The design and characterization of metal–organic complexes for optoelectronic applications is an active area of research. The metal–organic complex offers unique optical and electronic properties arising from the interplay between the inorganic metal and the organic ligand. The ability to modify chemical structure through control over metal–ligand interaction on a molecular level could directly impact the properties of the complex. When deposited in thin film form, this class of materials enable the fabrication of a wide variety of low-cost electronic and optoelectronic devices. These include light emitting diodes, solar cells, photodetectors, field-effect transistors as well as chemical and biological sensors. Here we present an overview of recent development in metal–organic complexes with controlled molecular structures and tunable properties. Advances in extending the control of molecular structures to solid materials for energy conversion and information technology applications will be highlighted.

/pdf/recent-progress-in-metal-organic-complexes-for-7man7jhubz.pdf

Recent progress in metal–organic complexes for optoelectronic applications

Polymers for flexible displays: From material selection to device applications

Degradation in organic light-emitting diodes (OLEDs) is a complex problem. Depending upon the materials and the device architectures used, the degradation mechanism can be very different. In this Progress Report, using examples in both small molecule and polymer OLEDs, the different degradation mechanisms in two types of devices are examined. Some of the extrinsic and intrinsic degradation mechanisms in OLEDs are reviewed, and recent work on degradation studies of both small-molecule and polymer OLEDs is presented. For small-molecule OLEDs, the operational degradation of exemplary fluorescent devices is dominated by chemical transformations in the vicinity of the recombination zone. The accumulation of degradation products results in coupled phenomena of luminance-efficiency loss and operating-voltage rise. For polymer OLEDs, it is shown how the charge-transport and injection properties affect the device lifetime. Further, it is shown how the charge balance is controlled by interlayers at the anode contact, and their effects on the device lifetime are discussed.

Degradation Mechanisms in Small-Molecule and Polymer Organic Light-Emitting Diodes

Advances in hole transport materials engineering for stable and efficient perovskite solar cells

Metal halide perovskites (MHPs) have numerous advantages as light emitters such as high photoluminescence quantum efficiency with a direct bandgap, very narrow emission linewidth, high charge-carrier mobility, low energetic disorder, solution processability, simple color tuning, and low material cost. Based on these advantages, MHPs have recently shown unprecedented radical progress (maximum current efficiency from 0.3 to 42.9 cd A-1 ) in the field of light-emitting diodes. However, perovskite light-emitting diodes (PeLEDs) suffer from intrinsic instability of MHP materials and instability arising from the operation of the PeLEDs. Recently, many researchers have devoted efforts to overcome these instabilities. Here, the origins of the instability in PeLEDs are reviewed by categorizing it into two types: instability of (i) the MHP materials and (ii) the constituent layers and interfaces in PeLED devices. Then, the strategies to improve the stability of MHP materials and PeLEDs are critically reviewed, such as A-site cation engineering, Ruddlesden-Popper phase, suppression of ion migration with additives and blocking layers, fabrication of uniform bulk polycrystalline MHP layers, and fabrication of stable MHP nanoparticles. Based on this review of recent advances, future research directions and an outlook of PeLEDs for display applications are suggested.

Improving the Stability of Metal Halide Perovskite Materials and Light-Emitting Diodes

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.

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

The authors report the effect of thermal treatment of hole-transporting interlayers between a polymeric hole injection layer and an emitting layer (EML) on the luminous efficiency and the lifetime performance in blue polymer light-emitting diodes. As the thermal annealing temperature of the interlayer increased, the hole mobility of the interlayer tended to decrease, which results in reducing the hole current injected into the EML in the devices. Hence, the device luminous efficiency decreased due to lower electron-hole balance. Nevertheless, the device lifetime increased, which can be attributed to the formation of the thicker interlayer and the better defined interlayer/EML interface.

http://phome.postech.ac.kr/user/pnel/publication/47.%20T.-W.%20Lee,%20M.-G.%20Kim,%20S.%20Y.%20Kim,%20S.%20H.%20Park,%20T.%20Y.%20Noh,%20T.-S.%20Oh,%20Applied%20Physics%20Letters,%2089,%20123505%20(2006)..pdf

Hole-transporting interlayers for improving the device lifetime in the polymer light-emitting diodes

The short device lifetime ofblue polymer light-emitting diodes (PLEDs) is still a bottleneck for commercialization of self emissive full-color displays. Since the cathode in the device has a dominant influence on the device lifetime, a systematic design ofthecathode structure is necessary. The operational lifetime ofblue PLEDs can be greatly improved by introducing a three-layer (BaF 2 /Ca/Al) cathode compared with conventional two-layer cathodes (BaF 2 /Al and Ba/Al). Therefore, the roles ofthe BaF 2 and Ca layers in terms of electron injection, luminous efficiency, and device lifetime are here investigated. For efficient electron injection, the BaF 2 layer should be deposited to the thickness of at least one monolayer (∼3 nm). However, it is found that the device lifetime does not show a strong relation with theelectron injection or luminous efficiency. In order to prolong the device lifetime, sufficient reaction between BaF 2 and the overlying Ca layer should take place during the deposition where the thickness of each layer is around that of a monolayer.

http://phome.postech.ac.kr/user/pnel/publication/76.%20T.-W.%20Lee,%20M.-G.%20Kim,%20S.%20H.%20Park,%20S.%20Y.%20Kim,%20O.%20Kwon,%20T.%20Noh,%20T.-L.%20Choi,%20J.%20H.%20Park,%20B.%20D.%20Chin,%20Advanced%20Functional%20Materials,%2019,%201863-1868%20(2009).pdf

Designing a Stable Cathode with Multiple Layers to Improve the Operational Lifetime of Polymer Light‐Emitting Diodes

We report the failure mode observed in polymer EL blue device. Such modes were analyzed by non-destructive and destructive ways. As a non-destructive way, investigation of mobility changes of hole and electron, measuring transient EL corresponding to life curve, had been done. We also observed compositional and morphological variation using TEM-EDX, STM, FT-IR, TOF-SIMS and reverse engineering method as a destructive way. Electron mobility has a higher dependency on life curve, which fact could be reflected on the formation of insoluble layer inside emitting layer on anode side. And such insoluble layer showed relatively ordered surface morphology, and might be a cross-linked layer through C-O-C bond cleavage process while EL operation. But, contrary to sulfur migration mechanism into insoluble layer insisted by CDT, we did confirm no obvious difference of sulfur composition between insoluble and emitting layers. Rather, there's some degree of Ba diffusion into emitting layer from decomposition of BaF 2 , but, which dose not have a major effect on device degradation.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S1946427400089958

Sang Hun Park

Papers

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

Hole-transporting interlayers for improving the device lifetime in the polymer light-emitting diodes

Designing a Stable Cathode with Multiple Layers to Improve the Operational Lifetime of Polymer Light‐Emitting Diodes

Failure Mode Investigation for Blue Polymer EL Device