John Hong

High Performance PbS Quantum Dot/Graphene Hybrid Solar Cell with Efficient Charge Extraction

Hybrid colloidal quantum dot (CQD) solar cells are fabricated from multilayer stacks of lead sulfide (PbS) CQD and single layer graphene (SG). The inclusion of graphene interlayers is shown to increase power conversion efficiency by 9.18%. It is shown that the inclusion of conductive graphene enhances charge extraction in devices. Photoluminescence shows that graphene quenches emission from the quantum dot suggesting spontaneous charge transfer to graphene. CQD photodetectors exhibit increased photoresponse and improved transport properties. We propose that the CQD/SG hybrid structure is a route to make CQD thin films with improved charge extraction, therefore resulting in improved solar cell efficiency.

Monolayer optical memory cells based on artificial trap-mediated charge storage and release

Monolayer transition metal dichalcogenides are considered to be promising candidates for flexible and transparent optoelectronics applications due to their direct bandgap and strong light-matter interactions. Although several monolayer-based photodetectors have been demonstrated, single-layered optical memory devices suitable for high-quality image sensing have received little attention. Here we report a concept for monolayer MoS2 optoelectronic memory devices using artificially-structured charge trap layers through the functionalization of the monolayer/dielectric interfaces, leading to localized electronic states that serve as a basis for electrically-induced charge trapping and optically-mediated charge release. Our devices exhibit excellent photo-responsive memory characteristics with a large linear dynamic range of ∼4,700 (73.4 dB) coupled with a low OFF-state current (<4 pA), and a long storage lifetime of over 104 s. In addition, the multi-level detection of up to 8 optical states is successfully demonstrated. These results represent a significant step toward the development of future monolayer optoelectronic memory devices.

A pseudo-capacitive chalcogenide-based electrode with dense 1-dimensional nanoarrays for enhanced energy density in asymmetric supercapacitors

To achieve the further development of supercapacitors (SCs), which have intensively received attention as a next-generation energy storage system, the rational design of active electrode materials with an electrochemically more favorable structure is one of the most important factors to improve the SC performance with high specific energy and power density. We propose and successfully grow copper sulfide (CuS) nanowires (NWs) as a chalcogenide-based electrode material directly on a Cu mesh current collector using the combination of a facile liquid–solid chemical oxidation process and an anion exchange reaction. We found that the as-prepared CuS NWs have well-arrayed structures with nanosized crystal grains, a high aspect ratio and density, as well as a good mechanical and electrical contact to the Cu mesh. The obtained CuS NW-based electrodes, which were binder- and conductive material-free, exhibit a much higher areal capacitance of 378.0 mF cm−2 and excellent cyclability of approximately 90.2% retention during 2000 charge/discharge cycles due to their unique structural, electrical, and electrochemical properties. Furthermore, for practical SC applications, an asymmetric supercapacitor is fabricated using active carbon as an anode and CuS NWs as a cathode, and exhibits the good capacitance retention of 91% during 2000 charge/discharge processes and the excellent volumetric energy density of 1.11 mW h cm−3 compared to other reported pseudo-capacitive SCs.

Highly Monodispersed PbS Quantum Dots for Outstanding Cascaded-Junction Solar Cells

High-performance cascaded-junction quantum dot solar cells (CJQDSCs) are fabricated from as-prepared highly monodispersed lead sulfide QDs. The cells have a high power conversion of 9.05% and a short-circuit current density of 32.51 mA cm–2. A reliable and effective stratagem for fabricating high-quality lead sulfide quantum dots (QD) is explored through a “monomer” concentration-controlled experiment. Robust QDSC performances with different band gaps are demonstrated from the as-proposed synthesis and processing stratagems. Various potential CJQDSCs can be envisioned from the band edge evolution of the QDs as a function of size and ligands reported here.

Strain-mediated interlayer coupling effects on the excitonic behaviors in an epitaxially-grown MoS2/WS2 van der Waals heterobilayer

van der Waals heterostructures composed of two different monolayer crystals have recently attracted attention as a powerful and versatile platform for studying fundamental physics, as well as having great potential in future functional devices because of the diversity in the band alignments and the unique interlayer coupling that occurs at the heterojunction interface. However, despite these attractive features, a fundamental understanding of the underlying physics accounting for the effect of interlayer coupling on the interactions between electrons, photons, and phonons in the stacked heterobilayer is still lacking. Here, we demonstrate a detailed analysis of the strain-dependent excitonic behavior of an epitaxially grown MoS2/WS2 vertical heterostructure under uniaxial tensile and compressive strain that enables the interlayer interactions to be modulated along with the electronic band structure. We find that the strain-modulated interlayer coupling directly affects the characteristic combined vibrational and excitonic properties of each monolayer in the heterobilayer. It is further revealed that the relative photoluminescence intensity ratio of WS2 to MoS2 in our heterobilayer increases monotonically with tensile strain and decreases with compressive strain. We attribute the strain-dependent emission behavior of the heterobilayer to the modulation of the band structure for each monolayer, which is dictated by the alterations in the band gap transitions. These findings present an important pathway toward designing heterostructures and flexible devices.

