Developing highly efficient catalysts for the oxygen reduction reaction (ORR) is key to the fabrication of commercially viable fuel cell devices and metal–air batteries for future energy applications. Herein, we review the most recent advances in the development of Pt-based and Pt-free materials in the field of fuel cell ORR catalysis. This review covers catalyst material selection, design, synthesis, and characterization, as well as the theoretical understanding of the catalysis process and mechanisms. The integration of these catalysts into fuel cell operations and the resulting performance/durability are also discussed. Finally, we provide insights into the remaining challenges and directions for future perspectives and research.

Recent advancements in Pt and Pt-free catalysts for oxygen reduction reaction

Although graphene films have a strong potential to replace indium tin oxide anodes in organic light-emitting diodes (OLEDs), to date, the luminous efficiency of OLEDs with graphene anodes has been limited by a lack of efficient methods to improve the low work function and reduce the sheet resistance of graphene films to the levels required for electrodes1,2,3,4. Here, we fabricate flexible OLEDs by modifying the graphene anode to have a high work function and low sheet resistance, and thus achieve extremely high luminous efficiencies (37.2 lm W–1 in fluorescent OLEDs, 102.7 lm W–1 in phosphorescent OLEDs), which are significantly higher than those of optimized devices with an indium tin oxide anode (24.1 lm W–1 in fluorescent OLEDs, 85.6 lm W–1 in phosphorescent OLEDs). We also fabricate flexible white OLED lighting devices using the graphene anode. These results demonstrate the great potential of graphene anodes for use in a wide variety of high-performance flexible organic optoelectronics. By replacing conventional indium tin oxide (ITO) anodes with high-work-function, low-sheet-resistance graphene anodes, researchers demonstrate flexible fluorescent organic LEDs with extremely high luminous efficiencies of 37.2 lm W–1 for fluorescent devices and 102.7 lm W–1 for phosphorescent devices. These values are significantly higher than those of optimized organic LEDs based on ITO anodes.

http://phome.postech.ac.kr/user/pnel/publication/95.%20T.-H.%20Han,%20Y.%20Lee,%20M.-R.%20Choi,%20S.-H.%20Woo,%20S.-H.%20Bae,%20B.%20H.%20Hong,%20J.-H.%20Ahn,%20T.-W.%20Lee,%20Nature.%20Photon.,%206,%20105-110%20(2012).pdf

Extremely efficient flexible organic light-emitting diodes with modified graphene anode

High particle uniformity, high photoluminescence quantum yields, narrow and symmetric emission spectral lineshapes and minimal single-dot emission intermittency (known as blinking) have been recognized as universal requirements for the successful use of colloidal quantum dots in nearly all optical applications. However, synthesizing samples that simultaneously meet all these four criteria has proven challenging. Here, we report the synthesis of such high-quality CdSe-CdS core-shell quantum dots in an optimized process that maintains a slow growth rate of the shell through the use of octanethiol and cadmium oleate as precursors. In contrast with previous observations, single-dot blinking is significantly suppressed with only a relatively thin shell. Furthermore, we demonstrate the elimination of the ensemble luminescence photodarkening that is an intrinsic consequence of quantum dot blinking statistical ageing. Furthermore, the small size and high photoluminescence quantum yields of these novel quantum dots render them superior in vivo imaging agents compared with conventional quantum dots. We anticipate these quantum dots will also result in significant improvement in the performance of quantum dots in other applications such as solid-state lighting and illumination.

/pdf/compact-high-quality-cdse-cds-core-shell-nanocrystals-with-1t891588k6.pdf

Compact high-quality CdSe–CdS core–shell nanocrystals with narrow emission linewidths and suppressed blinking

This review article attempts to provide a comprehensive review of the recent progress in the so-called resistive random access memories (RRAMs) First, a brief introduction is presented to describe the construction and development of RRAMs, their potential for broad applications in the fields of nonvolatile memory, unconventional computing and logic devices, and the focus of research concerning RRAMs over the past decade Second, both inorganic and organic materials used in RRAMs are summarized, and their respective advantages and shortcomings are discussed Third, the important switching mechanisms are discussed in depth and are classified into ion migration, charge trapping/de-trapping, thermochemical reaction, exclusive mechanisms in inorganics, and exclusive mechanisms in organics Fourth, attention is given to the application of RRAMs for data storage, including their current performance, methods for performance enhancement, sneak-path issue and possible solutions, and demonstrations of 2-D and 3-D crossbar arrays Fifth, prospective applications of RRAMs in unconventional computing, as well as logic devices and multi-functionalization of RRAMs, are comprehensively summarized and thoroughly discussed The present review article ends with a short discussion concerning the challenges and future prospects of the RRAMs

Recent progress in resistive random access memories: Materials, switching mechanisms, and performance

2.3. Ionic Liquids (ILs) 5374 2.4. Complexation Reactions on Oxidized CNTs 5375 2.5. Halogenation 5376 2.6. Cycloaddition Reactions 5377 2.7. Radical Additions 5379 2.8. Nucleophilic Additions 5381 2.9. Electrophilic Additions 5381 2.10. Electrochemical Modifications 5381 2.11. Plasma-Activation 5381 2.12. Mechanochemical Functionalizations 5382 3. Noncovalent Interactions 5382 3.1. Polynuclear Aromatic Compounds 5382 3.2. Interactions with Other Substances 5384 3.3. Interactions with Biomolecules 5385 4. Endohedral Filling 5386 4.1. Encapsulation of Fullerenes 5386 4.2. Encapsulation of Organic Substances 5387 4.3. Encapsulation of Inorganic Substances 5387 5. Decoration of CNTs with Metal Nanoparticles 5388 5.1. Covalent Linkage 5388 5.2. Direct Formation on Defect Sites 5388 5.3. Electroless Deposition 5388 5.4. Electrodeposition 5389 5.5. Chemical Decoration 5390 5.6. Deposition of Nanoparticles onto CNTs 5391 5.7. π-π Stacking and Electrostatic Interactions 5391 6. Concluding Remarks 5392 7. Acknowledgments 5392 8. References 5392

Current progress on the chemical modification of carbon nanotubes.

