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This review outlines problems and progress in development of solution-processed organic light-emitting diodes (SOLEDs) in industry and academia. Solution processing has several advantages such as low consumption of materials, low-cost processing, and large-area manufacturing. However, use of a solution process entails complications, such the needs for solvent resistivity and solution-processable materials, and yields SOLEDs that have limited luminous efficiency, severe roll-off characteristics, and short lifetime compared to OLEDs fabricated using thermal evaporation. These demerits impede production of practical SOLED displays. This review outlines the industrial demands for commercial SOLEDs and the current status of SOLED development in industries and academia, and presents research guidelines for development of SOLEDs that have high efficiency, long lifetime, good processability to achieve commercialization. This article is protected by copyright. All rights reserved.

Although several transparent conducting materials such as carbon nanotubes, graphene, and conducting polymers have been intensively explored as flexible electrodes in optoelectronic devices, their insufficient electrical conductivity, low work function, and complicated electrode fabrication processes have limited their practical use. Herein, a 2D titanium carbide (Ti3 C2 ) MXene film with transparent conducting electrode (TCE) properties, including high electrical conductivity (≈11 670 S cm-1 ) and high work function (≈5.1 eV), which are achieved by combining a simple solution processing with modulation of surface composition, is described. A chemical neutralization strategy of a conducting-polymer hole-injection layer is used to prevent detrimental surface oxidation and resulting degradation of the electrode film. Use of the MXene electrode in an organic light-emitting diode leads to a current efficiency of ≈102.0 cd A-1 and an external quantum efficiency of ≈28.5% ph/el, which agree well with the theoretical maximum values from optical simulations. The results demonstrate the strong potential of MXene as a solution-processable electrode in optoelectronic devices and provide a guideline for use of MXenes as TCEs in low-cost flexible optoelectronic devices.

Intrinsically, detrimental defects accumulating at the surface and grain boundaries limit both the performance and stability of perovskite solar cells. Small molecules and bulkier polymers with functional groups are utilized to passivate these ionic defects but usually suffer from volatility and precipitation issues, respectively. Here, starting from the addition of small monomers in the PbI2 precursor, a polymerization-assisted grain growth strategy is introduced in the sequential deposition method. With a polymerization process triggered during the PbI2 film annealing, the bulkier polymers formed will be adhered to the grain boundaries, retaining the previously established interactions with PbI2 . After perovskite formation, the polymers anchored on the boundaries can effectively passivate undercoordinated lead ions and reduce the defect density. As a result, a champion power conversion efficiency (PCE) of 23.0% is obtained, together with a prolonged lifetime where 85.7% and 91.8% of the initial PCE remain after 504 h continuous illumination and 2208 h shelf storage, respectively.

Although metal halide perovskite (MHP) light-emitting diodes (LEDs) have demonstrated great potential in terms of electroluminescence efficiency, the operational stability of MHP LEDs currently remains the biggest bottleneck toward their practical usage. Well-confined excitons/charge carriers in a dielectric/quantum well based on conventional spatial or potential confinement approaches substantially enhance radiative recombination in MHPs, but an increased surface-to-volume ratio and multiphase interfaces likely result in a high degree of surface or interface defect states, which brings about a critical environmentally/operationally vulnerable point on LED stability. Here, an effective solution is suggested to mitigate such drawbacks using strategically designed surface-2D/bulk-3D heterophased MHP nanograins for long-term-stable LEDs. The 2D surface-functionalized MHP renders significantly reduced trap density, environmental stability, and an ion-migration-immune surface in addition to a fast radiative recombination owing to its spatially and potentially confined charge carriers, simultaneously. As a result, heterophased MHP LEDs show substantial improvement in operational lifetime (T50 : >200 h) compared to conventional pure 3D or quasi-2D counterparts (T50 : < 0.2 h) as well as electroluminescence efficiency (surface-2D/bulk-3D: ≈7.70 ph per el% and pure 3D: ≈0.46 ph per el%).

Halide perovskite colloidal quantum dots (CQDs) have recently emerged as a promising candidate for CQD photovoltaics due to their superior optoelectronic properties to conventional chalcogenides CQDs. However, the low charge separation efficiency due to quantum confinement still remains a critical obstacle toward higher-performance perovskite CQD photovoltaics. Available strategies employed in the conventional CQD devices to enhance the carrier separation, such as the design of type-Ⅱ core-shell structure and versatile surface modification to tune the electronic properties, are still not applicable to the perovskite CQD system owing to the difficulty in modulating surface ligands and structural integrity. Herein, a facile strategy that takes advantage of conjugated small molecules that provide an additional driving force for effective charge separation in perovskite CQD solar cells is developed. The resulting perovskite CQD solar cell shows a power conversion efficiency approaching 13% with an open-circuit voltage of 1.10 V, short-circuit current density of 15.4 mA cm-2 , and fill factor of 74.8%, demonstrating the strong potential of this strategy toward achieving high-performance perovskite CQD solar cells.

This work was supported by the National Research Laboratory Program (R0A-2008-000-20057-0), the World Class University (WCU) program (R32-2008-000- 10051-0), the Pioneer Research Center Program (2009-0093758), and the National Research Foundation (NRF 2008-0062204) funded by the Korean government. Supporting Information is available online from Wiley InterScience or from the author.

We thank Dr C. M. Koo and Prof. D. G. Churchill for helpful discussions, and M. J. Woo for assisting with the powder diffractometer system. This work was supported by the Korea Research Foundation (KRF-2005-003-D00085), Creative Research Initiatives (Center for Time-Resolved Diffraction) of MOST/KOSEF, the second stage of the Brain Korea 21 Project, the Basic Research Program of the Korea Science &Engineering Foundation (R01-2005-000-10456-0), and the Korean Ministry of Science and Technology, the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Commerce, Industry and Energy, Korea. Supporting Information is available online from Wiley InterScience or from the author.

Liquid-crystalline peptide nanowires consisting of aromatic dipeptides are easily prepared by a sonication in a volatile organic solvent, report Hyotcherl Ihee, Sang Ouk Kim, and co-workers on p. 3924. The colloidal nematic liquid crystalline phase of the rigid nanowires allows for a macroscopically ordered morphology of the nanowires under an external electric field. The cover image presents a broad field view of nanowires oriented by an electric field between two electrodes.