>2</sub>SnI<sub>6</sub> 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 Cs<sub>2</sub>SnI<sub>6</sub> for
photovoltaic and related optoelectronic applications