Here, an investigation of three-dimensional (3D) morphologies for bulk heterojunction (BHJ) films based on regioregular poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is reported. Based on the results, it is demonstrated that optimized post-treatment, such as solvent annealing, forces the PCBM molecules to migrate or diffuse toward the top surface of the BHJ composite films, which induces a new vertical component distribution favorable for enhancing the internal quantum efficiency (ηIQE) of the devices. To investigate the 3D BHJ morphology, novel time-of-flight secondary-ion mass spectroscopy studies are employed along with conventional methods, such as UV-vis absorption, X-ray diffraction, and high-resolution transmission electron microscopy studies. The ηIQE of the devices are also compared after solvent annealing for different times, which clearly shows the effect of the vertical component distribution on the performance of BHJ polymer solar cells. In addition, the fabrication of high-performance P3HT:PCBM solar cells using the optimized solvent-annealing method is reported, and these cells show a mean power-conversion efficiency of 4.12% under AM 1.5G illumination conditions at an intensity of 100 mW cm−2.

Three-Dimensional Bulk Heterojunction Morphology for Achieving High Internal Quantum Efficiency in Polymer Solar Cells

A new approach to forming a gradient hole-injection layer in polymer light-emitting diodes (PLEDs) is demonstrated. Single spin-coating of hole-injecting conducting polymer compositions with a perfluorinated ionomer results in a work function gradient through the layer formed by self-organization, which leads to remarkably efficient single-layer PLEDs (ca. 21 cd A–1). The device lifetime is significantly improved (ca. 50 times) compared with the conventional hole-injection layer, poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonate). These results are a good example for demonstrating that the shorter lifetime of PLEDs compared with small-molecule-based organic LEDs (SM-OLEDs) is not mainly due to the inherent degradation of the polymeric emitter itself. Hence, the results open the way to further improvements of PLEDs for real applications to large-area, high-resolution, and full-color flexible displays.

Self-Organized Gradient Hole Injection to Improve the Performance of Polymer Electroluminescent Devices

Control of surface composition of a hole-injecting conducting polymer complex, poly(3,4-ethylenedioxy thiophene) (PEDOT) doped with a polystyrene sulfonate (PSS) has been conducted in the spin-cast films. We found that the work function of the polymeric complex films formed via single spin-coating can be greatly increased up to 5.44 eV by increasing the surface concentration of the PSS dopant. As a result, we improved the device efficiency and the lifetime of green emitting polymer light-emitting diodes (PLEDs). This implies that the PSS surface layer of the films spin-cast from the conducting polymer complexes plays a key role in making high performance PLEDs.

http://phome.postech.ac.kr/user/pnel/publication/67.%20T.-W.%20Lee,%20Y.%20Chung%20,%20Advanced%20Functional%20Materials,%2018,%202246-2252%20(2008).pdf

Control of the Surface Composition of a Conducting-Polymer Complex Film to Tune the Work Function**

The effect of dispersant structures for dispersing single-walled carbon nanotubes (SWCNTs) is investigated. The monomer 3-hexylthiophene is used as the starting material for the development of a series of oligomers that are used to disperse SWCNTs in an organic solvent. The series is obtained by varying the number of head groups, the regioregularity of head groups, and the head-to-tail ratios of the hexyl group in the oligomers. The SWCNT solutions are characterized with UV-vis-near-IR spectroscopy and transmission electron microscopy. An increase in the number of head groups improves the dispersity of SWCNTs, and a regioregular oligomer plays an important role in dispersing SWCNTs. Furthermore, Raman spectroscopy and X-ray photoelectron spectroscopy shows that the sulfur atom head groups enhance interactions between the thiophenes and the SWCNT walls. The analysis demonstrated that a well-designed thiophene oligomer could afford well-dispersed SWCNT solutions with long-term dispersion stability, even with an extremely low dispersant concentration (weight ratio of CNTs/dispersant is one and the dispersant concentration is 0.1 g L -1 ).

Design of Dispersants for the Dispersion of Carbon Nanotubes in an Organic Solvent

This letter introduces conducting polymer compositions which can be used for hole-injection layer in organic light-emitting diodes. The compositions are composed of poly (3,4-ethylenedioxythiophene) (PEDOT), polystyrene sulfonic acid (PSS) and a perfluorinated ionomer. The films based on these compositions showed much higher workfunction (∼5.3–5.7eV) than conventional PEDOT/PSS (∼5.0–5.2eV). When we fabricated blue polymer light-emitting diodes by using these compositions as a hole-injection layer, the luminescent efficiency was improved and the device lifetime was also enhanced relative to the device using the commercially available PEDOT/PSS. These compositions including perfluorinated ionomers can be one of the promising candidates for a hole-injection layer in organic light-emitting devices.

Youngsu Chung

Papers

Three-Dimensional Bulk Heterojunction Morphology for Achieving High Internal Quantum Efficiency in Polymer Solar Cells

Self-Organized Gradient Hole Injection to Improve the Performance of Polymer Electroluminescent Devices

Control of the Surface Composition of a Conducting-Polymer Complex Film to Tune the Work Function**

Design of Dispersants for the Dispersion of Carbon Nanotubes in an Organic Solvent

Hole-injecting conducting-polymer compositions for highly efficient and stable organic light-emitting diodes