Thermodynamically Stable Synthesis of Large‐Scale and Highly Crystalline Transition Metal Dichalcogenide Monolayers and their Unipolar n–n Heterojunction Devices

Transition metal dichalcogenide (TMDC) monolayers are considered to be potential materials for atomically thin electronics due to their unique electronic and optical properties. However, large-area and uniform growth of TMDC monolayers with large grain sizes is still a considerable challenge. This report presents a simple but effective approach for large-scale and highly crystalline molybdenum disulfide monolayers using a solution-processed precursor deposition. The low supersaturation level, triggered by the evaporation of an extremely thin precursor layer, reduces the nucleation density dramatically under a thermodynamically stable environment, yielding uniform and clean monolayer films and large crystal sizes up to 500 µm. As a result, the photoluminescence exhibits only a small full-width-half-maximum of 48 meV, comparable to that of exfoliated and suspended monolayer crystals. It is confirmed that this growth procedure can be extended to the synthesis of other TMDC monolayers, and robust MoS2 /WS2 heterojunction devices are easily prepared using this synthetic procedure due to the large-sized crystals. The heterojunction device shows a fast response time (≈45 ms) and a significantly high photoresponsivity (≈40 AW-1 ) because of the built-in potential and the majority-carrier transport at the n-n junction. These findings indicate an efficient pathway for the fabrication of high-performance 2D optoelectronic devices.

Synergistic Effects of a Multifunctional Graphene Based Interlayer on Electrochemical Behavior and Structural Stability

The ability to rationally design and manipulate the interfacial structure in lithium ion batteries (LIBs) is of utmost technological importance for achieving desired performance requirements as it provides synergistic effects to the electrochemical properties and cycling stability of electrode materials. However, despite considerable efforts and progress made in recent years through the interface engineering based on active electrode materials, relatively little attention has been devoted to address the physical aspects of the interface and interfacial layer between the anode materials layer and the current collector. Here, we propose and successfully grow unique graphene directly on a Cu current collector as an ideal interfacial layer using the modified chemical vapor deposition (CVD). The anode with an engineered graphene interlayer exhibits remarkably improved electrochemical performances, such as large reversible specific capacity (921.4 mAh g(-1) at current density of 200 mA g(-1)), excellent Coulombic efficiency (close to approximately 96%), and superior cycling capacity retention and rate properties compared to the bare Cu. These excellent electrochemical features are discussed in terms of multiple beneficial effects of graphene on interfacial stability and adhesion between the anode and the collector, oxidation or corrosion resistance of the graphene grown Cu current collector, and electrical contact conductance during the charge/discharge process.

Synergistic incorporation of hybrid heterobimetal–nitrogen atoms into carbon structures for superior oxygen electroreduction performance

Although Pt-based catalytic technology has led to significant advances in the development of electrocatalysts in fuel cells, Pt replacement with efficient and stable non-precious metal catalysts has a great technological significance for successful large-scale implementation of fuel cells. Here, we present the development of hybrid functional 1-dimensional carbon structures incorporated homogeneously with high contents of non-precious metal multi-dopants, consisting of iron, cobalt and nitrogen, as a promising alternative to Pt-based catalysts for the cathodic oxygen reduction reaction (ORR) through a modified electrospinning technique. These hybrid heterobimetal–nitrogen-incorporated carbon structures exhibit superior ORR electrocatalytic properties i.e., more positive reduction potential, high electroreduction current density, high electron transfer value (∼3.87) close to the perfect ORR and improved electrochemical stability with a very small decrease of ∼8 mV in half-wave potential. The observed enhancement in electrochemical performance can be ascribed to the increased amount of catalytically active sites with relatively high contents of heterometallic iron and cobalt atoms surrounded by nitrogen species and their homogeneous distribution on the catalyst surface.

Solubility-Dependent NiMoO4 Nanoarchitectures: Direct Correlation between Rationally Designed Structure and Electrochemical Pseudokinetics

Tailoring the binary metal oxide along with developing new synthetic methods for controlling resultant nanostructures in a predictive way is an essential requirement for achieving the further improved electrochemical performance of pseudocapacitors. Here, through a rational design of the supersaturation-mediated driving force for hydrothermal nucleation and crystal growth, we successfully obtain one-dimensional (1-D) nickel molybdenum oxide (NiMoO4) nanostructures with controlled aspect ratios. The morphology of the 1-D NiMoO4 nanostructures can be tuned from a low to a high aspect ratio (over a range of diameter sizes from 80 to 800 nm). Such a controllable structure provides a platform for understanding the electrochemical relationships in terms of fast relaxation times and improved ion-diffusion coefficients. We show that the 1-D NiMoO4 electrode with a high aspect ratio (HAR) exhibits a much higher specific capacitance of 1335 F g-1 at a current density of 1 A g-1 compared to the other electrodes with a relatively low aspect ratio, which is due to the unique physical and chemical structure being suitable for electrochemical kinetics. We further demonstrate that an asymmetric supercapacitor consisting of the tailored HAR-NiMoO4 electrode can achieve an energy density of 40.7 Wh kg-1 and a power density of 16 kW kg-1.

Balancing Charge Carrier Transport in a Quantum Dot P–N Junction toward Hysteresis-Free High-Performance Solar Cells

In a quantum dot solar cell (QDSC) that has an inverted structure, the QD layers form two different junctions between the electron transport layer (ETL) and the other semiconducting QD layer. Recent work on an inverted-structure QDSC has revealed that the junction between the QD layers is the dominant junction, rather than the junction between the ETL and the QD layers, which is in contrast to the conventional wisdom. However, to date, there have been a lack of systematic studies on the role and importance of the QD heterojunction structure on the behavior of the solar cell and the resulting device performance. In this study, we have systematically controlled the structure of the QD junction to balance charge transport, which demonstrates that the position of the junction has a significant effect on the hysteresis effect, fill factor, and solar cell performance and is attributed to balanced charge transport.